



Network Working Group                                 F. L. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                         20 October 2025
Expires: 23 April 2026


              Automatic Extended Route Optimization (AERO)
                      draft-templin-6man-aero3-47

Abstract

   This document specifies an Automatic Extended Route Optimization
   (AERO) service for IP internetworking over Overlay Multilink Network
   (OMNI) Interfaces.  AERO/OMNI use IPv6 Neighbor Discovery (IPv6 ND)
   for control plane messaging over the OMNI virtual link.  Router
   discovery and neighbor coordination are employed for network
   admission and to manage the OMNI link forwarding and routing systems.
   Secure multilink path selection, multinet traversal, mobility
   management, multicast forwarding, multihop operation and route
   optimization are naturally supported through dynamic neighbor cache
   updates on a per flow basis.  AERO is a widely-applicable service
   especially well-suited for air/land/sea/space mobility applications
   including aviation, intelligent transportation systems, mobile end
   user devices, space exploration and many others.

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 23 April 2026.

Copyright Notice

   Copyright (c) 2025 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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   4.  Automatic Extended Route Optimization (AERO)  . . . . . . . .  19
     4.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  19
     4.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  20
       4.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  20
       4.2.2.  AERO Addressing . . . . . . . . . . . . . . . . . . .  24
       4.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  25
       4.2.4.  Segment Routing Topologies (SRTs) . . . . . . . . . .  27
       4.2.5.  Segment Routing For OMNI Link Selection . . . . . . .  28
     4.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  28
     4.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  31
       4.4.1.  AERO Gateway Behavior . . . . . . . . . . . . . . . .  31
       4.4.2.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  31
       4.4.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  32
     4.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  32
       4.5.1.  AERO/OMNI Control Plane Messages  . . . . . . . . . .  34
       4.5.2.  OMNI Neighbor Window Synchronization  . . . . . . . .  36
     4.6.  OMNI Interface Encapsulation and Fragmentation  . . . . .  37
     4.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  39
     4.8.  OMNI Interface Data Origin Authentication . . . . . . . .  40
     4.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  40
     4.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  41
       4.10.1.  Client Forwarding Algorithm  . . . . . . . . . . . .  43
       4.10.2.  Proxy/Server and Relay Forwarding Algorithm  . . . .  44
       4.10.3.  Gateway Forwarding Algorithm . . . . . . . . . . . .  46
     4.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  48
     4.12. AERO Mobility Service Coordination  . . . . . . . . . . .  51
       4.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  51
     4.13. AERO Address Resolution, Multilink Forwarding and Route
            Optimization . . . . . . . . . . . . . . . . . . . . . .  53
       4.13.1.  Multilink Address Resolution . . . . . . . . . . . .  55
       4.13.2.  Multilink Forwarding . . . . . . . . . . . . . . . .  61
       4.13.3.  Mobile Ad-hoc Network (MANET) Forwarding . . . . . .  71
       4.13.4.  AERO Route Optimization  . . . . . . . . . . . . . .  72
     4.14. Neighbor Unreachability Detection (NUD) . . . . . . . . .  75



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     4.15. Mobility Management and Quality of Service (QoS)  . . . .  76
       4.15.1.  Mobility Update Messaging  . . . . . . . . . . . . .  76
       4.15.2.  Announcing Link-Layer Information Changes  . . . . .  77
       4.15.3.  Bringing New Links Into Service  . . . . . . . . . .  77
       4.15.4.  Deactivating Existing Links  . . . . . . . . . . . .  78
       4.15.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  78
       4.15.6.  Accommodating Path Changes . . . . . . . . . . . . .  79
     4.16. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  80
       4.16.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  80
       4.16.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  81
       4.16.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  82
     4.17. Operation over Multiple OMNI Links  . . . . . . . . . . .  82
     4.18. DNS Considerations  . . . . . . . . . . . . . . . . . . .  83
     4.19. Transition/Coexistence Considerations . . . . . . . . . .  84
     4.20. Proxy/Server-Gateway Bidirectional Forwarding
            Detection  . . . . . . . . . . . . . . . . . . . . . . .  84
     4.21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  85
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  85
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  85
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  86
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  88
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  90
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  90
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  92
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . .  99
     A.1.  Implementation Strategies for Route Optimization  . . . .  99
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . . 100
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . . 100
     A.4.  AERO Critical Infrastructure Considerations . . . . . . . 100
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . . 101
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . . 102
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 104
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 105

1.  Introduction

   Automatic Extended Route Optimization (AERO) fulfills the
   requirements of Distributed Mobility Management (DMM) [RFC7333] and
   route optimization [RFC5522] for air/land/sea/space mobility
   applications including aeronautical networking intelligent
   transportation systems, home network users, enterprise mobile device
   users, space exploration and many others.  AERO is a secure
   internetworking and mobility management service that employs the
   Overlay Multilink Network Interface (OMNI) [I-D.templin-6man-omni3]
   with its Non-Broadcast, Multiple Access (NBMA) virtual link model.






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   The OMNI link is an adaptation layer virtual overlay manifested by
   IPv6 encapsulation over a network-of-networks concatenation of
   underlay Internetworks.  Nodes on the link can exchange original IP
   packets or parcels (see: [I-D.templin-6man-parcels2] and
   [I-D.templin-intarea-parcels2]) as single-hop neighbors; both IP
   protocol versions (IPv4 and IPv6) are supported.  The OMNI Adaptation
   Layer (OAL) supports multilink operation for increased reliability
   and path optimization while providing fragmentation and reassembly
   services to support improved performance and Maximum Transmission
   Unit (MTU) diversity.  This specification provides a mobility service
   architecture companion to the OMNI specification.

   The AERO service connects Clients as OMNI link end systems via Proxy/
   Servers and Relays as intermediate systems as necessary; AERO further
   employs Gateways that interconnect diverse Internetworks as OMNI link
   segments through OAL forwarding at a layer below IP.  Each node's
   OMNI interface supports the operation of IPv6 Neighbor Discovery
   (IPv6 ND) [RFC4861] as the mobility service control message protocol.
   A Client's OMNI interface can be configured over multiple underlay
   interfaces, and therefore appears as a single interface with multiple
   link layer addresses.  Each link layer address is subject to change
   due to mobility and/or multilink fluctuations, and link layer address
   changes are signaled by ND messaging the same as for any IPv6 link.

   AERO provides a secure virtual link management service where mobile
   node Clients use Proxy/Servers acting as proxys and/or designated
   routers while correspondent nodes on foreign networks use any Relay
   on the link for efficient communications.  Foreign network
   correspondent nodes forward original IP packets destined to other
   AERO nodes via the nearest Relay, which forwards them through the
   cloud.  Mobile node Clients discover shortest paths to OMNI link
   neighbors through AERO route optimization.  Both unicast and
   multicast communications are supported.

   Correspondent nodes on foreign networks configure Provider Aggregated
   (PA) IP addresses from Foreign Network Prefixes (FNPs) advertised by
   Relays.  Mobile node Clients register Provider-Independent (PI)
   Mobile Network Prefixes (MNPs) with Mobility Anchor Point (MAP)
   Proxy/Servers to support global mobile Internetworking.  Clients also
   obtain adaptation layer addresses from Proxy Network Prefixes (PNPs)
   aggregated by First Hop Segment (FHS) Proxy/Servers.

   AERO Gateways peer with Proxy/Servers in a secured private BGP
   overlay routing instance to establish a Segment Routing Topology
   (SRT) virtual spanning tree over the underlay Internetworks of one or
   more disjoint administrative domains concatenated as a single unified
   OMNI link.  Each OMNI link instance is characterized by a set of
   Mobility Service Prefixes (MSPs) common to all mobile nodes and from



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   which MNP sub-prefixes are derived.  Relays provide an optimal route
   from correspondent nodes on foreign links/networks to mobile or fixed
   nodes on the local OMNI link.  From the perspective of underlay
   Internetworks, each Relay appears as the source of a route to the
   MSP; uplink traffic to mobile nodes is therefore naturally routed to
   the nearest Relay.

   AERO operates over OMNI links that span private-use Internetworks
   and/or public Internetworks such as the global IPv4 and IPv6
   Internets.  In both cases, Clients may be located behind Network
   Address Translators (NATs) on the path to their associated Proxy/
   Servers and/or peers.  A means for robust traversal of NATs while
   avoiding "triangle routing" and critical infrastructure traffic
   concentration through a service known as route optimization is
   therefore provided.

   AERO assumes the use of PIM Sparse Mode in support of multicast
   communication.  In support of Source Specific Multicast (SSM) when a
   Mobile Node is the source, AERO route optimization ensures that a
   shortest-path multicast tree is established with provisions for
   mobility and multilink operation.  In all other multicast scenarios
   there are no AERO dependencies.

   AERO provides a secure aeronautical internetworking service for both
   manned and unmanned aircraft, where the aircraft is treated as a
   mobile node (MN) that can connect airborne Internet of Things (IoT)
   sub-networks.  AERO is also applicable to a wide variety of other use
   cases.  For example, it can be used to coordinate the links of mobile
   nodes (e.g., cellphones, tablets, laptop computers, etc.) that
   connect into a home enterprise network via public access networks
   with Virtual Private Network (VPN) or open Internetwork services
   enabled according to the appropriate security model.  AERO also
   supports terrestrial vehicular, urban air mobility and mobile
   pedestrian communication services for intelligent transportation
   systems [RFC9365].  Other applicable use cases including home and
   small office networks, enterprise networks and many others represent
   additional large classes of potential AERO/OMNI users.

   Along with OMNI, AERO provides secured optimal routing support for
   the "6 M's of Modern Internetworking", including:

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






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

   3.  Mobility - a mobile node'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 nodes belonging to the same interest group,
       but without disturbing other nodes 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 large
       packets 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.

   The following numbered sections present the AERO specification.  The
   appendices at the end of the document are non-normative.

2.  Terminology

   The terminology in the normative references applies; especially, the
   OMNI specification terminology [I-D.templin-6man-omni3] and the IPv6
   Neighbor Discovery (IPv6 ND) [RFC4861] node variables, protocol
   constants and message types (including Router Solicitation (RS),
   Router Advertisement (RS), Neighbor Solicitation (NS), Neighbor
   Advertisement (NA), unsolicited NA (uNA) and Redirect) are cited
   extensively throughout.  AERO further introduces new "pseudo" IPv6 ND
   message types Multilink Initiate (MI), Multilink Respond (MR) and
   Multilink Control (NC) with formats identical to the standard uNA
   message but with different Code values.  These messages are used to
   control adaptation layer functions only and are never exposed to the
   network layer.






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   Throughout the document, the simple terms "(Proxy/)Client", "Proxy/
   Server", "Gateway" and "Relay" refer to "AERO/OMNI (Proxy/)Client",
   "AERO/OMNI Proxy/Server", "AERO/OMNI Gateway" and "AERO/OMNI Relay",
   respectively.  Capitalization is used to distinguish these terms from
   other common Internetworking uses in which they appear in lower case,
   and implies that the node in question both configures an OMNI
   interface and engages the OMNI Adaptation Layer (OAL).

   The terms "All-Routers multicast", "All-Nodes multicast", "Solicited-
   Node multicast" and "Subnet-Router anycast" are defined in [RFC4291].

   The term "IP" refers generically to either Internet Protocol version
   (IPv4 [RFC0791] or IPv6 [RFC8200]) for specification elements that
   apply equally to both.

   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 introduces an "adaptation layer" positioned
   below the network layer but 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 terms "IP jumbogram", "advanced jumbo (AJ)" and "IP parcel" refer
   to special packet formats that enable a new link model for the
   Internet as discussed in [I-D.templin-6man-parcels2]
   [I-D.templin-intarea-parcels2].

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

   IPv6 Neighbor Discovery (IPv6 ND)
      a control message service for coordinating neighbor relationships
      between nodes connected to a common link.  AERO uses the IPv6 ND
      messaging service specified in [RFC4861] in conjunction with the
      OMNI extensions specified in [I-D.templin-6man-omni3].

   IPv6 Prefix Delegation (IPv6 PD)
      a networking service for delegating IPv6 prefixes to nodes on the
      link.  AERO nodes apply the IPv6 PD service provided by DHCPv6
      [I-D.ietf-dhc-rfc8415bis][RFC9762] in conjunction with OMNI
      interface IPv6 ND.






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   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 network model.  Also known as "layer
      3", "IP layer", etc.

   L2
      The Data Link layer in the OSI network model.  Also known as
      "layer 2", "link layer", "sub-IP layer", etc.

   Adaptation Layer
      An encapsulation mid-layer that adapts L3 to a diverse collection
      of L2 underlay interfaces and their encapsulations.  (No layer
      number is assigned, since numbering was an artifact of the legacy
      reference model that need not carry forward in the modern
      architecture.)  The adaptation layer sees the network layer as
      "L3" and sees all link layer encapsulations as "L2
      encapsulations", which may include UDP, IP and true link layer
      (e.g., Ethernet, etc.) headers.

   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 Mobility Service over the OMNI
      link.  Physical and/or data link level security is assumed and
      sometimes referred to as "protected spectrum" for wireless
      domains.  Private enterprise networks and ground domain aviation
      service networks may provide multiple secured IP hops between the
      Client's 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.










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   Internetwork (INET)
      a connected network region with a coherent IP addressing plan that
      provides transit forwarding services between (M)ANETs and AERO/
      OMNI nodes that coordinate with the Mobility Service over
      unprotected media.  No physical and/or data link level security is
      assumed, therefore security must be applied by the network and/or
      higher layers.  The global public Internet itself is an example.

   End-user Network (ENET)
      a simple or complex "downstream" network tethered to a Client as a
      single logical unit that travels together.  The ENET could be as
      simple as a single link connecting a single end system, or as
      complex as a large network with many links, routers, bridges and
      end user devices.  The ENET 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.  ENETs can also support a recursively-
      descending chain of additional Clients such that the ENET of an
      upstream Client is seen as the (M)ANET of a downstream Client.

   *NET
      a "wildcard" term used when a given specification applies equally
      to all MANET/ANET/INET cases.  From the Client's perspective, *NET
      interfaces are "upstream" interfaces that connect the Client to
      the Mobility Service, while ENET interfaces are "downstream"
      interfaces that the Client uses to connect downstream *NETs which
      may connect other Clients.  Local communications between
      correspondents within the same *NET can often be conducted based
      on IPv6 ULAs [RFC4193] or MLAs [I-D.templin-6man-mla].

   underlay network/interface
      a *NET or ENET network/interface over which an OMNI interface is
      configured.  The OMNI interface is seen as a network layer (L3)
      interface by the IP layer, and the OMNI adaptation layer sees the
      underlay interface as a data link layer (L2) interface.  The
      underlay interface either connects directly to the physical/
      virtual communications media or coordinates with another node that
      hosts the media.

   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.





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   OMNI link
      the same as defined in [I-D.templin-6man-omni3].  The OMNI link
      employs IPv6 encapsulation to traverse intermediate systems in a
      spanning tree over underlay network segments the same as a bridged
      campus LAN.  AERO nodes on the OMNI link appear as single-hop
      neighbors at the network layer even though they may be separated
      by many underlay network hops; AERO nodes can employ Segment
      Routing [RFC8402][RFC8754] to navigate between different OMNI
      links, and/or to cause packets to visit selected waypoints within
      the same OMNI link.

   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 Ethernet (veth) 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 each link.  The OMNI interface configures a
      Maximum Transmission Unit (MTU) and an Effective MTU to Receive
      (EMTU_R) the same as any interface and also configures a randomly-
      initialized built-in Ethernet link layer address.  The OMNI
      interface assigns an LLA the same as for any IPv6 interface and
      assigns an MLA for adaptation layer addressing over its underlay
      networks.  The OMNI interface further configures any unicast or
      anycast PNP addresses acquired through address autoconfiguration
      as adaptation layer addresses that are not made visible to the
      network layer.  Since OMNI interface addresses are managed for
      uniqueness, 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 spanning
      multiple segments of an extended OMNI link.





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   OMNI Option
      a pseudo IPv6 ND option providing multilink parameters for the
      OMNI interface.  The OMNI option is appended to the end of an IPv6
      ND message during OAL encapsulation such that it appears
      immediately following the final message option.

   (network) partition
      frequently, underlay networks such as large corporate enterprise
      networks are sub-divided internally into separate isolated
      partitions (a technique also known as "network segmentation").
      Each partition is fully connected internally but disconnected from
      other partitions, and there is no requirement that separate
      partitions maintain consistent Internet Protocol and/or addressing
      plans.  (Each partition is seen as a separate OMNI link segment as
      discussed throughout this document.)

   (OMNI) L2 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 or ENET underlay network
      partition.  Common L2 encapsulation combinations include UDP/IP/
      Ethernet, etc. using a port/protocol/type number for OMNI.

   L2 address (L2ADDR)
      an address that appears in the L2 encapsulation for an underlay
      interface and also in IPv6 ND message OMNI options.  L2ADDR can be
      either an IP address for IP encapsulations or an IEEE EUI address
      [EUI] for direct data link encapsulation.  (When UDP/IP
      encapsulation is used, the UDP port number is regarded as an
      extension of the IP L2ADDR.)

   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 L2
      encapsulation, or following L2 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.  (Note that control message atomic
      fragments also omit the Extended Fragment Header over secured
      spanning tree links.)

   (L2) carrier packet
      an encapsulated OAL packet/fragment following L2 encapsulation or
      prior to L2 decapsulation.  OAL sources and destinations exchange
      carrier packets over underlay interfaces, and may be separated by
      one or more OAL intermediate systems.  OAL intermediate systems
      re-encapsulate OAL packets/fragments during forwarding by removing
      the L2 headers of carrier packets from a previous hop underlay
      network and replacing them with new L2 headers for the next hop
      underlay network.  Carrier packets may themselves be subject to
      fragmentation and reassembly in L2 underlay networks at a layer
      below the OAL.  Carrier packets sent over unsecured paths use OMNI
      protocol L2 encapsulations, while those sent over secured paths
      use L2 security encapsulations such as IPsec [RFC4301].

   OAL source
      an OMNI interface acts as an OAL source when it encapsulates
      original IP packets to form OAL packets, then performs OAL
      fragmentation and L2 encapsulation to create carrier packets.
      Every OAL source is also an OAL end system.

   OAL destination
      an OMNI interface acts as an OAL destination when it decapsulates
      carrier packets, then performs OAL reassembly/decapsulation to
      restore the original IP packet.  Every OAL destination is also an
      OAL end system.


















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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 OAL packet/fragment, then re-encapsulates in new L2
      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.

   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
      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 alternatively
      be used subject to certain limitations (see:
      [I-D.templin-6man-omni3]).  OMNI links that connect to the global
      Internet advertise their MSPs to interdomain routing peers.

   Mobile Network Prefix (MNP)
      a longer IP GUA prefix derived from an MSP (e.g.,
      2001:db8:1000:2000::/56, 2002:192.0.2.8::/48, etc.) and delegated
      to an AERO Client.

   Proxy Network Prefix (PNP)
      a ULA or GUA IPv6 prefix assigned to a Proxy/Server that connects
      local *NET Client groups to the rest of the OMNI link.  Each
      Proxy/Server assigns a unique PNP.  Clients request address
      delegations from the PNP to support adaptation layer addressing.
      Proxy/Servers use IP network address/prefix translation
      [RFC6145][RFC6146] [RFC6147][RFC6296] to translate MLAs configured
      internally by Clients into PNP addresses for external
      communications.

   Foreign Network Prefix (FNP)
      a global IP prefix not covered by a MSP and assigned to a link or
      network outside of the AERO/OMNI domain.  Relays advertise any of
      their associated FNPs into the AERO/OMNI routing system and
      forward packets between MNP mobile or fixed nodes on the OMNI link
      and FNP correspondent nodes on other links.



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   Subnet Router Anycast (SRA) Address
      An IPv6 address taken from an FNP/MNP/PNP in which the remainder
      of the address beyond the final bit of the prefix is set to the
      value "all-zeros".  For example, the SRA for 2001:db8:1::/48 is
      simply 2001:db8:1:: (i.e., with the 80 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].

   Interface Identifier (IID)
      the least significant 64 bits of an IPv6 address, as specified in
      the IPv6 addressing architecture [RFC4291].

   Proxy-Aggregated (PA) Address
      a PNP address delegated to a Client by an FHS Proxy/Server is
      considered Proxy-Aggregated (PA).  The Client configures the PA
      addresses as adaptation layer addressing and the FHS Proxy/Server
      translates between MLAs and PA addresses via Network Prefix
      Translation for IPv6 (NPTv6) [RFC6296].

   Provider-Independent (PI) Address
      a GUA allocated from an MNP delegated to a Client via a MAP Proxy/
      Server is considered Provider-Independent (PI) or "Proxy/Server-
      Independent".  The Client assigns PI addresses to (downstream)
      ENET interfaces and can sub-delegate the MNP to downstream ENET
      nodes.

   AERO node
      a node that is connected to an OMNI link and participates in the
      AERO internetworking and mobility service.

   (AERO) (Proxy/)Client
      an AERO node that configures an OMNI interface over one or more
      underlay interfaces and requests PNP address and MNP prefix
      delegations from AERO Proxy/Servers.  The Client assigns an LLA
      and MLA to the OMNI interface for use in IPv6 ND exchanges with
      other AERO nodes and forwards original IP packets to
      correspondents according to OMNI interface neighbor cache state.
      The Client coordinates with Proxy/Servers and/or other Clients
      over upstream ANET/INET interfaces and may also provide Proxy
      services for other Clients over downstream interfaces.

   (AERO) Proxy/Server
      an AERO node that provides a proxying service between AERO Clients
      and external peers on its Client-facing (M)ANET interfaces (i.e.,
      in the same fashion as for an enterprise network proxy) as well as
      designated router services for coordination with correspondents on
      its INET-facing interfaces.  (Proxy/Servers in the open INET
      instead configure only a single INET interface and no (M)ANET



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      interfaces.)  The Proxy/Server configures an OMNI interface and
      maintains BGP peerings with Gateways to provide a local anchor
      point for its stable and/or mobile Clients.  All Proxy/Servers
      configure a unique PNP and manage 1x1 mappings of internal MLAs
      and external PNP addresses according to [RFC6296].

   (AERO) Relay
      an AERO Proxy/Server that provides forwarding services between
      nodes reached via the OMNI link and correspondents on foreign
      links/networks.  AERO Relays maintain BGP peerings with Gateways
      the same as Proxy/Servers.  Relays also run a dynamic routing
      protocol to discover any Foreign Network Prefix (FNP) routes in
      service on other links/networks, advertise OMNI link MSP(s) to
      other links/networks, and redistribute FNPs discovered on other
      links/networks into the OMNI link BGP routing system.  (Relays
      that connect to major Internetworks such as the global IPv6 or
      IPv4 Internets can also be configured to advertise "default"
      routes into the OMNI link BGP routing system.)

   (AERO) Gateway
      a BGP hub autonomous system node that also provides OAL forwarding
      services for nodes on an OMNI link.  Gateways forward OAL packets/
      fragments between OMNI link segments as OAL intermediate systems
      while decrementing the OAL IPv6 header Hop Limit but without
      decrementing the network layer IP TTL/Hop Limit.  Gateways peer
      with Proxy/Servers and other Gateways to form an IPv6-based OAL
      spanning tree over all OMNI link segments and to discover the set
      of all FNP/MNP/PNP prefixes in service.  Gateways process OAL
      packets/fragments received over the secured spanning tree that are
      addressed to themselves, while forwarding all other OAL packets/
      fragments to the next hop also via the secured spanning tree.
      Gateways forward OAL packets/fragments received over the unsecured
      spanning tree to the next hop either via the unsecured spanning
      tree or via direct encapsulation if the next hop is on the same
      OMNI link segment.  It is important to note that all Gateways are
      also Proxy/Servers, but only those Proxy/Servers configured as
      intermediate nodes in the spanning tree are considered Gateways.

   First-Hop Segment (FHS) Client
      a Client that initiates communications with a target peer by
      sending control messages to establish reverse-path multilink
      forwarding state in OMNI link intermediate systems on the path to
      the target.  Note that in some arrangements the Client's (FHS)
      Proxy/Server (and not the Client itself) initiates the exchange.

   Last-Hop Segment (LHS) Client
      a Client that responds to a communications request from a source
      peer's initiation by returning a response message to establish



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      forward-path multilink forwarding state in OMNI link intermediate
      systems on the path to the source.  Note that in some arrangements
      the Client's (LHS) Proxy/Server (and not the Client itself)
      returns the response.

   First-Hop Segment (FHS) Proxy/Server
      a Proxy/Server for an FHS Client's underlay interface that
      forwards the Client's OAL packets into the segment routing
      topology.  FHS Proxy/Servers also act as intermediate forwarding
      systems to facilitate RS/RA exchanges between a Client and its MAP
      Proxy/Server.

   Last-Hop Segment (LHS) Proxy/Server
      a Proxy/Server for an underlay interface of an LHS Client that
      forwards OAL packets received from the segment routing topology to
      the Client over that interface.

   Mobility Anchor Point (MAP) Proxy/Server
      a Proxy/Server selected by a Client that injects the Client's MNP
      into the BGP routing system and provides both forwarding and
      mobility services for any *NET underlay interfaces that register
      the MNP.  Clients often select the first FHS Proxy/Server they
      coordinate with to serve in the MAP role as all FHS Proxy/Servers
      are equally capable candidates to serve as a MAP.  The Client can
      instead select any available Proxy/Server for the OMNI link as
      there is no requirement that the MAP must also be one of the
      Client's FHS Proxy/Servers.  This flexible arrangement supports a
      fully distributed mobility management service.

   Segment Routing Topology (SRT)
      a Multinet OMNI link forwarding region between FHS and LHS Proxy/
      Servers.  FHS/LHS Proxy/Servers and SRT Gateways span the OMNI
      link on behalf of communicating peer nodes.  The SRT maintains a
      spanning tree established through BGP peerings between Gateways
      and Proxy/Servers.  Each SRT leaf segment includes Gateways in a
      "hub" and Proxy/Servers in "spokes", while adjacent segments are
      interconnected by Gateway-Gateway peerings.  The BGP peerings are
      configured over both secured and unsecured underlay network paths
      such that a secured spanning tree is available for critical
      control messages while other messages can use the unsecured
      spanning tree.

   Mobile Node (MN)
      an AERO Client and all of its downstream-attached networks that
      move together as a single unit, i.e., an end system and its
      connected IoT sub-networks.





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   Mobile Router (MR)
      a MN's on-board router that forwards original IP packets between
      any downstream-attached networks and the OMNI link.  The MR is the
      MN entity that hosts the AERO Client.

   Address Resolution Source (ARS)
      the node nearest the original source that initiates OMNI link
      address resolution.  The ARS may be a Proxy/Server or Relay for
      the source, or may be the source Client itself.  The ARS is often
      (but not always) also the same node that becomes the FHS source
      during route optimization.

   Address Resolution Target (ART)
      the node toward which address resolution is directed.  The ART may
      be a Relay or the target Client itself.  The ART is often (but not
      always) also the same node that becomes the LHS target during
      route optimization.

   Address Resolution Responder (ARR)
      the node that responds to address resolution requests on behalf of
      the ART.  The ARR may be a Relay, the ART itself, or the ART's
      current MAP Proxy/Server.  Note that a MAP Proxy/Server can assume
      the ARR role even if it is located on a different SRT segment than
      the ART.  The MAP Proxy/Server assumes the ARR role only when it
      receives an RS message from the ART with the 'ARR' flag set (see:
      [I-D.templin-6man-omni3]).

   Potential Router List (PRL)
      a geographically and/or topologically referenced list of addresses
      of all Proxy/Servers within the same OMNI link segment.  Each OMNI
      link segment has its own PRL.

   Distributed Mobility Management (DMM)
      a BGP-based overlay routing service coordinated by Proxy/Servers
      and Gateways that tracks all Proxy/Server-to-Client associations.

   Mobility Service (MS)
      the collective set of all Proxy/Servers, Gateways and Relays that
      provide the AERO Service to Clients.

   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 of the Flow Label, Source
      Address and Destination Address fields enable efficient IPv6 flow
      classification.  The IPv6 Flow Label Specification is observed per
      [RFC6437] [RFC6438].




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   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.

   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 L2
      address, with the pair established based on adaptation layer IPv6
      ND control messaging.  The AFV also caches underlay interface
      pairwise Identification sequence number parameters to support
      carrier packet filtering.

   AERO Flow Vector Index (AFVI)
      A 2-octet 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 L2
      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.  In the 4-octet form, the maximum AFVI value is limited
      to (2**31-1) since the most significant bit is reserved as a flag.

3.  Requirements

   OMNI interfaces should limit the size of their IPv6 ND 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 exceed this size,
   the OMNI interface forwards the information as multiple smaller IPv6
   ND 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.







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   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.

4.  Automatic Extended Route Optimization (AERO)

   The following sections specify the operation of IP over OMNI links
   using the AERO service:

4.1.  AERO Node Types

   AERO (Proxy/)Clients can be deployed as fixed infrastructure nodes
   close to end systems, or as Mobile Nodes (MNs) that can change their
   network attachment points dynamically.  AERO Clients configure OMNI
   interfaces over underlay interfaces with addresses that may change
   due to mobility.  AERO Clients receive PNP addresses from their
   Proxy/Servers.  AERO Clients that obtain PI MNPs register them with
   the AERO service, and distribute the MNPs to ENETs (which may connect
   other Clients).  AERO Clients can also provide Proxy services for
   other Clients on downstream-attached networks.

   AERO Gateways, Proxy/Servers and Relays are infrastructure elements
   in *NET boundary (or standalone INET) deployments and hence have INET
   addresses that rarely (if ever) change.  Together, they provide
   access to the AERO service OMNI link virtual overlay for connecting
   AERO Clients.  AERO Gateways (together with Proxy/Servers and Relays)
   provide the secured backbone supporting infrastructure for an OMNI
   link Segment Routing Topology (SRT) spanning tree.

   AERO Gateways are Proxy/Servers deployed as OMNI link intermediate
   systems that forward packets both within the same SRT segment and
   between disjoint SRT segments based on an IPv6 encapsulation mid-
   layer known as the OMNI Adaptation Layer (OAL).  The OMNI interface
   and OAL provide an adaptation layer forwarding service that the
   network layer perceives as L2 bridging, since the inner IP TTL/Hop
   Limit is not decremented.  Each Gateway peers with Proxy/Servers,
   Relays and other Gateways in a dynamic routing protocol instance as a
   Distributed Mobility Management (DMM) service for the list of active
   MNPs (see: Section 4.2.3).  Gateways assign one or more Mobility
   Service Prefixes (MSPs) to the OMNI link and configure IPsec tunnels
   with Proxy/Servers, Relays and other Gateways; they further maintain
   forwarding table entries for each FNP/MNP/PNP prefix in service on
   the OMNI link.






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   AERO Proxy/Servers distributed across one or more SRT segments
   provide default forwarding and mobility/multilink services for AERO
   Client mobile nodes.  Each Proxy/Server acts as either an OMNI link
   intermediate system or end system according to the service model
   selected by the Client.  Each Proxy/Server also peers with Gateways
   in an adaptation layer dynamic routing protocol instance to advertise
   its list of associated MNPs (see Section 4.2.3).  MAP Proxy/Servers
   provide prefix delegation services and track the mobility/multilink
   profiles of each of their associated Clients, where each delegated
   prefix becomes an MNP taken from an MSP.  Proxy/Servers at *NET
   boundaries provide a primary forwarding service for (M)ANET Client
   communications with peers in external INETs.  Proxy/Servers in open
   INETs provide an authentication service for IPv6 ND messages but
   should be considered as a less preferred data plane forwarding
   service when a Client cannot forward directly to an INET peer.
   Source Clients securely coordinate with target Clients by sending
   control messages via a First-Hop Segment (FHS) Proxy/Server which
   forwards them over the SRT spanning tree to a Last-Hop Segment (LHS)
   Proxy/Server which finally forwards them to the target.

   AERO Relays are Proxy/Servers that provide forwarding services to
   exchange original IP packets between the OMNI link and fixed or
   mobile nodes on other links/networks.  Relays run a dynamic routing
   protocol to discover any FNP prefixes in service on foreign links/
   networks, and Relays that connect to larger Internetworks (such as
   the Internet) may originate default routes.  The Relay redistributes
   OMNI link MSP(s) into other links/networks, and redistributes FNPs
   via OMNI link Gateway BGP peerings.

4.2.  The AERO Service over OMNI Links

4.2.1.  AERO/OMNI Reference Model

   Figure 1 presents the basic OMNI link reference model:

















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                         +-----------------+
                         | AERO Gateway G1 |
                         | Nbr: S1, S2, P1 |
                         |(X1->S1; X2->S2) |
                         |      MSP M1     |
                         +--------+--------+
       +--------------+           |            +--------------+
       |  AERO P/S S1 |           |            |  AERO P/S S2 |
       |  Nbr: C1, G1 |           |            |  Nbr: C2, G1 |
       |  default->G1 |           |            |  default->G1 |
       |    X1->C1    |           |            |    X2->C2    |
       +-------+------+           |            +------+-------+
               |       OMNI link  |                   |
       X===+===+==================+===================+===+===X
           |                                              |
     +-----+--------+                            +--------+-----+
     |AERO Client C1|                            |AERO Client C2|
     |    Nbr: S1   |                            |   Nbr: S2    |
     | default->S1  |                            | default->S2  |
     |    MNP X1    |                            |    MNP X2    |
     +------+-------+                            +-----+--------+
            |                                          |
           .-.                                        .-.
        ,-(  _)-.     +-------+     +-------+      ,-(  _)-.
     .-(_  IP   )-.   |IP end |     |IP end |    .-(_  IP   )-.
   (__    ENET     )--|system |     |system |--(__    ENET     )
      `-(______)-'    +-------+     +-------+     `-(______)-'

                    Figure 1: AERO/OMNI Reference Model

   In this model:

   *  the OMNI link is an overlay network service configured over one or
      more underlay SRT segments which may be managed by diverse
      administrative domains using incompatible protocols and/or
      addressing plans.

   *  AERO Gateway G1 aggregates Mobility Service Prefix (MSP) M1,
      discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP
      via BGP peerings over secured tunnels to other Gateways in the SRT
      (not shown).  Together, the set of all Gateways provide the
      backbone for an SRT spanning tree for the OMNI link.









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   *  AERO Proxy/Servers S1 and S2 configure secured tunnels with
      Gateway G1 and also provide mobility, multilink, multicast and
      default router services for the MNPs of their associated Clients
      C1 and C2.  (Proxy/Servers that act as Relays can also advertise
      FNP routes for non-mobile correspondent nodes the same as for MNP
      Clients.)

   *  AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2,
      respectively.  They receive MNP delegations X1 and X2, and also
      act as default routers for their associated physical or internal
      virtual ENETs.  (While not shown, AERO Clients can also be
      recursively nested in an arbitrarily-deep chain of (Proxy/)Clients
      between a Proxy/Server and the ultimate IP end systems.)

   *  IP end systems attach to the ENETs served by Clients C1 and C2,
      respectively.  (Although not depicted here, there may be multiple
      Proxy/Client intermediate systems between Clients C1 and C2 and
      the ultimate IP end systems.)

   An OMNI link configured over a single underlay network appears as a
   single unified link with a consistent addressing plan; all nodes on
   the link can exchange carrier packets via simple L2 encapsulation
   (i.e., following any necessary NAT traversal) since the underlay is
   connected.  In common practice, however, OMNI links are often
   configured over an SRT spanning tree that bridges multiple distinct
   underlay network segments managed under different administrative
   authorities (e.g., as for worldwide aviation service providers such
   as ARINC, SITA, Inmarsat, etc.).  Individual underlay networks may
   also be partitioned internally, in which case each internal partition
   appears as a separate segment.

   The addressing plan of each SRT segment is consistent internally but
   will often bear no relation to the addressing plans of other
   segments.  Each segment is also likely to be separated from others by
   network security devices (e.g., firewalls, proxys, packet filtering
   gateways, etc.), and disjoint segments often have no common physical
   link connections.  Therefore, nodes can only be assured of exchanging
   carrier packets directly with correspondents in the same segment, and
   not with those in other segments.  The only means for joining the
   segments therefore is through inter-domain peerings between AERO
   Gateways.










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   The OMNI link spans multiple SRT segments using the OAL to provide
   the network layer with a virtual abstraction similar to a bridged
   campus LAN.  The OAL is an OMNI interface sublayer that inserts a
   mid-layer IPv6 encapsulation header for inter-segment forwarding
   (i.e., bridging) without decrementing the network layer TTL/Hop Limit
   of the original IP packet.  An example OMNI link SRT is shown in
   Figure 2:

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
  .                                                                           .
 .                                                                             .
 .     .-(::::::::)                .-(::::::::)               .-(::::::::)     .
 .  .-(::::::::::::)-.   +-+    .-(::::::::::::)-.   +-+   .-(::::::::::::)-.  .
 . (::::    FHS    :::)--|G|--(::: Intermediate ::)--|G|--(::::    LHS    :::) .
 .  `-(::::::::::::)-'   +-+    `-(::Segments::)-'   +-+   `-(::::::::::::)-'  .
 .     `-(::::::)-'                `-(::::::)-'               `-(::::::)-'     .
 .           |                                                      |          .
 .         +---+                                                  +---+        .
 .         |P/S|                                                  |P/S|        .
 .         +---+                                                  +---+        .
 .           |                                                      |          .
 .     .-(::::::::)                                          .-(::::::::)      .
 .  .-(: First Hop :)-.  +-------+             +-------+   .-(: Last Hop :)-.  .
 . (::::  Access  ::::)--| Source|             | Target|--(::::  Access  ::::) .
 .  `-(:: Network ::)-'  | Client|             | Client|     (:: Network ::)-' .
 .     `-(::::::)-'      +-------+             +-------+      `-(::::::)-'     .
 .                                                                             .
 .                                                                             .
 .         <--  Segment Routing Topology (SRT) Spanned by OMNI Link -->        .
   .                                                                          .
     . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

          Figure 2: OMNI Link Segment Routing Topology (SRT)

   In the Segment Routing Topology, a source Client connects via a first
   hop access network served by a First Hop Segment (FHS) Proxy/Server.
   The FHS Proxy/Server then forwards to an FHS Gateway which connects
   to an arbitrarily complex set of Intermediate Segments.  Adjacent
   intermediate Segments are joined by intermediate Gateways (not shown)
   that serve as adaptation layer IPv6 routers, with the final segment
   connected by a Last Hop Segment (LHS) Gateway.  The LHS Gateway then
   forwards to an LHS Proxy/Server which in turn connects to the last
   hop access network where the target Client resides.

   Gateway, Proxy/Server and Relay OMNI interfaces are configured over
   both secured tunnels and open INET underlay interfaces within their
   respective SRT segments.  Within each segment, Gateways configure
   "hub-and-spokes" BGP peerings with Proxy/Servers and Relays as



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   "spokes".  Adjacent SRT segments are joined by Gateway-to-Gateway
   peerings to collectively form a spanning tree over the entire SRT.
   The "secured spanning tree" supports authentication and integrity for
   critical control plane messages (and any trailing data plane message
   extensions).  The "unsecured spanning tree" conveys ordinary carrier
   packets without security codes and that must be examined by
   destinations according to data origin authentication procedures.
   AERO nodes can employ route optimization to cause carrier packets to
   take more direct paths between OMNI link neighbors without having to
   follow strict spanning tree paths.

   The network of networks concept emerged from the earliest days of
   Internetworking beginning in the 1960's [KAHN].  The concept has
   carried forward to the present day where the Internet has become
   successful beyond measure.  The AERO Multinet service concatenates
   SRT segments through Gateway-to-Gateway peerings as suggested in the
   "Catenet Model for Internetworking (IEN48)" [CERF].  Catenet
   suggested a concatenation of independent and diverse Internetwork
   "segments" to form a much larger network supporting end-to-end
   services.

   The Catenet vision originally articulated in the 1970's faded into
   obscurity as the Internet evolved in the decades that followed, and
   the adaptation layer was omitted from the architecture.  As a result,
   the Internet has evolved to become a monolithic public routing and
   addressing service interconnecting private domains leading to the
   rise of the middle and a diminished role for end-to-end [RFC3724].
   The adaptation layer manifested by AERO and OMNI now promises to
   restore the best aspects of end-to-end envisioned by Catenet through
   incremental deployment in the modern Internet.

4.2.2.  AERO Addressing

   AERO nodes on OMNI links assign a Link-Local Address (LLA) on the
   OMNI interface as required by [RFC4861].  AERO nodes do not use their
   LLAs for IPv6 ND messaging but instead use a Multilink Local Address
   (MLA).

   AERO nodes assign an MLA to the OMNI interface per
   [I-D.templin-6man-mla].  The node assigns an MLA to an OMNI interface
   the same as suggested for "sites" in the IPv6 scoped addressing
   architecture [RFC4007], i.e., as a single adaptation layer address
   assigned to a virtual interface configured over potentially multiple
   underlying interfaces.

   MLAs are considered as adaptation layer addresses in the architecture
   and can appear as OAL encapsulation addresses, but nodes may also use
   them as the Source and Destination Addresses of original IP packets



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   exchanged between peers within the same OMNI link segment.  Each
   original IP packet with MLA addresses is subject to OAL encapsulation
   with an IPv6 header that also uses MLA addresses.

   AERO Clients receive Proxy Network Prefixes (PNPs) and
   Addresses(PNPADDRs) as well as Mobile Network Prefix (MNP)
   delegations during Proxy/Server RS/RA exchanges.  AERO Clients cache
   per underlay interface PNPADDRs in the adaptation layer and assign
   MNPs to ENET interfaces.

   AERO MSPs, MNPs, FNPs and PNPs are typically based on GUAs, but in
   some cases may be based on IPv4 private addresses [RFC1918] or IPv6
   ULA-C's [RFC4193].

   AERO address selection rules are conducted per [RFC6724] as updated
   by [I-D.ietf-6man-rfc6724-update].

   See [I-D.templin-6man-omni3] for a full discussion of the various
   unicast, anycast and multicast addresses used by AERO nodes on OMNI
   links.

4.2.3.  AERO Routing System

   The AERO routing system comprises a private Border Gateway Protocol
   (BGP) [RFC4271] service coordinated between Gateways as interior
   nodes and Proxy/Servers and Relays as leaf nodes of a spanning tree.
   The service supports OAL packet/fragment forwarding at a layer below
   IP and does not interact with the public Internet BGP routing system,
   but supports redistribution of information for other networks
   connected by Relays.

   In a reference deployment, each Proxy/Server is configured as an
   Autonomous System Border Router (ASBR) for a stub Autonomous System
   (AS) using a 32-bit AS Number (ASN) [RFC4271] that is unique within
   the BGP instance, and each Proxy/Server further uses eBGP to peer
   with one or more Gateways but does not peer with other Proxy/Servers.
   Each SRT segment in the OMNI link must include one or more Gateways
   in a "hub" AS, which peer with the Proxy/Servers within that segment
   as "spoke" ASes.  All Gateways within the same segment are members of
   the same hub AS, and use iBGP to maintain a consistent view of all
   active routes currently in service.  The Gateways of different
   segments peer with one another using eBGP.

   Gateways maintain forwarding table entries for the PNP prefixes
   assigned to Proxy/Servers and the set of all FNP/MNP routes that are
   currently active; Gateways also maintain black-hole routes for the
   OMNI link MSPs so that OAL packets/fragments destined to non-existent
   more-specific routes are flushed from the routing system.  In this



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   way, Proxy/Servers and Relays have only partial topology knowledge
   (i.e., they only maintain routing information for their directly
   associated Clients and foreign links) and they forward all other OAL
   packets/fragments to Gateways which have full topology knowledge.

   Each OMNI link segment assigns a unique sub-prefix of the MSP known
   as the "SRT prefix".  For example, a first segment could assign
   2001:db8::/48, a second could assign 2001:db8:1::/48, a third could
   assign 2001:db8:2::/48, etc.  Within each segment, each Proxy/Server
   and Gateway configures a PNP within the segment's SRT prefix, e.g.,
   the PNPs 2001:db8::/64, 2001:db8:0:1::/64 2001:db8:0:2::/64 all
   belong to the SRT prefix 2001:db8::/48.

   The administrative authorities for each segment must therefore
   coordinate to assure mutually-exclusive PNP assignments, but internal
   provisioning of PNPs is an independent local consideration for each
   administrative authority.  For each SRT prefix, the Gateway(s) that
   connect that segment assign the all-zero's address of the prefix as a
   Subnet Router Anycast (SRA) address.  For example, the SRA address
   for 2001:db8::/48 is simply 2001:db8::. All Proxy/Servers also assign
   the SRA address taken from their uniquely-assigned more-specific PNP,
   e.g., the SRA address for the PNP 2001:db8:0:1::/64 is simply
   2001:db8:0:1::.

   SRT prefixes (and their PNP sub-prefixes) are statically represented
   in Gateway forwarding tables.  Gateways join multiple SRT segments
   into a unified OMNI link over multiple diverse network administrative
   domains.  They support a virtual bridging service by first
   establishing forwarding table entries for their SRT prefixes either
   via standard BGP routing or static routes.  For example, if three
   Gateways ('A', 'B' and 'C') from different segments serviced
   2001:db8::/48, 2001:db8:1::/48 and 2001:db8:2::/48 respectively, then
   the forwarding tables in each gateway appear as follows:

   A:  2001:db8::/48->local, 2001:db8:1::/48->B, 2001:db8:2::/48->C

   B:  2001:db8::/48->A, 2001:db8:1::/48->local, 2001:db8:2::/48->C

   C:  2001:db8::/48->A, 2000:db8:1::/48->B, 2001:db8:2::/48->local

   These forwarding table entries rarely change, since they correspond
   to fixed infrastructure elements in their respective segments.









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   FNP and MNP routes are instead dynamically advertised in the AERO
   routing system by Proxy/Servers and Relays that provide anchor points
   for their corresponding prefixes.  For example, if three Proxy/
   Servers ('D', 'E' and 'F') service the MNPs 2001:db8:1000:1::64/,
   2001:db8:1000:2::/64 and 2001:db8:1000:2::/48 then the routing system
   would include:

   D:  2001:db8:1000:1::/64

   E:  2001:db8:1000:2::/64

   F:  2001:db8:1000:3::/64

   A full discussion of the BGP-based routing system used by AERO is
   found in [I-D.ietf-rtgwg-atn-bgp].

4.2.4.  Segment Routing Topologies (SRTs)

   The distinct GUA prefixes in an OMNI link domain identify distinct
   Segment Routing Topologies (SRTs).  Each SRT is a mutually-exclusive
   OMNI link overlay instance using a distinct set of GUAs, and emulates
   a bridged campus LAN service for the OMNI link.  In some cases (e.g.,
   when redundant topologies are needed for fault tolerance and
   reliability) it may be beneficial to deploy multiple SRTs that act as
   independent overlay instances.  A communication failure in one
   instance therefore will not affect communications in other instances.

   Each SRT is identified by a distinct GUA prefix and assigns an IPv6
   SRA address used for OMNI interface determination in Safety-Based
   Multilink (SBM) as discussed in [I-D.templin-6man-omni3].  Each OMNI
   interface further applies Performance-Based Multilink (PBM)
   internally.

   The Gateways and Proxy/Servers of each independent SRT engage in BGP
   peerings to form a spanning tree with the Gateways in non-leaf nodes
   and the Proxy/Servers in leaf nodes.  The spanning tree is configured
   over both secured and unsecured underlay network paths.  The secured
   spanning tree is used to convey secured control messages (and
   sometimes data message extensions) between Proxy/Servers and
   Gateways, while the unsecured spanning tree forwards bulk data
   messages and/or unsecured control messages.

   Each SRT segment is identified by a unique GUA prefix used by all
   Proxy/Servers and Gateways in the segment.  Each AERO node must
   therefore discover an SRT prefix that correspondents can use to
   determine the correct segment, and must publish the SRT prefix in
   IPv6 ND messages.




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   Note: The distinct GUA prefixes in an OMNI link domain can be carried
   either in a common BGP routing protocol instance for all OMNI links
   or in distinct BGP routing protocol instances for different OMNI
   links.  In some SBM environments, such separation may be necessary to
   ensure that distinct OMNI links do not include any common
   infrastructure elements as single points of failure.  In other
   environments, carrying the GUAs of multiple OMNI links within a
   common routing system may be acceptable.

4.2.5.  Segment Routing For OMNI Link Selection

   Original IPv6 sources can direct IPv6 packets to an AERO node by
   including a standard IPv6 Segment Routing Header (SRH) [RFC8754] with
   the IPv6 SRA address for the selected OMNI link as either the IPv6
   Destination Address or as an intermediate hop Segment List IPv6
   address within the SRH.  This allows the original source to determine
   the specific OMNI link SRT an original IPv6 packet will traverse when
   there may be multiple alternatives.

   When an AERO node processes the SRH and forwards the original IPv6
   packet to the correct OMNI interface, the OMNI interface writes the
   next IPv6 address from the SRH into the IPv6 Destination Address and
   decrements Segments Left.  If decrementing would cause Segments Left
   to become 0, the OMNI interface deletes the SRH before forwarding.
   This form of Segment Routing supports SBM.

4.3.  OMNI Interface Characteristics

   OMNI interfaces are virtual interfaces configured over one or more
   underlay interfaces classified as follows:

   *  (M)ANET interfaces connect to a protected and secured ANET or an
      open MANET that connects to an INET via Proxy/Servers.  The
      (M)ANET interface may be either on the same L2 link segment as a
      Proxy/Server, or separated from a Proxy/Server by multiple IP
      hops.  (Note that NATs may appear internally within a (M)ANET and
      may require NAT traversal on the path to the Proxy/Server the same
      as for the INET case.)  MANETs are special cases of ANETs in which
      adaptation layer multihop forwarding may be necessary, and
      protected secured underlay links cannot always be assumed.

   *  INET interfaces connect to an INET either natively or through one
      or several IPv4 Network Address Translators (NATs).  Native INET
      interfaces have global IP addresses that are reachable from
      correspondent on the same INET.  NATed INET interfaces typically
      have private IP addresses and connect to a private network behind
      one or more NATs with the outermost NAT providing INET access.




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   *  ENET interfaces connect a Client's downstream-attached networks,
      where the Client provides forwarding services for ENET end system
      communications to remote peers.  An ENET can 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 ANET or INET.

   *  VPN interfaces use security encapsulations (e.g. IPsec tunnels)
      over underlay networks to connect Clients, Proxy/Servers and/or
      Gateways.  VPN interfaces provide security services at lower
      layers of the architecture (L2/L1) the same as for Direct point-
      to-point interfaces.

   *  Direct point-to-point interfaces securely connect Clients, Proxy/
      Servers and/or Gateways 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.

   OMNI interfaces use OAL encapsulation and fragmentation as discussed
   in Section 4.6.  OMNI interfaces use L2 encapsulation (see:
   Section 4.6) to exchange carrier packets with OMNI link neighbors
   over INET interfaces and IPsec tunnels as well as over ANET
   interfaces for which the Client and neighbor may be multiple IP hops
   away.  OMNI interfaces use link layer encapsulation only (i.e., and
   no other L2 encapsulations) over Direct underlay interfaces or
   (M)ANET interfaces when the Client and neighbor are known to be on
   the same underlay link.

   OMNI interfaces maintain an adaptation layer view of the neighbor
   cache for tracking per-neighbor state.  IP nodes that configure OMNI
   interfaces use ND messages including Router Solicitation (RS), Router
   Advertisement (RA), Neighbor Solicitation (NS), Neighbor
   Advertisement (NA), unsolicited Neighbor Advertisement (uNA) and
   Redirect to manage both the network and adaptation layer views of the
   neighbor cache.  The adaptation layer further uses a multilink
   forwarding message set termed Multilink Initiate (MI), Multilink
   Respond (MR) and Multilink Control (MC) which use the same ICMPv6
   Type value as the standard NA message but with different Code values.
   OMNI neighbors invoke per-flow OAL Identification window
   synchronization in their ND message exchanges to enable Source
   Address verification, header compression and robust fragmentation/
   reassembly.

   OMNI interfaces include OMNI options formatted as specified in
   [I-D.templin-6man-omni3] in the IP ND messages they forward.  The
   OMNI option includes parameters for coordinating the OMNI interface's
   underlay interfaces.



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   A Client's OMNI interface may be configured over multiple *NET
   underlay interfaces.  For example, common mobile handheld devices
   have both wireless local area network ("WLAN") and cellular wireless
   links.  These links are often used "one at a time" with low-cost WLAN
   preferred and highly-available cellular wireless as a standby, but a
   simultaneous-use capability could provide benefits.  In a more
   complex example, aircraft frequently have many wireless data link
   types (e.g.  satellite-based, cellular, terrestrial, air-to-air
   directional, etc.) with diverse performance and cost properties.

   If a Client's multiple *NET underlay interfaces are used "one at a
   time" (i.e., all other interfaces are in standby mode while one
   interface is active), then successive IPv6 ND messages all include
   OMNI option Interface Attributes, Traffic Selector and/or Neighbor
   Synchronization sub-options with the same underlay interface ifIndex.
   In that case, the Client would appear to have a single underlay
   interface but with a dynamically changing link layer address.

   If the Client has multiple active *NET underlay interfaces, then from
   the perspective of IPv6 ND it would appear to have multiple link
   layer addresses.  In that case, IPv6 ND message OMNI options MAY
   include sub-options with different underlay interface ifIndexes.

   Proxy/Servers on the open Internet include only a single INET
   underlay interface.  INET Clients therefore discover only the L2ADDR
   information for the Proxy/Server's INET interface.  Proxy/Servers on
   a (M)ANET/INET boundary include both (M)ANET and INET underlay
   interfaces.  (M)ANET Clients therefore must discover both the (M)ANET
   and INET L2ADDR information for their Proxy/Servers.

   Gateway and Proxy/Server OMNI interface connections to the SRT are
   configured over both secured IPsec tunnels for carrying IPv6 ND and
   BGP protocol control plane messages and open INET paths for carrying
   unsecured data plane messages.  The OMNI interface configures an MLA
   and acts as an OAL source to encapsulate original IP packets, then
   fragments the resulting OAL packets, performs L2 encapsulation and
   sends the resulting carrier packets over the secured or unsecured
   underlay paths.  Note that Gateway and Proxy/Server end-to-end
   transport protocol sessions used by the BGP run directly over the
   OMNI interface and use MLA IPv6 Source and Destination Addresses.











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4.4.  OMNI Interface Initialization

   AERO Proxy/Servers and Clients configure OMNI interfaces as their
   point of attachment to the OMNI link.  AERO nodes assign the MSPs for
   the link to their OMNI interfaces (i.e., as a "route-to-interface")
   to ensure that original IP packets with Destination Addresses covered
   by an MNP not explicitly associated with another interface are
   directed to an OMNI interface where address resolution is applied.

   OMNI interface initialization procedures for Gateways, Proxy/Servers
   and Clients are discussed in the following sections.

4.4.1.  AERO Gateway Behavior

   AERO Gateways configure an OMNI interface and assign both MLAs and
   PNPs with corresponding SRA PNPADDRs for their OMNI link SRT
   segments.  Gateways configure underlay interface secured tunnels with
   Proxy/Servers in the same SRT segment and other Gateways in the same
   (or an adjacent) SRT segment.  Gateways then engage in an adaptation
   layer BGP routing protocol session with neighbors over the secured
   spanning tree (see: Section 4.2.3).

4.4.2.  AERO Proxy/Server and Relay Behavior

   When a Proxy/Server enables an OMNI interface, it assigns both an LLA
   and MLA plus a PNP with the SRA PNPADDR appropriate for its OMNI link
   SRT segment.  The Proxy/Server also configures secured underlay
   interface tunnels and engages in adaptation layer BGP routing
   protocol sessions over the OMNI interface with one or more
   neighboring Gateways.

   The OMNI interface provides a single interface abstraction to the
   network layer, but internally serves as an NBMA nexus for exchanging
   carrier packets with other OMNI nodes over underlay interfaces and/or
   secured tunnels.  The Proxy/Server further configures a service to
   facilitate IPv6 ND exchanges with AERO Clients and manages per-Client
   Neighbor Cache Entries (NCEs) and IP forwarding table entries based
   on control message exchanges.

   Relays are simply Proxy/Servers that run a dynamic routing protocol
   to redistribute routes between the OMNI interface and foreign
   networks/links (see: Section 4.2.3).  The Relay provisions MNPs and
   advertises the MSP(s) for the OMNI link over its foreign network
   interface attachments.  The Relay further provides an OMNI link
   attachment point for FNP-based topologies.






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4.4.3.  AERO Client Behavior

   When a Client enables an OMNI interface, it assigns an LLA and a
   unique MLA to the OMNI interface.  The adaptation layer within the
   interface then issues an internally-generated RA message to the
   network layer to establish itself as a (virtual) default router for
   the OMNI link.  The network layer then issues a DHCPv6 Prefix
   Delegation (PD) request per [RFC9762].  When it receives the PD
   request (or after a brief timeout), the adaptation layer then sends
   OMNI-encapsulated RS messages to FHS Proxy/Servers which allocate a
   PNPADDR within a PNP and optionally coordinate with a MAP Proxy/
   Server that delegates one or more MNPs.  The MAP/FHS Proxy/Servers
   then return an RA message to the Client which may pass through one or
   more NATs in the path.

   When the Client sends initial RS messages, it will discover PNPADDRs
   in the corresponding RAs that it receives from FHS Proxy/Servers and
   can then associate them with the OMNI interface.  If the Client is
   operating outside the context of AERO infrastructure, however, it may
   continue using MLAs over its underlay or OMNI interfaces for peer-to-
   peer communications within the local *NET.  The Client can then
   continue indefinitely or at least until it encounters an
   infrastructure element that can delegate current PNPADDRs and/or
   MNPs.)

   A Client can further extend the OMNI link over its (downstream) ENET
   interfaces where it provides a first-hop router for end systems and
   other Clients connected to the ENET.  A downstream Client that
   connects via the ENET serviced by an upstream Client can in turn
   service further downstream ENETs that connect other end systems and
   Clients.  This OMNI link extension can be applied recursively over a
   "chain" of ENET Clients acting as Proxys.

4.5.  OMNI Interface Neighbor Cache Maintenance

   Each Client and Proxy/Server OMNI interface maintains a network layer
   conceptual Neighbor and Destination Cache per [RFC1256][RFC4861] the
   same as for any IP interface.  The OMNI interface network layer
   neighbor cache (NLNC) is maintained through static and/or dynamic
   neighbor cache entry (NLNCE) configurations.  The IP layer initiates
   and terminates IP ND messaging exchanges to manage the network layer
   view of the neighbor cache.

   Each OMNI interface also maintains an internal adaptation layer view
   of the neighbor cache (ALNC) that includes an entry (ALNCE) for each
   of its active OAL neighbors.  ALNCE state is potentially shared
   between multiple NLNCEs, and is managed for freshness/staleness the
   same as the NLNC.  The adaptation layer indexes the ALNC by the



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   neighbor's MLA and assigns a unique locally generated link-layer
   address for each distinct MLA.  The adaptation layer then exposes
   only the link-layer address through the inclusion of Source/Target
   Link Layer Address Options (S/TLLAOs) in IPv6 ND messages it delivers
   to the network layer.  IPv6 ND messages that update the ALNC include
   an OMNI option with zero or more sub-options.

   Each OMNI interface ALNCE is indexed by the IPv6 MLA of a neighbor
   found in an ND message and determines the context for Identification
   verification.  Clients and Proxy/Servers maintain NCEs through
   dynamic RS/RA message exchanges, and also maintain NCEs for any
   active correspondent peers through dynamic IPv6 NS/NA message
   exchanges.

   Clients establish NCEs and establish adaptation layer service
   profiles for their associated FHS and MAP Proxy/Servers through the
   exchange of RS/RA messages as specified in [I-D.templin-6man-omni3].
   When a Client and Proxy/Server establish NCEs, they set a
   ReachableTime timer to REACHABLE_TIME seconds.

   Both the Client and its MAP Proxy/Server have full knowledge of the
   Client's current underlay Interface Attributes and Traffic Selectors,
   while FHS Proxy/Servers acting in "proxy" mode have knowledge of only
   the individual Client underlay interfaces they service.

   When an Address Resolution Source (ARS) sends an NS(AR) message
   toward an Address Resolution Target (ART) Client/Relay, the OMNI link
   routing system directs the NS(AR) to a MAP Proxy/Server for the ART.
   The MAP then either acts as an Address Resolution Responder (ARR) on
   behalf of the ART or forwards the NS(AR) to the ART which acts as an
   ARR on its own behalf.  The ARR returns an NA(AR) response to the
   ARS, which creates or updates a NCE for the ART while caching L3 and
   L2 addressing information.  The ARS then (re)sets ReachableTime for
   the NCE to REACHABLE_TIME seconds and performs multilink forwarding
   ND message exchanges over specific underlay interface pairs to
   determine paths for sending carrier packets directly to the ART.  The
   ARS otherwise decrements ReachableTime while no further solicited ND
   messages arrive.

   Proxy/Servers add an additional state DEPARTED to the list of NCE
   states found in Section 7.3.2 of [RFC4861].  When a Client terminates
   its association, the Proxy/Server OMNI interface sets a DepartTime
   variable for the NCE to DEPART_TIME seconds.  DepartTime is
   decremented unless a new IPv6 ND message causes the state to return
   to REACHABLE.  While a NCE is in the DEPARTED state, the Proxy/Server
   forwards OAL packets/fragments destined to the target Client to the
   Client's new FHS/MAP Proxy/Server instead.




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   It is RECOMMENDED that REACHABLE_TIME be set to the default constant
   value 30 seconds as specified in [RFC4861].  It is RECOMMENDED that
   DEPART_TIME be set to the default constant value 10 seconds to accept
   any carrier packets that may be in flight.  When ReachableTime or
   DepartTime decrement to 0, the NCE is deleted.

   AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the number
   of NS(NUD) messages sent when a correspondent may have gone
   unreachable, the value MAX_RTR_SOLICITATIONS to limit the number of
   RS messages sent without receiving an RA and the value
   MAX_NEIGHBOR_ADVERTISEMENT to limit the number of solicited IPv6 ND
   advertisements that can be sent based on a single event.  It is
   RECOMMENDED that MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and
   MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in
   [RFC4861].

   Different values for the above constants MAY be administratively set;
   however, if different values are chosen, all nodes on the link MUST
   consistently configure the same values.

4.5.1.  AERO/OMNI Control Plane Messages

   OMNI interfaces use IPv6 ND messages as the secured control plane
   messaging service for all adaptation layer neighbor coordination
   exchanges.  IPv6 ND messages sent over OMNI interfaces include or
   omit the Source/Target Link Layer Address Option (S/TLLAO) as
   discussed in [I-D.templin-6man-omni3].  OMNI interfaces forward IPv6
   ND messages to and from the IP layer the same as for standard IPv6
   ND, but also append/remove a trailing OMNI pseudo-option during
   encapsulation/decapsulation [I-D.templin-6man-omni3].

   For each IPv6 ND message, the OMNI interface includes a trailing OMNI
   option following any other ND message options then completely
   populates all sub-option information.  If the OMNI interface includes
   an authentication sub-option, it calculates and includes a digital
   signature per the OMNI specification.  OMNI interfaces verify
   integrity and authenticity of each message received, and process the
   message further only following successful verification.

   OMNI options include per-neighbor information that provides multilink
   forwarding, link layer address and traffic selector information for
   the neighbor's underlay interfaces.  This information is stored in
   both the neighbor cache and AERO Flow Information Base (AFIB) as
   basis for the forwarding algorithm specified in Section 4.10.  The
   information is cumulative and reflects the union of the OMNI
   information from the most recent IPv6 ND messages received from the
   neighbor.




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   AERO/OMNI Clients send RS messages to cause Proxy/Servers to respond
   with RA messages that include autoconfiguration and addressing
   parameters as specified in [I-D.templin-6man-omni3].

   AERO nodes use NS/NA messages as follows:

   *  NS/NA(AR) messages are used for address resolution.  When an ARS
      prepares an NS(AR) it sets the Source Address to the IPv6 MLA
      assigned to the OMNI interface.  The ARS also sets the Target
      Address to the IP Destination Address of the invoking packet and
      sets the Destination Address to the solicited-node multicast
      address corresponding to the (unicast) Target Address.  After the
      ARS sends the NS(AR), an ARR with addressing information for the
      ART returns a unicast NA(AR) that contains current, consistent and
      authentic Target Address resolution information.  The ARR sets the
      NA(AR) Source Address to an IPv6 MLA assigned to the ART"s OMNI
      interface, sets the Destination Address to the Source Address of
      the NS(AR) and sets the Target Address to the Target Address of
      the NS(AR).  NS/NA(AR) messages must be secured.

   *  Other NS/NA message exchanges are used to determine target
      reachability (NS/NA(NUD)).  These messages include the same
      addresses as for NS/NA(AR) with the exception that the source sets
      the NS(NUD) Destination and Target Address to the same unicast
      address.  The target then returns a responsive NA(NUD).  NS/
      NA(NUD) messages used to establish or update NCE state be secured.

   *  Unsolicited NA messages (uNAs) are used to update a neighbor's
      cache when an underlay interface address changes due to a mobility
      event.  Nodes also use uNAs during Route Optimization.

   *  NS/NA(DAD) messages are not used in AERO, since Duplicate Address
      Detection is not required on OMNI links.

   AERO introduces three special-purpose IPv6 ND messages for Multilink
   Forwarding.  These messages use the same Type value and message
   formatting specifications as the standard NA messages but include
   different Code values.  The messages are:

   *  Multilink Initiate (MI) (Type 136; Code TBD1) - sent as an
      adaptation layer control message used to initiate state needed to
      support multilink forwarding.  Recipients of MI messages respond
      by returning a Multilink Respond (MR).

   *  Multilink Respond (MR) (Type 136; Code TBD2) - an adaptation layer
      control message multilink forwarding response to an MI message.
      Recipients of MR messages optionally return a Multilink Control
      (MC).



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   *  Multilink Control (MC) (Type 136; Code TBD3) - a muti-purpose
      secured standalone adaptation layer control message used to
      confirm a previous MI/MR exchange, establish multilink forwarding
      state, forward error/informational messages or transport passenger
      packets.

   Unlike standard IPv6 ND messages, the pseudo message set (MI/MR/MC)
   is used internally within the adaptation layer only and the messages
   are never exposed to the network layer; any MI/MR/MC messages
   accidentally exposed to the network layer would be silently discarded
   due to NA message validation rules per [RFC4861] since they include
   non-zero Code values.

   IPv6 ND (pseudo) messages sent on OMNI links that must be examined by
   transit OAL intermediate systems on the path require a special
   codepoint for recognition other than the IPv6 Destination Address.
   The OAL source therefore sets the DSCP field in the IPv6 OAL
   encapsulation header of such messages to the special value '111111'
   (see: [I-D.templin-6man-omni3]).  The control planes of transit OAL
   intermediate systems can then intercept and process these messages
   before forwarding them to the next OAL hop.

   IPv6 ND pseudo messages MI and MC set the Target Address to the
   Source Address of the subject packet, set the Source Address to the
   MLA of the source and set the Destination Address to the Destination
   Address of the subject packet.  IPv6 ND pseudo message MR sets the
   Target Address to the Destination Address of the subject packet, sets
   the Source Address to the MLA of the source and sets the Destination
   Address to the Target Address of the MI/MC.

   If necessary, IPv6 ND pseudo messages include an SRH extension to the
   OAL IPv6 header as discussed in [I-D.templin-6man-omni3].  The IPv6
   ND pseudo message R/S/O flags are unused; they should be set to 0 on
   transmission and ignored on reception.  As a result, the pseudo
   messages are distinguished from ordinary uNA messages by the message
   Code field value alone.

4.5.2.  OMNI Neighbor Window Synchronization

   In secured environments (e.g., between secured spanning tree
   neighbors, between neighbors on the same secured ANET, etc.), OMNI
   interface neighbors can exchange AERO control messages without
   including Identification values.  In environments where spoofing is
   considered a threat, OMNI interface neighbors instead invoke
   Identification window synchronization by including OMNI Neighbor
   Synchronization sub-options in IPv6 ND message exchanges to maintain
   send/receive window state in their respective neighbor caches as well
   as in AFIB entries of all OAL intermediate nodes in the forward and



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   reverse paths.

   In common arrangements, OAL Identification window synchronization is
   necessary for Client to Client, Client to Proxy/Server or Proxy/
   Server to Proxy/Server message exchanges conducted over unsecured
   Internetwork paths.  Conversely, Proxy/Server to Proxy/Server, Proxy/
   Server to Gateway and Gateway to Gateway message exchanges carried
   over the secured spanning tree do not require window synchronization.

   OAL end system and intermediate nodes verify Identification values of
   OAL packets that traverse the unsecured spanning tree according to
   their populated AFIB state.  This allows each OAL node to exclude
   spurious packets injected into the OMNI link from an off-path
   adversary.

4.6.  OMNI Interface Encapsulation and Fragmentation

   When the network layer forwards an original IP packet into an OMNI
   interface, the interface locates a NLNCE corresponding to the OAL
   destination.  The OMNI interface then invokes the OAL as discussed in
   [I-D.templin-6man-omni3] which removes the virtual Ethernet header
   and encapsulates the packet in an IPv6 header to form an OAL packet
   according to ALNCE information.

   Following encapsulation, the OAL source then fragments the OAL packet
   while including an identical Identification value for each fragment
   that must be within the window for the flow over the interface pair
   selected for the neighbor.  The OAL source includes any necessary OAL
   IPv6 extension headers including an identical SRH with each fragment
   if necessary.  The OAL source can instead invoke OAL header
   compression by replacing the full OAL IPv6 header, SRH and Extended
   Fragment Header with an OAL Compressed Header (OCH) that includes an
   AERO Flow Vector Index (AFVI) (see: [I-D.templin-6man-omni3]).

   For messages that will traverse unsecured paths, the OAL source
   finally performs L2 encapsulation on each resulting OAL fragment to
   form a carrier packet, with Source Address set to its own L2 address
   (e.g., 192.0.2.100) and Destination Address set to the L2 address of
   the next hop OAL intermediate system or destination (e.g.,
   192.0.2.1).  The carrier packet encapsulation format in the above
   example is shown in Figure 3:










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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |           L2 Headers          |
        ~       src = 192.0.2.100       ~
        |        dst = 192.0.2.1        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~   L2 IPv6 Extension Headers   ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        OAL IPv6 Header        |
        ~       Source Address (1)      ~
        |    Destination Address (2)    |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~   OAL IPv6 Extension Headers  ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |       Original IP Header      |
        ~     (first-fragment only)     ~
        ~       Source Address (3)      ~
        |    Destination Address (4)    |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~ Original Packet Body/Fragment ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: Carrier Packet Format

   In this format, the OAL source encapsulates the original IP header
   and packet body/fragment in an OAL IPv6 header.  The OAL source then
   adds an SRH (if necessary) plus Extended Fragment Header as OAL IPv6
   header extensions for each fragment and prepends L2 headers prepared
   as discussed in [I-D.templin-6man-omni3].  The OAL source sends each
   such carrier packet into the SRT unsecured spanning tree, where they
   may be forwarded over multiple OAL intermediate systems until they
   arrive at the OAL destination.  These carrier packets may themselves
   be subject to L2 fragmentation and reassembly along the concatenated
   network path segments.

   The OMNI link control plane service distributes Client MNP prefix
   information that may change occasionally due to regional node
   mobility, as well as more static information for Relay FNPs and per-
   segment PNPs that rarely change.  OMNI link Gateways and Proxy/
   Servers use the information to establish and maintain a forwarding
   plane spanning tree that connects all nodes on the link.  The
   spanning tree supports a virtual bridging service according to link
   layer (instead of network layer) information, but may often include
   longer paths than necessary.




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   Each OMNI interface therefore also includes an AERO Flow Information
   Base (AFIB) that caches AERO Flow Vectors (AFVs) which can provide
   both carrier packet Identification context and more direct forwarding
   "shortcuts" that avoid strict spanning tree paths.  As a result, the
   spanning tree is always available but OMNI interfaces can often use
   the AFIB entries established through route optimization to greatly
   improve performance and reduce load on critical infrastructure
   elements.

   For OAL packets/fragments undergoing L2 re-encapsulation at an OAL
   intermediate system, the OMNI interface performs L2 decapsulation
   followed by Identification verification and OAL reassembly only if
   the OAL packet/fragment is addressed to itself.  The OMNI interface
   then decrements the OAL IPv6 header Hop Limit and discards the
   packet/fragment if the Hop Limit reaches 0.  Otherwise, the OMNI
   interface updates the OAL addresses if necessary, includes an
   appropriate Identification, performs OAL fragmentation then for each
   OAL fragment performs L2 encapsulation to produce a carrier packet
   appropriate for next segment forwarding.

4.7.  OMNI Interface Decapsulation

   When an OAL node receives OAL packets/fragments addressed to another
   node, it discards the L2 headers and includes new L2 headers
   appropriate for the next hop in the forwarding path to the OAL
   destination.  The node then sends these new carrier packets into the
   next hop underlay interface.

   When an OAL node receives OAL packets/fragments addressed to itself,
   it performs L2 decapsulation, verifies the Identification, then
   performs OAL reassembly/decapsulation to obtain the original OAL
   packet or composite packet (see: [I-D.templin-6man-omni3]).  Next, if
   the enclosed original IP packet(s) are addressed either to itself or
   to a destination reached via an interface other than the OMNI
   interface, the OAL node replaces the OAL encapsulation IPv6 header
   with a virtual Ethernet header and forwards the original IP packet(s)
   to the network layer.

   If the original IP packet(s) are destined to another node reached by
   the OMNI interface, the OAL node instead decrements the Hop Limit,
   then performs L2 encapsulation and forwards these new carrier packets
   into an underlay interface for the next segment.

   Further OMNI link decapsulation details are specified in
   [I-D.templin-6man-omni3].  Further OMNI link forwarding procedures
   are specified in Section 4.10.





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4.8.  OMNI Interface Data Origin Authentication

   AERO nodes employ simple data origin authentication procedures.  In
   particular:

   *  AERO Gateways and Proxy/Servers accept carrier packets received
      from the secured spanning tree.

   *  AERO Proxy/Servers and Clients accept carrier packets and original
      IP packets that originate from within the same secured ANET.

   *  AERO Clients and Relays accept original IP packets from downstream
      network correspondents based on ingress filtering.

   *  AERO Clients, Relays, Proxy/Servers and Gateways verify carrier
      packet L2 encapsulation addresses according to
      [I-D.templin-6man-omni3].

   *  OAL end systems and intermediate systems forward/accept OAL
      packets/fragments with Identification values within the current
      window for the OAL source neighbor for a specific underlay
      interface pair and drop any packets with out-of-window
      Identification values.

   AERO nodes silently drop any packets that do not satisfy the above
   data origin authentication procedures.  Further security
   considerations are discussed in Section 7.

4.9.  OMNI Interface MTU

   The OMNI interface observes the link nature of tunnels, including the
   Maximum Transmission Unit (MTU), Effective MTU to Receive (EMTU_R)
   and the role of fragmentation and reassembly
   [I-D.ietf-intarea-tunnels].  The OMNI interface employs the OAL to
   accommodate multiple underlay links with diverse MTUs.  OMNI
   interface packet sizing considerations are specified in
   [I-D.templin-6man-omni3], where the OMNI interface MTU can
   essentially be considered "unlimited".













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   When the network layer presents an original IP packet to the OMNI
   interface, the OAL source encapsulates and fragments the packet if
   necessary.  When the network layer presents the OMNI interface with
   multiple original IP packets addressed to the same IPv6 flow, the OAL
   source can concatenate them as a single OAL composite packet as
   discussed in [I-D.templin-6man-omni3] before applying fragmentation.
   The OAL source then submits each OAL fragment for L2 encapsulation
   for transmission as a carrier packet over an underlay interface
   connected to either a physical link (e.g., Ethernet, WiFi, Cellular,
   etc.) or a virtual link such as an Internet or higher-layer tunnel.

4.10.  OMNI Interface Forwarding Algorithm

   Original IP packets enter a node's OMNI interface either from the
   network layer (i.e., from a local application or the IP forwarding
   system) while carrier packets enter from the link layer (i.e., from
   an OMNI interface neighbor).  All original IP packets and carrier
   packets entering a node's OMNI interface first undergo data origin
   authentication as discussed in Section 4.8.  Those that satisfy data
   origin authentication are processed further, while all others are
   dropped silently.

   Original IP packets that enter the OMNI interface from the network
   layer are forwarded to an OMNI interface neighbor using OAL
   encapsulation and fragmentation to produce carrier packets for
   transmission over underlay interfaces.  (If forwarding state
   indicates that the original IP packet should instead be forwarded
   back to the network layer, the packet is dropped to avoid looping).
   Carrier packets that enter the OMNI interface from the link layer are
   either re-encapsulated and re-admitted into the link layer, or
   reassembled and forwarded to the network layer where they are subject
   to either local delivery or IP forwarding.

   When the network layer of a router forwards an original IP packet
   into the OMNI interface, it decrements the TTL/Hop Limit following
   standard IP router conventions.  Once inside the OMNI interface,
   however, the OAL does not decrement the original IP packet TTL/Hop
   Limit further since its adaptation layer forwarding actions occur
   below the network layer.  The original IP packet's TTL/Hop Limit will
   therefore be the same when it exits the destination OMNI interface as
   when it first entered the source OMNI interface.

   When an OAL intermediate system receives a carrier packet, it
   performs L2 decapsulation to obtain the enclosed OAL packet/fragment.
   When the intermediate system forwards an OAL packet/fragment not
   addressed to itself (or one addressed to itself but that also
   includes an SRH with Segments Left greater than 0), it decrements the
   OAL Hop Limit without decrementing the network layer IP TTL/Hop



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   Limit.  If decrementing would cause the OAL Hop Limit to become 0,
   the OAL intermediate system drops the OAL packet/fragment.  This
   ensures that original IP packet(s) cannot enter an endless loop.

   OMNI interfaces may have multiple underlay interfaces and/or NCEs for
   neighbors with multiple underlay interfaces (see Section 4.3).  The
   OAL uses Interface Attributes and/or Traffic Selectors to select an
   outbound underlay interface for each OAL packet and also to select
   segment routing and/or link layer Destination Addresses based on the
   neighbor's target underlay interfaces.  AERO implementations SHOULD
   permit network management to dynamically adjust Traffic Selector
   values at runtime.

   If an OAL packet/fragment matches the Interface Attributes and/or
   Traffic Selectors of multiple outgoing interfaces and/or neighbor
   interfaces, the OMNI interface replicates the packet and sends a
   separate copy via each of the (outgoing / neighbor) interface pairs;
   otherwise, it sends a single copy via an interface with the best
   matching attributes/selectors.  (While not strictly required, the
   likelihood of successful reassembly is often greatest when the OMNI
   interface sends all fragments of the same fragmented OAL packet/
   fragment consecutively over the same underlay interface pair to avoid
   complicating factors such as delay variance and reordering.)  AERO
   nodes keep track of which underlay interfaces are currently
   "reachable" or "unreachable", and use only "reachable" interfaces for
   forwarding purposes.

   In addition to standard forwarding based on Interface Attributes and/
   or Traffic Selectors, nodes may employ a policy engine that would
   provide further guidance to the forwarding algorithm.  For example
   the policy engine may suggest a load balancing profile over multiple
   underlay interface pairs, with portions of a traffic flow spread
   between multiple paths according to Equal Cost MultiPath or Link
   Aggregation Groups (LAGs) [RFC6438] (note that Interface Attributes
   include an underlay interface group identifier).  Other policies may
   suggest the use of paths with the least cost, best performance, etc.
   This document therefore specifies mechanisms without mandating any
   particular policies.

   All Clients, Proxy/Servers and Gateways serve as OAL intermediate
   nodes for the purpose of forwarding OAL packets/fragments that
   include an SRH or OCH with non-zero AFVI over the unsecured spanning
   tree based on AFIB entries.  When an OAL intermediate node forwards
   an OAL packet/fragment with an L2 Source Address and AFVI that
   matches an AFV, the node first verifies that the Identification is in
   sequence.  The OAL intermediate node then rewrites the packet's AFVI
   with a value that will be recognized by the next OAL hop and forwards
   the packet.  (For OAL packets/fragments with uncompressed headers and



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   with AFVI set to 0, the OAL intermediate node instead forwards based
   on matching the OAL IPv6 Destination Address with a standard IPv6
   forwarding table entry after applying SRH processing if necessary.)
   The chain of OAL source, intermediate and destination nodes may
   therefore traverse many (Proxy/)Clients, Proxy/Servers and Gateways
   on the path.

   The following sections discuss the OMNI interface-specific forwarding
   algorithms for Clients, Proxy/Servers and Gateways.  In the following
   discussion, an original IP packet's Destination Address is said to
   "match" if it is the same as a cached address, or if it is covered by
   a cached FNP/MNP/PNP.

4.10.1.  Client Forwarding Algorithm

   When the network layer presents an original IP packet to a Client's
   OMNI interface, the Client either forwards the message to an FHS
   Proxy/Server serving as a default router or invokes address
   resolution if there is no NLNCE that matches the Destination.  In the
   latter case, the Client holds the packet in a queue until address
   resolution completes, then admits the packet into the OMNI interface.
   The OMNI interface then searches for an ALNCE that matches the
   corresponding OAL destination.  If there is a matching ALNCE for a
   neighbor reached via a *NET interface (i.e., an upstream interface),
   the Client selects one or more "reachable" neighbor interfaces in the
   entry for forwarding purposes.

   When a carrier packet enters a Client's OMNI interface from the link
   layer, the Client performs L2 decapsulation if necessary to obtain
   the OAL packet/fragment then examines the OAL Destination Address
   (i.e., after locating the correct AFV if the OAL packet header is
   OCH).  If the OAL Destination Address matches one of the Client's
   addresses and the packet includes an SRH with Segments Left greater
   than 0, the Client rewrites the OAL Destination Address and forwards
   the packet to the peer Client or Proxy/Server indicated by the next
   hop SRH address.  Otherwise, the Client (acting as an OAL
   destination) verifies that the Identification is in-window for the
   matching AFV, then reassembles/decapsulates as necessary and delivers
   the original IP packet to the network layer.  If the OAL Destination
   Address does not match, the Client drops the original IP packet and
   MAY return a network layer ICMP Destination Unreachable message
   subject to rate limiting (see: Section 4.11).

   Note: The forwarding table entries established in peer Clients are
   based on MLAs which also appear as OAL Source or Destination
   Addresses within (M)ANETs but Proxys/Servers may rewrite them as
   PNPADDRs over INETs.  The original IP packet Source and Destination
   Addresses instead use MNP or MLA addresses.



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   Note: Clients within MANETs support Client-to-Client multihop
   forwarding when necessary to reach destinations or FHS Proxy/Servers
   that may be multiple OAL hops away.  In this way, forwarding Clients
   act as OAL intermediate nodes and forward using OCH compression based
   on AFV state that is indexed by the AFVIs included in each OAL
   packet/fragment.

4.10.2.  Proxy/Server and Relay Forwarding Algorithm

   When the network layer admits an original IP packet into a Proxy/
   Server's OMNI interface, the OAL drops the packet to avoid looping if
   forwarding state indicates that it should be forwarded back to the
   network layer.  Otherwise, the OAL examines the IP Destination
   Address to determine if it matches the SRA PNPADDR of a neighboring
   Gateway found in the OMNI interface's network layer neighbor cache.
   If so, the Proxy/Server performs OAL encapsulation and fragmentation
   then performs L2 encapsulation and forwards the resulting carrier
   packet to the Gateway over a secured link (e.g., an IPsec tunnel,
   Direct link, etc.) to support control plane functions such as the
   operation of the BGP routing protocol.  If the IP Destination Address
   matches an FNP/MNP/PNP associated with a (foreign) Proxy/Server or
   Client, the (local) Proxy/Server instead assumes the Relay role and
   forwards the original IP packet in the same manner as for Client
   forwarding while including an SRH if necessary.  Specifically, if
   there is a matching NCE the Proxy/Server selects one or more
   "reachable" neighbor interfaces in the entry for forwarding purposes;
   otherwise, the Proxy/Server performs OAL encapsulation/fragmentation
   followed by L2 encapsulation and forwards the resulting carrier
   packets while invoking address resolution and multilink forwarding
   procedures per Section 4.13.

   When the Proxy/Server receives carrier packets on underlay interfaces
   that contain OAL packets/fragments with both a Source and Destination
   OAL Address that correspond to the same Client's MLA, the Proxy/
   Server drops the carrier packets regardless of their OMNI link point
   of origin.  The Proxy/Server also drops original IP packets received
   on underlay interfaces either directly from a *NET Client if the
   original IP Destination Address corresponds to the same Client's
   delegated MNP or PNPADDR.  Proxy/Servers also drop carrier packets
   that contain OAL packets/fragments with foreign OAL Destination
   Addresses (MLAs) that do not match one of their local *NET Clients.
   These checks are essential to prevent forwarding inconsistencies from
   accidentally or intentionally establishing endless loops that could
   congest nodes and/or *NET links.

   Proxy/Servers process carrier packets that contain OAL packets/
   fragments with OCH headers or with Destination Addresses that match
   their SRA PNPADDR and also include an SRH.  In the first case, the



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   Proxy/Server examines the L2 Source Address and AFVI to locate the
   corresponding AFV entry in the AFIB.  In the second case, the Proxy/
   Server applies standard SRH processing procedures.  The Proxy/Server
   then forwards them according to the AFV or IPv6 routing state while
   decrementing the OAL packet/fragment Hop Limit.

   For OAL packets/fragments with Destination Addresses that match their
   MLA or PNP prefix and also include an SRH, the Proxy/Server performs
   any necessary local processing then rewrites the OAL Destination
   Address according to the next hop SRH address.  For those that do not
   include an OCH or SRH with additional next hop addresses, the Proxy/
   Server instead performs L2 decapsulation, verifies the Identification
   and performs OAL reassembly to obtain the original IP packet.  For
   data packets addressed to its own SRA PNPADDR that arrived via the
   secured spanning tree, the Proxy/Server delivers the original IP
   packet to the network layer to support secured BGP routing protocol
   control messaging.  For data packets originating from one of its
   dependent Clients, the Proxy/Server instead performs OAL
   encapsulation/fragmentation followed by L2 encapsulation and sends
   the resulting carrier packets while invoking address resolution and
   multilink forwarding procedures per Section 4.13.  For IPv6 ND
   control messages, the Proxy/Server instead authenticates the message
   and processes it as specified in later sections of this document
   while updating neighbor cache and/or AFIB state accordingly.

   When the Proxy/Server receives carrier packets that contain OAL
   packets with OAL Destination Address set to the PNPADDR of one of its
   Client neighbors established through RS/RA exchanges, it accepts the
   carrier packets only if data origin authentication succeeds.  If the
   NCE state is DEPARTED, the Proxy/Server changes the OAL Destination
   Address to the PNPADDR of the Client's interface at its new Proxy/
   Server, decrements the OAL Hop Limit, then performs L2 encapsulation
   and forwards the resulting carrier packets into the spanning tree
   which will eventually deliver them to the new Proxy/Server.  If the
   neighbor cache state for the Client is REACHABLE and the Proxy/Server
   is a MAP responsible for serving as the Client's address resolution
   responder and/or default router, it verifies the Identification then
   submits the OAL packet/fragment for reassembly.  The Proxy/Server
   then decapsulates and processes the resulting IPv6 ND message or
   original IP packet accordingly.  Otherwise, the Proxy/Server changes
   the OAL Destination Address to the local Client's MLA, includes an
   SRH with intermediate hop MLAs if necessary, decrements the OAL Hop
   Limit, performs L2 encapsulation and forwards the carrier packet to
   the Client which then performs data origin verification and
   reassembly.  (In the mobility case, the Client may receive fragments
   of the same original IP packet from different Proxy/Servers but this
   will not interfere with correct reassembly.)




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   When the Proxy/Server receives carrier packets that contain OAL
   packets with OAL Source Address set to the MLA of one of its Client
   neighbors established through RS/RA exchanges, it accepts the carrier
   packets only if data origin authentication succeeds.  If OAL
   Destination Address is the PNPADDR of a target Client associated with
   a remote Proxy/Server, the local Proxy/Server rewrites the OAL Source
   Address to the local Client's PNPADDR then decrements the OAL Hop
   Limit, performs L2 encapsulation and forwards the carrier packet to
   the remote Client.

   When the Proxy/Server receives carrier packets that contain OAL
   packets with OAL Destination Address set to a FNP address that does
   not match the MSP, it accepts the carrier packets only if data origin
   authentication succeeds and if there is a network layer forwarding
   table entry for the FNP.  The Proxy/Server then performs L2
   decapsulation, verifies the Identification, performs OAL reassembly/
   decapsulation to obtain the original IP packet, then presents it to
   the network layer (as a Relay) where it will be delivered according
   to standard IP forwarding.

   When a Proxy/Server receives a carrier packet from the secured
   spanning tree, it considers the message as authentic without having
   to verify network or higher layer authentication signatures.

   If the Proxy/Server has multiple original IP packets to send to the
   same neighbor, it can concatenate them as a single OAL composite
   packet [I-D.templin-6man-omni3].

4.10.3.  Gateway Forwarding Algorithm

   When the network layer admits an original IP packet into the
   Gateway's OMNI interface, the OAL drops the packet if routing
   indicates that it should be forwarded back to the network layer to
   avoid looping.  Otherwise, the Gateway examines the IP Destination
   Address to determine if it matches the SRA PNPADDR of a neighboring
   Gateway or Proxy/Server by examining the OMNI interface's network
   layer neighbor cache.  If so, the Gateway performs OAL encapsulation/
   fragmentation followed by L2 encapsulation and forwards the resulting
   carrier packets to the neighboring Gateway or Proxy/Server over a
   secured link (e.g., an IPsec tunnel, etc.) to support the operation
   of control plane functions (including the BGP routing protocol)
   between OAL neighbors.

   Gateways forward OAL packets/fragments reassembled from spanning tree
   carrier packets while decrementing the OAL Hop Limit but not the
   original IP header TTL/Hop Limit.  Gateways send carrier packets that
   contain OAL packets/fragments with critical IPv6 ND control messages
   or BGP routing protocol control messages via the SRT secured spanning



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   tree, and may send other carrier packets via the secured/unsecured
   spanning tree or via more direct paths according to AFIB information.
   When the Gateway receives a carrier packet, it decapsulates to obtain
   the OAL packet/fragment then searches for an AFIB entry that matches
   the OAL AFVI or an IPv6 forwarding table entry that matches the OAL
   Destination Address.

   Gateways process carrier packets containing OAL packets/fragments
   with OAL Destination Addresses that do not match their MLAs in the
   same manner as for traditional IP forwarding within the OAL, i.e.,
   they forward packets not explicitly addressed to themselves.
   Gateways locally process OAL packets/fragments with OCH headers or
   full OAL headers with their MLA as the OAL Destination Address.  If
   the OAL packet/fragment contains an OCH or a full OAL header with an
   SRH extension, the Gateway forwards the OAL packet/fragment to the
   next hop while decrementing the OAL Hop Limit but without
   reassembling.  When the Gateway forwards the OAL packet/fragment, it
   either rewrites the OCH AFVI with the value it will represent to the
   next OAL hop or follows standard SRH processing procedures.

   If the OAL packet/fragment includes a full OAL header but does not
   include an AFVI, the Gateway instead examines the OAL packet.  The
   Gateway first determines whether the OAL packet includes an MI/MR/MC
   message then processes the message according to the multilink
   forwarding procedures discussed in Section 4.13.  If the carrier
   packets arrived over the secured spanning tree and the enclosed OAL
   packets/fragments are addressed to its SRT SRA PNPADDR, the Gateway
   instead reassembles then discards the OAL header and forwards the
   original IP packet to the network layer to support secured BGP
   routing protocol control messaging.  The Gateway instead drops all
   other OAL packets.

   Gateways forward OAL packets/fragments received in carrier packets
   that arrived from a first segment via the secured spanning tree to
   the next segment also via the secured spanning tree.  Gateways
   forward OAL packets/fragments received in carrier packets that
   arrived from a first segment via the unsecured spanning tree to the
   next segment also via the unsecured spanning tree.  Gateways
   configure an IPv6 routing table that determines the next hop for a
   given OAL Destination Address, where the secured/unsecured spanning
   tree is determined through the selection of the underlay interface to
   be used for transmission (e.g., an IPsec tunnel or an open INET
   interface).

   As for Proxy/Servers, Gateways must verify that the L2 Source
   Addresses of carrier packets not received from the secured spanning
   tree are "trusted" before forwarding according to an AFV (otherwise,
   the carrier packet must be dropped).



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4.11.  OMNI Interface Error Handling

   When an AERO node admits an original IP packet into the OMNI
   interface, it may receive link and/or network layer error
   indications.  The AERO node may also receive OMNI link error
   indications in OAL-encapsulated MC messages that include
   authentication signatures.

   A link layer error indication is an ICMP error message generated by a
   router in an underlay network on the path to the next OAL hop or by
   the next OAL hop itself.  The message includes an IP header with the
   address of the node that generated the error as the Source Address
   and with the link layer address of the AERO node as the Destination
   Address.

   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  Valid type values include "Destination
   Unreachable", "Packet Too Big", "Time Exceeded", "Parameter Problem"
   etc.  [RFC0792][RFC4443].

   The ICMP header is followed by the leading portion of the carrier
   packet that generated the error, also known as the "packet-in-error".
   For ICMPv6, [RFC4443] specifies that the packet-in-error includes:
   "As much of invoking packet as possible without the ICMPv6 packet
   exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes).  For
   ICMPv4, [RFC0792] specifies that the packet-in-error includes:
   "Internet Header + 64 bits of Original Data Datagram", however
   [RFC1812] Section 4.3.2.3 updates this specification by stating: "the
   ICMP datagram SHOULD contain as much of the original datagram as
   possible without the length of the ICMP datagram exceeding 576
   bytes".

   The link layer error message format is shown in Figure 4:


















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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~    IP Header of link layer    ~
        ~         error message         ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~          ICMP Header          ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        |                               |   P
        ~   carrier packet L2 and OAL   ~   a
        ~     encapsulation headers     ~   c
        |                               |   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        |                               |   t
        ~    original IP packet hdrs    ~
        ~    (first-fragment only)      ~   i
        |                               |   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |   e
        ~    Portion of the body of     ~   r
        ~    the original IP packet     ~   r
        ~       (all fragments)         ~   o
        |                               |   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

          Figure 4: OMNI Interface Link-Layer Error Message Format

   The AERO node rules for processing these link layer error messages
   are as follows:

   *  When an AERO node receives a link layer Parameter Problem message,
      it processes the message the same as described as for ordinary
      ICMP errors in the normative references [RFC0792][RFC4443].

   *  When an AERO node receives persistent link layer Packet Too Big
      messages, there may be a restricting link on the path or the next
      OAL hop may be experiencing reassembly cache congestion.  In both
      cases, the node should adaptively decrease the size of the OAL
      fragments it sends to this OAL next hop (note that the PTB
      messages could indicate either "hard" or "soft" errors).

   *  When an AERO node receives persistent link layer Time Exceeded
      messages, the IP ID field may be wrapping before earlier fragments
      awaiting reassembly have been processed.  In that case, the node
      should adaptively decrease the size of the OAL fragments it sends
      to this OAL next hop.





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   *  When an AERO node receives persistent link layer Destination
      Unreachable messages in response to carrier packets that it sends
      to one of its neighbor correspondents, the node should process the
      message as an indication that a path may be failing, and
      optionally initiate NUD over that path.  If it receives
      Destination Unreachable messages over multiple paths, the node
      should allow future carrier packets destined to the correspondent
      to flow through a default route and re-initiate route
      optimization.

   *  When an AERO Client receives persistent link layer Destination
      Unreachable messages in response to carrier packets that it sends
      to one of its neighbor Proxy/Servers, the Client should mark the
      path as unusable and use another path.  If it receives Destination
      Unreachable messages on many or all paths, the Client should
      associate with a new Proxy/Server and release its association with
      the old Proxy/Server as specified in Section 4.15.5.

   *  When an AERO Proxy/Server receives persistent link layer
      Destination Unreachable messages in response to carrier packets
      that it sends to one of its neighbor Clients, the Proxy/Server
      should mark the underlay path as unusable and use another underlay
      path.

   *  When an AERO Proxy/Server receives link layer Destination
      Unreachable messages in response to a carrier packet that it sends
      to one of its permanent neighbors, it treats the messages as an
      indication that the path to the neighbor may be failing.  However,
      the dynamic routing protocol should soon re-converge and correct
      the temporary outage.

   When an AERO Gateway receives a carrier packet for which the network
   layer Destination Address is covered by an MSP assigned to a black-
   hole route, the Gateway drops the carrier packet if there is no more-
   specific routing information for the destination and returns an OMNI
   interface Destination Unreachable message subject to rate limiting.

   AERO nodes include ICMPv6 error messages intended for an OAL source
   as sub-options in the OMNI option of secured MC messages.  When the
   OAL source receives the MC message, it can extract the ICMPv6 error
   message enclosed in the OMNI option and either process it locally or
   translate it into a network layer error to return to the original
   source.

   An AERO/OMNI intermediate system may discover that a transit packet
   has no matching AFIB state to support forwarding to the next
   adaptation layer hop.  In that case, the intermediate system should
   return a Destination Unreachable error sub-option in a secured MC



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   message.  The OAL source should process the message as an indication
   that AFIB multilink forwarding state for a particular flow must be
   refreshed.

4.12.  AERO Mobility Service Coordination

   AERO nodes observes the Router Discovery and Prefix Registration
   specifications and coordinate their autoconfiguration actions with
   the mobility service through RS/RA message exchanges as specified in
   [I-D.templin-6man-omni3].

4.12.1.  AERO Service Model

   Each AERO Proxy/Server on the OMNI link is configured to respond to
   Client address delegation requests for Proxy Aggregated (PA)
   addressing.  Each Proxy/Server aggregates a unique PNP that it
   manages independently of other Proxy/Servers, and ensures that only
   unique PNPADDRs are delegated to Clients.  Each Proxy/Server runs its
   own independent DHCPv6 server that shares operational fate with the
   Proxy/Server itself.  If the Proxy/Server goes down, the DHCPv6
   service is also disabled and the lease database must be refreshed
   after the Proxy/Server reboots.  Clients associate their PNPADDR
   delegations with the adaptation layer of the OMNI interface in
   association with the corresponding underlay interface for each Proxy/
   Server.

   Each AERO Proxy/Server on the OMNI link is configured to respond to
   Client prefix delegation/registration requests for Provider
   Independent (PI) addressing also based on the DHCPv6 service.  Each
   Proxy/Server is provisioned with a database of MNP-to-Client ID
   mappings for all Clients enrolled in the AERO service, as well as any
   information necessary to authenticate each Client.  The Client
   database is maintained by a central administrative authority for the
   OMNI link and securely distributed to all Proxy/Servers, e.g., via
   the Lightweight Directory Access Protocol (LDAP) [RFC4511], via
   static configuration, etc.  Clients receive the same PI service
   regardless of the Proxy/Servers they select and provision their PI
   prefixes for downstream-attached node addressing on ENET interfaces.
   (Note: an OMNI link can instead delegate non-correlated MNPs to
   Clients instead of maintaining a common synchronized database.  In
   that case, each Client may receive a different MNP delegation each
   time it registers with the OMNI domain and may need to renumber its
   downstream-attached ENETs.)

   Clients associate each of their *NET underlay interfaces with FHS
   Proxy/Servers.  Each FHS Proxy/Server locally services one or more of
   the Client's underlay interfaces, and the Client typically selects
   one among them to serve as the MAP Proxy/Server (the Client may



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   instead select a "third-party" MAP Proxy/Server that does not
   directly service any of its underlay interfaces).  All of the
   Client's other FHS Proxy/Servers forward proxyed copies of RS/RA
   messages between the MAP Proxy/Server and Client without assuming the
   MAP role functions themselves.

   Each Client typically associates with a single MAP Proxy/Server,
   while all other Proxy/Servers are candidates for providing the MAP
   role for other Clients.  A Client can select both an FHS and MAP
   Proxy/Server in a single message by including an SRH in the RS
   message OAL header when it already knows the MAP's address.  An FHS
   Proxy/Server assumes the MAP role when it receives an RS message with
   a Destination Address that matches its own MLA, SRA PNPADDR or link-
   scoped All-Routers multicast.  An FHS Proxy/Server assumes the proxy
   role when it receives an RS message with an SRH or with a Destination
   Address that matches another Proxy/Server.  (An FHS Proxy/Server can
   also assume the proxy role when it receives an RS message addressed
   to link-scoped All-Routers multicast if it can determine the SRA
   PNPADDR of another Proxy/Server to serve as a MAP.)

   AERO Clients and Proxy/Servers use IPv6 ND messages to maintain
   NLNCEs and ALNCEs.  AERO Proxy/Servers configure their OMNI
   interfaces as advertising NBMA interfaces, and therefore send unicast
   RA messages with a short Router Lifetime value (e.g., 2 *
   ReachableTime seconds) in response to a Client's RS message.
   Thereafter, Clients send additional RS messages to keep Proxy/Server
   state alive.

   AERO Clients and FHS/MAP Proxy/Servers include PNPADDR address
   delegation (and optionally also MNP prefix delegation) DHCPv6
   parameters in RS/RA messages.  The IPv6 ND messages are exchanged
   between the Client and any FHS Proxy/Servers acting as proxys for the
   MAP Proxy/Server as specified in [I-D.templin-6man-omni3] according
   to the address/prefix management schedule required by the service.
   If the Client knows its MNP in advance, it can include the MNP in its
   DHCPv6 prefix delegation request.  If the MAP Proxy/Server accepts
   the Client's MNP assertion (or if it delegates a new MNP for the
   Client), it injects the MNP into the routing system and establishes
   the necessary neighbor cache state.

   AERO Clients and their FHS Proxy/Servers on MANETs and open INETs
   must establish and maintain Identification synchronization windows in
   their RS/RA exchanges.  The window synchronization provides a well-
   managed Identification value that the Client and Proxy/Server can use
   for validating IPv6 ND messages with authentication signatures.






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   All Client and Proxy/Server behaviors for the exchange of RS/RA
   messages are conducted according to the Router Discovery and Prefix
   Delegation specifications found in [I-D.templin-6man-omni3].

4.13.  AERO Address Resolution, Multilink Forwarding and Route
       Optimization

   AERO nodes invoke address resolution, multilink forwarding and route
   optimization when they need to forward the initial original IP
   packets of flows to new neighbors over (M)ANET/INET interfaces as
   well as for continuous multilink forwarding coordination with
   existing neighbors.  As specified in [RFC4861], "when a node has a
   unicast packet to send to a neighbor, but does not know the
   neighbor's link-layer address, it performs address resolution".

   Possible Source and Destination Addresses for original IP packets
   that traverse a local (M)ANET/INET and/or the rest of the OMNI link
   include addresses taken from an FNP, MNP or MLA assigned to a Client.
   The flow is then identified by the 3-tuple consisting of the IPv6
   Source Address, Destination Address and Flow Label.

   The Address Resolution Source (ARS) considers all candidate original
   IP packet destinations as on-link on the OMNI interface and invokes
   address resolution if there is no NLNCE matching the destination.
   The first such address resolution may return prefix information
   sufficient to satisfy future resolutions for addresses covered by the
   same prefix locally and without the need for additional IPv6 ND
   messaging over the network.

   Address resolution is based on an IPv6 ND NS/NA(AR) messaging
   exchange between the ARS and the target neighbor as the Address
   Resolution Target (ART).  The ARS engages address resolution by
   sending NS(AR) messages over a selected underlay interface to
   determine adaptation and link-layer address mappings for the ART
   network layer address.  (The ARS may select any available underlay
   interface to carry the NS(AR) with a likely emphasis on the best
   performing, least congested, etc.)

   The ARS then discovers address resolution information from any OMNI
   Interface Attributes sub-options included in NA(AR) messages returned
   by the ART.  Both the ARS and ART can update their Destination and
   Neighbor Caches based on the address resolution information and cache
   any information received in Route Information Options (RIOs)
   [RFC4191] included in the NS/NA(AR) exchange.  Note that the NS/
   NA(AR) RIOs are included as adaptation layer information in the OMNI
   option per [I-D.templin-6man-omni3] and are not delivered to the
   network layer.




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   The original source or its current FHS/MAP Proxy/Server serves as the
   ARS.  Either the ART itself or the current LHS/MAP Proxy/Server (or
   Relay) for the ART serves as the Address Resolution Responder (ARR),
   i.e., the NA(AR) source.

   Address resolution is initiated by the first eligible ARS closest to
   the original source as follows:

   *  For Clients on VPN/IPsec and Direct interfaces, the Client's FHS
      Proxy/Server is the ARS.

   *  For Clients on (M)ANET interfaces, either the FHS Proxy/Server or
      the Client itself may be the ARS.

   *  For Clients on INET interfaces, the Client itself is the ARS.

   *  For FNP correspondent nodes on foreign links/networks serviced by
      a Relay, the Relay is the ARS.

   *  For Clients that engage the MAP Proxy/Server in "mobility anchor"
      mode, the MAP Proxy/Server is the ARS.

   *  For peer Clients within the same (M)ANET/ENET, address resolution
      and route optimization is through receipt of Redirect messages.

   The AERO routing system directs an address resolution request sent by
   the ARS to the ARR.  The ARR then returns an address resolution reply
   which must include information that is complete, current, consistent
   and authentic.  Both the ARS and ARR are then jointly responsible for
   periodically refreshing the address resolution, and for quickly
   informing each other of any changes.  Following address resolution,
   the ARS and ART perform subsequent multilink forwarding and route
   optimization exchanges to maintain optimal forwarding profiles for
   each distinct flow.

   During multilink forwarding and/or route optimization an IPv6 ND
   message source may attach a small number of original IP packets
   associated with the message exchange as composite packet extensions
   per [I-D.templin-6man-omni3].  The authentication signatures and/or
   lower-layer security features employed at the OAL source and each OAL
   intermediate system will provide authorization and integrity services
   for both the IPv6 ND messages and their IP packet attachments.  The
   final OAL intermediate system in the path will then securely forward
   the IPv6 ND message IP packet attachments to the target.

   The source can attach original IP packets to the subject IPv6 ND
   message, but this may cause the message size to exceed the IPv6
   minimum MTU and/or result in sub-optimal forwarding for the IP packet



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   attachments.  In that case, the source can instead create small MC
   "pilot" messages used to transport the original IP packets as
   attachments over shortest paths determined by routing.  The OAL
   source can attach as many IP packets as will fit without causing the
   OAL packet to exceed the adaptation layer path MTU using the
   composite packet construct discussed in [I-D.templin-6man-omni3].

   When the target Proxy/Server or Client receives a pilot MC, it
   removes all passenger attachment original IP packets then delivers
   them to the destination.  This service supports assured (but sub-
   optimal) short-term delivery of protocol data while neighbor
   coordination is in progress without creating network state.

   The address resolution, multilink forwarding and route optimization
   procedures are specified in the following sections.

4.13.1.  Multilink Address Resolution

   The IP layer engages address resolution over OMNI interfaces the same
   as specified in Section 7 of [RFC4861] including the sending and
   receiving of NS/NA(AR) messages as well as their implications for
   neighbor cache entry creation and state management.  The NS/NA(AR)
   messages include addresses and S/TLLAOs in the same manner as for any
   interface and as discussed in Section 4.5.1.  The OMNI interface
   therefore exhibits an IP layer behavior that is indistinguishable
   from an ordinary Ethernet interface while managing adaptation layer
   state at a layer below IP.

   Address Resolution over OMNI interfaces is therefore driven by
   network layer NS/NA(AR) messaging the same as for any IP interface.
   The OMNI interface removes the S/TLLAO upon transmission of all NS/
   NA(AR) messages and includes a TLLAO upon reception of NA(AR)
   messages while including a locally unique link-layer address for this
   neighbor.  The OMNI interface also caches any updated adaptation
   layer addressing information found in NS/NA(AR) messages in the ALNC.

   When one or more original IP packets for a flow 3-tuple are forwarded
   over an OMNI interface, the ARS checks the Destination Cache to
   determine whether there is a NCE that matches the Destination
   Address.  If there is a NCE in the REACHABLE state, the ARS invokes
   the OAL and forwards the resulting carrier packets according to the
   cached state then returns from processing.  If there is no NLNCE but
   the ARS is able to determine that adaptation layer mapping state for
   the IP destination is available, e.g., by snooping the ALNC, it can
   also admit the original IP packets into the interface without
   requiring an NS/NA(AR) exchange.





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   Otherwise, if there is no NLNCE the ARS creates one in the INCOMPLETE
   state.  The ARS then prepares an Address Resolution NS(AR) message to
   send toward an ART.  The resulting NS(AR) message must be sent
   securely and includes Source, Destination and Target Addresses as
   discussed in Section 4.5.1.

   When the ARS admits the NS(AR) message into the OMNI interface, the
   adaptation layer returns an immediate NA(AR) if the ALNC already
   contains fresh address resolution information for the FNP/MNP prefix
   that covers the IP Destination Address.  Otherwise, the adaptation
   layer prepares to forward the NS(AR) while removing the SLLAO (if
   present) since the locally-unique link-layer address has no useful
   information for prospective neighbors.

   The ARS then includes an OMNI option with an authentication sub-
   option (if necessary).  If the ARS can commit to managing its own
   dynamic mobility updates, it then includes Interface Attributes and/
   or Traffic Selectors for all of the source Client's underlay
   interfaces plus RIOs for any of the Client's MNPs.  The ARS then
   calculates and includes an authentication signature (if necessary)
   followed by the checksum, then submits the NS(AR) message for OAL
   encapsulation.

   When the ARS is a FHS Proxy/Server, it sets the OAL Source Address to
   the Client's PNPADDR and sets the OAL Destination Address to the FNP/
   MNP SRA GUA corresponding to the ART.  The ARS then performs L2
   encapsulation and sends the resulting carrier packet into the SRT
   secured spanning tree without decrementing the network layer TTL/Hop
   Limit field.

   When the ARS is a Client, it instead uses its own MLA as the OAL
   Source Address and the MLA of the next OAL hop as the OAL Destination
   Address while including an SRH with the MLAs of OAL intermediate
   systems if necessary ending with the MLA of the interface-specific
   FHS Proxy/Server.  If the Client is in a MANET or an open INET, it
   next calculates and includes an authentication signature.  In all
   *NET cases, the Client then includes an OAL IPv6 Extended Fragment
   Header with Identification set to an in-window value for this FHS
   Proxy/Server.  The ARS Client then performs L2 encapsulation and
   forwards the carrier packet to the FHS Proxy/Server.

   The FHS Proxy/Server then performs L2 decapsulation, verifies the
   Identification, verifies the NS(AR) checksum/authentication signature
   and confirms that the Client's claimed FNP/MNP RIO(s) and Source
   Address are correct.  The FHS Proxy/Server then changes the OAL
   Source Address to the Client's PNPADDR and changes the OAL
   Destination Address to the FNP/MNP SRA GUA corresponding to the
   NS(AR) Target Address.  The FHS Proxy/Server next removes the IPv6



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   SRH and Extended Fragment Header, performs L2 encapsulation and sends
   the resulting carrier packet into the secured spanning tree on behalf
   of the Client.

   Note: both the source and target Client/Relay and their MAP Proxy/
   Servers include current and accurate information for their multilink
   Interface Attributes profile.  The MAP Proxy/Servers can be trusted
   to provide an authoritative ARR response and/or mobility update
   message on behalf of the source/target if necessary.

4.13.1.1.  Relaying the NS(AR)

   When a Gateway receives carrier packets containing the NS(AR), it
   performs L2 decapsulation and determines the next hop by consulting
   its standard IPv6 forwarding table for the OAL header Destination
   Address.  The Gateway next decrements the OAL header Hop Limit,
   performs L2 encapsulation and sends the carrier packet(s) via the
   secured spanning tree the same as for any IPv6 router where they may
   traverse multiple intermediate OMNI link segments interconnected by
   Gateways.  The final Gateway will deliver the carrier packets via the
   secured spanning tree to the LHS/MAP Proxy/Server (or Relay) that
   services the ART.

4.13.1.2.  NS(AR) Processing at the ARR/ART

   When the LHS/MAP Proxy/Server (or Relay) of the ART receives the
   NS(AR) secured carrier packets with the FNP/MNP SRA GUA of the ART as
   the OAL Destination Address, it performs L2 decapsulation then either
   forwards the NS(AR) to the ART or processes it locally if it is
   acting as the ART's designated ARR.  The LHS/MAP Proxy/Server (or
   Relay) processes the message as follows:

   *  if the NS(AR) target matches a Client NCE in the DEPARTED state,
      the (old) MAP Proxy/Server resets the OAL Destination Address to
      the PNPADDR of the Client at its new MAP Proxy/Server.  The old
      MAP Proxy/Server then decrements the OAL header Hop Limit,
      performs L2 encapsulation and forwards the resulting carrier
      packet over the secured spanning tree.

   *  If the NS(AR) target matches a Client NCE in the REACHABLE state,
      the LHS/MAP Proxy/Server (or Relay) notes whether the NS(AR)
      arrived from the secured spanning tree.  If the message arrived
      via the secured spanning tree the LHS/MAP Proxy/Server (or Relay)
      verifies the NS(AR) checksum only; otherwise, it must also verify
      the message authentication signature.






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   *  If the LHS/MAP Proxy/Server maintains a Report List for the ART,
      it next records the NS(AR) Source Address in the Report List for
      this ART.  If the MAP Proxy/Server is the ART's designated ARR, it
      forwards any original IP packet(s) attached to the NS(AR)
      composite packet to the ART and prepares to return an NA(AR) as
      discussed below; otherwise, the LHS/MAP Proxy/Server determines
      the underlay interface for the ART and proceeds as follows:

      -  If the LHS/MAP Proxy/Server is also the LHS Proxy/Server on the
         underlay interface used to convey the NS(AR) to the ART, it
         includes an OAL IPv6 Extended Fragment Header with an in-window
         Identification for the ART Client plus an SRH and
         authentication signature if necessary then recalculates the
         OMNI checksum.  The Proxy/Server then changes the OAL
         Destination Address to the MLA of the ART, decrements the OAL
         Hop Limit, performs L2 encapsulation and forwards the resulting
         carrier packet over the underlay interface to the ART.

      -  If the MAP Proxy/Server is not the LHS Proxy/Server on the
         underlay interface used to convey the NS(AR) to the ART, it
         instead changes the OAL Destination Address to the PNPADDR of
         the Client aggregated by the LHS Proxy/Server for the selected
         ART interface.  The MAP Proxy/Server next decrements the OAL
         Hop Limit, performs L2 encapsulation and forwards the resulting
         carrier packet over the secured spanning tree.

      -  When the LHS Proxy/Server receives the carrier packets, it
         performs L2 decapsulation, verifies the NS(AR) checksum, then
         forwards to the ART while changing the OAL Destination Address
         to the ART's MLA as above.  The LHS Proxy/Server also includes
         an IPv6 Extended Fragment Header plus an SRH and authentication
         signature if necessary while recalculating the checksum the
         same as described above.

   *  If the NS(AR) target matches one of its FNP routes, the MAP/LHS
      Proxy/Server serves as both a Relay and an ARR, since the Relay
      forwards original IP packets toward FNP target nodes at the
      network layer.

   If the ARR is a Relay or the ART itself, it first creates or updates
   a ALNCE for the NS(AR) MLA while caching all Interface Attributes and
   Traffic Selector information in the ALNCE and caching any IPv6
   addresses for the original source found in the Interface Attributes
   in the Destination Cache.  The ARR then installs any RIO MNP prefixes
   in the ALNCE.  (If the NS(AR) does not include address resolution
   information, the ART will simply need to initiate another
   unidirectional address resolution request if it has return traffic to
   send back to the ARS.)



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   Next, the ARR prepares a solicited NA(AR) message to return to the
   ARS as an adaptation layer function without exposing the NS(AR) to
   the network layer.  The ARR includes RIOs for all of the ART's FNPs/
   MNPs, where the RIO for a destination that matches only "default"
   includes a /64 FNP that covers the address.  The ARR then includes
   Interface Attributes and Traffic Selector sub-options for all of the
   ART's underlay interfaces with current information for each interface
   including their PNPADDRs.  The ARR next sets the NA(AR) message R
   flag to 1 (as a router) and S flag to 1 (as a response to a
   solicitation) and sets the O flag to 1 (as an authoritative
   responder).

   The ARR finally includes an authentication signature, an IPv6
   Extended Fragment Header and an SRH with addressing information for
   the ARS MAP/FHS Proxy/Server if necessary.  The ARR next calculates
   the NA(AR) message checksum then submits the NA(AR) for encapsulation
   with OAL Source Address set to its own MLA and Destination Address
   set to either the MLA that appeared in the NS(AR) OAL source for
   (M)ANET traversal or the NS(AR) source itself for INET traversal
   while including an SRH if necessary.  The ARR then performs L2
   encapsulation and forwards the resulting carrier packet.

   When the ART's FHS Proxy/Server receives carrier packets sent by an
   ART acting as an ARR on its own behalf, it performs L2 decapsulation
   then verifies the OAL Identification and NA(AR) message checksum/
   authentication signature.  The Proxy/Server then verifies that any
   RIO information is acceptable, changes the OAL Source Address to the
   Client's PNPADDR and changes the OAL Destination Address to the
   PNPADDR of the NA(AR) Destination.  The Proxy/Server next decrements
   the OAL Hop Limit, removes the OAL Extended Fragment Header, performs
   L2 encapsulation and finally forwards the resulting carrier packet
   into the secured spanning tree.

4.13.1.3.  Relaying the NA(AR)

   When a Gateway receives NA(AR) carrier packets, it performs L2
   decapsulation and determines the next hop by consulting its standard
   IPv6 forwarding table for the OAL header Destination Address.  The
   Gateway then decrements the OAL header Hop Limit, performs L2
   encapsulation and forwards the resulting carrier packet via the SRT
   secured spanning tree where it may traverse multiple intermediate
   OMNI link segments interconnected by other Gateways.  The final-hop
   Gateway will deliver the carrier packets via the secured spanning
   tree to a Proxy/Server for the ARS.







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4.13.1.4.  Processing the NA(AR) at the ARS

   When the ARS receives NA(AR) carrier packets, it performs L2
   decapsulation then searches for an ALNCE that matches the MLA
   corresponding to the NA(AR) Source.  The ARS then processes the
   message the same as for standard IPv6 Address Resolution [RFC4861].
   In the process, it caches all OMNI option Interface Attributes and
   Traffic Selectors in the ALNCE for the NA(AR) MLA Source Address and
   caches any IPv6 addresses for the ART found in the Interface
   Attributes in the Destination Cache.  The ARS then caches any RIO MNP
   prefixes in the ALNCE indexed by the neighbor's MLA.  All included
   Interface Attributes sub-options plus RIOs together provide the
   address mapping information necessary to satisfy address resolution.

   The adaptation layer of the ARS then includes a TLLAO with a locally-
   unique link-local address for the source then delivers the NA(AR) to
   the network layer.  The network layer will then set the NLNCE for
   this neighbor to REACHABLE while caching the link-layer address.
   Future original IP packet transmissions over the OMNI interface will
   use this IP to link-layer address mapping the same as for any IPv6
   interface.

   When the ARS is a Client, the SRT secured spanning tree will first
   deliver the solicited NA(AR) message to the Client's FHS Proxy/
   Server, which includes an OAL Extended Fragment Header with an in-
   window Identification for this Client, and forwards the message to
   the Client.  If the Client is on a well-managed ANET, physical
   security and protected spectrum ensures security for the NA(AR)
   without needing an additional authentication signature or
   Identification; if the Client is in a MANET or in the open INET the
   Proxy/Server must instead include an Identification and
   authentication signature.  The Proxy/Server then includes an SRH if
   necessary and changes the OAL Destination Address to the MLA of the
   Client when it forwards the NA(AR).  The Proxy/Server then decrements
   the OAL Hop Limit, performs L2 encapsulation and forwards the
   resulting carrier packet over the underlay interface to the Client.
   When the Client receives the NA(AR), it caches the adaptation layer
   information as above then removes the OMNI option and forwards the
   NS(AR) to the network layer.  The Client is then responsible for
   informing the ART if any of its adaptation layer addressing changes
   (e.g., due to mobility) before the ALCNE expires.

4.13.1.5.  Reliability

   After the ARS transmits the first NS(AR), it should wait up to
   RETRANS_TIMER seconds to receive a responsive NA(AR).  The ARS can
   then retransmit the NS(AR) up to MAX_UNICAST_SOLICIT times before
   giving up.



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4.13.2.  Multilink Forwarding

   Following address resolution, the ARS and ART (i.e., the end system
   Clients or their respective Proxy/Servers) can assert per-flow
   multilink forwarding paths through underlay interface pairs serviced
   by the same Source/Destination Addresses by sending MI/MR/MC messages
   with OMNI Neighbor Synchronization sub-options and with the OMNI
   trailer (I)nitialize flag set to 1.  The MI/MR/MC messages establish
   per-flow multilink forwarding and header compression state in OAL
   intermediate systems in the path between the ARS and ART.  Note that
   either the ARS or ART can independently initiate multilink forwarding
   by sending MI messages on behalf of specific flows over underlay
   interface pairs.

   The source Client or FHS Proxy/Server sets the MI address fields as
   discussed in Section 4.5.1 then includes the Flow Label of the
   original IP packet set according to [RFC6437][RFC6438].  The source
   finally performs OAL encapsulation with Source and Destination
   addresses set the same as for address resolution while including its
   MLA in a Node Identification sub-option and including an SRH
   extension if necessary.  The flow 3-tuple is then identified by the
   MI Target Address, Destination Address and Flow Label.

   When the target Client or LHS Proxy/Server returns an MR, it sets the
   address fields as discussed in Section 4.5.1.  The target Client or
   LHS Proxy/Server then sets the Flow Label to the same value that
   appeared in the MI and finally performs OAL encapsulation while
   including an MLA and SRH extension if necessary.

   The multilink forwarding profile provides support for redundant paths
   that each OAL node can harness to its best advantage.  For example,
   OAL nodes can use traffic selectors to distribute different traffic
   types over available multilink paths, while other factors such as
   metrics, cost, provider, etc. can also provide useful decision
   points.  OAL nodes can also employ multilink forwarding for fault
   tolerance by sending redundant data over multiple paths
   simultaneously, or for load balancing where the individual packets of
   a single traffic flow are spread across multiple independent paths.
   OAL nodes that engage in multilink forwarding therefore must
   incorporate a policy engine that selects both inbound and outbound
   multilink paths for a given traffic profile at a given point in time.
   This specification therefore provides multilink forwarding mechanisms
   without mandating any specific multilink policy.

   All Client, Proxy/Server and Gateway nodes that configure OMNI
   interfaces and engage in multilink coordination include an additional
   forwarding table termed the AERO Flow Information Base (AFIB) that
   supports OAL packet/fragment forwarding based on original IP packet



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   flows over specific OMNI neighbor interface pairs.  The AFIB contains
   per-flow AERO Flow Vectors (AFVs) identified by the L2 address of the
   previous OAL hop plus a value known as the AFV Index (AFVI).  The
   AFVs cache uncompressed OAL header information to support forwarding
   of packets with compressed headers as well as previous/next-hop
   addressing and AFVI information.  The AFVs also cache window
   synchronization state (i.e., the starting sequence number and window
   size) for each specific flow.  Using the window synchronization
   state, simple Identification-based data origin authentication is
   enabled at each OAL source, intermediate system and target node.

   Client and Proxy/Server OMNI interfaces manage end system AFIB
   entries in conjunction with their internal ALNC, where the ALNCEs
   link to (possibly) multiple AFVs with one per flow over a specific
   FHS/LHS interface ifIndex pair.  When OMNI interface peers need to
   coordinate, they locate a NLNCE for the peer (established through
   address resolution) then use the ALNCE as a nexus that aggregates
   potentially many AVFs which cache AFVIs to support multilink
   forwarding on a per-flow basis.  Gateway OMNI interfaces and the OMNI
   interfaces of Clients or Proxy/Servers acting as OAL intermediate
   nodes manage transit AFIB entries independently of their internal
   neighbor caches.  These transit AFVs are indexed by the L2 address
   and AFVI supplied by the previous hop.

   OAL source, intermediate system and target nodes create or update
   AFVs/AFVIs when they process an MI/MR/MC initiation or response
   message with an OMNI Neighbor Synchronization sub-option with the SYN
   flag set (see: [I-D.templin-6man-omni3]).  The Target Address of the
   initiating MI (which is also the Destination Address of the
   responsive MR) is considered to reside in the "First Hop Segment
   (FHS)", while the Destination Address of the MI (which is also the
   Target Address of the responsive MR) is considered to reside in the
   "Last Hop Segment (LHS)".

   The FHS and LHS roles are determined on a per-flow and per-interface-
   pair basis.  After address resolution, either peer is equally capable
   of initiating multilink forwarding on behalf of a specific flow.  The
   peer that sends the initiating MI message with Neighbor
   Synchronization for a specific pair becomes the FHS peer while the
   one that returns the responsive MR becomes the LHS peer for that
   (flow, interface pair) only.  It is therefore commonplace that peers
   may assume the FHS role for some flows while assuming the LHS role
   for others, i.e., even though each peer maintains only a single NCE.

   When an OAL node sends/forwards an initiating MI or responsive MR
   with a Neighbor Synchronization sub-option with the SYN flag set, it
   creates or updates an AFV, caches the Identification window
   information, caches the MI/MR OAL IPv6 header and caches the flow



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   3-tuple for compression/decompression of the original IP packet
   header.  The OAL node also records the previous hop L2 address and
   AFVI, then generates a new next hop AFVI or updates the lifetime of
   an already-established AFVI.  The next hop AFVI should be selected
   within the range [1 - (2**16-1)] unless all values within that range
   are already in active use.  Otherwise, the AFVI must be selected
   within the range [2**16 - (2**31-1)] while the value 0 indicates
   "AFVI unspecified".  When the OAL node forwards future OAL packets/
   fragments that include the previous hop L2 address and AFVI, it can
   unambiguously locate the correct AFV and use the cached information
   to forward to the next OAL hop.

   OAL nodes cache AFVs for up to ReachableTime seconds following their
   initial creation.  If the node processes another MI/MR message
   specific to an AFV, it updates ReachableTime to REACHABLE_TIME
   seconds, i.e., the same as for NCEs.  If ReachableTime expires, the
   node deletes the AFV.

   The following sections provide the detailed specifications of these
   MI/MR/MC exchanges for all nodes along the forward and reverse paths.

4.13.2.1.  FHS Client-Proxy/Server MI Forwarding

   When an FHS OAL source has an original IP packet to send toward an
   LHS OAL target, it first performs address resolution resulting in the
   creation of an ALNCE for the MLA of the target then selects a source
   and target underlay interface pair.  The FHS source then uses its
   cached information for the target interface as LHS information then
   prepares an MI message with a Neighbor Synchronization sub-option
   while setting the MI Source, Target and Destination Addresses as
   specified above.

   The FHS source next creates an AFV then generates and assigns an AFVI
   for the flow over this interface pair; the AFVI must be unique for
   its communications to this next OAL hop.  The FHS source then
   includes an SRH with segment addressing information if necessary.
   The FHS source finally includes an OMNI Neighbor Synchronization sub-
   option with window synchronization parameters and prepares the MI
   message for transmission while also caching the window
   synchronization parameters in the local AFV.

   If the FHS source is the FHS Proxy/Server, it performs OAL
   encapsulation while setting the OAL Source Address to the Client's
   PNPADDR and setting the OAL Destination Address to the destination's
   PNPADDR.  The FHS Proxy/Server then performs L2 encapsulation and
   forwards the resulting carrier packet into the secured spanning tree
   which will deliver it to an FHS Gateway.




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   If the FHS source is the FHS Client, it instead includes an
   authentication signature and OAL Extended Fragment Header with an in-
   window Identification for its FHS Proxy/Server if necessary.  If FMT-
   Forward and FMT-Mode are both set, the Client sets the Neighbor
   Synchronization LHS ifIndex to the ifIndex of the target; otherwise,
   it sets the ifIndex to 0 to allow the FHS Proxy/Server to select the
   target ifIndex.  The FHS Client then calculates the IPv6 ND message
   checksum, performs OAL encapsulation while including an SRH if
   necessary with the PNPADDR of the destination as the final entry,
   sets the OAL Source Address to its own MLA and sets the OAL
   Destination Address to the MLA of the first hop toward the FHS Proxy/
   Server.  The FHS Client finally performs L2 encapsulation and
   forwards the resulting carrier packet to the FHS Proxy/Server.

   If there are multiple OAL hops between the Client and FHS Proxy/
   Server, the first OAL intermediate node receives the carrier packets
   containing the MI then also verifies the checksum and authentication
   signature.  The OAL intermediate node then caches the FHS/LHS Client
   addressing, AFVI and window synchronization information as previous
   hop information in a new or existing AFV.  The OAL intermediate hop
   then creates a new unique AFVI to forward to the next OAL hop, then
   both caches the AFVI and writes it into the IPv6 ND message AVI
   field, i.e., while over-writing the value supplied by the previous
   hop.  The OAL intermediate node then forwards to the next OAL hop
   toward the FHS Proxy/Server which then performs the same functions as
   the previous OAL hop.

   When the FHS Proxy/Server receives the carrier packets, it performs
   L2 decapsulation, verifies the Identification, and verifies the MI
   checksum and authentication signature.  The FHS Proxy/Server then
   creates an AFV (i.e., the same as the FHS Client had done) while
   caching the FHS/LHS Client addressing, AFVI and window
   synchronization information as previous hop information for this AFV.
   The FHS Proxy/Server next generates a new unique AFVI to forward to
   the next OAL hop, then both caches the AFVI in the AFV and writes it
   into the MI AFVI field.  The FHS Proxy/Server next calculates the MI
   checksum then sets the OAL Source Address to the Client's PNPADDR and
   OAL Destination Address to the destination's PNPADDR.  The FHS Proxy/
   Server finally decrements the OAL Hop Limit, removes the OAL Extended
   Fragment Header, performs L2 encapsulation and forwards the resulting
   carrier packet into the secured spanning tree.










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4.13.2.2.  FHS/intermediate/LHS Gateway MI Forwarding

   Gateways in the spanning tree forward OAL packets/fragments not
   explicitly addressed to themselves, while forwarding those that
   arrived via the secured spanning tree to the next hop also via the
   secured spanning tree and forwarding all others via the unsecured
   spanning tree.  When an FHS Gateway receives an MI packet over the
   secured spanning tree, it performs L2 decapsulation then verifies the
   MI checksum.  The FHS Gateway next creates an AFV based on the
   previous hop Neighbor Synchronization information, i.e., the same as
   the FHS Proxy/Server had done.  The FHS Gateway then generates a
   locally-unique AFVI for the next hop and both caches the value in the
   AFV and copies it into the MI AFVI.

   The FHS Gateway then examines the SRT prefixes corresponding to both
   the FHS and LHS.  If the FHS Gateway has a local interface connection
   to both the FHS and LHS (whether they are the same or different
   segments), the FHS/LHS Gateway caches the MI Neighbor Synchronization
   information in the AFV, and writes a new locally-unique AFVI for the
   next hop into the AFV and MI AFVI.  The FHS Gateway then decrements
   the OAL Hop Limit, performs L2 encapsulation and forwards the
   resulting carrier packet into the secured spanning tree.

   When the FHS and LHS Gateways are different, the LHS Gateway will
   receive carrier packets over the secured spanning tree from the FHS
   Gateway, noting there may be many intermediate Gateways in the path
   between FHS and LHS which will update their transit AFVs in the same
   fashion while selecting new locally-unique AFVIs for the next hop
   based on Neighbor Synchronization and SRH information.  The LHS
   Gateway then performs L2 decapsulation, verifies the Identification,
   verifies the MI checksum then creates an AFV (i.e., the same as all
   previous hop Gateways had done) while caching the Neighbor
   Synchronization information from the previous hop and creating a new
   AFVI for the next hop.  The LHS Gateway then decrements the OAL Hop
   Limit, performs L2 encapsulation and forwards the resulting carrier
   packet into the secured spanning tree.

4.13.2.3.  LHS Proxy/Server-Client MI/MR Processing

   When the LHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it performs L2 decapsulation, verifies the MI
   checksum then creates an AFV and caches the previous hop Neighbor
   Synchronization and addressing information.








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   If the MI Destination Address matches the target and the LHS Proxy/
   Server is configured to respond on the target's behalf, (i.e., if
   FMT-Forward is set) it next prepares to return a responsive MR.  The
   LHS Proxy/Server next creates or updates an NCE for the MI Source
   Address (if necessary) with state set to STALE.

   The LHS Proxy/Server then creates an MR while copying the Neighbor
   Synchronization sub-option from the MI and including responsive
   window synchronization information.  The LHS Proxy/Server sets the MR
   addresses as discussed in Section 4.5.1 then encapsulates the MR with
   OAL Source Address set to the MI OAL Destination Address and with OAL
   Destination Address set to the MI OAL Source Address.  If the LHS
   Proxy/Server will set the MR message SYN flag, it also writes a non-
   zero value in the MR AFVI field; otherwise, it writes the value 0.
   The LHS Proxy/Server then calculates the MR checksum, performs L2
   encapsulation and forwards the resulting carrier packet into the
   secured spanning tree.

   The LHS Proxy/Server then creates a locally-unique AFVI for the
   Client and both caches it in the newly-created AFV and writes it into
   the MR AFVI field.  If FMT-Forward is clear and FMT-Mode is set, the
   LHS Proxy/Server next resets the Neighbor Synchronization FHS ifIndex
   to 0.  The LHS Proxy/Server next includes an authentication signature
   in the MI if necessary, changes the OAL Destination Address to the
   MLA of the LHS Client and includes an SRH with the MLAs of
   intermediate systems if necessary.  The LHS Proxy/Server then
   decrements the OAL Hop Limit, includes an OAL Extended Fragment
   Header with an appropriate Identification value if necessary,
   performs L2 encapsulation and forwards the resulting carrier packet
   to the LHS Client.

   If there are multiple OAL hops between the LHS Proxy/Server and LHS
   Client, the first OAL intermediate node receives the carrier packet
   containing the MI then also verifies the checksum and authentication
   signature.  The OAL intermediate node then caches the FHS/LHS Client
   addressing, AFVI and window synchronization information as previous
   hop information in a new or existing AFV.  The OAL intermediate hop
   then creates a new unique AFVI to forward to the next OAL hop, then
   both caches the AFVI and writes it into the MI, i.e., while over-
   writing the value supplied by the previous hop.  The OAL intermediate
   node then forwards to the next OAL hop toward the LHS Client which
   then performs the same functions as the previous OAL hop.

   When the LHS Client receives the carrier packet, it performs L2
   decapsulation, verifies the Identification, then verifies the MI
   checksum/authentication signature.  The LHS Client then creates an
   ALNCE for the MI Source Address (if necessary) in the STALE state and
   caches the MI Neighbor Synchronization information in a new AFV



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   associated with the NCE corresponding to the MI Source Address.  If
   the LHS Client will request reverse path state establishment, it
   finally generates and assigns a locally-unique AFVI for a flow to be
   forwarded to the previous hop, which it caches in the new AFV.
   Otherwise, the LHS Client sets the reverse path AFVI to 0.

   The LHS Client then prepares an MR using the same procedures as for
   the LHS Proxy/Server above while including responsive window
   synchronization information, the new AFVI in the MR and with SRH
   addressing information necessary for OAL routing in the reverse path.
   The LHS Client includes an authentication signature if necessary,
   calculates the MR message checksum, then encapsulates the MR with OAL
   Source Address set to its own MLA and OAL Destination Address set to
   the MLA of the LHS Proxy/Server.  The LHS Client finally includes an
   OAL SRH and Extended Fragment Header with an appropriate
   Identification if necessary, performs L2 encapsulation and forwards
   the resulting carrier packet to the LHS Proxy/Server.

   If there are multiple OAL hops between the LHS Client and LHS Proxy/
   Server, the first OAL intermediate node receives the carrier packet
   containing the MR then also verifies the checksum and authentication
   signature.  If the MR Neighbor Synchronization sub-option SYN flag is
   set, the OAL intermediate node then caches the LHS/FHS Client
   addressing, AFVI and window synchronization information as previous
   hop information in a new or existing AFV.  The OAL intermediate hop
   then creates a new unique AFVI to forward to the next OAL hop, then
   both caches the AFVI and writes it into the MR, i.e., while over-
   writing the value supplied by the previous hop.  The OAL intermediate
   node then forwards to the next OAL hop toward the LHS Proxy/Server
   which then performs the same functions as the previous OAL hop.

4.13.2.4.  Reverse Path Forwarding of the MR

   When the LHS Proxy/Server receives the carrier packet, it performs L2
   decapsulation, verifies the Identification and verifies the MR
   checksum/authentication signature.  The LHS Proxy/Server then changes
   the OAL Source Address to the LHS Client's PNPADDR, changes the OAL
   Destination Address to the FHS Client's PNPADDR, removes the OAL
   Extended Fragment Header and SRH and forwards the MR over the reverse
   path toward the initiating FHS node, where it may traverse many
   intermediate Gateways.










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   Each Gateway along the reverse path processes the Neighbor
   Synchronization information in the MR message in the same way that
   the Gateways in the forward path had processed the MI.  If the FHS
   Proxy/Server forwards the message, it changes the OAL Destination
   Address to the MLA of the FHS Client and includes an SRH if
   necessary.  The MR will eventually arrive at the initiating FHS node
   as confirmation that AFV state is established in all end and
   intermediate systems in the forward path.

   Flow state is normally unidirectional from the source to the
   destination, however the LHS node can elect bidirectional state by
   setting the SYN flag in the MR Neighbor Synchronization sub-option.
   In that case, the forward and reverse paths between the FHS initiator
   and LHS responder may traverse different sets of intermediate nodes
   but the source and destination would need some way to coordinate the
   value used in the Flow Label.  The initiator should then complete the
   three-way handshake by returning an MC or a data packet with an in-
   window Identification value to confirm that the SYN was received.

   Under nominal conditions when the source and destination do not
   coordinate their flow 3-tuples, if AFV state is needed in the reverse
   path a separate MI/MR exchange between the LHS initiator and FHS
   responder based on asynchronous packet arrivals may be needed.  In
   that case, the forward and reverse path flows bear no relation to one
   another and will often include different flow 3-tuple information.

4.13.2.5.  OAL End System Exchanges Following Synchronization

   Following the initial MI/MR/MC exchange OAL end systems can begin
   exchanging ordinary carrier packets for synchronized flows that
   include AFVIs and with Identification values within their respective
   send windows without requiring security signatures and/or secured
   spanning tree traversal.  OAL end and intermediate systems can also
   consult their AFIBs when they receive carrier packets that contain
   OAL packets/fragments with AFVIs to unambiguously locate the correct
   AFV and can use the AFV state to forward OAL packets/fragments to the
   next hop.  OAL end systems must then perform continuous MI/MR/MC
   exchanges to update window state, register new flows for optimized
   multilink forwarding, confirm reachability and/or refresh AFIB cache
   state in the path before ReachableTime expires.

   While the OAL end systems continue to actively exchange OAL packets,
   they are jointly responsible for updating cache state and per-
   interface reachability before expiration.  Window synchronization
   state is performed on a per-flow basis and tracked in the AFVs which
   are also linked to the appropriate NCE.  However, the window
   synchronization exchange only confirms target Client reachability
   over the specific underlay interface pair.  Reachability for other



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   underlay interfaces that share the same NCE must be determined
   individually using additional MI/MR/MC messages that include Neighbor
   Synchronization information.

   OAL sources can then begin including OCHs in OAL packets/fragments
   with an AFVI that OAL intermediate systems can use for shortest-path
   forwarding based on AFVIs instead of spanning tree OAL IPv6
   addresses.  Forwarding based on the limited OCH information is
   supported since all OAL nodes in the path up to (and sometimes
   including) the OAL destination have already established AFVs.

   When a Proxy/Server receives OAL packets/fragments destined to a
   local SRT segment Client or forwards OAL packets/fragments received
   from a local segment Client, it first locates the correct AFV.  If
   the OAL packet/fragment includes a secured IPv6 ND message, the
   Proxy/Server uses the Client's ALNCE established through RS/RA
   exchanges to re-encapsulate while sending outbound secured carrier
   packets via the secured spanning tree and sending inbound secured
   carrier packets while including an authentication signature/checksum.
   For ordinary OAL packets/fragments, the Proxy/Server uses the same
   AFV if directed by AFVI and/or OAL addressing.  Otherwise it locates
   an AFV established through an MI/MR/MC exchange between the Client
   and the remote SRT segment peer, and forwards the OAL packet/
   fragments without first reassembling/decapsulating.

   When a source Client forwards OAL packets/fragments it can employ
   header compression according to the AFVs established through an
   MI/MR/MC exchange with a remote or local peer.  When a target Client
   receives carrier packets that contain OAL packets/fragments that
   match a local AFV, the Client first verifies the Identification then
   decompresses the headers if necessary, reassembles to obtain the OAL
   packet then decapsulates and delivers the original IP packet to the
   network layer.

   When synchronized peer Clients in the same SRT segment with FMT-
   Forward and FMT-Mode set discover each other's NATed L2ADDR
   addresses, they can exchange carrier packets that contain OAL
   packets/fragments directly with header compression using AFVIs
   discovered as above (see: Section 4.13.4.3).

   When the FHS Client or FHS Proxy/Server sends an MI for the purpose
   of establishing multilink forwarding state, it should wait up to
   RETRANS_TIMER seconds to receive a responsive MR.  The FHS node can
   then retransmit the MI up to MAX_UNICAST_SOLICIT times before giving
   up.






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4.13.2.6.  Dynamic Multilink Flow State Management

   Following address resolution, either the ARS or ART acting as an OAL
   source may immediately begin forwarding original IP packets for a
   flow as composite packet attachments to "pilot" MC messages without
   waiting for an initial MI/MR exchange.  The messages will include an
   authentication signature if necessary that covers the entire
   composite packet including the MC pilot and all original IP packet
   attachments.

   Each MC message OMNI option contains a Neighbor Synchronization sub-
   option with the SYN flag set to cause OAL intermediate systems in the
   forward path to create or update AFIB state.  The MC sets its
   Source/Destination/Target Addresses and Neighbor Synchronization
   flags the same as described for MI messages above.  All OAL
   intermediate systems in the forward path will process the MC message
   the same as specified for MI messages above.

   The OAL source should continue to forward additional original IP
   packets for the flow that arrive during a brief convergence interval
   as secured MC composite packets, e.g., up to N composite packets
   within a 1 second window.  The OAL source can then begin forwarding
   new IP packet arrivals for the flow via the unsecured spanning tree
   while applying OAL encapsulation, fragmentation and header
   compression.  The OAL source can continue forwarding under the
   optimistic expectation that AFIB state is securely established in the
   forward path.  As AFIB state becomes stale, the OAL source can resume
   forwarding secured MC composite packets during a new convergence
   interval to refresh/renew state as above before resuming via the
   unsecured spanning tree.

   The OAL source can also forward OAL-fragmented packets in this manner
   by including the fragment body preceded by an OMNI Fragment Header
   (OMNI-FH) immediately following the OAL-encapsulated MC message.
   Each fragment MUST be no larger than the minimum OAL Fragment Size
   (OFS) of 1024 octets to ensure they will transit the secured spanning
   tree without encountering a size restriction.  The fragment size is
   determined by the MC message payload length minus the initial payload
   length up to the beginning of the OMNI-FH, i.e., the fragment must
   appear as the final trailing component of the composite packet.

   Further information on OMNI fragmentation and reassembly can be found
   in [I-D.templin-6man-omni3].








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4.13.3.  Mobile Ad-hoc Network (MANET) Forwarding

   Clients with OMNI interfaces configured over underlay interfaces with
   indeterminant neighborhood properties may be connected to a Mobile
   Ad-hoc NETwork (MANET).  Each MANET may be either completely outside
   of the range of any OMNI link Proxy/Servers or may require multihop
   traversal between Clients acting as MANET routers to reach Proxy/
   Servers that connect to the rest of the OMNI link.  The former class
   of MANETs must operate in isolation solely based on the unique IPv6
   MLAs they configure locally.  The latter class allows MANET routers
   to extend infrastructure-based addressing information including MNPs
   over multiple OMNI link hops as discussed in the OMNI specification.

   MANET Clients configure their OMNI interfaces over one or more MANET
   interfaces where multihop forwarding may be necessary.  Routing
   protocols suitable for use over MANET interfaces include OSPFv3
   [RFC5340] with MANET Designated Router (OSPF-MDR) extensions
   [RFC5614], OLSRv2 [RFC7181], Babel [RFC8966], AODVv2
   [I-D.perkins-manet-aodvv2] and others.  Other services specific to
   MANET link-local and/or site-local operations (including SMF
   [RFC6621], DLEP [RFC8175] and others) are also considered in-scope.
   These services strive for optimal use of available radio bandwidth
   and power consumption in their control message transmissions, but
   efficient data plane operation is also essential.

   Clients must therefore reduce overhead through minimal encapsulation
   and effective header compression whenever possible.  For this reason,
   when the MANET routing protocol discovers a new route the Client
   configures a lesser-preferred forwarding table entry over the
   corresponding MANET interface and a more-preferred forwarding table
   entry over the OMNI interface.  This will cause the network layer to
   direct outbound packets to the OMNI interface, which can apply header
   compression and underlay MANET interface selection.

   Multilink Clients that connect a MANET to the rest of the OMNI link
   act as regular Clients for exchanges with external INETs, but act as
   Proxy/Servers over their MANET interfaces.  Each such Client
   therefore has at least two underlay interfaces, including both INET
   and MANET interfaces.  The Client therefore services the MANET as if
   it were a Proxy/server but presents itself as a Client to external
   facing INETs.  This class of Clients are also known as "Proxy/
   Clients".

   The process for a multihop Client to establish multilink forwarding
   and header compression AFV state in the MANET is conducted in the
   same fashion as described above and using the same MI/MR/MC or
   unidirectional MC message exchanges.  Each MANET forwarding node in
   the path creates or updates AFV state in the same fashion as for



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   intermediate Gateways in the secured spanning tree except that the
   MI/MR/MC messages require authentication signatures (unless
   neighboring MANET nodes configure IPsec tunnels) and an
   Identification that is within the window for its serving Proxy/Server
   if the destination is outside of the local MANET.  The MI/MR/MC
   messages extend from the initiating FHS MANET Client, then across any
   MANET hops over intermediate FHS Proxy/Clients, then to the FHS
   Proxy/Server, then across the secured SRT spanning tree to the LHS
   Proxy/Server, then finally across any intermediate LHS MANET hops to
   the responding LHS Client.  In all other ways, the MI/MR/MC or
   unidirectional MC exchanges are the same as discussed in
   Section 4.13.2.

   Following the MI/MR/MC exchanges, each MANET router in the forward
   (and optionally also reverse) path in both the FHS and LHS MANETs
   will have established AFVs containing multilink forwarding and header
   compression state for the flow.  The AFVs determine AFVI-based
   forwarding based on the OCH header contents, and each MANET router
   only forwards packets with in-window Identification values for the
   flow.  MANET routers maintain AFVs for up to ReachableTime seconds
   unless they are refreshed by a new MI/MR/MC message.  New window
   synchronization exchanges must also be performed periodically to
   avoid window exhaustion and/or spoofing based on predictable
   Identifications.

   Note: while the MANET routing protocol runs directly over the node's
   MANET interfaces to discover routing information, the node configures
   lesser-preferred forwarding table entries over the MANET interface
   and corresponding more-preferred forwarding table entries over the
   OMNI interface.  This causes the network layer to forward outbound
   packets via the OMNI interface which applies encapsulation,
   fragmentation and/or header compression as necessary before
   forwarding over the underlying MANET interface.  The OMNI protocol
   designator in the UDP port, IP protocol or Ethernet EtherType field
   will then cause the packets to visit the OMNI interface of each
   successive next-hop MANET node.

4.13.4.  AERO Route Optimization

4.13.4.1.  Proxy/Server-to-Proxy/Server Route Optimization

   When the FHS and LHS Proxy/Servers are both connected to an IPv6
   underlay for the same SRT segment, they can forward MI/MR/MC
   exchanges directly over the underlay without engaging SRT spanning
   tree hops.  This is made possible when the FHS and LHS Proxy/Servers
   inject their PNPs into the underlay network routing system as well as
   the SRT overlay routing system.




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   If the underlay is not secured, the FHS and LHS Proxy/Servers must
   include an authentication signature with their MI/MR/MC messages,
   which could either be the original authentication signature included
   by their respective Clients or a new signature included by the Proxy/
   Server itself.  If the Proxy/Server that processes the MI/MR/MC
   message determines that the message is authentic, it creates or
   updates an AFV entry according to the multilink forwarding
   parameters.  This establishes both AFVI and Identification window
   state to be used for future data traffic forwarding.

4.13.4.2.  Gateway-to-Proxy/Server Route Optimization

   When the LHS gateway and FHS Proxy/Server are both connected to an
   IPv6 underlay for the same SRT segment, they can forward MI/MR/MC
   exchanges directly over the underlay without engaging additional SRT
   spanning tree hops.  In this arrangement, the LHS Gateway acts the
   same as the FHS Proxy/Server as discussed in Section 4.13.4.1 and
   observes the requirement for including authentication signatures.

4.13.4.3.  Client-to-Client Route Optimization

   When the FHS/LHS Clients are both located on the same SRT segment,
   Client-to-Client route optimization is possible following the
   establishment of any necessary state in NATs in the path.  Both
   Clients will have already established state via their respective
   shared segment Proxy/Servers (and possibly also any shared segment
   Gateways) and can begin sending carrier packets directly via NAT
   traversal while avoiding any Proxy/Server and/or Gateway hops.

   When the FHS/LHS Clients on the same SRT segment perform initial
   MI/MR/MC exchanges to establish AFIB state, they first examine the
   FMT-Forward and FMT-Mode settings to determine whether direct-path
   forwarding is even possible for one or both Clients (direct-path
   forwarding is only possible when FMT-Forward and FMT-Mode are both
   set).  The MI/MR/MC messages then include an Interface Attributes
   sub-option (i.e., in addition to a Neighbor Synchronization sub-
   option) with the mapped L2ADDR information discovered during the RS/
   RA exchanges with their respective Proxy/Servers.  After the AFV
   paths have been established, both Clients can begin sending carrier
   packets via strict AFV paths while establishing a direct path for
   Client-to-Client route optimization.

   To establish the direct path, either Client (acting as the source)
   transmits a bubble to the mapped L2 address for the target Client
   which primes the local chain of NATs for reception of future carrier
   packets from that L2 address (see: [RFC4380] and
   [I-D.templin-6man-omni3]).  The source Client then prepares an MI
   message with its own MNP SRA GUA as the Source Address, with the MNP



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   SRA GUA of the target as the Target and Destination Address and with
   an OMNI option with an Interface Attributes sub-option.  The source
   Client then encapsulates the MI in an OAL header with its own MLA as
   the Source Address, with the MLA of the Proxy/Server as the
   Destination Address and with an in-window Identification for the
   target.  The source Client then performs L2 encapsulation and sends
   the resulting carrier packets to the Proxy/Server.

   When the Proxy/Server receives the carrier packets, it re-
   encapsulates and sends them as unsecured carrier packets according to
   AFIB state where they will eventually arrive at the target Client.
   The target Client then prepares an MR message with its own MNP SRA
   GUA as the Source and Target Address, with the MNP SRA GUA of the
   source Client as the Destination Address and with an OMNI option with
   an Interface Attributes sub-option.  The target Client then
   encapsulates the MR in an OAL header with its own MLA as the Source
   Address, with the MLA of the source Client as the Destination Address
   and with an in-window Identification for the source Client.  The
   target Client then performs L2 encapsulation then forwards the
   resulting carrier packet directly to the source Client.

   Following the initial MI/MR exchange, both Clients mark their
   respective (source, target) underlay interface pairs as "trusted" for
   no more than ReachableTime seconds.  The Clients can then begin
   exchanging ordinary data packets as OCH encapsulated carrier packets.
   While the Clients continue to exchange packets via the direct path
   avoiding all Proxy/Servers and Gateways, they should perform
   additional MI/MR exchanges via their local Proxy/Servers to refresh
   NCE state as well as send additional bubbles to the peer's L2ADDR if
   necessary to refresh NAT state.

   Note: these procedures are suitable for a widely-deployed but basic
   class of NATs.  Procedures for advanced NAT classes are outlined in
   [RFC6081], which provides mechanisms that can be employed equally for
   AERO using the corresponding sub-options specified by OMNI.

   Note: each communicating pair of Clients may need to maintain NAT
   state for peer to peer communications via multiple underlay interface
   pairs and/or multiple flows.  It is therefore important that L2ADDR
   information is maintained with the correct peer interface and that
   the NCE may cache information for multiple peer interfaces.

   Note: the source and target Client exchange L2ADDR information during
   the secured MI/MR/MC multilink route optimization exchange.  This
   allows for subsequent MI/MR/MC exchanges to proceed using only the
   Identification value as a data origin confirmation.  However, Client-
   to-Client peerings that require stronger security may also include
   authentication signatures for mutual authentication.



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4.13.4.4.  Intra-(M)ANET/ENET Route Optimization

   When a Client forwards an OAL packet (or an original IP packet) from
   another Client connected to one of its downstream ENETs to a peer
   within the same downstream ENET, the Client returns an IPv6 ND
   Redirect message to inform the source that the target can be reached
   directly.  The contents of the Redirect message are the same as
   specified in [RFC4861], and should also include any OMNI option RIOs
   with MNP information corresponding to the target.

   In the same fashion, when a Proxy/Server forwards an OAL packet (or
   original IP packet) from a Client connected to one of its downstream
   *NETs to a peer within the same downstream *NET, the Proxy/Server
   returns an IPv6 ND Redirect message.

   All other route optimization functions are conducted per the MI/MR/MC
   messaging discussed in the previous sections.

4.14.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) per
   [RFC4861] either reactively in response to persistent link layer
   errors (see Section 4.11) or proactively to confirm reachability.
   The NUD algorithm is based on periodic control message exchanges and
   may further be seeded by IPv6 ND hints of forward progress, but care
   must be taken to avoid inferring reachability based on spoofed
   information.

   NUD messaging is driven by the network layer under the same
   conditions as for any interface.  The NS/NA(NUD) messages follow the
   same forwarding and processing rules as for address resolution and
   confirm that the target neighbor is still reachable without making
   any statement about specific underlay interface pairs.  The messages
   may also update NLNCE/ALNCE state if they include address resolution
   information and an authentication signature.

   In order to test specific underlay interface pairs, the adaptation
   layer can independently send an MI message with an AFVI with the OMNI
   trailer (I)nitialize flag set to 0 in order to receive an MR
   response.  The MI messages may be sent while ordinary data packets
   are flowing either as independent messages or as pilot messages of
   composite packets.  The MI messages follow already-established paths
   without updating state and therefore need not include an
   authentication signature but should include an OMNI Nonce sub-option.

   When the source receives the MR, it marks the target underlay
   interface tested as "trusted".  Note that underlay interface states
   are maintained independently of the overall NCE REACHABLE state, and



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   that a single NCE may have multiple target underlay interfaces in
   various "trusted/untrusted" states while the NCE state as a whole
   remains REACHABLE.

4.15.  Mobility Management and Quality of Service (QoS)

   AERO is a fully Distributed Mobility Management (DMM) service in
   which each Proxy/Server is responsible for only a subset of the
   Clients on the OMNI link.  This is in contrast to a Centralized
   Mobility Management (CMM) service where there are only one or a few
   network mobility collective entities for large Client populations.
   Clients coordinate with their associated FHS and MAP Proxy/Servers
   via RS/RA exchanges to maintain the DMM profile, and the AERO routing
   system tracks all current Client/Proxy/Server peering relationships.

   MAP Proxy/Servers provide an address resolution aggregation point for
   their dependent Clients, while FHS Proxy/Servers provide a proxy
   conduit between the Client and both the MAP and OMNI link in general.
   Clients are responsible for maintaining neighbor relationships with
   their Proxy/Servers through periodic RS/RA exchanges, which also
   serves to confirm neighbor reachability.  When a Client's underlay
   interface attributes change, the Client is responsible for updating
   the MAP Proxy/Server through new RS/RA exchanges using the FHS Proxy/
   Server as a first-hop conduit.  The FHS Proxy/Server can also act as
   a proxy to perform some IPv6 ND exchanges on the Client's behalf
   without consuming bandwidth on the Client underlay interface.

   Note: when a Client's underlay interface address changes, the Client
   and/or its (former) FHS Proxy/Server for this interface must
   invalidate any AFVs based on the (changed) interface.  Future data
   packet forwarding will then trigger a new multilink forwarding MI/MR/
   MC exchange to re-populate new AFVs in the path.

   Mobility management considerations are specified in the following
   sections.

4.15.1.  Mobility Update Messaging

   Mobile Clients (and/or their MAP Proxy/Servers) accommodate mobility
   and/or multilink change events by sending secured uNA messages to
   each active neighbor as an OMNI link service.  When a node sends a
   uNA message to each specific neighbor on behalf of a mobile Client,
   it sets the IPv6 Source Address to its own MLA then sets the
   Destination and Target Address to the neighbor's unicast address.
   The uNA also includes an OMNI option with OMNI Interface Attributes
   and Traffic Selector sub-options for the mobile Client's underlay
   interfaces and includes an authentication signature if necessary.
   The node next sets the uNA R flag to 1, S flag to 0 and O flag to 1,



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   then encapsulates the message in an OAL header.  Following OAL and L2
   encapsulation, the carrier packet containing the uNA message will
   then follow the secured spanning tree and arrive at the specific
   neighbor.

   As discussed in Section 7.2.6 of [RFC4861], the transmission and
   reception of uNA messages is unreliable but provides a useful
   optimization.  In well-connected Internetworks with robust data links
   uNA messages will be delivered with high reliability, but in any case
   the node can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs to
   each neighbor to increase the likelihood that at least one will be
   received.  Alternatively, the node can send an MI(MM) message to
   request an MR(MM) response as discussed in Section 4.5.1.

4.15.2.  Announcing Link-Layer Information Changes

   When a Client needs to change its underlay Interface Attributes and/
   or Traffic Selectors for one or more underlay interfaces (e.g., due
   to a mobility event), the Client sends RS messages to its MAP Proxy/
   Server (via first-hop FHS Proxy/Servers if necessary).  Each RS
   includes an OMNI option with Interface Attributes and/or Traffic
   Selector sub-options for the ifIndex in question.

   Note that the first FHS Proxy/Server may change due to the underlay
   interface change.  If the Client RS includes an OMNI Proxy/Server
   Departure sub-option for the former FHS Proxy/Server, the new FHS
   Proxy/Server can send a departure indication (see Section 4.15.5);
   otherwise, any stale state in the former FHS Proxy/Server will simply
   expire after ReachableTime expires with no effect on the MAP Proxy/
   Server.

   Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with
   sending carrier packets containing user data in case one or more RAs
   are lost.  If all RAs are lost, the Client SHOULD re-associate with a
   new Proxy/Server.

   After performing the RS/RA exchange, the Client sends uNA messages to
   all neighbors the same as described in the previous section.

4.15.3.  Bringing New Links Into Service

   When a Client needs to bring new underlay interfaces into service
   (e.g., when it activates a new data link), it sends an RS message to
   the MAP Proxy/Server via a FHS Proxy/Server for the underlay
   interface (if necessary) with an OMNI option that includes an
   Interface Attributes sub-option with interface parameters and with
   link layer address information for the new link.  The Client then
   again sends uNA messages to all neighbors the same as described



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   above.

4.15.4.  Deactivating Existing Links

   When a Client needs to deactivate an existing underlay interface, it
   sends an uNA message toward the MAP Proxy/Server via an FHS Proxy/
   Server with an OMNI option with appropriate Interface Attributes
   values for the deactivated link.

   If the Client needs to send uNA messages over an underlay interface
   other than the one being deactivated, it MUST include Interface
   Attributes for any underlay interfaces being deactivated.  The Client
   then again sends uNA messages to all neighbors the same as described
   above.

   Note that when a Client deactivates an underlay interface, neighbors
   that receive the ensuing uNA messages need not purge all references
   for the underlay interface from their NCEs.  The Client may
   reactivate or reuse the underlay interface and/or its ifIndex at a
   later point in time, when it will send new RS messages to an FHS
   Proxy/Server with fresh interface parameters to update any neighbors.
   The manner in which the Client dynamically manages its local ifIndex
   to interface mappings is a local decision, but should not be done in
   a manner that could cause state inconsistencies in the network.

4.15.5.  Moving Between Proxy/Servers

   When a Client associates with a new MAP Proxy/Server, it sends RS
   messages to register its underlay interfaces with the new MAP while
   including the old MAP's SRA PNPADDR in the "Old MAP Proxy/Server
   PNPADDR" field of a Proxy/Server Departure OMNI sub-option.  When the
   new MAP Proxy/Server returns the RA message via the FHS Proxy/Server
   (acting as a proxy), the FHS Proxy/Server sends an uNA to the old MAP
   Proxy/Server (i.e., if the PNPADDR is non-zero and different from its
   own).  The FHS Proxy/Server encapsulates the uNA in an OAL header
   with the SRA PNPADDR of the new MAP as the Source Address and the SRA
   PNPADDR of the old MAP as the Destination Address, then performs L2
   encapsulation and forwards the resulting carrier packet via the
   secured spanning tree.












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   When the old MAP Proxy/Server receives the carrier packets, it
   decapsulates and reassembles if necessary to obtain the uNA then
   changes the Client's NCE state to DEPARTED, resets DepartTime and
   caches the new MAP Proxy/Server PNPADDR.  After a short delay (e.g.,
   2 seconds) the old MAP Proxy/Server withdraws the Client's MNP(s)
   from the routing system.  While in the DEPARTED state, the old MAP
   Proxy/Server forwards any carrier packets received via the secured
   spanning tree destined to the Client's MNP GUAs to the new MAP Proxy/
   Server's PNPADDR.  When DepartTime expires, the old MAP Proxy/Server
   deletes the Client's NCE.

   Mobility events may also cause a Client to change to a new FHS Proxy/
   Server over a specific underlay interface at any time such that a
   Client RS/RA exchange over the underlay interface will engage the new
   FHS Proxy/Server instead of the old.  The Client can arrange to
   inform the old FHS Proxy/Server of the departure by including a
   Proxy/Server Departure sub-option for the "Old FHS Proxy/Server
   PNPADDR", and the new FHS Proxy/Server will issue a uNA using the
   same procedures as outlined for the MAP above while using its own SRA
   PNPADDR as the Source Address.  This can often result in successful
   delivery of carrier packets that would otherwise be lost due to the
   mobility event.

   Clients SHOULD NOT move rapidly between MAP Proxy/Servers in order to
   avoid causing excessive oscillations in the AERO routing system.
   Examples of when a Client might wish to change to a different MAP
   Proxy/Server include a MAP Proxy/Server that has become unresponsive,
   topological movements of significant distance, movement to a new
   geographic region, movement to a new OMNI link segment, etc.

4.15.6.  Accommodating Path Changes

   After AFV state has been established for a flow, all OAL intermediate
   systems in the forward path will have AFVs with header compression
   state and (AFVI, L2ADDR) information for the next hop.  However,
   paths can fluctuate due to factors such as node mobility, routing
   changes, network membership, etc.  If an OAL intermediate system
   forwarding OAL packets with OCH headers detects that the next hop in
   the path has changed, it immediately reverts to sending the packets
   with header compression disabled by including full OAL IPv6 and
   Extended Fragment Headers (plus full original IP headers) in future
   packets.

   When the OAL destination begins to receive OAL packets with full
   headers (i.e., when it had previously received OCH compressed
   headers), it assumes that the network path for this flow has changed
   and begins sending MC messages to the OAL source.  The OAL
   destination sends the MC messages subject to rate limiting, and



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   includes a Neighbor Synchronization OMNI sub-option with both the ACK
   and RST flags set and with the most recent OAL packet Identification
   recorded in the Acknowledgment field.

   When the OAL source receives the MC messages with ACK and RST set, it
   re-initiates multilink forwarding for this flow by issuing a new
   MI/MR/MC or unidirectional MC exchange the same as for a new flow as
   specified in Section 4.13.2.  The AFV state in the former path then
   simply becomes stale and is soon purged by the former OAL
   intermediate nodes.

4.16.  Multicast

   Each Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6) [RFC3810]
   proxy service for its ENETs and/or hosted applications [RFC4605] and
   acts as a Protocol Independent Multicast - Sparse-Mode (PIM-SM, or
   simply "PIM") Designated Router (DR) [RFC7761] on the OMNI link.
   Proxy/Servers act as OMNI link PIM routers for Clients on ANET, VPN/
   IPsec or Direct interfaces, and Relays also act as OMNI link PIM
   routers on behalf of nodes on other links/networks.

   Clients on VPN/IPsec, Direct or (M)ANET underlay interfaces for which
   the *NET has deployed native multicast services forward IGMP/MLD
   messages into the *NET.  The IGMP/MLD messages may be further
   forwarded by a first-hop *NET access router acting as an IGMP/MLD-
   snooping switch [RFC4541], then ultimately delivered to a *NET (FHS)
   Proxy/Server.  The FHS Proxy/Server then acts as an ARS to send
   NS(AR) messages to an ARR for the multicast source.  Clients on *NET
   underlay interfaces without native multicast services instead send
   NS(AR) messages as an ARS to cause their FHS Proxy/Server to forward
   the message to an ARR.  When the ARR prepares an NA(AR) response, it
   initiates PIM protocol messaging according to the Source-Specific
   Multicast (SSM) and Any-Source Multicast (ASM) operational modes as
   discussed in the following sections.

4.16.1.  Source-Specific Multicast (SSM)

   When an ARS "X" (i.e., either a Client or Proxy/Server) acting as PIM
   router receives a Join/Prune message from a node on its downstream
   interfaces containing one or more ((S)ource, (G)roup) pairs, it
   updates its Multicast Routing Information Base (MRIB) accordingly.
   For each S belonging to a prefix reachable via X's non-OMNI
   interfaces, X then forwards the (S, G) Join/Prune to any PIM routers
   on those interfaces per [RFC7761].  The same as for unicast
   destinations, the 3-tuple of Source Address, Destination Address and
   Flow Label identifies a flow for multicast group G.





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   For each S belonging to a prefix reachable via X's OMNI interface, X
   sends an NS(AR) message (see: Section 4.13) into the secured spanning
   tree which delivers it to ARR "Y" that services S.  Y will then
   return an NA(AR) that includes an OMNI option with Interface
   Attributes, Traffic Selectors and RIOs for S.

   When X processes the NA(AR) it selects one or more underlay
   interfaces for S and performs an MI/MR/MC multilink forwarding
   exchange over the secured spanning tree while including a PIM Join/
   Prune message for each multicast group of interest in the OMNI
   option.  If S is located behind any Proxys "Z"*, each Z* then updates
   its MRIB accordingly and maintains the MNP SRA GUA of X as the next
   hop in the reverse path.  Since Gateways forward messages not
   addressed to themselves without examining them, this means that the
   (reverse) multicast tree path is simply from each Z* (and/or S) to X
   with no other multicast-aware routers in the path.

   Following the initial combined Join/Prune and MI/MR/MC messaging, X
   maintains a NCE for each S the same as if X was sending unicast data
   traffic to S.  In particular, X performs additional MI/MR/MC
   exchanges to keep the NCE alive for up to t_periodic seconds
   [RFC7761].  If no new Joins are received within t_periodic seconds, X
   allows the NCE to expire.  Finally, if X receives any additional
   Join/Prune messages for (S,G) it forwards the messages over the
   secured spanning tree.

   Client C that holds an MNP for source S may later depart from a first
   Proxy/Server Z1 and/or connect via a new Proxy/Server Z2.  In that
   case, Y sends an MC message to X the same as specified for unicast
   mobility in Section 4.15.  When X receives the MC message, it updates
   its NCE for the MLA for source S and sends new Join messages in
   MI/MR/MC exchanges addressed to the new target Client underlay
   interface connection for S.  There is no requirement to send any
   Prune messages to old Proxy/Server Z1 since source S will no longer
   source any multicast data traffic via Z1.  Instead, the multicast
   state for (S,G) in Proxy/Server Z1 will soon expire since no new
   Joins will arrive.

4.16.2.  Any-Source Multicast (ASM)

   When an ARS "X" acting as a PIM router receives Join/Prune messages
   from a node on its downstream interfaces containing one or more (*,G)
   pairs, it updates its Multicast Routing Information Base (MRIB)
   accordingly.  X first performs an NS/NA(AR) exchange to receive
   address resolution information for Rendezvous Point (RP) "R" for each
   G.  X then includes a copy of each Join/Prune message in the OMNI
   option of an MI/MR/MC message, then encapsulates the MI/MR/MC message
   in an OAL header and sends the message into the secured spanning



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   tree.

   For each source "S" that sends multicast traffic to group G via R,
   Client S* that aggregates S (or its Proxy/Server) encapsulates the
   original IP packets in PIM Register messages, includes the PIM
   Register messages in the OMNI options of MC messages, performs OAL
   encapsulation and fragmentation with Identification values within the
   receive window for Client R* that aggregates R, then performs L2
   encapsulation and forwards the resulting carrier packets.  Client R*
   may then elect to send a PIM Join to S* in the OMNI option of a MC
   over the secured spanning tree.  This will result in an (S,G) tree
   rooted at S* with R as the next hop so that R will begin to receive
   two copies of the original IP packet; one native copy from the (S, G)
   tree and a second copy from the pre-existing (*, G) tree that still
   uses MC PIM Register encapsulation.  R can then issue a MC PIM
   Register-stop message over the secured spanning tree to suppress the
   Register-encapsulated stream.  At some later time, if Client S* moves
   to a new Proxy/Server, it resumes sending original IP packets via MC
   PIM Register encapsulation via the new Proxy/Server.

   At the same time, as multicast listeners discover individual S's for
   a given G, they can initiate an (S,G) Join for each S under the same
   procedures discussed in Section 4.16.1.  Once the (S,G) tree is
   established, the listeners can send (S, G) Prune messages to R so
   that multicast original IP packets for group G sourced by S will only
   be delivered via the (S, G) tree and not from the (*, G) tree rooted
   at R.  All mobility considerations discussed for SSM apply.

4.16.3.  Bi-Directional PIM (BIDIR-PIM)

   Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate
   approach to ASM that treats the Rendezvous Point (RP) as a Designated
   Forwarder (DF).  Further considerations for BIDIR-PIM are out of
   scope.

4.17.  Operation over Multiple OMNI Links

   An AERO Client can connect to multiple OMNI links the same as for any
   data link service.  In that case, the Client maintains a distinct
   OMNI interface for each link, e.g., 'omni0' for the first link,
   'omni1' for the second, 'omni2' for the third, etc.  Each OMNI link
   would include its own distinct set of Gateways and Proxy/Servers,
   thereby providing redundancy in case of failures.

   Each OMNI link could utilize the same or different ANET/INET link
   layer connections.  The links can be distinguished at the link layer
   via the SRT prefix in a similar fashion as for Virtual Local Area
   Network (VLAN) tagging (e.g., IEEE 802.1Q) and/or through assignment



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   of distinct sets of MSPs on each link.  This gives rise to the
   opportunity for supporting multiple redundant networked paths (see:
   Section 4.2.4).

   The Client's network layer can select the outbound OMNI interface
   appropriate for a given traffic profile while (in the reverse
   direction) correspondent nodes must have some way of steering their
   original IP packets destined to a target via the correct OMNI link.

   In a first alternative, if each OMNI link services different MSPs the
   Client can receive a distinct MNP from each of the links.  IP routing
   will therefore assure that the correct OMNI link is used for both
   outbound and inbound traffic.  This can be accomplished using
   existing technologies and approaches, and without requiring any
   special supporting code in correspondent nodes or Gateways.

   In a second alternative, if each OMNI link services the same MSP(s)
   then each link could assign a distinct "OMNI link Anycast" address
   that is configured by all Gateways on the link.  Correspondent nodes
   can then perform Segment Routing to select the correct SRT, which
   will then direct the original IP packet over multiple hops to the
   target.

4.18.  DNS Considerations

   AERO Client MNs and INET correspondent nodes consult the Domain Name
   System (DNS) the same as for any Internetworking node.  When
   correspondent nodes and Client MNs use different IP protocol versions
   (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain
   A records for IPv4 address mappings to MNs which must then be
   populated in Relay NAT64 mapping caches.  In that way, an IPv4
   correspondent node can send original IPv4 packets to the IPv4 address
   mapping of the target MN, and the Relay will translate the IPv4
   header and Destination Address into an IPv6 header and IPv6
   Destination Address of the MN.

   When an AERO Client registers with an AERO Proxy/Server, the Proxy/
   Server can return the address(es) of DNS servers in RDNSS options
   [RFC6106].  The DNS server provides the IP addresses of other MNs and
   correspondent nodes in AAAA records for IPv6 or A records for IPv4.











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4.19.  Transition/Coexistence Considerations

   OAL encapsulation ensures that dissimilar INET partitions can be
   joined into a single unified OMNI link, even though the partitions
   themselves may have differing protocol versions and/or incompatible
   addressing plans.  However, a commonality can be achieved by
   incrementally distributing globally routable (i.e., native) IP
   prefixes to eventually reach all nodes (both mobile and fixed) in all
   OMNI link segments.  This can be accomplished by incrementally
   deploying AERO Gateways on each INET partition, with each Gateway
   distributing its MNPs and/or discovering FNPs on its INET links.

   This gives rise to the opportunity to eventually distribute native IP
   addresses to all nodes, and to present a unified OMNI link view even
   if the INET partitions remain in their current protocol and
   addressing plans.  In that way, the OMNI link can serve the dual
   purpose of providing a mobility/multilink service and a transition/
   coexistence service.  Alternatively, if an INET partition is
   transitioned to a native IP protocol version and addressing scheme
   compatible with the OMNI link MNP-based addressing scheme, the
   partition and OMNI link can be joined by Gateways.

   Relays that connect INETs/ENETs with dissimilar IP protocol versions
   may need to employ a network address and protocol translation
   function such as NAT64 [RFC6146].

4.20.  Proxy/Server-Gateway Bidirectional Forwarding Detection

   In environments where rapid failure recovery is essential, Proxy/
   Servers and Gateways SHOULD use Bidirectional Forwarding Detection
   (BFD) [RFC5880].  Nodes that use BFD can quickly detect and react to
   failures so that cached information is re-established through
   alternate nodes.  BFD control messaging is carried only over well-
   connected ground domain networks (i.e., and not low-end radio links)
   and can therefore be tuned for rapid response.

   Proxy/Servers and Gateways can maintain BFD sessions in parallel with
   their BGP peerings.  If a Proxy/Server or Gateway fails, BGP peers
   will quickly re-establish routes through alternate paths the same as
   for common BGP operational practice.











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4.21.  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 MN may be willing to
   sacrifice a modicum of efficiency in order to have time-varying MNPs
   that can be changed every so often to defeat adversarial tracking.

   The DHCPv6 service offers a way for Clients that desire time-varying
   MNPs to obtain short-lived prefixes (e.g., on the order of a small
   number of minutes).  In that case, the identity of the Client would
   not be bound to the MNP but rather to a Node Identification value
   (see: [I-D.templin-6man-omni3]) that can serve as a Client ID seed
   for MNP prefix delegation.  The Client would then be obligated to
   renumber its internal networks whenever its MNP changes.  This should
   not present problems for Clients with automated network renumbering
   services, however it can limit the durations of ongoing sessions that
   would prefer to use a constant address.

5.  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 v0.6 tagged on September
   22, 2025.  Future versions will be made available for public release.

6.  IANA Considerations

   The IANA is instructed to assign three new Codes in the "ICMPv6
   "Code" Fields - Type 136 - Neighbor Advertisement" registry of the
   https://www.iana.org/assignments/icmpv6-parameters registry group
   (registration procedure is Standards Action or IESG Approval).  The
   registry entries should appear as follows:

      Code   Message Name                         Reference
      ----   -------------                        ----------
      TBD1   Multilink Initiate (MI)              [RFCXXXX]
      TBD2   Multilink Respond (MR)               [RFCXXXX]
      TBD3   Multilink Control (MC)               [RFCXXXX]

               Figure 5: IPv6 ND Neighbor Solicitation Codes:





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   [RFCXXXX] above refers to this document, while [TBD1/TBD2/TBD3] may
   be any available values.  The values [250/251/252] are suggested to
   support maximum forward compatibility for future specifications that
   might define earlier values in this and/or other IPv6 ND message
   tables.

   The IANA assigned UDP port number "8060" for an experimental first
   edition of AERO [RFC6706].  The Overlay Multilink Network Interface
   (OMNI) specification [I-D.templin-6man-omni3] reclaims "8060" as the
   service port for AERO/OMNI UDP/IP encapsulation, therefore this
   document makes no IANA request.  (Note: although [RFC6706] was not
   widely implemented or deployed, it need not be obsoleted since it
   uses ICMPv6 message type '0' (Reserved) which implementations of this
   specification ignore.)

7.  Security Considerations

   AERO Gateways establish security associations with AERO Proxy/Servers
   and Relays within their local OMNI link segments using secured
   tunnels over underlay interfaces.  The AERO Gateways of all OMNI link
   segments in turn configure secured tunnels with neighboring AERO
   Gateways for other OMNI link segments in a secured spanning tree
   topology.  Applicable security services include IPsec [RFC4301] with
   IKEv2 [RFC7296], etc.  (Note that secured direct point-to-point links
   can also be used instead of or in addition to network layer
   security.)  Together, these services are responsible for assuring
   connectionless integrity and data origin authentication with optional
   protection against replays for control messages that traverse the
   secured spanning tree.

   To prevent unauthorized local applications from congesting the
   secured spanning tree, Proxy/Servers and Gateways configure local
   access controls to permit only the BGP protocol service daemon to
   source routing protocol control messages with the MLA assigned to the
   OMNI interface as the source over the secured spanning tree.  An
   implementation can employ a port/address filtering configuration that
   permits only TCP port 179 (as defined in the IANA "Service Names and
   Port Numbers" registry) when using the MLA assigned to the OMNI
   interface.  To prevent malicious Clients from congesting the secured
   spanning tree, Proxy/Servers should also rate-limit the secured IPv6
   ND messages they process for the same (source, target) pair, e.g., by
   applying IPv6 ND MAX_UNICAST_SOLICIT; MAX_NEIGHBOR_ADVERTISEMENT
   limits.

   To prevent spoofing, Proxy/Servers MUST silently discard without
   responding to any unsecured IPv6 ND messages with OMNI sub-options
   that would otherwise affect state.  Also, Proxy/Servers MUST silently
   discard without forwarding any original IP packets received from one



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   of their own Clients (whether directly or following OAL reassembly)
   with a Source Address that does not match the Client's MNP and/or a
   Destination Address that does match the Client's MNP.  Finally,
   Proxy/Servers MUST silently discard without forwarding any carrier
   packets that include an OAL packet/fragment with Source and
   Destination Addresses that both match the same MNP.

   AERO Clients that connect to secured ANETs need not apply additional
   security to their IPv6 ND messages, since the messages will be
   accepted and forwarded by a perimeter Proxy/Server that applies
   security over its INET-facing interface to the secured spanning tree
   (see above).  AERO Clients that connect to MANETs or open INETs can
   use network and/or transport layer security services such as VPNs
   (e.g., IPsec tunnels) or can by some other means establish a secured
   direct link to a Proxy/Server.  When a VPN or direct link may be
   impractical, however, INET Clients and Proxy/Servers SHOULD include
   and verify authentication signatures for IPv6 ND messages as
   specified in [I-D.templin-6man-omni3].

   End systems SHOULD apply transport or higher layer security services
   such as QUIC-TLS [RFC9000], TLS/SSL [RFC8446], DTLS [RFC6347], etc.
   to provide a level of protection comparable to critical secured
   Internet services.  End systems that require host-based VPN services
   SHOULD use network and/or transport layer security services such as
   IPsec, TLS/SSL, DTLS, etc.  AERO Proxy/Servers and Clients can also
   provide a network-based VPN service on behalf of end systems, e.g.,
   if the end system is located within a secured enclave and cannot
   establish a VPN on its own behalf.

   AERO Proxy/Servers and Gateways present targets for traffic
   amplification Denial of Service (DoS) attacks.  This concern is no
   different than for widely-deployed VPN security gateways in the
   Internet, where attackers could send spoofed packets to the gateways
   at high data rates.  This can be mitigated through the AERO/OMNI data
   origin authentication procedures, as well as connecting Proxy/Servers
   and Gateways over dedicated links with no connections to the Internet
   and/or when connections to the Internet are only permitted through
   well-managed firewalls.  Traffic amplification DoS attacks can also
   target an AERO Client's low data rate links.  This is a concern not
   only for Clients located on the open Internet but also for Clients in
   secured enclaves.  AERO Proxy/Servers and Proxys can institute rate
   limits that protect Clients from receiving carrier packet floods that
   could DoS low data rate links.

   AERO Relays must implement ingress filtering to avoid a spoofing
   attack in which spurious messages with ULA addresses are injected
   into an OMNI link from an outside attacker.  AERO Clients MUST ensure
   that their connectivity is not used by unauthorized nodes on their



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   ENETs to gain access to a protected network, i.e., AERO Clients that
   act as routers MUST NOT provide routing services for unauthorized
   nodes.  (This concern is no different than for ordinary hosts that
   receive an IP address delegation but then "share" the address with
   other nodes via some form of Internet connection sharing such as
   tethering.)

   The AERO service for MANET and open INET Clients depends on a public
   key distribution service in which Client public keys and identities
   are maintained in a shared database accessible to Proxy/Servers and
   potential correspondent peer nodes.  Similarly, each Client must be
   able to determine the public key of each Proxy/Server, e.g. by
   consulting an online database.

   The PRL contains only public information, but MUST be well-managed
   and secured from unauthorized tampering.  The PRL can be conveyed to
   the Client in a similar fashion as in [RFC5214] (e.g., through data
   link layer login messaging, secure upload of a static file, DNS
   lookups, etc.).

   Security considerations for IPv6 fragmentation and reassembly are
   discussed in [I-D.templin-6man-omni3].  In environments where
   spoofing is considered a threat, all OAL nodes SHOULD employ
   Identification window synchronization and OAL end systems SHOULD
   configure an (end-system-based) firewall.

   Security considerations for accepting link layer ICMP messages and
   reflected carrier packets are discussed throughout the document.

8.  Acknowledgements

   Discussions in the IETF, aviation standards communities and private
   exchanges helped shape some of the concepts in this work.
   Individuals who contributed insights include Mikael Abrahamsson,
   Felipe Magno de Almeida, Mark Andrews, Fred Baker, Amanda Baber, Bob
   Braden, Stewart Bryant, Scott Burleigh, Brian Carpenter, Wojciech
   Dec, Pavel Drasil, Ralph Droms, Adrian Farrel, Nick Green, Sri
   Gundavelli, Brian Haberman, Bernhard Haindl, Joel Halpern, Tom
   Herbert, Bob Hinden, Sascha Hlusiak, Lee Howard, Christian Huitema,
   Zdenek Jaron, Andre Kostur, Hubert Kuenig, Eliot Lear, Ted Lemon,
   Andy Malis, Satoru Matsushima, Tomek Mrugalski, Thomas Narten, Madhu
   Niraula, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa, Dave
   Thaler, Joe Touch, Bernie Volz, Ryuji Wakikawa, Tony Whyman, Lloyd
   Wood and James Woodyatt.  Members of the IESG also provided valuable
   input during their review process that greatly improved the document.
   Special thanks go to Stewart Bryant, Joel Halpern and Brian Haberman
   for their shepherding guidance during the publication of the AERO
   first edition.



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   This work has further been encouraged and supported by Boeing
   colleagues including Akash Agarwal, Kyle Bae, M.  Wayne Benson, Dave
   Bernhardt, Cam Brodie, John Bush, Balaguruna Chidambaram, Irene Chin,
   Bruce Cornish, Claudiu Danilov, Sean Dickson, Don Dillenburg, Joe
   Dudkowski, Wen Fang, Samad Farooqui, Anthony Gregory, Jeff Holland,
   Seth Jahne, Brian Jaury, Greg Kimberly, Ed King, Madhuri Madhava
   Badgandi, Laurel Matthew, Gene MacLean III, Kyle Mikos, Rob
   Muszkiewicz, Sean O'Sullivan, Satish Raghavendran, Vijay Rajagopalan,
   Kristina Ross, Greg Saccone, Ron Sackman, Bhargava Raman Sai Prakash,
   Rod Santiago, Madhanmohan Savadamuthu, Kent Shuey, Brian Skeen, Mike
   Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia Wilson,
   Julie Wulff, Yueli Yang, Eric Yeh and other members of the Boeing
   mobility, networking and autonomy teams.  Akash Agarwal, Kyle Bae,
   Wayne Benson, Madhuri Madhava Badgandi, Vijayasarathy Rajagopalan,
   Bhargava Raman Sai Prakash, Katie Tran and Eric Yeh are especially
   acknowledged for their work on the AERO implementation.  Chuck
   Klabunde is honored for his support and guidance, and we mourn his
   untimely loss.

   This work was inspired by the support and encouragement of countless
   outstanding colleagues, managers and program directors over the span
   of many decades.  Beginning in the late 1980s,' the Digital Equipment
   Corporation (DEC) Ultrix Engineering and DECnet Architects groups
   identified early issues with fragmentation and bridging links with
   diverse MTUs.  In the early 1990s, engagements at DEC Project Sequoia
   at UC Berkeley and the DEC Western Research Lab in Palo Alto included
   investigations into large-scale networked filesystems, ATM vs
   Internet and network security proxys.  In the mid-1990s to early
   2000s employment at the NASA Ames Research Center (Sterling Software)
   and SRI International supported early investigations of IPv6, ONR UAV
   Communications and the IETF.  An employment at Nokia where important
   IETF documents were published gave way to a present-day engagement
   with The Boeing Company.  The work matured at Boeing through major
   programs including Future Combat Systems, Advanced Airplane Program,
   DTN for the International Space Station, Mobility Vision Lab, CAST,
   Caravan, Airplane Internet of Things, the NASA UAS/CNS program, the
   FAA/ICAO ATN/IPS program and many others.  An attempt to name all who
   gave support and encouragement would double the current document size
   and result in many unintentional omissions - but to all a humble
   thanks.

   Earlier works on NBMA tunneling approaches are found in
   [RFC2529][RFC5214][RFC5569].

   Many of the constructs presented in this second edition of AERO are
   based on the author's earlier works, including:

   *  Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) [RFC5214]



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   *  The Subnetwork Encapsulation and Adaptation Layer (SEAL) [RFC5320]

   *  Virtual Enterprise Traversal (VET) [RFC5558]

   *  Routing and Addressing in Networks with Global Enterprise
      Recursion (RANGER) [RFC5720][RFC6139]

   *  The Internet Routing Overlay Network (IRON) [RFC6179]

   *  AERO, First Edition [RFC6706]

   Note that these works cite numerous earlier efforts that are not
   included here due to space limitations.  The authors of those earlier
   works are acknowledged for their insights.

   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 Commercial Airplanes (BCA)
   Airplane Internet of Things (AIoT) and autonomy programs.

   This work is aligned with the Boeing Information Technology (BIT)
   MobileNet program.

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

   Honoring life, liberty and the pursuit of happiness.

9.  References

9.1.  Normative References

   [I-D.ietf-dhc-rfc8415bis]
              Mrugalski, T., Volz, B., Richardson, M., Jiang, S., and T.
              Winters, "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6)", Work in Progress, Internet-Draft, draft-ietf-
              dhc-rfc8415bis-12, 4 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dhc-
              rfc8415bis-12>.








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   [I-D.templin-6man-omni3]
              Templin, F. L., "Transmission of IP Packets over Overlay
              Multilink Network (OMNI) Interfaces", Work in Progress,
              Internet-Draft, draft-templin-6man-omni3-63, 15 October
              2025, <https://datatracker.ietf.org/doc/html/draft-
              templin-6man-omni3-63>.

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

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [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>.

   [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>.

   [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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [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>.

   [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>.




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   [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>.

   [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>.

   [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>.

   [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>.

9.2.  Informative References

   [BGP]      Huston, G., "BGP in 2015, http://potaroo.net", January
              2016.

   [CERF]     Cerf, V., "The Catenet Model For Internetworking, IETF
              IEN48, http://www.postel.org/ien/pdf/ien048.pdf", July
              1978.

   [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.



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   [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-rtgwg-atn-bgp]
              Templin, F., Saccone, G., Dawra, G., Lindem, A., and V.
              Moreno, "A Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", Work in
              Progress, Internet-Draft, draft-ietf-rtgwg-atn-bgp-28, 17
              March 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-rtgwg-atn-bgp-28>.

   [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-mla]
              Templin, F., "IPv6 Addresses for Ad Hoc Networks", Work in
              Progress, Internet-Draft, draft-templin-6man-mla-27, 3
              April 2025, <https://datatracker.ietf.org/doc/html/draft-
              templin-6man-mla-27>.

   [I-D.templin-6man-parcels2]
              Templin, F., "IPv6 Parcels and Advanced Jumbos (AJs)",
              Work in Progress, Internet-Draft, draft-templin-6man-
              parcels2-27, 21 May 2025,
              <https://datatracker.ietf.org/doc/html/draft-templin-6man-
              parcels2-27>.








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   [I-D.templin-intarea-parcels2]
              Templin, F., "IPv4 Parcels and Advanced Jumbos (AJs)",
              Work in Progress, Internet-Draft, draft-templin-intarea-
              parcels2-18, 21 May 2025,
              <https://datatracker.ietf.org/doc/html/draft-templin-
              intarea-parcels2-18>.

   [KAHN]     Perry, T., "The Great Interconnector, IEEE Spectrum,
              https://spectrum.ieee.org/bob-kahn-2667754905", May 2024.

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

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, DOI 10.17487/RFC2236, November 1997,
              <https://www.rfc-editor.org/info/rfc2236>.

   [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>.

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529,
              DOI 10.17487/RFC2529, March 1999,
              <https://www.rfc-editor.org/info/rfc2529>.

   [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>.

   [RFC3724]  Kempf, J., Ed., Austein, R., Ed., and IAB, "The Rise of
              the Middle and the Future of End-to-End: Reflections on
              the Evolution of the Internet Architecture", RFC 3724,
              DOI 10.17487/RFC3724, March 2004,
              <https://www.rfc-editor.org/info/rfc3724>.






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   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

   [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>.

   [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>.

   [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>.

   [RFC4511]  Sermersheim, J., Ed., "Lightweight Directory Access
              Protocol (LDAP): The Protocol", RFC 4511,
              DOI 10.17487/RFC4511, June 2006,
              <https://www.rfc-editor.org/info/rfc4511>.

   [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>.

   [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>.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
              <https://www.rfc-editor.org/info/rfc5015>.



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   [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>.

   [RFC5320]  Templin, F., Ed., "The Subnetwork Encapsulation and
              Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320,
              February 2010, <https://www.rfc-editor.org/info/rfc5320>.

   [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>.

   [RFC5522]  Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
              Route Optimization Requirements for Operational Use in
              Aeronautics and Space Exploration Mobile Networks",
              RFC 5522, DOI 10.17487/RFC5522, October 2009,
              <https://www.rfc-editor.org/info/rfc5522>.

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

   [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569,
              January 2010, <https://www.rfc-editor.org/info/rfc5569>.

   [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>.

   [RFC5720]  Templin, F., "Routing and Addressing in Networks with
              Global Enterprise Recursion (RANGER)", RFC 5720,
              DOI 10.17487/RFC5720, February 2010,
              <https://www.rfc-editor.org/info/rfc5720>.

   [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>.

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







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   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, DOI 10.17487/RFC6106, November 2010,
              <https://www.rfc-editor.org/info/rfc6106>.

   [RFC6139]  Russert, S., Ed., Fleischman, E., Ed., and F. Templin,
              Ed., "Routing and Addressing in Networks with Global
              Enterprise Recursion (RANGER) Scenarios", RFC 6139,
              DOI 10.17487/RFC6139, February 2011,
              <https://www.rfc-editor.org/info/rfc6139>.

   [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>.

   [RFC6179]  Templin, F., Ed., "The Internet Routing Overlay Network
              (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011,
              <https://www.rfc-editor.org/info/rfc6179>.

   [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>.

   [RFC6621]  Macker, J., Ed., "Simplified Multicast Forwarding",
              RFC 6621, DOI 10.17487/RFC6621, May 2012,
              <https://www.rfc-editor.org/info/rfc6621>.

   [RFC6706]  Templin, F., Ed., "Asymmetric Extended Route Optimization
              (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
              <https://www.rfc-editor.org/info/rfc6706>.






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   [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>.

   [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>.

   [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>.

   [RFC7333]  Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
              Korhonen, "Requirements for Distributed Mobility
              Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
              <https://www.rfc-editor.org/info/rfc7333>.

   [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>.

   [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
              Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
              DOI 10.17487/RFC8175, June 2017,
              <https://www.rfc-editor.org/info/rfc8175>.

   [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>.

   [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>.

   [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>.



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   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [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>.

   [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>.

Appendix A.  Non-Normative Considerations

   AERO can be applied to a multitude of Internetworking scenarios, with
   each having its own adaptations.  The following considerations are
   provided as non-normative guidance:

A.1.  Implementation Strategies for Route Optimization

   Address resolution and route optimization as discussed in
   Section 4.13 results in the creation of NCEs.  The NCE state is set
   to REACHABLE for at most ReachableTime seconds.  In order to refresh
   the NCE lifetime before the ReachableTime timer expires, the
   specification requires implementations to issue a new NS/NA(AR)
   exchange to reset ReachableTime while data messages are still
   flowing.  However, the decision of when to initiate a new NS/NA(AR)
   exchange and to perpetuate the process is left as an implementation
   detail.

   One possible strategy may be to monitor the NCE watching for data
   messages for (ReachableTime - 5) seconds.  If any data messages have
   been sent to the neighbor within this timeframe, then send an NS(AR)
   to receive a new NA(AR).  If no data messages have been sent, wait
   for 5 additional seconds and send an immediate NS(AR) if any data
   packets are sent within this "expiration pending" 5 second window.
   If no additional data messages are sent within the 5 second window,
   reset the NCE state to STALE.

   The monitoring of the neighbor data traffic therefore becomes an
   ongoing process during the NCE lifetime.  If the NCE expires, future
   data messages will trigger a new NS/NA(AR) exchange while the
   messages themselves may be delivered over longer paths until route
   optimization state is re-established.



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A.2.  Implicit Mobility Management

   OMNI interface neighbors MAY provide a configuration option that
   allows them to perform implicit mobility management in which no IPv6
   ND messaging is used.  In that case, the Client only transmits
   carrier packets over a single interface at a time, and the neighbor
   always observes carrier packets arriving from the Client from the
   same L2 Source Address.

   If the Client's underlay interface address changes (either due to a
   readdressing of the original interface or switching to a new
   interface) the neighbor immediately updates the NCE for the Client
   and begins accepting and sending carrier packets according to the
   Client's new address.  This implicit mobility method applies to use
   cases such as cellphones with both WiFi and Cellular interfaces where
   only one of the interfaces is active at a given time, and the Client
   automatically switches over to the backup interface if the primary
   interface fails.

A.3.  Direct Underlying Interfaces

   When a Client's OMNI interface is configured over a Direct interface,
   the neighbor at the other end of the Direct link can receive original
   IP packets without any encapsulation.  In that case, the Client sends
   packets over the Direct link according to traffic selectors.  If the
   Direct interface is selected, then the Client's packets are
   transmitted directly to the peer without traversing an ANET/INET.  If
   other interfaces are selected, then the Client's packets are
   transmitted via a different interface, which may result in the
   inclusion of Proxy/Servers and Gateways in the communications path.
   Direct interfaces must be tested periodically for reachability, e.g.,
   via NUD.

A.4.  AERO Critical Infrastructure Considerations

   AERO Gateways can be either Commercial off-the Shelf (COTS) standard
   IP routers or virtual machines in the cloud.  Gateways must be
   provisioned, supported and managed by the INET administrative
   authority, and connected to the Gateways of other INETs via inter-
   domain peerings.  Cost for purchasing, configuring and managing
   Gateways is nominal even for very large OMNI links.










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   AERO INET Proxy/Servers can be standard dedicated server platforms,
   but most often will be deployed as virtual machines in the cloud.
   The only requirements for INET Proxy/Servers are that they can run
   the AERO/OMNI code and have at least one network interface connection
   to the INET.  INET Proxy/Servers must be provisioned, supported and
   managed by the INET administrative authority.  Cost for purchasing,
   configuring and managing cloud Proxy/Servers is nominal especially
   for virtual machines.

   AERO ANET Proxy/Servers are most often standard dedicated server
   platforms with one underlay interface connected to the ANET and a
   second interface connected to an INET.  As with INET Proxy/Servers,
   the only requirements are that they can run the AERO/OMNI code and
   have at least one interface connection to the INET.  ANET Proxy/
   Servers must be provisioned, supported and managed by the ANET
   administrative authority.  Cost for purchasing, configuring and
   managing Proxys is nominal, and borne by the ANET administrative
   authority.

   AERO Relays are simply Proxy/Servers connected to INETs and/or ENETs
   that provide forwarding services for non-MNP destinations.  The Relay
   connects to the OMNI link and engages in eBGP peering with one or
   more Gateways as a stub AS.  The Relay then injects its MNPs and/or
   non-MNP prefixes into the BGP routing system, and provisions the
   prefixes to its downstream-attached networks.  The Relay can perform
   ARS/ARR services the same as for any Proxy/Server, and can route
   between the MNP and non-MNP address spaces.

A.5.  AERO Server Failure Implications

   AERO Proxy/Servers do not present a single point of failure in the
   architecture since all Proxy/Servers on the link provide identical
   services and loss of a Proxy/Server does not imply immediate and/or
   comprehensive communication failures.  Proxy/Server failure can be
   quickly detected and conveyed by Bidirectional Forward Detection
   (BFD) and/or proactive NUD allowing Clients to migrate to new Proxy/
   Servers.

   If a Proxy/Server fails, peer carrier packet forwarding to Clients
   will continue by virtue of the NCEs that have already been
   established through address resolution and route optimization.  If a
   Client also experiences mobility events at roughly the same time the
   Proxy/Server fails, uNA messages may be lost but NCEs in the DEPARTED
   state will ensure that carrier packet forwarding to the Client's new
   locations will continue for up to DepartTime seconds.






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   If a Client is left without a Proxy/Server for a considerable length
   of time (e.g., greater than ReachableTime seconds) then existing NCEs
   will eventually expire and both ongoing and new communications will
   fail.  The original source will continue to retransmit until the
   Client has established a new Proxy/Server relationship, after which
   time communications can continue .

   Therefore, links that provide many Proxy/Servers with high
   availability profiles are responsive to loss of individual
   infrastructure elements, since Clients can quickly establish new
   Proxy/Server relationships in event of failures.

A.6.  AERO Client / Server Architecture

   The AERO architectural model is client / server in the control plane,
   with route optimization in the data plane.  The same as for common
   Internet services, the AERO Client discovers the addresses of AERO
   Proxy/Servers and connects to one or more of them.  The AERO service
   is analogous to common Internet services such as google.com,
   yahoo.com, cnn.com, etc.  However, there is only one AERO service for
   the link and all Proxy/Servers provide identical services.

   Common Internet services provide differing strategies for advertising
   server addresses to clients.  The strategy is conveyed through the
   DNS resource records returned in response to name resolution queries.
   As of January 2020 Internet-based 'nslookup' services were used to
   determine the following:

   *  When a client resolves the domainname "google.com", the DNS always
      returns one A record (i.e., an IPv4 address) and one AAAA record
      (i.e., an IPv6 address).  The client receives the same addresses
      each time it resolves the domainname via the same DNS resolver,
      but may receive different addresses when it resolves the
      domainname via different DNS resolvers.  But, in each case,
      exactly one A and one AAAA record are returned.

   *  When a client resolves the domainname "ietf.org", the DNS always
      returns one A record and one AAAA record with the same addresses
      regardless of which DNS resolver is used.

   *  When a client resolves the domainname "yahoo.com", the DNS always
      returns a list of 4 A records and 4 AAAA records.  Each time the
      client resolves the domainname via the same DNS resolver, the same
      list of addresses are returned but in randomized order (i.e.,
      consistent with a DNS round-robin strategy).  But, interestingly,
      the same addresses are returned (albeit in randomized order) when
      the domainname is resolved via different DNS resolvers.




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   *  When a client resolves the domainname "amazon.com", the DNS always
      returns a list of 3 A records and no AAAA records.  As with
      "yahoo.com", the same three A records are returned from any
      worldwide Internet connection point in randomized order.

   The above example strategies show differing approaches to Internet
   resilience and service distribution offered by major Internet
   services.  The Google approach exposes only a single IPv4 and a
   single IPv6 address to clients.  Clients can then select whichever IP
   protocol version offers the best response, but will always use the
   same IP address according to the current Internet connection point.
   This means that the IP address offered by the network must lead to a
   highly-available server and/or service distribution point.  In other
   words, resilience is predicated on high availability within the
   network and with no client-initiated failovers expected (i.e., it is
   all-or-nothing from the client's perspective).  However, Google does
   provide for worldwide distributed service distribution by virtue of
   the fact that each Internet connection point responds with a
   different IPv6 and IPv4 address.  The IETF approach is like google
   (all-or-nothing from the client's perspective), but provides only a
   single IPv4 or IPv6 address on a worldwide basis.  This means that
   the addresses must be made highly-available at the network level with
   no client failover possibility, and if there is any worldwide service
   distribution it would need to be conducted by a network element that
   is reached via the IP address acting as a service distribution point.

   In contrast to the Google and IETF philosophies, Yahoo and Amazon
   both provide clients with a (short) list of IP addresses with Yahoo
   providing both IP protocol versions and Amazon as IPv4-only.  The
   order of the list is randomized with each name service query
   response, with the effect of round-robin load balancing for service
   distribution.  With a short list of addresses, there is still
   expectation that the network will implement high availability for
   each address but in case any single address fails the client can
   switch over to using a different address.  The balance then becomes
   one of function in the network vs function in the end system.















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   The same implications observed for common highly-available services
   in the Internet apply also to the AERO client/server architecture.
   When an AERO Client connects to one or more ANETs, it discovers one
   or more AERO Proxy/Server addresses through the mechanisms discussed
   in earlier sections.  Each Proxy/Server address presumably leads to a
   fault-tolerant clustering arrangement such as supported by Linux-HA,
   Extended Virtual Synchrony or Paxos.  Such an arrangement has
   precedence in common Internet service deployments in lightweight
   virtual machines without requiring expensive hardware deployment.
   Similarly, common Internet service deployments set service IP
   addresses on service distribution points that may relay requests to
   many different servers.

   For AERO, the expectation is that a combination of the Google/IETF
   and Yahoo/Amazon philosophies would be employed.  The AERO Client
   connects to different ANET access points and can receive 1-2 Proxy/
   Server ULAs at each point.  It then selects one AERO Proxy/Server
   address, and engages in RS/RA exchanges with the same Proxy/Server
   from all ANET connections.  The Client remains with this Proxy/Server
   unless or until the Proxy/Server fails, in which case it can switch
   over to an alternate Proxy/Server.  The Client can likewise switch
   over to a different Proxy/Server at any time if there is some reason
   for it to do so.  So, the AERO expectation is for a balance of
   function in the network and end system, with fault tolerance and
   resilience at both levels.

Appendix B.  Change Log

   << RFC Editor - remove prior to publication >>

   Differences from earlier versions:

   Draft -46 to -47
      *  Clarified the role of S/TLLAOs.

   Draft -44 to -46
      *  Address resolution clarifications and harmonization with
         RFC9762.  Address resolution on the ARS now considers all
         addresses on the OMNI link as on-link, so that NS/NA(AR) always
         precede new communications.  In many cases, however, the NS/
         NA(AR) exchange can be satisfied by the local adaptation layer
         without having to disturb the network.

      *  Removed major sections on RS/RA mobility service coordination
         that were redundant with the OMNI spec.

      *  Removed stale text from mobility sections.




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      *  Simplified and clarified section on NUD.

      *  Updated SRH contents specification.

      *  Removed obsolete "OFH" references.

   Draft -42 to -44
      *  Globally changed "AERO Forwarding" to "AERO Flow".

      *  Globally replaced "packet/parcel" with simply "packet".

   Draft -39 to -42
      *  Removed all references to L2 fragmentation/reassembly.

      *  Removed unused RFC citations.

   Draft -37 to -39
      *  Replaced Origin with Interface Attributes.

      *  Removed references to specific MLA candidate types.

   Draft -36 to -37
      *  Segment Routing for the adaptation layer per [RFC8754].

   Draft -35 to -36
      *  Discussion of DHCPv6 service model for OMNI links.

   Draft -34 to -35
      *  Further clarification on unidirectional nature of flows.

      *  Introduced "Proxy/Client" archetype.

Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA 98124
   United States of America
   Email: fltemplin@acm.org











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