



Network Working Group                                 F. L. Templin, Ed.
Internet-Draft                                        The Boeing Company
Intended status: Standards Track                            1 March 2026
Expires: 2 September 2026


    Transmission of IP Packets over Overlay Multilink Network (OMNI)
                               Interfaces
                      draft-templin-6man-omni3-80

Abstract

   Mobile nodes that operate in air/land/sea/space domains (e.g.,
   aircraft of various configurations, terrestrial vehicles, seagoing
   vessels, space systems, enterprise wireless devices, pedestrians with
   cell phones, etc.) configure mobile routers to connect end user
   network peers with Internetwork correspondents over wireless and/or
   wired-line data links.  This document presents a multilink virtual
   interface specification that supports mobile node coordination with a
   network-based mobility service, fixed node correspondents and/or
   other mobile node peers.  The virtual interface provides an
   adaptation layer service that supports secure global mobile
   Internetworking.  This document specifies the transmission of IP
   packets over Overlay Multilink Network (OMNI) Interfaces.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 2 September 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   4.  Overlay Multilink Network (OMNI) Interface Model  . . . . . .  19
   5.  OMNI Interface Maximum Transmission Unit (MTU)  . . . . . . .  26
   6.  The OMNI Adaptation Layer (OAL) . . . . . . . . . . . . . . .  27
     6.1.  OAL Source Encapsulation and Fragmentation  . . . . . . .  28
     6.2.  Underlay Encapsulation and Re-Encapsulation . . . . . . .  32
     6.3.  Reassembly and Decapsulation  . . . . . . . . . . . . . .  36
     6.4.  OMNI-Encoded IPv6 Extension Headers . . . . . . . . . . .  37
     6.5.  OMNI Full and Compressed Headers  . . . . . . . . . . . .  40
     6.6.  UDP/IP Encapsulation Avoidance  . . . . . . . . . . . . .  45
     6.7.  OAL Identification Window Maintenance . . . . . . . . . .  46
     6.8.  OAL Fragmentation Reports and Retransmissions . . . . . .  51
     6.9.  OMNI Interface MTU Feedback Messaging . . . . . . . . . .  53
     6.10. OAL Composite Packets . . . . . . . . . . . . . . . . . .  55
     6.11. OAL Bubbles . . . . . . . . . . . . . . . . . . . . . . .  57
     6.12. OAL Requirements  . . . . . . . . . . . . . . . . . . . .  57
     6.13. OAL Fragmentation Security Implications . . . . . . . . .  58
     6.14. Control/Data Plane Considerations . . . . . . . . . . . .  59
   7.  Ethernet-Compatible Link Layer Frame Format . . . . . . . . .  60
   8.  OMNI Addressing . . . . . . . . . . . . . . . . . . . . . . .  61
   9.  Node Identification . . . . . . . . . . . . . . . . . . . . .  63
   10. Address Mapping - Unicast . . . . . . . . . . . . . . . . . .  63
     10.1.  The OMNI Option  . . . . . . . . . . . . . . . . . . . .  65
     10.2.  OMNI Sub-Options . . . . . . . . . . . . . . . . . . . .  68
       10.2.1.  NULL Sub-Option  . . . . . . . . . . . . . . . . . .  69
       10.2.2.  CGA  . . . . . . . . . . . . . . . . . . . . . . . .  70
       10.2.3.  RSA Signature  . . . . . . . . . . . . . . . . . . .  71
       10.2.4.  Timestamp  . . . . . . . . . . . . . . . . . . . . .  72
       10.2.5.  Nonce  . . . . . . . . . . . . . . . . . . . . . . .  73
       10.2.6.  Trust Anchor . . . . . . . . . . . . . . . . . . . .  74
       10.2.7.  Certificate  . . . . . . . . . . . . . . . . . . . .  74
       10.2.8.  Hashed Message Authentication Code (HMAC)  . . . . .  75
       10.2.9.  Node Identification  . . . . . . . . . . . . . . . .  76
       10.2.10. Neighbor Synchronization . . . . . . . . . . . . . .  78
       10.2.11. Interface Attributes . . . . . . . . . . . . . . . .  80



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       10.2.12. Traffic Selector . . . . . . . . . . . . . . . . . .  84
       10.2.13. Geo Coordinates  . . . . . . . . . . . . . . . . . .  86
       10.2.14. PIM-SM Message . . . . . . . . . . . . . . . . . . .  86
       10.2.15. Fragmentation Report (FRAGREP) . . . . . . . . . . .  87
       10.2.16. Proxy/Server Control . . . . . . . . . . . . . . . .  89
       10.2.17. Prefix Information Option (PIO)  . . . . . . . . . .  90
       10.2.18. Route Information Option (RIO) . . . . . . . . . . .  91
       10.2.19. DHCPv6 Message . . . . . . . . . . . . . . . . . . .  91
   11. Address Mapping - Multicast . . . . . . . . . . . . . . . . .  92
   12. Multilink Conceptual Sending Algorithm  . . . . . . . . . . .  92
     12.1.  Multiple OMNI Interfaces . . . . . . . . . . . . . . . .  93
     12.2.  Client-Proxy/Server Loop Prevention  . . . . . . . . . .  94
   13. OAL Segment Routing . . . . . . . . . . . . . . . . . . . . .  95
   14. Router Discovery and Prefix Delegation  . . . . . . . . . . .  97
     14.1.  Client-Proxy/Server Window Synchronization . . . . . . . 108
     14.2.  IP Multihop and IPv4-Only Networks . . . . . . . . . . . 109
   15. Secure Redirection  . . . . . . . . . . . . . . . . . . . . . 113
   16. Proxy/Server Resilience . . . . . . . . . . . . . . . . . . . 114
   17. Detecting and Responding to Proxy/Server Failures . . . . . . 114
   18. OMNI Interfaces on Open Internetworks . . . . . . . . . . . . 115
   19. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . . . 117
   20. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 117
     20.1.  Protocol Numbers . . . . . . . . . . . . . . . . . . . . 117
     20.2.  IEEE 802 Numbers . . . . . . . . . . . . . . . . . . . . 117
     20.3.  IPv4 Special-Purpose Address . . . . . . . . . . . . . . 118
     20.4.  Segment Routing Header TLVs  . . . . . . . . . . . . . . 118
     20.5.  IANA OUI Ethernet Numbers  . . . . . . . . . . . . . . . 118
     20.6.  Overlay Multilink Network (OMNI) Interface Registry
            Group  . . . . . . . . . . . . . . . . . . . . . . . . . 118
       20.6.1.  OMNI Option Sub-Types (New Registry) . . . . . . . . 119
       20.6.2.  OMNI Node Identification ID-Types (New Registry) . . 119
       20.6.3.  OMNI Geo Coordinates Types (New Registry)  . . . . . 120
     20.7.  Additional Considerations  . . . . . . . . . . . . . . . 120
   21. Security Considerations . . . . . . . . . . . . . . . . . . . 121
   22. Implementation Status . . . . . . . . . . . . . . . . . . . . 122
   23. Document Updates  . . . . . . . . . . . . . . . . . . . . . . 122
   24. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 123
   25. References  . . . . . . . . . . . . . . . . . . . . . . . . . 124
     25.1.  Normative References . . . . . . . . . . . . . . . . . . 124
     25.2.  Informative References . . . . . . . . . . . . . . . . . 127
   Appendix A.  VDL Mode 2 Considerations  . . . . . . . . . . . . . 138
   Appendix B.  Client-Proxy/Server Isolation Through Link-Layer
           Address Mapping . . . . . . . . . . . . . . . . . . . . . 139
   Appendix C.  IPv4 as an OAL Encapsulation Service . . . . . . . . 140
   Appendix D.  Change Log . . . . . . . . . . . . . . . . . . . . . 140
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 141





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1.  Introduction

   Mobile nodes that operate in air/land/sea/space domains (e.g.,
   aircraft of various configurations, terrestrial vehicles, seagoing
   vessels, space systems, enterprise wireless devices, pedestrians with
   cellphones, etc.) configure mobile routers that connect end user
   network peers with Internetwork correspondents over multiple
   interface connections to wireless and/or wired-line data links.
   These data links often have diverse performance, cost and
   availability properties that can change dynamically due to mobility
   patterns, flight phases, proximity to infrastructure, etc.  The
   mobile router acts as a Client of a network-based Mobility Service
   (MS) by configuring a virtual interface over its underlay interface
   data link connections to support secure global mobile
   Internetworking.

   Each Client configures a virtual network interface (termed the
   "Overlay Multilink Network (OMNI) Interface") as a thin layer over
   its underlay interfaces which may themselves connect to virtual or
   physical links.  The OMNI interface therefore exposes a single
   interface abstraction to the IP layer which behaves according to the
   Non-Broadcast, Multiple Access (NBMA) interface principle, while each
   underlay interface appears as a link layer communication channel in
   the architecture.  The OMNI interface appears as an ordinary network
   interface and internally employs the "OMNI Adaptation Layer (OAL)" to
   efficiently accommodate original IP packets of all sizes over diverse
   underlay interfaces with heterogeneous properties.

   The OMNI interface connects to a virtual overlay termed the "OMNI
   link" which spans one or more concatenated Internetwork underlays
   such as private-use infrastructures (e.g., enterprise networks,
   operator networks, etc.) and/or the global public Internet itself.
   Together, OMNI and the OAL provide foundational elements necessary to
   support the "6 M's of Modern Internetworking", including:

   1.  Multilink - a Client's ability to coordinate multiple diverse
       underlay interfaces as a single logical unit (i.e., the OMNI
       interface) to achieve the required communications performance and
       reliability objectives.

   2.  Multinet - the ability to span the OMNI link over an end to end
       topology connecting multiple diverse administrative domain
       network segments while maintaining seamless communications
       between mobile Clients and correspondents such as air traffic
       controllers, fleet administrators, other mobile Clients, etc.






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   3.  Mobility - a Client's ability to change network points of
       attachment (e.g., moving between wireless base stations) which
       may result in an underlay interface address change, but without
       disruptions to ongoing communication sessions with peers over the
       OMNI link.

   4.  Multicast - the ability to send a single network transmission
       that reaches multiple Clients belonging to the same interest
       group, but without disturbing other Clients not subscribed to the
       interest group.

   5.  Multihop - a mobile Client peer-to-peer relaying capability
       useful when multiple forwarding hops between peers may be
       necessary to reach a target peer or an infrastructure access
       point connection to the OMNI link.

   6.  (Performance) Maximization - the ability to exchange packets of
       all sizes between peers without loss due to a link size
       restriction, and to adaptively adjust packet sizes to maintain
       the best performance profile for each independent traffic flow.

   Client OMNI interfaces coordinate with the MS and/or OMNI link peers
   through secure IPv6 Neighbor Discovery (ND) control message exchanges
   [RFC4861].  The MS consists of a distributed set of service elements
   (including fixed or mobile Proxy/Servers and other supporting
   infrastructure) that also configure OMNI interfaces.  Automatic
   Extended Route Optimization (AERO) in particular provides a candidate
   MS compatible with the OMNI architecture [I-D.templin-6man-aero3].
   AERO discusses details of ND messaging for address resolution,
   multilink forwarding, route optimization, mobility management, and
   multinet traversal, while fundamental aspects of OMNI link operation
   and router discovery are discussed in this document.

   Clients that connect to multiple distinct OMNI links configure a
   corresponding OMNI interface for each link, e.g., omni0, omni1,
   omni2, etc.  Each OMNI interface is configured over a distinct set of
   underlay interfaces and provides a nexus for Safety-Based Multilink
   (SBM) operation within an OMNI domain.  The IP layer applies SBM
   routing to select a specific OMNI interface which then applies
   Performance-Based Multilink (PBM) to select appropriate underlay
   interfaces.  Applications select SBM topologies based on IP layer
   Segment Routing [RFC8402], while each OMNI interface applies PBM
   internally based on OAL Multinet traversal.

   Each OMNI interface assigns an external Link-Local Address (LLA) for
   use by the network layer the same as for any IPv6 interface and also
   assigns a different internal LLA for use by the adaptation layer.
   The adaptation layer then presents the appearance of a virtual router



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   on the same link as the network layer in the role of a virtual host
   allowing for the exchange of ICMPv6 control messages per
   [RFC4443][RFC4861].  The OMNI interface also assigns a unique
   Multilink Local Address (MLA) to support multilink operations.

   Each OMNI link assigns one or more IP Global Unicast Address (GUA)
   Mobility Service Prefixes (MSPs).  The MS then delegates network
   layer Mobile Network Prefixes (MNPs) taken from an MSP to Client end
   systems according to the "prefix-per-Client" model [RFC9663]
   [RFC9762].

   Clients receive MNP delegations from Proxy/Servers through OMNI-
   encapsulated ICMPv6 control message exchanges over Access Networks
   (ANETs), Mobile Ad-hoc Networks (MANETs) and/or open Internetworks
   (INETs).  Clients sub-delegate MNPs to downstream-attached End-user
   Networks (EUNs) independently of the underlay interfaces selected for
   upstream data transport.  Each Client acts as a fixed or mobile
   router on behalf of EUN peers, and uses OMNI interface control
   messaging to coordinate with Proxy/Servers and/or other Clients.  The
   Client registers with Proxy/Servers over each of the OMNI interface's
   (M)ANET/INET underlay interfaces to enroll each interface with the MS
   (see: Section 14).  The Client can also provide (proxyed) multihop
   forwarding services for a recursively extended chain of other Clients
   and end systems connected via downstream-attached *NETs.

   Proxy/Servers on the link delegate MNPs to Clients via the Dynamic
   Host Configuration Protocol for IPv6 (DHCPv6) service.  DHCPv6
   messaging is carried as OMNI extensions to encapsulated ICMPv6 router
   discovery messages since each Proxy/Server supports both DHCPv6
   Server and IPv6 router functions.

   Peer-to-peer neighbor coordination over the OMNI link proceeds
   according to either the "on-link" or "off-link" IPv6 Neighbor
   Discovery model.  For IP destinations that match on-link prefixes,
   the initiating peer's network layer invokes address resolution to
   create a network layer neighbor cache entry and also cause the
   interface to create a corresponding adaptation layer cache entry.
   The network layer then engages the OMNI interface as a "virtual
   Ethernet" [VETH] or "TAP" [TUNTAP] interface including the mapping of
   network layer addresses to link-layer addresses.  The on-link model
   may be more suitable for general purpose systems and/or when multiple
   OMNI interfaces are necessary, but requires careful coordination
   between the network and adaptation layers.

   For IP destinations that do not match on-link prefixes the network
   layer forwards all original IP packets to a virtual router function
   within the OMNI interface without disturbing the network layer
   neighbor cache.  The OMNI interface then invokes address resolution



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   as an adaptation layer function.  This off-link model can be
   coordinated over virtual Ethernet and TAP interfaces the same as for
   the on-link model, or can instead use a "TUN" interface [TUNTAP]
   without mapping network layer addresses to link-layer addresses.  The
   off-link model may be more applicable for end user equipment such as
   cellphones where administrative privileges may not support creation
   of virtual Ethernet interfaces, or when operation may be simplified
   by avoiding coordination between the network and adaptation layer.
   Implementations are free to select the on- or off-link model on a
   per-prefix basis independently of other nodes on the link, as the
   external appearance is identical in both cases.

   The OMNI link model is suitable for a wide range of use cases.  For
   example, the International Civil Aviation Organization (ICAO) Working
   Group-I Mobility Subgroup is developing a future Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS)
   and has issued a liaison statement requesting IETF adoption [ATN] in
   support of ICAO Document 9896 [ATN-IPS].  The IETF IP Wireless Access
   in Vehicular Environments (ipwave) working group has further included
   problem statement and use case analysis for OMNI in [RFC9365].  Still
   other communities of interest include AEEC, RTCA Special Committee
   228 (SC-228) and NASA programs that examine commercial aviation,
   Urban Air Mobility (UAM) and Unmanned Air Systems (UAS).  Pedestrians
   with handheld mobile devices using 5G/6G wireless services, home and
   small office networks, enterprise networks and many others represent
   additional large classes of potential OMNI users.

   This document specifies the transmission of original IP packets and
   control messages over OMNI interfaces.  The operation of both IP
   protocol versions (i.e., IPv4 [RFC0791] and IPv6 [RFC8200]) is
   specified as the network layer data plane, while OMNI interfaces use
   ICMPv6 messaging in the control plane independently of the data plane
   protocol(s).  OMNI interfaces also provide an adaptation layer based
   on encapsulation and fragmentation for transmission over
   heterogeneous underlay interfaces as an OAL sublayer between L3 and
   an underlay configured over any L2 media.  OMNI and the OAL are
   specified in detail throughout the remainder of this document.

2.  Terminology

   The terminology in the normative references applies; especially, the
   terms "link" and "interface" are the same as defined in the IPv6
   [RFC8200] and IPv6 Neighbor Discovery (ND) [RFC4861][RFC9762]
   specifications.  This document assumes the following IPv6 ND control
   plane message types: Router Solicitation (RS), Router Advertisement
   (RA), Neighbor Solicitation (NS), Neighbor Advertisement (NA),
   unsolicited NA (uNA) and Redirect.




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   The terms "All-Routers multicast", "All-Nodes multicast" and "Subnet-
   Router anycast" are the same as defined in [RFC4291].  Also, IPv6 ND
   state names, variables and constants including REACHABLE,
   ReachableTime and REACHABLE_TIME are the same as defined in
   [RFC4861].

   The term "IP" is used to refer collectively to either Internet
   Protocol version (i.e., IPv4 [RFC0791] or IPv6 [RFC8200]) when a
   specification at the layer in question applies equally to either
   version.

   The terms (Proxy/)Client and Proxy/Server are intentionally
   capitalized to denote an instance of a particular node type that also
   configures an OMNI interface and engages the OMNI Adaptation Layer.

   The terms "application layer (L5 and higher)", "transport layer
   (L4)", "network layer (L3)", "(data) link layer (L2)" and "physical
   layer (L1)" are used consistently with common Internetworking
   terminology, with the understanding that reliable delivery protocol
   users of UDP are considered as transport layer elements.  The OMNI
   specification further defines an "adaptation layer" positioned below
   the network layer and above the link layer, which may include
   physical links and Internet- or higher-layer tunnels.  A (network)
   interface is a node's attachment to a link (via L2), and an OMNI
   interface is therefore a node's attachment to an OMNI link (via the
   adaptation layer).

   The following terms are defined within the scope of this document:

   GUA, ULA, LLA, MLA
      A Globally-Unique (GUA), Unique-Local (ULA) or Link-Local (LLA)
      Address per the IPv6 addressing architecture [RFC4193] [RFC4291],
      or a Multilink-Local Address (MLA) per [I-D.templin-6man-mla].
      IPv4 prefixes other than those reserved for special purposes
      [RFC6890] are also considered as GUA prefixes.

   L3
      The Network layer in the OSI reference model, also known as "layer
      3" or the "IP layer".  The Network layer engages the Adaptation
      layer via OMNI interfaces.

   Adaptation layer
      An IPv6 encapsulation and fragmentation mid-layer that adapts L3
      to a diverse collection of underlay interfaces.  The adaptation
      layer then engages an underlay network that performs UDP/IP, IP,
      or NULL encapsulation for transmission over underlay interface
      attachments to L2 media.




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   L2
      The Data Link layer in the OSI reference model, also known as
      "layer 2" or "link layer".

   Access Network (ANET)
      a connected network region (e.g., an aviation radio access
      network, corporate enterprise network, satellite service provider
      network, cellular operator network, residential WiFi network,
      etc.) that connects Clients to the rest of the OMNI link.
      Physical and/or data link level security is assumed (sometimes
      referred to as "protected spectrum" for wireless domains).  ANETs
      such as private enterprise networks and ground domain aviation
      service networks often provide multiple secured IP hops between
      the Client's physical point of connection and the nearest Proxy/
      Server.

   Mobile Ad-hoc NETwork (MANET)
      a connected ANET region for which links often have undetermined
      connectivity properties, lower layer security services cannot
      always be assumed and multihop forwarding between Clients acting
      as MANET routers may be necessary.  Clients nested deeply within a
      MANET often require adaptation layer proxy services from
      "upstream" peer Proxy/Clients located progressively nearer the
      MANET border.  The OMNI link model naturally supports MANET
      Internetworking [I-D.templin-manet-inet].

   Internetwork (INET)
      a connected network region with a coherent IP addressing plan that
      provides transit forwarding services between (M)ANETs and/or OMNI
      nodes that coordinate with the Mobility Service over unprotected
      media.  Since physical and/or data link level security cannot
      always be assumed, security must be applied by higher layers in
      the architecture as necessary.  The global public Internet itself
      is an example.

   End-user Network (EUN)
      a simple or complex "downstream" network tethered to a Client as a
      single logical unit that travels together.  The EUN could be as
      simple as a single link connecting a small number of hosts, or as
      complex as a large network with many links, routers, bridges and
      end user devices.  The EUN provides an "upstream" link for
      arbitrarily many low-, medium- or high-end devices dependent on
      the Client for their upstream connectivity, i.e., as Internet of
      Things (IoT) entities.

   *NET
      a "wildcard" term used when a given specification applies equally
      to all MANET/ANET/INET cases.  From the Client's perspective, *NET



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      interfaces are "upstream" interfaces that connect the Client to
      the Mobility Service, while EUN interfaces are "downstream"
      interfaces that the Client uses to connect downstream EUNs and/or
      other Clients.  Local communications between correspondents within
      the same *NET can often be conducted based on IPv6 MLAs
      [I-D.templin-6man-mla].

   underlay network/interface
      a *NET interface over which an OMNI interface is configured.  The
      network layer engages the OMNI interface as an ordinary network
      interface, and the adaptation layer engages each underlay
      interface as a link layer conduit.  The underlay includes UDP/IP,
      IP or NULL encapsulations for data units transferred between the
      adaptation and link layers.

   MANET Interface
      a node's underlay interface to a local network with indeterminant
      neighborhood properties over which multihop relaying may be
      necessary.  All MANET interfaces used by AERO/OMNI are IPv6
      interfaces and therefore must configure a Maximum Transmission
      Unit (MTU) no smaller than the IPv6 minimum MTU (1280 octets) even
      if lower-layer fragmentation is needed.

   OMNI link
      a Non-Broadcast, Multiple Access (NBMA) virtual overlay configured
      over one or more INETs and their connected (M)ANETs/EUNs.  An OMNI
      link may comprise multiple distinct "segments" joined by "bridges"
      the same as for any link; the addressing plans in each segment may
      be mutually exclusive and managed by different administrative
      entities.  Proxy/Servers and other infrastructure elements extend
      the link to support communications between Clients as single-hop
      neighbors.

   OMNI link segment
      a Proxy/Server and all of its constituent Clients within any
      attached *NETs is considered as a leaf OMNI link segment, with
      each leaf interconnected via links and "bridge" nodes in
      intermediate OMNI link segments.  When the *NETs of multiple leaf
      segments overlap (e.g., due to network mobility), they can combine
      to form larger *NETs with no changes to Client-to-Proxy/Server
      relationships.  The OMNI link consists of the concatenation of all
      OMNI link leaf and intermediate segments as a loop-free spanning
      tree.

   OMNI interface
      a node's virtual interface to an OMNI link, and configured over
      one or more underlay interfaces.  If there are multiple OMNI links
      in an OMNI domain, a separate OMNI interface is configured for



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      each link.  The OMNI interface configures a Maximum Transmission
      Unit (MTU) and an Effective MTU to Receive (EMTU_R) the same as
      any interface.  The OMNI interface assigns an "external" LLA and
      Ethernet link-layer address the same as for any IPv6 interface,
      assigns a different "internal" LLA and link-layer address to
      support a virtual internal router entity, and assigns an MLA for
      adaptation layer addressing over underlay interfaces.  Since OMNI
      interface MLAs are managed for uniqueness and LLAs are used for
      node-local operations only, OMNI interfaces assume Optimistic
      Duplicate Address Detection (DAD) per [RFC4429].

   OMNI Adaptation Layer (OAL)
      an OMNI interface sublayer service that encapsulates original IP
      packets admitted into the interface in an IPv6 header and/or
      subjects them to fragmentation and reassembly.  The OAL is also
      responsible for generating MTU-related control messages as
      necessary, and for providing addressing context for OMNI link SRT
      traversal.  The OAL presents a new layer in the Internet
      architecture known simply as the "adaptation layer".  The OMNI
      link is an example of a limited domain [RFC8799] at the adaptation
      layer although its segments may be joined over open Internetworks
      in the underlay.

   OMNI Option
      a pseudo IPv6 ND option providing multilink parameters for the
      OMNI interface as specified in Section 10.  The OMNI option is
      appended to the end of a control message during OAL encapsulation
      such that it appears immediately following the final message
      option or composite packet extension.

   (OMNI) Client
      a network platform/device mobile router or end system that
      configures one or more OMNI interfaces over distinct sets of
      underlay interfaces grouped as logical OMNI link units.  The
      Client coordinates with the Mobility Service via upstream networks
      over *NET interfaces, and may also serve as a Proxy for other
      Clients on downstream *NETs.  The Client's upstream network
      interface addresses and performance characteristics may change
      over time (e.g., due to node mobility, link quality, etc.) while
      other Clients on downstream networks can engage the (upstream)
      Client as a Proxy.

   (OMNI) Proxy/Server
      a segment management topology edge node that configures an OMNI
      interface and connects Clients to the Mobility Service.  As a
      server, the Proxy/Server responds directly to some Client IPv6 ND
      messages.  As a proxy, the Proxy/Server forwards other Client IPv6
      ND messages to other Proxy/Servers and Clients.  As a router, the



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      Proxy/Server provides a forwarding service for ordinary data
      messages that may be essential in some environments and a last
      resort in others.  Proxy/Servers at (M)ANET boundaries configure
      both an (M)ANET downstream interface and *NET upstream interface,
      while INET-based Proxy/Servers configure only an INET interface.

   First-Hop Segment (FHS) Proxy/Server
      a Proxy/Server connected to the source Client's *NET that forwards
      OAL packets sent by the source into the segment management
      topology.  FHS Proxy/Servers act as intermediate forwarding
      systems to facilitate RS/RA-based Prefix Delegation exchanges
      between Clients and Mobility Anchor Point (MAP) Proxy/Servers.

   Last-Hop Segment (LHS) Proxy/Server
      a Proxy/Server connected to the target Client's *NET that forwards
      OAL packets received from the segment management topology to the
      target.

   Mobility Anchor Point (MAP) Proxy/Server
      a Proxy/Server selected by the Client that provides a designated
      router service for any *NET underlay networks that register the
      Client's Mobile Network Prefix (MNP).  Since all Proxy/Servers
      provide equivalent services, Clients normally select the first FHS
      Proxy/Server they coordinate with to serve as the MAP.  However,
      the MAP can instead be any available Proxy/Server for the OMNI
      link, i.e., and not necessarily one of the Client's FHS Proxy/
      Servers.  This flexible arrangement supports a fully distributed
      mobility management service.

   Segment Routing Topology (SRT)
      a multinet forwarding region configured over one or more INETs
      between the FHS Proxy/Server and LHS Proxy/Server, where each INET
      appears as a segment in a virtual overlay link.  The SRT spans the
      OMNI link on behalf of communicating peer nodes in a manner
      outside the scope of this document (see:
      [I-D.templin-6man-aero3]).

   Mobility Service (MS)
      a mobile routing service that tracks Client movements and ensures
      that Clients remain continuously reachable even across mobility
      events.  The MS consists of the set of all Proxy/Servers plus all
      other OMNI link supporting infrastructure nodes.  Specific MS
      details are out of scope for this document, with an example found
      in [I-D.templin-6man-aero3].

   Mobility Service Prefix (MSP)
      an aggregated IP GUA prefix (e.g., 2001:db8::/32,
      2002:192.0.2.0::/40, etc.) assigned to the OMNI link and from



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      which more-specific Mobile Network Prefixes (MNPs) are delegated,
      where IPv4 MSPs are represented as "6to4 prefixes" per [RFC3056].
      OMNI link administrators typically obtain MSPs from an Internet
      address registry, however private-use prefixes can also be used
      subject to certain limitations (see: Section 8).  OMNI links that
      connect to the global Internet advertise their MSPs to their
      interdomain routing peers.

   Mobile Network Prefix (MNP)
      a longer IP prefix delegated from an MSP (e.g.,
      2001:db8:1000:2000::/56, 2002:192.0.2.8::/46, etc.) and assigned
      to a Client.  Clients receive MNPs from MAP Proxy/Servers and sub-
      delegate them to routers in downstream EUNs.

   Foreign Network Prefix (FNP)
      a global IP prefix not covered by a MSP and assigned to a link or
      network outside of the OMNI domain.

   Subnet Router Anycast (SRA) Address
      An IPv6 address taken from an FNP/MNP in which the remainder of
      the address beyond the prefix is set to the value "all-zeros".
      For example, the SRA for 2001:db8:1::/64 is simply 2001:db8:1::
      (i.e., with the 64 least significant bits set to 0).  For IPv4,
      the IPv6 SRA corresponding to the IPv4 prefix 192.0.2.0/24 is
      2002:192.0.2.0::/40 per [RFC3056].

   original IP packet
      a whole IP packet or fragment admitted into the OMNI interface by
      the network layer prior to OAL encapsulation/fragmentation, or an
      IP packet delivered to the network layer by the OMNI interface
      following OAL reassembly/decapsulation.

   OAL packet
      an original IP packet encapsulated in an OAL IPv6 header with an
      IPv6 Extended Fragment Header extension that includes an 8 octet
      (64-bit) OAL Identification value.  Each OAL packet is then
      subject to fragmentation by the source and reassembly by the
      destination.

   OAL fragment
      a portion of an OAL packet following fragmentation but prior to
      underlay encapsulation, or following underlay decapsulation but
      prior to OAL reassembly.








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   (OAL) atomic fragment
      an OAL packet that can be forwarded without fragmentation, but
      still includes an IPv6 Extended Fragment Header with an 8 octet
      (64-bit) OAL Identification value and with Index and More
      Fragments both set to 0.

   carrier packet
      an OAL packet or fragment submitted for underlay interface
      encapsulation.  OAL nodes exchange carrier packets over underlay
      interfaces in a hop-by-hop fashion beginning with the OAL source,
      then continuing over any OAL intermediate nodes and ending with
      the OAL destination.  Each intermediate hop removes the underlay
      encapsulations of the previous hop and inserts underlay
      encapsulations appropriate for the next hop.  Carrier packets may
      themselves be subject to fragmentation and reassembly in some IPv4
      underlay networks at a layer below the OAL.  Carrier packets sent
      over unsecured paths use OMNI protocol underlay encapsulations,
      while those sent over secured paths use security encapsulations
      such as IPsec [RFC4301].  (The term "carrier" honors agents of the
      service postulated by [RFC1149] and [RFC6214].)

   OAL source
      a node that configures an OMNI interface acts as an OAL source
      when it encapsulates original IP packets to form OAL packets, then
      performs OAL fragmentation and encapsulation to create carrier
      packets.  Every OAL source is also an OMNI link ingress.

   OAL destination
      a node that configures an OMNI interface acts as an OAL
      destination when it decapsulates carrier packets (following
      reassembly if necessary), then performs OAL reassembly/
      decapsulation to derive the original IP packet.  Every OAL
      destination is also an OMNI link egress.


















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   OAL intermediate system
      an OMNI interface acts as an OAL intermediate system when it
      decapsulates carrier packets received from a first segment to
      obtain the original OAL packet/fragment, then re-encapsulates in
      new underlay headers and sends these new carrier packets into the
      next segment.  OAL intermediate systems decrement the Hop Limit in
      OAL packets/fragments during forwarding, and discard the OAL
      packet/fragment if the Hop Limit reaches 0.  OAL intermediate
      systems do not decrement the TTL/Hop Limit of the original IP
      packet, which can only be updated by the network and higher
      layers.  OAL intermediate systems along the path explicitly
      addressed by the OAL IPv6 Destination (e.g., Proxys, etc.) are
      regarded as "endpoint" intermediate systems while those not
      explicitly addressed (e.g., MANET routers, AERO Gateways, etc.)
      are regarded as "transit" intermediate systems.

   Multilink
      a Client OMNI interface's manner of managing multiple diverse *NET
      underlay interfaces as a single logical unit.  The OMNI interface
      provides a single unified interface to the network layer, while
      underlay interface selections are performed on a per-flow basis
      considering traffic selectors such as Traffic Class, Flow Label,
      application policy, signal quality, cost, etc.  Multilink
      selections are coordinated in both the outbound and inbound
      directions based on source/target underlay interface pairs.

   Multinet
      an intermediate system's manner of spanning multiple diverse IP
      Internetwork and/or private enterprise network "segments" through
      OAL encapsulation.  Multiple diverse Internetworks (such as the
      global public IPv4 and IPv6 Internets) can serve as transit
      segments in an end-to-end OAL forwarding path through intermediate
      system concatenation of SRT network segments.  This OAL
      concatenation capability provides benefits such as supporting
      IPv4/IPv6 transition and coexistence, joining multiple diverse
      operator networks into a cooperative single service network, etc.
      See: [I-D.templin-6man-aero3] for further information.

   Multihop
      an iterative relaying of carrier packets between Clients over an
      OMNI underlay interface technology (such as omnidirectional
      wireless) without support of fixed infrastructure.  Multihop
      services entail Client-to-Client relaying within a Mobile/
      Vehicular Ad-hoc Network (MANET/VANET) for Vehicle-to-Vehicle
      (V2V) communications and/or for Vehicle-to-Infrastructure (V2I)
      "range extension" where Clients within range of communications
      infrastructure elements provide forwarding services for other
      Clients.



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   Mobility
      any action that results in a change to a Client underlay interface
      address.  The change could be due to, e.g., a handover to a new
      wireless base station, loss of link due to signal fading, an
      actual physical node movement, etc.

   Safety-Based Multilink (SBM)
      A means for ensuring fault tolerance through redundancy by
      connecting multiple OMNI interfaces within the same domain to
      independent routing topologies (i.e., multiple independent OMNI
      links).  SBM can also include non-terrestrial physical link types
      such as low earth orbit satellite services in a hyperconnected
      service model.

   Performance Based Multilink (PBM)
      A means for selecting one or more underlay interface(s) for
      carrier packet transmission and reception within a single OMNI
      interface.

   OMNI Domain
      The set of all SBM/PBM OMNI links that collectively provides
      services for a common set of MSPs.  All OMNI links within the same
      domain configure, advertise and respond to the SRA address(es)
      corresponding to the MSP(s) assigned to the domain.

   flow
      a sequence of packets sent from a particular source to a
      particular unicast, anycast, or multicast destination that a node
      desires to label as a flow.  The 3-tuple (Source Address,
      Destination Address, Flow Label) enables efficient IPv6 flow
      classification.  All packets of the same flow will receive
      identical forwarding services over the OMNI link and must
      therefore have compatible traffic classifications.  The IPv6 Flow
      Label Specification is observed per [RFC6437][RFC6438].

   AERO Flow Information Base (AFIB)
      A multilink forwarding table on each OAL source, destination and
      intermediate system that includes AERO Flow Vectors (AFV) with
      both next hop forwarding instructions and context for
      reconstructing compressed headers for specific underlay interface
      pairs used to transport flows from a source to a destination.
      See: [I-D.templin-6man-aero3] for further discussion.

   AERO Flow Vector (AFV)
      An AFIB entry that includes soft state for each underlay interface
      pairwise communication flow from source to destination.  AFVs are
      identified by an AFV Index (AFVI) paired with the previous hop
      underlay address, with the pair established based on an IPv6 ND



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      solicitation and solicited IPv6 ND advertisement response.  The
      AFV also caches underlay interface pairwise Identification
      sequence number parameters to support carrier packet filtering.
      See: [I-D.templin-6man-aero3] for further discussion.

   AERO Flow Vector Index (AFVI)
      A 2 or 4 octet integer value supplied by a previous hop OAL node
      when it requests a next hop OAL node to create an AFV.  (The AFVI
      is always processed as a 4 octet value, but compressed headers may
      omit the 2 most significant octets when they encode the value 0.)
      The next hop OAL node caches the AFVI and underlay address
      supplied by the previous hop as header compression/decompression
      state for future OAL packets with compressed headers.  The
      previous hop OAL node must ensure that the AFVI values it assigns
      to the next hop via a specific underlay interface are distinct and
      reused only after their useful lifetimes expire.  The special
      value 0 means that no AFVI is asserted.

   underlay encapsulation
      the OMNI protocol encapsulation of OAL packets/fragments in an
      outer header or headers to form carrier packets that can be routed
      within the scope of the local *NET underlay network partition.
      Common underlay encapsulation combinations include UDP, IP and/or
      Ethernet using a port/protocol/type number for OMNI.

   underlay-extended (UNX) address
      an address that appears in the OMNI protocol underlay
      encapsulation for an underlay interface and also in control
      message OMNI options.  UNX can be either an IP address for
      (UDP/)IP encapsulations or an IEEE EUI address [EUI] for direct
      data link encapsulation.  (When UDP/IP encapsulation is used, the
      UDP port number is considered an extension of the IP UNX.)

   OAL Fragment Size (OFS)
      the current OAL source fragmentation size for a given flow which
      must be no smaller than 1024 octets and should be no larger than
      65279 octets to allow sufficient space for OAL and underlay
      encapsulations.  (OFS pertains to the fragment payload immediately
      following the fragment header; if OAL extension headers are
      included following the first fragment header a slightly larger
      minimum OFS may be necessary to accommodate maximum-sized
      packets.)  Each OAL source maintains OFS in an AERO Flow Vector
      (AFV) for each independent flow.  The OAL source discovers larger
      OFS sizes through dynamic probing the same as defined for Maximum
      Packet Size (MPS) probing per Section 4.4 of [RFC8899] and should
      adaptively maintain the best possible OFS for each flow according
      to current network conditions.




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3.  Requirements

   OMNI interfaces maintain the same Conceptual Data Structures as for
   any IPv6 interface, including the Neighbor Cache, Destination Cache,
   Prefix List and Default Router List [RFC4861].  These data structures
   are affected by the exchange of IPv6 ND messages in the same manner
   as for any IPv6 interface.  The OMNI interface also internally
   configures an extension to the Neighbor Cache that includes
   adaptation layer information managed in parallel with network layer
   information.  This document refers to the distinct neighbor cache
   views as the Adaptation Layer Neighbor Cache (ALNC) and Network Layer
   Neighbor Cache (NLNC).

   Client and Proxy/Server OMNI interfaces maintain per-destination
   state in Destination Cache Entries (DCEs) as a level of indirection
   into per-neighbor state in Neighbor Cache Entries (NCEs) which
   include both network layer (NLNCE) and adaptation layer (ALNCE)
   information.  The IPv6 ND Protocol Constants associated with these
   caches defined in Section 10 of [RFC4861] apply also to this
   document.

   OMNI interfaces should limit the size of their control plane messages
   (plus any original IP packet attachments) to the adaptation layer
   path MTU which may be as small as the minimum IPv6 link MTU minus
   encapsulation overhead.  If there are sufficient OMNI parameters and/
   or IP packet attachments that would cause a control message to exceed
   this size, the OMNI interface should forward the information as
   multiple smaller messages and the recipient accepts the union of all
   information received.  This allows the messages to travel without
   loss due to a size restriction over secured control plane paths that
   include IPsec tunnels [RFC4301], secured direct point-to-point links
   and/or unsecured paths that require an authentication signature.

   L3, the adaptation layer and the underlay each include distinct
   packet Identification numbering spaces.  The adaptation layer employs
   an extended Identification numbering space that is distinct from the
   L3 and underlay spaces, with an Identification value appearing in an
   IPv6 Extended Fragment Header [I-D.templin-6man-ipid-ext2] or an OMNI
   Compressed Header (OCH) (see: Section 6.5) in each adaptation layer
   encapsulation.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.





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4.  Overlay Multilink Network (OMNI) Interface Model

   The IP layer engages the OMNI interface as a virtual interface
   configured over one or more underlay interfaces, which may be
   physical (e.g., an aeronautical radio link, a cellular wireless link,
   etc.) or virtual (e.g., an internet-layer or higher-layer "tunnel").
   The OMNI interface architectural layering model is the same as in
   [RFC5558][RFC7847], and augmented as shown in Figure 1.  The network
   layer therefore engages the OMNI interface as a single L3 interface
   nexus for multiple underlay interfaces that appear as L2
   communication channels in the architecture.

                                     +----------------------------+
                                     |    Upper Layer Protocol    |
              Session-to-IP    +---->|                            |
              Address Binding  |     +----------------------------+
                               +---->|           IP (L3)          |
              IP Address       +---->|                            |
              Binding          |     +----------------------------+
                               +---->|       OMNI Interface       |
              Logical-to-      +---->|   (OMNI Adaptation Layer)  |
              Physical         |     +----------------------------+
              Interface        +---->|  L2  |  L2  |       |  L2  |
              Binding                |(IF#1)|(IF#2)| ..... |(IF#n)|
                                     +------+------+       +------+
                                     |  L1  |  L1  |       |  L1  |
                                     |      |      |       |      |
                                     +------+------+       +------+

           Figure 1: OMNI Interface Architectural Layering Model

   Each underlay interface provides an L2/L1 abstraction according to
   one of the following models:

   *  INET interfaces connect to an INET either natively or through IP
      Network Address Translators (NATs).  Native INET interfaces have
      global IP addresses that are reachable from any INET
      correspondent.  NATed INET interfaces typically configure private
      IP addresses and connect to a private network behind one or more
      NATs with the outermost NAT providing INET access.

   *  (M)ANET interfaces connect to a (M)ANET that is connected to the
      open INET by Proxy/Servers.  The (M)ANET interface may be either
      on the same link segment as a Proxy/Server, or separated from a
      Proxy/Server by multiple adaptation layer and/or underlay/L2 hops.
      (Note that NATs may appear internally within a (M)ANET or on the
      Proxy/Server itself and may require NAT traversal the same as for
      the INET case.)



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   *  EUN interfaces connect a Client's downstream-attached networks,
      where the Client provides forwarding services for EUN end system
      communications to remote peers.  An EUN may be as simple as a
      small IoT sub-network that travels with a mobile Client to as
      complex as a large private enterprise network that the Client
      connects to a larger *NET.

   *  VPN interfaces use security encapsulations (e.g. IPsec tunnels)
      over underlay networks to connect Client, Proxy/Server or other
      critical infrastructure nodes.  VPN interfaces provide security
      services at sub-network layers of the architecture with securing
      properties similar to Direct point-to-point interfaces.

   *  Direct point-to-point interfaces securely connect Clients, Proxy/
      Servers and/or other critical infrastructure nodes over physical
      or virtual media that does not transit any open Internetwork
      paths.  Examples include a line-of-sight link between a remote
      pilot and an unmanned aircraft, a fiberoptic link between
      gateways, etc.

   The OMNI interface forwards original IP packets from the network
   layer using the OMNI Adaptation Layer (OAL) (see: Section 5) as an
   encapsulation and fragmentation sublayer service.  This "OAL source"
   then further encapsulates the resulting OAL packets/fragments in
   underlay network headers (e.g., UDP/IP, IP-only, Ethernet-only, etc.)
   to create encapsulated "carrier packets" for transmission over
   underlay interfaces.  The target OMNI interface then receives the
   carrier packets from underlay interfaces and performs decapsulation
   to obtain OAL packets/fragments.

   If the resulting OAL packets/fragments are addressed to itself, the
   OMNI interface performs reassembly/decapsulation as an "OAL
   destination" and delivers the original IP packet to the network
   layer.  If the OAL packets/fragments are addressed to another node,
   the OMNI interface instead re-encapsulates them in new underlay
   network headers as an "OAL intermediate system" then forwards the
   resulting carrier packets over an underlay interface.  The OAL source
   and OAL destination appear as "neighbors" on the OMNI link, while OAL
   intermediate systems provide a virtual bridging service that joins
   the segments of a (multinet) Segment Routing Topology (SRT).











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   The OMNI interface transports carrier packets over either secured or
   unsecured underlay interfaces to access the secured/unsecured OMNI
   link spanning trees as discussed further throughout the document.
   Carrier packets that carry control plane messages over secured
   underlay interfaces use sub-network securing services such as IPsec,
   direct encapsulation over secured point-to-point links, etc.  Carrier
   packets that carry data plane messages over unsecured underlay
   interfaces instead use encapsulations appropriate for public or
   private Internetworks.

   The OMNI interface and its OAL can forward original IP packets over
   underlay interfaces while including/omitting various lower layer
   encapsulations including OAL, UDP, IP and (ETH)ernet or other link
   layer header.  The network layer can also engage underlay interfaces
   directly while bypassing the OMNI interface entirely when necessary.
   This architectural flexibility may be beneficial for underlay
   interfaces (e.g., some aviation data links) for which encapsulation
   overhead is a primary consideration.  OMNI interfaces that send
   original IP packets directly over underlay interfaces without
   invoking the OAL can only reach peers located on the same OMNI link
   segment.  Source Clients can instead use the OAL to coordinate with
   target Clients in the same or different OMNI link segments by sending
   initial carrier packets to a First-Hop Segment (FHS) Proxy/Server.
   The FHS Proxy/Sever then sends the carrier packets into the SRT
   spanning tree, which transports them to a Last-Hop Segment (LHS)
   Proxy/Server for the target Client.

   The OMNI interface encapsulation/decapsulation layering possibilities
   are shown in Figure 2 below.  An imaginary vertical lines drawn
   between the Network Layer at the top of the figure and Underlay
   Interfaces at the bottom of the figure then allowed to slide
   horizontally either to the right or left illustrates the various
   encapsulation/decapsulation layering combination possibilities.
   Common combinations include IP-only (i.e., direct access to underlay
   interfaces with or without using the OMNI interface), IP/IP, IP/UDP/
   IP, IP/UDP/IP/ETH, IP/OAL/UDP/IP, IP/OAL/UDP/ETH, etc.















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    +------------------------------------------------------------+  ^
    |              Network Layer (Original IP packets)           |  ^
    +--+---------------------------------------------------------+ L3
       |         OMNI Interface (virtual sublayer nexus)         |  v
       +--------------------------+------------------------------+  -
                                  |      OAL Encaps/Decaps       |  ^
                                  +------------------------------+ OAL
                                  |        OAL Frag/Reass        |  v
                     +------------+---------------+--------------+  -
                     | UDP Encaps/Decaps/Compress |                 ^
                +----+---+------------+--------+--+  +--------+     |
                | IP E/D |            | IP E/D |     | IP E/D |    U/L
           +----+-----+--+----+    +--+----+---+     +---+----+--+ L2
           |ETH E/D|  |ETH E/D|    |ETH E/D|             |ETH E/D|  |
    +------+-------+--+-------+----+-------+-------------+-------+  v
    |                    Underlay Interfaces                     |
    +------------------------------------------------------------+

                     Figure 2: OMNI Interface Layering

   The OMNI/OAL model gives rise to a number of opportunities:

   *  Clients coordinate with the MS and receive MNP delegations through
      ICMPv6 control plane message exchanges with Proxy/Servers and
      hence assured unique.  Since the MLAs assigned to the OMNI
      interface are managed for uniqueness, Duplicate Address Detection
      (DAD) and Multicast Listener Discovery (MLD) messaging is serviced
      locally within the OMNI interface without disturbing other nodes
      on the link.

   *  underlay interfaces on the same L2 link segment as a Proxy/Server
      can omit UDP/IP underlay encapsulations for communications
      coordinated entirely over the OMNI interface.

   *  as underlay interface properties change (e.g., link quality, cost,
      availability, etc.), any active interface can be used to update
      the profiles of multiple additional interfaces in a single
      message.  This allows for timely adaptation and service continuity
      under dynamically changing conditions.

   *  coordinating underlay interfaces in this way allows them to be
      represented in a unified MS profile with provisions to support the
      "6 M's of Modern Internetworking".

   *  header compression and path MTU determination is conducted on a
      per-flow basis, with each flow adapting to the best performance
      profiles and path selections.




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   *  exposing a single virtual interface abstraction to the network
      layer allows for multilink operation (including QoS based link
      selection, carrier packet replication, load balancing, etc.) in
      the underlay while still permitting network layer traffic shaping
      based on, e.g., Traffic Class, IP address range, transport
      protocol port number, Flow Label, etc.

   *  the OMNI interface supports multinet traversal over the SRT when
      communications across different administrative domain network
      segments are necessary.  This mode of operation would not be
      possible via direct communications over the underlay interfaces
      themselves.

   *  the OAL supports lossless and adaptive path MTU mitigations not
      available for communications directly over the underlay interfaces
      themselves.  The OAL supports "packing" of multiple original IP
      payload packets within a single OAL "composite packet" and also
      supports transmission of IP packets of all sizes up to and
      including (advanced) jumbograms.

   *  the OAL assigns per-packet Identification values that allow for
      adaptation/link layer reliability and data origin authentication.

   *  The network layer engages the OMNI interface as a point of
      connection to the OMNI link; if there are multiple OMNI links, the
      network layer will engage multiple OMNI interfaces.

   *  Multiple independent OMNI interfaces can be used for increased
      fault tolerance through Safety-Based Multilink (SBM), with
      Performance-Based Multilink (PBM) applied within each interface.

   *  Multiple independent OMNI links can be joined together into a
      single link without requiring renumbering of infrastructure
      elements, since the MNPs assigned by Proxy/Servers of the
      different links will be mutually exclusive.

   *  The concept of OMNI endpoint intermediate systems supports logical
      partitioning (or "clustering") within MANETs without requiring
      address aggregation.  Instead, MANET routing within each
      partition/cluster is based on MLA host routes (i.e., MLA::/128)
      that are not redistributed into other partitions/clusters.  A
      hierarchy of partitions/clusters then connects MANET Clients to an
      FHS Proxy/Server.  Packet forwarding between distinct partitions/
      clusters is accomplished using the Segment Routing Header (SRH).







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   *  Clients can configure OMNI interfaces in parallel with physical
      link types such as non-terrestrial and/or low-earth orbit
      satellite services in a hyperconnected architecture.  Each
      interface configures a distinct set of IP addresses so that upper
      layers experience a multi-addressed network layer.

   Figure 3 depicts the architectural model for a source Client with an
   attached EUN connecting to the OMNI link via multiple independent
   *NETs.  The Client's OMNI interface forwards adaptation layer
   encapsulated ICMPv6 solicitation messages over available *NET
   underlay interfaces using any necessary underlay encapsulations.  The
   ICMPv6 messages traverse the *NETs until they reach an FHS Proxy/
   Server (FHS#1, FHS#2, ..., FHS#n), which returns an ICMPv6
   advertisement message and/or forwards a proxyed version of the
   message over the SRT to an LHS Proxy/Server near the target Client
   (LHS#1, LHS#2, ..., LHS#m).  The Hop Limit in ICMPv6 messages is not
   decremented due to encapsulation; hence, the source and target Client
   OMNI interfaces appear to be attached to a shared NBMA link.

































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                           +--------------+
                           |Source Client |
                           +--------------+        (:::)-.
                           |OMNI interface|<-->.-(:: EUN ::)
                           +----+----+----+      `-(::::)-'
                  +--------|IF#1|IF#2|IF#n|------ +
                 /         +----+----+----+        \
                /                 |                 \
               /                  |                  \
              v                   v                   v
           (:::)-.              (:::)-.              (:::)-.
      .-(::*NET:::)        .-(::*NET:::)        .-(::*NET:::)
        `-(::::)-'           `-(::::)-'           `-(::::)-'
         +-----+              +-----+              +-----+
    ...  |FHS#1|  .........   |FHS#2|   .........  |FHS#n|  ...
   .     +--|--+              +--|--+              +--|--+     .
   .        |                    |                    |
   .        \                    v                    /        .
   .         \                                       /         .
   .           v                 (:::)-.           v           .
   .                        .-(::::::::)                       .
   .                    .-(::: Segment :::)-.                  .
   .                  (:::::   Routing  :::::)                 .
   .                     `-(:: Topology ::)-'                  .
   .                         `-(:::::::-'                      .
   .                  /          |          \                  .
   .                 /           |           \                 .
   .                v            v            v
   .     +-----+              +-----+              +-----+     .
    ...  |LHS#1|  .........   |LHS#2|   .........  |LHS#m|  ...
         +--|--+              +--|--+              +--|--+
             \                   |                    /
              v                  v                   v
                       <-- Target Clients -->

       Figure 3: Source/Target Client Coordination over the OMNI Link

   After the initial control message exchange, the source Client (as
   well as any nodes on its attached EUNs) can send carrier packets to
   the target Client via the OMNI interface.  OMNI interface multilink
   services will forward the carrier packets via FHS Proxy/Servers for
   the correct underlay *NETs.  The FHS Proxy/Server then re-
   encapsulates the carrier packets and forwards them over the SRT which
   delivers them to an LHS Proxy/Server, and the LHS Proxy/Server in
   turn re-encapsulates and forwards them to the target Client.  (Note
   that when the source and target Client are on the same SRT segment,
   the FHS and LHS Proxy/Servers may be one and the same.)




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   Mobile Clients select a MAP Proxy/Server (not shown in the figure),
   which will often be one of their FHS Proxy/Servers but may instead be
   any Proxy/Server on the OMNI link.  Clients then register all of
   their *NET underlay interfaces with the MAP Proxy/Server via per
   interface FHS Proxy/Servers in a pure proxy role.  The MAP Proxy/
   Server then advertises the Client's MLA and MNPs into the OMNI link
   routing system, and the Client can quickly migrate to a new MAP
   Proxy/Server if the former becomes unresponsive.

   Clients therefore use Proxy/Servers as bridges into the SRT to reach
   OMNI link correspondents via a spanning tree established in a manner
   outside the scope of this document.  Proxy/Servers forward critical
   MS control messages via the secured spanning tree and forward other
   messages via the unsecured spanning tree (see: Security
   Considerations).  When AERO route optimization is applied, Clients
   can instead forward directly to correspondents in the same SRT
   segment to reduce Proxy/Server and/or Gateway load.

   Note: Original IP packets sent into an OMNI interface will receive
   consistent consideration according to their size as discussed in the
   following sections, while those sent directly over underlay
   interfaces that exceed the underlay network path MTU are dropped with
   an ordinary ICMP Packet Too Big (PTB) message returned.  These PTB
   messages are subject to loss the same as for any non-OMNI IP
   interface [RFC2923].

5.  OMNI Interface Maximum Transmission Unit (MTU)

   The OMNI interface observes the link nature of tunnels, including the
   Maximum Transmission Unit (MTU), Effective MTU to Send (EMTU_S),
   Effective MTU to Receive (EMTU_R) and the role of fragmentation and
   reassembly [I-D.ietf-intarea-tunnels].  The OMNI interface is
   configured over one or more underlay interfaces as discussed in
   Section 4, where underlay links and network paths may have diverse
   MTUs.  OMNI interface considerations for accommodating original IP
   packets of various sizes are discussed in the following sections.

   IPv6 underlay interfaces are REQUIRED to configure a minimum MTU of
   1280 octets and a minimum EMTU_R of 1500 octets [RFC8200].
   Therefore, the minimum IPv6 path MTU is 1280 octets since routers on
   the path are not permitted to perform network fragmentation even
   though the destination is required to reassemble more.  The network
   therefore MUST forward original IP packets as large as 1280 octets
   without generating an IPv6 Path MTU Discovery (PMTUD) Packet Too Big
   (PTB) message [RFC8201].






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   IPv4 underlay interfaces are REQUIRED to configure a minimum MTU of
   68 octets [RFC0791] and a minimum EMTU_R of 576 octets
   [RFC0791][RFC1122].  However, links that configure small MTUs are
   likely to have low-end performance and occur only at the extreme
   network edges while higher-performance interior network links
   commonly configure MTUs no smaller than 1280 octets and EMTU_Rs no
   smaller than 1500 octets [RFC3819].

   The OMNI interface itself sets an "unlimited" MTU of (2**32 - 1)
   octets.  The network layer therefore unconditionally admits all
   original IP packets into the OMNI interface, where the adaptation
   layer accommodates them if possible according to their size.  The OAL
   source then invokes adaptation layer encapsulation/fragmentation
   services to transform all original IP packets no larger than 65535
   octets into OAL packets/fragments.  The OAL source then applies
   underlay encapsulation to form carrier packets and finally forwards
   the carrier packets via underlay interfaces.

   When the OAL source performs IPv6 encapsulation and fragmentation
   (see: Section 6), the Payload Length field limits the maximum-sized
   original IP packet that the OAL can accommodate while applying IPv6
   fragmentation to (2**16 - 1) = 65535 octets (i.e., not including the
   OAL encapsulation header lengths).  The OAL source is also permitted
   to forward packets larger than this size as a best-effort delivery
   service if the path can accommodate them through "jumbo-in-jumbo"
   encapsulation; otherwise, the OAL source discards the packet and
   arranges to return a PTB "hard error" to the original source (see:
   Section 6.9).

   Each OMNI interface therefore sets a minimum EMTU_R of 65535 octets
   (plus the length of the OAL encapsulation headers), and each OAL
   destination must consistently either accept or reject still larger
   whole packets that arrive over any of its underlay interfaces
   according to their size.  If an underlay interface presents a whole
   packet larger than the OAL destination is prepared to accept (e.g.,
   due to a buffer size restriction), the OAL destination discards the
   packet and arranges to return a PTB "hard error" to the OAL source
   which in turn forwards a translated PTB to the original source (see:
   Section 6.9).

6.  The OMNI Adaptation Layer (OAL)

   The OMNI interface forwards original IP packets from the network
   layer for transmission over underlay interfaces.  The OMNI Adaptation
   Layer (OAL) acting as the OAL source then replaces the virtual
   Ethernet header with an IPv6 encapsulation header to form OAL
   packets.  The OMNI interface then applies source fragmentation to
   break these OAL packets into IPv6 fragments suitable for underlay



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   encapsulation and transmission as carrier packets.

   The carrier packets then traverse one or more underlay networks
   spanned by OAL intermediate systems in the SRT.  Each successive OAL
   intermediate system then re-encapsulates by removing the first
   underlay network encapsulations and appending encapsulations
   appropriate for the next underlay network.  (This process supports
   the multinet concatenation capability needed for joining multiple
   diverse networks.)  Following any forwarding by OAL intermediate
   systems, the carrier packets eventually arrive at the OAL
   destination.

   When the OAL destination receives the carrier packets, it discards
   the underlay encapsulations and reassembles the resulting OAL
   fragments into an OAL packet as described in Section 6.3.  The OAL
   destination next replaces the OAL packet IPv6 encapsulation header
   with a virtual Ethernet header to obtain the original IP packet for
   delivery to the network layer via the OMNI interface.  The OAL source
   may be either the source Client or its FHS Proxy/Server, while the
   OAL destination may be either the LHS Proxy/Server or the target
   Client.  Proxy/Servers (and SRT Gateways per
   [I-D.templin-6man-aero3]) may also serve as OAL intermediate systems.

   The OAL presents an OMNI sublayer abstraction similar to ATM
   Adaptation Layer 5 (AAL5).  Unlike AAL5 which performs segmentation
   and reassembly with fixed-length 53 octet cells over ATM networks,
   however, the OAL uses IPv6 encapsulation, fragmentation and
   reassembly with larger variable-length cells over heterogeneous
   networks.  (The OAL also does not include a trailing CRC since each
   IPv6 fragment is covered by hop-by-hop link layer integrity checks.)
   Detailed operations of the OAL are specified in the following
   sections.

6.1.  OAL Source Encapsulation and Fragmentation

   When the network layer forwards an original IP packet into the OMNI
   interface, it either sets the TTL/Hop Limit for locally-generated
   packets or decrements the TTL/Hop Limit according to standard IP
   forwarding rules.  The OAL source next creates an "OAL packet" by
   replacing the virtual Ethernet header with an IPv6 encapsulation
   header per [RFC2473].  The OAL source sets the IPv6 encapsulation
   header Version to "OMNI-IP6" (see: Section 6.2) and Next Header to
   TBD1 (see: IANA Considerations).








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   When the OAL source performs IPv6 encapsulation, it sets the IPv6
   header "Flow Label" as specified in [RFC6438].  The OAL source next
   copies the "Type of Service/Traffic Class Differentiated Service Code
   Point (DSCP)" [RFC2474][RFC2983] and "Explicit Congestion
   Notification (ECN)" [RFC3168] values in the original packet's IP
   header into the corresponding fields of the OAL IPv6 header.

   For original IP packets with DSCP '111111' (including ordinary
   network control/data plane messages), the OAL source resets the OAL
   IPv6 encapsulation header DSCP to '110111'.  The OAL source instead
   sets the IPv6 encapsulation header DSCP to '111111' for adaptation
   layer control plane messages that must be processed by all OAL
   intermediate systems on the path including both endpoints and
   transits.  These DSCP values belong to the "Pool 2 Experimental and
   Local Use (EXP/LU)" range reserved in [RFC2474] and correspond to
   Network/Internetwork Control precedence in [RFC0791].

   The OAL source next sets the IPv6 header Payload Length to the length
   of the original IP packet and sets Hop Limit to a value that is
   sufficiently large to support loop-free forwarding over multiple
   concatenated OAL intermediate hops.  The OAL source next selects OAL
   IPv6 Source and Destination Addresses associated with its own OMNI
   interface and the OMNI interface of the target.  (See: Section 8 for
   Source and Destination Address selection requirements.)

   The OAL source next inserts any necessary extension headers following
   the IPv6 header as specified in Section 6.4.  For OAL data plane
   packets, the source first inserts any per-fragment extension headers
   (e.g., Hop-by-Hop, Routing, etc.) then inserts an IPv6 Extended
   Fragment Header (see: [I-D.templin-6man-ipid-ext2]) with an extended
   OAL packet Identification.  Note that the extension header insertions
   could cause the IPv6 Payload Length to exceed 65535 octets by a small
   amount when the original IP packet is (nearly) the maximum length.

   The OAL source then source-fragments the OAL packet if necessary
   according to an OAL Fragment Size (OFS) maintained in AERO Flow
   Vectors (AVFs) for each independent flow.  (The OAL source
   encapsulates payloads that are no larger than the OFS and original IP
   packets larger than 65535 octets as "atomic fragments".)  OAL
   fragments prepared by the source must not be fragmented further by
   OAL intermediate systems on the path to the OAL destination.

   OAL packets that contain original IP packets no larger than 65535
   octets are subject to OAL source fragmentation using the IPv6
   Extended Fragment Header (EFH) fragmentation specification
   [I-D.templin-6man-ipid-ext2] with the exception that the IPv6 Payload
   Length may exceed 65535 by at most the length of the extension
   headers.  For each independent flow, the OAL source MUST set OFS to a



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   size no smaller than 1024 octets and thereafter adjust OFS according
   to dynamic network control message feedback.  The OAL source SHOULD
   limit OFS to a size no larger than 65279 octets (i.e., 256 octets
   less than the maximum length IPv6 payload packet to allow room for
   encapsulation) unless it has assurance that the path can accommodate
   a larger size.  (Note: the minimum size ensures that OAL fragments
   can be accommodated over any potential combination of IPv4/IPv6
   underlay network paths where transit for smaller sizes is assured
   without probing, while the maximum size observes the 65535 octet
   limitation for conventional IP packets.)

   When the OAL source performs fragmentation, it SHOULD produce the
   minimum number of fragments under current OFS constraints.  The
   fragments produced MUST be non-overlapping and the portion of each
   non-final fragment following the IPv6 Extended Fragment Header MUST
   be equal in length while that of the final fragment MAY be smaller
   and MUST NOT be larger.

   For each consecutive OAL fragment beginning with the first, the OAL
   source then writes a monotonically-increasing "ordinal" value between
   0 and 63 in the Extended Fragment Header Index field.  Specifically,
   the OAL source writes the ordinal value '0' for the first fragment,
   '1' for the first non-first fragment, '2' for the next, '3' for the
   next, etc. up to the final fragment.  The final fragment may assign
   an ordinal as large as '63' such that at most 64 fragments are
   possible.

   The OAL source finally encapsulates the fragments in any underlay
   headers necessary to form carrier packets for transmission over
   underlay interfaces, while retaining the fragments and their ordinal
   numbers (i.e., #0, #1, #2, etc.) for a brief period to support
   adaptation layer retransmissions (see: Section 6.8).  OAL fragment
   and carrier packet formats are shown in Figure 4.


















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        +----------+-------------------------+---------------+
        |OAL Header| Original Packet Headers |    Frag #0    |
        +----------+-------------------------+---------------+
        +----------+----------------+
        |OAL Header|     Frag #1    |
        +----------+----------------+
        +----------+----------------+
        |OAL Header|     Frag #2    |
        +----------+----------------+
                    ....
        +----------+----------------+
        |OAL Header|   Frag #(N-1)  |
        +----------+----------------+
        a) OAL fragmentation


        +----------+-----------------------------+
        |OAL Header|     Original IP packet      |
        +----------+-----------------------------+
        b) An OAL atomic fragment


        +--------+----------+----------------+
        |U/L Hdrs|OAL Header|     Frag #i    |
        +--------+----------+----------------+
        c) OAL carrier packet after underlay encapsulation

                Figure 4: OAL Fragments and Carrier Packets

   After establishing AFV state in the forward path for a given flow,
   the OAL source dynamically manages the per-flow OFS by continually
   probing the forward path to the OAL destination beginning with a size
   no smaller than 1024 octets and increasing to progressively larger
   sizes per [RFC8899].  In this process, the OAL source acts as a
   datagram packetization layer for the flow when it applies OAL
   encapsulation, fragmentation and header compression.

   The OAL source creates a probe by setting the P flag in the Type 1
   OMNI Compressed Header (OCH1) (see: Section 6.5) of a probe packet
   for the flow.  For probes that advance the OFS to a larger size, the
   probe packet can include discard data (e.g., an IP packet with Next
   Header/Protocol set to 59 ("No Next Header"), a UDP packet with
   service port number set to 9 ("discard"), etc.) or live protocol data
   with null padding.  For probes that confirm the current OFS, the
   probe packet can instead entirely include live protocol data.  The
   OAL source then admits the probe for underlay encapsulation and
   transmission.




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   When the OAL destination receives the probe, it returns an OAL-
   encapsulated secured control message to the OAL source with an OMNI
   option that includes a FRAGREP sub-option (see: Section 10.2.15).
   The OAL destination then returns the secured control message to the
   OAL source without marking it for examination by OAL intermediate
   systems.

   When the OAL source receives the secured control message, it first
   determines that the message is authentic.  The OAL source can then
   tentatively advance OFS for this flow to the probe size but should
   maintain an ongoing stream of additional probes for the flow to
   confirm the current OFS and/or to advance to still larger OFS values.
   The OAL source may additionally receive MTU soft error feedback from
   an OMNI destination or intermediate system and should compensate
   accordingly as discussed in Section 6.9.

6.2.  Underlay Encapsulation and Re-Encapsulation

   The OAL source or intermediate system next encapsulates each OAL
   fragment (with either full or compressed headers) in underlay
   encapsulation headers to create a carrier packet.  The OAL source or
   intermediate system (i.e., the underlay source) includes a full/
   compressed IP header if there may be routers on the path to the
   underlay destination.  The source appends any additional underlay IP
   encapsulation sublayers (e.g., IPsec AH/ESP, an IP Hop-by-Hop option
   for jumbo-in-jumbo encapsulation, etc.) and also includes a UDP
   header if NATs and/or filtering middleboxes might occur on the path.
   The underlay source finally includes an actual L2 header such as an
   Ethernet header for Ethernet-compatible links.

   The underlay source encapsulates the OAL information immediately
   following the innermost underlay header.  The underlay source next
   interprets the first 4 bits following the underlay headers as a Type
   field that determines the type of OAL header that follows.  The
   underlay source sets Type to (OMNI-IP6) for an uncompressed IPv6 OMNI
   Full Header or (OMNI-OCH1/2) for an OMNI Compressed Header (OCH1/2)
   as specified in Section 6.5.  Other Type values may also appear as
   specified in Section 6.5.

   The underlay source prepares the underlay encapsulation headers for
   OAL packets/fragments as follows:










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   *  For UDP/IP encapsulation, the underlay source sets the UDP source
      port to 8060 (i.e., the port number reserved for AERO/OMNI).  When
      the underlay destination is a Proxy/Server or Gateway, the
      underlay source sets the UDP Destination Port to 8060; otherwise,
      the underlay source sets the UDP Destination Port to its cached
      port number value for the peer.  The underlay source next sets the
      UDP Length the same as specified in [I-D.ietf-tsvwg-udp-options].

   *  The underlay source then sets the IP {Protocol, Next Header} to
      '17' (the UDP protocol number) and sets the {Total, Payload}
      Length the same as specified in the base IP protocol
      specifications for ordinary IP packets (see: [RFC0791], [RFC8200]
      and [I-D.ietf-tsvwg-udp-options]).  The underlay source then
      continues to set the remaining IP header fields as discussed
      below.

   *  For raw IP encapsulation, the underlay source sets the IP
      {Protocol, Next Header} to TBD1 (see: IANA Considerations) and
      sets the {Total, Payload} Length the same as specified in
      [RFC0791] or [RFC8200].  The underlay source then continues to set
      the remaining IP header fields as discussed below.

   *  For IPsec AH/ESP encapsulation, the underlay source sets the
      appropriate IP or UDP header to indicate AH/ESP then sets the AH/
      ESP Next Header field to TBD1 the same as for raw IP
      encapsulation.

   *  For direct encapsulations over Ethernet-compatible links, the
      underlay source prepares an Ethernet Header with EtherType set to
      TBD2 (see: Section 20.2) (see: Section 7).

   *  For OAL packet/fragment encapsulations over secured underlay
      interface connections to the secured spanning tree, the underlay
      source applies any underlay security encapsulations according to
      the protocol (e.g., IPsec).  These secured carrier packets are
      then subject to lower layer security services.















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   When an underlay source includes a UDP header, it SHOULD calculate
   and include a UDP checksum in carrier packets with full OAL headers
   to prevent mis-delivery and/or detect IPv4 reassembly corruption; the
   underlay source MAY set UDP checksum to 0 (disabled) in carrier
   packets with compressed OAL headers (see: Section 6.5) or when
   reassembly corruption is not a concern.  If the underlay source
   discovers that a path is dropping carrier packets with UDP checksums
   disabled, it should supply UDP checksums in future carrier packets
   sent to the same underlay destination.  If the underlay source
   discovers that a path is dropping carrier packets that do not include
   a UDP header, it should include a UDP header in future carrier
   packets.

   When an underlay source sends carrier packets with compressed OAL
   headers and with UDP checksums disabled, mis-delivery due to
   corruption of the AFVI is possible but unlikely since the corrupted
   index would somehow have to match valid state in the (sparsely-
   populated) AERO Flow Information Base (AFIB).  In the unlikely event
   that a match occurs, an OAL intermediate system or destination may
   receive carrier packets that contain a mis-delivered OAL fragment but
   can immediately reject any with incorrect Identifications.  If the
   Identification value is somehow accepted, the OAL destination may
   submit the mis-delivered OAL fragment to the reassembly cache where
   it will most likely be rejected due to incorrect reassembly
   parameters.  If a reassembly that includes the mis-delivered OAL
   fragment somehow succeeds (or, for atomic fragments) the OAL
   destination will verify any included checksums to detect corruption.
   Finally, any spurious data that somehow eludes all prior checks will
   be detected and rejected by end-to-end upper layer integrity checks.
   See: [RFC6935] [RFC6936] for further discussion.

   For UDP/IP or raw IP encapsulations, when the underlay source is also
   the OAL source it next copies the DSCP, ECN and Flow Label values
   from the OAL header into the underlay IP header.  The underlay source
   then sets the IP TTL/Hop Limit the same as for any host (i.e., it
   does not copy the Hop Limit value from the OAL header) and finally
   sets the IP Source and Destination Addresses to direct the carrier
   packet to the next OAL hop.  For carrier packets subject to re-
   encapsulation, the OAL intermediate system removes the underlay
   header(s) then prepares to act as the underlay source for the next
   hop.










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   The underlay source first decrements the OAL header Hop Limit and
   discards the OAL packet/fragment if the value reaches 0.  Otherwise,
   the underlay source copies the DSCP value from OAL IPv6 header into
   the next segment underlay IP header while setting the next segment
   underlay IP Source and Destination Addresses the same as above.  The
   underlay source then copies the ECN value from the previous segment
   underlay IP header into both the OAL full/compressed header and the
   next segment underlay IP header.

   The underlay source then prepares to forward the carrier packets to
   the next OAL intermediate system or destination.  For underlay
   encapsulations over IPv4, if the carrier packet is no larger than
   1280 octets the underlay source sets the IPv4 Don't Fragment (DF) bit
   to 0 and includes a suitable IPv4 Identification value; otherwise,
   the underlay source sets DF to 1.  This ensures that all IPv4 carrier
   packets no larger than 1280 octets will be delivered to the underlay
   destination even if a small amount of fragmentation occurs in the
   path (see: [RFC3819] for IPv4 link MTU expectations according to
   their performance characteristics).

   For IPv4 carrier packets that set DF to 1 and for all IPv6 carrier
   packets, delivery is best-effort according to the available path MTU
   in the spirit of [RFC2473] and [RFC4213].  Since carrier packet
   transmissions are not within the scope of an explicit tunnel required
   to pass the IPv6 minimum MTU, however, there is no need for the
   underlay source to apply source fragmentation since the 1024 octet
   minimum OFS is operationally assured over all IPv4 and IPv6 paths.
   The underlay source should therefore ignore any ICMPv6 Packet Too Big
   or IPv4 Fragmentation Needed messages returned from the network in
   response to any of its large carrier packet transmissions since the
   OAL source engages in active probing per [RFC8899].

   The underlay source then sends the resulting carrier packets over one
   or more underlay interfaces.  Underlay interfaces often connect
   directly to physical media on the local platform (e.g., an aircraft
   with a radio frequency link, a laptop computer with WiFi, etc.), but
   in some configurations the physical media may be hosted on a separate
   Local Area Network (LAN) node.  In that case, the OMNI interface can
   establish a Layer-2 VLAN or a point-to-point tunnel (at a layer below
   the underlay interface) to the node hosting the physical media.  The
   OMNI interface may also apply encapsulation at the underlay interface
   layer (e.g., as for a tunnel virtual interface) such that carrier
   packets would appear "double-encapsulated" on the LAN; the node
   hosting the physical media in turn removes the LAN encapsulation
   prior to transmission or inserts it following reception.  Finally,
   the underlay interface must monitor the node hosting the physical
   media (e.g., through periodic keepalives) so that it can convey up-
   to-date Interface Attribute information to the OMNI interface.



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6.3.  Reassembly and Decapsulation

   For both IPv4 and IPv6, OAL intermediate systems and destinations
   MUST configure a minimum EMTU_R of 1500 octets on all unsecured
   underlay interfaces.  (Secured underlay interfaces instead use an
   EMTU_R specific to the underlay security service such as IPsec.)  OAL
   intermediate systems and destinations are permitted to configure a
   larger underlay interface EMTU_R in order to pass still larger
   carrier packets.

   OAL destinations MUST configure an adaptation layer EMTU_R of 65535
   octets to support reassembly of fragmented OAL packets of all sizes.
   OAL nodes must further recognize and honor the extended
   Identifications included in the IPv6 Extended Fragment Header
   [I-D.templin-6man-ipid-ext2].

   When an OMNI interface processes a carrier packet received on an
   underlay interface, it copies the ECN value from the underlay IP
   encapsulation header into the OAL header but does not copy the DSCP
   value from the underlay IP header into the OAL header according to
   the differentiated services pipe model for tunnels [RFC2983].  The
   OMNI interface next discards the underlay headers and examines the
   OAL header of the enclosed OAL packet/fragment according to the value
   in the Type field as discussed in Section 6.2

   If the OAL packet/fragment is addressed to a different node, the OMNI
   interface (acting as an OAL intermediate system) decrements the OAL
   Hop Limit as discussed in Section 6.2 then performs underlay
   encapsulation and forwards the resulting carrier packet.  If the OAL
   packet/fragment is addressed to itself, the OMNI interface (acting as
   an OAL destination) accepts or drops based on the (Source,
   Destination, Flow Label, Identification)-tuple.

   The OAL destination next drops all ordinal OAL non-first fragments
   that would overlap or leave "holes" with respect to other ordinal
   fragments already received.  The OAL destination updates a checklist
   of accepted ordinal fragments of the same OAL packet but admits all
   accepted fragments into the reassembly cache.













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   The OAL destination then reassembles the original OAL packet after
   all fragments have arrived.  The reassembled OAL packet may exceed
   65535 by as much as the size of the OAL encapsulation extension
   headers.  The OAL destination does not write this (too-large) value
   into the OAL header Payload Length field, but instead retains the
   value during reassembly.  When reassembly is complete, the OAL
   destination finally replaces the OAL IPv6 encapsulation header with a
   virtual Ethernet header.  The OAL destination's OMNI interface then
   delivers the original IP packet to the network layer.  The original
   IP packet may therefore be as large as 65535 octets.

   When an OAL path traverses an IPv6 network with routers that perform
   adaptation layer forwarding based on full IPv6 headers with OAL
   addresses, the OAL intermediate system at the head of the IPv6 path
   forwards the OAL packet/fragment the same as an ordinary IPv6 packet
   without decapsulating and delivering to the network layer.  Once
   within the IPv6 network, these OAL packets/fragments may traverse
   arbitrarily-many IPv6 hops before arriving at an OAL intermediate
   system which may again encapsulate the OAL packets/fragments as
   carrier packets for transmission over underlay interfaces.

   Note: carrier packets often traverse paths with underlying links that
   use integrity checks such as CRC-32 which provide adequate hop-by-hop
   integrity assurance for payloads up to ~9K octets [CRC].  However,
   other paths may traverse links (such as fragmenting tunnels over IPv4
   - see: [RFC4963]) that do not include adequate checks.

6.4.  OMNI-Encoded IPv6 Extension Headers

   The IPv6 specification [RFC8200] defines extension headers that
   follow the base IPv6 header, while Upper Layer Protocols (ULPs) are
   specified in other documents.  Each extension header present is
   identified by a "Next Header" octet in the previous (extension)
   header and encodes a "Next Header" field in the first octet that
   identifies the next extension header or ULP instance.  The OMNI
   specification supports encoding of IPv6 extension header chains
   immediately following the underlay UDP, IP or Ethernet header even if
   the underlay IP protocol version is IPv4.

   The OAL source prepares an OMNI extension header chain by setting the
   first 4 bits of the first IPv6 extension header in the chain to a
   Type value for the extension header itself immediately following the
   underlay protocol header.  The source then sets the next 4 bits to a
   Next value that identifies either a terminating ULP or the next
   extension header in the chain.  The source then sets the first 8 bits
   of each subsequent IPv6 extension header in the chain to the standard
   Next Header encoding as shown in Figure 5:




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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~               Underlay UDP, IP or Ethernet Header             ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Type |  Next |           Extension Header #1                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |           Extension Header #2                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |           Extension Header #3                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             ...                         ...                          ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |           Extension Header #N                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~  OMNI Full/Compressed, IPv6/IPv4, TCP/UDP, ICMPv6, ESP, etc.  ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: OMNI Extension Header Chains

   The following Type/Next values are currently defined:

      0 (OMNI-RES1) - Reserved for experimentation.

      1 (OMNI-OCH1) - OMNI Compressed Header, Type 1 per Section 6.5.

      2 (OMNI-OCH2) - OMNI Compressed Header, Type 2 per Section 6.5.

      3 (OMNI-RES2) - Reserved for experimentation.

      4 (OMNI-IP4) - IPv4 header per [RFC0791].

      5 (OMNI-HBH) - Hop-by-Hop Options per Section 4.3 of [RFC8200].

      6 (OMNI-IP6) - IPv6 header per [RFC8200].

      7 (OMNI-RH) - Routing Header per Section 4.4 of [RFC8200].

      8 (OMNI-FH) - Fragment Header per Section 4.5 of [RFC8200].

      9 (OMNI-DO) - Destination Options per Section 4.6 of [RFC8200].

      10 (OMNI-AH) - Authentication Header per [RFC4302].

      11 (OMNI-ESP) - Encapsulating Security Payload per [RFC4303].

      12 (OMNI-NNH) - No Next Header per Section 4.7 of [RFC8200].




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      13 (OMNI-TCP) - TCP Header per [RFC9293].

      14 (OMNI-UDP) - UDP Header per [RFC0768].

      15 (OMNI-ULP) - Upper Layer Protocol shim (see below).

   Entries OMNI-OCH1 through OMNI-AH in the above list follow the
   convention that the OMNI Type/Version appears in the first 4 bits of
   the extension header (or IP header) itself.  Conversely, entries
   OMNI-ESP through OMNI-UDP represent commonly-used ULPs which do not
   encode a Type/Version in the first 4 bits.

   Entries OMNI-HBH, OMNI-RH, OMNI-FH, OMNI-DO and OMNI-AH represent
   true IPv6 extension headers encoded for OMNI, which may be chained.
   Source and destination processing of OMNI extension headers follows
   exactly per their definitions in the normative references, with the
   exception of the special (Type, Next) coding in the first 8 bits of
   the first extension header.

   When a ULP not found in the above table immediately follows the
   underlay UDP, IP or Ethernet header, the source includes a 2 octet
   "Type 1 ULP Shim" before the ULP where both the first 4-bit (Type)
   and next 4-bit (Next) fields encode the special value 15 (OMNI-ULP).
   The source then includes a Next Header field that encodes the IP
   protocol number of the ULP.  The source then includes the ULP data
   immediately after the shim as shown in Figure 6.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Type=15|Next=15|  Next Header  |   Upper Layer Protocol        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 6: OMNI Upper Layer Protocol (ULP) Shim (Type 1)

   When a ULP "OMNI-(N)" found in the above table immediately follows
   the underlay UDP, IP or Ethernet header, the source includes a 1
   octet "Type 2 ULP Shim" before the ULP where the first 4 bits encode
   the special Type value 15 (OMNI-ULP) and the next 4 bits encode the
   Next ULP type "N" taken from the table above.  The source then
   includes the ULP data immediately after the shim as shown in
   Figure 7.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Type=15| Next=N|          Upper Layer Protocol                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 7: OMNI Upper Layer Protocol (ULP) Shim (Type 2)





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   When a ULP not found in the above table follows a first OMNI
   extension header, the source sets the extension header Next field to
   OMNI-ULP (15) and includes a 1 octet "Type 3 ULP Shim" that encodes
   the IP protocol number for the Next Header of the ULP data that
   follows as shown in Figure 8.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |           Upper Layer Protocol                ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 8: OMNI Upper Layer Protocol (ULP) Shim (Type 3)

   When a ULP "OMNI-(N)" found in the above table follows a first OMNI
   extension header, the source sets the extension header Next field to
   the ULP Type "N" and does not include a shim.  The ULP then begins
   immediately after the first OMNI extension header.

   When a ULP of any kind follows a non-first OMNI extension header, the
   source sets the extension header Next Header field to the IP protocol
   number for the ULP and does not include a shim.  The ULP then begins
   immediately after the non-first OMNI extension header.

   Note: The underlay UDP header (when present) is logically considered
   as the first extension header in the chain.  If an Advanced Jumbo
   extension header is also present, its Jumbo Payload length includes
   the length of the UDP header.

   Note: After a node parses the extension header chain, it changes the
   "Type/Next" field in the first extension header back to the correct
   "Next Header" value before processing the first extension header.

6.5.  OMNI Full and Compressed Headers

   OAL sources that send OAL packets with full OMNI IPv6 Headers include
   a Segment Routing Header (SRH) as an extension per [RFC8754].  The
   Segment List elements include the adaptation layer addresses of the
   Client itself and of any OAL intermediate systems on the path.
   Clients discover their local Segment List elements in their RS/RA
   exchanges with FHS Proxy/Servers, where each partition border OAL
   intermediate system in the RS message forwarding path records its
   address before forwarding to the next partition border OAL
   intermediate system.  See: Section 13 for further discussion.

   The SRH is followed by an IPv6 Extended Fragment Header to support
   segment-by-segment forwarding based on an AERO Flow Information Base
   (AFIB) in each OAL node in the path.  OAL sources, intermediate
   systems and destinations establish AFIB header compression state
   based on secured "pilot" control messages.  OAL nodes should apply



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   OMNI Header Compression for subsequent data plane messages to
   significantly reduce header overhead and suppress advisory ICMPv6
   Parameter Problem messages (see: [I-D.templin-6man-aero3]).

   OAL sources apply header compression in order to avoid transmission
   of redundant data found in the original IP packet and OAL
   encapsulation headers; the resulting compressed headers are often
   significantly smaller than the original IP packet header itself even
   when OAL encapsulation is applied.  Header compression is limited to
   the OAL IPv6 encapsulation header plus extensions along with the base
   original IP packet header; it does not extend to include any
   extension headers of the original IP packet which appear as upper
   layer payload immediately following the compressed headers.

   Each OAL node establishes AFIB soft state entries known as AERO Flow
   Vectors (AFVs) which support both OAL packet/fragment forwarding and
   OAL/IPv6 header compression/decompression.  OAL nodes locate each AFV
   by an AERO Flow Vector Index (AFVI) which in conjunction with the
   previous hop underlay address provides compression/decompression and
   next hop forwarding context.

   When an OAL source sends carrier packets that contain OAL packets/
   fragments to a next hop, it includes a full IPv6 header with an SRH
   containing segment addressing information followed by an Extended
   Fragment Header.  The first 4 bits following the underlay headers
   must encode the Type OMNI-IP6 to signify that an uncompressed IPv6
   header (plus any extensions) is present.

   When AFV state is available, the OAL source should omit significant
   portions of the OAL header (plus extensions) and original IP packet
   header by applying OMNI header compression.  For OAL first fragments
   (including atomic fragments), the OAL source uses OMNI Compressed
   Header, Type 1 (OCH1) Format (a) as shown in Figure 9:

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      | Type  |A|F|M|P| Traffic Class | OAL Hop Limit |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                OAL Identification (4 octets)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | L3 Next Header|  L3 Hop Limit |     AFVI (2 or 4 octets)      ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~  Payload Len (0 or 2 octets)  ~  IPv4 Ident. (0 or 2 octets)  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~ IPv4 Checksum (0 or 2 octets) ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 9: OMNI Compressed Header (OCH1) Format (a)




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   The format begins with a 4-bit Type followed by 4 flag bits followed
   by an 1 octet Traffic Class (copied into the OAL header from the
   original IP packet header) followed by an 1 octet OAL Hop Limit.  The
   OAL source sets Type to OMNI-OCH1, sets Hop Limit to the uncompressed
   OAL header Hop Limit and sets the ECN bits in the Traffic Class field
   the same as for an uncompressed IP header.  The OAL source next sets
   (F)ormat to 0 then sets (M)ore Fragments the same as for an
   uncompressed Extended Fragment Header.

   The header next includes the 4 least significant octets of the OAL
   Identification followed by the Next Header (or Protocol) and Hop
   Limit (or TTL) values from the original (L3) IP packet header.  These
   values are followed by 2 octet AFVI if the (A) flag is set to 0;
   otherwise a 4 octet AFVI.  The format then includes a 2 octet Payload
   Length only if the underlay headers do not include a length field.
   The format finally includes 2 octet Identification and Header
   Checksum values for IPv4 original packets only.  (Compression
   therefore applies to the original IP packet header plus the OAL IPv6
   header along with its SRH and Extended Fragment Header in a unified
   concatenation.)

   The OAL source finally includes the payload of the OAL first fragment
   (i.e., beginning after the original IP header) immediately following
   the OCH1 header, and the underlay header length field (if present) is
   reduced by the difference in length between the compressed and full-
   length headers.  The OAL destination can determine the payload length
   by examining the underlay header length field if present; otherwise,
   the OCH1 header itself includes a 2 octet Payload Length field that
   encodes the length of the packet payload that follows the OCH1.
   (Note that OAL first fragments and atomic packets are logically
   considered ordinal fragment 0 even though the format does not include
   an Index field.)

   When the OAL source needs to probe the OAL Fragment Size (OFS) for a
   given flow, it sets the (P)robe flag and includes a probe message of
   the desired size following the OCH1 header.  Upon receipt, the OAL
   destination returns a secured control message reply to the OAL
   source.  When the OAL source receives the control message, it can
   either maintain its current OFS for this flow or advance to a larger
   OFS according to the probe size.

   For OAL non-first fragments (i.e., those with non-zero Index), the
   OAL uses OMNI Compressed Header, Type 1 (OCH1) Format (b) as shown in
   Figure 10:







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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type  |A|F|M|Resvd|   Index   | Traffic Class | OAL Hop Limit |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Identification (4 octets)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     AFVI (2 or 4 octets)      ~  Payload Len (0 or 2 octets)  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 10: OMNI Compressed Header (OCH1) Format (b)

   The format begins with a 4-bit Type followed by 3 flags followed by a
   3-bit Reserved field (set to 0) followed by a 6-bit ordinal fragment
   Index.  All other fields up to and including the Payload Length (if
   present) are included the same as for an OCH1 first fragment.

   The OAL source sets Type to OMNI-OCH1, sets Hop Limit to the
   uncompressed OAL header Hop Limit value, sets (Index, (A)FVI, (M)ore
   Fragments, Identification) to their appropriate values as a non-first
   fragment and sets (F)ormat to 1.  The OAL source also sets Index to a
   monotonically increasing ordinal value beginning with 1 for the first
   non-first fragment, 2 for the second non-first fragment, 3 for the
   third non-first fragment, etc., up to at most 63 for the final
   fragment.

   The OAL source then includes a non-first fragment body immediately
   following the OCH1 header, and reduces the underlay header length
   field (if present) by the difference in length between the compressed
   headers and full-length original IP header with OAL IPv6 header plus
   extensions.  The OAL destination will then be able to determine the
   Payload Length by examining the underlay header length field if
   present; otherwise by examining the 2 octet OCH1 Payload Length the
   same as for first fragments.

   The OCH1 Format (a) is used for all original IPv6 packets that do not
   include a Fragment Header as well as for original IPv4 packets that
   set IHL to 5, DF to 1 and (MF; Fragment Offset) to 0 (the OCH1 Format
   (b) is used for non-first fragments in all IP protocol cases).

   For other "non-atomic" original IP packets and first fragments, the
   OAL source uses the "Type 2" OMNI Compressed Header (OCH2) formats
   shown in Figure 11 and Figure 12:










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                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      | Type  |A|F|M|P| Traffic Class | OAL Hop Limit |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  OAL Identification (4 octets)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | L3 Next Header| L3 Hop Limit  |     AFVI (2 or 4 octets)      ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~  Payload Len (0 or 2 octets)  |      Fragment Information     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       IPv6 Identification                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 11: OMNI Compressed Header, Type 2 (OCH2) Format (a)

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      | Type  |A|F|M|P|Type of Service| OAL Hop Limit |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  OAL Identification (4 octets)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | L3 Next Header| L3 Hop Limit  |     AFVI (2 or 4 octets)      ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~  Payload Len (0 or 2 octets)  |Version|  IHL  |   Fragment    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Information  |      IPv4 Identification      |  Checksum (1) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Checksum (2) |            Options            |    Padding    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 12: OMNI Compressed Header, Type 2 (OCH2) Format (b)

   In both of the above OCH2 formats, the leading octets include the
   same information that would appear in a corresponding OCH1 format (a)
   header.  The (F) flag is set to 0 for OCH2 format (a) or 1 for OCH2
   format (b), while all other flags are processed the same as for OCH1
   format (a).

   The remainder of the OCH2 format (a) includes fields that would
   appear in an uncompressed IPv6 header per [RFC8200] plus
   Fragmentation Information.  For the standard IPv6 Fragment Header,
   Fragment Information consists of the 13-bit Fragment Offset followed
   by the 3 IPv6 Fragment Header flag bits.  For the EFH, Fragmentation
   Information consists of the NH-Cache followed by the 2 EFH flag bits
   followed by the 6-bit Index.

   The remainder of OCH2 format (b) includes fields that would appear in
   an uncompressed IPv4 header per [RFC0791] with the Options and
   Padding lengths calculated based on IHL.  In both cases, the Source
   and Destination Addresses are not transmitted.



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   When an OAL destination or intermediate system receives a carrier
   packet, it determines the length of the encapsulated OAL information
   and verifies that the innermost underlay next header field indicates
   OMNI (see: Section 6.2), then processes any included OMNI underlay
   extension headers as specified in Section 6.4.  The OAL destination
   then examines the Next Header field of the final underlay extension
   header.  If the Next Header field contains the value TBD1, and the
   4-bit Type that follows encodes a value OMNI-IP6, OMNI-OCH1 or OMNI-
   OCH2 the OAL node processes the remainder of the OAL header as a full
   or compressed header as specified above.

   When an OAL node forwards an OAL packet, it determines the AFVI for
   the next OAL hop by using the AFVI included in the OCH to search for
   a matching AFV.  The OAL node then writes the next hop AFVI into the
   OCH (while adjusting the AFVI length if necessary) and forwards the
   OAL packet to the next hop.  This same AFVI re-writing progression
   begins with the OAL source then continues over all OAL intermediate
   systems in the path and finally ends at the OAL destination.

   When the OAL destination receives the OAL packet, it reconstructs the
   OAL and original IP headers based on the information cached in the
   AFV combined with the received information in the OCH1/2.  For non-
   atomic fragments, the OAL destination then adds the resulting OAL
   fragment to the reassembly cache if the Identification is acceptable.
   Following OAL reassembly if necessary, the OAL destination delivers
   the original IP packet to the network layer.

   For all OCH1/2 types, the OAL source sets all Reserved fields and
   bits to 0 on transmission and the destination node ignores the values
   on reception.  For both OCH1/2, ECN information is compiled for first
   fragments, and not for non-first fragments.

   Finally, if an IPv6 Hop-by-Hop (HBH) and/or Routing Header extension
   header is required to appear as per-fragment extensions with each OAL
   fragment that uses OCH1 format (b) or OCH2 compression the OAL source
   inserts an OMNI-HBH and/or OMNI-RH header as the first extension(s)
   following the underlay header and before the OMNI-OCH1/2 as discussed
   in Section 6.4.

6.6.  UDP/IP Encapsulation Avoidance

   When an OAL node is unable to determine whether the next OAL hop is
   connected to the same underlay link, it should perform carrier packet
   underlay encapsulation for initial packets sent via the next hop over
   a specific underlay interface by including full UDP/IP headers and
   with the UDP port numbers set as discussed in Section 6.2.  The node
   can thereafter attempt to send an IPv6 ND solicitation message to the
   next OAL hop in carrier packet(s) that omit the UDP header and set



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   the IP protocol number to TBD1.  If the OAL node receives an IPv6 ND
   advertisement, it can omit the UDP header in subsequent packets.  The
   node can further attempt to send an IPv6 ND solicitation in carrier
   packet(s) that omit both the UDP and IP headers and set EtherType to
   TBD2.  If the source receives an IPv6 ND advertisement, it can begin
   omitting both the UDP and IP headers in subsequent packets.

   Note: in the above, "next OAL hop" refers to the first OAL node
   encountered on the optimized path to the destination over a specific
   underlay interface as determined through route optimization (e.g.,
   see: [I-D.templin-6man-aero3]).  The next OAL hop could be a Proxy/
   Server, Gateway or the OAL destination itself.

6.7.  OAL Identification Window Maintenance

   The OAL encapsulates each original IP packet as an OAL packet then
   performs fragmentation to produce one or more carrier packets with
   the same 8 octet Identification value.  In environments where
   spoofing is not considered a threat, OMNI interfaces send OAL packets
   with Identifications beginning with an unpredictable Initial Send
   Sequence (ISS) value [RFC7739] monotonically incremented (modulo
   2**64) for each successive OAL packet sent to either a specific
   neighbor or to any neighbor.  (The OMNI interface may later change to
   a new unpredictable ISS value as long as the Identifications are
   assured unique within a timeframe that would prevent the fragments of
   a first OAL packet from becoming associated with the reassembly of a
   second OAL packet.)  In other environments, OMNI interfaces should
   maintain explicit per-flow send and receive windows to detect and
   exclude spurious carrier packets that might clutter the reassembly
   cache as discussed below.

   OMNI interface neighbors use a window synchronization service similar
   to TCP [RFC9293] to maintain unpredictable ISS values incremented
   (modulo 2**64) for each successive OAL packet and re-negotiate
   windows often enough to maintain an unpredictable profile.  OMNI
   interface neighbors exchange control messages that include OMNI
   Neighbor Synchronization sub-options that include TCP-like
   information fields and flags to manage streams of OAL packets instead
   of streams of octets.  As a link layer service, the OAL provides low-
   persistence best-effort retransmission with no mitigations for
   duplication, reordering or deterministic delivery.  Since the service
   model is best-effort and only control message sequence numbers are
   acknowledged, OAL nodes can select unpredictable new initial sequence
   numbers outside of the current window without delaying for the
   Maximum Segment Lifetime (MSL).






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   OMNI interface end systems and intermediate systems maintain current
   and previous per-flow window state in IPv6 ND NCEs and/or AFVs to
   support dynamic rollover to a new window while still sending OAL
   packets and accepting carrier packets from the previous windows.
   OMNI interface neighbors synchronize windows through asymmetric and/
   or symmetric control message exchanges.  When OMNI end and
   intermediate systems receive a control message with new per-flow
   window information, it resets the previous window state based on the
   current window then resets the current window based on new and/or
   pending information.

   The control message OMNI option Neighbor Synchronization sub-option
   includes TCP-like information fields including Sequence Number,
   Acknowledgement Number, Scale, Window and flags (see: Section 10).
   Window Scaling is applied the same as specified in [RFC7323].  OMNI
   interface neighbors and intermediate systems maintain the following
   TCP-like state variables on a per-interface-pair basis (i.e., through
   a combination of NCE and/or AFV state):

       Send Sequence Variables (current, previous and pending)

         SND.NXT - send next
         SND.WND - send window
         ISS     - initial send sequence number

       Receive Sequence Variables (current and previous)

         RCV.NXT - receive next
         RCV.WND - receive window
         IRS     - initial receive sequence number

   OMNI interface neighbors "OAL A" and "OAL B" exchange control
   messages per [RFC4861] with OMNI options that include TCP-like
   information fields in a Neighbor Synchronization.  When OAL A
   synchronizes with OAL B, it maintains both a current and previous
   SND.WND beginning with a new unpredictable ISS and monotonically
   increments SND.NXT for each successive OAL packet transmission.  OAL
   A initiates synchronization by including the new ISS in the Sequence
   Number of an authentic control message with the SYN flag set and with
   Scale set to S (up to 14) and Window set to W (up to 2**16) while
   creating a NCE in the INCOMPLETE state if necessary.  OAL A caches
   the new ISS as pending, uses the new ISS as the Identification for
   OAL encapsulation, then sends the resulting OAL packet to OAL B and
   waits up to RetransTimer milliseconds to receive a control message
   response with the ACK flag set (retransmitting up to
   MAX_UNICAST_SOLICIT times if necessary).





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   When OAL B receives the SYN, it creates a NCE in the STALE state and
   also an AFV if necessary, resets its RCV variables and caches the
   source's send window size as its receive window size.  OAL B then
   prepares a control message with the ACK flag set, with the
   Acknowledgement Number set to OAL A's next sequence number, and with
   Scale set to S and Window set to W.  Since OAL B does not assert an
   ISS of its own, it uses the IRS it has cached for OAL A as the
   Identification for OAL encapsulation then sends the ACK to OAL A.

   When OAL A receives the ACK, it notes that the Identification in the
   OAL header matches its pending ISS.  OAL A then sets the NCE state to
   REACHABLE and resets its SND variables based on the Scale, Window and
   Acknowledgement Number (which must include the sequence number
   following the pending ISS).  OAL A can then begin sending OAL packets
   to OAL B with Identification values within the (new) current SND.WND
   for this interface pair for up to ReachableTime milliseconds or until
   the NCE is updated by a new control message exchange.  This implies
   that OAL A must send a new SYN before sending more than N OAL packets
   within the current SND.WND, i.e., even if ReachableTime is not
   nearing expiration.  After OAL B returns the ACK, it accepts carrier
   packets received from OAL A via this interface pair within either the
   current or previous RCV.WND as well as any new authentic control
   messages with the SYN flag set received from OAL A even if outside
   the windows.

   OMNI interface neighbors can employ asymmetric window synchronization
   as described above using 2 independent (SYN -> ACK) exchanges (i.e.,
   a 4-message exchange), or they can employ symmetric window
   synchronization using a modified version of the TCP "3-way handshake"
   as follows:

   *  OAL A prepares a SYN with an unpredictable ISS not within the
      current SND.WND and with Scale set to S and Window set to W.  OAL
      A caches the new ISS and window size as pending information, uses
      the pending ISS as the Identification for OAL encapsulation, then
      sends the resulting OAL packet to OAL B and waits up to
      RetransTimer milliseconds to receive an ACK response
      (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).

   *  OAL B receives the SYN, then resets its RCV variables based on the
      Sequence Number while caching OAL A's send window size as its
      receive window size.  OAL B then selects a new unpredictable ISS
      outside of its current window, then prepares a response with
      Sequence Number set to the pending ISS and Acknowledgement Number
      set to OAL A's next sequence number.  OAL B then sets both the SYN
      and ACK flags, sets Scale and Window to chosen values S' and W'
      and sets the OPT flag according to whether an explicit concluding
      ACK is optional or mandatory.  OAL B then uses the pending ISS as



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      the Identification for OAL encapsulation, sends the resulting OAL
      packet to OAL A and waits up to RetransTimer milliseconds to
      receive an acknowledgement (retransmitting up to
      MAX_UNICAST_SOLICIT times if necessary).

   *  OAL A receives the SYN/ACK, then resets its SND variables based on
      the Acknowledgement Number (which must include the sequence number
      following the pending ISS).  OAL A then resets its RCV variables
      based on the Sequence Number and OAL B's advertised send window
      S'/W' and marks the NCE as REACHABLE.  If the OPT flag is clear,
      OAL A next prepares an immediate unsolicited control message with
      the ACK flag set, the Acknowledgement Number set to OAL B's next
      sequence number, with Scale set to S' and Window set W', and with
      the OAL encapsulation Identification to SND.NXT, then sends the
      resulting OAL packet to OAL B.  If the OPT flag is set and OAL A
      has OAL packets queued to send to OAL B, it can optionally begin
      sending their carrier packets under the current SND.WND as
      implicit acknowledgements instead of returning an explicit ACK.

   *  OAL B receives the implicit/explicit acknowledgement(s) then
      resets its SND state based on the pending/advertised values and
      marks the NCE as REACHABLE.  Note that OAL B sets the OPT flag in
      the SYN/ACK to assert that it will interpret timely receipt of
      carrier packets within the (new) current window as an implicit
      acknowledgement.  Potential benefits include reduced delays and
      control message overhead, but use case analysis is outside the
      scope of this specification.)

   Following synchronization, OAL A and OAL B hold updated NCEs and
   AFVs, and can exchange OAL packets with Identifications set to
   SND.NXT for each flow while the state remains REACHABLE and there is
   available window capacity.  (Intermediate systems that establish AFVs
   for the per-flow window synchronization exchanges can also use the
   Identification window for source validation.)  Either neighbor may at
   any time send a new SYN to assert a new ISS.  For example, if OAL A's
   current SND.WND for OAL B is nearing exhaustion and/or ReachableTime
   is nearing expiration, OAL A can continue sending OAL packets under
   the current SND.WND while also sending a SYN with a new unpredictable
   ISS.  When OAL B receives the SYN, it resets its RCV variables and
   may optionally return either an asymmetric ACK or a symmetric SYN/ACK
   to also assert a new ISS.  While sending SYNs, both neighbors
   continue to send OAL packets with Identifications set to the current
   SND.NXT for each interface pair then reset the SND variables after an
   acknowledgement is received.

   While the optimal symmetric exchange is efficient, anomalous
   conditions such as receipt of old duplicate SYNs can cause confusion
   for the algorithm as discussed in Section 3.5 of [RFC9293].  For this



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   reason, the OMNI Neighbor Synchronization sub-option includes an RST
   flag which OAL nodes set in solicited control message responses to
   ACKs received with incorrect acknowledgement numbers.  The RST
   procedures (and subsequent synchronization recovery) are conducted
   exactly as specified in [RFC9293].

   OMNI interfaces that employ the window synchronization procedures
   described above observe the following requirements:

   *  OMNI interfaces MUST select new unpredictable ISS values that are
      at least a full window outside of the current SND.WND.

   *  OMNI interfaces MUST set the Scale and Window fields in SYN
      messages as a non-negotiable advertised send window size.

   *  OMNI interfaces MUST send control messages used for window
      synchronization securely while using unpredictable initial
      Identification values until synchronization is complete.

   It is essential to understand that the above window synchronization
   operations between nodes OAL(A) and OAL(B) are conducted in control
   message exchanges over multihop paths with potentially many OAL(i)
   intermediate hops in the forward and reverse paths (which may be
   disjoint).  Each such forward path OAL(i) caches the Sequence Number,
   Scale and Window values advertised from OAL(A) to OAL(B) in its AFV
   entry indexed by the previous hop underlay address and AFVI, while
   each such reverse path OAL(i) caches the Sequence Number, Scale,
   Window and AFVI advertised from OAL(B) to OAL(A).  (The forward/
   reverse path OAL(i) nodes then select new unique next-hop AFVIs
   before forwarding.)

   While multiple independent paths may exist between nodes OAL(A) and
   OAL(B), the synchronized Sequence Numbers between the two nodes apply
   collectively to all paths.  Nodes OAL(A) and OAL(B) therefore perform
   initial synchronization through control message exchanges with the
   SYN flag set over a first path for which intermediate nodes cache the
   Sequence Number, Scale and Window values in their AFVs.  However,
   control message exchanges that establish and maintain alternate paths
   include the current Sequence Number and residual window size but with
   the SYN flag clear.

   Each neighbor pair can therefore dynamically coordinate multiple
   independent paths from a single Sequence Number space in this way.
   When nodes OAL(A) and OAL(B) need to re-synchronize they again
   advertise new Sequence Number, Scale and Window size values with the
   SYN flag set.  The nodes must then exchange additional control
   messages using the new values and with the SYN flag clear to
   establish or maintain alternate paths.



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   Note: Although OMNI interfaces employ TCP-like window synchronization
   and support ACK responses to SYNs, all other aspects of the IPv6 ND
   protocol (e.g., control message exchanges, NCE state management,
   timers, retransmission limits, etc.) are honored exactly per
   [RFC4861].  OMNI interfaces further manage per-interface-pair window
   synchronization parameters in one or more AFVs for each neighbor
   pair.

   Note: Recipients of OAL-encapsulated control messages index the NCE
   based on the message Source Address, which also determines the
   carrier packet Identification window.  However, control messages may
   contain a message Source Address that does not match the OMNI
   encapsulation Source Address when the recipient acts as a proxy.

   Note: OMNI interface neighbors apply separate send and receive
   windows for all of their (multilink) underlay interface pairs that
   exchange carrier packets.  Each interface pair represents a distinct
   underlay network path, and the set of paths traversed may be highly
   diverse when multiple interface pairs are used.  OMNI intermediate
   systems therefore become aware of each distinct set of interface pair
   window synchronization parameters based on periodic control message
   updates to their respective AFVs.

6.8.  OAL Fragmentation Reports and Retransmissions

   When the OAL destination experiences reassembly congestion for a
   specific flow (e.g., when excessive numbers of reassembly failures
   are occurring), it can send an OAL Fragmentation Report (FRAGREP)
   message to the OAL source to recommend a reduced Maximum Receive Unit
   (MRU) for the flow (see: Section 10.2.15).  When the OAL source
   receives the FRAGREP, it caches the new MRU for the flow and returns
   "soft errors" to original sources that send larger packets (see:
   Section 6.9).  When the OAL destination experiences reassembly
   congestion for all flows from the same OAL source, it can return
   FRAGREP messages with Flow Label set to 0 as indication that all
   flows are affected.

   When the round-trip delay from the original source to the final
   destination is long while the round-trip time from the OAL source to
   the OAL destination is significantly shorter, the OAL source can
   maintain a short-term cache of the OAL fragments it sends to OAL
   destinations for each flow in case timely best-effort selective
   retransmission is requested.  The OAL destination in turn maintains a
   checklist for (Source, Destination, Flow Label, Identification)-
   tuples of recently received OAL fragments and notes the ordinal
   numbers of OAL fragments already received (i.e., as ordinals #0, #1,
   #2, #3, etc.).  The timeframe for maintaining the OAL source and
   destination caches determines the link persistence (see: [RFC3366]).



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   If the OAL destination notices some fragments missing after most
   other fragments within the same link persistence timeframe have
   already arrived, it may issue an Automatic Repeat Request (ARQ) with
   Selective Repeat (SR) by sending an unsolicited control message to
   the OAL source.  The OAL destination creates a control message with
   an OMNI option with one or more FRAGREP sub-options that include
   Bitmaps for fragments received and missing from this OAL source (see:
   Section 10.2.15).  The OAL destination includes an authentication
   signature if necessary, performs OAL encapsulation (with its own
   address as the OAL Source Address and the Source Address of the
   message that prompted the unsolicited control message as the OAL
   Destination Address) and sends the message to the OAL source.

   If an OAL intermediate system or OAL destination processes an OAL
   fragment for which corruption is detected, it may similarly issue an
   immediate ARQ/SR the same as described above.  The FRAGREP provides
   an immediate (rather than time-bounded) indication to the OAL source
   that a fragment has been lost.

   When the OAL source receives the control message, it authenticates
   the message then examines any enclosed FRAGREPs.  For each (Source,
   Destination, Flow Label, Identification)-tuple, the OAL source
   determines whether it still holds the corresponding OAL fragments in
   its cache and retransmits any for which the Bitmap indicates a loss
   event.  For example, if the Bitmap indicates that ordinal fragments
   #3, #7, #10 and #13 from the OAL packet with Identification
   0x0123456789abcdef are missing the OAL source only retransmits those
   fragments.  When the OAL destination receives the retransmitted OAL
   fragments, it admits them into the reassembly cache and updates its
   checklist.  If some fragments are still missing, the OAL destination
   may send a small number of additional ARQ/SR control messages within
   the link persistence timeframe.

   The OAL therefore provides a link layer low-to-medium persistence
   ARQ/SR service consistent with [RFC3366] and Section 8.1 of
   [RFC3819].  The service provides the benefit of timely best-effort
   link layer retransmissions which may reduce OAL fragment loss and
   avoid some unnecessary end-to-end delays.  This best-effort network-
   based service therefore complements transport and higher layer end-
   to-end protocols responsible for true reliability.











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6.9.  OMNI Interface MTU Feedback Messaging

   When the OMNI interface forwards original IP packets from the network
   layer, it invokes the OAL and returns internally-generated Path MTU
   Discovery (PMTUD) ICMPv4 "Fragmentation Needed and Don't Fragment
   Set" [RFC1191] or ICMPv6 "Packet Too Big (PTB)" [RFC8201] messages as
   necessary.  This document refers to both message types as "PTBs" and
   introduces a distinction between PTB "hard" and "soft" errors as
   discussed below.

   Ordinary PTB messages are hard errors that always indicate loss due
   to a real MTU restriction has occurred.  However, the OMNI interface
   can also forward original IP packets via OAL encapsulation and
   fragmentation while at the same time returning PTB soft error
   messages (subject to rate limiting) to the original source to suggest
   smaller sizes due to factors such as link performance
   characteristics, excessive numbers of fragments needed, reassembly
   congestion, etc.

   This ensures that the path MTU is adaptive and reflects the current
   path used for a given data flow.  The OMNI interface can therefore
   continuously forward original IP packets without loss while returning
   PTB soft error messages that recommend smaller sizes.  Original
   sources that receive the soft errors in turn reduce the size of the
   original IP packets they send the same as for hard errors, but not
   necessarily due to a loss event.  The original source can then resume
   sending larger packets if the soft errors subside.

   OAL intermediate systems that experience fragment loss and OAL end
   systems that experience reassembly cache congestion can return
   unsolicited control messages that include OMNI encapsulated PTB soft
   error messages to OAL sources that originate fragments (subject to
   rate limiting).  The OAL node creates a secured ICMPv6 PTB control
   message with MTU set to a reduced value and with the leading portion
   an OAL first fragment containing the header of an original IP packet
   for which the source must be notified (see: Section 10).

   The OAL node that sends the control message encapsulates the leading
   portion of the OAL first fragment (beginning with the OAL header) in
   the PTB "packet in error" field and signs the message if an
   authentication signature is necessary.  The OAL node then performs
   OAL encapsulation (with its own address as the Source Address and the
   Source Address of the message that prompted the control message
   response as the Destination Address) and sends the message to the OAL
   source.  (Note that OAL intermediate systems forward control messages
   via the secured spanning tree while OAL source and destination end
   systems include an authentication signature when necessary.)




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   The OAL source prepares the PTB soft error by first setting the Type
   field to 2 for IPv6 [RFC4443] or "Packet Too Big" for IPv4 (see:
   [I-D.templin-6man-ipid-ext2]).  The OAL source then sets the Code
   field to "PTB Soft Error (no loss)" if the OAL destination forwarded
   the original IP packet successfully or "PTB Soft Error (loss)" if it
   was dropped (see: [I-D.templin-6man-ipid-ext2]).  The OAL source next
   sets the PTB Destination Address to the original IP packet Source
   Address, and sets the PTB Source Address to one of its OMNI interface
   addresses that is reachable from the perspective of the original
   source.

   The OAL source then sets the MTU field to a value smaller than the
   original IP packet size but no smaller than 1280, writes as much of
   the original IP packet first fragment as possible into the "packet in
   error" field such that the entire PTB including the IP header is no
   larger than 1280 octets for IPv6 or 576 octets for IPv4.  The OAL
   source then calculates and sets the ICMP Checksum and returns the PTB
   to the original source.

   An original sources that receives these PTB soft errors first
   verifies that the ICMP Checksum is correct and the packet-in-error
   contains the leading portion of one of its recent packet
   transmissions.  The original source can then adaptively tune the size
   of the original IP packets it sends to produce the best possible
   throughput and latency, with the understanding that these parameters
   may fluctuate over time due to factors such as congestion, mobility,
   network path changes, etc.  Original sources should therefore
   consider receipt or absence of soft errors as hints of when
   decreasing or increasing packet sizes may provide better performance.

   The OMNI interface supports continuous transmission and reception of
   packets of various sizes in the face of dynamically changing network
   conditions.  Moreover, since PTB soft errors do not indicate a hard
   limit, original sources that receive soft errors can resume sending
   larger packets without waiting for the recommended 10 minutes
   specified for PTB hard errors [RFC1191][RFC8201].  The OMNI interface
   therefore provides an adaptive service that accommodates MTU
   diversity especially well-suited for air/land/sea/space mobile
   Internetworking.

   Note: when the OAL source receives persistent Fragmentation Reports
   for a given flow (see: Section 6.8), it should return PTB soft errors
   to the original source (subject to rate limiting) the same as if it
   had received PTB soft errors from the OAL destination.  When the
   original source is likely to retransmit an entire original IP packet
   on its own behalf in case of loss, the OAL destination can elect to
   return only PTB soft errors and refrain from returning Fragmentation
   Reports.



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   Note: the OAL source may receive control messages that include both a
   PTB soft error and Fragmentation Report(s).  If so, the OAL source
   both returns PTB soft errors to the original source (subject to rate
   limiting) and retransmits any missing fragments if it is configured
   to do so.

6.10.  OAL Composite Packets

   The OAL source ordinarily includes a 40 octet IPv6 encapsulation
   header for each original IP packet during OAL encapsulation.  The OAL
   source then performs fragmentation such that a copy of the 40 octet
   IPv6 header plus a 16 octet IPv6 Extended Fragment Header is included
   in each OAL fragment (when a Routing Header is added, the OAL
   encapsulation headers become larger still).  However, these
   encapsulations may represent excessive overhead in some environments.

   OAL header compression as discussed in Section 6.5 can significantly
   reduce encapsulation overhead, however a complementary technique
   known as "packing" (see: [I-D.ietf-intarea-tunnels]) supports
   encapsulation of multiple original IP packets and/or control messages
   within a single OAL "composite packet".

   When the OAL source has multiple original IP packets to send to the
   same OAL destination with total length no larger than the OAL
   destination EMTU_R, it can concatenate them into a composite packet
   encapsulated in a single OAL header.  Within the OAL composite
   packet, the IP header of the first original IP packet (iHa) followed
   by its data (iDa) is concatenated immediately following the OAL
   header.  The IP header of the next original packet (iHb) followed by
   its data (iDb) is then concatenated immediately following the first,
   with each remaining original IP packet concatenated in succession.
   The OAL composite packet format is transposed from
   [I-D.ietf-intarea-tunnels] and shown in Figure 13:


















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                   <------- Original IP packets ------->
                   +-----+-----+
                   | iHa | iDa |
                   +-----+-----+
                         |
                         |     +-----+-----+
                         |     | iHb | iDb |
                         |     +-----+-----+
                         |           |
                         |           |     +-----+-----+
                         |           |     | iHc | iDc |
                         |           |     +-----+-----+
                         |           |           |
                         v           v           v
        +----------+-----+-----+-----+-----+-----+-----+
        |  OAL Hdr | iHa | iDa | iHb | iDb | iHc | iDc |
        +----------+-----+-----+-----+-----+-----+-----+
        <-- OAL composite packet with single OAL Hdr -->

                   Figure 13: OAL Composite Packet Format

   When the OAL source prepares a composite packet, it applies OAL
   fragmentation then applies underlay encapsulation and sends the
   resulting carrier packets to the OAL destination.  When the OAL
   destination receives the composite packet it first reassembles if
   necessary.  The OAL destination then selectively extracts each
   original IP packet (e.g., by setting pointers into the composite
   packet buffer and maintaining a reference count, by copying each
   packet into a separate buffer, etc.) and forwards each one to the
   network layer.  During extraction, the OAL determines the IP protocol
   version of each successive original IP packet 'j' by examining the 4
   most-significant bits of iH(j), and determines the length of each one
   by examining the rest of iH(j) according to the IP protocol version.

   When an OAL source prepares a composite packet that includes a
   control message as the first original IP packet (i.e., iHa/iDa) it
   includes any additional original IP packets in concatenated
   succession then includes a trailing OMNI option.  If the OMNI option
   contains an authentication sub-option, the OAL source calculates the
   authentication signature over the entire length of the composite
   packet.  (A second common use case entails a path MTU probe beginning
   with an unsigned control message followed by a suitably large NULL
   packet (e.g., an IP packet with padding octets added beyond the IP
   header and with {Protocol, Next Header} set to 59 ("No Next Header"),
   a UDP/IP packet with port number set to 9 ("discard") [RFC0863],
   etc.)





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   The OAL source can also apply this composite packet packing technique
   at the same time it performs OCH1 header compression as discussed in
   Section 6.5.  Note that this technique can only be applied for
   original IP packets of a single flow, such as for a stream of packets
   for the flow that are queued for transmission service at roughly the
   same time.

6.11.  OAL Bubbles

   OAL sources may send NULL OAL packets known as "bubbles" for example
   to establish Network Address Translator (NAT) state on the path to
   the OAL destination.  The OAL source prepares a bubble by crafting an
   OAL header with appropriate IPv6 Source and Destination ULAs, with
   the IPv6 Next Header field set to the value 59 ("No Next Header" -
   see: [RFC8200]) and with 0 or more octets of NULL protocol data
   immediately following the IPv6 header.

   The OAL source includes a randomly-chosen Identification value then
   encapsulates the OAL packet in underlay headers destined to either
   the mapped address of the OAL destination's first-hop ingress NAT or
   the underlay address of the OAL destination itself.  When the OAL
   source sends the resulting carrier packet, any egress NATs in the
   path toward the underlay destination will establish state based on
   the activity.  At the same time, the bubbles themselves will be
   harmlessly discarded by either an ingress NAT on the path to the OAL
   destination or by the OAL destination itself.

   The bubble concept for establishing NAT state originated in [RFC4380]
   and was later updated by [RFC6081].  OAL bubbles may be employed by
   mobility services such as AERO.

6.12.  OAL Requirements

   In light of the above, OAL sources, destinations and intermediate
   systems observe the following normative requirements:

   *  OAL sources MUST forward original IP packets either larger than
      the OMNI interface minimum EMTU_R or smaller than the minimum OFS
      as atomic fragments (i.e., and not as multiple fragments).

   *  OAL sources MUST perform OAL fragmentation such that all non-final
      fragments are equal in length while the final fragment may be
      smaller.

   *  OAL sources MUST produce non-final fragments with payloads no
      smaller than the minimum OFS during fragmentation.





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   *  OAL intermediate systems SHOULD and OAL destinations MUST
      unconditionally drop any non-final OAL fragments with payloads
      smaller than the minimum OFS.

   *  OAL destinations MUST drop any new OAL fragments that would
      overlap with other fragments and/or leave holes smaller than the
      minimum OFS between fragments that have already been received.

   Note: Certain legacy network hardware of the past millennium was
   unable to accept IP fragment "bursts" resulting from a fragmentation
   event - even to the point that the hardware would reset itself when
   presented with a burst.  This does not seem to be a common problem in
   the modern era, where fragmentation and reassembly can be readily
   demonstrated at line rate (e.g., using tools such as 'iperf3') even
   over fast links on ordinary hardware platforms.  Even so, while the
   OAL destination is reporting reassembly congestion (see: Section 6.9)
   the OAL source could impose "pacing" by inserting an inter-fragment
   delay and increasing or decreasing the delay according to congestion
   indications.

6.13.  OAL Fragmentation Security Implications

   As discussed in Section 3.7 of [RFC8900], there are 4 basic threats
   concerning IPv6 fragmentation; each of which is addressed by
   effective mitigations as follows:

   1.  Overlapping fragment attacks - reassembly of overlapping
       fragments is forbidden by [RFC8200]; therefore, this threat does
       not apply to the OAL.

   2.  Resource exhaustion attacks - this threat is mitigated by
       providing a sufficiently large OAL reassembly cache and
       instituting "fast discard" of incomplete reassemblies that may be
       part of a buffer exhaustion attack.  The reassembly cache should
       be sufficiently large so that a sustained attack does not cause
       excessive loss of good reassemblies but not so large that (timer-
       based) data structure management becomes computationally
       expensive.  The cache should also be indexed based on the arrival
       underlay interface such that congestion experienced over a first
       underlay interface does not cause discard of incomplete
       reassemblies for uncongested underlay interfaces.










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   3.  Attacks based on predictable fragment Identification values - in
       environments where spoofing is possible, this threat is mitigated
       through the use of Identification windows beginning with
       unpredictable values per Section 6.7.  By maintaining windows of
       acceptable Identifications, OAL neighbors can quickly discard
       spurious carrier packets that might otherwise clutter the
       reassembly cache.

   4.  Evasion of Network Intrusion Detection Systems (NIDS) - since the
       OAL source employs a robust OFS, network-based firewalls can
       inspect and drop OAL fragments containing malicious data thereby
       disabling reassembly by the OAL destination.  However, each OAL
       destination should also employ a (host-based) firewall.

   IPv4 includes a 2 octet (16-bit) Identification (IP ID) field with
   only 65535 unique values such that even at moderate data rates the
   field could wrap and apply to new carrier packets while the fragments
   of old carrier packets using the same IP ID are still alive in the
   network [RFC4963].  However, IPv4 links that configure a small MTU
   are likely to occur only at extreme network edges where low data rate
   links occur [RFC3819].  Since IPv6 provides a 4 octet (32-bit)
   Identification value, IP ID wraparound for IPv6 fragmentation may
   only be a concern at extreme data rates (e.g., 1Tbps or more).  These
   limitations are fully addressed through the 8 octet (64-bit) Extended
   Identification format supported by [I-D.templin-6man-ipid-ext2].

   Unless the path is secured at the network layer or below (i.e., in
   environments where spoofing is possible), OMNI interfaces MUST NOT
   send OAL packets/fragments with Identification values outside the
   current window and MUST secure control messages used for address
   resolution or window state synchronization.  OAL destinations SHOULD
   therefore discard without reassembling any out-of-window OAL
   fragments received over an unsecured path.

6.14.  Control/Data Plane Considerations

   The above sections primarily concern data plane aspects of the OMNI
   interface service and describe the data plane service model offered
   to the network layer.  OMNI interfaces also internally employ a
   control plane service based on control messaging.  These control
   plane messages are first subject to OAL encapsulation then forwarded
   over secured underlay interfaces (e.g., IPsec tunnels, secured direct
   point-to-point links, etc.) or over unsecured underlay interfaces and
   with an authentication signature included.

   OMNI interfaces must send all control plane messages as "atomic OAL
   packets".  This means that these messages must not be subjected to
   OAL fragmentation and reassembly, although they may be subjected to



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   underlay network fragmentation and reassembly along some paths.
   Fragmentation security concerns for large control messages are
   documented in [RFC6980].

7.  Ethernet-Compatible Link Layer Frame Format

   When the OMNI interface forwards original IP packets from the network
   layer it first invokes OAL encapsulation and fragmentation, then
   wraps each resulting OAL packet/fragment in any necessary underlay
   headers to produce carrier packets according to the native frame
   format of the underlay interface.  For example, for Ethernet-
   compatible interfaces the frame format is specified in [RFC2464], for
   aeronautical radio interfaces the frame format is specified in
   standards such as ICAO Doc 9776 (VDL Mode 2 Technical Manual), for
   various forms of tunnels the frame format is found in the appropriate
   tunneling specification, etc.

   When the OMNI interface encapsulates an OAL packet/fragment directly
   over an Ethernet-compatible link layer, the over-the-wire
   transmission format is shown in Figure 14:

      +--- ~~~ ---+-------~~~------+---------~~~---------+--- ~~~ ---+
      |  eth-hdr  | OMNI Ext. Hdrs | OAL Packet/Fragment | eth-trail |
      +--  ~~~ ---+-------~~~------+---------~~~---------+--- ~~~ ---+
                  |<-------   Ethernet Payload  -------->|

                   Figure 14: OMNI Ethernet Frame Format

   The format includes a standard Ethernet Header ("eth-hdr") with
   EtherType TBD2 (see: Section 20.2) followed by an Ethernet Payload
   that includes zero or more OMNI Extension Headers followed by an OAL
   (or native IPv6/IPv4) Packet/Fragment.  The Ethernet Payload is then
   followed by a standard Ethernet Trailer ("eth-trail").

   The first OMNI extension header and the OAL Packet/Fragment both
   begin with a 4-bit "Type/Version" as discussed in Section 6.2.  When
   "Type/Version" encodes an OMNI extension header type, the length of
   the extension headers is limited by [I-D.ietf-6man-eh-limits] and the
   length of the OAL Packet/Fragment is determined by the IP header
   fields that follow the extension headers.

   When "Type/Version" encodes OMNI-OCH1/2, OMNI-IP4 or OMNI-IP6 the
   length of the OAL Packet/Fragment is determined by the {Total,
   Payload} Length field found in the full/compressed header according
   to the specific protocol rules.






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   See Figure 2 for a map of the various underlay encapsulation layering
   possibilities.  For any layering combination, the final layer (e.g.,
   UDP, IP, Ethernet, etc.) must have an assigned number and frame
   format representation that is compatible with the selected underlay
   interface.

8.  OMNI Addressing

   OMNI addressing observes the IPv6 addressing architecture [RFC4291]
   requirements: "IPv6 addresses of all types are assigned to
   interfaces, not nodes.  An IPv6 unicast address refers to a single
   interface.  Since each interface belongs to a single node, any of
   that node's interfaces' unicast addresses may be used as an
   identifier for the node [...]".  OMNI addressing further follows the
   IPv6 address selection policies specified in [RFC6724] as updated by
   [I-D.ietf-6man-rfc6724-update].

   Each OMNI interface is configured over a set of underlay interfaces
   as a virtual data link layer for the OAL.  OMNI nodes assign IP
   addresses to their underlay interfaces according to the native *NET
   autoconfiguration service(s) or through manual configuration.  OMNI
   nodes assign IPv6 addresses to their OMNI interfaces as specified in
   this section.

   [RFC4861] requires that hosts and routers assign Link-Local Addresses
   (LLAs) to all interfaces including OMNI interfaces, and that routers
   use their LLAs as the Source Address for RA and Redirect messages.
   OMNI nodes assign different external and internal LLAs to their OMNI
   interfaces but need not test them for uniqueness over the entire OMNI
   link since each (locally-unique) LLA is mapped to an assured
   globally-unique Multilink Local Address (MLA).  OMNI nodes that
   assign multiple external LLAs to an OMNI interface (e.g., as
   suggested by [I-D.link-6man-gulla]) map all LLAs to the (singular)
   OMNI interface MLA.

   This specification further requires that each OMNI interface must
   assign a unique MLA per the address format specified in [RFC9374] and
   within the architecture of [I-D.templin-6man-mla].  The node assigns
   the MLA to an OMNI interface configured over its set of underlay
   interfaces per the IPv6 scoped addressing architecture "site"
   abstraction [RFC4007].  For OMNI interfaces configured over MANET
   underlay interfaces, the node also assigns the same MLA to each MANET
   interface.  The node regards the OMNI interface MLA assignment as an
   adaptation layer address in the architecture and regards the underlay
   interface MLA assignments as node-local anycast addresses, with each
   underlay interface distinguished by its ifIndex.





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   The OMNI link extends across any underlying Internetworks to include
   all Proxy/Servers and other service nodes.  All Clients are also
   considered to be connected to the OMNI link, however unnecessary
   encapsulations are omitted whenever possible to conserve bandwidth
   (see: Section 12).  An OMNI domain consists of one or more OMNI links
   joined together to provide service for a common set of MSPs.

   OMNI domains include one or more OMNI links that together coordinate
   a common set of MSPs delegated from an IP GUA prefix space [RFC4291]
   from which the MS delegates MNPs to support Client EUN addressing.

   For IPv6, MSPs are assigned to an OMNI domain by IANA and/or an
   associated Regional Internet Registry [IPV6] such that the link(s)
   can be connected to the global IPv6 Internet without causing routing
   inconsistencies.  Instead of GUAs, an OMNI link could use ULAs with
   the 'L' bit set to 0 (i.e., from the "ULA-C" prefix fc00::/8)
   [RFC4193], however this would require IPv6 NAT if the domain were
   ever connected to the global IPv6 Internet.

   For IPv4, MSPs are assigned to an OMNI domain by IANA and/or an
   associated RIR [IPV4] such that the link(s) can be connected to the
   global IPv4 Internet without causing routing inconsistencies.  An
   OMNI *NET could instead use private IPv4 prefixes (e.g., 10.0.0.0/8,
   etc.)  [RFC6890], however this would require IPv4 NAT at the *NET
   boundary.  OMNI interfaces advertise IPv4 MSPs into IPv6 routing
   systems as "6to4 prefixes" [RFC3056] (e.g., the IPv6 prefix for the
   IPv4 MSP "V4ADDR/24" is 2002:V4ADDR::/40).

   IPv4 routers that configure OMNI interfaces advertise the prefix
   TBD3/N (see: IANA Considerations) into the routing systems of their
   connected *NETs and assign the IPv4 OMNI anycast address TBD3.1 to
   their *NET interfaces.  IPv6 routers that configure OMNI interfaces
   advertise the prefix 2002:TBD3::/(N+16) into the routing systems of
   their connected *NETs and assign the IPv6 OMNI anycast address
   2002:TBD3:: to their *NET interfaces.

   OMNI interfaces use their OMNI IPv6 and IPv4 anycast addresses to
   support control plane Service Discovery in the spirit of [RFC7094],
   i.e., the addresses are not intended for use in supporting longer
   term data plane flows.  Specific applications for OMNI IPv6 and IPv4
   anycast addresses are discussed throughout the document as well as in
   [I-D.templin-6man-aero3].









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9.  Node Identification

   OMNI Clients and Proxy/Servers that connect over open Internetworks
   include a unique node identification value for themselves in the IPv6
   Source Address and/or in an OMNI option of their control messages
   (see: Section 10.2.9).  Each node configures and includes an MLA as a
   node identification as discussed in Section 8.  (The Universally
   Unique IDentifier (UUID) [RFC9562] is another example of a node
   identifier which can be self-generated by a node without supporting
   infrastructure with very low probability of collision.)

   When a Client is truly outside the context of any infrastructure, it
   may have no topology-aggregated addressing information at all.  In
   that case, the Client can use an MLA as an IPv6 Source/Destination
   Address for sustained communications in Vehicle-to-Vehicle (V2V) and
   (multihop) Vehicle-to-Infrastructure (V2I) scenarios.  The Client can
   also propagate the MLA into the multihop routing tables of
   (collective) Mobile/Vehicular Ad-hoc Networks (MANETs/VANETs) using
   only the vehicles themselves as communications relays.  MLAs provide
   an especially useful node identification construct since they appear
   as properly-formed IPv6 addresses.

10.  Address Mapping - Unicast

   OMNI interfaces maintain network layer conceptual Neighbor and
   Destination Caches per [RFC1256][RFC4861] the same as for any IP
   interface.  The network layer maintains state through static and/or
   dynamic Neighbor/Destination Cache Entry (NCE/DCE) configurations.

   Each OMNI interface also maintains internal ALNCEs that supplement
   the NLNCEs for each of its active neighbors.  For each peer,
   neighbors also maintain AERO Flow Vectors (AFVs) as ALNCE state to
   map neighbor per-interface-pair parameters.

   When a Client's network layer sends or receives IPv6 Neighbor
   Discovery (ND) messages over an OMNI interface, it follows the
   procedures in [RFC4861] using the Source/Target Link-Layer Address
   Option (S/TLLAO) format defined for Ethernet [RFC2464].  The OAL then
   removes the S/TLLAO at the adaptation layer before transmission since
   the locally-assigned Ethernet address has no significance to external
   neighbors.  On receipt of Neighbor Advertisement and Redirect
   messages, the OAL inserts a TLLAO with the OMNI interface internal
   link-layer address then re-calculates the ICMPv6 message checksum and
   forwards it to the network layer.

   When a Client's network layer sends or receives an ordinary IP packet
   over an OMNI interface, the OAL consults the IP Destination to OAL
   IPv6 address mappings established by earlier control message



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   exchanges.  On transmission, the OAL uses the IP destination address
   to determine the Destination Address for an OAL encapsulation header
   while including an SRH extension.  On reception, the OAL uses the
   IPv6 encapsulation header Source Address to determine the source
   address for the virtual Ethernet header.

   The OMNI interface must therefore maintain ALNCEs that map IP
   Destination addresses to MLAs while exposing only the OMNI interface
   internal link-layer address to the IP layer.  When the OMNI interface
   discovers a new neighbor (e.g., when it creates a new NCE based on
   receipt of an IPv6 ND message), it maps the MLA to the OMNI interface
   internal link-layer address.  When the OMNI interface discards an
   existing neighbor, it deletes the now-expired NCE.

   An OAL destination or intermediate system may also need to return
   ICMPv6 error messages (e.g., Destination Unreachable, Packet Too Big,
   Time Exceeded, Parameter Problem, etc.)  [RFC4443] to the OAL source.
   The OAL destination or intermediate system marks the ICMPv6 error
   message as an OMNI control message and includes a trailing OMNI
   option with any necessary authentication sub-options as discussed
   below.  The OAL source can then confirm that the message originated
   from a trusted OAL node on the path.

   When the OAL forwards control messages from the network layer to the
   underlay, it replaces the Ethernet header with an adaptation layer
   IPv6 encapsulation header (plus an SRH extension) and a pseudo IPv6
   ND option trailer encoding OMNI link-specific information.  When the
   OAL forwards IPv6 ND messages from the underlay to the network layer,
   it performs decapsulation by parsing and removing the trailer while
   replacing the adaptation layer IPv6 header with a local Ethernet
   header.

   When the OAL forwards a control message from the network layer to the
   underlay, it can verify the control message checksum to ensure
   integrity in the local network stack as an OPTIONAL first step before
   performing OAL encapsulation.  When the OAL alters a network layer
   control message or creates an internally-generated control message
   that it will forward to the underlay, it sets the control message
   checksum to 0 since an OAL checksum will properly cover the same data
   (see: Section 10.1).











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   When the OAL receives an OAL-encapsulated control message from the
   underlay, it ignores the control message checksum if it will process
   the message internally since integrity is already verified by an OAL
   checksum in the trailing OMNI option (see below).  For control
   messages that it will forward to the network layer that have the
   control message checksum set to 0, the OAL instead calculates and
   rewrites the checksum following OAL decapsulation.  The network layer
   will then verify the control message checksum independently of the
   OAL the same as for any IPv6 interface.

   Hence, this document defines a new pseudo IPv6 ND option type termed
   the "OMNI option" designed for these purposes.  Since the pseudo-
   option is inserted and removed by the adaptation layer and never
   exposed to the network layer, it does not require a formal IPv6 ND
   option number assignment.

10.1.  The OMNI Option

   During OAL IPv6 encapsulation of each control message, the OAL source
   appends a single OMNI (pseudo-)option as a contiguous block of data
   immediately following the end of the (composite) packet.  The OAL
   source then sets the DSCP value as specified in Section 6.1 to mark
   the message as control.  If the composite packet does not end on an
   integral 8 octet boundary, the OAL source inserts padding octets
   following the final composite packet element to ensure 8 octet
   alignment before appending the rest of the OMNI option.  The OAL
   source then adds the OMNI option length (including padding) to the
   OAL Payload Length.

   During decapsulation of each control message, the OAL destination
   processes the OMNI option contents then removes the option before
   delivering the original control message (plus any additional original
   IP packets from the composite packet) to the network layer.  The OAL
   instead consumes control messages specific to the adaptation layer
   internally without delivering them to the network layer.

   The OMNI option therefore appears as a trailer in all OAL (composite)
   control packets.  The OMNI option format is shown in Figure 15.













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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       ~             OMNI Sub-Options (0 or more octets)               ~
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  OMNI Length  |  Auth Offset  |         OAL Checksum          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 15: OMNI Option Format

   In this format:

   *  Padding is included if necessary to begin the OMNI option on an
      even 8 octet boundary.

   *  OMNI Sub-Options is a variable-length concatenation of 0 or more
      sub-options formatted as specified in Section 10.2 such that the
      total length of all sub-options is an integer multiple of 8
      octets.  If an HMAC or RSA Signature authentication sub-option is
      included, it must appear as the final sub-option immediately
      preceded by any "authentication helper" sub-options necessary to
      support authentication.

   *  OMNI Length is a 1 octet field that encodes the length in 8 octet
      units of the immediately preceding OMNI Sub-Options data block.
      If no sub-options are present, OMNI Length must encode the value
      0.

   *  Auth Offset is a 1 octet field that encodes the offset in 8 octet
      units to the beginning of the first authentication (or
      "authentication helper") sub-option relative to the beginning of
      the OMNI sub-options.  If authentication sub-options are present,
      Auth Offset must encode a value smaller than OMNI Length;
      otherwise, it must encode a value no smaller than OMNI Length.
      For example, when OMNI sub-options includes 20 8-octet units and a
      first authentication/helper sub-option begins at the 12th 8-octet
      unit, OMNI Length encodes the value 20 and Auth Offset encodes the
      value 12.  If there are no authentication sub-options, Auth Offset
      instead encodes the value 20 or larger.

   *  OAL Checksum is a 2 octet field used to ensure integrity and
      protect against misdelivery.  If the OMNI option includes an
      authentication sub-option, the OAL source calculates and includes
      the authentication signature first.  The OAL source then



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      calculates the OAL Checksum beginning with a pseudo-header of the
      OAL IPv6 header per Section 8.1 of [RFC8200].  The pseudo-header
      includes the OAL IPv6 Source and Destination Addresses, where the
      MLAs of the source and its FHS or LHS egress peer (e.g., a Client
      and its Proxy/Server) are used if there are SRH intermediate hops
      between them.  The pseudo-header also sets Next Header to 41 (for
      IPv6 encapsulation), and sets Upper-Layer Packet Length to the OAL
      IPv6 Payload Length minus the length of any extension headers
      present between the OAL IPv6 header and the encapsulated control
      message.  The checksum then extends over the length of the message
      beginning with the first octet of the OMNI-encapsulated control
      message and continuing over the OMNI option up to and including
      the OMNI Length and Auth Offset fields.  The OAL source then
      writes the resulting value into the OAL Checksum field.  The OAL
      destination verifies the checksum upon message receipt and
      processes the message further only if the checksum is correct.

   OMNI encapsulated control messages exchanged over unsecured *NETs
   between peer Clients or Clients and their Proxy/Servers use either
   public-key based digital signatures per SEcure Neighbor Discovery
   (SEND) [RFC3971][RFC9374] or Hashed Message Authentication Codes
   (HMAC) per [RFC8754][RFC2104] as an adaptation layer authentication
   service.  Since the adaptation layer already applies authentication
   from within the OMNI interface, the network layer need not also apply
   IPv6 ND message authentication over the OMNI interface.  The OMNI
   option therefore provides sub-options to support either SEND or HMAC
   as adaptation layer authentication services.  Alternate
   authentication sub-option types may be specified in future documents.

   Although originally specified to operate with Cryptographically
   Generated Addresses (CGAs) per [RFC3972], SEND notes that: "This
   specification also allows a node to use non-CGAs with certificates
   that authorize their use.  However, the details of such use are
   beyond the scope of this specification and are left for future work."
   OMNI is based on an alternate cryptographically generated IPv6
   address type (the MLA), and therefore does not use CGAs.

   The OMNI Sub-Options may include full or partial information for the
   neighbor.  The OMNI interface therefore retains the union of the most
   recently received information in the corresponding NCE.











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   OMNI interface Clients such as aircraft typically have multiple
   wireless data link types (e.g. satellite-based, cellular,
   terrestrial, air-to-air directional, etc.) with diverse performance,
   cost and availability properties.  The OMNI interface would therefore
   appear to have multiple L2 connections, and may include information
   for multiple underlay interfaces in a single OMNI option.  OMNI
   interfaces manage their dynamically-changing multilink profiles by
   including OMNI sub-options as discussed in Section 10.2.

10.2.  OMNI Sub-Options

   The OMNI option includes a Sub-Options block containing zero or more
   individual sub-options.  Each successive sub-option is concatenated
   immediately following its predecessor.  All sub-options are encoded
   as follows:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        |    Sub-Type   |   Sub-Length  | Sub-Option Data ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                        Figure 16: Sub-Option Format

   *  Sub-Type is a 1 octet field that encodes the sub-option type.
      Sub-option types defined in this document include:

           Sub-Option Name             Sub-Type
           Null                           0
           CGA                            1
           RSA Signature                  2
           Timestamp                      3
           Nonce                          4
           Trust Anchor                   5
           Certificate                    6
           HMAC                           7
           Node Identification            8
           Neighbor Synchronization       9
           Interface Attributes          10
           Traffic Selector              11
           Geo Coordinates               12
           PIM-SM Message                13
           Fragmentation Report          14
           Proxy/Server Control          15
           Prefix Information Option     16
           Route Information Option      17
           DHCPv6 Message                18

                                  Figure 17




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      Sub-Types 19-252 are reserved for future sub-option assignments.
      Sub-Types 253 and 254 are reserved for experimentation while Sub-
      Type 255 is reserved by IANA.

   *  Sub-Length is a 1 octet field that encodes the length of the sub-
      option (including the type and length fields) in units of 8
      octets.  The value 0 is invalid; nodes MUST silently discard an ND
      packet that contains a sub-option with Sub-Length set to 0.

   *  Sub-Option Data is a block of data with format determined by Sub-
      Type and length determined by Sub-Length.

   The OAL source codes all sub-options in a single OMNI option in the
   same control message, with each sub-option concatenated immediately
   following the previous and with padding added if necessary to end
   each sub-option on an even 8 octet boundary.  If the total size of
   all sub-options would cause the control message to exceed the path
   MTU, the OAL source includes as many sub-options as possible and
   codes any remaining sub-options in additional control messages.

   The OAL destination processes the OMNI option in a received control
   message beginning with the first sub-option while skipping over and
   ignoring any NULL or unrecognized sub-options.  If an individual sub-
   option length would cause processing to exceed the OMNI option
   instance and/or control message lengths, the OAL destination drops
   the message.

   Control messages that require OMNI authentication services include an
   RSA or HMAC authentication sub-option as the final sub-option
   immediately preceded by any "authentication helper" sub-options.  A
   single control message includes at most one OMNI authentication
   service sub-option; if multiple appear (or if the sub-option is non-
   final) the message is dropped.

   Note: large objects that exceed the maximum Sub-Option Data length
   are not supported under the current specification; if this proves too
   limiting in practice, future specifications may define support for
   fragmenting large sub-options across multiple control messages.

   The following sub-option types and formats are defined in this
   document:

10.2.1.  NULL Sub-Option

   The NULL Sub-Option is skipped over and ignored on receipt regardless
   of the contents of the Sub-Option Data following the Sub-Length
   octet.  The format and contents of the sub-option are shown in
   Figure 18:



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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=0   |  Sub-Length   |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 18: NULL Sub-Option

   *  Sub-Type is set to 0.  Multiple instances may appear in the same
      OMNI option.

   *  Sub-Length is set to the number of 8 octet units in the sub-option
      including the Sub-Option Data that follows.

   *  Sub-Option Data includes non-offensive padding data which must be
      ignored on receipt, where any non-null padding is subject to the
      OAL checksum and authentication.

   Note that an authorized intermediate system can convert any sub-
   option into a NULL sub-option simply by writing the value 0 into its
   Sub-Type and resetting the OAL checksum.  Unauthorized intermediate
   systems cannot perform this conversion without invalidating
   authentication.

10.2.2.  CGA

   The OAL source includes a Cryptographically Generated Address (CGA)
   OMNI sub-option formatted as specified in Section 5.1 of [RFC3971]
   except that OMNI interfaces use MLAs (instead of CGAs) which are also
   cryptographically-generated [RFC9374].  The OMNI CGA sub-option has
   the following format:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=1   |  Sub-Length   |   Pad Length  |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                          CGA Parameters                       ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                              Figure 19: CGA

   *  Sub-Type is set to 1.  The CGA sub-option may appear at most once
      in any OMNI option; if multiple appear the message is dropped.





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   *  Sub-Length is set to the number of 8 octet units in the sub-option
      including the Sub-Option Data that follows.

   Among other parameters, the sub-option includes a public key as
   specified in [RFC3972] applied to the MLA Source Address according to
   [RFC9374].

   The CGA option must appear in all secured OMNI control messages that
   also include an RSA Signature sub-option.

10.2.3.  RSA Signature

   The OMNI RSA Signature sub-option includes a public key-based
   authentication signature extending over the length of the OMNI-
   encapsulated control message.  When present, the RSA Signature sub-
   option must appear as the final OMNI sub-option and must be
   immediately preceded by any "authentication helper" sub-options such
   as CGA, Nonce, Timestamp, etc.

   The OAL source fully populates the OMNI option and can then calculate
   a digital signature to include in an OMNI RSA Signature sub-option as
   discussed below.  The OAL source sets Auth Offset to the offset of
   the first "authentication helper" sub-option relative to the
   beginning of the OMNI sub-options.

   The RSA Signature sub-option is formatted as shown in Figure 20:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=2   |  Sub-Length   |           Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                           Key Hash                            ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Digital Signature                       ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           Padding                             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 20: RSA Signature

   *  Sub-Type is set to 2.  The RSA Signature sub-option may appear at
      most once in any OMNI option; if multiple appear the message is
      dropped.





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   *  Sub-Length is set to the length of the sub-option in 8 octet
      units.  The Key Hash is always the most significant (leftmost) 128
      bits of the secure hash of the public key used for constructing
      the signature while the length of the Digital Signature plus
      Padding is constrained by the remaining available space for this
      sub-option.

   After fully populating the OMNI sub-options, the OAL source
   constructs the Digital Signature according to Section 5.2 of
   [RFC3971] except beginning with the 128-bit Context ID value
   specified for MLAs in Section 3 of [RFC9374] instead of the CGA
   Message Type value.

   The signature then continues over the OAL IPv6 Source and Destination
   Addresses, where the MLAs of the source and its FHS or LHS egress
   peer are used if there are SRH intermediate hops between them.  The
   signature next continues over the encapsulated control message
   including all control message header and option contents.  The
   signature next continues over any composite packet extensions then
   extends over the entire OMNI option up to the beginning of the RSA
   sub-option itself.  The signature finally concludes by covering the
   trailing OMNI Length and Auth Offset fields that follow the RSA sub-
   option.

   After calculating the signature, the node writes the value into the
   Digital Signature field before calculating the trailing OAL Checksum.

   Note that the control message MLA Source Address encodes security
   suite information that determines the cryptographic algorithms and
   Digital Signature length [RFC9374].  The OMNI RSA Signature sub-
   option can therefore also carry non-RSA Digital Signatures.

10.2.4.  Timestamp

   The OAL source includes an OMNI Timestamp sub-option in control
   messages to ensure that unsolicited advertisements and redirects have
   not been replayed.  If multiple Timestamp sub-options appear the
   message is dropped.

   The Timestamp sub-option is processed exactly the same as in
   Section 5.3.1 of [RFC3971].  The OMNI Timestamp sub-option has the
   following format:









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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=3   |  Sub-Length   |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                       Reserved (6 octets)                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                       Timestamp (8 octets)                    +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 21: Timestamp

   *  Sub-Type is set to 3.  The Timestamp sub-option may appear at most
      once in any OMNI option; if multiple appear the message is
      dropped.

   *  Sub-Length is set to 2.

10.2.5.  Nonce

   The OAL source includes an OMNI Nonce sub-option to ensure that an
   IPv6 ND advertisement is a fresh response to one of its earlier
   solicitations.

   The Nonce sub-option is processed exactly the same as in
   Section 5.3.2 of [RFC3971] per the following format:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=4   |  Sub-Length   |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
       |                                                               ~
       ~                         Nonce (N octets)                      ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Figure 22: Nonce

   *  Sub-Type is set to 4.  The Nonce sub-option may appear at most
      once in any OMNI option; if multiple appear the message is
      dropped.

   *  Sub-Length is set to the number of 8 octet units in the sub-option
      including the Sub-Option Data that follows, where N is ((Sub-
      Length * 8) - 2).







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10.2.6.  Trust Anchor

   The OAL source includes a Trust Anchor OMNI sub-option the same as in
   Section 6.4.3 of [RFC3971] per the following format:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=5   |  Sub-Length   |  Name Type    |  Pad  Length  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Trust Anchor Body                       ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 23: Trust Anchor

   *  Sub-Type is set to 5.  The Trust Anchor sub-option may appear at
      most once in any OMNI option; if multiple appear the message is
      dropped.

   *  Sub-Length is set to the number of 8 octet units in the sub-option
      including the Sub-Option Data that follows.

10.2.7.  Certificate

   The OAL source includes a Certificate OMNI sub-option the same as in
   Section 6.4.4 of [RFC3971] per the following format:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=6   |  Sub-Length   |  Cert Type    |    Reserved   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                        Certificate Body                       ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 24: Certificate

   *  Sub-Type is set to 6.  The Certificate sub-option may appear at
      most once in any OMNI option; if multiple appear the message is
      dropped.

   *  Sub-Length is set to the number of 8 octet units in the sub-option
      including the Sub-Option Data that follows.




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10.2.8.  Hashed Message Authentication Code (HMAC)

   The OAL source may include a Hashed Message Authentication Code
   (HMAC) sub-option.  When present, the HMAC sub-option appears as the
   final sub-option and must be immediately preceded by any
   "authentication helper" sub-options such as Nonce, Timestamp, etc.
   the same as specified for RSA Signature above.

   The OAL source fully populates the OMNI option and can then calculate
   a digital signature to include in an OMNI HMAC sub-option as
   discussed below.  The OAL source sets Auth Offset to the offset of
   the first "authentication helper" sub-option relative to the
   beginning of the OMNI sub-options.

   The format of the HMAC option is taken directly from Section 2.1.2 of
   [RFC8754] as shown in Figure 25:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=7   |  Sub-Length   |D|           Reserved          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      HMAC Key ID (4 octets)                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                        HMAC (variable)                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                             Padding                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 25: Hashed Message Authentication Code (HMAC)

   Sub-Type is set to 7, and Sub-Length is set to the number of 8 octet
   units in the sub-option (note that this type value differs from the
   HMAC type defined for TLV options in [RFC8754] since the two
   (sub-)option numbering spaces are independent).  The HMAC sub-option
   may appear at most once in any OMNI option; if multiple appear the
   message is dropped.

   The HMAC digest is encoded and processed the same as specified in
   [RFC2104] and Section 2.1.2 of [RFC8754].  The HMAC secure hash
   algorithm (e.g., SHA-256, SHA3-256, etc.) is determined by consulting
   local network configuration indexed by the HMAC Key ID.

   The OAL node applies the HMAC over the OAL IPv6 Source and
   Destination Addresses, where the MLAs of the source and its FHS or
   LHS egress peer are used if there are SRH intermediate hops between
   them.  The HMAC then continues over the encapsulated control message
   including all control message header and option contents.  The HMAC



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   next continues over any composite packet extensions then extends over
   the entire OMNI option up to and including the HMAC key ID.  The HMAC
   finally concludes by covering the trailing OMNI Length and Auth
   Offset fields that follow the HMAC sub-option.

   After calculating the HMAC digest, the OAL node writes the value into
   the HMAC field then includes padding octets if necessary for 8 octet
   alignment before calculating the trailing OAL Checksum.  Only FHS and
   LHS OAL nodes need to include and verify the HMAC, since intermediate
   SRT hops engage the secured spanning tree.  The HMAC is inserted and
   calculated by the FHS/LHS ingress node then verified and removed by
   the FHS/LHS egress node.

10.2.9.  Node Identification

   The OAL source may include the Node Identification sub-option as
   supplementary identification information in addition to the control
   message Source Address.  If multiple instances appear in the same
   OMNI option, the first instance of a specific ID-Type is processed
   and all other instances of the same ID-Type are ignored.  (A single
   control message can therefore include multiple distinct Node
   Identifications - each with a different ID-Type.)

   The format and contents of the sub-option are shown in Figure 26:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=8   |  Sub-Length   |   Pad Length  |    ID-Type    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                    Node Identification Value                  ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 26: Node Identification

   *  Sub-Type is set to 8.  Multiple instances are processed as
      discussed above.

   *  Sub-Length is set to the number of 8 octet units in the sub-option
      including the Sub-Option Data that follows.

   *  Pad Length encodes the number of trailing padding octets required
      to end the sub-option on an even 8 octet boundary.  The ID-Type
      field is always present, and the maximum Node Identification Value
      length is limited by the remaining available space in this OMNI
      option.



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   *  ID-Type is a 1 octet field that encodes the type of the Node
      Identification Value.  The following ID-Type values are currently
      defined:

      -  0 - Multilink Local Address (MLA).  A special-purpose IPv6
         address assigned to an OMNI interface for adaptation layer
         addressing as discussed in Section 8.  Indicates that Node
         Identification Value contains a 16 octet MLA.

      -  1 - Universally Unique IDentifier (UUID) [RFC9562].  Indicates
         that Node Identification Value contains a 16 octet UUID.

      -  2 - Network Access Identifier (NAI) [RFC7542].  Indicates that
         Node Identification Value contains an N octet NAI.

      -  3 - Fully-Qualified Domain Name (FQDN) [RFC1035].  Indicates
         that Node Identification Value contains an N octet FQDN.

      -  4 - IPv4 Address.  Indicates that Node Identification contains
         a 4 octet IPv4 address.  The IPv4 address type is determined
         with reference to the IANA IPv4 Address Space Registry [IPV4].

      -  5 - Router ID (RID).  Indicates that Node Identification
         contains an N octet router ID other than an IPv4 or IPv6
         address.  May be useful for some MANET routing protocols that
         define their own RID formats.

      -  6 - IPv6 Address.  Indicates that Node Identification contains
         a general-purpose 16 octet IPv6 address that is not an MLA.
         The IPv6 address type is determined according to the IPv6
         addressing architecture [RFC4291] with reference to the IANA
         IPv6 Global Unicast Address Assignments Registry [IPV6].

      -  7 - 252 - Unassigned.

      -  253 - 254 - reserved for experimentation, as recommended in
         [RFC3692].

      -  255 - reserved by IANA.

   *  Node Identification Value is an N octet field encoded according to
      the appropriate the "ID-Type" reference above.  The Node
      Identification Value is followed by the number of padding octets
      indicated by Pad Length.







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   The OAL source encodes Node Identification Values used for DHCPv6
   messaging purposes as DHCP Unique IDentifiers (DUIDs) using the
   "DUID-EN for OMNI" format with enterprise number 45282 (see:
   Section 20) as shown in Figure 27:

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |         DUID-Type (2)         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Enterprise Number (45282)                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    ID-Type    |                                               |
       +-+-+-+-+-+-+-+-+                                               ~
       |                                                               ~
       ~                   Node Identification Value                   ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 27: DUID-EN for OMNI Format

   In this format, the OAL source codes the ID-Type and Node
   Identification Value fields from the OMNI sub-option following a 6
   octet DUID-EN header, then includes the entire "DUID-EN for OMNI" in
   a DHCPv6 message per [RFC8415].

10.2.10.  Neighbor Synchronization

   The OAL source includes a Neighbor Synchronization OMNI sub-option in
   control messages that establish or update neighbor state between
   Clients and their Proxy/Servers or peers.  Each control message
   includes at most one Neighbor Synchronization sub-option which must
   be specific to the underlying interface pair over which ND messages
   are exchanged.

   The Neighbor Synchronization sub-option is formatted as follows:

















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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=9   |  Sub-Length   |O|P|        Reserved           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Source ifIndex                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Destination ifIndex                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       |       |C|E|U|A|P|R|S|F|                               |
       | Scale | Rsrvd |W|C|R|C|S|S|Y|I|            Window             |
       |       |       |R|E|G|K|H|T|N|N|                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                        Sequence Number                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                     Acknowledgment Number                     ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 28: Neighbor Synchronization

   *  Sub-Type is set to 9.  If multiple instances appear in the same
      OMNI option, the first is processed and all others are ignored.
      Sub-Length is set to 2, 3 or 4 depending on whether a Sequence
      Number and/or Acknowledgement Number is included - see below.

   *  the next 2 octets include functional and reserved flags.
      Neighbors set the (O)ptional (OPT) flag as discussed in
      Section 6.7 in a SYN/ACK synchronization message that does not
      require a responsive ACK.  OAL intermediate systems set the (P)ath
      Change (PCH) flag in control messages used to report a change in a
      path established by multilink forwarding.

   *  the next 8 octets of Sub-Option Data includes the 4 octet ifIndex
      of the control message source node's underlay interface followed
      by the 4 octet ifIndex of the destination node's underlay
      interface.













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   *  the remainder of Sub-Option Data is modeled from the Transmission
      Control Protocol (TCP) header specified in Section 3.1 of
      [RFC9293].  The data begins with a 4-bit window Scale, followed by
      a 4-bit Reserved field, followed by 8 flags, followed by a 2 octet
      Window size.  An 8 octet Sequence Number follows only if the SYN
      flag is set, and an 8 octet Acknowledgement Number follows only if
      the ACK flag is set.  (If the SYN/ACK flag settings and Sub-Length
      value are inconsistent, the message MUST be discarded.)
      Intermediate nodes always cache the Sequence Number, window Scale
      and Window size values when the SYN flag is set.

10.2.11.  Interface Attributes

   The Interface Attributes sub-option provides neighbors with
   forwarding information for the multilink conceptual sending algorithm
   discussed in Section 12.  Neighbors use the forwarding information to
   select among candidate underlay interfaces that can be used to
   forward carrier packets to the neighbor based on factors such as
   traffic selectors and link metrics.  Interface Attributes further
   include link-layer address information to be used for either direct
   INET encapsulation for targets in the local SRT segment or spanning
   tree forwarding for targets in remote SRT segments.

   The OAL source includes Interface Attributes for some/all of a source
   or target Client's underlay interfaces in control solicitation and
   response messages that exchange peer-to-peer Client information (see:
   [I-D.templin-6man-aero3]).  The first Interface Attributes sub-option
   included MUST correspond to the interface used to transmit the
   control message.  At most one Interface Attributes sub-option for
   each distinct ifIndex may be included; if a control message includes
   multiple Interface Attributes sub-options for the same ifIndex, the
   first is processed and all others are ignored.  OMNI nodes that
   receive control messages can use all of the included Interface
   Attributes and/or Traffic Selectors to formulate a map of the
   prospective source or target node as well as to seed the information
   to be populated in future neighbor exchanges.

   OMNI Clients and Proxy/Servers also include Interface Attributes sub-
   options in RS/RA messages used to initialize, discover and populate
   routing and addressing information.  Each RS message MUST contain
   exactly one Interface Attributes sub-option with an ifIndex
   corresponding to the Client's underlay interface used to transmit the
   message, and each RA message MUST echo the same Interface Attributes
   sub-option with any (proxyed) information populated by the FHS Proxy/
   Server to provide operational context.






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   When an FHS Proxy/Server receives an RS message destined to an
   anycast underlay address, it MUST include an additional Interface
   Attributes sub-option with ifIndex '0' that encodes its own unicast
   underlay address relative to the Client's underlay interface in the
   solicited RA response.  Any additional Interface Attributes sub-
   options that appear in RS/RA messages (i.e., besides those for the
   Client's own ifIndex and ifIndex '0') are ignored.

   The Interface Attributes sub-option is formatted as shown below:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=10  |  Sub-Length   |      SRT      |      FMT      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            ifIndex                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            ifType                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            ifProvider                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            ifMetric                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            ifGroup                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            LHS-MLA                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            LHS-UNX                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           Padding                             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 29: Interface Attributes

   *  Sub-Type is set to 10.  Multiple instances are processed as
      discussed above.  Sub-Length is set to the number of 8 octet units
      in the option.  The SRT and FMT fields are specified below in
      conjunction with the LHS fields.

   *  Sub-Option Data contains an "Interface Attributes" option encoded
      as follows:

      -  ifIndex is a 4 octet index value corresponding to a specific
         underlay interface.  Client OMNI interfaces MUST number each
         distinct underlay interface with a unique non-zero ifIndex
         value assigned by network management per [RFC2863] and include
         the value in this field.  The ifIndex value '0' denotes
         "unspecified".





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      -  ifType is a 4 octet type value corresponding to this underlay
         interface.  The value is coded per the IANA
         "https://www.iana.org/assignments/smi-numbers registry group
         Interface Types (ifType)" registry according to [RFC8892].

      -  ifProvider is a 4 octet provider identifier corresponding to
         this underlay interface.  This document defines the single
         provider identifier value '0' (undefined).  Future documents
         may define other values.

      -  ifMetric encodes a 4 octet interface metric.  Lower values
         indicate higher priorities, and the highest value indicates an
         interface that should not be selected.  The ifMetric setting
         provides an instantaneous indication of the interface
         bandwidth, link quality, signal strength, cost, etc.; hence,
         its value may change in successive control messages.

      -  ifGroup is a 4 octet identifier for a Link Aggregation Group
         (LAG) [IEEE802.1AX] corresponding to the underlay interface
         identified by ifIndex.  Interface attributes for ifIndex
         members of the same group will encode the same value in
         ifGroup.  This document defines the single ifGroup value '0'
         meaning "no group assigned".  Future documents will specify the
         setting of other values.

      -  SRT is a 1 octet Segment Routing Topology control field.  The
         field contains 8 Reserved flags which must be set to 0 on
         transmission.  Future specifications may define new SRT control
         flags.

      -  FMT - a 1 octet "Forward/Mode/Type" code interpreted as
         follows:

         o  The most significant 2 bits (i.e., "FMT-Forward" and "FMT-
            Mode") are interpreted in conjunction with one another.
            When FMT-Forward is clear, the LHS Proxy/Server performs OAL
            reassembly and decapsulation to obtain the original IP
            packet before forwarding.  If the FMT-Mode bit is clear, the
            LHS Proxy/Server then forwards the original IP packet at L3;
            otherwise, it invokes the OAL to reassemble, re-fragment and
            re-encapsulate then sends the resulting carrier packets to
            the Client via the selected underlay interface.  When FMT-
            Forward is set, the LHS Proxy/Server forwards unmodified OAL
            fragments to the Client without reassembling.  If FMT-Mode
            is clear, all carrier packets destined to the Client must
            always be sent via the LHS Proxy/Server; otherwise the
            Client is eligible for direct forwarding over the open INET
            where it may be located behind one or more NATs.



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         o  The least significant 6 bits ("FMT-Type") determines the
            type of underlay encapsulation needed to reach the target
            Client interface within its local *NET.  When the most
            significant bit (msb) of FMT-Type is set, the interface has
            been determined to reside behind a Network Address
            Translator (NAT) as discovered during Client exchanges with
            their Proxy/Servers.  The least significant 5 bits of FMT-
            Type encode an underlay encapsulation type value as follows:

            +  0 - underlay encapsulation type is unspecified.  No UNX
               address is included and the msb is ignored.

            +  1 - Client interface is within a MANET where multihop
               forwarding occurs as an adaptation layer service.  No UNX
               address is included and the msb is ignored.

            +  2 - underlay encapsulation type is EUI-48 only.  UNX is 6
               octets in length and encodes an EUI-48 address [EUI].

            +  3 - underlay encapsulation type is EUI-64 only.  UNX is 8
               octets in length and encodes an EUI-64 address [EUI].

            +  4 - underlay encapsulation type is IPv4 only.  UNX is 4
               octets in length and encodes an IPv4 address.

            +  6 - underlay encapsulation type is IPv6 only.  UNX is 16
               octets in length and encodes an IPv6 address.

            +  7 - underlay encapsulation type is UDP/IPv4.  UNX is 6
               octets in length and encodes a 4 octet IPv4 address
               followed by a 2 octet UDP port number.

            +  8 - underlay encapsulation type is UDP/IPv6.  UNX is 18
               octets in length and encodes a 16 octet IPv6 address
               followed by a 2 octet UDP port number.

            +  5, [9 - 31] - Reserved for future use.

      -  LHS-MLA is the 16 octet MLA of the LHS Proxy/Server for the
         specified target Client interface.











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      -  LHS-UNX is L octets in length according to FMT-Type as
         discussed above.  LHS-UNX identifies the LHS Client's *NET
         interface which may connect to an open INET or a private *NET
         behind one or more NATs.  When LHS-UNX includes an IPv4 or IPv6
         address, it appears in network byte order in ones-complement
         "obfuscated" form per [RFC4380].  Trailing padding is added
         following LHS-UNX if necessary to end the sub-option on an even
         8 octet boundary.

      -  The LHS information therefore satisfies per-interface address
         resolution and SRT/FMT/LHS together inform the OMNI interface
         forwarding algorithm.  If the FHS and LHS SRT segments are one
         and the same, the source can address the target Client either
         via its Proxy/Server or through direct underlay encapsulation
         (while engaging NAT traversal in the underlay if necessary)
         according to FMT.  If the target Client is located on a
         different SRT segment, the path from the source must employ a
         combination of route optimization and spanning tree hop
         traversals.

10.2.12.  Traffic Selector

   The Traffic Selector sub-option provides flow binding information for
   the multilink conceptual sending algorithm discussed in Section 12.
   The sub-option includes an augmented traffic selector per [RFC6088]
   as ancillary information for an Interface Attributes sub-option with
   the same ifIndex value, or as discrete information for the included
   ifIndex when no Interface Attributes sub-option is present.

   All packets of the same flow should include compatible traffic
   selector profiles, as the flow (i.e., and not individual packets)
   determines path selection.

   Control messages may include multiple Traffic Selectors for some or
   all of the source/target Client's underlay interfaces (see:
   [I-D.templin-6man-aero3] for further discussion).  Any included
   Traffic Selector sub-options MUST appear in the format shown below:














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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=11  |  Sub-Length   |   TS Format   |   Pad Length  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            ifIndex                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |A|B|C|D|E|F|G|H|I|J|K|L|M|N|            Reserved               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 (A)Start Source Address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 (B)End Source Address                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 (C)Start Destination Address                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 (D)End Destination Address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     (E)Start IPsec SPI                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      (F)End IPsec SPI                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   (G)Start Source port        |   (H)End Source port          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   (I)Start Destination port   |   (J)End Destination port     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  (K)Start DS  |  (L)End DS    |(M)Start Prot. | (N) End Prot. |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 30: Traffic Selector

   *  Sub-Type is set to 11.  Multiple instances with the same or
      different ifIndex values may appear in the same OMNI option.  When
      multiple instances appear, all are processed and the cumulative
      information from all is accepted.  Sub-Length is set to the number
      of 8 octet units in the sub-option.

   *  Sub-Option data begins with 1 octet "TS Format" and "Pad Length"
      fields, where Pad Length indicates the number of trailing padding
      octets.  These fields are followed by a 4 octet ifIndex value
      corresponding to a specific underlay interface.

   *  When TS Format encodes the value 1 or 2, the Traffic Selector body
      encodes an IPv4 or IPv6 traffic selector per [RFC6088] beginning
      with 14 flag bits ("A-N"); when TS Format encodes any other value
      the Traffic Selector block is skipped and processing resumes
      beginning with the next Traffic Selector block (note that future
      specifications may define new TS Formats).




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   *  The Traffic Selector block elements then follow an 18-bit Reserved
      field and encode the information corresponding to any set flag
      bit(s) in order the same as specified in [RFC6088].

10.2.13.  Geo Coordinates

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=12  |  Sub-Length   |   Pad Length  |    Geo Type   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                        Geo Coordinates                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                             Padding                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 31: Geo Coordinates

   *  Sub-Type is set to 12.  If multiple instances with different Geo
      Types appear in the same OMNI option all are processed.

   *  Sub-Length is set to the number of 8 octet units in the sub-
      option, including the Sub-Option Data.

   *  Pad Length is the length in octets of the trailing padding.

   *  Geo Type is a 1 octet field that encodes a type designator that
      determines the format and contents of the Geo Coordinates field
      that follows.  The following types are currently defined:

      -  0 - NULL, i.e., the Geo Coordinates field is zero-length.

   *  Geo Coordinates is a type-specific format field of length up to
      the remaining available space for this OMNI option.  Padding is
      added if necessary according to Geo Type to cause the option to
      end on an 8 octet boundary.

   *  New Geo Coordinate formats to be specified in future documents and
      may include attributes such as latitude/longitude, altitude,
      heading, speed, etc.

10.2.14.  PIM-SM Message

   The Protocol Independent Multicast - Sparse Mode (PIM-SM) Message
   sub-option may be included in the OMNI options of control messages.
   The PIM-SM message sub-option is formatted per Section 4.9 of
   [RFC7761] and as shown in Figure 32:




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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=13  |  Sub-Length   |PIM Ver| Type  |  Pad Length   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                         PIM-SM Message                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 32: PIM-SM Message Option Format

   *  Sub-Type is set to 13.  If multiple instances appear in a single
      OMNI option all are processed.

   *  Sub-Length is set to the number of 8 octet units in the sub-
      option, and Pad Length indicates the length of the trailing
      padding.  The length of the entire PIM-SM message is therefore
      limited by the remaining available space for this OMNI option.

   *  The PIM-SM message is coded exactly as specified in Section 4.9 of
      [RFC7761], except that the Checksum field is omitted since message
      integrity is already assured by the OMNI option checksum.  The
      "PIM Ver" field encodes the value 2, and the "Type" field encodes
      the PIM message type.  (See Section 4.9 of [RFC7761] for a list of
      PIM-SM message types and formats.)

10.2.15.  Fragmentation Report (FRAGREP)

   Fragmentation Report (FRAGREP) sub-options may be included in the
   OMNI options of unsolicited IPv6 ND control messages sent from an OAL
   destination to an OAL source on behalf of a specific flow.  The
   message is formatted and processed the same as specified for the
   Fragmentation Report option in [I-D.templin-6man-ipid-ext2].

   The message consists of the 20-bit Flow Label value for the source's
   flow, followed by the 11 most significant bits of the 16-bit Maximum
   Receive Unit (MRU) for this flow followed by a (L)oss indication.
   The MRU field is then followed by an Identification for the specific
   packet from the OAL source that triggered the flow plus an optional
   Bitmap marking the ordinal positions of individual fragments received
   and missing.









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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=14  |  Sub-Length   |    Fragment Payload Length    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Flow Label               |L|P|       MRU         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                   Identification (64 bits)                    +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                       Bitmap (64 bits)                        +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 33: Fragmentation Report (FRAGREP)

   *  Sub-Type is set to 14.  If multiple instances appear in the same
      OMNI option all are processed.  Sub-Length is set to 3 if a Bitmap
      field is included; otherwise set to 2.

   *  Fragment Payload Length is the payload length of the invoking
      fragment beyond the (Extended) Fragment Header.

   *  Flow Label, L, P and MRU are 4 octets that include the same
      information as for the Fragmentation Report option in
      [I-D.templin-6man-ipid-ext2].

   *  Identification includes the 8 octet Identification value found in
      a received OAL fragment.

   *  Bitmap (optional) includes a 64-bit checklist of up to 64 ordinal
      fragments for this Identification, with each bit set to 1 for a
      fragment received or 0 for a fragment corrupted, lost or still in
      transit.  For example, for a 20-fragment OAL packet with ordinal
      fragments #3, #10, #13 and #17 missing or corrupted and all other
      fragments received or still in transit, Bitmap(i) encodes the
      following:

           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
           |1|1|1|0|1|1|1|1|1|1|0|1|1|0|1|1|1|0|1|1|0|0|0|...
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                                  Figure 34








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10.2.16.  Proxy/Server Control

   OMNI Clients include a Proxy/Server Control sub-option in RS messages
   when they associate with a current FHS and/or MAP Proxy/Server and/or
   need to send a departure indication to an old FHS and/or MAP Proxy/
   Server.  When the FHS Proxy/Server forwards an RS to a different
   Proxy/Server acting as the MAP, the MAP also echoes the Proxy/Server
   Control sub-option in the responsive RA message.

   Proxy/Servers also include the Proxy/Server Control sub-option in
   control messages sent as departure signals to departed FHS/MAP Proxy/
   Servers as specified in [I-D.templin-6man-aero3].

   The Proxy/Server Control sub-option is formatted as shown below:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=15  |   Sub-Length  |M|P|N|A|R|      Reserved       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Reserved                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~             Departed MAP Proxy/Server MLA (16 octets)         ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~             Departed FHS Proxy/Server MLA (16 octets)         ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 35: Proxy/Server Control

   *  Sub-Type is set to 15.  If multiple instances appear the first
      instance is processed and all others are ignored.  Sub-Length is
      set to 5 if departed Proxy/Server MLAs are included; otherwise,
      set to 1.

   *  Sub-Option Data contains 5 functional flags followed by 11
      Reserved flags followed by 4 Reserved octets followed optionally
      by departed Proxy/Server MLAs.

   *  Clients set the (M)ap flag if the message must be forwarded to
      and/or processed by a MAP Proxy/Server.

   *  Clients set the (P)roxy flag to 0, and the FHS Proxy/Server resets
      the P flag to 1 as it forwards an RS message to a MAP Proxy/
      Server.





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   *  Clients set the (N)eighbor Unreachability Detection (NUD),
      (A)ddress Resolution Responder (ARR) and (R)eport (RPT) flags in
      RS messages to control the operation of their FHS/MAP Proxy/
      Servers as discussed in Section 14.

   *  Clients finally include departed Proxy/Server MLAs if the Client
      has departed from a former FHS and/or MAP Proxy/Server.  The
      departed FHS/MAP Proxy/Server address is set to an MLA for a
      departure; otherwise, set to ::/128.

10.2.17.  Prefix Information Option (PIO)

   OMNI Proxy/Servers include Prefix Information Option (PIO) sub-
   options in RA messages used to convey adaptation layer addressing
   information.  The format corresponds to the PIO specified in
   [RFC4861] and [RFC9762] as shown below:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=16  |  Sub-Length   | Prefix Length |L|A|R|P| Rsvd1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Valid Lifetime                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Preferred Lifetime                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Reserved2                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 36: Prefix Information Option (PIO)

   *  Sub-Type is set to 16.  If multiple instances appear in the same
      OMNI option, all are processed.  Sub-Length is set to the number
      of 8 octet units in the sub-option.

   *  Sub-Option Data contains the same information as described in
      Section 4.6.2 of [RFC4861] and as updated by [RFC9762] with the
      exception that the included prefix is variable length as described
      in Section 2.3 of [RFC4191].  The Prefix field is 0, 8, or 16
      octets according to Sub-Length and encodes an IPv6 prefix of
      length indicated by Prefix Length.








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10.2.18.  Route Information Option (RIO)

   OMNI nodes include Route Information Option (RIO) sub-options in NS/
   NA messages used for Address Resolution.  The format is the same as
   specified in [RFC4191] as shown below:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=17  | Sub-Length=N  | Prefix Length |Resvd|Prf|Resvd|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Route Lifetime                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 37: Route Information Option (RIO)

   *  Sub-Type is set to 17.  If multiple instances appear in the same
      OMNI option, all are processed.  Sub-Length is set to the number
      of 8 octet units in the sub-option.

   *  Sub-Option Data contains the same information as described in
      Section 2.3 of [RFC4191], where the Prefix field is 0, 8, or 16
      octets according to Sub-Length.

10.2.19.  DHCPv6 Message

   The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) sub-option
   may be included in the OMNI options of Client RS messages and Proxy/
   Server RA messages.  The DHCPv6 sub-option is formatted per Section 8
   of [RFC8415] as shown in Figure 38:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Sub-Type=18  | Sub-length=N  |  Pad Length   |    Reserved   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    msg-type   |               transaction-id                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               ~
       ~                        DHCPv6 options                         ~
       ~                 (variable number and length)                  ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                            Padding                            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 38: DHCPv6 Message




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   *  Sub-Type is set to 18.  At most 1 instance may appear in a single
      OMNI option.  If multiple instances appear, the first is processed
      and all others are ignored.

   *  Sub-Length encodes the number of 8 octet units in the sub-option,
      and Pad Length encodes the number of trailing padding octets.
      Reserved is a 1 octet field set to 0 on transmission and ignored
      on reception.

   *  'msg-type', 'transaction-id' and 'DHCPv6 options' are coded
      according to [RFC8415].  A Client's DHCPv6 message sub-option in
      an RS message is copied from a network layer DHCPv6 solicitation,
      and a Proxy/Server's DHCPv6 message sub-option in the RA message
      reply is copied from the DHCPv6 server reply.  The Client and
      DHCPv6 server include their MLAs as DUID values as discussed in
      Section 10.2.9.

11.  Address Mapping - Multicast

   The multicast address mapping of the native underlay interface
   applies.  The Client mobile router also serves as an IGMP/MLD Proxy
   for its EUNs and/or hosted applications per [RFC4605].

   The Client uses Multicast Listener Discovery (MLDv2) [RFC9777] to
   coordinate with Proxy/Servers, and underlay network elements use MLD
   snooping [RFC4541].  The Client can also employ multicast routing
   protocols to coordinate with network-based multicast sources as
   specified in [I-D.templin-6man-aero3].

   NS messages used for Duplicate Address Detection (DAD) include the
   unspecified address as the Source, the address being checked as the
   Target, and the solicited-node multicast address of the target as the
   Destination [RFC4862].  Since only MLAs that are already managed for
   uniqueness are assigned to the OMNI interface, the interface discards
   all NS(DAD) messages generated by the network layer.

   Since the OMNI link model is NBMA, OMNI links support other link-
   scoped multicasting through iterative unicast transmissions to
   individual multicast group members (i.e., unicast/multicast
   emulation).

12.  Multilink Conceptual Sending Algorithm

   The Client's network layer selects the outbound OMNI interface
   according to SBM considerations when forwarding original IP packets
   from local or EUN applications to external correspondents.  Each OMNI
   interface maintains a NLNC maintained the same as discussed in
   [RFC4861], but also includes ALNC state for multilink coordination.



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   For each original IP packet it forwards, the OMNI interface selects
   one or more source underlay interfaces based on PBM factors (e.g.,
   traffic selectors, cost, performance, message size, etc.) and one or
   more target underlay interfaces for the neighbor based on Interface
   Attributes received in control messages (see: Section 10.2.10).
   Multilink forwarding may also direct carrier packet replication
   across multiple underlay interface pairs for increased reliability at
   the expense of duplication.  The set of all Interface Attributes and
   Traffic Selectors received in control messages determines the
   multilink forwarding profile for selecting target underlay
   interfaces.

   When the OMNI interface forwards an original IP packet over a
   selected source underlay interface, it first employs OAL
   encapsulation and fragmentation as discussed in Section 5, then
   performs underlay encapsulation as directed by the appropriate AFV.
   The OMNI interface also performs underlay encapsulation (following
   OAL encapsulation) when the nearest Proxy/Server is located multiple
   hops away as discussed in Section 14.2.

   OMNI interface multilink service designers MUST observe the BCP
   guidance in Section 15 [RFC3819] in terms of implications for
   reordering when original IP packets from the same flow may be spread
   across multiple underlay interfaces having diverse properties.

12.1.  Multiple OMNI Interfaces

   Clients may connect to multiple independent OMNI links (and/or
   multiple independent physical links) within the same or different
   OMNI domains to support SBM.  The Client configures a separate
   interface for each distinct link so that multiple interfaces (e.g.,
   omni0, omni1, satcom2, etc.) are exposed to the network layer.  Each
   OMNI interface is configured over a separate set of underlying
   interfaces and configures one or more OMNI link Subnet Router Anycast
   (SRA) addresses; the Client injects the corresponding SRA prefixes
   into the EUN routing system.  Multiple distinct OMNI links can
   therefore be used to support fault tolerance, load balancing,
   reliability, hyperconnectivity, etc.













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   Applications in EUNs can use Segment Routing to select the desired
   OMNI interface based on SBM considerations.  The application writes
   an OMNI link SRA address into the original IP packet's Destination
   Address, and writes the actual destination (along with any additional
   intermediate hops) into the Segment Routing Header.  Standard IP
   routing directs the packet to the Client's mobile router entity,
   where the OMNI link SRA address identifies the correct OMNI interface
   for next hop forwarding.  When the Client receives the packet, it
   replaces the IP Destination Address with the next Address found in
   the Segment Routing Header and forwards the message via the OMNI
   interface identified by the SRA address.

   Note: The Client need not configure its OMNI interface indexes in
   one-to-one correspondence with the global OMNI Link-IDs configured
   for OMNI domain administration since the Client's indexes (i.e.,
   omni0, omni1, omni2, etc.) are used only for its own local interface
   management.

12.2.  Client-Proxy/Server Loop Prevention

   After a Proxy/Server has registered an MNP for a Client (see:
   Section 14), the Proxy/Server will forward all original IP packets
   (or carrier packets) destined to an address within the MNP to the
   Client.  Under normal circumstances, the Client will then forward the
   resulting original IP packet to the correct destination within its
   connected (downstream) EUNs.

   If at some later time the Client loses state (e.g., after a reboot),
   it may begin returning original IP packets (or carrier packets) with
   destinations corresponding to its MNP to the Proxy/Server as its
   default router.  The Proxy/Server therefore drops any original IP
   packets received from the Client with a Destination Address that
   corresponds to the Client's MNP, and drops any carrier packets with
   both Source and Destination Address corresponding to the same
   Client's MNP regardless of their origin.

   Proxy/Servers support "hairpinning" for packets with MLA Source and
   Destination Addresses that would convey useful data from a source
   Client to a target Client both located in the same OMNI link segment.
   Proxy/Servers support this hairpinning according to [RFC6296],
   however direct forwarding between peer nodes within the same OMNI
   link segment is preferred whenever possible.









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13.  OAL Segment Routing

   As discussed in Section 12.1, Segment Routing per [RFC8754] can be
   used by the network layer to select an OMNI link when there may be
   multiple alternatives.  Once an OMNI link is selected, the OAL also
   applies Segment Routing internally to navigate multiple adaptation
   layer hops as discussed below.

   The Segment Routing Header (SRH) is included as an extension to the
   OAL IPv6 encapsulation header and includes a Segment List with the
   MLAs of intermediate OAL hops.  The SRH also includes a new TLV
   termed the AERO Flow Vector Index (AFVI) with the following format
   per Section 2.1 of [RFC8754]:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |I|          Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 AERO Flow Vector Index (AFVI)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 39: AERO Flow Vector Index (AFVI) TLV

   In this format:

   *  Type is set to TBD4 (see: IANA considerations).

   *  Length is set to 6.

   *  I is an "Initialize" flag.  When I is set to 1, the message is
      used to establish new AERO Flow Vector (AFV) state.  When I is set
      to 0, the message follows existing AFV state.  The I flag is
      followed by a 15 bit Reserved field which must be set to 0 (future
      specifications may define new values).

   *  AERO Flow Vector Index (AFVI) is a 32-bit field set by the OAL
      source for a specific flow and rewritten by each transit or
      endpoint OAL intermediate system on the path ending at the OAL
      destination.  The special value 0 denotes "AFVI unspecified".

   The FHS Client includes an OAL SRH to either discover and initialize
   the OAL intermediate systems on the forward path to the LHS Client or
   to follow a forward path previously established.  When OAL header
   compression is applied the SRH is omitted and only the AFVI is
   transmitted.

   For forward path discovery and initialization, the FHS Client
   includes its own MLA as Segment List[0], sets Segments Left and Last
   Entry to 0, sets I to 1 and sets AFVI to a locally-unique value that



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   can ideally be represented in 2 octets (i.e., a value less than
   2**16).  If no locally-unique values remain within the 2 octet limit,
   the OAL source instead selects a unique value that can be represented
   in 4 octets.

   The FHS Client then includes the AFVI TLV in an SRH extension to the
   OAL header, calculates and includes an HMAC TLV if necessary and
   forwards the OAL IPv6 ND solicitation toward the FHS Proxy/Server.
   (The FHS Proxy/Server then either processes the message locally or
   forwards it on to the addressed LHS Client or MAP Proxy/Server.)

   When an OAL intermediate system forwards the solicitation, it first
   verifies the HMAC then caches the AFVI and UNX address from the
   previous hop as AFV state for future forwarding purposes.  If
   Segments Left is 0, the intermediate system then includes its own MLA
   as the new Segment List[0] and increments Last Entry.  All
   intermediate systems rewrite the AFVI with their own unique value
   (i.e., the same as the OAL source had done) and recalculate the HMAC
   signature if necessary.  The intermediate system then forwards the
   solicitation toward the FHS Proxy/Server and/or LHS Client.

   When the I flag is set, each OAL intermediate system creates new AFV
   state if none previously existed or if it has stale AFV state older
   than (REACHABLE_TIME / 2) seconds.  The OAL intermediate system
   refreshes the AFV state timer when it forwards a fresh secured
   control message for the flow.  The OAL intermediate system retains
   any stale state up to REACHABLE_TIME seconds to accommodate any
   packets with stale AFVIs that may still be flowing.

   Note that only FHS and LHS intermediate systems act as endpoints by
   adding their MLAs to the Segment List since all interdomain
   intermediate systems are globally addressable and can act as
   transits.  For packets destined to targets in other OMNI link
   segments, the source adds the MLA of the LHS Proxy/Server as the
   penultimate and the MLA of the target as the ultimate Segment List
   entries.

   The FHS Proxy/Server must also remove all FHS intermediate system
   MLAs from the Segment List when it forwards a message toward the LHS
   Proxy/Server and the LHS Proxy/Server must remove all LHS
   intermediate system MLAs when it forwards a message toward the FHS
   Proxy/server.  The LHS Proxy/Server must then append its cached list
   of LHS intermediate system MLAs when it forwards a message toward an
   LHS Client and the FHS Proxy/Server must append its cached list of
   FHS intermediate system MLAs when it forwards a message toward an FHS
   Client.  (The FHS/LHS Proxy/Server then sets Segments Left and Last
   Entry to non-zero values specific to their appended lists.)




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   Therefore, the FHS/LHS Proxy/Servers must cache the Segment Lists for
   their local domain Clients so that the only Segment List entries
   exposed in the interdomain area are those of the FHS/LHS/MAP Proxy/
   Servers themselves.  Clients in turn must cache the Segment List
   including the FHS/LHS intermediate system addresses for future
   message transmissions to peers.

   When the LHS Client receives a solicitation message, it returns a
   response that includes an OAL SRH with Segment List set to the
   reverse path list of n intermediate systems discovered in the
   solicitation and with Segments Left and Last Entry set to n.  If the
   response will also establish AFV state in the reverse path, the LHS
   Client sets I to 1 and sets AFVI to a new locally-unique value for
   the previous OAL intermediate system; otherwise, the LHS Client sets
   both I and AFVI to 0.  The LHS Client includes an HMAC signature if
   necessary, then forwards the response to the previous hop OAL
   intermediate system on the path to the FHS Client.

   When an OAL intermediate node forwards the response, it first
   verifies the HMAC then (if I is 1) caches the AFVI and UNX address
   from the previous hop as AFV state for future forwarding purposes.
   FHS and LHS intermediate nodes then set the OAL destination to the
   next Segment List entry and decrement Segments Left.  Intermediate
   nodes then rewrite AFVI with their own unique value and include a new
   HMAC signature if necessary.  The node then forwards the
   advertisement to the next hop found in the Segment List.

   The OAL peers (i.e., the FHS and LHS Clients) can then begin sending
   OAL packets with OCH headers that include the AFVI which each
   forwarding OAL intermediate system in the path can use to determine
   the next OAL hop.  The network layer header, full OAL header and SRH
   therefore need not appear in these header-compressed OAL packets
   since the hop-by-hop AFVI values provide sufficient forwarding
   information.  If either OAL peer needs to send a packet with full
   headers, it can include an OAL SRH with AFVI TLV with the I flag set
   to 0.  OAL intermediate nodes will forward the packet based on
   matching cached AFVI values instead of initializing new AFVI values.

14.  Router Discovery and Prefix Delegation

   Clients that configure an OMNI interface also honor the specification
   for using RA messages to signal the availability of DHCPv6 Prefix
   Delegation (PD) [RFC9762].  When a Client initializes an OMNI
   interface, the adaptation layer within the interface delivers a
   locally-generated RA message to the network layer per [RFC4861].  The
   RA message includes the OMNI interface internal LLA as the Source
   Address, the Client's MLA as the Destination Address, an SLLAO set to
   the OMNI interface's internal link-layer address and standard RA



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   message body contents with the M, O flags both set to 1.  The RA
   message also includes Prefix Information Option(s) with Prefix set to
   ::/0 or a more-specific prefix, with [A=0; P=1] and with L=0 to
   engage the "off-link" model or L=1 to engage the "on-link" model as
   discussed in Section 1.  After the initial RA message delivery, the
   adaptation layer continuously delivers additional RAs before the
   Router Lifetime or any prefix lifetimes expire.

   The receipt of these RA messages causes the network layer to regard
   prefix ranges as on/off-link over the OMNI interface and to install
   the OMNI interface internal LLA in the Default Router List.  This
   will cause all packets to a new destination to invoke address
   resolution over the OMNI interface while regarding the local
   adaptation layer as a virtual default router.  The receipt of the RA
   message will also cause the network layer to issue a DHCPv6 Solicit
   message for Prefix Delegation over the OMNI interface.  The OMNI
   interface therefore maps the link-scoped multicast address
   "All_DHCP_Relay_Agents_and_Servers - (ff02::1:2)" [RFC8415] to the
   link-layer address of its internal virtual router function to support
   prefix delegation services.

   Following the initial locally-generated RA and receipt of a DHCPv6
   Solicit message for Prefix Delegation per [RFC9762] (or after a brief
   delay), Clients engage the OMNI link at the adaptation layer by
   sending OAL encapsulated RS messages with OMNI options to cause
   Proxy/Servers to process the message and respond.  Each RS message is
   received by an FHS Proxy/Server, which may in turn forward a proxyed
   copy to a MAP Proxy/Server located in a local or remote SRT segment
   according to an included Proxy/Server Control OMNI sub-option.  The
   MAP Proxy/Server then returns an OAL encapsulated RA message either
   directly to the Client or via the original FHS Proxy/Server acting as
   a proxy.

   Clients send RS messages under the conditions specified in
   Section 6.3.7 of [RFC4861] which includes not only interface/link
   initialization conditions but also mobility factors.  In particular,
   Clients may send RS messages when the OMNI interface or an underlay
   interface changes state, or when the Client moves to a new link and
   needs to discover addressing parameters for its new topological
   orientation.  The Client's RS/RA exchanges therefore maintain the
   most current OMNI link state information even across unanticipated
   mobility events.

   To initiate Router Discovery and Address/Prefix Delegation, the
   Client's adaptation layer uses the OMNI interface MLA as the IPv6
   Source Address for RS messages it sends to candidate FHS Proxy/
   Servers.  The adaptation layer also includes its MLA as the OAL
   encapsulation source while also including OMNI authentication,



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   Interface Attributes and any other sub-options.  (In particular, the
   adaptation layer includes a DHCPv6 message sub-option that
   encapsulates the Client's soliciting DHCPv6 Prefix Delegation
   request.)  When the FHS Proxy/Server receives the RS, it can
   optionally consult an MLA registration service to determine whether
   the Client is authorized to use its claimed MLA and discards the RS
   message if authorization fails.  Otherwise, the FHS Proxy/Server
   provides Router Discovery services for the Client per the remainder
   of this section.

   To support Client to service coordination, the OMNI option provides
   flag bits in OMNI Proxy/Server Control sub-options as discussed in
   Section 10.2.16.  Clients set or clear Proxy/Server control flags in
   RS messages as directives to the Mobility Service FHS/MAP Proxy/
   Servers.  Proxy/Servers interpret the flags as follows:

   *  When an FHS Proxy/Server forwards or processes an RS with the NUD
      flag set, it responds directly to future NS Neighbor
      Unreachability Detection (NUD) messages with the Client as the
      target by returning NA(NUD) replies; otherwise, it forwards
      NS(NUD) messages to the Client.

   *  When the MAP Proxy/Server receives an RS with the ARR flag set, it
      responds directly to future NS Address Resolution (NS(AR))
      messages with the Client as the target by returning NA(AR)
      replies; otherwise, it forwards NS(AR) messages to the Client.

   *  When the MAP Proxy/Server receives an RS with the RPT flag set, it
      maintains a Report List of recent NS(AR) message sources for the
      source or target Client and sends unsolicited IPv6 ND control
      messages to all list members if any aspects of the Client's
      underlay interfaces change.

   Mobility Service Proxy/Servers function according to the NUD, ARR and
   RPT flag settings received in the most recent RS message to support
   dynamic Client updates.

   Clients and FHS Proxy/Servers include an authentication signature as
   an OMNI sub-option in their RS/RA exchanges when necessary but always
   include a valid OAL Checksum.  Clients and Proxy/Servers use the
   information included in RS/RA messages to establish NCE state and
   OMNI link autoconfiguration information as discussed in this section.

   For each underlay interface, the Client sends RS messages with OMNI
   options to coordinate with potentially different FHS Proxy/Servers
   for each interface but normally only with one MAP Proxy/Server at a
   given time.  All Proxy/Servers accept original IP packets addressed
   to their MLAs or link-scoped multicast, OAL packets addressed to



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   their anycast/unicast MLA and carrier packets addressed to their
   anycast/unicast UNX addresses.  The Client typically selects one MAP
   Proxy/Server among any of its FHS Proxy/Servers, but may instead
   select any other Proxy/Server on the OMNI link to serve as the MAP.

   Example UNX address discovery methods appear in [RFC5214] and include
   data link login parameters, name service lookups, static
   configuration, a DHCP option, a static "hosts" file, etc.  In the
   absence of other information, the Client can resolve the DNS Fully-
   Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where
   "linkupnetworks" is a constant text string and "[domainname]" is a
   DNS suffix for the OMNI link (e.g., "example.com").  The name
   resolution will return a set of DNS resource records to populate a
   Potential Router List (PRL) that contains addresses of Proxy/Servers
   for the local OMNI link segment.  When the underlay *NET does not
   support standard unicast server-based name resolution [RFC1035] the
   Client can engage a multicast service such as mDNS [RFC6762] within
   the local OMNI link segment.

   Each FHS Proxy/Server configures an MLA then advertises its UNX
   address(es) for discovery as above.  Each FHS Proxy/Server may
   service one or more of a Client's underlay interfaces which each
   become associated with the Proxy/Server's MLA.  The ifIndex included
   in the Neighbor Synchronization sub-option is used to determine which
   Client underlay interface is selected.

   Clients configure OMNI interfaces that observe the properties
   discussed in previous sections.  The OMNI interface and its underlay
   interfaces are said to be in either the "UP" or "DOWN" state
   according to administrative actions in conjunction with the interface
   connectivity status.  An OMNI interface transitions to UP/DOWN
   through administrative action and/or through underlay interface state
   transitions.  When a first underlay interface transitions to UP, the
   OMNI interface also transitions to UP.  When all underlay interfaces
   transition to DOWN, the OMNI interface also transitions to DOWN.
















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   When a Client OMNI interface transitions to UP, the interface returns
   a locally-generated RA to the network layer, and the network layer
   will issue a DHCPv6 Solicit message for Prefix Delegation.  The OMNI
   interface then appends a single OMNI option trailer for each RS
   message while sending an interface-specific copy of the message over
   each underlay interface.  These OMNI RS messages will register an
   initial set of underlay interfaces that are also UP and optionally
   also request an MNP delegation.  The Client sends additional RS
   messages to refresh lifetimes and to register/deregister underlay
   interfaces as they transition to UP or DOWN.  Guidelines for sending
   additional RS messages to generate corresponding RAs are found in
   Section 8.3.4 of [RFC5214], and are further extended to include
   proactive responses to mobility events.

   The Client's OMNI interface sends initial RS messages over an UP
   underlay interface with source set to its MLA [RFC4861].  The Client
   further sets the RS Destination Address to either the MLA of a
   specific (MAP) Proxy/Server or the link-scoped All-Routers multicast
   address ff02::2 [RFC4291].  The Client's OMNI interface then appends
   an OMNI option per Section 10 with Proxy/Server Control sub-options
   with the MAP, NUD, ARR and RPT flags set or cleared as necessary.
   When the Client needs to coordinate with a MAP Proxy/Server other
   than the FHS Proxy/Server for a specific underlay interface, it sets
   the RS Destination Address to the MLA of the MAP.

   If the Client will synchronize with this FHS Proxy/Server it also
   includes a Neighbor Synchronization sub-option with FHS ifIndex set
   to the ifIndex of its own underlay interface and with LHS ifIndex set
   to 0 (i.e., the default ifIndex configured by all Proxy/Servers).
   The Client also sets Sequence Number to a randomly-chosen 8 octet
   value, sets AFVI to a randomly-chosen initial value and sets the Flow
   Label in the IPv6 header to 0.  The resulting exchange will establish
   symmetric Identification windows for the Client and FHS Proxy/Server.
   (Note that the Client may omit the Neighbor Synchronization sub-
   option if it only wants to test reachability for certain Proxy/
   Servers without establishing state.)

   The Client next includes an Interface Attributes sub-option for the
   underlay interface with LHS-MLA set to ::/128.  The Client then
   includes any other necessary OMNI sub-options such as Traffic
   Selectors, authentication, Timestamp, Nonce, etc.  The OMNI interface
   finally sets or clears the Interface Attributes FMT-Forward and FMT-
   Mode bits according to its desired FHS Proxy/Server service model for
   this interface as described in Section 10.2.10.

   The Client next prepares to forward the RS over the underlay
   interface using OAL encapsulation while including authentication sub-
   options if necessary followed by the OMNI option trailing fields.



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   The Client also includes a Segment Routing Header (SRH) extension to
   the OAL header with its own MLA as Segment List[0], as discussed in
   Section 13.  The Client then sets the OAL Source Address to its own
   MLA and sets the OAL Destination Address to the known MLA of the FHS
   Proxy/Server, site-scoped All-Routers multicast (ff05::2) [RFC4291]
   or an anycast address.  When the Client requires underlay
   encapsulation, it next includes the discovered FHS Proxy/Server UNX
   address or an anycast address as the underlay destination then
   forwards the resulting carrier packet into the underlay network.  The
   Client also caches its RS message transmissions in the OAL to match
   against any received RA messages.

   When an FHS Proxy/Server receives a carrier packet containing an RS
   it sets aside the underlay and OAL headers then verifies the checksum
   and authentication signature while also consulting an MLA
   registration service based on the Client's claimed certificate.  If
   the RS message authenticity/integrity is verified, the FHS Proxy/
   Server then creates/updates an ALNCE indexed by the Client's MLA
   found in the RS Source Address.  The FHS Proxy/Server then caches the
   OMNI Interface Attributes and any Traffic Selector sub-options while
   also caching the AFVI plus underlay (UDP/IP) and OAL Source/
   Destination Address information.

   The FHS Proxy/Server next caches the RS Proxy/Server Control NUD flag
   and Neighbor Synchronization parameters if present (see:
   Section 10.1).  If the RS includes a Proxy/Server Control sub-option
   with the M flag set and the RS is addressed either to itself or
   Multicast/Anycast it assumes the MAP role.  If the RS includes an
   OMNI DHCPv6 Solicit sub-option, the FHS/MAP Proxy/Server uses the
   encapsulated message to coordinate an MNP Prefix Delegation exchange
   with the local DHCPv6 server.  The FHS/MAP Proxy/Server then assumes
   the MAP role as a mobility service entry point for injecting the
   Client's MLA and MNP(s) into the OMNI link routing system.  The FHS/
   MAP Proxy/Server then caches all of the delegated prefixes as L3
   addressing information for this Client MLA, and caches the RS NUD,
   ARR and RPT flags to determine its role in processing NS(AR) messages
   and generating unsolicited IPv6 ND control messages (see:
   Section 10.1).













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   The FHS/MAP Proxy/Server then prepares to return an RA message
   directly to the Client by first populating the Cur Hop Limit, Flags,
   Router Lifetime, Reachable Time and Retrans Timer fields with values
   appropriate for the OMNI link.  The RA message also includes a PIO
   that encodes the MSP and with (A=0; L=0; P=1) to cause the Client to
   regard all addresses covered by the MSP as reachable via the OMNI
   interface (while making no statement about on/off-link properties)
   and with per-Client Prefix Delegation enabled.  The FHS/MAP Proxy/
   Server then sets the RA Source Address to its own MLA and sets the RA
   Destination Address to the RS Source Address.

   The FHS/MAP Proxy/Server next includes an SRH with the RS Segments
   written into the Segment List as specified in Section 13.  The FHS/
   MAP Proxy/Server then includes an OMNI option with a Neighbor
   Synchronization sub-option with responsive window synchronization
   information.  The FHS/MAP Proxy/Server also includes a DHCPv6 Reply
   sub-option, then includes a copy of the Client's original Interface
   Attributes sub-option with its INET-facing interface information
   written in the SRT/FMT/LHS fields.  The Proxy/Server sets the LHS-MLA
   field to its own MLA and sets LHS-UNX to the address it observes in
   the RS message underlay source address.  If the Proxy/Server observes
   a different UNX address than the one supplied by the Client, it sets
   the NAT indication in FMT-Type.

   The FHS/MAP Proxy/Server next sets or clears the FMT-Forward and FMT-
   Mode flags if necessary to convey its capabilities to the Client,
   noting that it should honor the Client's stated preferences for those
   parameters if possible or override otherwise.  The FMT-Forward/Mode
   flags thereafter remain fixed unless and until a new RS/RA exchange
   establishes different values (see: Section 10.2.10 for further
   discussion).  If the FHS/MAP Proxy/Server's Client-facing interface
   is different than its INET-facing interface, the Proxy/Server next
   includes a second Interface Attributes sub-option with ifIndex set to
   '0', with a unicast underlay address for its Client-facing interface
   in the LHS-UNX field and with its own MLA in the LHS-MLA.

   The FHS/MAP Proxy/Server then includes any other necessary OMNI sub-
   options such as an authentication sub-option, Nonce, Timestamp, etc.
   The FHS/MAP Proxy/Server then calculates the authentication
   signature/checksum and performs OAL encapsulation while setting the
   OAL Source Address to its own MLA and Destination Address to the OAL
   Source Address that appeared in the RS.  The FHS/MAP Proxy/Server
   then performs underlay encapsulation with Source and Destination
   address information reversed from the RS underlay information and
   returns the resulting carrier packet to the Client over the same
   underlay interface the RS arrived on.





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   When an FHS Proxy/Server receives an RS addressed to the MLA of a
   different MAP Proxy/Server, it acts as a proxy for the target MAP.
   The FHS Proxy/Server first processes the RS message locally the same
   as described above except that it does not process the DHCPv6 sub-
   option.  The FHS Proxy/Server then sets the P flag in the Proxy/
   Server Control option, sets the OAL Source Address to its own MLA and
   sets the OAL Destination to the MLA of the MAP provided by the
   Client.  The FHS Proxy/Server next creates or updates an ALNCE for
   the Client (i.e., based on the Client's MLA) and caches the OAL
   Source Address, Neighbor Synchronization and Interface Attributes
   information as above.

   The FHS Proxy/Server then caches the RS SRH Segment List to include
   in the return RA message and writes the RS UNX address and its own
   MLA into the corresponding Interface Attributes fields.  The FHS
   Proxy/Server next calculates and includes the OAL checksum then
   performs underlay encapsulation and sends the resulting carrier
   packet into the SRT secured spanning tree.

   When the MAP Proxy/Server receives the carrier packet, it performs
   underlay decapsulation and OAL decapsulation to obtain the proxyed
   RS, verifies the checksum, then performs prefix delegation based on
   the Client's supplied DHCPv6 sub-option to obtain or update any MNPs
   for the Client.  The MAP Proxy/Server then creates/updates an ALNCE
   indexed by the Client's MLA found in the RS Source Address and caches
   any state (including delegated MNPs, the ARR and RPT flags, Interface
   Attributes information and Traffic Selectors).  The MAP Proxy/Server
   finally caches any delegated MNPs as L3 information for this Client
   and also injects them into the OMNI link routing protocol.

   The MAP Proxy/Server then returns an OMNI encapsulated RA that echoes
   the Client's (proxyed) Interface Attributes and Proxy/Server Control
   sub-options and with any other RA parameters the same as specified
   for the FHS/MAP Proxy/Server case above while also including a DHCPv6
   Reply sub-option with the prefix delegation transaction results.  The
   MAP Proxy/Server sets the RA Source Address to its own MLA and sets
   the Destination Address to the RS Source Address (i.e., the MLA of
   the Client).  The OMNI interface of the MAP Proxy/Server then sets
   the OAL Source Address to its own MLA and Destination Address to the
   MLA of the FHS Proxy/Server.  The MAP Proxy/Server then calculates
   the OAL checksum and encapsulates the RA as an OAL packet.  The MAP
   Proxy/Server finally performs underlay encapsulation and sends the
   resulting carrier packet into the secured spanning tree.

   When the FHS Proxy/Server receives the carrier packet it performs
   underlay decapsulation, verifies the checksum then updates the OMNI
   interface ALNCE for the Client and creates/updates an ALNCE for the
   MAP.  The FHS Proxy/Server then sets the P flag in the RA flags field



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   [RFC4389] and proxys the RA by changing the OAL source to its MLA and
   changing the OAL Destination Address to the MLA indicated by the
   cached FHS SRH node sequence discovered in the RS message.

   The FHS Proxy/Server next includes a Neighbor Synchronization sub-
   option with responses to its cached solicitations from the Client.
   The FHS Proxy/Server also includes an Interface Attributes sub-option
   with ifIndex '0' and with its Client-facing interface unicast
   underlay address if necessary (see above) and includes authentication
   sub-options if necessary.  The FHS Proxy/Server next includes a
   unique AFVI for this Client then calculates the OAL authentication
   signature and checksum.  The FHS Proxy/Server then includes an SRH
   extension to the OAL header with an AFVI, with the MLAs of endpoint
   intermediate systems on the reverse path to the Client and with the
   OAL Destination Address adjusted accordingly.  The FHS Proxy/Server
   finally performs underlay encapsulation with addresses taken from the
   Client's ALNCE and sends the resulting carrier packet via the same
   underlay interface over which the RS was received.

   When the Client receives the carrier packet, it performs underlay
   decapsulation followed by OAL decapsulation to obtain the RA message.
   The Client next verifies the OAL checksum and authentication
   signature, then matches the RA with its previously-sent RS by
   comparing the RS Sequence Number with the RA Acknowledgement Number
   and also comparing the Nonce and/or Timestamp values.  If the values
   match, the Client then creates/updates OMNI interface ALNCEs for both
   the MAP and FHS Proxy/Server and caches the information in the RA
   message.  The Client next discovers the MLA associated with its
   interface over this FHS Proxy/Server by examining the proxyed
   Interface Attributes sub-option.

   If the Client has multiple underlay interfaces, it creates additional
   FHS Proxy/Server ALNCEs as necessary when it receives RAs over those
   interfaces (noting that multiple of the Client's underlay interfaces
   may be serviced by the same or different FHS Proxy/Servers).  If the
   RA message includes OMNI DHCPv6 Reply sub-option with the results of
   prefix delegation transactions, the Client returns the DHCPv6 Reply
   response to the network layer.  This will cause the network layer to
   provision the delegated prefixes on downstream-facing links per
   [RFC9762].

   For each underlay interface, the Client next caches the (filled-out)
   Interface Attributes for its own ifIndex including the SRT/FMT/LHS
   and Segment List information that it received in an RA message over
   that interface so that it can include them in future control messages
   to provide neighbors with accurate address resolution parameters.
   (If the message includes an Interface Attributes sub-option with
   ifIndex '0', the Client also caches UNX as the underlay network-local



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   unicast address of the FHS Proxy/Server via that underlay interface.)
   The Client then consults FMT-Type and the UNX address to determine
   whether there may be NATs on the path to the FHS Proxy/Server.  The
   Client finally caches the Neighbor Synchronization responsive window
   synchronization parameters for use in future control message
   exchanges via this FHS Proxy/Server.

   Following the initial exchange, the FHS Proxy/Server MAY later send
   additional periodic and/or event-driven unsolicited RA messages per
   [RFC4861].  (The unsolicited RAs may be initiated either by the FHS
   Proxy/Server itself or by the MAP via the FHS as a proxy.)  The
   Client then continuously manages its underlay interfaces according to
   their states as follows:

   *  When an underlay interface transitions to UP, the Client sends an
      RS over the underlay interface with an OMNI option with sub-
      options as specified above.

   *  When an underlay interface transitions to DOWN, the Client sends
      unsolicited IPv6 ND control messages over any UP underlay
      interface with an OMNI option containing Interface Attributes sub-
      options for the DOWN underlay interface with ifMetric set to
      'ffffffff'.  The Client sends isolated unsolicited IPv6 ND control
      messages when reliability is not thought to be a concern (e.g., if
      redundant transmissions are sent on multiple underlay interfaces),
      or may instead send an IPv6 ND solicitation message to receive a
      solicited reply.

   *  When the Router Lifetime for the MAP Proxy/Server nears
      expiration, the Client sends an RS over any underlay interface to
      receive a fresh RA from the MAP.  If no RA messages are received
      over a first underlay interface (i.e., after retrying), the Client
      marks the underlay interface as DOWN and should attempt to contact
      the MAP Proxy/Server via a different underlay interface.  If the
      MAP Proxy/Server is unresponsive over additional underlay
      interfaces, the Client sends an RS message with Destination
      Address set to the MLA of another Proxy/Server which will then
      assume the MAP role.

   *  When all of a Client's underlay interfaces have transitioned to
      DOWN (or if a prefix delegation lifetime expires), the MAP Proxy/
      Server withdraws the Client's MLA and MNPs the same as if it had
      received a message with a release indication.

   The Client is responsible for retrying each RS exchange up to
   MAX_RTR_SOLICITATIONS times separated by RTR_SOLICITATION_INTERVAL
   seconds until an RA is received.  If no RA is received over an UP
   underlay interface (i.e., even after attempting to contact alternate



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   Proxy/Servers), the Client can either declare this underlay interface
   as DOWN or continue to use the interface to support any peer-to-peer
   local communications with peers located in the same *NET.  When
   changing to a new FHS/MAP Proxy/Server, the Client also includes a
   Proxy/Server Control OMNI sub-option in new RS messages; the (new)
   FHS Proxy/Server will in turn send unsolicited IPv6 ND control
   messages to the departed FHS and/or MAP Proxy/Server to announce the
   Client's departure as specified in [I-D.templin-6man-aero3].

   Note: IPv6 ND messaging is internally-generated by the adaptation
   layer or in response to a network layer event such as receipt of a
   DHCPv6 Solicit message for Prefix Delegation.  Most RS/RA messaging
   occurs at the adaptation layer only, with only locally-generated RA
   and DHCPv6 Reply messages returned to the network layer.  The
   adaptation layer therefore behaves as a singular IPv6 router from the
   perspective of the adaptation layer.

   Note: The Router Lifetime value in RA messages indicates the time
   before which the Client must send another RS message over this
   underlay interface (e.g., 600 seconds), however that timescale may be
   significantly longer than the lifetime the MS has committed to retain
   the prefix registration (e.g., REACHABLE_TIME seconds).  Proxy/
   Servers are therefore responsible for updating MS state on a shorter
   timescale than the Client may be required to do on its own behalf.

   Note: On certain multicast-capable underlay interfaces, Clients
   should send periodic unsolicited multicast NA messages and Proxy/
   Servers should send periodic unsolicited multicast RA messages as
   "beacons" that can be heard by other nodes on the link.  If a node
   fails to receive a beacon after a timeout value specific to the link,
   it can initiate Neighbor Unreachability Detection (NUD) exchanges to
   test reachability.

   Note: Although the Client's FHS Proxy/Server is a first-hop segment
   node from its own perspective, the Client stores the Proxy/Server's
   FMT, SRT, and addresses as last-hop segment (LHS) information to
   supply to neighbors.  This allows both the Client and MAP Proxy/
   Server to supply the information to neighbors that will perceive it
   as LHS information on the return path to the Client.

   Note: The MAP Proxy/Server injects Client MLA and MNPs into the OMNI
   link routing system by simply creating a route-to-interface
   forwarding table entry for MLA::/128 and MNP::/N via the OMNI
   interface.  The dynamic routing protocol will notice the new entries
   and propagate the routes to its peers.  If the MAP receives
   additional RS messages, it need not re-create the forwarding table
   entries (nor disturb the dynamic routing protocol) if the entries are
   already present.  If the MAP ceases to receive RS messages from any



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   of the Client's interfaces, it removes the Client MLA and MNP(s) from
   the forwarding table (i.e., after a short delay) which also results
   in their removal from the routing system.

   Note: If the Client's initial RS message includes an anycast underlay
   Destination Address, the FHS Proxy/Server returns the solicited RA
   using the same anycast address as the underlay source while including
   an Interface Attributes sub-option with ifIndex '0' and its true
   unicast address in the UNX.  When the Client sends additional RS
   messages, it includes this FHS Proxy/Server unicast address as the
   underlay Destination Address and the FHS Proxy/Server returns the
   solicited RA using the same unicast address as the underlay Source
   Address.  This will ensure that RS/RA exchanges are not impeded by
   any NATs on the path while avoiding long-term exposure of messages
   that use an anycast address as the source.

   Note: Clients should set the NUD, ARR and RPT flags consistently in
   successive RS messages and only change those settings when an FHS/MAP
   Proxy/Server service profile update is necessary.

14.1.  Client-Proxy/Server Window Synchronization

   The RS/RA exchanges discussed above observe the principles specified
   in Section 6.7.  Window synchronization is conducted between the
   Client and each FHS Proxy/Server used to contact the MAP Proxy/
   Server, i.e., and not between the Client and the MAP.  This is due to
   the fact that the MAP Proxy/Server is responsible only for forwarding
   messages via the secured spanning tree to FHS Proxy/Servers, and is
   not responsible for forwarding messages directly to the Client over
   unsecured networks.

   When a Client sends an RS to perform window synchronization via a new
   FHS Proxy/Server, it includes an OMNI Neighbor Synchronization sub-
   option with window synchronization parameters with FHS ifIndex set to
   its own interface index, with LHS ifIndex set to 0, with the SYN flag
   set and ACK flag clear, and with an initial Sequence Number.  The
   Client also includes an Interface Attributes sub-option then performs
   OAL encapsulation and underlay encapsulation and sends the resulting
   carrier packet to the FHS Proxy/Server.  When the FHS Proxy/Server
   receives the carrier packet, it performs underlay decapsulation, then
   extracts the RS message and caches the Neighbor Synchronization
   parameters.  In the process, the FHS Proxy/Server removes the
   Neighbor Synchronization sub-option itself, since the path to the MAP
   Proxy/Server is not included in window synchronization.

   The FHS Proxy/Server then performs underlay encapsulation and sends
   the resulting carrier packet via the secured spanning tree to the MAP
   Proxy/Server, which updates the Client's Interface Attributes and



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   returns a unicast RA message.  The MAP Proxy/Server performs OAL
   encapsulation followed by underlay encapsulation and sends the
   resulting carrier packet via the secured spanning tree to the FHS
   Proxy/Server.  The FHS Proxy/Server then proxys the message as
   discussed in the previous section and includes a Neighbor
   Synchronization sub-option with responsive window synchronization
   information.  The FHS Proxy/Server then forwards the message to the
   Client via OAL encapsulation which updates its window synchronization
   information for the FHS Proxy/Server as necessary.

   Following the initial RS/RA-driven window synchronization, the Client
   can re-assert new windows with specific FHS Proxy/Servers by
   performing RS/RA exchanges between its own MLA and the MLAs of the
   FHS Proxy/Servers at any time without having to disturb the MAP.
   When the Client also needs to refresh MAP state, it can set the RS
   Destination Address to the MAP MLA and include an SRH to support FHS
   Proxy/Server RS forwarding.

   This window synchronization is necessary only for MANET and INET
   Clients that must include authentication signatures with their IPv6
   ND messages; Clients in secured ANETs can omit window
   synchronization.  When Client-to-Proxy/Server window synchronization
   is used, subsequent control messages exchanged between peers include
   IPv6 Extended Fragment Headers in the OAL encapsulations with in-
   window Identification values to support message authentication.  No
   header compression state is maintained by OAL intermediate systems,
   which only maintain state for per-flow data plane windows.

14.2.  IP Multihop and IPv4-Only Networks

   On some *NETs, a Client may be located multiple intermediate OAL hops
   away from the nearest OMNI link Proxy/Server.  Clients in multihop
   networks perform route discovery through the application of an
   adaptation layer routing protocol (e.g., a MANET routing protocol
   over omnidirectional wireless interfaces, etc.).  Example routing
   protocols optimized for MANET operations include OSPFv3 [RFC5340]
   with MANET Designated Router (OSPF-MDR) extensions [RFC5614], OLSRv2
   [RFC7181], Babel [RFC8966], AODVv2 [I-D.perkins-manet-aodvv2] and
   others.  Clients employ the routing protocol according to the link
   model found in [RFC5889] and subnet model articulated in [RFC5942].
   For unique identification within the MANET, Clients use an MLA either
   directly as a Router ID or as an extended identifier for a shorter
   Router ID.

   The Client configures the top-level MLA prefix (e.g., 2001:30::/28
   [RFC9374]) on the OMNI interface and configures the MANET routing
   protocol to populate discovered MLA-specific routes in an alternate
   kernel routing table.  The OAL then engages the Netlink socket



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   [NETLINK] (or an alternate operating system primitive) to monitor the
   table.  When the MANET routing protocol adds, modifies or removes an
   MLA route the OAL adds/modifies/removes a corresponding route within
   the OMNI interface to allow MLA-addressed packets forwarded by the
   network layer to flow through the OMNI interface instead of directly
   via a MANET interface.  The OMNI interface virtual router entity then
   engages OAL encapsulation and header compression before forwarding an
   OAL packet or fragment via the correct MANET underlay interface.  The
   next hop's OMNI interface will then receive the packets from its
   MANET interface due to the OAL encapsulation port/protocol/ethertype.

   MANETs can be compartmentalized internally with some nodes configured
   as simple Clients and others (that may have both "upstream" and
   "downstream" underlay interfaces) configured as cluster heads that
   act as Proxy/Clients.  These cluster heads configure and listen to
   the same multicast and anycast addresses as for the downstream
   interfaces of Proxy/Servers in order to act as endpoint OAL
   intermediate node proxys for other downstream Clients.  Clusters
   within clusters based on these cluster head Proxy/Clients can then be
   recursively nested to arbitrary depths as long as at least one
   ultimate Proxy/Client configures an upstream interface that can
   directly address a Proxy/Server with outside Internetwork
   connectivity.

   A Client located potentially multiple OAL hops away from the nearest
   Proxy can optionally discover Proxy MLAs using name resolution
   services such as mDNS as discussed in Section 14, where multicast
   exchanges can be propagated by Simplified Multicast Forwarding (SMF)
   [RFC6621].  The Client next prepares an RS message, sets the Source
   Address to its MLA, and sets the Destination Address to link-scoped
   All-Routers multicast (ff02::2) or a discovered Proxy MLA.  The OMNI
   interface then employs OAL encapsulation, sets the OAL Source Address
   to its MLA and sets the OAL Destination Address to the MLA of the
   Proxy, the site-scoped All-Routers multicast address (ff05::2) or the
   OMNI IPv6 anycast address.

   For IPv6-enabled *NETs where the underlay interface observes the
   MANET properties discussed above, the Client injects its MLA into the
   IPv6 multihop routing system and forwards the RS message without
   further encapsulation.  Otherwise, the Client encapsulates the
   message in UDP/IPv6 underlay headers with Source Address set to the
   underlay interface IPv6 address and Destination Address set to the
   discovered unicast or anycast address of a Proxy.  The Client then
   forwards the message into the IPv6 multihop routing system which
   conveys it to the nearest Proxy.






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   For IPv4-only *NETs, the Client encapsulates the RS message in UDP/
   IPv4 underlay headers with Source Address set to the underlay
   interface IPv4 address and Destination Address set to the discovered
   unicast address of a Proxy/Server or the OMNI IPv4 anycast address.
   The Client then forwards the message into the IPv4 multihop routing
   system which conveys it to the nearest Proxy/Server advertising the
   corresponding IPv4 prefix.  If the nearest Proxy/Server is too busy,
   it should forward (without Proxying) the OAL-encapsulated RS to
   another nearby Proxy/Server connected to the same IPv4 (multihop)
   network that configures the OMNI IPv6 anycast address.  (In
   environments where reciprocal RS forwarding cannot be supported, the
   first Proxy/Server should instead return an RA based on its own
   MSP(s).)

   When an OAL intermediate node that participates in the routing
   protocol receives the encapsulated RS, it appends its MLA to the RS
   SRH Segment List, caches the AFVI value and rewrites the SRH AFVI
   with a new unique value and forwards the message according to its OAL
   IPv6 forwarding table (note that an OAL intermediate system could be
   a fixed infrastructure element such as a roadside unit or another
   MANET/VANET Client).  This process repeats iteratively over
   successive OAL intermediate nodes until the RS message is received by
   a penultimate OAL hop within single-hop communications range of a
   Proxy/Server, which forwards the message to the Proxy/Server final
   hop.

   When a Proxy/Server that configures the OMNI IPv6 anycast address
   receives the message, it decapsulates the RS and assumes either the
   MAP or FHS role (in which case, it may forward the RS to a candidate
   MAP).  The MAP/FHS Proxy/Server then prepares an RA message using the
   same addressing disciplines as discussed in Section 14 and forwards
   the RA either to the FHS Proxy/Server or directly to the Client.

   When the MAP or FHS Proxy/Server forwards the RA to the Client, it
   encapsulates the message in an OAL header, includes an SRH copied
   from the RS as specified in Section 13 and includes underlay
   encapsulation headers if necessary.  The Proxy/Server then forwards
   the message to an OAL node within communications range, which
   forwards the message according to the next OAL hop.  The multihop
   forwarding process within the *NET continues repetitively until the
   message arrives at the original Client, which decapsulates and
   performs autoconfiguration the same as if it had received the RA
   directly from a Proxy/Server on the same physical link.

   MANETs often include Clients that configure multiple interfaces, with
   downstream interfaces internal to the MANET and upstream interfaces
   connected to external *NETs.  Such Clients can provide proxy services
   to enable router discovery for peer Clients that connect only



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   internally within the MANET.  These Proxy/Clients first perform
   router discovery to associate with true Proxy/Servers located on
   upstream *NETs.  The Proxy/Clients also subscribe to the site-scoped
   all-routers multicast group (i.e., ff05::2) and advertise
   reachability for the OMNI IPv6 anycast address over their MANET
   interfaces.

   When a source Client sends an initial RS message seeking service,
   MANET routing will direct the RS to one or more nearby Proxy/Clients
   which in turn forward the RS to one or more upstream interface
   Proxys.  Each such Proxy/Client writes its MLA as the final Segment
   List IPv6 address at the end of the RS SRH and also includes a new
   unique AFVI.  The natural progression continues from innermost Proxy/
   Clients to outermost Proxy/Clients until the RS message reaches a
   Proxy/Server.  By that time, the SRH Segment List will contain an
   ordered list of MLAs of all Proxy/Clients in the reverse path.

   The MANET Proxy/Client model recursively extends to include
   arbitrarily many layers of nested MANET clusters between the source
   Client and external Proxy/Servers.  When the source Client's first-
   hop Proxy/Client forwards an RS, it forwards to the next upstream
   Proxy/Client in succession toward the Proxy/Server while recording
   its MLA in the SRH as above.  The progression continues until the RS
   reaches an ultimate upstream Proxy/Client that can directly contact a
   Proxy/Server via underlay encapsulation over an upstream *NET
   interface.

   The Proxy/Server processes the RS and returns an RA while including
   its own MLA in the OAL Source Address and the MLA of the outermost
   Proxy/Client in the OAL Destination Address.  The Proxy/Server also
   includes the RS SRH information as an SRH extension to the RA OAL
   header with the ordered Segment List of Proxy/Client MLAs plus a
   unique AFVI value.  When the outermost Proxy/Client receives the RA,
   it forwards the message to the MLA of the next hop Proxy/Client in
   succession based on the SRH information until the message arrives at
   the source Client.  The source Client can then update its ALNCE
   information based on the information returned by the Proxy/Server.
   The Client also retains the Segment List information in the ALNCE for
   inclusion in OAL SRH headers for future multihop forwarding purposes.

   When the Proxy/Server returns an RA, each upstream Proxy/Client
   forwards the RA through the recursively descending chain of
   downstream Proxy/Clients on the path to the source Client.  Each
   Proxy/Client rewrites the OAL Destination Address according to the
   SRH next hop MLA address for the next downstream Proxy/Client hop
   toward the source Client and also includes a new unique AFVI value.





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   Note that this service model applies equally for MANETs that have
   only Proxy/Client access to external *NET Proxy/Servers as well as
   those that have some mix of Proxy/Clients and true Proxy/Servers at
   the MANET border.  True Proxy/Servers at the MANET border will
   service MANET Client router discovery requests the same as for any
   *NET, while external Proxy/Servers will discover potentially many
   MANET Clients all using the same UNX address belonging to a single
   Proxy/Client.  This arrangement ensures that MANET-internal Clients
   are able to access external Internetworking services the same as for
   MANET border Clients that also have direct connections to external
   *NETs.

   Note: When the RS message includes an anycast underlay encapsulation
   Destination Address, the FHS Proxy/Server must use the same anycast
   addresses as the underlay encapsulation Source Address to support
   forwarding of the RA message plus any initial data messages.  The FHS
   Proxy/Server then sends the resulting carrier packets over any NATs
   on the path.  When the outermost (Proxy/)Client receives the RA, it
   will discover the FHS Proxy/Server unicast underlay encapsulation
   address and can send future carrier packets using the unicast
   (instead of anycast) addresses to populate NAT state in the forward
   path.  (If the Client does not have immediate data to send to the FHS
   Proxy/Server, it can instead send an OAL "bubble" - see:
   Section 6.11.)  After the Client begins using unicast underlay
   encapsulation addresses in this way, the FHS Proxy/Server should also
   begin using the same unicast address in the reverse direction.

   Note: When an OMNI interface configures an MLA, any nodes that
   forward an encapsulated RS message with the MLA as the OAL source
   must not consider the message as being specific to a particular OMNI
   link segment.  MLAs can therefore also serve as the Source and
   Destination Addresses of unencapsulated IPv6 data communications
   within the local routing region; if the MLAs are injected into the
   local network routing protocol their prefix length must be set to 128
   per [RFC5889].

15.  Secure Redirection

   If the *NET link model is multiple access, the FHS Proxy/Server is
   responsible for assuring that address duplication cannot corrupt the
   neighbor caches of other nodes on the link through the use of the
   DHCPv6 address delegation service.  When the Client sends an RS
   message on a multiple access *NET, the Proxy/Server verifies that the
   Client is authorized to use the MLA Source Address and responds with
   an RA (or forwards the RS to the MAP) only if the Client is
   authorized.





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   After verifying Client authorization and returning an RA, the Proxy/
   Server MAY return IPv6 ND Redirect messages in response to subsequent
   data plane packet transmissions to direct Clients located on the same
   *NET to exchange OAL packets directly without transiting the Proxy/
   Server.  The Redirect messages use MLAs instead of LLAs to uniquely
   identify peers on the link, and include Interface Attribute OMNI sub-
   options for address resolution purposes.

   Following secure redirection, the Clients can exchange OAL packets
   according to their unicast underlay addresses discovered from the
   Redirect message instead of using the dogleg path through the Proxy/
   Server.  In some *NETs, however, such direct communications may be
   undesirable and continued use of the dogleg path through the Proxy/
   Server may provide better performance.  In that case, the Proxy/
   Server can refrain from sending Redirects, and/or Clients can ignore
   them.

16.  Proxy/Server Resilience

   *NETs SHOULD deploy Proxy/Servers in Virtual Router Redundancy
   Protocol (VRRP) [RFC5798] configurations so that service continuity
   is maintained even if one or more Proxy/Servers fail.  Using VRRP,
   the Client is unaware which of the (redundant) FHS Proxy/Servers is
   currently providing service, and any service discontinuity will be
   limited to the failover time supported by VRRP.  Widely deployed
   public domain implementations of VRRP are available.

   Proxy/Servers SHOULD use high availability clustering services so
   that multiple redundant systems can provide coordinated response to
   failures.  As with VRRP, widely deployed public domain
   implementations of high availability clustering services are
   available.  Note that special-purpose and expensive dedicated
   hardware is not necessary, and public domain implementations can be
   used even between lightweight virtual machines in cloud deployments.

17.  Detecting and Responding to Proxy/Server Failures

   In environments where fast recovery from Proxy/Server failure is
   essential, FHS Proxy/Servers SHOULD use proactive Neighbor
   Unreachability Detection (NUD) in a manner that parallels
   Bidirectional Forwarding Detection (BFD) [RFC5880] to track MAP
   Proxy/Server reachability.  FHS Proxy/Servers can then quickly detect
   and react to failures so that cached information is re-established
   through alternate paths.  Proactive NUD control messaging is carried
   only over well-connected ground domain networks (i.e., and not low-
   end links such as aeronautical radios) and can therefore be tuned for
   rapid response.




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   FHS Proxy/Servers perform proactive NUD for MAP Proxy/Servers for
   which there are currently active Clients.  If a MAP Proxy/Server
   fails, the FHS Proxy/Server can quickly inform Clients of the outage
   by sending multicast RA messages.  The FHS Proxy/Server sends RA
   messages to Clients with Source Address set to the GUA of the MAP,
   with Destination Address set to link-scoped All-Nodes multicast
   (ff02::1) [RFC4291] and with Router Lifetime set to 0.

   The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA
   messages separated by small delays [RFC4861].  Any Clients that have
   been using the (now defunct) MAP Proxy/Server will receive the RA
   messages.

18.  OMNI Interfaces on Open Internetworks

   Client OMNI interfaces configured over IPv6-enabled underlay
   interfaces on an open Internetwork without an OMNI-aware first-hop
   router receive IPv6 RA messages with no OMNI options, while OMNI
   interfaces configured over IPv4-only underlay interfaces receive no
   IPv6 RA messages at all (but may receive IPv4 RA messages per
   [RFC1256]).  Client OMNI interfaces that receive RA messages with
   OMNI options configure addresses, on-link prefixes, etc. on the
   underlay interface that received the RA according to standard IPv6 ND
   and address resolution conventions [RFC4861][RFC4862].  Client OMNI
   interfaces configured over IPv4-only underlay interfaces configure
   IPv4 address information on the underlay interfaces using mechanisms
   such as DHCPv4 [RFC2131].

   Client OMNI interfaces configured over underlay interfaces connected
   to open Internetworks can apply lower layer security services such as
   VPNs (e.g., IPsec tunnels) to connect to a Proxy/Server, or can
   establish a secured direct point-to-point link to the Proxy/Server
   through some other means (see: Section 4).  In environments where
   lower layer security may be impractical or undesirable, Client OMNI
   interfaces can instead send control messages with OMNI sub-options
   that include authentication parameters.

   OMNI interfaces use UDP/IP as underlay encapsulation headers for
   transmission over open Internetworks with UDP service port number
   8060 for both IPv4 and IPv6 underlay interfaces.  The OMNI interface
   submits original IP packets for OAL encapsulation, then encapsulates
   the resulting OAL fragments in UDP/IP underlay headers to form
   carrier packets.  (The first 4 bits following the UDP header
   determine whether the OAL headers are uncompressed/compressed as
   discussed in Section 6.5.)  The OMNI interface sets the UDP length to
   the encapsulated OAL fragment length and sets the IP length to an
   appropriate value at least as large as the UDP datagram.




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   When necessary, sources include an OMNI option with an authentication
   sub-option in control messages.  Procedures for including OMNI
   authentication sub-options are discussed in Section 10.

   After establishing a secured underlay link or preparing for UDP/IP
   encapsulation, OMNI interfaces send RS/RA messages for Client-Proxy/
   Server coordination (see: Section 14) and peer-to-peer control
   solicitation and response messages for multilink forwarding, route
   optimization, and mobility management (see:
   [I-D.templin-6man-aero3]).  These control plane messages must be
   authenticated while other control and data plane messages are
   delivered the same as for ordinary best effort traffic with Source
   Address and/or Identification window-based data origin verification.
   Transport and higher layer protocol sessions over OMNI interfaces
   that connect over open Internetworks without an explicit underlay
   link security services should therefore employ security at their
   layers to ensure authentication, integrity and/or confidentiality.

   Clients should avoid using INET Proxy/Servers as general-purpose
   routers for steady streams of carrier packets that do not require
   authentication.  Clients should therefore perform route optimization
   to coordinate with other INET nodes that can provide forwarding
   services (or preferably coordinate with peer Clients directly)
   instead of burdening the Proxy/Server.  Procedures for coordinating
   with peer Clients and discovering INET nodes that can provide better
   forwarding services are discussed in [I-D.templin-6man-aero3].

   Clients that attempt to contact peers over INET underlay interfaces
   often encounter NATs in the path.  OMNI interfaces accommodate NAT
   traversal using UDP/IP encapsulation and the mechanisms discussed in
   [I-D.templin-6man-aero3].  FHS Proxy/Servers include UNX information
   in an Interface Attributes sub-option in RA messages to allow Clients
   to detect the presence of NATs.

   Note: Following the initial control message exchange, OMNI interfaces
   configured over INET underlay interfaces maintain neighbor
   relationships by transmitting periodic control messages with OMNI
   options that include authentication signatures.

   Note: OMNI interfaces configured over INET underlay interfaces should
   employ the Identification window synchronization mechanisms specified
   in Section 6.7 in order to exclude spurious carrier packets that
   might otherwise clutter the reassembly cache.  This is especially
   important in environments where carrier packet spoofing and/or
   corruption is a threat.






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   Note: NATs may be present on the path from a Client to its FHS Proxy/
   Server, but never on the path from the FHS Proxy/Server to the MAP
   where only INET and/or spanning tree hops occur.  Therefore, the FHS
   Proxy/Server does not communicate Client origin information to the
   MAP where it would serve no purpose.

19.  Time-Varying MNPs

   In some use cases, it is desirable, beneficial and efficient for the
   Client to receive a constant MNP that travels with the Client
   wherever it moves.  For example, this would allow air traffic
   controllers to easily track aircraft, etc.  In other cases, however
   (e.g., intelligent transportation systems), the Client may be willing
   to sacrifice a modicum of efficiency in order to have time-varying
   MNPs that can be changed occasionally to defeat adversarial tracking.

   The OMNI link prefix delegation service allows Clients that desire
   time-varying MNPs to obtain short-lived prefixes to send RS messages
   with an OMNI option with DHCPv6 IA_PD sub-options.  The Client would
   then be obligated to renumber its internal networks whenever its MNPs
   change.  This should not present a challenge for Clients with
   automated network renumbering services, but may disrupt persistent
   sessions that would prefer to use a constant address.

20.  IANA Considerations

   The following IANA actions are requested in accordance with
   [RFC8126].  Both existing registries and new registries specific to
   OMNI are affected.  Existing registries should be updated according
   to standard IANA practices.  New registries should be created under a
   new registry group for "Overlay Multilink Network (OMNI) Interface".

20.1.  Protocol Numbers

   The IANA is instructed to allocate an Internet Protocol number TBD1
   from the https://www.iana.org/assignments/protocol-numbers registry
   for the Overlay Multilink Network (OMNI) Interface as a non IPv6
   Extension Header protocol.  Guidance is found in [RFC5237]
   (registration procedure is IESG Approval or Standards Action).

20.2.  IEEE 802 Numbers

   During final publication stages, the IESG will be requested to
   procure an IEEE EtherType value TBD2 for OMNI according to the
   statement found at https://www.ietf.org/about/groups/iesg/statements/
   ethertypes/.





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   Following this procurement, the IANA is instructed to register the
   value TBD2 in the Ethertypes registry of the
   https://www.iana.org/assignments/ieee-802-numbers registry group for
   "Overlay Multilink Network (OMNI) Interface encapsulation on Ethernet
   networks".  Guidance is found in [RFC7042] (registration procedure is
   Expert Review).

20.3.  IPv4 Special-Purpose Address

   The IANA is instructed to assign TBD3/N as an "OMNI IPv4 anycast"
   address/prefix in the https://www.iana.org/assignments/iana-ipv4-
   special-registry registry in a similar fashion as for [RFC3068].  The
   assignment also automatically provides the basis for an OMNI IPv6
   subnet router anycast address configured as 2002:TBD3::. The IANA is
   requested to assist the author's efforts to obtain a TBD3/N public
   IPv4 prefix, whether through an RIR allocation, a delegation from the
   "Current Recovered IPv4 Pool" of the
   https://www.iana.org/assignments/ipv4-recovered-address-space
   registry or through an unspecified third party donation.

20.4.  Segment Routing Header TLVs

   The IANA is instructed to allocate a new Type value TBD4 with
   Description "AFVI TLV" and Reference [RFCXXXX] in the "Segment
   Routing Header TLVs" registry found in
   https://www.iana.org/assignments/ipv6-parameters (registration
   procedure is "IETF Review").

20.5.  IANA OUI Ethernet Numbers

   The IANA is instructed to allocate one Ethernet unicast address TBD5
   (suggested value '00-52-14') in the "IANA Unicast 48-bit MAC
   Addresses" registry in the https://www.iana.org/assignments/ethernet-
   numbers registry group (registration procedure is Expert Review).
   The registration should appear as follows:

   Addresses      Usage                                         Reference
   ---------      -----                                         ---------
   00-52-14       Overlay Multilink Network (OMNI) Interface    [RFCXXXX]

             Figure 40: IANA Unicast 48-bit MAC Addresses

20.6.  Overlay Multilink Network (OMNI) Interface Registry Group

   The IANA is instructed to create a new 'omni-interface' registry
   group for "Overlay Multilink Network (OMNI) Interface".  The registry
   group must include the following new registries:




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20.6.1.  OMNI Option Sub-Types (New Registry)

   The OMNI option defines a 1 octet Sub-Type field, for which IANA is
   instructed to create and maintain a new registry entitled "OMNI
   Option Sub-Type Values".  Initial values are given below
   (registration procedure is RFC required):

      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        NULL                           [RFCXXXX]
      1        CGA                            [RFCXXXX]
      2        RSA Signature                  [RFCXXXX]
      3        Timestamp                      [RFCXXXX]
      4        Nonce                          [RFCXXXX]
      5        Trust Anchor                   [RFCXXXX]
      6        Certificate                    [RFCXXXX]
      7        HMAC                           [RFCXXXX]
      8        Node Identification            [RFCXXXX]
      9        Neighbor Synchronization       [RFCXXXX]
      10       Interface Attributes           [RFCXXXX]
      11       Traffic Selector               [RFCXXXX]
      12       Geo Coordinates                [RFCXXXX]
      13       PIM-SM Message                 [RFCXXXX]
      14       Fragmentation Report           [RFCXXXX]
      15       Proxy/Server Control           [RFCXXXX]
      16       Prefix Information Option      [RFCXXXX]
      17       Route Information Option       [RFCXXXX]
      18       DHCPv6 Message                 [RFCXXXX]
      19-252   Unassigned                     [RFCXXXX]
      253-254  Reserved for Experimentation   [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

                   Figure 41: OMNI Option Sub-Type Values

20.6.2.  OMNI Node Identification ID-Types (New Registry)

   The OMNI Node Identification sub-option (see: Section 10.2.9)
   contains a 1 octet ID-Type field, for which IANA is instructed to
   create and maintain a new registry entitled "OMNI Node Identification
   ID-Type Values".  Initial values are given below (registration
   procedure is RFC required):










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      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        MLA                            [RFCXXXX]
      1        UUID                           [RFCXXXX]
      2        Network Access Identifier      [RFCXXXX]
      3        FQDN                           [RFCXXXX]
      4        IPv4 Address                   [RFCXXXX]
      5        Unassigned                     [RFCXXXX]
      6        IPv6 Address                   [RFCXXXX]
      7-252    Unassigned                     [RFCXXXX]
      253-254  Reserved for Experimental Use  [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

             Figure 42: OMNI Node Identification ID-Type Values

20.6.3.  OMNI Geo Coordinates Types (New Registry)

   The OMNI Geo Coordinates sub-option (see: Section 10.2.13) contains
   an 1 octet Type field, for which IANA is instructed to create and
   maintain a new registry entitled "OMNI Geo Coordinates Type Values".
   Initial values are given below (registration procedure is RFC
   required):

      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        NULL                           [RFCXXXX]
      1-252    Unassigned                     [RFCXXXX]
      253-254  Reserved for Experimental Use  [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

                    Figure 43: OMNI Geo Coordinates Type

20.7.  Additional Considerations

   IANA has assigned UDP port number "8060" for an earlier experimental
   version of AERO [RFC6706].  This document reclaims UDP port number
   "8060" for 'aero' as the service port for OMNI interface UDP/IP
   encapsulation.  (Note that, although [RFC6706] is not widely
   implemented or deployed, any messages coded to that specification can
   be easily distinguished and ignored since they include an invalid
   ICMPv6 message type number '0'.)  IANA is therefore instructed to
   update the reference for UDP port number "8060" from "RFC6706" to
   "RFCXXXX" (i.e., this document) while retaining the existing name
   'aero'.

   IANA has assigned a 4 octet Private Enterprise Number (PEN) code
   "45282" in the https://www.iana.org/assignments/enterprise-numbers
   registry.  This document is the normative reference for using code



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   "45282" in DHCP Unique IDentifiers based on Enterprise Numbers
   ("DUID-EN for OMNI Interfaces") (see: Section 9).  IANA is therefore
   instructed to change the enterprise designation for PEN code "45282"
   from "LinkUp Networks" to "Overlay Multilink Network (OMNI)
   Interface".

   IANA has assigned the ifType code "301 - omni - Overlay Multilink
   Network (OMNI) Interface" in accordance with Section 6 of [RFC8892].
   The registration appears under the IANA
   https://www.iana.org/assignments/smi-numbers registry group Interface
   Types (ifType)" registry, and the IANA is instructed to change the
   reference to [RFCXXXX] (i.e., this document).

   No further IANA actions are required.

21.  Security Considerations

   Security considerations for IPv4 [RFC0791], IPv6 [RFC8200] and IPv6
   Neighbor Discovery [RFC4861] apply.  For end-to-end peer exchanges
   not fully protected by security associations, OMNI interfaces SHOULD
   use a modified version of SEcure Neighbor Discovery (SEND) or a
   Hashed Message Authentication Code (HMAC) per Section 10.1 as an
   adaptation layer service to ensure authentic exchanges.  (Alternate
   OMNI interface authentication service types may be specified in
   future documents.)  These services provide authentication for
   unsecured FHS and LHS *NETs, while intermediate hops are protected by
   the secured spanning tree (see below).

   OMNI interfaces configured over secured ANET interfaces inherit the
   physical and/or link layer security services (i.e., protected
   spectrum and/or [MACSEC]) of the connected networks.  OMNI interfaces
   configured over open *NET interfaces include message authentication
   codes as above; they can instead (or in addition) use symmetric
   securing services such as IPsec tunnels [RFC4301] or can by some
   other means establish a direct point-to-point link secured at lower
   layers.

   OMNI link mobility services MUST support strong authentication for
   control plane messages and forwarding path integrity for data plane
   messages.  In particular, the AERO service [I-D.templin-6man-aero3]
   constructs a secured spanning tree with Proxy/Servers as leaf nodes
   and secures the spanning tree links with network layer security
   services based on IPsec [RFC4301] with IKEv2 [RFC7296].  (Note that
   direct point-to-point links secured at lower layers can also be used
   instead of or in addition to network layer security.)  Together,
   these services provide connectionless integrity and data origin
   authentication with optional protection against replays.




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   Control plane messages that affect the routing system or neighbor
   state either include authentication signatures or are constrained to
   travel only over secured spanning tree paths; in both cases, the
   messages are protected by network (and/or lower-layer) security.
   Other control and data plane messages can travel over unsecured route
   optimized paths that do not strictly follow the spanning tree,
   therefore end-to-end sessions should employ transport or higher layer
   security services (e.g., TLS/SSL [RFC8446], DTLS [RFC6347], etc.).
   Additionally, the OAL Identification value can provide a first level
   of data origin authentication to mitigate off-path spoofing.

   Identity-based key verification infrastructure services such as iPSK
   may be necessary for verifying the identities claimed by Clients.
   This requirement should be harmonized with the manner in which
   identifiers such as MLAs are certified in a given operational
   environment.

   Security considerations for specific access network interface types
   are covered under the corresponding IP-over-(foo) specification
   (e.g., [RFC2464], [RFC2492], etc.).

   Security considerations for IPv6 fragmentation and reassembly are
   discussed in Section 6.13.  In environments where spoofing is
   considered a threat, OMNI nodes SHOULD employ Identification window
   synchronization and OAL destinations SHOULD configure an (end-system-
   based) firewall.

22.  Implementation Status

   AERO/OMNI Release-3.2 was tagged on March 30, 2021, and was subject
   to internal testing.  The implementation is not planned for public
   release.

   A write-from-scratch reference implementation is under active
   internal development, with release version v0.pre8 tagged on January
   16, 2026.  Future versions will be made available for public release.

23.  Document Updates

   This document suggests that the following could be updated through
   future IETF initiatives:

   *  [RFC1191]

   *  [RFC4443]

   *  [RFC8200]




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   *  [RFC8201]

   Updates can be through, e.g., standards action, the errata process,
   etc. as appropriate.

24.  Acknowledgements

   The first version of this document was prepared per the consensus
   decision at the 7th Conference of the International Civil Aviation
   Organization (ICAO) Working Group-I Mobility Subgroup on March 22,
   2019.  Consensus to take the document forward to the IETF was reached
   at the 9th Conference of the Mobility Subgroup on November 22, 2019.
   Attendees and contributors included: Guray Acar, Danny Bharj,
   Francois D'Humieres, Pavel Drasil, Nikos Fistas, Giovanni Garofolo,
   Bernhard Haindl, Vaughn Maiolla, Tom McParland, Victor Moreno, Madhu
   Niraula, Brent Phillips, Liviu Popescu, Jacky Pouzet, Aloke Roy, Greg
   Saccone, Robert Segers, Michal Skorepa, Michel Solery, Stephane
   Tamalet, Fred Templin, Jean-Marc Vacher, Bela Varkonyi, Tony Whyman,
   Fryderyk Wrobel and Dongsong Zeng.

   The following individuals are acknowledged for their useful comments:
   Felipe Magno de Almeida, Amanda Baber, Scott Burleigh, Stuart Card,
   Donald Eastlake, Adrian Farrel, Michael Matyas, Robert Moskowitz,
   Madhu Niraula, Greg Saccone, Stephane Tamalet, Eliot Lear, Eduard
   Vasilenko, Eric Vyncke.  Pavel Drasil, Zdenek Jaron and Michal
   Skorepa are especially recognized for their many helpful ideas and
   suggestions.  Akash Agarwal, Madhuri Madhava Badgandi, Sean Dickson,
   Don Dillenburg, Joe Dudkowski, Vijayasarathy Rajagopalan, Ron
   Sackman, Bhargava Raman Sai Prakash and Katherine Tran are
   acknowledged for their hard work on the implementation and technical
   insights that led to improvements for the spec.

   Discussions on the IETF 6man and atn mailing lists during the fall of
   2020 suggested additional points to consider.  The authors gratefully
   acknowledge the list members who contributed valuable insights
   through those discussions.  Eric Vyncke and Erik Kline were the
   intarea ADs, while Bob Hinden and Ole Troan were the 6man WG chairs
   at the time the document was developed; they are all gratefully
   acknowledged for their many helpful insights.  Many of the ideas in
   this document have further built on IETF experiences beginning in the
   1990s, with insights from colleagues including Ron Bonica, Brian
   Carpenter, Ralph Droms, Tom Herbert, Bob Hinden, Christian Huitema,
   Thomas Narten, Dave Thaler, Joe Touch, Pascal Thubert, and many
   others who deserve recognition.

   Early observations on IP fragmentation performance implications were
   noted in the 1986 Digital Equipment Corporation (DEC) "qe reset"
   investigation, where fragment bursts from NFS UDP traffic triggered



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   hardware resets resulting in communication failures.  Jeff Chase,
   Fred Glover and Chet Juzsczak of the Ultrix Engineering Group led the
   investigation, and determined that setting a smaller NFS mount block
   size reduced the amount of fragmentation and suppressed the resets.
   Early observations on L2 media MTU issues were noted in the 1988 DEC
   FDDI investigation, where Raj Jain, KK Ramakrishnan and Kathy Wilde
   represented architectural considerations for FDDI networking in
   general including FDDI/Ethernet bridging.  Jeff Mogul (who led the
   IETF Path MTU Discovery working group) and other DEC colleagues who
   supported these early investigations are also acknowledged.

   Throughout the 1990's and into the 2000's, many colleagues supported
   and encouraged continuation of the work.  Beginning with the DEC
   Project Sequoia effort at the University of California, Berkeley,
   then moving to the DEC research lab offices in Palo Alto CA, then to
   Sterling Software at the NASA Ames Research Center, then to SRI in
   Menlo Park, CA, then to Nokia in Mountain View, CA and finally to the
   Boeing Company in 2005 the work saw continuous advancement through
   the encouragement of many.  Those who offered their support and
   encouragement are gratefully acknowledged.

   This work is aligned with the NASA Safe Autonomous Systems Operation
   (SASO) program under NASA contract number NNA16BD84C.

   This work is aligned with the FAA as per the SE2025 contract number
   DTFAWA-15-D-00030.

   This work is aligned with the Boeing Information Technology (BIT)
   Mobility Vision Lab (MVL) program.

   This work is aligned with the Boeing/Virginia Tech National Security
   Institute (VTNSI) 5G MANET research program.

   Honoring life, liberty and the pursuit of happiness.

25.  References

25.1.  Normative References

   [I-D.templin-6man-ipid-ext2]
              Templin, F. and T. Herbert, "IPv6 Extended Fragment Header
              (EFH)", Work in Progress, Internet-Draft, draft-templin-
              6man-ipid-ext2-27, 12 January 2026,
              <https://datatracker.ietf.org/doc/html/draft-templin-6man-
              ipid-ext2-27>.






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   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              DOI 10.17487/RFC2003, October 1996,
              <https://www.rfc-editor.org/info/rfc2003>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <https://www.rfc-editor.org/info/rfc3971>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              DOI 10.17487/RFC4007, March 2005,
              <https://www.rfc-editor.org/info/rfc4007>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.



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   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC6088]  Tsirtsis, G., Giarreta, G., Soliman, H., and N. Montavont,
              "Traffic Selectors for Flow Bindings", RFC 6088,
              DOI 10.17487/RFC6088, January 2011,
              <https://www.rfc-editor.org/info/rfc6088>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <https://www.rfc-editor.org/info/rfc8028>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.








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   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/info/rfc9293>.

25.2.  Informative References

   [ATN]      Maiolla, V., "The OMNI Interface - An IPv6 Air/Ground
              Interface for Civil Aviation, IETF Liaison Statement
              #1676, https://datatracker.ietf.org/liaison/1676/", 3
              March 2020.

   [ATN-IPS]  "ICAO Document 9896 (Manual on the Aeronautical
              Telecommunication Network (ATN) using Internet Protocol
              Suite (IPS) Standards and Protocol), Draft Edition 3
              (work-in-progress)", 10 December 2020.

   [CRC]      Jain, R., "Error Characteristics of Fiber Distributed Data
              Interface (FDDI), IEEE Transactions on Communications",
              August 1990.

   [EUI]      "IEEE Guidelines for Use of Extended Unique Identifier
              (EUI), Organizationally Unique Identifier (OUI), and
              Company ID, https://standards.ieee.org/wp-
              content/uploads/import/documents/tutorials/eui.pdf", 3
              August 2017.

   [I-D.gont-dhcwg-dhcpv6-iids]
              Gont, F., "A Method for Generating Semantically Opaque
              IPv6 Interface Identifiers (IIDs) with Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", Work in
              Progress, Internet-Draft, draft-gont-dhcwg-dhcpv6-iids-00,
              10 February 2025, <https://datatracker.ietf.org/doc/html/
              draft-gont-dhcwg-dhcpv6-iids-00>.







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   [I-D.herbert-ipv4-eh]
              Herbert, T., "IPv4 Extension Headers and Flow Label", Work
              in Progress, Internet-Draft, draft-herbert-ipv4-eh-03, 22
              February 2024, <https://datatracker.ietf.org/doc/html/
              draft-herbert-ipv4-eh-03>.

   [I-D.ietf-6man-eh-limits]
              Herbert, T., "Limits on Sending and Processing IPv6
              Extension Headers", Work in Progress, Internet-Draft,
              draft-ietf-6man-eh-limits-19, 27 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-eh-
              limits-19>.

   [I-D.ietf-6man-rfc6724-update]
              Buraglio, N., Chown, T., and J. Duncan, "Prioritizing
              known-local IPv6 ULAs through address selection policy",
              Work in Progress, Internet-Draft, draft-ietf-6man-rfc6724-
              update-25, 11 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              rfc6724-update-25>.

   [I-D.ietf-intarea-tunnels]
              Touch, J. D. and M. Townsley, "IP Tunnels in the Internet
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-intarea-tunnels-15, 9 May 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
              tunnels-15>.

   [I-D.ietf-tsvwg-udp-options]
              Touch, J. D. and C. M. Heard, "Transport Options for UDP",
              Work in Progress, Internet-Draft, draft-ietf-tsvwg-udp-
              options-45, 16 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
              udp-options-45>.

   [I-D.ietf-v6ops-ula-usage-considerations]
              Jiang, S., Liu, B., and N. Buraglio, "Considerations For
              Using Unique Local Addresses", Work in Progress, Internet-
              Draft, draft-ietf-v6ops-ula-usage-considerations-05, 11
              December 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-v6ops-ula-usage-considerations-05>.

   [I-D.link-6man-gulla]
              Linkova, J., "Using Prefix-Specific Link-Local Addresses
              to Improve SLAAC Robustness", Work in Progress, Internet-
              Draft, draft-link-6man-gulla-01, 3 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-link-6man-
              gulla-01>.



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   [I-D.perkins-manet-aodvv2]
              Perkins, C. E., Dowdell, J., Steenbrink, L., and V.
              Pritchard, "Ad Hoc On-demand Distance Vector Version 2
              (AODVv2) Routing", Work in Progress, Internet-Draft,
              draft-perkins-manet-aodvv2-06, 20 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-perkins-
              manet-aodvv2-06>.

   [I-D.templin-6man-aero3]
              Templin, F., "Automatic Extended Route Optimization
              (AERO)", Work in Progress, Internet-Draft, draft-templin-
              6man-aero3-52, 23 January 2026,
              <https://datatracker.ietf.org/doc/html/draft-templin-6man-
              aero3-52>.

   [I-D.templin-6man-mla]
              Templin, F., "IPv6 Addresses for Ad Hoc Networks", Work in
              Progress, Internet-Draft, draft-templin-6man-mla-30, 11
              November 2025, <https://datatracker.ietf.org/doc/html/
              draft-templin-6man-mla-30>.

   [I-D.templin-manet-inet]
              Templin, F. and D. J. Jakubisin, "MANET Internetworking:
              Problem Statement and Gap Analysis", Work in Progress,
              Internet-Draft, draft-templin-manet-inet-02, 12 January
              2026, <https://datatracker.ietf.org/doc/html/draft-
              templin-manet-inet-02>.

   [IANA-CGA] Postel, J., "Cryptographically Generated Addresses (CGA)
              Message Type Name Space, https://www.iana.org/assignments/
              cga-message-types/cga-message-types.xhtml", 15 March 2023.

   [IEEE802.1AX]
              "Institute of Electrical and Electronics Engineers, Link
              Aggregation, IEEE Standard 802.1AX-2008,
              https://standards.ieee.org/ieee/802.1AX/6768/", 29 May
              2020.

   [IPV4]     Postel, J., "IPv4 Address Space Registry,
              https://www.iana.org/assignments/ipv4-address-space/ipv4-
              address-space.xhtml", 14 December 2020.

   [IPV6]     Postel, J., "IPv6 Global Unicast Address Assignments,
              https://www.iana.org/assignments/ipv6-unicast-address-
              assignments/ipv6-unicast-address-assignments.xhtml", 14
              December 2020.





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   [MACSEC]   Seaman, M., "IEEE Standard for Local and metropolitan area
              networks Media Access Control (MAC) Security (IEEE Std.
              802.1AE), https://1.ieee802.org/security/802-1ae-2018/",
              September 2018.

   [NETLINK]  "https://en.wikipedia.org/wiki/Netlink", 1 January 2026.

   [RFC0863]  Postel, J., "Discard Protocol", STD 21, RFC 863,
              DOI 10.17487/RFC0863, May 1983,
              <https://www.rfc-editor.org/info/rfc863>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1149]  Waitzman, D., "Standard for the transmission of IP
              datagrams on avian carriers", RFC 1149,
              DOI 10.17487/RFC1149, April 1990,
              <https://www.rfc-editor.org/info/rfc1149>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [RFC1256]  Deering, S., Ed., "ICMP Router Discovery Messages",
              RFC 1256, DOI 10.17487/RFC1256, September 1991,
              <https://www.rfc-editor.org/info/rfc1256>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
              <https://www.rfc-editor.org/info/rfc2464>.






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   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2492]  Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM
              Networks", RFC 2492, DOI 10.17487/RFC2492, January 1999,
              <https://www.rfc-editor.org/info/rfc2492>.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, DOI 10.17487/RFC2675, August 1999,
              <https://www.rfc-editor.org/info/rfc2675>.

   [RFC2863]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group
              MIB", RFC 2863, DOI 10.17487/RFC2863, June 2000,
              <https://www.rfc-editor.org/info/rfc2863>.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,
              <https://www.rfc-editor.org/info/rfc2923>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <https://www.rfc-editor.org/info/rfc2983>.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, DOI 10.17487/RFC3056, February
              2001, <https://www.rfc-editor.org/info/rfc3056>.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, DOI 10.17487/RFC3068, June 2001,
              <https://www.rfc-editor.org/info/rfc3068>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3366]  Fairhurst, G. and L. Wood, "Advice to link designers on
              link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
              DOI 10.17487/RFC3366, August 2002,
              <https://www.rfc-editor.org/info/rfc3366>.

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692,
              DOI 10.17487/RFC3692, January 2004,
              <https://www.rfc-editor.org/info/rfc3692>.



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   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,
              <https://www.rfc-editor.org/info/rfc3819>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213,
              DOI 10.17487/RFC4213, October 2005,
              <https://www.rfc-editor.org/info/rfc4213>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006,
              <https://www.rfc-editor.org/info/rfc4380>.

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
              2006, <https://www.rfc-editor.org/info/rfc4389>.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
              <https://www.rfc-editor.org/info/rfc4429>.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
              <https://www.rfc-editor.org/info/rfc4541>.






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   [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
              "Internet Group Management Protocol (IGMP) / Multicast
              Listener Discovery (MLD)-Based Multicast Forwarding
              ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
              August 2006, <https://www.rfc-editor.org/info/rfc4605>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,
              <https://www.rfc-editor.org/info/rfc5213>.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              DOI 10.17487/RFC5214, March 2008,
              <https://www.rfc-editor.org/info/rfc5214>.

   [RFC5237]  Arkko, J. and S. Bradner, "IANA Allocation Guidelines for
              the Protocol Field", BCP 37, RFC 5237,
              DOI 10.17487/RFC5237, February 2008,
              <https://www.rfc-editor.org/info/rfc5237>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC5558]  Templin, F., Ed., "Virtual Enterprise Traversal (VET)",
              RFC 5558, DOI 10.17487/RFC5558, February 2010,
              <https://www.rfc-editor.org/info/rfc5558>.

   [RFC5614]  Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
              Extension of OSPF Using Connected Dominating Set (CDS)
              Flooding", RFC 5614, DOI 10.17487/RFC5614, August 2009,
              <https://www.rfc-editor.org/info/rfc5614>.

   [RFC5798]  Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
              Version 3 for IPv4 and IPv6", RFC 5798,
              DOI 10.17487/RFC5798, March 2010,
              <https://www.rfc-editor.org/info/rfc5798>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.




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   [RFC5889]  Baccelli, E., Ed. and M. Townsley, Ed., "IP Addressing
              Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889,
              September 2010, <https://www.rfc-editor.org/info/rfc5889>.

   [RFC5942]  Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
              Model: The Relationship between Links and Subnet
              Prefixes", RFC 5942, DOI 10.17487/RFC5942, July 2010,
              <https://www.rfc-editor.org/info/rfc5942>.

   [RFC6081]  Thaler, D., "Teredo Extensions", RFC 6081,
              DOI 10.17487/RFC6081, January 2011,
              <https://www.rfc-editor.org/info/rfc6081>.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
              <https://www.rfc-editor.org/info/rfc6145>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/info/rfc6147>.

   [RFC6214]  Carpenter, B. and R. Hinden, "Adaptation of RFC 1149 for
              IPv6", RFC 6214, DOI 10.17487/RFC6214, April 2011,
              <https://www.rfc-editor.org/info/rfc6214>.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
              <https://www.rfc-editor.org/info/rfc6296>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6543]  Gundavelli, S., "Reserved IPv6 Interface Identifier for
              Proxy Mobile IPv6", RFC 6543, DOI 10.17487/RFC6543, May
              2012, <https://www.rfc-editor.org/info/rfc6543>.

   [RFC6621]  Macker, J., Ed., "Simplified Multicast Forwarding",
              RFC 6621, DOI 10.17487/RFC6621, May 2012,
              <https://www.rfc-editor.org/info/rfc6621>.




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   [RFC6706]  Templin, F., Ed., "Asymmetric Extended Route Optimization
              (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
              <https://www.rfc-editor.org/info/rfc6706>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <https://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980,
              DOI 10.17487/RFC6980, August 2013,
              <https://www.rfc-editor.org/info/rfc6980>.

   [RFC7042]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
              IETF Protocol and Documentation Usage for IEEE 802
              Parameters", RFC 7042, DOI 10.17487/RFC7042, October 2013,
              <https://www.rfc-editor.org/info/rfc7042>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,
              <https://www.rfc-editor.org/info/rfc7094>.

   [RFC7181]  Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
              "The Optimized Link State Routing Protocol Version 2",
              RFC 7181, DOI 10.17487/RFC7181, April 2014,
              <https://www.rfc-editor.org/info/rfc7181>.




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   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,
              <https://www.rfc-editor.org/info/rfc7323>.

   [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
              Routable Cryptographic Hash Identifiers Version 2
              (ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September
              2014, <https://www.rfc-editor.org/info/rfc7343>.

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,
              <https://www.rfc-editor.org/info/rfc7542>.

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC7847]  Melia, T., Ed. and S. Gundavelli, Ed., "Logical-Interface
              Support for IP Hosts with Multi-Access Support", RFC 7847,
              DOI 10.17487/RFC7847, May 2016,
              <https://www.rfc-editor.org/info/rfc7847>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.




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   [RFC8504]  Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <https://www.rfc-editor.org/info/rfc8504>.

   [RFC8799]  Carpenter, B. and B. Liu, "Limited Domains and Internet
              Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
              <https://www.rfc-editor.org/info/rfc8799>.

   [RFC8892]  Thaler, D. and D. Romascanu, "Guidelines and Registration
              Procedures for Interface Types and Tunnel Types",
              RFC 8892, DOI 10.17487/RFC8892, August 2020,
              <https://www.rfc-editor.org/info/rfc8892>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
              <https://www.rfc-editor.org/info/rfc8900>.

   [RFC8966]  Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
              <https://www.rfc-editor.org/info/rfc8966>.

   [RFC9365]  Jeong, J., Ed., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              RFC 9365, DOI 10.17487/RFC9365, March 2023,
              <https://www.rfc-editor.org/info/rfc9365>.

   [RFC9374]  Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
              "DRIP Entity Tag (DET) for Unmanned Aircraft System Remote
              ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023,
              <https://www.rfc-editor.org/info/rfc9374>.

   [RFC9562]  Davis, K., Peabody, B., and P. Leach, "Universally Unique
              IDentifiers (UUIDs)", RFC 9562, DOI 10.17487/RFC9562, May
              2024, <https://www.rfc-editor.org/info/rfc9562>.

   [RFC9602]  Krishnan, S., "Segment Routing over IPv6 (SRv6) Segment
              Identifiers in the IPv6 Addressing Architecture",
              RFC 9602, DOI 10.17487/RFC9602, October 2024,
              <https://www.rfc-editor.org/info/rfc9602>.






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   [RFC9663]  Colitti, L., Linkova, J., Ed., and X. Ma, Ed., "Using
              DHCPv6 Prefix Delegation (DHCPv6-PD) to Allocate Unique
              IPv6 Prefixes per Client in Large Broadcast Networks",
              RFC 9663, DOI 10.17487/RFC9663, October 2024,
              <https://www.rfc-editor.org/info/rfc9663>.

   [RFC9762]  Colitti, L., Linkova, J., Ma, X., Ed., and D. Lamparter,
              "Using Router Advertisements to Signal the Availability of
              DHCPv6 Prefix Delegation to Clients", RFC 9762,
              DOI 10.17487/RFC9762, June 2025,
              <https://www.rfc-editor.org/info/rfc9762>.

   [RFC9777]  Haberman, B., Ed., "Multicast Listener Discovery Version 2
              (MLDv2) for IPv6", STD 101, RFC 9777,
              DOI 10.17487/RFC9777, March 2025,
              <https://www.rfc-editor.org/info/rfc9777>.

   [TUNTAP]   Krasnyansky, M., "Universal TUN/TAP device driver,
              https://docs.kernel.org/networking/tuntap.html", January
              2000.

   [VETH]     Interfaces, K., "veth(4) - Linux manual page,
              https://man7.org/linux/man-pages/man4/veth.4.html", May
              2025.

Appendix A.  VDL Mode 2 Considerations

   ICAO Doc 9776 is the "Technical Manual for VHF Data Link Mode 2"
   (VDLM2) that specifies an essential radio frequency data link service
   for aircraft and ground stations in worldwide civil aviation air
   traffic management.  The VDLM2 link type is "multicast capable"
   [RFC4861], but with considerable differences from common multicast
   links such as Ethernet and IEEE 802.11.

   First, the VDLM2 link data rate is only 31.5Kbps - multiple orders of
   magnitude less than most modern wireless networking gear.  Second,
   due to the low available link bandwidth only VDLM2 ground stations
   (i.e., and not aircraft) are permitted to send broadcasts, and even
   so only as compact link layer "beacons".  Third, aircraft employ the
   services of ground stations by performing unicast RS/RA exchanges
   upon receipt of beacons instead of listening for multicast RA
   messages and/or sending multicast RS messages.

   This beacon-oriented unicast RS/RA approach is necessary to conserve
   the already-scarce available link bandwidth.  Moreover, since the
   numbers of beaconing ground stations operating within a given spatial
   range must be kept as sparse as possible, it would not be feasible to
   have different classes of ground stations within the same region



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   observing different protocols.  It is therefore highly desirable that
   all ground stations observe a common language of RS/RA as specified
   in this document.

   Note that links of this nature may benefit from compression
   techniques that reduce the bandwidth necessary for conveying the same
   amount of data.  The IETF lpwan working group is considering possible
   alternatives: [https://datatracker.ietf.org/wg/lpwan/documents].

Appendix B.  Client-Proxy/Server Isolation Through Link-Layer Address
             Mapping

   Per [RFC4861], IPv6 ND control messages may be sent to either a
   multicast or unicast link-scoped IPv6 Destination Address.  However,
   IPv6 ND messaging should be coordinated between the Client and Proxy/
   Server only without invoking other nodes on the underlay network.
   This implies that Client-Proxy/Server control messaging should be
   isolated and not overheard by other nodes on the link.

   To support Client-Proxy/Server isolation on some links, Proxy/Servers
   can maintain an OMNI-specific unicast link-layer address ("MSADDR").
   For Ethernet-compatible links, this specification reserves one
   Ethernet unicast address TBD5 (see: IANA Considerations).  For non-
   Ethernet statically-addressed links MSADDR is reserved per the
   assigned numbers authority for the link-layer addressing space.  For
   still other links, MSADDR may be dynamically discovered through other
   means, e.g., link layer beacons.

   Clients map the L3 addresses of all IPv6 ND messages they send (i.e.,
   both multicast and unicast) to MSADDR instead of to an ordinary
   unicast or multicast link-layer address.  In this way, all of the
   Client's IPv6 ND messages will be received by Proxy/Servers that are
   configured to accept carrier packets destined to MSADDR.  Note that
   multiple Proxy/Servers on the link could be configured to accept
   carrier packets destined to MSADDR, e.g., as a basis for supporting
   redundancy.

   Therefore, Proxy/Servers must accept and process carrier packets
   destined to MSADDR, while all other devices must not process carrier
   packets destined to MSADDR.  This model has well-established
   operational experience in Proxy Mobile IPv6 (PMIP)
   [RFC5213][RFC6543].









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Appendix C.  IPv4 as an OAL Encapsulation Service

   Throughout the document, IPv6 encapsulation per [RFC2473] is assumed
   as the OMNI Adaptation Layer (OAL) encapsulation service.  At first
   glance, the minimum 40 octets needed for encapsulation may seem
   excessive however the full OAL encapsulation headers rarely appear
   over the wire due to effective header compression.

   Still, the question may arise as to whether IPv4 encapsulation per
   [RFC2003] could be applied instead with OMNI encapsulation Type OMNI-
   IP4.  After all, the IPv4 header is significantly smaller than even
   the smallest IPv6 header plus extensions.  However, IPv4 provides
   only 32-bit addresses useful for OAL addressing whereas IPv6 provides
   128-bits allowing for full MLA addresses along with their security
   and uniqueness properties.

   IPv4 as an OAL encapsulation service may therefore be suitable for
   small networks where adaptation layer routers operate based on 32-bit
   router IDs deployed through well-managed assignments.  However, IPv4
   does not honor the Flow Label and IPv4 links could configure MTUs as
   small as 68 octets.  An OAL IPv4 header plus extensions would also
   not be as compressible as for IPv6, therefore resulting in extra cost
   for carrying uncompressible IPv4 header information in the data
   plane.

Appendix D.  Change Log

   << RFC Editor - remove prior to publication >>

   Differences from earlier versions:

   Draft -75 to -80
      *  Final version; future updates to appear in amendments draft.

      *  MLA routes now routable via the OMNI link.

      *  MLA routes now discovered within the OMNI interface.

      *  ICMPv6 error messages now appear as standalone control messages
         and not appended to an IPv6 ND message.

      *  Specified AFV state management procedures.

   Draft -74 to -75
      *  OMNI option now includes "OMNI Length" and "Auth Offset".

   Draft -70 to -74
      *  Support marking non IPv6 ND messages as control.



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      *  OMNI interface LLA clarifications.

   Draft -69 to -70
      *  Eliminated distinction between "gen/sec blocks" in OMNI option.

      *  Restored the DHCPv6 message sub-option.

      *  Included rules for processing IPv6 ND message checksums.

      *  Corrections to HMAC/SEND authentication and OMNI Checksum.

      *  Further clarification on Segment Routing.

Author's Address

   Fred L. Templin (editor)
   The Boeing Company
   P.O. Box 3707
   Seattle, WA 98124
   United States of America
   Email: fltemplin@acm.org






























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