6lo Working Group                                               C. Gomez
Internet-Draft                                                       UPC
Updates: 8138, 8724, 9008 (if approved)                      A. Minaburo
Intended status: Standards Track                              Consultant
Expires: 5 April 2025                                       October 2024


  Transmission of SCHC-compressed packets over IEEE 802.15.4 networks
                     draft-ietf-6lo-schc-15dot4-07

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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   This Internet-Draft will expire on 4 April 2025.

Copyright Notice

   Copyright (c) 2024 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
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   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 . . . . . . . . . . . . . . . . . .   4
     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  . . . . . . . . . . . . .   6
     3.2.  SCHC architecture concepts  . . . . . . . . . . . . . . .   8
       3.2.1.  SCHC Stratum and Discriminator  . . . . . . . . . . .   8
       3.2.2.  Single-instance networks  . . . . . . . . . . . . . .   8
       3.2.3.  Multiple-instance networks  . . . . . . . . . . . . .   9
     3.3.  Network topologies  . . . . . . . . . . . . . . . . . . .   9
     3.4.  Single-hop communication  . . . . . . . . . . . . . . . .   9
     3.5.  Multihop communication  . . . . . . . . . . . . . . . . .  10
       3.5.1.  Straightforward Route-Over (SRO)  . . . . . . . . . .  10
       3.5.2.  Tunneled, RPL-based Route-Over (TRO)  . . . . . . . .  12
       3.5.3.  Pointer-based Route-Over (PRO)  . . . . . . . . . . .  16
       3.5.4.  Mesh-Under  . . . . . . . . . . . . . . . . . . . . .  18
   4.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  20
     4.1.  Single-hop or SRO frame format  . . . . . . . . . . . . .  20
       4.1.1.  SCHC Dispatch . . . . . . . . . . . . . . . . . . . .  21
       4.1.2.  SCHC Header . . . . . . . . . . . . . . . . . . . . .  21
       4.1.3.  SCHC-compressed Header  . . . . . . . . . . . . . . .  21
       4.1.4.  Padding . . . . . . . . . . . . . . . . . . . . . . .  22
     4.2.  TRO frame format  . . . . . . . . . . . . . . . . . . . .  22
     4.3.  PRO frame format  . . . . . . . . . . . . . . . . . . . .  23
     4.4.  Mesh-Under frame format . . . . . . . . . . . . . . . . .  25
     4.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  27
   5.  Enabling the transition protocol stack  . . . . . . . . . . .  27
   6.  SCHC compression for IPv6, UDP, and CoAP headers  . . . . . .  28
     6.1.  SCHC compression for IPv6 and UDP headers . . . . . . . .  28
       6.1.1.  Compression of IPv6 addresses . . . . . . . . . . . .  29
       6.1.2.  UDP checksum field  . . . . . . . . . . . . . . . . .  30
     6.2.  SCHC compression for CoAP headers . . . . . . . . . . . .  30
   7.  Neighbor Discovery  . . . . . . . . . . . . . . . . . . . . .  31



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   8.  Fragmentation and reassembly  . . . . . . . . . . . . . . . .  31
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  32
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  32
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     12.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Appendix A.  Header compression examples  . . . . . . . . . . . .  36
     A.1.  Single-hop or SRO frame format  . . . . . . . . . . . . .  37
     A.2.  TRO frame format  . . . . . . . . . . . . . . . . . . . .  37
     A.3.  PRO frame format  . . . . . . . . . . . . . . . . . . . .  37
     A.4.  Mesh-Under frame format . . . . . . . . . . . . . . . . .  37
     A.5.  Enabling the transition protocol stack  . . . . . . . . .  37
   Appendix B.  Analysis of route-over multihop approaches . . . . .  39
     B.1.  SRO . . . . . . . . . . . . . . . . . . . . . . . . . . .  39
     B.2.  TRO . . . . . . . . . . . . . . . . . . . . . . . . . . .  40
     B.3.  PRO . . . . . . . . . . . . . . . . . . . . . . . . . . .  41
     B.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Appendix C.  Relationship with RFC 7973 . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

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.

   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



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   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.

   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].

   This specification updates RFC 8138, RFC 8724, and RFC 9008.

2.  Terminology

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.











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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.)

   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].



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        +------------+          +------------+
        | 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.

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 two possible protocol stacks, where 6LoWPAN header
   compression is used to compress IPv6/UDP headers while SCHC
   compresses CoAP headers (see Figure 2 and Section 5.1), and ii)
   enables the transmission of SCHC-compressed UDP/CoAP headers over
   6LoWPAN-compressed IPv6 packets (see Figure 3 and Section 5.2).
   However, note that the greatest header compression performance can be
   achieved by using SCHC to also compress the UDP header.

   RFC 8824 defines how SCHC can be used to compress CoAP headers,
   including Object Security for Constrained RESTful Environments
   (OSCORE)-protected messages [RFC8613].  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-intarea-schc-protocol-numbers].
   Figure 2 (left) shows the resulting protocol stack, where 6LoWPAN
   header compression is applied to UDP and IPv6.  When Datagram
   Transport Layer Security (DTLS) [RFC9147] is preferred to protect
   SCHC-compressed CoAP messages, the DTLS layer sits between the SCHC
   and UDP layers (Figure 2, right).




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

         Figure 2: Transition protocol stacks where 6LoWPAN header
       compression is applied to UDP and IPv6.  The leftmost protocol
        stack supports the use of OSCORE, whereas the rightmost one
       corresponds to the use of DTLS to protect SCHC-compressed CoAP
          messages.  HC and Frag stand for Header Compression and
                        Fragmentation, respectively.

   Finally, the "transition" protocol stack enabled by this document,
   which allows the transmission of 6LoWPAN-compressed IPv6 packets
   containing SCHC-compressed UDP/CoAP data units, is shown in Figure 3
   (rightmost).


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





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       Figure 3: Traditional 6LoWPAN-based protocol stack over IEEE
    802.15.4 (left), alternative protocol stack using SCHC for header
      compression (middle), and transition protocol stack using SCHC
     for header compression of UDP/CoAP headers (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 Header Instance", "SCHC Packet
   Instance", 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-instance
   networks and Multiple-instance networks are introduced.  Note: in the
   present document, "Single-instance networks" and "Multiple-instance
   networks" are used for brevity to refer to "Single-instance SCHC-Lo
   networks" and "Multiple-instance SCHC-Lo networks".

3.2.1.  SCHC Stratum and Discriminator

   When SCHC is used to compress IPv6 packets over IEEE 802.15.4
   networks, the 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).

   In both Single-instance and Multiple-instance 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-instance networks

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

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








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3.2.3.  Multiple-instance networks

   In Multiple-instance networks, at least some of the network nodes
   that use SCHC for C/D have more than one SCHC Packet Instances, and
   thus one SoR associated to each SCHC Packet Instance.  Therefore, in
   Multiple-instance networks, the SCHC Header cannot generally be fully
   compressed (i.e., in compressed form, the SCHC Header requires more
   than 0 bits to be transmitted over the air).

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

3.4.  Single-hop communication

   In order to support the transmission of SCHC-compressed packets
   between two endpoints that are single-hop neighbors, both endpoints
   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.











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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 endpoints.  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-instance
   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 this case, 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-instance
   networks, a not fully compressed SCHC Header MUST be used.

   Figure 4 illustrates an example Single-instance 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 endpoints
     (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 4: Rules stored by each node in an example Single-instance
                          network using SRO.

   Figure 5 illustrates an example Multiple-instance 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 4, which correspond to a SCHC
   Packet Instance called I1 in this example, there is a second RuleID
   2, which corresponds to communication between A and B, in a second
   SCHC Packet Instance (I2).
























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                                                   Host E
                    (RuleID 2, I2)                /
                    (RuleID 1, I1)      +--------+
                    (RuleID 2, I1)  --- |Internet|
                    (RuleID 3, I1) /    +--------+
                   6LBR -----------
                 /      \
                /        \
              6LR         6LR -------------+               Endpoints | Instance
(RuleID 1, I1) |         | (RuleID 1, I1)  |      RuleID 1:   A, B        I1
(RuleID 2, I1) |         | (RuleID 2, I1)  |      RuleID 2:   A, C        I1
(RuleID 3, I1) |         | (RuleID 3, I1)  |      RuleID 3:   A, E        I1
(RuleID 2, I2) |         | (RuleID 2, I2)  |      RuleID 2:   A, B        I2
               |         |                 |
              Host A      Host B         Host C
        (RuleID 1, I1)    (RuleID 1, I1)   (RuleID 2, I1)
        (RuleID 2, I1)    (RuleID 2, I2)
        (RuleID 3, I1)
        (RuleID 2, I2)


     Figure 5: Rules stored by each node in an example Multiple-
                     instance 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 endpoints.  A 6LR
   is relieved to store Rules used by pairs of endpoints 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-instance 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-instance
   network, a not fully compressed SCHC Header MUST be used.

   Figure 6 illustrates the Rules that need to be stored by the nodes in
   TRO, based on the same example Single-instance network and endpoint
   pairs shown in Figure 4.


                                                 Host E
                                                /
                    (RuleID 1)        +--------+
                    (RuleID 2)    --- |Internet|
                    (RuleID 3)   /    +--------+
                   6LBR ---------
                 /      \
                /        \
              6LR         6LR ------------+                    Pair of endpoints
     (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 6: Rules stored by each node in an example Single-instance
                          network using TRO.

   Figure 7 illustrates an example Multiple-instance 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 6, which correspond to a SCHC
   Packet Instance called I1 in this example, there is a second RuleID
   2, which corresponds to communication between A and B, in a second
   SCHC Packet Instance (I2).














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                                                  Host E
                    (RuleID 2, I2)               /
                    (RuleID 1, I1)      +--------+
                    (RuleID 2, I1)  --- |Internet|
                    (RuleID 3, I1) /    +--------+
                   6LBR -----------
                 /      \
                /        \
              6LR         6LR -------------+               Endpoints | Instance
    (No Rules) |         | (No Rules)      |      RuleID 1:   A, B        I1
               |         |                 |      RuleID 2:   A, C        I1
               |         |                 |      RuleID 3:   A, E        I1
               |         |                 |      RuleID 2:   A, B        I2
               |         |                 |
              Host A      Host B         Host C
        (RuleID 1, I1)    (RuleID 1, I1)   (RuleID 2, I1)
        (RuleID 2, I1)    (RuleID 2, I2)
        (RuleID 3, I1)
        (RuleID 2, I2)


     Figure 7: Rules stored by each node in an example Multiple-
                     instance network using TRO.

   RFC 9008 describes how the communication between a 6LN and another
   endpoint (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 6 (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 8: 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 4.3.

3.5.3.  Pointer-based Route-Over (PRO)

   In the previous 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 the
   endpoints, which in addition does not require 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
   destination address of a SCHC-compressed packet, decrement its Hop



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   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 as an endpoint with other endpoints.  A 6LBR MUST store
   the Rules used by any SCHC-Lo network node for communication with
   external nodes.

   In a PRO Single-instance network, a RuleID MAY be used to identify
   different Rules used by different pairs of endpoints.  In a PRO
   Multiple-instance network, a not fully compressed SCHC Header MUST be
   used.

   Figure 9 illustrates the Rules that are stored by the nodes in an
   example Single-instance 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 endpoints.


                                                      Host E
                                                  /
                                        +--------+- Host F
                   (RuleID 3)       --- |Internet|
                   (RuleID 4)      /    +--------+
                   6LBR -----------
                 /      \
                /        \
              6LR         6LR ------------+                   Pair of endpoints
     (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 9: Rules stored by each node in an example Single-instance
   network using PRO.  In this example, both RuleID 2 and RuleID 3
               are used by two pairs of endpoints each.

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








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   Figure 10 illustrates an example Multiple-instance 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 9, which correspond to a SCHC
   Packet Instance called I1 in this example, there is an additional
   RuleID 2, which corresponds to communication between A and D, in a
   second SCHC Packet Instance (I2).


                                               Host E
                                              /
                                    +--------+- Host F
                   (RID 3, I1)  --- |Internet|
                   (RID 4, I1) /    +--------+
                   6LBR -------
                 /      \
                /        \
              6LR         6LR ------------+                Endpoints | Instance
     (no Rules)/|         | (no Rules)    |            RID 1:  A, B       I1
              / |         |               |            RID 2:  A, C       I1
             /  |         |               |            RID 2:  D, B       I1
            /   |         |               |            RID 3:  A, E       I1
       Host D  Host A     Host B         Host C        RID 4:  B, F       I1
   (RID 2, I1)  (RID 1, I1)  (RID 1, I1)  (RID 2, I1)  RID 2:  A, D       I2
   (RID 2, I2)  (RID 2, I1)  (RID 2, I1)
                (RID 3, I1)  (RID 3, I1)
                (RID 2, I2)


     Figure 10: Rules stored by each node in an example Multiple-
        instance network using PRO.  'RID' stands for RuleID.

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 4.3.

   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 endpoints.

   In this case, a RuleID MAY be reused across disjoint pairs of
   endpoints, to identify different Rules used by such disjoint pairs of
   endpoints.






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

   Figure 11 illustrates the Rules that need to be stored by the nodes
   when SCHC is used for header compression in a Single-Instance Mesh-
   Under network, based on the same example network and endpoint pairs
   shown in Figure 9.  Note that, in this example, the network exploits
   the fact that Mesh-under allows a given RuleID to be reused by
   different pairs of endpoints, even if the Rules sharing the same
   RuleID are different.  Nodes denoted "m" in Figure 8 correspond to
   Mesh-Under forwarders [RFC 6606].


                                                     Host E
                                                  /
                                        +--------+- Host F
                   (RuleID 3)       --- |Internet|
                   (RuleID 4)      /    +--------+
                   6LBR -----------
                  /     \
                 /       \
                m         m --------------+                  Pair of endpoints
     (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 11: Rules stored by each node in an example Single-
   Instance network using Mesh-Under.  In this example, RuleID 2 is
                used by different pairs of endpoints.

   Figure 12 illustrates an example Multiple-instance 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 9, which correspond to a SCHC
   Packet Instance called I1 in this example, there is an additional
   RuleID 2, which corresponds to communication between A and D, in a
   second SCHC Packet Instance (I2).




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                                                Host E
                                              /
                                    +--------+- Host F
                   (RID 3, I1)  --- |Internet|
                   (RID 4, I1) /    +--------+
                   6LBR -------
                 /      \
                /        \
                m         m --------------+                Endpoints | Instance
     (no Rules)/|         | (no Rules)    |            RID 1:  A, B       I1
              / |         |               |            RID 2:  A, C       I1
             /  |         |               |            RID 2:  D, B       I1
            /   |         |               |            RID 3:  A, E       I1
       Host D  Host A     Host B         Host C        RID 4:  B, F       I1
   (RID 2, I1)  (RID 1, I1)  (RID 1, I1)  (RID 2, I1)  RID 2:  A, D       I2
   (RID 2, I2)  (RID 2, I1)  (RID 2, I1)
                (RID 3, I1)  (RID 2, I2)
                (RID 2, I2)


     Figure 12: Rules stored by each node in an example Multiple-
     instance 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.

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 Header, a SCHC Packet (i.e. a
   SCHC-compressed packet (RFC 8724), and Padding bits, if any).
   Figure 13 illustrates the described frame format.


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

                                    <------ SCHC Packet ----->
           +---------------+--------+--------------+---------+ - - - - +
           | SCHC Dispatch |SCHC Hdr| Cmprd Header | Payload | Padding |
           +---------------+--------+--------------+---------+ - - - - +






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   Figure 13: 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-
   compressed header (SCHC Header in Figure 13, 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 Header

   The SCHC Header ("SCHC Hdr" in Figure 9 and subsequent figures)
   determines the SCHC Packet Instance to be used to decompress the next
   field (SCHC-compressed header, see 4.1.3).  As described in the SCHC
   architecture draft, the SCHC Header comprises a RuleID and a
   compression residue [draft-ietf-schc-architecture].

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

4.1.3.  SCHC-compressed Header

   The SCHC-compressed Header ("Cmprd Header" in Figure 13) corresponds
   to a packet header that has been compressed by using SCHC.  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.







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4.1.4.  Padding

   If SCHC header compression leads to a SCHC Packet size of a non-
   integer number of bytes, padding bits of value equal to zero MUST be
   appended to the SCHC Packet 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
   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).

   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 14:


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

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



   Figure 14: 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 15:




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

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



   Figure 15: 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 16 (note
   that it does not include the source routing header that is present in
   the Downward frame format):


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

                                                   <-  SCHC pkt ->
       +-- ... -+- ... -+--- ... --+---- ... -+----+-----+-------+ - - +
       |11110001| RPI-  | IP-in-IP | 01000100 |SCHC|Cmprd|payload| pad |
       |Page 1  | 6LoRH |  6LoRH   |SCHCDsptch| Hdr| Hdr |       |     |
       +-- ... -+- ... -+--- ... --+---- ... -+----+-----+-------+ - - +
                                     (Page 1)

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



  Figure 16: 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 17:











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

                          <----- SCHC Packet ----->
           +--------------+-------------+---------+ - - - - +
           |  PRO Header  | Cmprd Header| Payload | Padding |
           +--------------+-------------+---------+ - - - - +
                   v              <->
                   |               |
                   +---------------+
                     SCHC Pointer

        Figure 17: frame format for SCHC-compressed packets in PRO.

   The PRO Header format is shown in Figure 18:



          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 18: PRO Header format.

   The first field in Figure 18 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.



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   The next field is the SCHC Header ("SCHC Hdr" in Figure 18), which
   has been defined in section 4.1.2.  As shown in Figure 18, in the PRO
   Header, the SCHC 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).

   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 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 19:









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

                                                   <---- SCHC Packet --->
    +-----------+----------+-------------+--------+-----------+---------+ - - +
    | Mesh Type | Mesh Hdr | SCHC Dsptch |SCHC Hdr| Cmprd Hdr | Payload | pad |
    +-----------+----------+-------------+--------+-----------+---------+ - - +



   Figure 19: 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 20:


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

                                                        <-- SCHC Packet ->
   +-------+-------+-------+-------+----------+--------+---------+-------+ - - +
   | M Typ | M Hdr | F Typ | F Hdr | SCHC Dsp |SCHC Hdr|Cmprd Hdr|Payload| Pad |
   +-------+-------+-------+-------+----------+--------+---------+-------+ - - +



   Figure 20: 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 21:


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

                                                        <-- SCHC Packet ->
   +-------+-------+-------+-------+----------+--------+---------+-------+ - - +
   | M Typ | M Hdr | B Typ | B Hdr | SCHC Dsp |SCHC Hdr|Cmprd Hdr|Payload| Pad |
   +-------+-------+-------+-------+----------+--------+---------+-------+ - - +










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      Figure 21: 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

   The different transmission alternatives enabled by the present
   document are shown in Figure 22:


   +-------------+----------------------------------------------------------+
   |  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   |
   +-------------+-----------+----------------+--------------+--------------+

   Figure 22: Summary of alternatives for the transmission of SCHC-
     compressed packets over IEEE 802.15.4 enabled by the present
                document, and corresponding artifacts

5.  Enabling the transition protocol stack

   In order to enable the transition protocol stack, (i.e., supporting
   SCHC-compressed UDP/CoAP headers over 6LoWPAN-compressed IPv6
   packets), the present document exploits the work that is being done
   by the INTAREA WG, to define a new Internet Protocol Number for SCHC
   [I-D.ietf-intarea-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 23.  Padding is added as needed to align the format to an
   octet boundary.




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


       Figure 23: Protocol data unit encapsulation and format for the
      transition protocol stack using a SCHC Internet Protocol Number

   For networks using the transition protocol stack based on RPL
   routing, the formats defined in RFC 8138 may also be used for the
   sake of efficiency, as shown in Figure 24.  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 Packet).


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


    Figure 24: Protocol data unit encapsulation and format for the
   transition protocol stack using a SCHC Internet Protocol Number
              and RFC 8138-compressed routing artifacts

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.





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   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 endpoint 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 pair of endpoints.

   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 IPv6 header compression, one Rule MUST be
   defined for each direction between the two involved C/D endpoints.
   In such a Rule, the IPv6 DevPrefix and IPv6 DevIID FIDs MUST refer to
   the destination address (i.e., the destination endpoint takes 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



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   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.

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
   endpoint it communicates with before such communication occurs
   [I-D.ietf-schc-architecture].  Therefore, in such cases, direction
   information will be specific to each pair of endpoints.





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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-barthel-schc-oam-
   schc-03].  Thus, it will be possible to compress the IPv6 header and
   the ICMPv6 headers 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 Packet (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:


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

       Figure 25: Details of the 6LoWPAN Dispatch Type Field request








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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, and Peter Yee made comments that helped shape this
   document.

   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








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   [I-D.ietf-intarea-schc-protocol-numbers]
              Moskowitz, R., Card, S. W., Wiethuechter, A., and P.
              Thubert, "Protocol Numbers for SCHC", Work in Progress,
              Internet-Draft, draft-ietf-intarea-schc-protocol-numbers-
              02, 8 April 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-intarea-schc-protocol-numbers-02>.

   [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-02, 11 April
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              schc-architecture-02>.

   [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>.

   [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>.




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   [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>.

   [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>.






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   [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>.

12.2.  Informative References

   [I-D.barthel-schc-oam-schc]
              Barthel, D. and L. Toutain, "Static Context Header
              Compression (SCHC) for the Internet Control Message
              Protocol (ICMPv6)", Work in Progress, Internet-Draft,
              draft-barthel-schc-oam-schc-04, 22 July 2024,
              <https://datatracker.ietf.org/doc/html/draft-barthel-schc-
              oam-schc-04>.




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   [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 26.


    +----------------+--+--+--+-------------+------+----------++------+
    |       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-* ||      |
    +================+==+==+==+=============+======+==========++======+




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        Figure 26: 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

   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






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   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 27.


    +----------------+--+--+--+-------------+------+----------++------+
    |       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 27: 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




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   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

   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.

   TO-DO: align with latest descriptions of SRO, TRO and 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
   network nodes (including intermediate nodes) need to store all the
   Rules in use in the network.  Therefore, it will be suitable for
   rather small networks and/or where nodes have sufficient memory.
   Also, SCHC context should be ideally and actually be as static as



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   possible, in order to avoid frequent network- wide stored SCHC
   context updates.

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.

   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
   this approach 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 this approach 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 this approach 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, pure intermediate nodes do not have to
   store SCHC context.

   Note that this approach 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.










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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.

   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, pure intermediate nodes do not have to
   store SCHC context.

   A potential advantage of PRO is that, in contrast with TRO, paths for
   intranetwork communication are not necessarily constrained to
   traversing a root node.  Therefore, for intranetwork communication,
   the chances of using optimal paths are greater.  Another feature is
   that the routing solution to be used is not tied to RPL non-storing
   mode.

B.4.  Summary

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

   TO-DO: to be completed.



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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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