path: root/doc/rfc/rfc9363.txt

Internet Engineering Task Force (IETF)                       A. Minaburo
Request for Comments: 9363                                        Acklio
Category: Standards Track                                     L. Toutain
ISSN: 2070-1721                                           IMT Atlantique
                                                              March 2023


     A YANG Data Model for Static Context Header Compression (SCHC)

Abstract

   This document describes a YANG data model for the Static Context
   Header Compression (SCHC) compression and fragmentation Rules.

   This document formalizes the description of the Rules for better
   interoperability between SCHC instances either to exchange a set of
   Rules or to modify the parameters of some Rules.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9363.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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 and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Terminology
   4.  SCHC Rules
     4.1.  Compression Rules
     4.2.  Identifier Generation
     4.3.  Convention for Field Identifier
     4.4.  Convention for Field Length
     4.5.  Convention for Field Position
     4.6.  Convention for Direction Indicator
     4.7.  Convention for Target Value
     4.8.  Convention for Matching Operator
       4.8.1.  Matching Operator Arguments
     4.9.  Convention for Compression Decompression Actions
       4.9.1.  Compression Decompression Action Arguments
     4.10. Fragmentation Rule
       4.10.1.  Fragmentation Mode
       4.10.2.  Fragmentation Header
       4.10.3.  Last Fragment Format
       4.10.4.  Acknowledgment Behavior
       4.10.5.  Timer Values
       4.10.6.  Fragmentation Parameter
       4.10.7.  Layer 2 Parameters
   5.  Rule Definition
     5.1.  Compression Rule
     5.2.  Fragmentation Rule
     5.3.  YANG Tree
   6.  YANG Data Model
   7.  IANA Considerations
     7.1.  URI Registration
     7.2.  YANG Module Name Registration
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Example
   Acknowledgments
   Authors' Addresses

1.  Introduction

   SCHC is a compression and fragmentation mechanism for constrained
   networks defined in [RFC8724].  It is based on a static context
   shared by two entities at the boundary of the constrained network.
   [RFC8724] provides an informal representation of the Rules used
   either for compression/decompression (C/D) or fragmentation/
   reassembly (F/R).  The goal of this document is to formalize the
   description of the Rules to offer:

   *  the same definition on both ends, even if the internal
      representation is different, and

   *  an update of the other end to set up some specific values (e.g.,
      IPv6 prefix, destination address, etc.).

   [LPWAN-ARCH] illustrates the exchange of Rules using the YANG data
   model.

   This document defines a YANG data model [RFC7950] to represent both
   compression and fragmentation Rules, which leads to common
   representation for values for all the Rules' elements.

2.  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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   This section defines the terminology and acronyms used in this
   document.  It extends the terminology of [RFC8376].

   App:  Low-Power WAN (LPWAN) Application, as defined by [RFC8376].  An
      application sending/receiving packets to/from the Dev.

   Bi:  Bidirectional.  Characterizes a Field Descriptor that applies to
      headers of packets traveling in either direction (Up and Dw; see
      this glossary).

   CDA:  Compression/Decompression Action.  Describes the pair of
      actions that are performed at the compressor to compress a header
      field and at the decompressor to recover the original value of the
      header field.

   Context:  A set of Rules used to compress/decompress headers.

   Dev:  Device, as defined by [RFC8376].

   DevIID:  Device Interface Identifier.  The IID that identifies the
      Dev interface.

   DI:  Direction Indicator.  This field tells which direction of packet
      travel (Up, Dw, or Bi) a Field Descriptor applies to.  This allows
      for asymmetric processing, using the same Rule.

   Dw:  Downlink direction for compression/decompression, from SCHC C/D
      in the network to SCHC C/D in the Dev.

   FID:  Field Identifier or Field ID.  This identifies the protocol and
      field a Field Descriptor applies to.

   FL:  Field Length.  This is the length of the original packet header
      field.  It is expressed as a number of bits for header fields of
      fixed lengths or as a type (e.g., variable, token length, ...) for
      Field Lengths that are unknown at the time of Rule creation.  The
      length of a header field is defined in the corresponding protocol
      specification (such as IPv6 or UDP).

   FP:  Field Position.  When a field is expected to appear multiple
      times in a header, the Field Position specifies the occurrence
      this Field Descriptor applies to (for example, first Uri-Path
      option, second Uri-Path, etc. in a Constrained Application
      Protocol (CoAP) header), counting from 1.  The value 0 is special
      and means "don't care" (see Section 7.2 of [RFC8724]).

   IID:  Interface Identifier.  See the IPv6 addressing architecture
      [RFC7136].

   L2 Word:  This is the minimum subdivision of payload data that the
      Layer 2 (L2) will carry.  In most L2 technologies, the L2 Word is
      an octet.  In bit-oriented radio technologies, the L2 Word might
      be a single bit.  The L2 Word size is assumed to be constant over
      time for each device.

   MO:  Matching Operator.  An operator used to match a value contained
      in a header field with a value contained in a Rule.

   RuleID:  Rule Identifier.  An identifier for a Rule.  SCHC C/D on
      both sides share the same RuleID for a given packet.  A set of
      RuleIDs are used to support SCHC F/R functionality.

   TV:  Target Value.  A value contained in a Rule that will be matched
      with the value of a header field.

   Up:  Uplink direction for compression/decompression, from the Dev
      SCHC C/D to the network SCHC C/D.

4.  SCHC Rules

   SCHC compression is generic; the main mechanism does not refer to a
   specific protocol.  Any header field is abstracted through a Field
   Identifier (FID), a position (FP), a direction (DI), and a value that
   can be a numerical value or a string.  [RFC8724] and [RFC8824]
   specify fields for IPv6 [RFC8200], UDP [RFC0768], and CoAP [RFC7252],
   including options defined for no server response [RFC7967] and Object
   Security for Constrained RESTful Environments (OSCORE) [RFC8613].
   For the latter, [RFC8824] splits this field into subfields.

   SCHC fragmentation requires a set of common parameters that are
   included in a Rule.  These parameters are defined in [RFC8724].

   The YANG data model enables the compression and the fragmentation
   selection using the feature statement.

4.1.  Compression Rules

   [RFC8724] proposes an informal representation of the compression
   Rule.  A compression context for a device is composed of a set of
   Rules.  Each Rule contains information to describe a specific field
   in the header to be compressed.

     +-----------------------------------------------------------------+
     |                      Rule N                                     |
    +-----------------------------------------------------------------+|
    |                    Rule i                                       ||
   +-----------------------------------------------------------------+||
   |  (FID)            Rule 1                                        |||
   |+-------+--+--+--+------------+-----------------+---------------+|||
   ||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
   |+-------+--+--+--+------------+-----------------+---------------+|||
   ||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
   |+-------+--+--+--+------------+-----------------+---------------+|||
   ||...    |..|..|..|   ...      | ...             | ...           ||||
   |+-------+--+--+--+------------+-----------------+---------------+||/
   ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
   |+-------+--+--+--+------------+-----------------+---------------+|/
   |                                                                 |
   \-----------------------------------------------------------------/

                Figure 1: Compression Decompression Context

4.2.  Identifier Generation

   Identifiers used in the SCHC YANG data model are from the identityref
   statement to ensure global uniqueness and easy augmentation if
   needed.  The principle to define a new type based on a group of
   identityref is the following:

   *  Define a main identity ending with the keyword base-type.

   *  Derive all the identities used in the data model from this base
      type.

   *  Create a typedef from this base type.

   The example below (Figure 2) shows how an identityref is created for
   Reassembly Check Sequence (RCS) algorithms used during SCHC
   fragmentation.

     identity rcs-algorithm-base-type {
       description
         "Identify which algorithm is used to compute RCS.
          The algorithm also defines the size of the RCS field.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     identity rcs-crc32 {
       base rcs-algorithm-base-type;
       description
         "CRC32 defined as default RCS in RFC 8724.  This RCS is
          4 bytes long.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     typedef rcs-algorithm-type {
       type identityref {
         base rcs-algorithm-base-type;
       }
       description
         "Define the type for RCS algorithm in Rules.";
     }

         Figure 2: Principle to Define a Type Based on identityref

4.3.  Convention for Field Identifier

   In the process of compression, the headers of the original packet are
   first parsed to create a list of fields.  This list of fields is
   matched against the Rules to find the appropriate Rule and apply
   compression.  [RFC8724] does not state how the Field ID value is
   constructed.  In examples, identification is done through a string
   indexed by the protocol name (e.g., IPv6.version, CoAP.version,
   etc.).

   The current YANG data model includes field definitions found in
   [RFC8724] and [RFC8824].

   Using the YANG data model, each field MUST be identified through a
   global YANG identityref.

   A YANG Field ID for the protocol is always derived from the fid-base-
   type.  Then, an identity for each protocol is specified using the
   naming convention fid-<<protocol name>>-base-type.  All possible
   fields for this protocol MUST derive from the protocol identity.  The
   naming convention is "fid-" followed by the protocol name and the
   field name.  If a field has to be divided into subfields, the field
   identity serves as a base.

   The full field-id definition is found in Section 6.  A type is
   defined for the IPv6 protocol, and each field is based on it.  Note
   that the Diffserv bits derive from the Traffic Class identity.

4.4.  Convention for Field Length

   The Field Length is either an integer giving the size of a field in
   bits or a specific function.  [RFC8724] defines the "var" function,
   which allows variable-length fields (whose length is expressed in
   bytes), and [RFC8824] defines the "tkl" function for managing the
   CoAP Token Length field.

   The naming convention is "fl-" followed by the function name.

   The Field Length function can be defined as an identityref, as
   described in Section 6.  Therefore, the type for the Field Length is
   a union between an integer giving the size of the length in bits and
   the identityref.

4.5.  Convention for Field Position

   The Field Position is a positive integer that gives the occurrence
   times of a specific field from the header start.  The default value
   is 1 and is incremented at each repetition.  Value 0 indicates that
   the position is not important and is not considered during the Rule
   selection process.

   The Field Position is a positive integer.  The type is uint8.

4.6.  Convention for Direction Indicator

   The Direction Indicator is used to tell if a field appears in both
   directions (Bi) or only uplink (Up) or Downlink (Dw).  The naming
   convention is "di" followed by the Direction Indicator name.

   The type is "di-type".

4.7.  Convention for Target Value

   The Target Value is a list of binary sequences of any length, aligned
   to the left.  In the Rule, the structure will be used as a list, with
   the index as a key.  The highest index value is used to compute the
   size of the index sent in residue for the match-mapping Compression
   Decompression Action (CDA).  The index can specify several values:

   *  For equal and most significant bits (MSBs), the Target Value
      contains a single element.  Therefore, the index is set to 0.

   *  For match-mapping, the Target Value can contain several elements.
      Index values MUST start from 0 and MUST be contiguous.

   If the header field contains text, the binary sequence uses the same
   encoding.

4.8.  Convention for Matching Operator

   The Matching Operator (MO) is a function applied between a field
   value provided by the parsed header and the Target Value.  [RFC8724]
   defines 4 MOs.

   The naming convention is "mo-" followed by the MO name.

   The type is "mo-type".

4.8.1.  Matching Operator Arguments

   They are viewed as a list, built with a tv-struct (see Section 4.7).

4.9.  Convention for Compression Decompression Actions

   The Compression Decompression Action (CDA) identifies the function to
   use for compression or decompression.  [RFC8724] defines 7 CDAs.

   The naming convention is "cda-" followed by the CDA name.

4.9.1.  Compression Decompression Action Arguments

   Currently no CDA requires arguments, but some CDAs may require one or
   several arguments in the future.  They are viewed as a list of
   target-value type.

4.10.  Fragmentation Rule

   Fragmentation is optional in the data model and depends on the
   presence of the "fragmentation" feature.

   Most of the fragmentation parameters are listed in Appendix D of
   [RFC8724].

   Since fragmentation Rules work for a specific direction, they MUST
   contain a mandatory Direction Indicator.  The type is the same as the
   one used in compression entries, but bidirectional MUST NOT be used.

4.10.1.  Fragmentation Mode

   [RFC8724] defines 3 fragmentation modes:

   *  No ACK: This mode is unidirectional; no acknowledgment is sent
      back.

   *  ACK Always: Each fragmentation window must be explicitly
      acknowledged before going to the next.

   *  ACK on Error: A window is acknowledged only when the receiver
      detects some missing fragments.

   The type is "fragmentation-mode-type".  The naming convention is
   "fragmentation-mode-" followed by the fragmentation mode name.

4.10.2.  Fragmentation Header

   A data fragment header, starting with the RuleID, can be sent in the
   fragmentation direction.  [RFC8724] indicates that the SCHC header
   may be composed of the following (cf. Figure 3):

   *  a Datagram Tag (DTag) identifying the datagram being fragmented if
      the fragmentation applies concurrently on several datagrams.  This
      field is optional, and its length is defined by the Rule.

   *  a Window (W) used in ACK-Always and ACK-on-Error modes.  In ACK-
      Always, its size is 1.  In ACK-on-Error, it depends on the Rule.
      This field is not needed in No-ACK mode.

   *  a Fragment Compressed Number (FCN) indicating the fragment/tile
      position within the window.  This field is mandatory on all modes
      defined in [RFC8724], and its size is defined by the Rule.

   |-- SCHC Fragment Header ----|
            |-- T --|-M-|-- N --|
   +-- ... -+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~
   | RuleID | DTag  | W |  FCN  | Fragment Payload | padding (as needed)
   +-- ... -+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~

                Figure 3: Data Fragment Header from RFC 8724

4.10.3.  Last Fragment Format

   The last fragment of a datagram is sent with a Reassembly Check
   Sequence (RCS) field to detect residual transmission errors and
   possible losses in the last window.  [RFC8724] defines a single
   algorithm based on Ethernet CRC computation.

   The naming convention is "rcs-" followed by the algorithm name.

   For ACK-on-Error mode, the All-1 fragment may just contain the RCS or
   can include a tile.  The following parameters define the behavior:

   *  all-1-data-no: The last fragment contains no data, just the RCS.

   *  all-1-data-yes: The last fragment includes a single tile and the
      RCS.

   *  all-1-data-sender-choice: The last fragment may or may not contain
      a single tile.  The receiver can detect if a tile is present.

   The naming convention is "all-1-data-" followed by the behavior
   identifier.

4.10.4.  Acknowledgment Behavior

   The acknowledgment fragment header goes in the opposite direction of
   data.  [RFC8724] defines the header, which is composed of the
   following (see Figure 4):

   *  a DTag (if present).

   *  a mandatory window, as in the data fragment.

   *  a C bit giving the status of RCS validation.  In case of failure,
      a bitmap follows, indicating the received tile.

   |--- SCHC ACK Header ----|
            |-- T --|-M-| 1 |
   +-- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
   | RuleID |  DTag | W |C=1| padding as needed                (success)
   +-- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~

   +-- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
   | RuleID |  DTag | W |C=0|Compressed Bitmap| pad. as needed (failure)
   +-- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~

           Figure 4: Acknowledgment Fragment Header for RFC 8724

   For ACK-on-Error, SCHC defines when an acknowledgment can be sent.
   This can be at any time defined by the Layer 2, at the end of a
   window (FCN all-0), or as a response to receiving the last fragment
   (FCN all-1).  The naming convention is "ack-behavior" followed by the
   algorithm name.

4.10.5.  Timer Values

   The state machine requires some common values to handle fragmentation
   correctly.

   *  The Retransmission Timer gives the duration before sending an ACK
      request (cf. Section 8.2.2.4 of [RFC8724]).  If specified, the
      value MUST be strictly positive.

   *  The Inactivity Timer gives the duration before aborting a
      fragmentation session (cf.  Section 8.2.2.4 of [RFC8724]).  The
      value 0 explicitly indicates that this timer is disabled.

   [RFC8724] does not specify any range for these timers.  [RFC9011]
   recommends a duration of 12 hours.  In fact, the value range should
   be between milliseconds for real-time systems to several days for
   worse-than-best-effort systems.  To allow a large range of
   applications, two parameters must be specified:

   *  the duration of a tick.  It is computed by this formula: 2^(tick-
      duration)/10^6.  When tick-duration is set to 0, the unit is the
      microsecond.  The default value of 20 leads to a unit of 1.048575
      seconds.  A value of 32 leads to a tick-duration of about 1 hour
      11 minutes.

   *  the number of ticks in the predefined unit.  With the default
      tick-duration value of 20, the timers can cover a range between
      1.0 second and 19 hours, as recommended in [RFC9011].

4.10.6.  Fragmentation Parameter

   The SCHC fragmentation protocol specifies the number of attempts
   before aborting through the parameter:

   *  max-ack-requests (cf. Section 8.2.2.4 of [RFC8724])

4.10.7.  Layer 2 Parameters

   The data model includes two parameters needed for fragmentation:

   *  l2-word-size: [RFC8724] base fragmentation, in bits, on a Layer 2
      Word that can be of any length.  The default value is 8 and
      corresponds to the default value for the byte-aligned Layer 2.  A
      value of 1 will indicate that there is no alignment and no need
      for padding.

   *  maximum-packet-size: defines the maximum size of an uncompressed
      datagram.  By default, the value is set to 1280 bytes.

   They are defined as unsigned integers; see Section 6.

5.  Rule Definition

   A Rule is identified by a unique Rule Identifier (RuleID) comprising
   both a RuleID value and a RuleID length.  The YANG grouping rule-id-
   type defines the structure used to represent a RuleID.  A length of 0
   is allowed to represent an implicit Rule.

   Three natures of Rules are defined in [RFC8724]:

   *  Compression: A compression Rule is associated with the RuleID.

   *  No-compression: This identifies the default Rule used to send a
      packet integrally when no-compression Rule was found (see
      Section 6 of [RFC8724]).

   *  Fragmentation: Fragmentation parameters are associated with the
      RuleID.  Fragmentation is optional, and the feature
      "fragmentation" should be set.

   The YANG data model respectively introduces these three identities :

   *  nature-compression

   *  nature-no-compression

   *  nature-fragmentation

   The naming convention is "nature-" followed by the nature identifier.

   To access a specific Rule, the RuleID length and value are used as a
   key.  The Rule is either a compression or a fragmentation Rule.

5.1.  Compression Rule

   A compression Rule is composed of entries describing its processing.
   An entry contains all the information defined in Figure 1 with the
   types defined above.

   The compression Rule described Figure 1 is defined by compression-
   content.  It defines a list of compression-rule-entry, indexed by
   their Field ID, position, and direction.  The compression-rule-entry
   element represents a line in Figure 1.  Their type reflects the
   identifier types defined in Section 4.1.

   Some checks are performed on the values:

   *  When MO is ignore, no Target Value is needed; for other MOs, there
      MUST be a Target Value present.

   *  When MSB MO is specified, the matching-operator-value must be
      present.

5.2.  Fragmentation Rule

   A fragmentation Rule is composed of entries describing the protocol
   behavior.  Some on them are numerical entries, others are identifiers
   defined in Section 4.10.

5.3.  YANG Tree

   The YANG data model described in this document conforms to the
   Network Management Datastore Architecture defined in [RFC8342].

   module: ietf-schc
     +--rw schc
        +--rw rule* [rule-id-value rule-id-length]
           +--rw rule-id-value                   uint32
           +--rw rule-id-length                  uint8
           +--rw rule-nature                     nature-type
           +--rw (nature)?
              +--:(fragmentation) {fragmentation}?
              |  +--rw fragmentation-mode
              |  |       schc:fragmentation-mode-type
              |  +--rw l2-word-size?             uint8
              |  +--rw direction                 schc:di-type
              |  +--rw dtag-size?                uint8
              |  +--rw w-size?                   uint8
              |  +--rw fcn-size                  uint8
              |  +--rw rcs-algorithm?            rcs-algorithm-type
              |  +--rw maximum-packet-size?      uint16
              |  +--rw window-size?              uint16
              |  +--rw max-interleaved-frames?   uint8
              |  +--rw inactivity-timer
              |  |  +--rw ticks-duration?   uint8
              |  |  +--rw ticks-numbers?    uint16
              |  +--rw retransmission-timer
              |  |  +--rw ticks-duration?   uint8
              |  |  +--rw ticks-numbers?    uint16
              |  +--rw max-ack-requests?         uint8
              |  +--rw (mode)?
              |     +--:(no-ack)
              |     +--:(ack-always)
              |     +--:(ack-on-error)
              |        +--rw tile-size?          uint8
              |        +--rw tile-in-all-1?      schc:all-1-data-type
              |        +--rw ack-behavior?       schc:ack-behavior-type
              +--:(compression) {compression}?
                 +--rw entry*
                         [field-id field-position direction-indicator]
                    +--rw field-id                    schc:fid-type
                    +--rw field-length                schc:fl-type
                    +--rw field-position              uint8
                    +--rw direction-indicator         schc:di-type
                    +--rw target-value* [index]
                    |  +--rw index    uint16
                    |  +--rw value?   binary
                    +--rw matching-operator           schc:mo-type
                    +--rw matching-operator-value* [index]
                    |  +--rw index    uint16
                    |  +--rw value?   binary
                    +--rw comp-decomp-action          schc:cda-type
                    +--rw comp-decomp-action-value* [index]
                       +--rw index    uint16
                       +--rw value?   binary

                 Figure 5: Overview of the SCHC Data Model

6.  YANG Data Model

   <CODE BEGINS> file "ietf-schc@2023-03-01.yang"
   module ietf-schc {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-schc";
     prefix schc;

     organization
       "IETF IPv6 over Low Power Wide-Area Networks (lpwan) Working
        Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/lpwan/about/>
        WG List:  <mailto:lp-wan@ietf.org>
        Editor:   Laurent Toutain
          <mailto:laurent.toutain@imt-atlantique.fr>
        Editor:   Ana Minaburo
          <mailto:ana@ackl.io>";
     description
       "Copyright (c) 2023 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.
        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Revised BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).
        This version of this YANG module is part of RFC 9363
        (https://www.rfc-editor.org/info/rfc9363); see the RFC itself
        for full legal notices.
        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.
        ***************************************************************
        Generic data model for the Static Context Header Compression
        Rule for SCHC, based on RFCs 8724 and 8824.  Including
        compression, no-compression, and fragmentation Rules.

        This module is a YANG data model for SCHC Rules (RFCs 8724 and
        8824).  RFC 8724 describes compression Rules in an abstract
        way through a table.
    |-----------------------------------------------------------------|
    |  (FID)            Rule 1                                        |
    |+-------+--+--+--+------------+-----------------+---------------+|
    ||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||
    |+-------+--+--+--+------------+-----------------+---------------+|
    ||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||
    |+-------+--+--+--+------------+-----------------+---------------+|
    ||...    |..|..|..|   ...      | ...             | ...           ||
    |+-------+--+--+--+------------+-----------------+---------------+|
    ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||
    |+-------+--+--+--+------------+-----------------+---------------+|
    |-----------------------------------------------------------------|
        This module specifies a global data model that can be used for
        Rule exchanges or modification.  It specifies both the data
        model format and the global identifiers used to describe some
        operations in fields.
        This data model applies to both compression and fragmentation.";

     revision 2023-03-01 {
       description
         "Initial version from RFC 9363.";
       reference
         "RFC 9363 A YANG Data Model for Static Context Header
                   Compression (SCHC)";
     }

     feature compression {
       description
         "SCHC compression capabilities are taken into account.";
     }

     feature fragmentation {
       description
         "SCHC fragmentation capabilities are taken into account.";
     }

     // -------------------------
     //  Field ID type definition
     //--------------------------
     // generic value TV definition

     identity fid-base-type {
       description
         "Field ID base type for all fields.";
     }

     identity fid-ipv6-base-type {
       base fid-base-type;
       description
         "Field ID base type for IPv6 headers described in RFC 8200.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-version {
       base fid-ipv6-base-type;
       description
         "IPv6 version field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-trafficclass {
       base fid-ipv6-base-type;
       description
         "IPv6 Traffic Class field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-trafficclass-ds {
       base fid-ipv6-trafficclass;
       description
         "IPv6 Traffic Class field: Diffserv field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification,
          RFC 3168 The Addition of Explicit Congestion Notification
                   (ECN) to IP";
     }

     identity fid-ipv6-trafficclass-ecn {
       base fid-ipv6-trafficclass;
       description
         "IPv6 Traffic Class field: ECN field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification,
          RFC 3168 The Addition of Explicit Congestion Notification
                   (ECN) to IP";
     }

     identity fid-ipv6-flowlabel {
       base fid-ipv6-base-type;
       description
         "IPv6 Flow Label field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-payload-length {
       base fid-ipv6-base-type;
       description
         "IPv6 Payload Length field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-nextheader {
       base fid-ipv6-base-type;
       description
         "IPv6 Next Header field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-hoplimit {
       base fid-ipv6-base-type;
       description
         "IPv6 Next Header field.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-devprefix {
       base fid-ipv6-base-type;
       description
         "Corresponds to either the source address or the destination
          address prefix of RFC 8200 depending on whether it is an
          uplink or a downlink message.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-deviid {
       base fid-ipv6-base-type;
       description
         "Corresponds to either the source address or the destination
          address IID of RFC 8200 depending on whether it is an uplink
          or a downlink message.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-appprefix {
       base fid-ipv6-base-type;
       description
         "Corresponds to either the source address or the destination
          address prefix of RFC 8200 depending on whether it is an
          uplink or a downlink message.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-ipv6-appiid {
       base fid-ipv6-base-type;
       description
         "Corresponds to either the source address or the destination
          address IID of RFC 8200 depending on whether it is an uplink
          or a downlink message.";
       reference
         "RFC 8200 Internet Protocol, Version 6 (IPv6) Specification";
     }

     identity fid-udp-base-type {
       base fid-base-type;
       description
         "Field ID base type for UDP headers described in RFC 768.";
       reference
         "RFC 768 User Datagram Protocol";
     }

     identity fid-udp-dev-port {
       base fid-udp-base-type;
       description
         "UDP source or destination port, if uplink or downlink
          communication, respectively.";
       reference
         "RFC 768 User Datagram Protocol";
     }

     identity fid-udp-app-port {
       base fid-udp-base-type;
       description
         "UDP destination or source port, if uplink or downlink
          communication, respectively.";
       reference
         "RFC 768 User Datagram Protocol";
     }

     identity fid-udp-length {
       base fid-udp-base-type;
       description
         "UDP length.";
       reference
         "RFC 768 User Datagram Protocol";
     }

     identity fid-udp-checksum {
       base fid-udp-base-type;
       description
         "UDP length.";
       reference
         "RFC 768 User Datagram Protocol";
     }

     identity fid-coap-base-type {
       base fid-base-type;
       description
         "Field ID base type for UDP headers described.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-version {
       base fid-coap-base-type;
       description
         "CoAP version.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-type {
       base fid-coap-base-type;
       description
         "CoAP type.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-tkl {
       base fid-coap-base-type;
       description
         "CoAP token length.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-code {
       base fid-coap-base-type;
       description
         "CoAP code.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-code-class {
       base fid-coap-code;
       description
         "CoAP code class.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-code-detail {
       base fid-coap-code;
       description
         "CoAP code detail.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-mid {
       base fid-coap-base-type;
       description
         "CoAP message ID.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-token {
       base fid-coap-base-type;
       description
         "CoAP token.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option {
       base fid-coap-base-type;
       description
         "Generic CoAP option.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-if-match {
       base fid-coap-option;
       description
         "CoAP option If-Match.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-uri-host {
       base fid-coap-option;
       description
         "CoAP option Uri-Host.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-etag {
       base fid-coap-option;
       description
         "CoAP option ETag.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-if-none-match {
       base fid-coap-option;
       description
         "CoAP option if-none-match.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-observe {
       base fid-coap-option;
       description
         "CoAP option Observe.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-uri-port {
       base fid-coap-option;
       description
         "CoAP option Uri-Port.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-location-path {
       base fid-coap-option;
       description
         "CoAP option Location-Path.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-uri-path {
       base fid-coap-option;
       description
         "CoAP option Uri-Path.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-content-format {
       base fid-coap-option;
       description
         "CoAP option Content Format.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-max-age {
       base fid-coap-option;
       description
         "CoAP option Max-Age.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-uri-query {
       base fid-coap-option;
       description
         "CoAP option Uri-Query.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-accept {
       base fid-coap-option;
       description
         "CoAP option Accept.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-location-query {
       base fid-coap-option;
       description
         "CoAP option Location-Query.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-block2 {
       base fid-coap-option;
       description
         "CoAP option Block2.";
       reference
         "RFC 7959 Block-Wise Transfers in the Constrained
                   Application Protocol (CoAP)";
     }

     identity fid-coap-option-block1 {
       base fid-coap-option;
       description
         "CoAP option Block1.";
       reference
         "RFC 7959 Block-Wise Transfers in the Constrained
                   Application Protocol (CoAP)";
     }

     identity fid-coap-option-size2 {
       base fid-coap-option;
       description
         "CoAP option Size2.";
       reference
         "RFC 7959 Block-Wise Transfers in the Constrained
                   Application Protocol (CoAP)";
     }

     identity fid-coap-option-proxy-uri {
       base fid-coap-option;
       description
         "CoAP option Proxy-Uri.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-proxy-scheme {
       base fid-coap-option;
       description
         "CoAP option Proxy-Scheme.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-size1 {
       base fid-coap-option;
       description
         "CoAP option Size1.";
       reference
         "RFC 7252 The Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-no-response {
       base fid-coap-option;
       description
         "CoAP option No response.";
       reference
         "RFC 7967 Constrained Application Protocol (CoAP)
                   Option for No Server Response";
     }

     identity fid-oscore-base-type {
       base fid-coap-option;
       description
         "OSCORE options (RFC8613) split in suboptions.";
       reference
         "RFC 8824 Static Context Header Compression (SCHC) for the
                   Constrained Application Protocol (CoAP)";
     }

     identity fid-coap-option-oscore-flags {
       base fid-coap-option;
       description
         "CoAP option OSCORE flags.";
       reference
         "RFC 8824 Static Context Header Compression (SCHC) for the
                   Constrained Application Protocol (CoAP) (see
                   Section 6.4)";
     }

     identity fid-coap-option-oscore-piv {
       base fid-coap-option;
       description
         "CoAP option OSCORE flags.";
       reference
         "RFC 8824 Static Context Header Compression (SCHC) for the
                   Constrained Application Protocol (CoAP) (see
                   Section 6.4)";
     }

     identity fid-coap-option-oscore-kid {
       base fid-coap-option;
       description
         "CoAP option OSCORE flags.";
       reference
         "RFC 8824 Static Context Header Compression (SCHC) for the
                   Constrained Application Protocol (CoAP) (see
                   Section 6.4)";
     }

     identity fid-coap-option-oscore-kidctx {
       base fid-coap-option;
       description
         "CoAP option OSCORE flags.";
       reference
         "RFC 8824 Static Context Header Compression (SCHC) for the
                   Constrained Application Protocol (CoAP)(see
                   Section 6.4)";
     }

     //----------------------------------
     // Field Length type definition
     //----------------------------------

     identity fl-base-type {
       description
         "Used to extend Field Length functions.";
     }

     identity fl-variable {
       base fl-base-type;
       description
         "Residue length in bytes is sent as defined for CoAP.";
       reference
         "RFC 8824 Static Context Header Compression (SCHC) for the
                   Constrained Application Protocol (CoAP) (see
                   Section 5.3)";
     }

     identity fl-token-length {
       base fl-base-type;
       description
         "Residue length in bytes is sent as defined for CoAP.";
       reference
         "RFC 8824 Static Context Header Compression (SCHC) for the
                   Constrained Application Protocol (CoAP) (see
                   Section 4.5)";
     }

     //---------------------------------
     // Direction Indicator type
     //---------------------------------

     identity di-base-type {
       description
         "Used to extend Direction Indicators.";
     }

     identity di-bidirectional {
       base di-base-type;
       description
         "Direction Indicator of bidirectionality.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.1)";
     }

     identity di-up {
       base di-base-type;
       description
         "Direction Indicator of uplink.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.1)";
     }

     identity di-down {
       base di-base-type;
       description
         "Direction Indicator of downlink.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.1)";
     }

     //----------------------------------
     // Matching Operator type definition
     //----------------------------------

     identity mo-base-type {
       description
         "Matching Operator: used in the Rule selection process
          to check if a Target Value matches the field's value.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.2)";
     }

     identity mo-equal {
       base mo-base-type;
       description
         "equal MO.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.3)";
     }

     identity mo-ignore {
       base mo-base-type;
       description
         "ignore MO.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.3)";
     }

     identity mo-msb {
       base mo-base-type;
       description
         "MSB MO.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.3)";
     }

     identity mo-match-mapping {
       base mo-base-type;
       description
         "match-mapping MO.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.3)";
     }

     //------------------------------
     // CDA type definition
     //------------------------------

     identity cda-base-type {
       description
         "Compression Decompression Actions. Specify the action to
          be applied to the field's value in a specific Rule.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.2)";
     }

     identity cda-not-sent {
       base cda-base-type;
       description
         "not-sent CDA.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.4)";
     }

     identity cda-value-sent {
       base cda-base-type;
       description
         "value-sent CDA.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.4)";
     }

     identity cda-lsb {
       base cda-base-type;
       description
         "Least Significant Bit (LSB) CDA.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.4)";
     }

     identity cda-mapping-sent {
       base cda-base-type;
       description
         "mapping-sent CDA.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.4)";
     }

     identity cda-compute {
       base cda-base-type;
       description
         "compute-* CDA.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.4)";
     }

     identity cda-deviid {
       base cda-base-type;
       description
         "DevIID CDA.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.4)";
     }

     identity cda-appiid {
       base cda-base-type;
       description
         "Application Interface Identifier (AppIID) CDA.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context
                   Header Compression and Fragmentation (see
                   Section 7.4)";
     }

     // -- type definition

     typedef fid-type {
       type identityref {
         base fid-base-type;
       }
       description
         "Field ID generic type.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     typedef fl-type {
       type identityref {
         base fl-base-type;
       }
       description
         "Function used to indicate Field Length.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     typedef di-type {
       type identityref {
         base di-base-type;
       }
       description
         "Direction in LPWAN network: up when emitted by the device,
          down when received by the device, or bi when emitted or
          received by the device.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     typedef mo-type {
       type identityref {
         base mo-base-type;
       }
       description
         "Matching Operator (MO) to compare field values with
          Target Values.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     typedef cda-type {
       type identityref {
         base cda-base-type;
       }
       description
         "Compression Decompression Action to compress or
          decompress a field.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     // -- FRAGMENTATION TYPE
     // -- fragmentation modes

     identity fragmentation-mode-base-type {
       description
         "Define the fragmentation mode.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     identity fragmentation-mode-no-ack {
       base fragmentation-mode-base-type;
       description
         "No-ACK mode.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     identity fragmentation-mode-ack-always {
       base fragmentation-mode-base-type;
       description
         "ACK-Always mode.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     identity fragmentation-mode-ack-on-error {
       base fragmentation-mode-base-type;
       description
         "ACK-on-Error mode.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     typedef fragmentation-mode-type {
       type identityref {
         base fragmentation-mode-base-type;
       }
       description
         "Define the type used for fragmentation mode in Rules.";
     }

     // -- Ack behavior

     identity ack-behavior-base-type {
       description
         "Define when to send an Acknowledgment.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     identity ack-behavior-after-all-0 {
       base ack-behavior-base-type;
       description
         "Fragmentation expects ACK after sending All-0 fragment.";
     }

     identity ack-behavior-after-all-1 {
       base ack-behavior-base-type;
       description
         "Fragmentation expects ACK after sending All-1 fragment.";
     }

     identity ack-behavior-by-layer2 {
       base ack-behavior-base-type;
       description
         "Layer 2 defines when to send an ACK.";
     }

     typedef ack-behavior-type {
       type identityref {
         base ack-behavior-base-type;
       }
       description
         "Define the type used for ACK behavior in Rules.";
     }

     // -- All-1 with data types

     identity all-1-data-base-type {
       description
         "Type to define when to send an Acknowledgment message.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     identity all-1-data-no {
       base all-1-data-base-type;
       description
         "All-1 contains no tiles.";
     }

     identity all-1-data-yes {
       base all-1-data-base-type;
       description
         "All-1 MUST contain a tile.";
     }

     identity all-1-data-sender-choice {
       base all-1-data-base-type;
       description
         "Fragmentation process chooses to send tiles or not in All-1.";
     }

     typedef all-1-data-type {
       type identityref {
         base all-1-data-base-type;
       }
       description
         "Define the type used for All-1 format in Rules.";
     }

     // -- RCS algorithm types

     identity rcs-algorithm-base-type {
       description
         "Identify which algorithm is used to compute RCS.
          The algorithm also defines the size of the RCS field.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     identity rcs-crc32 {
       base rcs-algorithm-base-type;
       description
         "CRC32 defined as default RCS in RFC 8724.  This RCS is
          4 bytes long.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     typedef rcs-algorithm-type {
       type identityref {
         base rcs-algorithm-base-type;
       }
       description
         "Define the type for RCS algorithm in Rules.";
     }

     // --------  RULE ENTRY DEFINITION ------------

     grouping tv-struct {
       description
         "Defines the Target Value element.  If the header field
          contains a text, the binary sequence uses the same encoding.
          field-id allows the conversion to the appropriate type.";
       leaf index {
         type uint16;
         description
           "Index gives the position in the matching list.  If only one
            element is present, index is 0.  Otherwise, index is the
            order in the matching list, starting at 0.";
       }
       leaf value {
         type binary;
         description
           "Target Value content as an untyped binary value.";
       }
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     grouping compression-rule-entry {
       description
         "These entries define a compression entry (i.e., a line),
          as defined in RFC 8724.
      +-------+--+--+--+------------+-----------------+---------------+
      |Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|
      +-------+--+--+--+------------+-----------------+---------------+
          An entry in a compression Rule is composed of 7 elements:
          - Field ID: the header field to be compressed
          - Field Length : either a positive integer or a function
          - Field Position: a positive (and possibly equal to 0)
            integer
          - Direction Indicator: an indication in which direction the
            compression and decompression process is effective
          - Target Value: a value against which the header field is
            compared
          - Matching Operator: the comparison operation and optional
            associate parameters
          - Comp./Decomp. Action: the compression or decompression
            action and optional parameters
         ";
       leaf field-id {
         type schc:fid-type;
         mandatory true;
         description
           "Field ID, identify a field in the header with a YANG
            identity reference.";
       }
       leaf field-length {
         type union {
         type uint8;
         type schc:fl-type;
           }
         mandatory true;
         description
           "Field Length, expressed in number of bits if the length is
            known when the Rule is created or through a specific
            function if the length is variable.";
       }
       leaf field-position {
         type uint8;
         mandatory true;
         description
           "Field Position in the header is an integer.  Position 1
            matches the first occurrence of a field in the header,
            while incremented position values match subsequent
            occurrences.
            Position 0 means that this entry matches a field
            irrespective of its position of occurrence in the
            header.
            Be aware that the decompressed header may have
            position-0 fields ordered differently than they
            appeared in the original packet.";
       }
       leaf direction-indicator {
         type schc:di-type;
         mandatory true;
         description
           "Direction Indicator, indicate if this field must be
            considered for Rule selection or ignored based on the
            direction (bidirectional, only uplink, or only
            downlink).";
       }
       list target-value {
         key "index";
         uses tv-struct;
         description
           "A list of values to compare with the header field value.
            If Target Value is a singleton, position must be 0.
            For use as a matching list for the mo-match-mapping Matching
            Operator, index should take consecutive values starting
            from 0.";
       }
       leaf matching-operator {
         type schc:mo-type;
         must "../target-value or derived-from-or-self(.,
                                                      'mo-ignore')" {
           error-message
             "mo-equal, mo-msb, and mo-match-mapping need target-value";
           description
             "target-value is not required for mo-ignore.";
         }
         must "not (derived-from-or-self(., 'mo-msb')) or
               ../matching-operator-value" {
           error-message "mo-msb requires length value";
         }
         mandatory true;
         description
           "MO: Matching Operator.";
         reference
           "RFC 8724 SCHC: Generic Framework for Static Context Header
                     Compression and Fragmentation (see Section 7.3)";
       }
       list matching-operator-value {
         key "index";
         uses tv-struct;
         description
           "Matching Operator Arguments, based on TV structure to allow
            several arguments.
            In RFC 8724, only the MSB Matching Operator needs arguments
            (a single argument, which is the number of most significant
            bits to be matched).";
       }
       leaf comp-decomp-action {
         type schc:cda-type;
         must "../target-value or
                   derived-from-or-self(., 'cda-value-sent') or
                   derived-from-or-self(., 'cda-compute') or
                   derived-from-or-self(., 'cda-appiid') or
                   derived-from-or-self(., 'cda-deviid')" {
           error-message
             "cda-not-sent, cda-lsb, and cda-mapping-sent need
              target-value";
           description
             "target-value is not required for some CDA.";
         }
         mandatory true;
         description
           "CDA: Compression Decompression Action.";
         reference
           "RFC 8724 SCHC: Generic Framework for Static Context Header
                     Compression and Fragmentation (see Section 7.4)";
       }
       list comp-decomp-action-value {
         key "index";
         uses tv-struct;
         description
           "CDA arguments, based on a TV structure, in order to allow
            for several arguments.  The CDAs specified in RFC 8724
            require no argument.";
       }
     }

     // --Rule nature

     identity nature-base-type {
       description
         "A Rule, identified by its RuleID, is used for a single
          purpose.  RFC 8724 defines 3 natures:
          compression, no-compression, and fragmentation.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation (see Section 6)";
     }

     identity nature-compression {
       base nature-base-type;
       description
         "Identify a compression Rule.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation (see Section 6)";
     }

     identity nature-no-compression {
       base nature-base-type;
       description
         "Identify a no-compression Rule.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation (see Section 6)";
     }

     identity nature-fragmentation {
       base nature-base-type;
       description
         "Identify a fragmentation Rule.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation (see Section 6)";
     }

     typedef nature-type {
       type identityref {
         base nature-base-type;
       }
       description
         "Defines the type to indicate the nature of the Rule.";
     }

     grouping compression-content {
       list entry {
         must "derived-from-or-self(../rule-nature,
                                           'nature-compression')" {
           error-message "Rule nature must be compression";
         }
         key "field-id field-position direction-indicator";
         uses compression-rule-entry;
         description
           "A compression Rule is a list of Rule entries, each
            describing a header field.  An entry is identified
            through a field-id, its position in the packet, and
            its direction.";
       }
       description
         "Define a compression Rule composed of a list of entries.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     grouping fragmentation-content {
       description
         "This grouping defines the fragmentation parameters for
          all the modes (No ACK, ACK Always, and ACK on Error) specified
          in RFC 8724.";
       leaf fragmentation-mode {
         type schc:fragmentation-mode-type;
         must "derived-from-or-self(../rule-nature,
                                           'nature-fragmentation')" {
           error-message "Rule nature must be fragmentation";
         }
         mandatory true;
         description
           "Which fragmentation mode is used (No ACK, ACK Always, or
            ACK on Error).";
       }
       leaf l2-word-size {
         type uint8;
         default "8";
         description
           "Size, in bits, of the Layer 2 Word.";
       }
       leaf direction {
         type schc:di-type;
         must "derived-from-or-self(., 'di-up') or
               derived-from-or-self(., 'di-down')" {
           error-message
             "Direction for fragmentation Rules are up or down.";
         }
         mandatory true;
         description
           "MUST be up or down, bidirectional MUST NOT be used.";
       }
       // SCHC Frag header format
       leaf dtag-size {
         type uint8;
         default "0";
         description
           "Size, in bits, of the DTag field (T variable from
            RFC 8724).";
       }
       leaf w-size {
         when "derived-from-or-self(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from-or-self(../fragmentation-mode,
                                   'fragmentation-mode-ack-always') ";
         type uint8;
         description
           "Size, in bits, of the window field (M variable from
            RFC 8724).";
       }
       leaf fcn-size {
         type uint8;
         mandatory true;
         description
           "Size, in bits, of the FCN field (N variable from
            RFC 8724).";
       }
       leaf rcs-algorithm {
         type rcs-algorithm-type;
         default "schc:rcs-crc32";
         description
           "Algorithm used for RCS.  The algorithm specifies the RCS
            size.";
       }
       // SCHC fragmentation protocol parameters
       leaf maximum-packet-size {
         type uint16;
         default "1280";
         description
           "When decompression is done, packet size must not
            strictly exceed this limit, expressed in bytes.";
       }
       leaf window-size {
         type uint16;
         description
           "By default, if not specified, the FCN value is 2^w-size - 1.
            This value should not be exceeded.  Possible FCN values
            are between 0 and window-size - 1.";
       }
       leaf max-interleaved-frames {
         type uint8;
         default "1";
         description
           "Maximum of simultaneously fragmented frames.  Maximum value
            is 2^dtag-size.  All DTag values can be used, but more than
            max-interleaved-frames MUST NOT be active at any time.";
       }
       container inactivity-timer {
         leaf ticks-duration {
           type uint8;
           default "20";
           description
             "Duration of one tick in microseconds:
                 2^ticks-duration/10^6 = 1.048s.";
         }
         leaf ticks-numbers {
           type uint16 {
             range "0..max";
           }
           description
             "Timer duration = ticks-numbers*2^ticks-duration / 10^6.";
         }

         description
           "Duration in seconds of the Inactivity Timer; 0 indicates
            that the timer is disabled.

            Allows a precision from microsecond to year by sending the
            tick-duration value. For instance:

           tick-duration: smallest value   <-> highest value

           20: 00y 000d 00h 00m 01s.048575<->00y 000d 19h 05m 18s.428159
           21: 00y 000d 00h 00m 02s.097151<->00y 001d 14h 10m 36s.856319
           22: 00y 000d 00h 00m 04s.194303<->00y 003d 04h 21m 13s.712639
           23: 00y 000d 00h 00m 08s.388607<->00y 006d 08h 42m 27s.425279
           24: 00y 000d 00h 00m 16s.777215<->00y 012d 17h 24m 54s.850559
           25: 00y 000d 00h 00m 33s.554431<->00y 025d 10h 49m 49s.701119

            Note that the smallest value is also the incrementation
            step.";
       }
       container retransmission-timer {
         leaf ticks-duration {
           type uint8;
           default "20";
           description
             "Duration of one tick in microseconds:
                 2^ticks-duration/10^6 = 1.048s.";
         }
         leaf ticks-numbers {
           type uint16 {
             range "1..max";
           }
           description
             "Timer duration = ticks-numbers*2^ticks-duration / 10^6.";
         }
         when "derived-from-or-self(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from-or-self(../fragmentation-mode,
                                   'fragmentation-mode-ack-always') ";
         description
           "Duration in seconds of the Retransmission Timer.
            See the Inactivity Timer.";
       }
       leaf max-ack-requests {
         when "derived-from-or-self(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from-or-self(../fragmentation-mode,
                                   'fragmentation-mode-ack-always') ";
         type uint8 {
           range "1..max";
         }
         description
           "The maximum number of retries for a specific SCHC ACK.";
       }
       choice mode {
         case no-ack;
         case ack-always;
         case ack-on-error {
           leaf tile-size {
             when "derived-from-or-self(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type uint8;
             description
               "Size, in bits, of tiles.  If not specified or set to 0,
                tiles fill the fragment.";
           }
           leaf tile-in-all-1 {
             when "derived-from-or-self(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type schc:all-1-data-type;
             description
               "Defines whether the sender and receiver expect a tile in
                All-1 fragments or not, or if it is left to the sender's
                choice.";
           }
           leaf ack-behavior {
             when "derived-from-or-self(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type schc:ack-behavior-type;
             description
               "Sender behavior to acknowledge, after All-0 or All-1 or
                when the LPWAN allows it.";
           }
         }
         description
           "RFC 8724 defines 3 fragmentation modes.";
       }
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     // Define RuleID.  RuleID is composed of a RuleID value and a
     // RuleID length

     grouping rule-id-type {
       leaf rule-id-value {
         type uint32;
         description
           "RuleID value.  This value must be unique, considering its
            length.";
       }
       leaf rule-id-length {
         type uint8 {
           range "0..32";
         }
         description
           "RuleID length, in bits.  The value 0 is for implicit
            Rules.";
       }
       description
         "A RuleID is composed of a value and a length, expressed in
          bits.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }

     // SCHC table for a specific device.

     container schc {
       list rule {
         key "rule-id-value rule-id-length";
         uses rule-id-type;
         leaf rule-nature {
           type nature-type;
           mandatory true;
           description
             "Specify the Rule's nature.";
         }
         choice nature {
           case fragmentation {
             if-feature "fragmentation";
             uses fragmentation-content;
           }
           case compression {
             if-feature "compression";
             uses compression-content;
           }
           description
             "A Rule is for compression, for no-compression, or for
              fragmentation.";
         }
         description
           "Set of compression, no-compression, or fragmentation
            Rules identified by their rule-id.";
       }
       description
         "A SCHC set of Rules is composed of a list of Rules that are
          used for compression, no-compression, or fragmentation.";
       reference
         "RFC 8724 SCHC: Generic Framework for Static Context Header
                   Compression and Fragmentation";
     }
   }
   <CODE ENDS>

                       Figure 6: SCHC YANG Data Model

7.  IANA Considerations

   This document registers one URI and one YANG data model.

7.1.  URI Registration

   IANA registered the following URI in the "IETF XML Registry"
   [RFC3688]:

   URI:  urn:ietf:params:xml:ns:yang:ietf-schc
   Registrant Contact:  The IESG.
   XML:  N/A; the requested URI is an XML namespace.

7.2.  YANG Module Name Registration

   IANA has registered the following YANG data model in the "YANG Module
   Names" registry [RFC6020].

   name:  ietf-schc
   namespace:  urn:ietf:params:xml:ns:yang:ietf-schc
   prefix:  schc
   reference:  RFC 9363

8.  Security Considerations

   The YANG module specified in this document defines a schema for data
   that is designed to be accessed via network management protocols such
   as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF layer
   is the secure transport layer, and the mandatory-to-implement secure
   transport is Secure Shell (SSH) [RFC6242].  The lowest RESTCONF layer
   is HTTPS, and the mandatory-to-implement secure transport is TLS
   [RFC8446].

   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.

   There are a number of data nodes defined in this YANG module that are
   writable/creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability:

   /schc:  All the data nodes may be modified.  The Rule contains
      sensitive information, such as the application IPv6 address where
      the device's data will be sent after decompression.  An attacker
      may try to modify other devices' Rules by changing the application
      address and may block communication or allows traffic
      eavesdropping.  Therefore, a device must be allowed to modify only
      its own rules on the remote SCHC instance.  The identity of the
      requester must be validated.  This can be done through
      certificates or access lists.  Modification may be allowed
      regarding the Field Descriptor (i.e., IPv6 addresses field
      descriptors should not be modified, but UDP dev port could be
      changed).

   Some of the readable data nodes in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to these data nodes.  These are the subtrees and data
   nodes and their sensitivity/vulnerability:

   /schc:  By reading a module, an attacker may learn the traffic
      generated by a device and can also learn about application
      addresses or REST API.

9.  References

9.1.  Normative References

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

   [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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <https://www.rfc-editor.org/info/rfc7136>.

   [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>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

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

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

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [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>.

9.2.  Informative References

   [LPWAN-ARCH]
              Pelov, A., Thubert, P., and A. Minaburo, "LPWAN Static
              Context Header Compression (SCHC) Architecture", Work in
              Progress, Internet-Draft, draft-ietf-lpwan-architecture-
              02, 30 June 2022, <https://datatracker.ietf.org/doc/html/
              draft-ietf-lpwan-architecture-02>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/info/rfc7967>.

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
              <https://www.rfc-editor.org/info/rfc8376>.

   [RFC9011]  Gimenez, O., Ed. and I. Petrov, Ed., "Static Context
              Header Compression and Fragmentation (SCHC) over LoRaWAN",
              RFC 9011, DOI 10.17487/RFC9011, April 2021,
              <https://www.rfc-editor.org/info/rfc9011>.

Appendix A.  Example

   The informal Rules given Figure 7 are represented in XML, as shown in
   Figure 8.

   /-------------------------\
   |Rule 6/3            110  |
   |---------------+---+--+--+----------------+-------+----------------\
   |IPV6.VER       |  4| 1|BI|               6|EQUAL  |NOT-SENT        |
   |IPV6.TC        |  8| 1|BI|               0|EQUAL  |NOT-SENT        |
   |IPV6.FL        | 20| 1|BI|               0|IGNORE |NOT-SENT        |
   |IPV6.LEN       | 16| 1|BI|                |IGNORE |COMPUTE-LENGTH  |
   |IPV6.NXT       |  8| 1|BI|              58|EQUAL  |NOT-SENT        |
   |IPV6.HOP_LMT   |  8| 1|BI|             255|IGNORE |NOT-SENT        |
   |IPV6.DEV_PREFIX| 64| 1|BI|200104701f2101d2|EQUAL  |NOT-SENT        |
   |IPV6.DEV_IID   | 64| 1|BI|0000000000000003|EQUAL  |NOT-SENT        |
   |IPV6.APP_PREFIX| 64| 1|BI|                |IGNORE |VALUE-SENT      |
   |IPV6.APP_IID   | 64| 1|BI|                |IGNORE |VALUE-SENT      |
   \---------------+---+--+--+----------------+-------+----------------/
   /-------------------------\
   |Rule 12/11     00001100  |
   !=========================+=========================================\
   !^ Fragmentation mode : NoAck   header dtag 2 Window  0 FCN  3  UP ^!
   !^ No Tile size specified                                          ^!
   !^ RCS Algorithm: RCS_CRC32                                        ^!
   \===================================================================/
   /-------------------------\
   |Rule 100/8     01100100  |
   | NO-COMPRESSION RULE     |
   \-------------------------/

                          Figure 7: Rules Example

   <?xml version='1.0' encoding='UTF-8'?>
     <schc xmlns="urn:ietf:params:xml:ns:yang:ietf-schc">
     <rule>
       <rule-id-value>6</rule-id-value>
       <rule-id-length>3</rule-id-length>
       <rule-nature>nature-compression</rule-nature>
       <entry>
         <field-id>fid-ipv6-version</field-id>
         <field-length>4</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-equal</matching-operator>
         <comp-decomp-action>cda-not-sent</comp-decomp-action>
         <target-value>
           <index>0</index>
           <value>AAY=</value>
         </target-value>
       </entry>
       <entry>
         <field-id>fid-ipv6-trafficclass</field-id>
         <field-length>8</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-equal</matching-operator>
         <comp-decomp-action>cda-not-sent</comp-decomp-action>
         <target-value>
           <index>0</index>
           <value>AA==</value>
         </target-value>
       </entry>
       <entry>
         <field-id>fid-ipv6-flowlabel</field-id>
         <field-length>20</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-ignore</matching-operator>
         <comp-decomp-action>cda-not-sent</comp-decomp-action>
         <target-value>
           <index>0</index>
           <value>AA==</value>
         </target-value>
       </entry>
       <entry>
         <field-id>fid-ipv6-payload-length</field-id>
         <field-length>16</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-ignore</matching-operator>
         <comp-decomp-action>cda-compute</comp-decomp-action>
       </entry>
       <entry>
         <field-id>fid-ipv6-nextheader</field-id>
         <field-length>8</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-equal</matching-operator>
         <comp-decomp-action>cda-not-sent</comp-decomp-action>
         <target-value>
           <index>0</index>
           <value>ADo=</value>
         </target-value>
       </entry>
       <entry>
         <field-id>fid-ipv6-hoplimit</field-id>
         <field-length>8</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-ignore</matching-operator>
         <comp-decomp-action>cda-not-sent</comp-decomp-action>
         <target-value>
           <index>0</index>
           <value>AP8=</value>
         </target-value>
       </entry>
       <entry>
         <field-id>fid-ipv6-devprefix</field-id>
         <field-length>64</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-equal</matching-operator>
         <comp-decomp-action>cda-not-sent</comp-decomp-action>
         <target-value>
           <index>0</index>
           <value>IAEEcB8hAdI=</value>
         </target-value>
       </entry>
       <entry>
         <field-id>fid-ipv6-deviid</field-id>
         <field-length>64</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-equal</matching-operator>
         <comp-decomp-action>cda-not-sent</comp-decomp-action>
         <target-value>
           <index>0</index>
           <value>AAAAAAAAAAM=</value>
         </target-value>
       </entry>
       <entry>
         <field-id>fid-ipv6-appprefix</field-id>
         <field-length>64</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-ignore</matching-operator>
         <comp-decomp-action>cda-value-sent</comp-decomp-action>
       </entry>
       <entry>
         <field-id>fid-ipv6-appiid</field-id>
         <field-length>64</field-length>
         <field-position>1</field-position>
         <direction-indicator>di-bidirectional</direction-indicator>
         <matching-operator>mo-ignore</matching-operator>
         <comp-decomp-action>cda-value-sent</comp-decomp-action>
       </entry>
     </rule>
     <rule>
       <rule-id-value>12</rule-id-value>
       <rule-id-length>11</rule-id-length>
       <rule-nature>nature-fragmentation</rule-nature>
       <direction>di-up</direction>
       <rcs-algorithm>rcs-crc32</rcs-algorithm>
       <dtag-size>2</dtag-size>
       <fcn-size>3</fcn-size>
       <fragmentation-mode>
            fragmentation-mode-no-ack
       </fragmentation-mode>
     </rule>
     <rule>
       <rule-id-value>100</rule-id-value>
       <rule-id-length>8</rule-id-length>
       <rule-nature>nature-no-compression</rule-nature>
     </rule>
    </schc>

                 Figure 8: XML Representation of the Rules

Acknowledgments

   The authors would like to thank Dominique Barthel, Carsten Bormann,
   Ivan Martinez, and Alexander Pelov for their careful reading and
   valuable inputs.  A special thanks for Joe Clarke, Carl Moberg, Tom
   Petch, Martin Thomson, and Éric Vyncke for their explanations and
   wise advice when building the model.

Authors' Addresses

   Ana Minaburo
   Acklio
   1137A avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex
   France
   Email: ana@ackl.io


   Laurent Toutain
   Institut MINES TELECOM; IMT Atlantique
   2 rue de la Chataigneraie CS 17607
   35576 Cesson-Sevigne Cedex
   France
   Email: Laurent.Toutain@imt-atlantique.fr