



6lo Working Group                                               C. Gomez
Internet-Draft                                                       UPC
Updates: 8138, 8724, 9008 (if approved)                      A. Minaburo
Intended status: Standards Track                              Consultant
Expires: 17 April 2026                                      October 2025


  Transmission of SCHC-compressed packets over IEEE 802.15.4 networks
                     draft-ietf-6lo-schc-15dot4-11

Abstract

   A framework called Static Context Header Compression and
   fragmentation (SCHC) has been designed with the primary goal of
   supporting IPv6 over Low Power Wide Area Network (LPWAN) technologies
   [RFC8724].  One of the SCHC components is a header compression
   mechanism.  If used properly, SCHC header compression allows a
   greater compression ratio than that achievable with traditional
   6LoWPAN header compression [RFC6282].  For this reason, it may make
   sense to use SCHC header compression in some 6LoWPAN environments,
   including IEEE 802.15.4 networks.  This document specifies how a
   SCHC-compressed packet can be carried over IEEE 802.15.4 networks.
   The document also enables the transmission of SCHC-compressed UDP/
   CoAP headers over 6LoWPAN-compressed IPv6 packets.

Status of This Memo

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   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 4 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 . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements language . . . . . . . . . . . . . . . . . .   5
     2.2.  Background on previous specifications . . . . . . . . . .   5
     2.3.  New term  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Protocol stacks . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Main protocol stack . . . . . . . . . . . . . . . . .   5
       3.1.2.  Transition protocol stacks  . . . . . . . . . . . . .  10
     3.2.  SCHC architecture concepts  . . . . . . . . . . . . . . .  12
       3.2.1.  SCHC Stratum and Discriminator  . . . . . . . . . . .  12
       3.2.2.  Single-end point networks . . . . . . . . . . . . . .  13
       3.2.3.  Multiple-end point networks . . . . . . . . . . . . .  13
     3.3.  Network topologies  . . . . . . . . . . . . . . . . . . .  13
     3.4.  Single-hop communication  . . . . . . . . . . . . . . . .  14
     3.5.  Multihop communication  . . . . . . . . . . . . . . . . .  14
       3.5.1.  Straightforward Route-Over (SRO)  . . . . . . . . . .  14
       3.5.2.  Tunneled, RPL-based Route-Over (TRO)  . . . . . . . .  16
       3.5.3.  Pointer-based Route-Over (PRO)  . . . . . . . . . . .  20
       3.5.4.  Mesh-Under  . . . . . . . . . . . . . . . . . . . . .  22
   4.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  24
     4.1.  Single-hop or SRO frame format  . . . . . . . . . . . . .  24
       4.1.1.  SCHC Dispatch . . . . . . . . . . . . . . . . . . . .  25
       4.1.2.  SCHC Stratum Header . . . . . . . . . . . . . . . . .  25
       4.1.3.  SCHC Payload  . . . . . . . . . . . . . . . . . . . .  27
       4.1.4.  User payload  . . . . . . . . . . . . . . . . . . . .  27
       4.1.5.  Padding . . . . . . . . . . . . . . . . . . . . . . .  27
     4.2.  TRO frame format  . . . . . . . . . . . . . . . . . . . .  27
     4.3.  PRO frame format  . . . . . . . . . . . . . . . . . . . .  29
     4.4.  Mesh-Under frame format . . . . . . . . . . . . . . . . .  31
     4.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  32
   5.  Enabling the TPS  . . . . . . . . . . . . . . . . . . . . . .  33
     5.1.  SCHC C/D for the TPS: joint UDP/CoAP header
           compression . . . . . . . . . . . . . . . . . . . . . . .  34
     5.2.  SCHC C/D for the TPS: multiple SCHC Strata  . . . . . . .  36
   6.  SCHC compression for IPv6, UDP, and CoAP headers  . . . . . .  40
     6.1.  SCHC compression for IPv6 and UDP headers . . . . . . . .  40



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       6.1.1.  Compression of IPv6 addresses . . . . . . . . . . . .  41
       6.1.2.  UDP checksum field  . . . . . . . . . . . . . . . . .  41
     6.2.  SCHC compression for CoAP headers . . . . . . . . . . . .  42
   7.  Neighbor Discovery  . . . . . . . . . . . . . . . . . . . . .  42
   8.  Fragmentation and reassembly  . . . . . . . . . . . . . . . .  42
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  43
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  43
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  44
     12.2.  Informative References . . . . . . . . . . . . . . . . .  47
   Appendix A.  Header compression examples  . . . . . . . . . . . .  47
     A.1.  Single-hop or SRO frame format  . . . . . . . . . . . . .  48
     A.2.  TRO frame format  . . . . . . . . . . . . . . . . . . . .  48
     A.3.  PRO frame format  . . . . . . . . . . . . . . . . . . . .  48
     A.4.  Mesh-Under frame format . . . . . . . . . . . . . . . . .  49
     A.5.  Enabling the transition protocol stack  . . . . . . . . .  49
   Appendix B.  Analysis of route-over multihop approaches . . . . .  51
     B.1.  SRO . . . . . . . . . . . . . . . . . . . . . . . . . . .  51
     B.2.  TRO . . . . . . . . . . . . . . . . . . . . . . . . . . .  51
     B.3.  PRO . . . . . . . . . . . . . . . . . . . . . . . . . . .  52
     B.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  53
   Appendix C.  Relationship with RFC 7973 . . . . . . . . . . . . .  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

1.  Introduction

   RFC 6282 is the main specification for IPv6 over Low power Wireless
   Personal Area Network (6LoWPAN) IPv6 header compression [RFC6282].
   That RFC was designed assuming IEEE 802.15.4 as the layer below the
   6LoWPAN adaptation layer, and it has also been reused by the IPv6
   over Networks of Resource-constrained Nodes (6lo) working group (with
   proper adaptations) for IPv6 header compression over many other
   technologies relatively similar to IEEE 802.15.4 in terms of
   characteristics such as physical layer bit rate, layer 2 maximum
   payload size, etc.  Examples of such technologies comprise BLE, DECT-
   ULE, ITU G.9959, MS/TP, NFC, and PLC.  RFC 6282 provides additional
   functionality, such as a mechanism for UDP header compression.

   In the best cases, RFC 6282 allows to compress a 40-byte IPv6 header
   down to a 2-byte compressed header (for link-local interactions) or a
   3-byte compressed header (when global IPv6 addresses are used).  On
   the other hand, RFC 6282 typically compresses a UDP header to a size
   of 2 to 4 bytes.  Therefore, in advantageous conditions, a 48-byte
   uncompressed IPv6/UDP header may be compressed down to a 4- to 6-byte
   format (when using link-local addresses) or a 5- to 7-byte format
   (for global interactions) by using RFC 6282.




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   Recently, a framework called Static Context Header Compression (SCHC)
   has been designed with the primary goal of supporting IPv6 over Low
   Power Wide Area Network (LPWAN) technologies [RFC8724].  SCHC
   comprises header compression and fragmentation functionality tailored
   to the extraordinary constraints of LPWAN technologies, which are
   more severe than those exhibited by IEEE 802.15.4 or other relatively
   similar technologies.  SCHC header compression allows a greater
   compression ratio than that of RFC 6282.  If used properly, SCHC
   allows to compress an IPv6/UDP header down to e.g. a single byte.  In
   addition, SCHC can be used to compress Constrained Application
   Protocol (CoAP) headers [RFC7252][RFC8824], which further increases
   the achievable performance improvement of using SCHC header
   compression, since there is no 6LoWPAN header compression mechanism
   defined for CoAP.  Therefore, it may make sense to use SCHC header
   compression in some 6LoWPAN environments, including IEEE 802.15.4
   networks, considering its greater efficiency.

   This document specifies how a SCHC-compressed packet can be carried
   over IEEE 802.15.4 networks.  In order to ease a transition from
   existing 6LoWPAN/6Lo implementations to support SCHC header
   compression, the document also enables the transmission of SCHC-
   compressed UDP/CoAP headers over 6LoWPAN-compressed IPv6 packets.
   Further transition approaches are also described.

   The mechanism to be used to provide the SCHC header compression
   context to the nodes in an IEEE 802.15.4 network is out of the scope
   of this document.  Techniques intended to allow communication between
   nodes that only use 6LoWPAN for header compression and nodes that
   only use SCHC for header compression are out of the scope of this
   document.

   Note that, as per this document, and while SCHC defines fragmentation
   mechanisms as well, 6LoWPAN/6lo fragmentation is used when necessary
   to transport SCHC-compressed packets over IEEE 802.15.4 networks
   [RFC4944][RFC8930][RFC8931].

   In order to properly adapt to the requirements of supporting SCHC-
   compressed packets over IEEE 802.15.4 networks, this specification
   updates RFC 8138, RFC 8724, and RFC 9008.

2.  Terminology










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2.1.  Requirements language

   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
   BCP14 [RFC2119], [RFC8174], when, and only when, they appear in all
   capitals, as shown here.

2.2.  Background on previous specifications

   The reader is expected to be familiar with the terms and concepts
   defined in specifications of 6LoWPAN frame formats [RFC4944],
   Neighbor Discovery for 6LoWPANs [RFC6775][RFC8505], RPL [RFC6550] and
   companion documents [RFC6553][RFC6554][RFC9008], 6LoWPAN Routing
   Header [RFC8138], SCHC [RFC8724], SCHC for CoAP [RFC8824], and SCHC
   architecture [I-D.ietf-schc-architecture].

   RFC 8724 defines the Rule concept, whereby a Rule may be used to
   support header compression or fragmentation functionality.  In the
   present document, Rules are only used for header compression.

   RFC 6775 defines the term 6LoWPAN Node (6LN) as the following: "A
   6LoWPAN node is any host or router participating in a LoWPAN.  This
   term is used when referring to situations in which either a host or
   router can play the role described."  In this document, as in RFC
   9008, 6LN acts as a leaf.

2.3.  New term

   SCHC-Lo network: a 6LoWPAN network where SCHC is used for header
   compression/decompression.  Note: "SCHC-Lo" is pronounced as "sheek-
   low", since it inherits the pronunciation of "SCHC" as "sheek" in
   English (see RFC 8724).

3.  Architecture

3.1.  Protocol stacks

3.1.1.  Main protocol stack

   The traditional 6LoWPAN-based protocol stack for constrained devices
   (Figure 1, left) places the 6LoWPAN adaptation layer between IPv6 and
   an underlying technology such as IEEE 802.15.4.  Suitable upper layer
   protocols include CoAP [RFC7252] and UDP.  (Note that, while CoAP has
   also been specified over TCP, and TCP may play a significant role in
   IoT environments [RFC9006], 6LoWPAN header compression has not been
   defined for TCP, as of the writing.)




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   6LoWPAN can be envisioned as a set of two main sublayers, where the
   upper one provides header compression, while the lower one offers
   fragmentation.

   This document defines an alternative approach for packet header
   compression over IEEE 802.15.4, which leads to a modified protocol
   stack (Figure 1, right).  Fragmentation functionality remains the one
   defined by 6LoWPAN [RFC4944] and 6lo [RFC8930][RFC8931].



        +------------+          +------------+
        | CoAP, other|          | CoAP, other|
        +------------+          +------------+
        | UDP, other |          | UDP, other |
        +------------+          +------------+
        |    IPv6    |          |    IPv6    |
        +------------+          +------------+
        | 6LoWPAN HC |          |  SCHC HC   |  <-- NEW
        +------------+          +------------+
        |6LoWPAN Frag|          |6LoWPAN Frag|
        +------------+          +------------+
        |  802.15.4  |          |  802.15.4  |
        +------------+          +------------+


        Figure 1: Traditional 6LoWPAN-based protocol stack over IEEE
       802.15.4 (left) and alternative protocol stack using SCHC for
         header compression (right).  HC and Frag stand for Header
                Compression and Fragmentation, respectively.

   SCHC header compression may be applied to the headers of different
   protocols or sets of protocols.  Some examples include: i) IPv6
   packet headers, ii) joint IPv6 and UDP packet headers, iii) joint
   IPv6, UDP and CoAP packet headers, etc.
















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   SCHC header compression can also be used at various layers of a
   protocol stack [draft-ietf-schc-arch].  For example, when CoAP is
   used at the application layer, CoAP headers can be compressed by
   means of SCHC [RFC8824][draft-ietf-schc-8824-update].  Figure 2
   illustrates the corresponding protocol stacks when SCHC is used to
   compress IPv6/UDP headers, and separate SCHC Strata [draft-ietf-schc-
   arch] are also used to compress CoAP headers, when CoAP is secured by
   means of Datagram Transport Layer Security (DTLS) [RFC9147]
   (Figure 2, left) or Object Security for Constrained RESTful
   Environments (OSCORE) [RFC8613] (Figure 2, right) [RFC8824].  Note
   that, when OSCORE is used to protect CoAP, both the CoAP inner and
   outer headers can be compressed by means of SCHC, which requires one
   SCHC Stratum for the CoAP inner header and another one for the CoAP
   outer header.



                                  +------------+
                                  | CoAP inner |
           +------------+         +------------+
           |    CoAP    |         |   SCHC HC  |
           +------------+         +------------+
           |   SCHC HC  |         | CoAP outer |
           +------------+         +------------+
           |    DTLS    |         |   SCHC HC  |
           +------------+         +------------+
           |     UDP    |         |     UDP    |
           +------------+         +------------+
           |    IPv6    |         |    IPv6    |
           +------------+         +------------+
           |   SCHC HC  |         |   SCHC HC  |
           +------------+         +------------+
           |6LoWPAN Frag|         |6LoWPAN Frag|
           +------------+         +------------+
           |  802.15.4  |         |  802.15.4  |
           +------------+         +------------+


     Figure 2: 6LoWPAN-based protocol stack over IEEE 802.15.4 using a
         SCHC Stratum for header compression of IPv6/UDP, and also
       separate SCHC Strata for CoAP header compression, when CoAP is
      secured by means of DTLS (left) and OSCORE (right).  HC and Frag
       stand for Header Compression and Fragmentation, respectively.








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   Figures 3, 4 and 5 illustrate the SCHC-Lo network scenarios
   corresponding to a 6LN communicating with an external host on the
   Internet, and the protocol stacks corresponding to each relevant node
   (6LN, 6LBR, and external host).  SCHC Context at different SCHC
   Strata may come from different provisioning domains.



            6LN                6LBR                          External host

         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         +--------+                                           +--------+
         |  UDP   |                                           |  UDP   |
         +--------+     +----------------+                    +--------+
         |  IPv6  |     |      IPv6      |                    |  IPv6  |
         +--------+     +--------+-------+                    +--------+
         |SCHC HC |     |SCHC HC |       |                    |        |
         +--------+     +--------+       +                    +        +
         |6Lo Frag|     |6Lo Frag|       |                    |        |
         +--------+     +--------+       +                    +        +
         |802.15.4|     |802.15.4|       |                    |        |
         +--------+     +--------+-------+                    +--------+
             |               |        |                           |
             +---------------+        +---------------------------+
              SCHC-Lo network                   Internet


        Figure 3: Scenario and protocol stacks for end-to-end
   communication between a 6LN in a SCHC-Lo network and an external
     host on the Internet, without end-to-end security for CoAP.
          (Note: the figure has been adapted from RFC 8824.)



















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            6LN                6LBR                          External host

         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         +--------+                                           +--------+
         |  SCHC  |                                           |  SCHC  |
         +--------+                                           +--------+
         |  DTLS  |                                           |  DTLS  |
         +--------+                                           +--------+
         .  udp   .                                           .  udp   .
         ..........     ..................                    ..........
         .  ipv6  .     .      ipv6      .                    .  ipv6  .
         ..........     ..................                    ..........
         .  schc  .     .  schc  .       .                    .        .
         ..........     ..........       .                    .        .
         .6lo frag.     .6lo frag.       .                    .        .
         ..........     ..........       .                    .        .
         .802.15.4.     .802.15.4.       .                    .        .
         ..........     ..................                    ..........
             |               |        |                           |
             +---------------+        +---------------------------+
              SCHC-Lo network                   Internet


        Figure 4: Scenario and protocol stacks for end-to-end
   communication between a 6LN in a SCHC-Lo network and an external
  host on the Internet, when CoAP is secured with DTLS.  (Note: the
               figure has been adapted from RFC 8824.)























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         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         |  Inner |                                           |  Inner |
         +--------+                                           +--------+
         |  SCHC  |                                           |  SCHC  |
         |  Inner |                                           |  Inner |
         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         |  Outer |                                           |  Outer |
         +--------+                                           +--------+
         |  SCHC  |                                           |  SCHC  |
         |  Outer |                                           |  Outer |
         +--------+                                           +--------+
         .  udp   .                                           .  udp   .
         ..........     ..................                    ..........
         .  ipv6  .     .      ipv6      .                    .  ipv6  .
         ..........     ..................                    ..........
         .  schc  .     .  schc  .       .                    .        .
         ..........     ..........       .                    .        .
         .6lo frag.     .6lo frag.       .                    .        .
         ..........     ..........       .                    .        .
         .802.15.4.     .802.15.4.       .                    .        .
         ..........     ..................                    ..........
             |               |        |                           |
             +---------------+        +---------------------------+
              SCHC-Lo network                   Internet


        Figure 5: Scenario and protocol stacks for end-to-end
   communication between a 6LN in a SCHC-Lo network and an external
   host on the Internet, when CoAP is secured with OSCORE.  (Note:
             the figure has been adapted from RFC 8824.).

3.1.2.  Transition protocol stacks

   In order to ease a transition from existing 6LoWPAN implementations
   to support SCHC header compression, the present document also: i)
   illustrates protocol stacks where 6LoWPAN header compression is used
   to compress IPv6/UDP headers while SCHC compresses CoAP headers (see
   Figure 6), and ii) enables the transmission of SCHC-compressed UDP/
   CoAP headers over 6LoWPAN-compressed IPv6 packets (see Figure 7 and
   Section 5).  Note that the greatest header compression performance
   can be achieved by using SCHC to also compress the UDP header.

   RFC 8824 and draft-ietf-schc-8824-update define how SCHC can be used
   to compress CoAP headers.  On the other hand, it is possible to carry
   SCHC-compressed CoAP headers over UDP by means of using SCHC UDP
   ports [I-D.ietf-schc-protocol-numbers].  Figure 6 (left) shows the



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   corresponding protocol stack, where 6LoWPAN header compression is
   applied to UDP and IPv6.  When DTLS is preferred to protect SCHC-
   compressed CoAP messages, the DTLS layer sits between the SCHC
   Stratum below CoAP and the UDP layer (Figure 6, middle).  Figure 6
   (right) shows the protocol stack when OSCORE is used to protect CoAP
   messages, and SCHC is used to compress both CoAP inner and outer
   headers.


                                                      +------------+
                                                      | CoAP inner |
                                 +------------+       +------------+
                                 |    CoAP    |       |   SCHC HC  |
            +------------+       +------------+       +------------+
            |    CoAP    |       |    SCHC    |       | CoAP outer |
            +------------+       +------------+       +------------+
            |    SCHC    |       |    DTLS    |       |   SCHC HC  |
            +------------+       +------------+       +------------+
            |     UDP    |       |     UDP    |       |     UDP    |
            +------------+       +------------+       +------------+
            |    IPv6    |       |    IPv6    |       |    IPv6    |
            +------------+       +------------+       +------------+
            | 6LoWPAN HC |       | 6LoWPAN HC |       | 6LoWPAN HC |
            +------------+       +------------+       +------------+
            |6LoWPAN Frag|       |6LoWPAN Frag|       |6LoWPAN Frag|
            +------------+       +------------+       +------------+
            |  802.15.4  |       |  802.15.4  |       |  802.15.4  |
            +------------+       +------------+       +------------+

         Figure 6: Transition protocol stacks where 6LoWPAN header
     compression is applied to UDP and IPv6: without security for CoAP
       (left), using DTLS (middle), and using OSCORE (right).  HC and
            Frag stand for Header Compression and Fragmentation,
                               respectively.

   Finally, the transition protocol stack (TPS) enabled by this document
   (Section 5), which allow the transmission of 6LoWPAN-compressed IPv6
   packets containing SCHC-compressed UDP/CoAP data units, is shown in
   Figure 7, in three different variants: single SCHC Stratum for joint
   UDP/CoAP SCHC header compression (left), two SCHC Strata -one below
   UDP and another one below CoAP- (middle), and three SCHC Strata -one
   below UDP, one below the CoAP outer layer, and one below the CoAP
   inner layer- (right).  Note that the rightmost protocol stack in
   Figure 7 corresponds to use of OSCORE-protected CoAP.







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                                                      +------------+
                                                      | CoAP inner |
                                                      +------------+
                                                      |   SCHC HC  |
                                 +------------+       +------------+
                                 |    CoAP    |       | CoAP outer |
            +------------+       +------------+       +------------+
            |    CoAP    |       |   SCHC HC  |       |   SCHC HC  |
            +------------+       +------------+       +------------+
            |     UDP    |       |     UDP    |       |     UDP    |
            +------------+       +------------+       +------------+
            |   SCHC HC  |       |   SCHC HC  |       |   SCHC HC  |
            +------------+       +------------+       +------------+
            |    IPv6    |       |    IPv6    |       |    IPv6    |
            +------------+       +------------+       +------------+
            | 6LoWPAN HC |       | 6LoWPAN HC |       | 6LoWPAN HC |
            +------------+       +------------+       +------------+
            |6LoWPAN Frag|       |6LoWPAN Frag|       |6LoWPAN Frag|
            +------------+       +------------+       +------------+
            |  802.15.4  |       |  802.15.4  |       |  802.15.4  |
            +------------+       +------------+       +------------+

      Figure 7: TPS variants using SCHC for header compression of UDP/
       CoAP headers (right): one SCHC Stratum (left), two SCHC Strata
      (middle), and three SCHC Strata (right).  HC and Frag stand for
            Header Compression and Fragmentation, respectively.

3.2.  SCHC architecture concepts

   This section describes how SCHC architecture concepts (such as "SCHC
   Stratum", "Discriminator", "SCHC Stratum Header end point", "SCHC
   Payload end point", and "Set of Rules" (SoR)) [draft-ietf-schc-
   architecture] are applied when SCHC is used to compress IPv6 packet
   headers over IEEE 802.15.4 networks.  In addition, the concepts of
   Single-end point networks and Multiple-end point networks are
   introduced.  Note: in the present document, "Single-end point
   networks" and "Multiple-end point networks" are used for brevity to
   refer to "Single-end point SCHC-Lo networks" and "Multiple-end point
   SCHC-Lo networks".

3.2.1.  SCHC Stratum and Discriminator

   When SCHC is used to compress IPv6 packets over IEEE 802.15.4
   networks, a SCHC Stratum is located on top of layer 2 and below layer
   3 (that is, at layer 2.5).  Note that the compressed data of the SCHC
   Stratum may also comprise upper layer packet headers.  For example,
   SCHC may be used to compress IP headers, IP/UDP headers or IP/UDP/
   CoAP headers (all at once).



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   In both Single-end point and Multiple-end point networks, the
   Discriminator is a 6LoWPAN Dispatch Type set to the SCHC Dispatch or
   to the SCHC Pointer Dispatch (see Section 4).

3.2.2.  Single-end point networks

   In Single-end point networks, all network nodes that use SCHC for C/D
   have a single SCHC Payload end point, and thus a single SoR for SCHC
   Packet C/D.  For this reason, in Single-end point networks, the SCHC
   Stratum Header is fully compressed (i.e., the SCHC Stratum Header
   requires 0 bits to be transmitted over the air).

   In Single-end point networks, all network nodes that use SCHC for C/D
   have a single SCHC Stratum Header end point, and therefore a single
   SoR for SCHC Stratum Header C/D, which in this case comprises a
   single, implicit Rule for SCHC Stratum Header C/D.

3.2.3.  Multiple-end point networks

   In Multiple-endpoint networks, at least some of the network nodes
   that use SCHC for C/D have more than one SCHC Payload end point, and
   thus one SoR associated to each SCHC Payload end point.  Therefore,
   in Multiple-end point networks, the SCHC Stratum Header end point
   cannot generally be fully compressed (i.e., in compressed form, a
   SCHC Stratum Header of more than 0 bits is generally required to be
   transmitted over the air).

   In Multiple-end point networks, all network nodes that use SCHC for
   C/D have a single SCHC Stratum Header end point, and therefore a
   single SoR for SCHC Stratum Header C/D, which may comprise several
   Rules for SCHC Stratum Header C/D.

3.3.  Network topologies

   IEEE 802.15.4 supports two main network topologies: the star
   topology, and the peer-to-peer (i.e., mesh) topology.

   SCHC has been designed for LPWAN technologies, which are typically
   based on a star topology where constrained devices (e.g., sensors)
   communicate with a less constrained, central network gateway [RFC
   8376].  However, as stated in [draft-ietf-schc-architecture], SCHC is
   generic and it can also be used in networking environments beyond the
   ones originally considered for SCHC.

   SCHC compression is applicable to both star topology and mesh
   topology IEEE 802.15.4 networks.  The mechanism to be used to provide
   the SCHC header compression context to the nodes in an IEEE 802.15.4
   network is out of the scope of this document.



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3.4.  Single-hop communication

   In order to support the transmission of SCHC-compressed packets
   between two IEEE 802.15.4 nodes that are single-hop neighbors, both
   nodes MUST store the Rules intended for the communication between
   those two endpoints.

   The frame format to be used to carry a SCHC-compressed packet in
   single-hop communication is described in Section 4.1.

3.5.  Multihop communication

   6LoWPAN defines two approaches for multihop communication: Route-Over
   and Mesh-Under [RFC6606].  In Route-Over, routing is performed at the
   IP layer.  In Mesh-Under, routing functionality is located at the
   adaptation layer, below IP.  This section describes how SCHC-
   compressed packets are transmitted over a multihop IEEE 802.15.4
   network, for both Route-Over and Mesh-Under.

3.5.1.  Straightforward Route-Over (SRO)

   SCHC header compression MAY be used in a Route-Over network in a
   straightforward approach, whereby all routers (i.e., all 6LRs and
   6LBRs) MUST store all the Rules in use by any nodes in the SCHC-Lo
   network, whereas a host MUST store the Rules defined for its
   communication with other nodes.  This approach is called
   Straightforward Route-Over (SRO).  In this case, 6LoWPAN routers are
   able to decompress (if needed) received packet headers and compress
   packet headers before being forwarded.  In SRO, in Single-end point
   networks, a RuleID and the Rule it identifies MUST be unique SCHC-Lo
   network-wide (note: the means to ensure so are out of the scope of
   this document).  In order to simplify the management of RuleIDs in
   the SCHC-Lo network, in SRO, all nodes in the SCHC-Lo network MAY
   share the same SoR.  In SRO, in Multiple-endpoint networks, a not
   fully compressed SCHC Stratum Header MUST be used.

   Figure 8 illustrates an example Single-end point network with the
   Rules that need to be stored by the nodes in SRO.  In this example,
   RuleID 1 is intended for communication between Host A and Host B,
   RuleID 2 is intended for communication between Host A and Host C, and
   RuleID 3 is used for the communication between Host A and an external
   node called Host E.









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                                                 Host E
                                                /
                    (RuleID 1)        +--------+
                    (RuleID 2)    --- |Internet|
                    (RuleID 3)   /    +--------+
                   6LBR ---------
                 /      \
                /        \
              6LR         6LR ------------+                      Pair of nodes
     (RuleID 1) |         | (RuleID 1)    |          RuleID 1:       A, B
     (RuleID 2) |         | (RuleID 2)    |          RuleID 2:       A, C
     (RuleID 3) |         | (RuleID 3)    |          RuleID 3:       A, E
                |         |               |
             Host A      Host B         Host C
              (RuleID 1)    (RuleID 1)     (RuleID 2)
              (RuleID 2)
              (RuleID 3)

     Figure 8: Rules stored by each node in an example Single-end
                       point network using SRO.

   Figure 9 illustrates an example Multiple-end point network with the
   Rules that need to be stored by the nodes in SRO.  In this example,
   in addition to the Rules used in Figure 8, which correspond to a SCHC
   Payload end point called E1 in this example, there is a second RuleID
   2, which corresponds to communication between A and B, in a second
   SCHC Payload end point (E2).  Note that, for simplicity, Figure 9
   shows the same end point identifier (e.g., E1 or E2) for two end
   points that share a Rule.






















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                                                   Host E
                    (RuleID 2, E2)                /
                    (RuleID 1, E1)      +--------+
                    (RuleID 2, E1)  --- |Internet|
                    (RuleID 3, E1) /    +--------+
                   6LBR -----------
                 /      \
                /        \
              6LR         6LR -------------+                 Nodes | End point
(RuleID 1, E1) |         | (RuleID 1, E1)  |      RuleID 1:   A, B      E1
(RuleID 2, E1) |         | (RuleID 2, E1)  |      RuleID 2:   A, C      E1
(RuleID 3, E1) |         | (RuleID 3, E1)  |      RuleID 3:   A, E      E1
(RuleID 2, E2) |         | (RuleID 2, E2)  |      RuleID 2:   A, B      E2
               |         |                 |
              Host A      Host B         Host C
        (RuleID 1, E1)    (RuleID 1, E1)   (RuleID 2, E1)
        (RuleID 2, E1)    (RuleID 2, E2)
        (RuleID 3, E1)
        (RuleID 2, E2)


    Figure 9: Rules stored by each node in an example Multiple-end
                       point network using SRO.

   The frame format to be used to carry a SCHC-compressed packet in SRO
   is described in Section 4.1.

3.5.2.  Tunneled, RPL-based Route-Over (TRO)

   In a Route-Over network that uses the IPv6 Routing Protocol for Low-
   Power and Lossy Networks (RPL) [RFC6550], the RPL non-storing mode
   [RFC6550, RFC 6554] and [RFC8138] MAY be exploited in order to
   efficiently transmit SCHC-compressed packets.  In this approach,
   packets sent by a 6LN are tunneled to the root, and packets intended
   for 6LNs are tunneled from the root (note: a tunnel is not needed
   when the root itself is the source).  Traffic between two 6LNs
   traverses an Upward tunnel to the root and a Downward tunnel from the
   root.  The present document defines the described approach as
   Tunneled, RPL-based Route-Over approach (TRO).

   In TRO, each 6LoWPAN node (i.e., a host, a 6LR or a 6LBR) MUST store
   the Rules defined for its communication with other peer nodes.  A 6LR
   is relieved to store Rules used by nodes that do not include the 6LR
   itself.  A 6LBR MUST store all the Rules used by all nodes in the
   SCHC-Lo network.






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   In a TRO Single-end point network, a RuleID and the Rule it
   identifies MUST be unique SCHC-Lo network-wide (note: the means to
   ensure so are out of the scope of this document).  In a TRO Multiple-
   end point network, a not fully compressed SCHC Stratum Header MUST be
   used.

   Figure 10 illustrates the Rules that need to be stored by the nodes
   in TRO, based on the same example Single-end point network and sets
   of peer nodes shown in Figure 8.


                                                 Host E
                                                /
                    (RuleID 1)        +--------+
                    (RuleID 2)    --- |Internet|
                    (RuleID 3)   /    +--------+
                   6LBR ---------
                 /      \
                /        \
              6LR         6LR ------------+                      Pair of nodes
     (no Rules) |         | (no Rules)    |           RuleID 1:      A, B
                |         |               |           RuleID 2:      A, C
                |         |               |           RuleID 3:      A, E
                |         |               |
             Host A      Host B         Host C
              (RuleID 1)    (RuleID 1)     (RuleID 2)
              (RuleID 2)
              (RuleID 3)

    Figure 10: Rules stored by each node in an example Single-end
                       point network using TRO.

   Figure 11 illustrates an example Multiple-end point network with the
   Rules that need to be stored by the nodes in TRO.  In this example,
   in addition to the Rules used in Figure 10, which correspond to a
   SCHC Payload end point called E1 in this example, there is a second
   RuleID 2, which corresponds to communication between A and B, in a
   second SCHC Payload end point (E2).













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                                                  Host E
                    (RuleID 2, E2)               /
                    (RuleID 1, E1)      +--------+
                    (RuleID 2, E1)  --- |Internet|
                    (RuleID 3, E1) /    +--------+
                   6LBR -----------
                 /      \
                /        \
              6LR         6LR -------------+                 Nodes | End point
    (No Rules) |         | (No Rules)      |      RuleID 1:   A, B      E1
               |         |                 |      RuleID 2:   A, C      E1
               |         |                 |      RuleID 3:   A, E      E1
               |         |                 |      RuleID 2:   A, B      E2
               |         |                 |
              Host A      Host B         Host C
        (RuleID 1, E1)    (RuleID 1, E1)   (RuleID 2, E1)
        (RuleID 2, E1)    (RuleID 2, E2)
        (RuleID 3, E1)
        (RuleID 2, E2)


   Figure 11: Rules stored by each node in an example Multiple-end
                       point network using TRO.

   RFC 9008 describes how the communication between a 6LN and another
   node (another 6LN or the root of the same RPL domain, or an external
   node, e.g., on the Internet) is performed.  For the sake of
   description clarity, Figure 12 (adapted from Figure 3 in RFC 9008)
   provides a reference topology including nodes referred to in the
   remainder of this subsection.





















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                        +------------+
                        |  INTERNET  |---------+
                        +------------+         |
                                             Z |
                                         +-------+
                                         | 6LBR  |
                             +-----------|(root) |--------+
                             |           +-------+        |
                             |                            |
                             | Y                          |X
                         +---|---+                    +---|---+
                         |  6LR  |                    |  6LR  |
                 +-------|       |--+              +--|       |--+
                 |       +-------+  |              |  +-------+  |
                 | W                |  V           |             |
             +---|---+          +---|---+          |             |
             |  6LR  |          |  6LR  |          |             |
             |       |          |       |          |             |
             +---|---+          +-|---|-+          |             |
                 |                |   |            |             |
                 |           +----+   |            |             |
              U  |         T |        | S        R |           Q |
           +-----+-+   +-------+  +---|--+     +---|---+     +---|---+
           |  RAL  |   | RUL   |  | RAL  |     |  RAL  |     | RUL   |
           |  6LN  |   |  6LN  |  | 6LN  |     |  6LN  |     |  6LN  |
           +-------+   +-------+  +------+     +-------+     +-------+

      Figure 12: Reference topology to support the description of TRO.

   In RPL non-storing mode, for Downward traffic, the root adds a
   source-routing header.  The root also performs IPv6-in-IPv6
   encapsulation, except when the root itself is the packet source.  The
   IPv6-in-IPv6 encapsulation terminates at the 6LN (if it is a RAL,
   e.g., U, S or R) or at the last 6LR, e.g., V or X, (if the 6LN is a
   RUL, e.g., T or Q).  For Upward traffic, IPv6-in-IPv6 encapsulation
   is performed by the first 6LR, e.g. V or X, when the 6LN is a RUL,
   e.g., T or Q, that sends a packet to an external node or to another
   6LN in the same RPL domain, but not to the root.  When the 6LN is a
   RAL (e.g., U, S or R) that sends packets to the same destinations,
   IPv6-in-IPv6 encapsulation may be performed (by the RAL itself).  The
   destination in the outer header of the IPv6-in-IPv6 encapsulation for
   Upward traffic is the root.









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   This document updates RFC 9008 by specifying that, in TRO, when a 6LN
   transmits an IPv6 packet whose header is compressed by means of SCHC
   instead of 6LoWPAN header compression (RFC 6282), the SCHC-compressed
   packet MUST be tunneled by means of IPv6-in-IPv6 encapsulation up to
   the root.  This applies regardless of the inner, SCHC-compressed
   packet destination.

   For Upward traffic, when the 6LN is a RAL (e.g., U, S or R), the 6LN
   itself performs the IPv6-in-IPv6 encapsulation.  However, if the 6LN
   is a RUL (e.g., T or Q), IPv6-in-IPv6 encapsulation is performed by
   the first 6LR (e.g., E or C, respectively).  In the latter case, in
   order to enable efficient packet transmission in the first hop from
   the 6LN, the first 6LR SHOULD be provided with SCHC Rules allowing
   efficient header compression of packets sent by that 6LN.

   For Downward traffic, when the 6LN is a RUL (e.g., G or J), in order
   to enable efficient packet transmission in the last hop to the 6LN,
   the last 6LR (e.g., V or X, respectively) SHOULD be provided with
   SCHC Rules allowing efficient header compression of packets sent to
   that 6LN.

   Not providing such SCHC Rules to the first or last 6LR (for Upward or
   Downward traffic, respectively) should only happen if it is not
   practical or possible to do so (e.g., due to lack of available memory
   at the 6LR).

   For the sake of efficiency, RFC 8138 MUST be used to compress IPv6-
   in-IPv6 headers, the RPL Option (RFC 6553) and the source routing
   header (RPL Routing Header type 3, RFC 6554).

   The frame format to be used to carry a SCHC-compressed packet in TRO
   is described in Section 4.2.

3.5.3.  Pointer-based Route-Over (PRO)

   In the previous SCHC-Lo route-over approach, TRO, intermediate nodes
   do not have to know the IPv6 destination address of a SCHC-compressed
   IPv6 packet to be able to forward it.  Another approach where
   intermediate nodes do not have to store the compression/decompression
   Rules used by other nodes, which in addition does not require the
   artifacts used in TRO (i.e., IPv6-in-IPv6 encapsulation, non-storing
   mode RPL and RFC 8138 compression), is called Pointer-based Route-
   Over (PRO).

   In PRO, a pointer (called "SCHC Pointer") is prepended to the SCHC-
   compressed packet, in order to indicate the location and length of
   the Hop Limit and the destination address residues in the SCHC-
   compressed header.  Therefore, a 6LR is able to determine the IPv6



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   destination address of a SCHC-compressed packet, decrement its Hop
   Limit and route the packet, without the need to store the
   corresponding Rules.  Note that, in PRO, each 6LoWPAN node (i.e., a
   host, a 6LR, or a 6LBR) MUST store the Rules defined for its
   communication with other peer nodes.  A 6LBR MUST store the Rules
   used by any SCHC-Lo network node for communication with external
   nodes.

   In a PRO Single-end point network, a RuleID MAY be used to identify
   different Rules used by different sets of peer nodes within the SCHC-
   Lo network.  In a PRO Multiple-end point network, a not fully
   compressed SCHC Stratum Header MUST be used.

   Figure 13 illustrates the Rules that are stored by the nodes in an
   example Single-end point network based using PRO.  Note that, in this
   example, the SCHC-Lo network exploits the fact that PRO allows a
   given RuleID to be used by different pairs of nodes.


                                                      Host E
                                                  /
                                        +--------+- Host F
                   (RuleID 3)       --- |Internet|
                   (RuleID 4)      /    +--------+
                   6LBR -----------
                 /      \
                /        \
              6LR         6LR ------------+                   Pair of nodes
     (no Rules)/|         | (no Rules)    |           RuleID 1:       A, B
              / |         |               |           RuleID 2:       A, C
             /  |         |               |           RuleID 2:       D, B
            /   |         |               |           RuleID 3:       A, E
       Host D  Host A     Host B         Host C       RuleID 4:       B, F
   (RuleID 2)   (RuleID 1)  (RuleID 1)    (RuleID 2)
                (RuleID 2)  (RuleID 2)
                (RuleID 3)  (RuleID 4)


    Figure 13: Rules stored by each node in an example Single-end
     point network using PRO.  In this example, both RuleID 2 and
            RuleID 3 are used by two pairs of nodes each.

   PRO is compatible with RPL storing mode, as well as with other
   routing protocols.

   Figure 14 illustrates an example Multiple-end point network with the
   Rules that need to be stored by the nodes in PRO.  In this example,
   in addition to the Rules used in Figure 13, which correspond to a



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   SCHC Packet Instance called E1 in this example, there is an
   additional RuleID 2, which corresponds to communication between A and
   D, in a second SCHC Payload end point (E2).


                                               Host E
                                              /
                                    +--------+- Host F
                   (RID 3, E1)  --- |Internet|
                   (RID 4, E1) /    +--------+
                   6LBR -------
                 /      \
                /        \
              6LR         6LR ------------+                   Nodes  |  End point
     (no Rules)/|         | (no Rules)    |            RID 1:  A, B        E1
              / |         |               |            RID 2:  A, C        E1
             /  |         |               |            RID 2:  D, B        E1
            /   |         |               |            RID 3:  A, E        E1
       Host D  Host A     Host B         Host C        RID 4:  B, F        E1
   (RID 2, E1)  (RID 1, E1)  (RID 1, E1)  (RID 2, E1)  RID 2:  A, D        E2
   (RID 2, E2)  (RID 2, E1)  (RID 2, E1)
                (RID 3, E1)  (RID 3, E1)
                (RID 2, E2)


   Figure 14: Rules stored by each node in an example Multiple-end
          point network using PRO.  'RID' stands for RuleID.

   The frame format to be used to carry a SCHC-compressed packet in PRO
   is described in Section 4.3.

3.5.4.  Mesh-Under

   When Mesh-Under is used in a SCHC-Lo network, Mesh-Under operates as
   described in RFC 4944.  The frame format to be used to carry a SCHC-
   compressed packet in the Mesh-Under approach is described in
   Section 4.4.

   For header compression in a Mesh-Under SCHC-Lo network, a SCHC-Lo
   network node MUST store the Rules defined for its communication with
   other peer nodes.










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   In Mesh-Under, in a Single-end point network, a RuleID MAY be used to
   identify different Rules used by different sets of peer nodes.  In a
   Mesh-Under Multiple-end point network, a fully compressed SCHC
   Stratum Header MAY be used as long as it is possible to determine the
   SCHC Payload end point needed to decompress a SCHC-compressed packet
   based on the packet source identifier (which is present in the Mesh-
   Under header [RFC 4944]).

   Figure 15 illustrates the Rules that need to be stored by the nodes
   when SCHC is used for header compression in a Single-end point Mesh-
   Under network, based on the same example network and node pairs shown
   in Figure 13.  Note that, in this example, the network exploits the
   fact that Mesh-under allows a given RuleID to be reused by different
   sets of peer nodes, even if the Rules sharing the same RuleID are
   different.  Nodes denoted "m" in Figure 15 correspond to Mesh-Under
   forwarders [RFC 6606].


                                                     Host E
                                                  /
                                        +--------+- Host F
                   (RuleID 3)       --- |Internet|
                   (RuleID 4)      /    +--------+
                   6LBR -----------
                  /     \
                 /       \
                m         m --------------+                    Pair of nodes
     (no Rules)/|         | (no Rules)    |           RuleID 1:    A, B
              / |         |               |           RuleID 2:    A, C
             /  |         |               |           RuleID 2:    D, B
            /   |         |               |           RuleID 3:    A, E
       Host D  Host A     Host B         Host C       RuleID 4:    B, F
   (RuleID 2)   (RuleID 1)  (RuleID 1)    (RuleID 2)
                (RuleID 2)  (RuleID 2)
                (RuleID 3)  (RuleID 4)


    Figure 15: Rules stored by each node in an example Single-end
    point network using Mesh-Under.  In this example, RuleID 2 is
                  used by different pairs of nodes.

   Figure 16 illustrates an example Multiple-end point network with the
   Rules that need to be stored by the nodes in PRO.  In this example,
   in addition to the Rules used in Figure 13, which correspond to a
   SCHC Payload end point called E1 in this example, there is an
   additional RuleID 2, which corresponds to communication between A and
   D, in a second SCHC Payload end point (E2).




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                                                Host E
                                              /
                                    +--------+- Host F
                   (RID 3, E1)  --- |Internet|
                   (RID 4, E1) /    +--------+
                   6LBR -------
                 /      \
                /        \
                m         m --------------+                   Nodes | End point
     (no Rules)/|         | (no Rules)    |            RID 1:  A, B       E1
              / |         |               |            RID 2:  A, C       E1
             /  |         |               |            RID 2:  D, B       E1
            /   |         |               |            RID 3:  A, E       E1
       Host D  Host A     Host B         Host C        RID 4:  B, F       E1
   (RID 2, E1)  (RID 1, E1)  (RID 1, E1)  (RID 2, E1)  RID 2:  A, D       E2
   (RID 2, E2)  (RID 2, E1)  (RID 2, E1)
                (RID 3, E1)  (RID 2, E2)
                (RID 2, E2)


   Figure 16: Rules stored by each node in an example Multiple-end
      point network using Mesh-Under.  'RID' stands for RuleID.

4.  Frame Format

   This section defines the frame formats that can be used when a SCHC-
   compressed packet is carried over IEEE 802.15.4.  Such formats are
   carried as IEEE 802.15.4 frame payload.  Note that the SCHC Stratum
   Header formats to support CoAP header C/D based on additional SCHC
   Strata over UDP (e.g., when CoAP is secured by means of DTLS or
   OSCORE, see Figure 2) are defined in Section 5.2.

4.1.  Single-hop or SRO frame format

   This subsection defines the frame format for carrying SCHC-compressed
   packets over IEEE 802.15.4 for single-hop communication (see 3.3) or
   when SRO is used for multihop communication (see 3.4.1).  This format
   comprises a SCHC Dispatch Type, a SCHC datagram, and Padding bits, if
   any.  The SCHC datagram is composed of a SCHC Stratum Header (which
   in some cases is fully elided), a SCHC Payload (i.e., the SCHC-
   compressed header of the packet being carried over IEEE 802.15.4),
   and user payload (i.e., the payload of the packet being carried over
   IEEE 802.15.4) [draft-ietf-schc-architecture].  Figure 17 illustrates
   the described frame format.







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      <--------------- IEEE 802.15.4 frame payload -------------------->

                      <----------- SCHC datagram ------------>
      +---------------+--------+--------------+--------------+ - - - - +
      | SCHC Dispatch |SCHC Hdr| SCHC Payload | user payload | Padding |
      +---------------+--------+--------------+--------------+ - - - - +


   Figure 17: Encapsulated, SCHC-compressed packet, for single-hop
       or SRO transmission.  Padding bits are added if needed.

4.1.1.  SCHC Dispatch

   Adding SCHC header compression to the panoply of header compression
   mechanisms used in 6LoWPAN/6Lo environments creates the need to
   signal when a packet header has been compressed by using SCHC.  To
   this end, the present document specifies the SCHC Dispatch.  The SCHC
   Dispatch indicates that the next field in the frame format is a SCHC
   Sratum header ("SCHC Hdr" in Figure 17, see 4.1.2)).

   This document defines the SCHC Dispatch as a 6LoWPAN Dispatch Type
   for SCHC header compression [RFC4944].  With the aim to minimize
   overhead, the present document allocates a 1-byte pattern in Page 0
   [RFC8025] for the SCHC Dispatch Type:

   SCHC Dispatch Type bit pattern: 01000100 (Page 0) (Note: to be
   confirmed by IANA))

4.1.2.  SCHC Stratum Header

   The SCHC Stratum Header ("SCHC Hdr" in Figure 17 and subsequent
   figures) determines the SCHC Payload end point to be used to
   decompress the next field (SCHC Payload, see 4.1.3).

   The SCHC Stratum Header format, and some examples of possible
   corresponding Rules for SCHC Stratum Header C/D, are shown in
   Figure 18.














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      Uncompressed SCHC Stratum Header format:
      +------------------+
      | SCHC Instance ID |
      +------------------+

      Compressed SCHC Stratum Header format:
      +--------+- - - - - - - - - - -+
      | RuleID | Compression Residue |
      +--------+- - - - - - - - - - -+

      Example C/D Rules for the SCHC Stratum Header:

      RuleID 1
      +-------------+--+---+--+-----+------+-----------+
      |     FID     |FL|POS|DI| TV  |  MO  |     CDA   |
      +-------------+--+---+--+-----+------+-----------+
      | SCHC.instid | 8| 1 |Bi|value|equal | not-sent  |
      +-------------+--+---+--+-----+------+-----------+

      RuleID 2
      +-------------+--+---+--+-----+------+-----------+
      |     FID     |FL|POS|DI| TV  |  MO  |    CDA    |
      +-------------+--+---+--+-----+------+-----------+
      | SCHC.instid | 8| 1 |Bi|0x00 |MSB(7)|    LSB    |
      +-------------+--+---+--+-----+------+-----------+

           Figure 18: SCHC Stratum Header Format and examples of
            corresponding C/D Rules for the SCHC Stratum Header

   The uncompressed SCHC Stratum Header format comprises a single field,
   called the SCHC Instance ID.  This field is an unsigned integer that
   identifies the session between SCHC end points in two or more peer
   nodes using a common SoR.  The SCHC Instance ID size is RECOMMENDED
   to be between 1 and 8 bits.

   As described in the SCHC architecture draft, in compressed form, the
   SCHC Stratum Header comprises a RuleID and a compression residue
   [draft-ietf-schc-architecture].  The RuleID size of the compressed
   SCHC Stratum Header is RECOMMENDED to be between 0 and 8 bits.  In
   the examples shown in Figure 18, the best match between a SCHC
   Instance ID and the Rules with RuleID 1 and RuleID 2 lead to
   compression residues of 0 bits and 1 bit, respectively.









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   In Single-end point networks, the SCHC Stratum Header MUST be fully
   compressed, i.e., its size in compressed form is 0 bits.  In
   Multiple-end point networks, the SCHC Stratum Header cannot always be
   fully compressed; in this case, the RuleID size (of the Rule used to
   compress the SCHC Stratum Header) is RECOMMENDED to be between 1 and
   8 bits.

4.1.3.  SCHC Payload

   The SCHC Payload is a packet header that has been compressed by using
   a SCHC Payload end point.  It is the compressed form of the header of
   the original packet being carried over IEEE 802.15.4.  As defined in
   [RFC8724], a SCHC-compressed header comprises a RuleID, and a
   compression residue.  As per the present specification, a RuleID size
   between 1 and 16 bits is RECOMMENDED.  In order to decide the RuleID
   size to be used in a SCHC-Lo network, the trade-off between
   (compressed) header overhead and the number of Rules needs to be
   carefully assessed.

4.1.4.  User payload

   The user payload is the payload of the original packet being carried
   over IEEE 802.15.4, which is unaffected by the SCHC Stratum [draft-
   ietf-schc-architecture].

4.1.5.  Padding

   If SCHC header compression leads to a SCHC datagram size of a non-
   integer number of bytes, padding bits of value equal to zero MUST be
   appended to the SCHC datagram as appropriate to align to an octet
   boundary.

4.2.  TRO frame format

   This subsection defines the frame formats for carrying SCHC-
   compressed packets over IEEE 802.15.4 in TRO (see 3.3.2).  Such
   formats are based on RFC 8138; however, instead of RFC 6282 header
   compression, this specification uses SCHC header compression.
   Accordingly, this specification updates RFC 8138 by stating that a
   6LoRH header MUST always be placed before the LOWPAN_IPHC as defined
   in RFC 6282 [RFC6282] or the SCHC Dispatch, followed by the SCHC
   Stratum Header and the SCHC-compressed packet, as defined in the
   present specification.

   Since 6LoRH uses Dispatch Types in Page 1, the present specification
   also defines a SCHC Dispatch Type in Page 1, with the same bit
   pattern as the one in Page 0: 01000100 (to be confirmed by IANA).




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   In the TRO frame formats, the SCHC Header is preceded by the SCHC
   Dispatch (in this case, in Page 1).

   The frame format for Downward transmission, except when the SCHC-
   compressed packet source is a RPL root, is shown in Figure 19:


    <----------------- IEEE 802.15.4 frame payload -------------------------->
                                                     <- SCHC datagram ->
    +-- ... -+-- ... --+- ... -+--- ... --+---- ... -+----+-----+------+ - - +
    |11110001|SRH-6LoRH| RPI-  | IP-in-IP | 01000100 |SCHC|SCHC | user | pad |
    |Page 1  |         | 6LoRH |  6LoRH   |SCHCDsptch| Hdr| Pld |  pld |     |
    +-- ... -+-- ... --+- ... -+--- ... --+---- ... -+----+-----+------+ - - +
                                            (Page 1)

                                          <-------- This specification ------->

   Figure 19: Downward frame format for SCHC-compressed packets in
               TRO, when the source is not a RPL root.

   The frame format for Downward transmission, when the SCHC-compressed
   packet source is a RPL root, is shown in Figure 20:


         <---------------- IEEE 802.15.4 frame payload ---------------->
                                               <- SCHC datagram ->
         +-- ... -+-- ... --+- ... -+---- ... -+----+-----+------+ - - +
         |11110001|SRH-6LoRH| RPI-  | 01000100 |SCHC|SCHC | user | pad |
         |Page 1  |         | 6LoRH |SCHCDsptch| Hdr| Pld |  pld |     |
         +-- ... -+-- ... --+- ... -+---- ... -+----+-----+------+ - - +
                                      (Page 1)

                                    <----- This specification ----->




    Figure 20: Downward frame format for SCHC-compressed packets in
                  TRO, when the source is a RPL root.

   The frame format for Upward transmission is shown in Figure 21 (note
   that it does not include the source routing header that is present in
   the Downward frame format):








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        <--------------- IEEE 802.15.4 frame payload ------------------>

                                               <- SCHC datagram ->
        +-- ... -+- ... -+--- ... --+---- ... -+----+-----+------+ - - +
        |11110001| RPI-  | IP-in-IP | 01000100 |SCHC|SCHC | user | pad |
        |Page 1  | 6LoRH |  6LoRH   |SCHCDsptch| Hdr| Pld |  pld |     |
        +-- ... -+- ... -+--- ... --+---- ... -+----+-----+------+ - - +
                                      (Page 1)

                                    <----- This specification ---->


   Figure 21: Upward frame format for SCHC-compressed packets in TRO.

4.3.  PRO frame format

   This subsection describes the frame format for carrying SCHC-
   compressed packets over IEEE 802.15.4 in PRO (see 3.3.3).  Such
   format is shown in Figure 22:


              <------------ IEEE 802.15.4 frame payload ------------->

              +--------------+--------------+--------------+ - - - - +
              |  PRO Header  | SCHC Payload | user payload | Padding |
              +--------------+--------------+--------------+ - - - - +
                      v              <->
                      |               |
                      +---------------+
                        SCHC Pointer


        Figure 22: frame format for SCHC-compressed packets in PRO.

   The PRO Header format is shown in Figure 23:



           0 1 2 3 4 5 6 7 0 1 2 3 4 .... 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3
          +---------------+-+ - - - +----+-------------+-+-------------+
          |      SCHC     |C|       |    |             |H|             |
          |     Pointer   |I|  DCI  |SCHC| Bit Pointer |L|   Address   |
          |    Dispatch   |D|       | Hdr|             |M|    Length   |
          +---------------+-+ - - - +----+-------------+-+-------------+


                      Figure 23: PRO Header format.




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   The first field in Figure 23 is defined as the SCHC Pointer Dispatch,
   which signals the start of a PRO Header format.  This document
   defines the SCHC Pointer Dispatch as a 6LoWPAN Dispatch Type
   [RFC4944] for SCHC header compression.

   With the aim to minimize header overhead, the present document
   allocates a 1-byte pattern in the 6LoWPAN Dispatch Type Page 0
   [RFC8025] for the SCHC Pointer Dispatch Type:

   SCHC Pointer Dispatch Type bit pattern: 01000101 (Page 0) (Note: to
   be confirmed by IANA))

   The next field in the PRO Header is the Context IDentifier (CID)
   flag, which is set to 1 to signal that the Destination Context
   Identifier (DCI) field (see PRO_header_format) is present in the
   frame.  When CID is set to 0, the DCI field is not present.

   The DCI field is optional.  When present, it has a size of 4 bits.
   Similarly to RFC 6282, this field identifies the prefix of the IPv6
   destination address.  How such prefix context is distributed and
   maintained is out of the scope of the present document.

   The next field is the SCHC Stratum Header ("SCHC Hdr" in Figure 22),
   which has been defined in section 4.1.2.  As shown in Figure 22, in
   the PRO Header, the SCHC Stratum Header is not immediately followed
   by the SCHC Packet.

   The Bit pointer gives the starting position of the Hop Limit followed
   by the IPv6 destination address in the SCHC residue of the SCHC-
   compressed IPv6 header (in bits), starting after the Address Length
   field and before the first field of the SCHC-compressed IPv6 header
   (i.e., the RuleID).  For example, if the Hop Limit and the IPv6
   destination address residue are the only residues in a SCHC-
   compressed IPv6 packet header (i.e., such residue starts right after
   the RuleID in the SCHC-compressed header), then the Bit pointer will
   have a value of RuleID length in bits.

   The Hop Limit (HLM) flag is 1 bit that indicates the length of the
   Hop Limit field residue in the SCHC-compressed IPv6 header.  When HLM
   equals 0, the Hop Limit compression residue has a size of 4 bits.  In
   this case, the 4 most significant bits of the uncompressed Hop Limit
   field are equal to 0.  Therefore, Hop Limit compression applies only
   to Hop Limit values between 15 and 0.  When HLM is set to 1, the Hop
   Limit compression residue has a size of 8 bits (i.e., it is
   uncompressed).






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   Address Length indicates the size of the IPv6 destination address
   residue (in bits).  It can be up to 128 bits to allow representing
   the complete destination address, if needed.

   PRO requires a special SCHC Rule design where the FIDs of the IPv6
   Destination and Source addresses are swapped (see 6.1.1).

4.4.  Mesh-Under frame format

   This subsection describes the frame formats for carrying SCHC-
   compressed packets over IEEE 802.15.4 in the Mesh-Under approach (see
   3.3.3).  Note that the formats are provided in this section for the
   sake of clarity and completeness, since they are the same as those
   defined for Mesh-Under in RFC 4944, except for the fact that SCHC-
   compressed packets are carried.

   The frame format for a SCHC-compressed packet to be sent by means of
   Mesh-Under, when fragmentation is not needed, is shown in Figure 24:


    <-------------------- IEEE 802.15.4 frame payload -------------------->

                                         <------ SCHC datagram ----->
    +-----------+----------+-------------+--------+--------+--------+ - - +
    | Mesh Type | Mesh Hdr | SCHC Dsptch |SCHC Hdr|SCHC Pld|User pld| pad |
    +-----------+----------+-------------+--------+--------+--------+ - - +


   Figure 24: Encapsulated, SCHC-compressed packet, for Mesh-Under
   transmission (without fragmentation).  Padding bits are added if
                               needed.

   The frame format for a SCHC-compressed packet to be sent by means of
   Mesh-Under, which also requires fragmentation, is shown in Figure 25:


   <----------------------- IEEE 802.15.4 frame payload ----------------------->

                                              <----- SCHC datagram ------>
   +-------+-------+-------+-------+----------+--------+--------+--------+ - - +
   | M Typ | M Hdr | F Typ | F Hdr | SCHC Dsp |SCHC Hdr|SCHC Pld|User pld| Pad |
   +-------+-------+-------+-------+----------+--------+--------+--------+ - - +









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   Figure 25: Encapsulated, SCHC-compressed packet, for Mesh-Under
    transmission (with fragmentation).  Padding bits are added if
                               needed.

   The frame format for a SCHC-compressed packet to be sent by means of
   Mesh-Under, which also requires a broadcast header to support mesh
   broadcast/multicast, is shown in Figure 26:


   <----------------------- IEEE 802.15.4 frame payload ----------------------->

                                              <------ SCHC datagram ----->
   +-------+-------+-------+-------+----------+--------+--------+--------+ - - +
   | M Typ | M Hdr | B Typ | B Hdr | SCHC Dsp |SCHC Hdr|SCHC Pld|User pld| Pad |
   +-------+-------+-------+-------+----------+--------+--------+--------+ - - +


      Figure 26: Encapsulated, SCHC-compressed packet, for mesh
       broadcast/multicast in Mesh-Under transmission (without
   fragmentation).  Padding bits are added if needed.  'B Dsp' and
    'B Hdr' stand for 'Broadcast Dispatch' and 'Broadcast Header',
                            respectively.

   As in RFC 4944, when more than one LoWPAN header is used in the same
   packet, they MUST appear in the following order: Mesh Addressing
   Header, Broadcast Header, Fragmentation Header.

4.5.  Summary

   A summary of the formats and main features for the different
   transmission alternatives enabled by the present documentis shown in
   Figure 27:


   +-------------+----------------------------------------------------------+
   |  Single-hop |                        Multihop                          |
   +-------------+-------------------------------------------+--------------+
   |             |                Route-Over                 |              |
   |             +-----------+----------------+--------------+  Mesh-Under  |
   |             |    SRO    |      TRO       |     PRO      |              |
   +-------------+-----------+----------------+--------------+--------------+
   |SCHC Dispatch| SCHC Disp |IP-in-IP, 6LoRH,|SCHC Ptr Disp,| Mesh Headers,|
   |             |           | SCHC Dispatch  | SCHC Pointer | SCHC Dispatch|
   +-------------+-----------+----------------+--------------+--------------+
   |   see 4.1   |  see 4.1  |    see 4.2     |   see 4.3    |    see 4.4   |
   +-------------+-----------+----------------+--------------+--------------+





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       Figure 27: Summary of formats and main features for the
     transmission of SCHC- compressed packets over IEEE 802.15.4
     enabled by the present document, and corresponding artifacts

5.  Enabling the TPS

   This section describes two main approaches to enable the TPS, i.e.,
   the protocol stack that keeps using 6LoWPAN/6lo header compression
   [RFC6282][RFC8138] for the IPv6 header, while using SCHC for UDP and
   CoAP header compression (Figure 7, Section 3.1.2).  The first
   approach is based on using a single SCHC Stratum for joint UDP/CoAP
   header C/D.  The second one is based on using at least two SCHC
   Strata (one of them for UDP header C/D, the other(s) for CoAP header
   C/D, including OSCORE).  The functionality associated to these two
   approaches is described in subsection 5.1 and subsection 5.2,
   respectively.

   SCHC uses a SCHC Stratum Header to identify the SCHC-compressed
   protocol header(s), along with further information to support SCHC
   operation (when needed).  SCHC may also need a Discriminator to
   identify the SoR to be used for header decompression [draft-ietf-
   schc-architecture].

   In order to support SCHC-compressed UDP/CoAP headers over 6LoWPAN-
   compressed IPv6 packets, the present document exploits the work that
   is being done by the SCHC WG to define a new Internet Protocol Number
   for SCHC [I-D.ietf-schc-protocol-numbers].  In this approach, the NH
   field of the RFC 6282-compressed IPv6 header format is set to 0.  The
   Next Header field of the IPv6 header remains an 8-bit (uncompressed)
   field carrying the SCHC Internet Protocol Number.  The resulting
   protocol encapsulation and corresponding format for an unfragmented
   packet, which is carried as IEEE 802.15.4 frame payload, is shown in
   Figure 28.  Padding is added as needed to align the format to an
   octet boundary.


       <---------------- IEEE 802.15.4 frame payload ------------------>
       +-----------------------+------------------+--------------+ - - +
       | RFC6282-compressed    | SCHC-compressed  |              |     |
       |     IPv6 header       | UDP/CoAP headers | CoAP Payload | Pad |
       |(NH=0,Next Header=SCHC)| (includes SCHC   |              |     |
       |                       | Stratum Header)  |              |     |
       +-----------------------+------------------+--------------+ - - +

    Figure 28: Protocol data unit encapsulation and format for the
              TPS, using a SCHC Internet Protocol Number





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   For RPL-based networks that use the TPS, the formats defined in RFC
   8138 may also be used for the sake of efficiency, as shown in
   Figure 29.  In this figure, the first field is the Page switch with
   value 1, followed by RFC 8138-compressed routing artifacts, then
   followed by the RFC 6282-compressed IPv6 header (which indicates that
   the next header data unit is a SCHC datagram).



    <------------------------- IEEE 802.15.4 frame payload ------------------------>
    +--------+------------+------------------+----------------+--------------+ - - +
    |11110001|8138-cmprssd|  6282-compressed | SCHC-comprssd  |              |     |
    |(Page 1)|  routing   |   IPv6 header    | UDP/CoAP hdrs  | CoAP Payload | Pad |
    |        | artifacts  |(NH=0,NxtHdr=SCHC)| (incl. SCHC    |              |     |
    |        |            |                  | Stratum Header)|              |     |
    +--------+------------+------------------+----------------+--------------+ - - +


    Figure 29: Protocol data unit encapsulation and format for the
  TPS using a SCHC Internet Protocol Number and RFC 8138-compressed
                          routing artifacts

5.1.  SCHC C/D for the TPS: joint UDP/CoAP header compression

   Over the IP layer, SCHC compression may be used for UDP only, UDP and
   CoAP jointly, or any other protocol or combination of protocols.
   This section describes joint UDP/CoAP C/D for the TPS, based on a
   single SCHC Stratum.

   The SCHC-compressed UDP/CoAP headers field has the format detailed in
   Figure 30.  Such field comprises in turn two fields: the SCHC Stratum
   Header for UDP and CoAP, and the corresponding SCHC Payload (i.e., a
   RuleID followed by the compression residue of the UDP/CoAP header).
   If there is a single SoR for UDP/CoAP header C/D, the SCHC Stratum
   Header for UDP and CoAP is fully elided (i.e., it requires zero bits
   when the packet is transmitted).















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       <---------------- IEEE 802.15.4 frame payload ------------------>
       +-----------------------+------------------+--------------+ - - +
       | RFC6282-compressed    | SCHC-compressed  |              |     |
       |     IPv6 header       | UDP/CoAP headers | CoAP Payload | Pad |
       |(NH=0,Next Header=SCHC)|(includes SCHC    |              |     |
       |                       | Stratum Header(s)|              |     |
       +-----------------------+------------------+--------------+ - - +
                               /                  \
                      |-------/                    \-----------|
                      +------------------+---------------------+
                      |   SCHC Stratum   |     SCHC Payload    |
                      |       Header     | (RuleID + cmp. rsd. |
                      | for UDP and CoAP | of UDP/CoAP header) |
                      +------------------+---------------------+


        Figure 30: Detailed view of the SCHC-compressed UDP and CoAP
     headers.  A single SCHC Stratum is used jointly for UDP and CoAP.

   The SCHC Stratum Header for joint UDP and CoAP header C/D, and the
   Rule to compress/decompress the SCHC Stratum Header itself for
   devices that only support the TPS, are defined in Figure 31.  When a
   TPS-only device transmits a CoAP data unit, the SCHC Stratum Header
   is fully compressed and it incurs no transmission overhead (i.e., it
   is compressed down to 0 bits when transmitted), since the SoR of the
   SCHC Stratum end point contains exactly one Rule.  When receiving a
   data unit, a TPS-only device also assumes that the SCHC Stratum
   Header is fully compressed (down to 0 bits).

   A SCHC-Lo network may comprise TPS-only nodes and other nodes that
   also use 6LoWPAN/6lo to compress IPv6 headers (and routing protocol
   artifacts when needed) but support other protocol combinations on top
   of IPv6, in addition to UDP/CoAP.  The latter nodes MUST also use/
   assume a fully compressed SCHC Stratum Header (down to 0 bits when
   transmitted) to send/receive UDP/CoAP data units to/from nodes that
   only implement the TPS, but will need to use/assume a not fully
   compressed SCHC Stratum Header when sending/receiving to/from other
   devices that support further protocols atop IPv6.  In that case, the
   uncompressed SCHC Stratum Header format will also be the one shown in
   Figure 31, but using the appropriate Protocol ID and Port number
   values.  In such a mixed network, a receiving node can determine
   whether the SCHC Stratum Header has been fully compressed (down to 0
   bits) based on prior knowledge that the sender is a TPS-only node.
   In this case, the IPv6 address of the sender is used as a
   Discriminator.






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    +-----------+-----------+
    |Protocol ID|Port number| Non-Compressed SCHC Stratum Header for joint UDP/CoAP C/D
    +-----------+-----------+
    Protocol ID = 17 (UDP)
    Port number = 5683 (CoAP)

    +---------+- - - - - - - - - - -+
    | Rule ID | Compression Residue | SCHC-Compressed Stratum Header for joint UDP/CoAP C/D
    +---------+- - - - - - - - - - -+
    Note: for devices that only implement the TPS (i.e., the only protocols carried over IP are UDP and CoAP), the SCHC-Compressed Stratum Header is fully
    compressed (down to 0 bits when transmitted over the air) since there is only one Rule in the SoR for the SCHC Stratum end point for such
    devices.

    Rule to compress/decompress the SCHC Stratum Header for joint UDP/CoAP header C/D for devices that only implement the TPS:

    RuleID
    +--------------+--+---+--+----+------+----------+
    |      FID     |FL|POS|DI| TV |  MO  |   CDA    |
    +--------------+--+---+--+----+------+----------+
    | SCHC.proto   | 8| 1 |Bi| 17 |equal | not-sent |
    +--------------+--+---+--+----+------+----------+
    | SCHC.portnum |16| 1 |Bi|5683|equal | not-sent |
    +--------------+--+---+--+----+------+----------+


   Figure 31: SCHC Stratum Header for joint UDP/CoAP header C/D in
    non-compressed and in SCHC-compressed form, and corresponding
                                Rule.

5.2.  SCHC C/D for the TPS: multiple SCHC Strata

   This section describes SCHC C/D for the TPS, based on using a SCHC
   Stratum below UDP, for UDP header C/D, and at least another one,
   between UDP and CoAP, for CoAP header C/D.

   When only one SCHC Stratum is used for CoAP header C/D (e.g., when
   OSCORE is not used), the SCHC-compressed UDP/CoAP headers field
   comprises four fields (Figure 32): the SCHC Stratum Header for UDP,
   the corresponding SCHC Payload (i.e., a RuleID followed by the
   compression residue of the UDP header), the SCHC Stratum Header for
   CoAP, and the SCHC Payload for the latter (i.e., a RuleID followed by
   the compression residue of the CoAP header).  If there is a single
   SoR for UDP header C/D or CoAP header C/D, the corresponding SCHC
   Stratum Header is fully elided.







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  <------------------------ IEEE 802.15.4 frame payload ------------------------->
  +--------+------------+------------------+----------------+--------------+ - - +
  |11110001|8138-cmprssd|  6282-compressed | SCHC-comprssd  |              |     |
  |(Page 1)|  routing   |   IPv6 header    | UDP/CoAP hdrs  | CoAP Payload | Pad |
  |        | artifacts  |(NH=0,NxtHdr=SCHC)| (incl. SCHC    |              |     |
  |        |            |                  | Stratum Hdr(s))|
  +--------+------------+------------------+----------------+--------------+ - - +
                                          /                  \
                                 |-------/                    \--------|
                                 +---------+--------+---------+--------+
                                 |  SCHC   |  SCHC  |  SCHC   | SCHC   |
                                 | Stratum | Payload| Stratum | Payload|
                                 | Header  | (UDP)  |  Header | (CoAP) |
                                 | for UDP |        | for CoAP|        |
                                 +---------+--------+---------+--------+

     Figure 32: Detailed view of the SCHC-compressed UDP and CoAP
     headers.  Two separate SCHC Strata are used to support SCHC-
       compressed UDP headers and SCHC-compressed CoAP headers,
                            respectively.

   The SCHC Stratum Header for UDP header C/D, and the Rule to compress/
   decompress that SCHC Stratum Header for devices that only support the
   TPS, are defined in Figure 33.  The SCHC Stratum Header for CoAP
   header C/D, and the Rule to compress/decompress that SCHC Stratum
   Header for devices that only support the TPS, are defined in
   Figure 34.


  +-------------+
  | Protocol ID | Non-Compressed SCHC Stratum Header for UDP
  +-------------+
  Protocol ID = 17 (UDP)

  +.........+- - - - - - - - - - -+
  | Rule ID | Compression Residue | SCHC-Compressed Stratum Header for UDP
  +.........+- - - - - - - - - - -+
  Note: for devices that only implement the TPS (i.e., the only protocol carried over IPv6 is UDP), the SCHC-Compressed Stratum Header is fully compressed     (down to 0 bits when transmitted over the air) since there is only one Rule in the SoR of the SCHC Stratum for such devices.

  Rule to compress the SCHC Stratum Header for UDP header C/D:

  RuleID
  +------------+--+---+--+----+------+----------+
  |     FID    |FL|POS|DI| TV |  MO  |   CDA    |
  +------------+--+---+--+----+------+----------+
  | SCHC.proto | 8| 1 |Bi| 17 |equal | not-sent |
  +------------+--+---+--+----+------+----------+




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      Figure 33: SCHC Stratum Header for UDP header C/D in non-
    compressed and SCHC- compressed form, and corresponding Rule.


  +-------------+
  | Port number | Non-Compressed SCHC Stratum Header for CoAP
  +-------------+
  Port number = 5683 (CoAP)

  +.........+- - - - - - - - - - -+
  | Rule ID | Compression Residue | SCHC-Compressed Stratum Header for CoAP
  +.........+- - - - - - - - - - -+
  Note: for devices that only implement the TPS (i.e., the only protocol carried over UDP is CoAP), the SCHC-Compressed Stratum Header is fully compressed (down to 0 bits when transmitted over the air) since there is only one Rule in the SoR of the SCHC Stratum for such devices.

  Rule to compress the SCHC Stratum Header for CoAP header C/D:

  RuleID
  +--------------+--+---+--+----+------+----------+
  |      FID     |FL|POS|DI| TV |  MO  |   CDA    |
  +--------------+--+---+--+----+------+----------+
  | SCHC.portnum | 8| 1 |Bi|5683|equal | not-sent |
  +--------------+--+---+--+----+------+----------+

      Figure 34: SCHC Stratum Header for CoAP header C/D in non-
   compressed and in SCHC-compressed form, and corresponding Rule.

   When CoAP is protected with OSCORE, one SCHC Stratum is used below
   UDP (for UDP header C/D), a second one is used between UDP and the
   CoAP outer header (for CoAP outer header C/D), and a third one is
   used between the CoAP outer header and the CoAP inner header (for
   CoAP inner header C/D).

   In this case, the SCHC-compressed UDP/CoAP headers field comprises
   six fields (Figure 35): the SCHC Stratum Header for UDP, the
   corresponding SCHC Payload (i.e., a RuleID followed by the
   compression residue of the UDP header), the SCHC Stratum Header for
   CoAP outer header, the SCHC Payload for the latter (i.e., a RuleID
   followed by the compression residue of the CoAP outer header), the
   SCHC Stratum Header for CoAP inner header, and the SCHC Payload for
   the latter (i.e., a RuleID followed by the compression residue of the
   CoAP inner header).  If there is a single SoR for UDP header C/D,
   CoAP outer header C/D, or CoAP inner header C/D, the corresponding
   SCHC Stratum Header is fully elided.








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  <------------------------ IEEE 802.15.4 frame payload ------------------------>
  +--------+------------+------------------+----------------+--------------+ - - +
  |11110001|8138-cmprssd|  6282-compressed | SCHC-comprssd  |              |     |
  |(Page 1)|  routing   |   IPv6 header    | UDP/CoAP hdrs  | CoAP Payload | Pad |
  |        | artifacts  |(NH=0,NxtHdr=SCHC)| (incl. SCHC    |              |     |
  |        |            |                  | Stratum Hdr(s))|
  +--------+------------+------------------+----------------+--------------+ - - +
                                          /                  \
                       |-----------------/                    \-----------------|
                       +---------+--------+---------+--------+---------+--------+
                       |  SCHC   |  SCHC  |  SCHC   | SCHC   | SCHC    | SCHC   |
                       | Stratum | Payload| Stratum | Payload| Stratum | Payload|
                       | Header  | (UDP)  |  Header | (CoAP  | Header  | (CoAP  |
                       | for UDP |        | for CoAP|  outer)| for CoAP| inner) |
                       +---------+--------+---------+--------+---------+--------+

     Figure 35: Detailed view of the SCHC-compressed UDP and CoAP

   When OSCORE is used to protect CoAP in the TPS, the SCHC Stratum
   Headers for UDP and CoAP outer header C/D, and the Rules to compress/
   decompress those SCHC Stratum Headers for devices that only support
   the TPS, are the ones already illustrated in Figures 33 and 34.  The
   SCHC Stratum Header for CoAP inner header C/D, and the Rule to
   compress/decompress that SCHC Stratum Header, are shown in Figure 36.


  +-------------+
  |Option number| Non-Compressed SCHC Stratum Header for CoAP inner header
  +-------------+
  Option number = 9 (OSCORE)

  +.........+- - - - - - - - - - -+
  | Rule ID | Compression Residue | SCHC-Compressed Stratum Header for CoAP inner
  +.........+- - - - - - - - - - -+
  Note: for devices that only implement the TPS and use OSCORE, the SCHC-Compressed Stratum Header for CoAP inner header C/D is fully compressed (down to 0 bits when transmitted over the air) since there is only one Rule in the SoR of that SCHC Stratum.

  Rule to compress the SCHC Stratum Header for CoAP inner header C/D:

  RuleID
  +--------------+--+---+--+----+------+----------+
  |      FID     |FL|POS|DI| TV |  MO  |   CDA    |
  +--------------+--+---+--+----+------+----------+
  | SCHC.optnum  |16| 1 |Bi|  9 |equal | not-sent |
  +--------------+--+---+--+----+------+----------+

   Figure 36: SCHC Stratum Header for CoAP inner header C/D in non-
     compressed and SCHC-compressed form, and corresponding Rule.




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6.  SCHC compression for IPv6, UDP, and CoAP headers

   SCHC header compression may be applied to the headers of different
   protocols or sets of protocols.  Some examples include: i) IPv6
   packet headers, ii) joint IPv6 and UDP packet headers, iii) joint
   IPv6, UDP and CoAP packet headers, etc.

   This section describes how IPv6, UDP, and CoAP header fields are
   compressed.

6.1.  SCHC compression for IPv6 and UDP headers

   IPv6 and UDP header fields MUST be compressed as per Section 10 of
   RFC 8724.

   IPv6 addresses are split into two 64-bit-long fields; one for the
   prefix and one for the Interface Identifier (IID).

   To allow for a single Rule being used for both directions, RFC 8724
   identifies IPv6 addresses and UDP ports by their role (Dev or App)
   and not by their position in the header (source or destination).
   This optimization can be used as is in some IEEE 802.15.4 networks
   (e.g., an IEEE 802.15.4 star topology where the peripheral devices
   (Devs) send/receive packets to/from a network-side entity (App)).

   However, in some types of 6LoWPAN environments (e.g., when a sender
   and its destination are both peer nodes in a mesh topology network),
   additional functionality is needed to allow use of the Dev and App
   roles for C/D.  In this case, each SCHC C/D entity needs to know its
   role (Dev or App) in addition to the Rule(s), and corresponding
   RuleIDs, for each node it communicates with before such communication
   occurs [I-D.ietf-schc-architecture].  In such cases, the terms Uplink
   and Downlink that have been defined in RFC 8724 need to be understood
   in the context of each specific set of peer nodes.

   RFC 8724 (Section 7.1) states that "In a Rule, the Field Descriptors
   are listed in the order in which the fields appear in the packet
   header".  The present specification updates RFC 8724 by stating that,
   in order to allow IPv6 header compression in PRO, the Field
   Descriptors of the IPv6 destination address (i.e., IPv6 DevPrefix and
   IPv6 DevIID) MUST appear before the Field Descriptors of the IPv6
   source address (i.e., IPv6 AppPrefix and IPv6 AppIID), while the rest
   of fields appear in the same order as in the IPv6 packet header.

   In PRO, in order to support SCHC-based IPv6 header compression, one
   Rule MUST be defined for each direction between the involved C/D
   nodes.  In such a Rule, the IPv6 DevPrefix and IPv6 DevIID FIDs MUST
   refer to the destination address (i.e., the destination node takes



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   the "Dev" role) of the SCHC-compressed IPv6 header.  This allows a
   6LR to read the compression residue of the Hop Limit and IPv6
   destination address fields of the SCHC-compressed header by means of
   the Bit Pointer.

6.1.1.  Compression of IPv6 addresses

   Compression of IPv6 source and destination prefixes MUST be performed
   as per Section 10.7.1 of RFC 8724.  Additional guidance is given in
   the present section.

   Compression of IPv6 source and destination IIDs MUST be performed as
   per Section 10.7.2 of RFC 8724.  One particular consideration when
   SCHC C/D is used in IEEE 802.15.4 networks is that, in contrast with
   some LPWAN technologies, IEEE 802.15.4 data frame headers include
   both source and destination fields.  If the Dev or App IID are based
   on an L2 address, in some cases the IID can be reconstructed with
   information coming from the L2 header.  Therefore, in those cases,
   DevIID and AppIID CDAs can be used.

   RFC 8724 states that "If the Rule is intended to compress packets
   with different prefix values, match-mapping SHOULD be used"
   (Section 10.7.1 of RFC 8724) and "If several IIDs are possible, then
   the TV contains the list of possible IIDs, the MO is set to "match-
   mapping" and the CDA is set to "mapping-sent"" (Section 10.7.2 of RFC
   8724).  However, the present specification updates RFC 8724 by
   stating that, in PRO, a source node MUST NOT use the match-mapping
   operator or the "mapping-sent" CDA to compress the IPv6 destination
   address prefix or the IPv6 destination IID, because 6LRs do not store
   SCHC context, and therefore do not have the match-mapping index
   meaning information.

6.1.2.  UDP checksum field

   RFC 8724 states that "a SCHC compressor MAY elide the UDP checksum
   when another layer guarantees at least equal integrity protection for
   the UDP payload and the pseudo-header".

   IEEE 802.15.4 frames carry a 16-bit Frame Check Sequence (FCS), which
   is computed by means of a 16-bit ITU-T CRC algorithm.  Considering
   the FCS size, the greater error detection capabilities of CRC
   compared with checksum, and the fact that the IEEE 802.15.4 FCS will
   be checked at each hop in an IEEE 802.15.4 multihop network, the UDP
   checksum MUST be elided when using SCHC to compress UDP headers.







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6.2.  SCHC compression for CoAP headers

   CoAP header fields MUST be compressed as per Sections 4 to 6 of RFC
   8824.  Additional guidance is given in this section.

   For CoAP header compression/decompression, the SCHC Rules description
   uses direction information in order to reduce the number of Rules
   needed to compress headers.

   As stated in 5.1, in some types of 6LoWPAN environments (e.g., when a
   sender and its destination are both peer nodes in a mesh topology
   network), each SCHC C/D entity needs to know its role (Dev or App),
   in addition to the Rule(s), and corresponding RuleIDs, for each node
   it communicates with before such communication occurs
   [I-D.ietf-schc-architecture].  Therefore, in such cases, direction
   information will be specific to each set of peer nodes.

7.  Neighbor Discovery

   A number of optimizations have been developed in order to efficiently
   support IPv6 Neighbor Discovery (ND) in 6LoWPAN environments (6LoWPAN
   ND) [RFC 6775][RFC 8505].  SCHC can also be used to compress 6LoWPAN
   ND packets.  At the time of this writing, compression of ICMPv6
   headers is being specified in the SCHC WG [draft-ietf-schc-
   icmpv6-compression].  Thus, it will be possible to compress the IPv6
   header and the ICMPv6 header of a packet carrying a 6LoWPAN ND
   message.

8.  Fragmentation and reassembly

   After applying SCHC header compression to a packet intended for
   transmission, if the size of the resulting SCHC datagram (Section 4)
   exceeds the IEEE 802.15.4 frame payload space available, such SCHC
   Packet MUST be fragmented, carried and reassembled by means of the
   fragmentation and reassembly functionality defined by 6LoWPAN
   [RFC4944] or 6Lo [RFC8930][RFC8931].

   In a Route-Over SCHC-Lo network, the 6LoWPAN fragment forwarding
   technique called Virtual Reassembly Buffer (VRB) [RFC8930] SHOULD be
   used.  However, VRB might not be the best approach for a particular
   SCHC-Lo network, e.g., if at least one of the caveats described in
   Section 6 of RFC 8930 is unacceptable or cannot be addressed.

9.  IANA Considerations

   This document requests the allocation of the 6LoWPAN Dispatch Type
   Field Bit Patterns, on the Pages and with the Header Types shown
   next:



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            +--------------+--------+-----------------+-------------+
            | Bit Pattern  |  Page  |   Header Type   |  Reference  |
            +--------------+--------+-----------------+-------------+
            |   01000100   |    0   |      SCHC       |  [RFCthis]  |
            +--------------+--------+-----------------+-------------+
            |   01000100   |    1   |      SCHC       |  [RFCthis]  |
            +--------------+--------+-----------------+-------------+
            |   01000101   |    0   |   SCHC Pointer  |  [RFCthis]  |
            +--------------+--------+-----------------+-------------+

       Figure 37: Details of the 6LoWPAN Dispatch Type Field request

10.  Security Considerations

   This document does not define SCHC header compression functionality
   beyond the one defined in RFC 8724.  Therefore, the security
   considerations in section 12.1 of RFC 8724 and in section 9 of RFC
   8824 apply.

   As a safety measure, a SCHC decompressor implementing the present
   specification MUST NOT reconstruct a packet larger than 1500 bytes
   [RFC8724].

   IEEE 802.15.4 networks support link-layer security mechanisms such as
   encryption and authentication.  As in RFC 8824, the use of a
   cryptographic integrity-protection mechanism to protect the SCHC-
   compressed headers is REQUIRED.

   The addition of the pointer used in PRO creates new attack
   opportunities.  A malicious node might be able to modify the related
   fields (i.e., Bit Pointer or Address Length) to prevent a router from
   correctly reconstructing the IPv6 destination field of a SCHC-
   compressed IPv6 packet, thus preventing delivery of the packet to its
   intended destination.  Appropriate use of link-layer security should
   significantly reduce the probability of the described threat.

11.  Acknowledgments

   Ana Minaburo and Laurent Toutain suggested for the first time the use
   of SCHC in environments where 6LoWPAN has traditionally been used.
   Flavien Moullec is a contributor to this document.  Laurent Toutain,
   Pascal Thubert, Dominique Barthel, Guangpeng Li, Carsten Bormann,
   Nathan Lecorchet, Stuart Cheshire, Kiran Makhijani, Georgios Z.
   Papadopoulos, Peter Yee, and Alexander Pelov made comments that
   helped shape this document.






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   Carles Gomez has been funded in part by the Spanish Government
   through project PID2019-106808RA-I00 and PID2023-146378NB-I00, and by
   Secretaria d'Universitats i Recerca del Departament d'Empresa i
   Coneixement de la Generalitat de Catalunya 2017 through grant SGR 376
   and 2021 throught grant SGR 00330.

12.  References

12.1.  Normative References

   [I-D.ietf-schc-architecture]
              Pelov, A., Thubert, P., and A. Minaburo, "Static Context
              Header Compression (SCHC) Architecture", Work in Progress,
              Internet-Draft, draft-ietf-schc-architecture-04, 6
              February 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-schc-architecture-04>.

   [I-D.ietf-schc-protocol-numbers]
              Moskowitz, R., Thubert, P., Gomez, C., and A. Minaburo,
              "Protocol Numbers for SCHC", Work in Progress, Internet-
              Draft, draft-ietf-schc-protocol-numbers-03, 10 October
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              schc-protocol-numbers-03>.

   [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>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.






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   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,
              <https://www.rfc-editor.org/info/rfc6553>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <https://www.rfc-editor.org/info/rfc6554>.

   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing",
              RFC 6606, DOI 10.17487/RFC6606, May 2012,
              <https://www.rfc-editor.org/info/rfc6606>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7973]  Droms, R. and P. Duffy, "Assignment of an Ethertype for
              IPv6 with Low-Power Wireless Personal Area Network
              (LoWPAN) Encapsulation", RFC 7973, DOI 10.17487/RFC7973,
              November 2016, <https://www.rfc-editor.org/info/rfc7973>.

   [RFC8025]  Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
              RFC 8025, DOI 10.17487/RFC8025, November 2016,
              <https://www.rfc-editor.org/info/rfc8025>.

   [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
              Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
              February 2017, <https://www.rfc-editor.org/info/rfc8065>.

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <https://www.rfc-editor.org/info/rfc8138>.




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   [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>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/info/rfc8724>.

   [RFC8824]  Minaburo, A., Toutain, L., and R. Andreasen, "Static
              Context Header Compression (SCHC) for the Constrained
              Application Protocol (CoAP)", RFC 8824,
              DOI 10.17487/RFC8824, June 2021,
              <https://www.rfc-editor.org/info/rfc8824>.

   [RFC8930]  Watteyne, T., Ed., Thubert, P., Ed., and C. Bormann, "On
              Forwarding 6LoWPAN Fragments over a Multi-Hop IPv6
              Network", RFC 8930, DOI 10.17487/RFC8930, November 2020,
              <https://www.rfc-editor.org/info/rfc8930>.

   [RFC8931]  Thubert, P., Ed., "IPv6 over Low-Power Wireless Personal
              Area Network (6LoWPAN) Selective Fragment Recovery",
              RFC 8931, DOI 10.17487/RFC8931, November 2020,
              <https://www.rfc-editor.org/info/rfc8931>.

   [RFC9008]  Robles, M.I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes, and IPv6-
              in-IPv6 Encapsulation in the RPL Data Plane", RFC 9008,
              DOI 10.17487/RFC9008, April 2021,
              <https://www.rfc-editor.org/info/rfc9008>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.




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12.2.  Informative References

   [I-D.ietf-schc-8824-update]
              Tiloca, M., Toutain, L., Martínez, I., and A. Minaburo,
              "Static Context Header Compression (SCHC) for the
              Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-schc-8824-update-05,
              7 July 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-schc-8824-update-05>.

   [I-D.ietf-schc-icmpv6-compression]
              Barthel, D. and L. Toutain, "Static Context Header
              Compression (SCHC) for the Internet Control Message
              Protocol (ICMPv6)", Work in Progress, Internet-Draft,
              draft-ietf-schc-icmpv6-compression-02, 13 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-schc-
              icmpv6-compression-02>.

   [RFC9006]  Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
              Guidance in the Internet of Things (IoT)", RFC 9006,
              DOI 10.17487/RFC9006, March 2021,
              <https://www.rfc-editor.org/info/rfc9006>.

Appendix A.  Header compression examples

   Uplink packet

   Source address: fd00::202:2:2:2 with port 8765
   Destination address: 2001::1 with port 5678
   Payload: "Hello 1" 68 65 6C 6C 6F 20 31

   Uncompressed IPv6/UDP packet:
   60 00 00 00 00 17 00 40    FD 00 00 00 00 00 00 00
   02 02 00 02 00 02 00 02    20 01 00 00 00 00 00 00
   00 00 00 00 00 00 00 01    22 3D 16 2E 00 0F 33 68
   68 65 6C 6C 6F 20 31

   IPv6/UDP header length: 48 bytes
   Total length:           55 bytes

   In this example, for SCHC compression of IPv6/UDP headers, RuleID
   0x20 is used.  The Rule corresponding to RuleID 0x20 is shown in
   Figure 38.








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    +----------------+--+--+--+-------------+------+----------++------+
    |       FID      |FL|FP|DI|      TV     |  MO  |    CDA   || Sent |
    |                |  |  |  |             |      |          ||[bits]|
    +----------------+--+--+--+-------------+------+----------++------+
    |IPv6 Version    |4 |1 |Bi|6            |ignore| not-sent ||      |
    |IPv6 Diffserv   |8 |1 |Bi|0            |equal | not-sent ||      |
    |IPv6 Flow Label |20|1 |Bi|0            |equal | not-sent ||      |
    |IPv6 Length     |16|1 |Bi|             |ignore|compute-* ||      |
    |IPv6 Next Header|8 |1 |Bi|17           |equal | not-sent ||      |
    |IPv6 Hop Limit  |8 |1 |Bi|64           |ignore| not-sent ||      |
    |IPv6 DevPrefix  |64|1 |Bi|FD00::/64    |equal | not-sent ||      |
    |IPv6 DevIID     |64|1 |Bi|             |ignore|value-sent|| 64   |
    |IPv6 AppPrefix  |64|1 |Bi|2001::/64    |equal | not-sent ||      |
    |IPv6 AppIID     |64|1 |Bi|::1          |equal | not-sent ||      |
    +================+==+==+==+=============+======+==========++======+
    |UDP DevPort     |16|1 |Bi|8765         |equal | not-sent ||      |
    |UDP AppPort     |16|1 |Bi|5678         |equal | not-sent ||      |
    |UDP Length      |16|1 |Bi|             |ignore|compute-* ||      |
    |UDP checksum    |16|1 |Bi|             |ignore|compute-* ||      |
    +================+==+==+==+=============+======+==========++======+

        Figure 38: Illustration of an example Rule with RuleID 0x20

A.1.  Single-hop or SRO frame format

   SCHC-compressed packet:
   44 20 02 02 00 02 00 02
   00 02 68 65 6C 6C 6F 20
   31

   Header length: 10 bytes
   SCHC Dispatch: 44 (01000100)
   SCHC RuleID: 0x20 (1 byte)
   SCHC residue: 02 02 00 02 00 02 00 02
   Payload: 68 65 6C 6C 6F 20 31
   Total length: 17 bytes

A.2.  TRO frame format

   TO-DO

A.3.  PRO frame format

   SCHC-compressed packet:
   45 88 40 20 02 02 00 02
   00 02 00 02 68 65 6C 6C
   6F 20 31




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   Header length: 12 bytes
   SCHC Pointer Dispatch: 45 (01000101)
   SCHC Pointer: 88 40
   SCHC Pointer P: 1
   SCHC Pointer Bit Pointer: 8
   SCHC Address length: 64 bits
   SCHC RuleID: 0x20 (1 byte)
   SCHC residue: 02 02 00 02 00 02 00 02
   Payload: 68 65 6C 6C 6F 20 31
   Total length: 19 bytes

A.4.  Mesh-Under frame format

   TO-DO

A.5.  Enabling the transition protocol stack

   Uplink packet

   Source address: fe80::201:1:1:1 with port 46487
   Destination address: fe80::1 with port 5683
   Payload (Temperature value): DA 8C E8 75 15 66 3B 00 1B 37
   SCHC protocol number: 145 (0x91)

   Uncompressed IPv6/UDP/CoAP packet:
   60 0D 4E 65 00 25 11 40    FE 80 00 00 00 00 00 00
   02 01 00 01 00 01 00 01    FE 80 00 00 00 00 00 00
   00 00 00 00 00 00 00 01    B5 97 16 33 00 25 00 38
   50 02 B6 F7 BA 74 65 6D    70 65 72 61 74 75 72 D1
   EA 00 FF DA 8C E8 75 15    66 3B 00 1B 37

   IPv6/UDP/CoAP header length: 67 bytes
   Total length: 77 bytes

   In this example, for SCHC compression of UDP/CoAP headers, RuleID
   0x22 is used.  The Rule corresponding to RuleID 0x22 is shown in
   Figure 39.














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    +----------------+--+--+--+-------------+------+----------++------+
    |       FID      |FL|FP|DI|      TV     |  MO  |    CDA   || Sent |
    |                |  |  |  |             |      |          ||[HEX] |
    +----------------+--+--+--+-------------+------+----------++------+
    |UDP DevPort     |16|1 |Bi|             |ignore|value-sent||B5 97 |
    |UDP AppPort     |16|1 |Bi|5683         |equal | not-sent ||      |
    |UDP Length      |16|1 |Bi|             |ignore|compute-* ||      |
    |UDP checksum    |16|1 |Bi|             |ignore|compute-* ||      |
    +================+==+==+==+=============+======+==========++======+
    |CoAP Version    |16|1 |Bi|1            |equal | not sent ||      |
    |CoAP Type       |16|1 |Up|01           |equal | not sent ||      |
    |CoAP TKL        |32|1 |Bi|0x00         |equal | not sent ||      |
    |CoAP Code       |8 |1 |Up|0.02         |equal | not-sent ||      |
    |CoAP MID        |16|1 |Bi|             |ignore|value-sent||B6 F7 |
    |CoAP OptUri-Path|10|1 |Up|/temperature |equal | not-sent ||      |
    |CoAP Opt No-Resp|1 |1 |Up|00           |equal | not-sent ||      |
    |CoAP Opt EndOpt |8 |1 |Up|0xFF         |equal | not-sent ||      |
    +================+==+==+==+=============+======+==========++======+


        Figure 39: Illustration of an example Rule with RuleID 0x22

IPv6 packet (with uncompressed header) carrying the SCHC-compressed UDP/CoAP headers:
60 0D 4E 65 00 25 91 40    FE 80 00 00 00 00 00 00
02 01 00 01 00 01 00 01    FE 80 00 00 00 00 00 00
00 00 00 00 00 00 00 01    22 B5 97 B6 F7 DA 8C E8
75 15 66 3B 00 1B 37

   Compressed packet (IPv6 using 6LoWPAN + UDP/CoAP using SCHC):
   6A 11 0D 4E 65 91 02 01    00 01 00 01 00 01 00 00
   00 00 00 00 00 01 22 B5    97 B6 F7 DA 8C E8 75 15
   66 3B 00 1B 37



















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   Header length: 27 bytes
   IPHC: 6A 11
     Dispatch: 011
     TF: 01
     NH: 0
     HLIM: 10
     CID: 0
     SAC: 0
     SAM: 01
     M: 0
     DAC: 0
     DAM: 01

     Traffic Class: 0D4E65
     Next Header: 91
     Src. Address: 201:1:1:1
     Dst. Address: ::1

   Next Header: 91 (SCHC)
   SCHC RuleID: 0x22
   SCHC Residue:
     UDP Dev Port: B5 97 (46487)
     CoAP MID: B6 F7 (46839)

   Total length: 37 bytes

Appendix B.  Analysis of route-over multihop approaches

   This section provides an analysis of the features, pros and cons of
   the route-over multihop approaches defined in this document: i) SRO,
   ii) TRO, and iii) PRO.

B.1.  SRO

   SRO incurs the lowest header overhead among the considered Route-Over
   approaches, as it only requires the SCHC Dispatch (1 byte).  However,
   it is the most demanding approach in terms of memory usage, since all
   SCHC-Lo network routers (i.e., 6LRs and 6LBRs) need to store all the
   Rules in use in the SCHC-Lo network.  Therefore, it will be suitable
   for rather small networks and/or where nodes have sufficient memory.
   Also, SCHC context should be as static as possible, in order to avoid
   frequent stored SCHC context updates on the SCHC-Lo network routers.

B.2.  TRO

   TRO incurs a header overhead that includes a fixed part (a Page
   Switch plus the SCHC Dispatch, of 1 byte each), plus a variable part
   that comprises RFC 8138-compressed routing artifacts.



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   Regarding the latter, in a Downward transmission, it would include
   the SRH-6LoRH (of variable size, of 4 bytes in the best case, or
   e.g., 8 bytes as in Fig. 20 of RFC 8138), the RPI-6LoRH (3 bytes in
   the best case) and the IP-in-IP header (not present if the source is
   the Root, at least 3 bytes otherwise).  In the cases considered, and
   when the Root is not the packet source, the total header overhead of
   TRO would be of at least 12-16 bytes.

   For upward transmission, the variable part of the header overhead for
   this approach would include only the RPI-6LoRH (at least, 3 bytes)
   and the IP-in-IP header (at least, 3 bytes).  Therefore, in the cases
   considered, the total header overhead of TRO would be of at least 8
   bytes.

   Note that, while the overhead of TRO may appear to be relatively
   high, tunnel-based structures like the one assumed in TRO may exist
   already in a network deployment.  Therefore, in such cases, the
   additional overhead of TRO may be actually lower.

   An advantage of TRO is that a node only has to store the Rules for
   the communications it is involved in as an endpoint, which minimizes
   memory requirements and the impact of potential SCHC context updates.
   For example, 6LRs do not have to store SCHC context.

   Note that TRO requires the network to use RPL, non-storing mode.
   Furthermore, the paths for communication between two nodes in the
   same network or with external nodes will need to traverse the Root.
   For communication with external nodes, traversing the Root will be
   needed anyway, therefore this feature does not pose any issue.
   However, this constraint will preclude the usage of optimal routes in
   some cases.

B.3.  PRO

   PRO incurs the PRO header overhead (i.e., between 3 and 3.5 bytes).
   In addition, with PRO, the Hop Limit field will have to be carried
   fully inline (1 byte) or compressed down to a minimum size of 4 bits.
   Furthermore, PRO introduces a limit to the achievable IPv6
   destination address compression performance, as described next (note
   that the size of the destination address compression residue will
   depend on and will need to be planned for the intended use case of
   the network):

   A.- In special cases (e.g., if there is only one possible destination
   that is known beforehand), there will not be a destination address
   residue.





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   B.- For a given destination prefix known by the network nodes (e.g.,
   when prefix contexts are used, or if there can only be one
   destination prefix), if there can be several possible destinations in
   that network, the destination address residue will be up to 8 bytes
   (it could be less depending on how the addresses in that network are
   built, for example, it could be just 2 bytes).

   C.- For destination prefixes not covered by prefix contexts or a
   priori knowledge by the nodes, the destination address residue will
   have to be the whole address (16 bytes), since an intermediate node
   does not know which is the destination prefix.

   An advantage of PRO, as in TRO, is that a node only has to store the
   Rules for the communications it is involved in as an endpoint, which
   minimizes memory requirements and the impact of potential SCHC
   context updates.  For example, 6LRs do not have to store SCHC
   context.  An exception is a 6LBR, which has to store the Rules for
   the communications of other endpoints with external nodes (if any).

   A potential advantage of PRO is that, in contrast with TRO, paths for
   intranetwork communication are not necessarily constrained to
   traversing a root node.  Another feature is that the routing solution
   to be used is not tied to RPL non-storing mode.  However, the routing
   solution may involve other constraints and/or trade-offs.

B.4.  Summary

   Assessing the suitability of the different SCHC-Lo route-over
   multihop approaches requires considering the following dimensions:
   network size, node memory capabilities, header overhead, routing
   constraints / path optimality, and intra- or inter-network
   communication.

   SRO is best suited for small and static-SCHC-context networks, such
   as a small home or a small office network (SRO may be used in larger
   networks as well, although with a trade-off with header compression
   performance and/or SCHC context management cost).  PRO and TRO offer
   greater network scalability.  TRO's best applicability is in networks
   where upwards traffic is dominant and RPL deployments are already in
   place and (e.g., a smart grid network).  PRO does not require RPL and
   can be a better fit when non-upwards traffic is significant (e.g.,
   between any 2 nodes within the same network, as in a large home
   network.)








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Appendix C.  Relationship with RFC 7973

   As reported in RFC 7973, IEEE assigned an Ethertype (with value
   0xA0ED) for "IPv6 datagrams using LoWPAN encapsulation".  As per RFC
   7973, any IPv6 datagram using the Dispatch octet as defined in
   Section 5.1 of RFC 4944, subsequently updated by RFC 6282, is
   regarded as using LoWPAN encapsulation.

   The present document also uses LoWPAN encapsulation, as it uses the
   Dispatch octet as described in RFC 7973.  Therefore, the
   functionality described in the present document can also benefit from
   the mentioned Ethertype.

Authors' Addresses

   Carles Gomez
   UPC
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain
   Email: carles.gomez@upc.edu


   Ana Minaburo
   Consultant
   Rue de Rennes
   35510 Cesson-Sevigne
   France
   Email: anaminaburo@gmail.com






















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