lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: 20 November 2022         Institut MINES TELECOM; IMT Atlantique
                                                             19 May 2022


        Data Model for Static Context Header Compression (SCHC)
                draft-ietf-lpwan-schc-yang-data-model-10

Abstract

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

Status of This Memo

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

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 20 November 2022.

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   Copyright (c) 2022 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SCHC rules  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Compression Rules . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Identifier generation . . . . . . . . . . . . . . . . . .   4
     2.3.  Field Identifier  . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Field length  . . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Field position  . . . . . . . . . . . . . . . . . . . . .   8
     2.6.  Direction Indicator . . . . . . . . . . . . . . . . . . .   8
     2.7.  Target Value  . . . . . . . . . . . . . . . . . . . . . .  10
     2.8.  Matching Operator . . . . . . . . . . . . . . . . . . . .  10
       2.8.1.  Matching Operator arguments . . . . . . . . . . . . .  12
     2.9.  Compression Decompression Actions . . . . . . . . . . . .  12
       2.9.1.  Compression Decompression Action arguments  . . . . .  13
     2.10. Fragmentation rule  . . . . . . . . . . . . . . . . . . .  13
       2.10.1.  Fragmentation mode . . . . . . . . . . . . . . . . .  13
       2.10.2.  Fragmentation Header . . . . . . . . . . . . . . . .  14
       2.10.3.  Last fragment format . . . . . . . . . . . . . . . .  15
       2.10.4.  Acknowledgment behavior  . . . . . . . . . . . . . .  17
       2.10.5.  Fragmentation Parameters . . . . . . . . . . . . . .  18
       2.10.6.  Layer 2 parameters . . . . . . . . . . . . . . . . .  19
   3.  Rule definition . . . . . . . . . . . . . . . . . . . . . . .  19
     3.1.  Compression rule  . . . . . . . . . . . . . . . . . . . .  21
     3.2.  Fragmentation rule  . . . . . . . . . . . . . . . . . . .  24
     3.3.  YANG Tree . . . . . . . . . . . . . . . . . . . . . . . .  29
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   5.  Security considerations . . . . . . . . . . . . . . . . . . .  30
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   7.  YANG Module . . . . . . . . . . . . . . . . . . . . . . . . .  30
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  53
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

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 a non formal representation of the rules used
   either for compression/decompression (or C/D) or fragmentation/
   reassembly (or 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.

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



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   *  ...

   This document defines a YANG module to represent both compression and
   fragmentation rules, which leads to common representation for values
   for all the rules elements.

2.  SCHC rules

   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 a non formal representation of the rules used
   either for compression/decompression (or C/D) or fragmentation/
   reassembly (or 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.

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

   *  ...

   This document defines a YANG module to represent both compression and
   fragmentation rules, which leads to common representation for values
   for all the rules elements.

   SCHC compression is generic, the main mechanism does not refer to a
   specific protocol.  Any header field is abstracted through an ID, a
   position, a direction, and a value that can be a numerical value or a
   string.  [RFC8724] and [RFC8824] specify fields for IPv6, UDP, CoAP
   and OSCORE.

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

   The YANG model allows to select the compression or the fragmentation
   using the feature command.












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

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

            Figure 1: Feature for compression and fragmentation.

2.1.  Compression Rules

   [RFC8724] proposes a non formal 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 2: Compression Decompression Context

2.2.  Identifier generation

   Identifier used in the SCHC YANG Data Model are from the identityref
   statement to ensure to be globally unique and be easily augmented 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.




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   *  derive all the identities used in the Data Model from this base
      type.

   *  create a typedef from this base type.

   The example (Figure 3) shows how an identityref is created for RCS
   algorithms used during SCHC fragmentation.

    // -- 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.";
     }

     identity rcs-RFC8724 {
       base rcs-algorithm-base-type;
       description
         "CRC 32 defined as default RCS in RFC8724. RCS is 4 byte-long";
     }

     typedef rcs-algorithm-type {
       type identityref {
         base rcs-algorithm-base-type;
       }
       description
         "type used in rules.";
     }

         Figure 3: Principle to define a type based on identityref.

2.3.  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,...).

   The current YANG Data Model includes fields definitions found in
   [RFC8724], [RFC8824].

   Using the YANG model, each field MUST be identified through a global
   YANG identityref.  A YANG field ID for the protocol always derives
   from the fid-base-type.  Then an identity for each protocol is
   specified using the naming convention fid-<<protocol name>>-base-



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   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
   sub-fields, the field identity serves as a base.

   The full field-id definition is found in Section 7.  The example
   Figure 4 gives the first field ID definitions.  A type is defined for
   IPv6 protocol, and each field is based on it.  Note that the DiffServ
   bits derives from the Traffic Class identity.

     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";
     }

     identity fid-ipv6-version {
       base fid-ipv6-base-type;
       description
         "IPv6 version field from RFC8200";
     }

     identity fid-ipv6-trafficclass {
       base fid-ipv6-base-type;
       description
         "IPv6 Traffic Class field from RFC8200";
     }

     identity fid-ipv6-trafficclass-ds {
       base fid-ipv6-trafficclass;
       description
         "IPv6 Traffic Class field from RFC8200,
          DiffServ field from RFC3168";
     }
     ...

             Figure 4: Definition of identityref for field IDs

   The type associated to this identity is fid-type (cf.  Figure 5)







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     typedef fid-type {
       type identityref {
         base fid-base-type;
       }
       description
         "Field ID generic type.";
     }

                  Figure 5: Type definition for field IDs

2.4.  Field length

   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.

     identity fl-base-type {
       description
         "Used to extend field length functions.";
     }

     identity fl-variable {
       base fl-base-type;
       description
         "Residue length in Byte is sent as defined
          for CoAP in RFC 8824 (cf. 5.3).";
     }

     identity fl-token-length {
       base fl-base-type;
       description
         "Residue length in Byte is sent as defined
          for CoAP in RFC 8824 (cf. 4.5).";
     }

            Figure 6: Definition of identityref for Field Length

   The field length function can be defined as an identityref as shown
   in Figure 6.

   Therefore, the type for field length is a union between an integer
   giving in bits the size of the length and the identityref (cf.
   Figure 7).




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     typedef fl-type {
       type union {
         type int64; /* positive integer, expressing length in bits */
         type identityref { /* function */
           base fl-base-type;
         }
       }
       description
         "Field length either a positive integer expressing the size in
          bits or a function defined through an identityref.";
     }

                 Figure 7: Type definition for field Length

2.5.  Field position

   Field position is a positive integer which gives the position of a
   field, the default value is 1, and incremented at each repetition.
   value 0 indicates that the position is not important and is not
   considered during the rule selection process.

   Field position is a positive integer.  The type is an uint8.

2.6.  Direction Indicator

   The Direction Indicator (di) is used to tell if a field appears in
   both direction (Bi) or only uplink (Up) or Downlink (Dw).
























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     identity di-base-type {
       description
         "Used to extend direction indicators.";
     }

     identity di-bidirectional {
       base di-base-type;
       description
         "Direction Indication of bidirectionality in
          RFC 8724 (cf. 7.1).";
     }

     identity di-up {
       base di-base-type;
       description
         "Direction Indication of uplink defined in
          RFC 8724 (cf. 7.1).";
     }

     identity di-down {
       base di-base-type;
       description
         "Direction Indication of downlink defined in
          RFC 8724 (cf. 7.1).";
     }

        Figure 8: Definition of identityref for direction indicators

   Figure 8 gives the identityref for Direction Indicators.  The naming
   convention is "di" followed by the Direction Indicator name.

   The type is "di-type" (cf.  Figure 9).

     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, bi when emitted or
          received by the device.";
     }

             Figure 9: Type definition for direction indicators







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2.7.  Target Value

   The Target Value is a list of binary sequences of any length, aligned
   to the left.  Figure 10 shows the definition of a single element of a
   Target Value.  In the rule, the structure will be used as a list,
   with index as a key.  The highest index value is used to compute the
   size of the index sent in residue for the match-mapping CDA.  The
   index allows to specify several values:

   *  For Equal and LSB, Target Value contains a single element.
      Therefore, the index is set to 0.

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

     grouping tv-struct {
       description
         "Defines the target value element. Always a binary type,
          strings must be converted to binary. field-id allows the
          conversion to the appropriate type.";
       leaf value {
         type binary;
         description
           "Target Value";
       }
       leaf index {
         type uint16;
         description
           "Index gives the position in the matching-list. If only one
            element is present, index is 0. Otherwise, indicia is the
            the order in the matching list, starting at 0.";
       }
     }

                   Figure 10: Definition of target value

2.8.  Matching Operator

   Matching Operator (MO) is a function applied between a field value
   provided by the parsed header and the target value.  [RFC8724]
   defines 4 MO as listed in Figure 11.










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     identity mo-base-type {
       description
         "Used to extend Matching Operators with SID values";
     }

     identity mo-equal {
       base mo-base-type;
       description
         "Equal MO as defined in RFC 8724 (cf. 7.3)";
     }

     identity mo-ignore {
       base mo-base-type;
       description
         "Ignore MO as defined in RFC 8724 (cf. 7.3)";
     }

     identity mo-msb {
       base mo-base-type;
       description
         "MSB MO as defined in RFC 8724 (cf. 7.3)";
     }

     identity mo-match-mapping {
       base mo-base-type;
       description
         "match-mapping MO as defined in RFC 8724 (cf. 7.3)";
     }

         Figure 11: Definition of identityref for Matching Operator

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

   The type is "mo-type" (cf.  Figure 12)

     typedef mo-type {
       type identityref {
         base mo-base-type;
       }
       description
         "Matching Operator (MO) to compare fields values with
          target values";
     }

              Figure 12: Type definition for Matching Operator






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2.8.1.  Matching Operator arguments

   They are viewed as a list, built with a tv-struct (see chapter
   Section 2.7).

2.9.  Compression Decompression Actions

   Compression Decompression Action (CDA) identifies the function to use
   for compression or decompression.  [RFC8724] defines 6 CDA.

   Figure 14 shows some CDA definition, the full definition is in
   Section 7.

     identity cda-base-type {
       description
         "Compression Decompression Actions.";
     }

     identity cda-not-sent {
       base cda-base-type;
       description
         "not-sent CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-value-sent {
       base cda-base-type;
       description
         "value-sent CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-lsb {
       base cda-base-type;
       description
         "LSB CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-mapping-sent {
       base cda-base-type;
       description
         "mapping-sent CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-compute {
       base cda-base-type;
       description
         "compute-* CDA as defined in RFC 8724 (cf. 7.4)";
     }
       ....



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     Figure 13: Definition of identityref for Compresion Decompression
                                   Action

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

     typedef cda-type {
       type identityref {
         base cda-base-type;
       }
       description
         "Compression Decompression Action to compression or
          decompress a field.";
     }

       Figure 14: Type definition for Compresion Decompression Action

2.9.1.  Compression Decompression Action arguments

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

2.10.  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 Annex 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.

2.10.1.  Fragmentation mode

   [RFC8724] defines 3 fragmentation modes:

   *  No Ack: this mode is unidirectionnal, 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.





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   Figure 15 shows the definition for identifiers from these three
   modes.

     identity fragmentation-mode-base-type {
       description
         "fragmentation mode.";
     }

     identity fragmentation-mode-no-ack {
       base fragmentation-mode-base-type;
       description
         "No-ACK of RFC8724.";
     }

     identity fragmentation-mode-ack-always {
       base fragmentation-mode-base-type;
       description
         "ACK-Always of RFC8724.";
     }

     identity fragmentation-mode-ack-on-error {
       base fragmentation-mode-base-type;
       description
         "ACK-on-Error of RFC8724.";
     }

     typedef fragmentation-mode-type {
       type identityref {
         base fragmentation-mode-base-type;
       }
       description
         "type used in rules";
     }

           Figure 15: Definition of fragmentation mode identifer

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

2.10.2.  Fragmentation Header

   A data fragment header, starting with the rule ID can be sent on the
   fragmentation direction.  The SCHC header may be composed of (cf.
   Figure 16):

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



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   *  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 on the window.  This field is mandatory on all modes
      defined in [RFC8724], its size is defined by the rule.

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

                Figure 16: Data fragment header from RFC8724

2.10.3.  Last fragment format

   The last fragment of a datagram is sent with an RCS (Reassembly Check
   Sequence) field to detect residual transmission error and possible
   losses in the last window.  [RFC8724] defines a single algorithm
   based on Ethernet CRC computation.  The identity of the RCS algorithm
   is shown in Figure 17.

     identity rcs-algorithm-base-type {
       description
         "Identify which algorithm is used to compute RCS.
          The algorithm also defines the size of the RCS field.";
     }

     identity rcs-RFC8724 {
       base rcs-algorithm-base-type;
       description
         "CRC 32 defined as default RCS in RFC8724. RCS is 4 byte-long";
     }

     typedef rcs-algorithm-type {
       type identityref {
         base rcs-algorithm-base-type;
       }
       description
         "type used in rules.";
     }

                     Figure 17: type definition for RCS

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




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   For Ack-on-Error mode, the All-1 fragment may just contain the RCS or
   can include a tile.  The parameters defined in Figure 18 allows to
   define the behavior:

   *  all1-data-no: the last fragment contains no data, just the RCS

   *  all1-data-yes: the last fragment includes a single tile and the
      RCS

   *  all1-data-sender-choice: the last fragment may or may not contain
      a single tile.  The receiver can detect if a tile is present.

     identity all1-data-base-type {
       description
         "Type to define when to send an Acknowledgment message.";
     }

     identity all1-data-no {
       base all1-data-base-type;
       description
         "All1 contains no tiles.";
     }

     identity all1-data-yes {
       base all1-data-base-type;
       description
         "All1 MUST contain a tile.";
     }

     identity all1-data-sender-choice {
       base all1-data-base-type;
       description
         "Fragmentation process chooses to send tiles or not in all1.";
     }

     typedef all1-data-type {
       type identityref {
         base all1-data-base-type;
       }
       description
         "Type used in rules.";
     }

                     Figure 18: type definition for RCS

   The naming convention is "all1-data" followed by the behavior
   identifier.




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2.10.4.  Acknowledgment behavior

   The acknowledgment fragment header goes in the opposite direction of
   data.  The header is composed of (see Figure 19):

   *  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 19: Acknowledgment fragment header for RFC8724

   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 following identifiers (cf.  Figure 20) define the
   acknowledgment behavior.






















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     identity ack-behavior-base-type {
       description
         "Define when to send an Acknowledgment .";
     }

     identity ack-behavior-after-All0 {
       base ack-behavior-base-type;
       description
         "Fragmentation expects Ack after sending All0 fragment.";
     }

     identity ack-behavior-after-All1 {
       base ack-behavior-base-type;
       description
         "Fragmentation expects Ack after sending All1 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
         "Type used in rules.";
     }

                   Figure 20: bitmap generation behavior

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

2.10.5.  Fragmentation Parameters

   The state machine requires some common values to handle
   fragmentation:

   *  retransmission-timer expresses, in seconds, the duration before
      sending an ack request (cf. section 8.2.2.4. of [RFC8724]).  If
      specified, value must be higher or equal to 1.







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   *  inactivity-timer expresses, in seconds, 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.

   *  max-ack-requests expresses the number of attempts before aborting
      (cf. section 8.2.2.4. of [RFC8724]).

   *  maximum-packet-size rexpresses, in bytes, the larger packet size
      that can be reassembled.

   They are defined as unsigned integers, see Section 7.

2.10.6.  Layer 2 parameters

   The data model includes two parameters needed for fragmentation:

   *  l2-word-size: [RFC8724] base fragmentation on a layer 2 word which
      can be of any length.  The default value is 8 and correspond to
      the default value for 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 a uncompressed
      datagram.  By default, the value is set to 1280 bytes.

   They are defined as unsigned integer, see Section 7.

3.  Rule definition

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

   Three types of rules are defined in [RFC8724]:

   *  Compression: a compression rule is associated with the rule ID.

   *  No compression: this identifies the default rule used to send a
      packet in extenso when no compression rule was found (see
      [RFC8724] section 6).

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






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     grouping rule-id-type {
       leaf rule-id-value {
         type uint32;
         description
           "Rule ID value, this value must be unique, considering its
            length.";
       }
       leaf rule-id-length {
         type uint8 {
           range "0..32";
         }
         description
           "Rule ID length, in bits. The value 0 is for implicit
            rules.";
       }
       description
         "A rule ID is composed of a value and a length, expressed in
          bits.";
     }

     // SCHC table for a specific device.

     container schc {
       list rule {
         key "rule-id-value rule-id-length";
         uses rule-id-type;
         choice nature {
           case fragmentation {
             if-feature "fragmentation";
             uses fragmentation-content;
           }
           case compression {
             if-feature "compression";
             uses compression-content;
           }
           case no-compression {
             description
               "RFC8724 requires a rule for uncompressed headers.";
           }
           description
             "A rule is for compression, for no-compression or for
              fragmentation.";
         }
         description
           "Set of rules compression, no compression or fragmentation
            rules identified by their rule-id.";
       }
       description



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         "a SCHC set of rules is composed of a list of rules which are
          used for compression, no-compression or fragmentation.";
     }
   }

                  Figure 21: Definition of a SCHC Context

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

3.1.  Compression rule

   A compression rule is composed of entries describing its processing
   (cf.  Figure 22).  An entry contains all the information defined in
   Figure 2 with the types defined above.

   The compression rule described Figure 2 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 represent a line of the table Figure 2.  Their type reflects
   the identifier types defined in Section 2.1

   Some checks are performed on the values:

   *  target value must be present for MO different from ignore.

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

     grouping compression-rule-entry {
       description
         "These entries defines 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. The content
            is a YANG identifer.
          - Field Length : either a positive integer of a function
            defined as a YANG id.
          - Field Position: a positive (and possibly equal to 0)
            integer.
          - Direction Indicator: a YANG identifier giving the direction.
          - Target value: a value against which the header Field is
            compared.



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          - Matching Operator: a YANG id giving the operation,
            parameters may be associated to that operator.
          - Comp./Decomp. Action: A YANG id giving the compression or
            decompression action, parameters may be associated to that
            action.
         ";
       leaf field-id {
         type schc:fid-type;
         mandatory true;
         description
           "Field ID, identify a field in the header with a YANG
            referenceid.";
       }
       leaf field-length {
         type schc:fl-type;
         mandatory true;
         description
           "Field Length, expressed in number of bits or through a
            function defined as a YANG referenceid.";
       }
       leaf field-position {
         type uint8;
         mandatory true;
         description
           "Field position in the header is an integer. Position 1
            matches the first occurence of a field in the header,
            while incremented position values match subsequent
            occurences.
            Position 0 means that this entry matches a field
            irrespective of its position of occurence 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, a YANG referenceid to say if the packet
            is bidirectional, up or down";
       }
       list target-value {
         key "index";
         uses tv-struct;
         description
           "A list of value to compare with the header field value.
            If target value is a singleton, position must be 0.



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            For use as a matching list for the mo-match-mapping matching
            operator, positions should take consecutive values starting
            from 1.";
       }
       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";
       }
       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;
         mandatory true;
         description
           "CDA: Compression Decompression Action.";
       }
       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.";
       }
     }

     grouping compression-content {



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       list entry {
         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 identifed
            through a field-id, its position in the packet and
            its direction.";
       }
       description
         "Define a compression rule composed of a list of entries.";
     }

                Figure 22: Definition of a compression entry

3.2.  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 2.10.

   The definition of a Fragmentation rule is divided into three sub-
   parts (cf.  Figure 24):

   *  parameters such as the fragmentation-mode, the l2-word-size and
      the direction.  Since Fragmentation rules are always defined for a
      specific direction, the value must be either di-up or di-down (di-
      bidirectional is not allowed).

   *  parameters defining the Fragmentation header format (dtag-size,
      w-size, fcn-size and rcs-algorithm).

   *  Protocol parameters for timers (inactivity-timer, retransmission-
      timer).  [RFC8724] do not specified any range for these timers.
      [RFC9011] recommends a duration of 12 hours.  In fact, the value
      range sould be between milli-seconds for real time systems to
      several days.  Figure 23 shows the two parameters defined for
      timers:

      -  the duration of a tick is computed through this formula 2^tick-
         duration/10^6.  When tick-duration is set to 0, the unit is the
         micro-second.  The default value of 20 leads to a unit of about
         1.05 second.  A value of 32 leads to a tick duration of about
         1.19 hours.

      -  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 sec and 19 hours covering [RFC9011] recommandation.



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   *  Protocol behavior (maximum-packet-size, max-interleaved-frames,
      max-ack-requests).  If these parameters are specific to a single
      fragmentation mode, they are grouped in a structure dedicated to
      that Fragmentation mode.  If some parameters can be found in
      several modes, typically ACK-Always and ACK-on-Error, they are
      defined in a common part and a when statement indicates which
      modes are allowed.

     grouping timer-duration {
       leaf ticks-duration {
         type uint8;
         default "20";
         description
           "duration of one tick in micro-seconds:
               2^ticks-duration/10^6 = 1.048s";
       }
       leaf ticks-numbers {
         type uint16;
         description
           "timer duration = ticks-numbers * 2^ticks / 10^6";
       }
       description
         "used by inactivity and retransmission timer. Allows a
          precision from micro-second to year by sending the
          tick-duration value.
          For instance:

          tick-duration /  smallest value          highest value
          v
          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,
          so the timer precision.
         ";
     }

                      Figure 23: Timer duration values









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     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;
         mandatory true;
         description
           "which fragmentation mode is used (noAck, AckAlways,
            AckonError)";
       }
       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
           "Should be up or down, bidirectionnal is forbiden.";
       }
       // SCHC Frag header format
       leaf dtag-size {
         type uint8;
         default "0";
         description
           "Size, in bits, of the DTag field (T variable from
            RFC8724).";
       }
       leaf w-size {
         when "derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-always') ";
         type uint8;
         description
           "Size, in bits, of the window field (M variable from
            RFC8724).";
       }



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       leaf fcn-size {
         type uint8;
         mandatory true;
         description
           "Size, in bits, of the FCN field (N variable from RFC8724).";
       }
       leaf rcs-algorithm {
         type rcs-algorithm-type;
         default "schc:rcs-RFC8724";
         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 2^w-size - 1. Should not exceed
            this value. 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 at most
            max-interleaved-frames must be active at any time.";
       }
       container inactivity-timer {
         uses timer-duration;
         description
           "Duration is seconds of the inactivity timer, 0 indicates
            that the timer is disabled.";
       }
       container retransmission-timer {
         uses timer-duration;
         when "derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from(../fragmentation-mode,



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                                   'fragmentation-mode-ack-always') ";
         description
           "Duration in seconds of the retransmission timer.";
       }
       leaf max-ack-requests {
         when "derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from(../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(../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-All1 {
             when "derived-from(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type schc:all1-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(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type schc:ack-behavior-type;
             description
               "Sender behavior to acknowledge, after All-0, All-1 or
                when the LPWAN allows it.";
           }
         }
         description
           "RFC 8724 defines 3 fragmentation modes.";



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       }
     }

                    Figure 24: Fragmentation Parameters

3.3.  YANG Tree

module: ietf-schc
 +-rw schc
   +-rw rule* [rule-id-value rule-id-length]
      +--rw rule-id-value                   uint32
      +--rw rule-id-length                  uint8
      +--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-All1?       schc:all1-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 value?   binary
         |     |  +--rw index    uint16
         |     +--rw matching-operator           schc:mo-type



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         |     +--rw matching-operator-value* [index]
         |     |  +--rw value?   binary
         |     |  +--rw index    uint16
         |     +--rw comp-decomp-action          schc:cda-type
         |     +--rw comp-decomp-action-value* [index]
         |        +--rw value?   binary
         |        +--rw index    uint16
         +--:(no-compression)

                              Figure 25

4.  IANA Considerations

   This document has no request to IANA.

5.  Security considerations

   This document does not have any more Security consideration than the
   ones already raised in [RFC8724] and [RFC8824].

6.  Acknowledgements

   The authors would like to thank Dominique Barthel, Carsten Bormann,
   Alexander Pelov for their careful reading and valuable inputs.  A
   special thanks for Carl Moberg for his patience and wise advices when
   building the model.

7.  YANG Module

   <code begins> file ietf-schc@2022-02-15.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:p-wan@ietf.org>
        Editor:   Laurent Toutain
          <mailto:laurent.toutain@imt-atlantique.fr>
        Editor:   Ana Minaburo
          <mailto:ana@ackl.io>";
     description
       "
        Copyright (c) 2021 IETF Trust and the persons identified as



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        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 Simplified 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 XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); 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 Static Context Header Compression Rule
        for SCHC, based on RFC 8724 and RFC8824. Include compression,
        no compression and fragmentation rules.

        This module is a YANG model for SCHC rules (RFC 8724 and
        RFC8824). RFC 8724 describes compression rules in a 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 proposes a global data model that can be used for
        rule exchanges or modification. It proposes 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.";




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     revision 2022-02-15 {
       description
         "Initial version from RFC XXXX ";
       reference
         "RFC XXX: 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";
     }

     identity fid-ipv6-version {
       base fid-ipv6-base-type;
       description
         "IPv6 version field from RFC8200";
     }

     identity fid-ipv6-trafficclass {
       base fid-ipv6-base-type;
       description
         "IPv6 Traffic Class field from RFC8200";
     }

     identity fid-ipv6-trafficclass-ds {
       base fid-ipv6-trafficclass;



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       description
         "IPv6 Traffic Class field from RFC8200,
          DiffServ field from RFC3168";
     }

     identity fid-ipv6-trafficclass-ecn {
       base fid-ipv6-trafficclass;
       description
         "IPv6 Traffic Class field from RFC8200,
          ECN field from RFC3168";
     }

     identity fid-ipv6-flowlabel {
       base fid-ipv6-base-type;
       description
         "IPv6 Flow Label field from RFC8200";
     }

     identity fid-ipv6-payloadlength {
       base fid-ipv6-base-type;
       description
         "IPv6 Payload Length field from RFC8200";
     }

     identity fid-ipv6-nextheader {
       base fid-ipv6-base-type;
       description
         "IPv6 Next Header field from RFC8200";
     }

     identity fid-ipv6-hoplimit {
       base fid-ipv6-base-type;
       description
         "IPv6 Next Header field from RFC8200";
     }

     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 if it is
                 respectively an uplink or a downlink message.";
     }

     identity fid-ipv6-deviid {
       base fid-ipv6-base-type;
       description
         "corresponds to either the source address or the destination



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          address prefix of RFC 8200. Depending if it is respectively
          an uplink or a downlink message.";
     }

     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 if it is respectively
          a downlink or an uplink message.";
     }

     identity fid-ipv6-appiid {
       base fid-ipv6-base-type;
       description
         "corresponds to either the source address or the destination
          address prefix of RFC 8200. Depending if it is respectively
          a downlink or an uplink message.";
     }

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

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

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

     identity fid-udp-length {
       base fid-udp-base-type;
       description
         "UDP length from RFC 768";
     }

     identity fid-udp-checksum {
       base fid-udp-base-type;



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       description
         "UDP length from RFC 768";
     }

     identity fid-coap-base-type {
       base fid-base-type;
       description
         "Field ID base type for UDP headers described in RFC 7252";
     }

     identity fid-coap-version {
       base fid-coap-base-type;
       description
         "CoAP version from RFC 7252";
     }

     identity fid-coap-type {
       base fid-coap-base-type;
       description
         "CoAP type from RFC 7252";
     }

     identity fid-coap-tkl {
       base fid-coap-base-type;
       description
         "CoAP token length from RFC 7252";
     }

     identity fid-coap-code {
       base fid-coap-base-type;
       description
         "CoAP code from RFC 7252";
     }

     identity fid-coap-code-class {
       base fid-coap-code;
       description
         "CoAP code class from RFC 7252";
     }

     identity fid-coap-code-detail {
       base fid-coap-code;
       description
         "CoAP code detail from RFC 7252";
     }

     identity fid-coap-mid {
       base fid-coap-base-type;



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       description
         "CoAP message ID from RFC 7252";
     }

     identity fid-coap-token {
       base fid-coap-base-type;
       description
         "CoAP token from RFC 7252";
     }

     identity fid-coap-option-if-match {
       base fid-coap-base-type;
       description
         "CoAP option If-Match from RFC 7252";
     }

     identity fid-coap-option-uri-host {
       base fid-coap-base-type;
       description
         "CoAP option URI-Host from RFC 7252";
     }

     identity fid-coap-option-etag {
       base fid-coap-base-type;
       description
         "CoAP option Etag from RFC 7252";
     }

     identity fid-coap-option-if-none-match {
       base fid-coap-base-type;
       description
         "CoAP option if-none-match from RFC 7252";
     }

     identity fid-coap-option-observe {
       base fid-coap-base-type;
       description
         "CoAP option Observe from RFC 7641";
     }

     identity fid-coap-option-uri-port {
       base fid-coap-base-type;
       description
         "CoAP option Uri-Port from RFC 7252";
     }

     identity fid-coap-option-location-path {
       base fid-coap-base-type;



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       description
         "CoAP option Location-Path from RFC 7252";
     }

     identity fid-coap-option-uri-path {
       base fid-coap-base-type;
       description
         "CoAP option Uri-Path from RFC 7252";
     }

     identity fid-coap-option-content-format {
       base fid-coap-base-type;
       description
         "CoAP option Content Format from RFC 7252";
     }

     identity fid-coap-option-max-age {
       base fid-coap-base-type;
       description
         "CoAP option Max-Age from RFC 7252";
     }

     identity fid-coap-option-uri-query {
       base fid-coap-base-type;
       description
         "CoAP option Uri-Query from RFC 7252";
     }

     identity fid-coap-option-accept {
       base fid-coap-base-type;
       description
         "CoAP option Accept from RFC 7252";
     }

     identity fid-coap-option-location-query {
       base fid-coap-base-type;
       description
         "CoAP option Location-Query from RFC 7252";
     }

     identity fid-coap-option-block2 {
       base fid-coap-base-type;
       description
         "CoAP option Block2 from RFC 7959";
     }

     identity fid-coap-option-block1 {
       base fid-coap-base-type;



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       description
         "CoAP option Block1 from RFC 7959";
     }

     identity fid-coap-option-size2 {
       base fid-coap-base-type;
       description
         "CoAP option size2 from RFC 7959";
     }

     identity fid-coap-option-proxy-uri {
       base fid-coap-base-type;
       description
         "CoAP option Proxy-Uri from RFC 7252";
     }

     identity fid-coap-option-proxy-scheme {
       base fid-coap-base-type;
       description
         "CoAP option Proxy-scheme from RFC 7252";
     }

     identity fid-coap-option-size1 {
       base fid-coap-base-type;
       description
         "CoAP option Size1 from RFC 7252";
     }

     identity fid-coap-option-no-response {
       base fid-coap-base-type;
       description
         "CoAP option No response from RFC 7967";
     }

     identity fid-coap-option-oscore-flags {
       base fid-coap-base-type;
       description
         "CoAP option oscore flags (see RFC 8824, section 6.4)";
     }

     identity fid-coap-option-oscore-piv {
       base fid-coap-base-type;
       description
         "CoAP option oscore flags (see RFC 8824, section 6.4)";
     }

     identity fid-coap-option-oscore-kid {
       base fid-coap-base-type;



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       description
         "CoAP option oscore flags (see RFC 8824, section 6.4)";
     }

     identity fid-coap-option-oscore-kidctx {
       base fid-coap-base-type;
       description
         "CoAP option oscore flags (see RFC 8824, 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 Byte is sent as defined
          for CoAP in RFC 8824 (cf. 5.3).";
     }

     identity fl-token-length {
       base fl-base-type;
       description
         "Residue length in Byte is sent as defined
          for CoAP in RFC 8824 (cf. 4.5).";
     }

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

     identity di-base-type {
       description
         "Used to extend direction indicators.";
     }

     identity di-bidirectional {
       base di-base-type;
       description
         "Direction Indication of bidirectionality in
          RFC 8724 (cf. 7.1).";
     }



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     identity di-up {
       base di-base-type;
       description
         "Direction Indication of uplink defined in
          RFC 8724 (cf. 7.1).";
     }

     identity di-down {
       base di-base-type;
       description
         "Direction Indication of downlink defined in
          RFC 8724 (cf. 7.1).";
     }

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

     identity mo-base-type {
       description
         "Used to extend Matching Operators with SID values";
     }

     identity mo-equal {
       base mo-base-type;
       description
         "Equal MO as defined in RFC 8724 (cf. 7.3)";
     }

     identity mo-ignore {
       base mo-base-type;
       description
         "Ignore MO as defined in RFC 8724 (cf. 7.3)";
     }

     identity mo-msb {
       base mo-base-type;
       description
         "MSB MO as defined in RFC 8724 (cf. 7.3)";
     }

     identity mo-match-mapping {
       base mo-base-type;
       description
         "match-mapping MO as defined in RFC 8724 (cf. 7.3)";
     }

     //------------------------------



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

     identity cda-base-type {
       description
         "Compression Decompression Actions.";
     }

     identity cda-not-sent {
       base cda-base-type;
       description
         "not-sent CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-value-sent {
       base cda-base-type;
       description
         "value-sent CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-lsb {
       base cda-base-type;
       description
         "LSB CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-mapping-sent {
       base cda-base-type;
       description
         "mapping-sent CDA as defined in RFC 8724 (cf. 7.4).";
     }

     identity cda-compute {
       base cda-base-type;
       description
         "compute-length CDA as defined in RFC 8724 (cf. 7.4)";
     }

     identity cda-deviid {
       base cda-base-type;
       description
         "deviid CDA as defined in RFC 8724 (cf. 7.4)";
     }

     identity cda-appiid {
       base cda-base-type;
       description
         "appiid CDA as defined in RFC 8724 (cf. 7.4)";



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     }

     // -- type definition

     typedef fid-type {
       type identityref {
         base fid-base-type;
       }
       description
         "Field ID generic type.";
     }

     typedef fl-type {
       type union {
         type int64; /* positive integer, expressing length in bits */
         type identityref { /* function */
           base fl-base-type;
         }
       }
       description
         "Field length either a positive integer expressing the size in
          bits or a function defined through an identityref.";
     }

     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, bi when emitted or
          received by the device.";
     }

     typedef mo-type {
       type identityref {
         base mo-base-type;
       }
       description
         "Matching Operator (MO) to compare fields values with
          target values";
     }

     typedef cda-type {
       type identityref {
         base cda-base-type;
       }
       description



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         "Compression Decompression Action to compression or
          decompress a field.";
     }

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

     identity fragmentation-mode-base-type {
       description
         "fragmentation mode.";
     }

     identity fragmentation-mode-no-ack {
       base fragmentation-mode-base-type;
       description
         "No-ACK of RFC8724.";
     }

     identity fragmentation-mode-ack-always {
       base fragmentation-mode-base-type;
       description
         "ACK-Always of RFC8724.";
     }

     identity fragmentation-mode-ack-on-error {
       base fragmentation-mode-base-type;
       description
         "ACK-on-Error of RFC8724.";
     }

     typedef fragmentation-mode-type {
       type identityref {
         base fragmentation-mode-base-type;
       }
       description
         "type used in rules";
     }

     // -- Ack behavior

     identity ack-behavior-base-type {
       description
         "Define when to send an Acknowledgment .";
     }

     identity ack-behavior-after-All0 {
       base ack-behavior-base-type;
       description



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         "Fragmentation expects Ack after sending All0 fragment.";
     }

     identity ack-behavior-after-All1 {
       base ack-behavior-base-type;
       description
         "Fragmentation expects Ack after sending All1 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
         "Type used in rules.";
     }

     // -- All1 with data types

     identity all1-data-base-type {
       description
         "Type to define when to send an Acknowledgment message.";
     }

     identity all1-data-no {
       base all1-data-base-type;
       description
         "All1 contains no tiles.";
     }

     identity all1-data-yes {
       base all1-data-base-type;
       description
         "All1 MUST contain a tile.";
     }

     identity all1-data-sender-choice {
       base all1-data-base-type;
       description
         "Fragmentation process chooses to send tiles or not in all1.";
     }




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     typedef all1-data-type {
       type identityref {
         base all1-data-base-type;
       }
       description
         "Type used 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.";
     }

     identity rcs-RFC8724 {
       base rcs-algorithm-base-type;
       description
         "CRC 32 defined as default RCS in RFC8724. RCS is 4 byte-long";
     }

     typedef rcs-algorithm-type {
       type identityref {
         base rcs-algorithm-base-type;
       }
       description
         "type used in rules.";
     }

     // --------- TIMER DURATION -------------------

     grouping timer-duration {
       leaf ticks-duration {
         type uint8;
         default "20";
         description
           "duration of one tick in micro-seconds:
               2^ticks-duration/10^6 = 1.048s";
       }
       leaf ticks-numbers {
         type uint16;
         description
           "timer duration = ticks-numbers * 2^ticks / 10^6";
       }
       description
         "used by inactivity and retransmission timer. Allows a
          precision from micro-second to year by sending the



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          tick-duration value.
          For instance:

          tick-duration /  smallest value          highest value
          v
          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,
          so the timer precision.
         ";
     }

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

     grouping tv-struct {
       description
         "Defines the target value element. Always a binary type,
          strings must be converted to binary. field-id allows the
          conversion to the appropriate type.";
       leaf value {
         type binary;
         description
           "Target Value";
       }
       leaf index {
         type uint16;
         description
           "Index gives the position in the matching-list. If only one
            element is present, index is 0. Otherwise, indicia is the
            the order in the matching list, starting at 0.";
       }
     }

     grouping compression-rule-entry {
       description
         "These entries defines 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:



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          - Field ID: The header field to be compressed. The content
            is a YANG identifer.
          - Field Length : either a positive integer of a function
            defined as a YANG id.
          - Field Position: a positive (and possibly equal to 0)
            integer.
          - Direction Indicator: a YANG identifier giving the direction.
          - Target value: a value against which the header Field is
            compared.
          - Matching Operator: a YANG id giving the operation,
            parameters may be associated to that operator.
          - Comp./Decomp. Action: A YANG id giving the compression or
            decompression action, parameters may be associated to that
            action.
         ";
       leaf field-id {
         type schc:fid-type;
         mandatory true;
         description
           "Field ID, identify a field in the header with a YANG
            referenceid.";
       }
       leaf field-length {
         type schc:fl-type;
         mandatory true;
         description
           "Field Length, expressed in number of bits or through a
            function defined as a YANG referenceid.";
       }
       leaf field-position {
         type uint8;
         mandatory true;
         description
           "Field position in the header is an integer. Position 1
            matches the first occurence of a field in the header,
            while incremented position values match subsequent
            occurences.
            Position 0 means that this entry matches a field
            irrespective of its position of occurence 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



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           "Direction Indicator, a YANG referenceid to say if the packet
            is bidirectional, up or down";
       }
       list target-value {
         key "index";
         uses tv-struct;
         description
           "A list of value 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, positions should take consecutive values starting
            from 1.";
       }
       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";
       }
       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;
         mandatory true;
         description
           "CDA: Compression Decompression Action.";
       }
       list comp-decomp-action-value {
         key "index";



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         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.";
       }
     }

     grouping compression-content {
       list entry {
         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 identifed
            through a field-id, its position in the packet and
            its direction.";
       }
       description
         "Define a compression rule composed of a list of entries.";
     }

     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;
         mandatory true;
         description
           "which fragmentation mode is used (noAck, AckAlways,
            AckonError)";
       }
       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;



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         description
           "Should be up or down, bidirectionnal is forbiden.";
       }
       // SCHC Frag header format
       leaf dtag-size {
         type uint8;
         default "0";
         description
           "Size, in bits, of the DTag field (T variable from
            RFC8724).";
       }
       leaf w-size {
         when "derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-always') ";
         type uint8;
         description
           "Size, in bits, of the window field (M variable from
            RFC8724).";
       }
       leaf fcn-size {
         type uint8;
         mandatory true;
         description
           "Size, in bits, of the FCN field (N variable from RFC8724).";
       }
       leaf rcs-algorithm {
         type rcs-algorithm-type;
         default "schc:rcs-RFC8724";
         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 2^w-size - 1. Should not exceed
            this value. Possible FCN values are between 0 and



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            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 at most
            max-interleaved-frames must be active at any time.";
       }
       container inactivity-timer {
         uses timer-duration;
         description
           "Duration is seconds of the inactivity timer, 0 indicates
            that the timer is disabled.";
       }
       container retransmission-timer {
         uses timer-duration;
         when "derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-always') ";
         description
           "Duration in seconds of the retransmission timer.";
       }
       leaf max-ack-requests {
         when "derived-from(../fragmentation-mode,
                                   'fragmentation-mode-ack-on-error')
               or
               derived-from(../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(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type uint8;
             description
               "Size, in bits, of tiles. If not specified or set to 0,



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                tiles fill the fragment.";
           }
           leaf tile-in-All1 {
             when "derived-from(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type schc:all1-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(../fragmentation-mode,
                                'fragmentation-mode-ack-on-error')";
             type schc:ack-behavior-type;
             description
               "Sender behavior to acknowledge, after All-0, All-1 or
                when the LPWAN allows it.";
           }
         }
         description
           "RFC 8724 defines 3 fragmentation modes.";
       }
     }

     // Define rule ID. Rule ID is composed of a RuleID value and a
     // Rule ID Length

     grouping rule-id-type {
       leaf rule-id-value {
         type uint32;
         description
           "Rule ID value, this value must be unique, considering its
            length.";
       }
       leaf rule-id-length {
         type uint8 {
           range "0..32";
         }
         description
           "Rule ID length, in bits. The value 0 is for implicit
            rules.";
       }
       description
         "A rule ID is composed of a value and a length, expressed in
          bits.";
     }




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     // SCHC table for a specific device.

     container schc {
       list rule {
         key "rule-id-value rule-id-length";
         uses rule-id-type;
         choice nature {
           case fragmentation {
             if-feature "fragmentation";
             uses fragmentation-content;
           }
           case compression {
             if-feature "compression";
             uses compression-content;
           }
           case no-compression {
             description
               "RFC8724 requires a rule for uncompressed headers.";
           }
           description
             "A rule is for compression, for no-compression or for
              fragmentation.";
         }
         description
           "Set of rules compression, no compression or fragmentation
            rules identified by their rule-id.";
       }
       description
         "a SCHC set of rules is composed of a list of rules which are
          used for compression, no-compression or fragmentation.";
     }
   }
   <code ends>

                                 Figure 26

8.  Normative References

   [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>.








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   [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>.

   [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>.

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























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