Data Model for Static Context Header Compression (SCHC)
draft-ietf-lpwan-schc-yang-data-model-05

Document Type Active Internet-Draft (lpwan WG)
Authors Ana Minaburo  , Laurent Toutain 
Last updated 2021-09-09
Replaces draft-toutain-lpwan-schc-yang-data-model
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lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: 13 March 2022            Institut MINES TELECOM; IMT Atlantique
                                                        9 September 2021

        Data Model for Static Context Header Compression (SCHC)
                draft-ietf-lpwan-schc-yang-data-model-05

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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 March 2022.

Copyright Notice

   Copyright (c) 2021 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
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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SCHC rules  . . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Compression Rules . . . . . . . . . . . . . . . . . . . .   3
     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  . . . . . . . . . . . . . . . . . . . . . .   9
     2.8.  Matching Operator . . . . . . . . . . . . . . . . . . . .  10
       2.8.1.  Matching Operator arguments . . . . . . . . . . . . .  11
     2.9.  Compression Decompression Actions . . . . . . . . . . . .  11
       2.9.1.  Compression Decompression Action arguments  . . . . .  12
     2.10. Fragmentation rule  . . . . . . . . . . . . . . . . . . .  12
       2.10.1.  Fragmentation mode . . . . . . . . . . . . . . . . .  12
       2.10.2.  Fragmentation Header . . . . . . . . . . . . . . . .  13
       2.10.3.  Last fragment format . . . . . . . . . . . . . . . .  14
       2.10.4.  Acknowledgment behavior  . . . . . . . . . . . . . .  16
       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  . . . . . . . . . . . . . . . . . . .  23
     3.3.  YANG Tree . . . . . . . . . . . . . . . . . . . . . . . .  26
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   5.  Security considerations . . . . . . . . . . . . . . . . . . .  28
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   7.  YANG Module . . . . . . . . . . . . . . . . . . . . . . . . .  28
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  50
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51

1.  Introduction

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 this constrained network.
   Draft [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.

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   *  an update 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] specifies fields for IPv6, UDP, CoAP
   and OSCORE.  [I-D.barthel-lpwan-oam-schc] describes ICMPv6 header
   compression and [I-D.ietf-lpwan-schc-compound-ack] includes a new
   fragmentation behavior.

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

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 1: Compression Decompression Context

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2.2.  Identifier generation

   Identifier used un 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.

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

   *  create a typedef from this base type.

   The example (Figure 2) 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 RSC.
          The algorithm also defines the size if the RSC field.";
     }

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

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

         Figure 2: Principle to define a type based on identityref.

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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 one and apply
   compression.  The link between the list given by the parsed fields
   and the rules is done through a field ID.  [RFC8724] do not state how
   the field ID value can be 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], and [I-D.barthel-lpwan-oam-schc].

   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-
   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 3 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.

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

     identity fid-ipv6-base-type {
       base fid-base-type;
       description
         "Field IP 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 3: Definition of identityref for field IDs

   The type associated to this identity is fid-type (cf.  Figure 4)

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

                  Figure 4: Type definition for field IDs

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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 in byte 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 defined in
         for CoAP in RFC 8824 (cf. 5.3)";
     }

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

            Figure 5: Definition of identityref for Field Length

   As for field ID, field length function can be defined as an
   identityref as shown in Figure 5.

   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 6).

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

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                 Figure 6: 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, but if the field is repeated several
   times, the value is higher.  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).

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

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

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

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

        Figure 7: Definition of identityref for direction indicators

   Figure 7 gives the identityref for Direction Indicators.  The naming
   convention is "di" followed by the Direction Indicator name.

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   The type is "direction-indicator-type" (cf.  Figure 8).

  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 8: Type definition for direction indicators

2.7.  Target Value

   The Target Value is a list of binary sequences of any length, aligned
   on the left.  Figure 9 gives the definition of a single element of a
   Target Value.  In the rule, this will be used as a list, with
   position as a key.  The highest position value is used to compute the
   size of the index sent in residue for LSB CDA.  The position allows
   to specify several values:

   *  For Equal and LSB, a single value is used, such as for the equal
      or LSB CDA, the position is set to 0.

   *  For match-mapping, several of these values can be contained in a
      Target Value field.  In the data model, this is generalized by
      adding a position, which orders the list of values.  Position
      values must start from 0 and be contiguous.

  grouping tv-struct {
    description
      "Define 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;
    }
    leaf position {
      type uint16;
      description
        "If only one element position is 0, otherwise position is the
         matching list.";
    }
  }

                 Figure 9: Definition of target value

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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 10.

     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 RFC 8724 (cf. 7.3)";
     }

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

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

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

         Figure 10: Definition of identityref for Matching Operator

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

   The type is "matching-operator-type" (cf.  Figure 11)

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

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           Figure 11: Type definition for Matching Operator

2.8.1.  Matching Operator arguments

   Some Matching Operator such as MSB can take some values.  Even if
   currently LSB is the only MO takes only one argument, in the future
   some MO may require several arguments.  They are viewed as a list of
   target-values-type.

2.9.  Compression Decompression Actions

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

   Figure 13 gives 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 defines in RFC 8724 (cf. 7.4)";
     }

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

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

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

       ....

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     Figure 12: 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 13: Type definition for Compresion Decompression Action

2.9.1.  Compression Decompression Action arguments

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

2.10.  Fragmentation rule

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

   Most of parameters for fragmentation are defined in Annex D of
   [RFC8724].

   Since fragmentation rules work for a specific direction, they contain
   a mandatory direction.  The type is the same as the one used in
   compression entries, but the use of bidirectional is forbidden.

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.

   Figure 14 give the definition for identifiers from these three modes.

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

     identity fragmentation-mode-no-ack {
       base fragmentation-mode-base-type;
       description
         "No Ack of RFC 8724.";
     }

     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 14: 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, directly following the rule ID can be sent on
   the fragmentation direction.  The direction is mandatory and must be
   up or down. bidirectional is forbidden.  The SCHC header may be
   composed of (cf.  Figure 15):

   *  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 and depends on the rule in Ack-on-Error.
      This field is not need 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 15: 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 16.

     // -- RCS algorithm types

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

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

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

                     Figure 16: type definition for RCS

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   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 parameters defined in Figure 17 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.

    // -- 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 choose to send tiles or not in all1.";
     }

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

                     Figure 17: type definition for RCS

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

2.10.4.  Acknowledgment behavior

   A cknowledgment fragment header goes in the opposite direction of
   data.  The header is composed of (see Figure 18):

   *  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 received fragment/tile.  The size of
      the bitmap is given by the FCN value.

   NOTE: IN THE DATA MODEL THERE IS A max-window-size FIELD TO LIMIT THE
   BITMAP SIZE, BUT IS NO MORE IN RFC8724!  DO WE KEEP IT?

   |--- 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 18: Acknowledgment fragment header for RFC8724

   For Ack-on-Error, SCHC defined when 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 at the end of the fragment (FCN All-1).  The following
   identifiers (cf.  Figure 19) define the acknowledgment behavior.

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   // -- Ack behavior

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

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

     typedef ack-behavior-type {
       type identityref {
         base ack-behavior-base-type;
       }
       description
         "Type used in rules";
     }

                   Figure 19: bitmap generation behavior

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

   For Ack-onError, [RFC8724] allows a single bitmap in an acknowledment
   fragment, and [I-D.ietf-lpwan-schc-compound-ack] proposes to
   acknowledge several windows on a single ack fragment.  The following
   identifiers (cf.  Figure 20) define the behavior.

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     identity bitmap-format-base-type {
       description
         "Define how the bitmap is defined in ACK messages.";
     }

     identity bitmap-RFC8724 {
       base bitmap-format-base-type;
       description
         "Bitmap as defined in RFC8724.";
     }

     identity bitmap-compound-ack {
       base bitmap-format-base-type;
       description
         "Compound Ack.";
     }

     typedef bitmap-format-type {
       type identityref {
         base bitmap-format-base-type;
       }
       description
         "type used in rules";
     }

                   Figure 20: bitmap generation behavior

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

2.10.5.  Fragmentation Parameters

   The state machine requires some common values to handle
   fragmentation:

   *  retransmission-timer gives 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.

   *  inactivity-timer gives in seconds the duration before aborting
      (cf. section 8.2.2.4. of [RFC8724]), value of 0 explicitly
      indicates that this timer is disabled.

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

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

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   The are defined as unsigned integer, 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 either a C/D or an F/R rule.  A rule is identified by the
   rule ID value and its associated length.  The YANG grouping rule-id-
   type defines the structure used to represent a rule ID.  Length of 0
   is allowed to represent an implicit rule.

   Three types of rules are defined in [RFC8724]:

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

   *  No compression: nothing is associated to the rule ID.

   *  Fragmentation: fragmentation parameters are associated to 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 combined with the length";
    }
    leaf rule-id-length {
      type uint8 {
        range "0..32";
      }
      description
        "Rule ID length in bits, value 0 is for implicit rules";
    }
    description
      "A rule ID is composed of a value and a length in bit";
  }

  // 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 {
          uses compression-content;
        }
        case no-compression {
          description
            "RFC8724 allows a rule for uncompressed headers";
        }
        description
          "A rule is either for compression, no compression or 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 rule which are either
       compression or fragmentation";
  }
}

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               Figure 21: Definition of a SCHC Context

   To access to a specific rule, rule-id and its specific length is used
   as a key.  The rule is either a compression or a fragmentation rule.

   Each context can be identified though a version id.

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 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 represent a line of the table Figure 1.  Their type reflects
   the identifier types defined in Section 2.1

   Some controls are made 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 and fragmentation parameters.

       +-------+--+--+--+------------+-----------------+---------------+
       |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 YANF 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, paramters may be
       associated to that operator.
       - Comp./Decomp. Action: A YANG id giving the compression or decompression
       action, paramters may be associated to that action.
      ";
    leaf field-id {
      type schc:fid-type;

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      mandatory true;
      description
        "Field ID, identify a field in the header with a YANG refenceid.";
    }
    leaf field-length {
      type schc:fl-type;
      mandatory true;
      description
        "Field Length in bit or through a function defined as a YANG referenceid";
    }
    leaf field-position {
      type uint8;
      mandatory true;
      description
        "Field position in the header is a integer. If the field is not repeated
         in the header the value is 1, and incremented for each repetition of the field. Position
         0 means that the position is not important and order may change when decompressed";
    }
    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 "position";
      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 matching-list, should be consecutive position
         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 require 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 {

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      key "position";
      uses tv-struct;
      description
        "Matching Operator Arguments, based on TV structure to allow several arguments.
         In RFC 8724, only MSB define a single argument: length in bits  ";
    }
    leaf comp-decomp-action {
      type schc:cda-type;
      mandatory true;
      description
        "CDA: Compression Decompression Action";
    }
    list comp-decomp-action-value {
      key "position";
      uses tv-struct;
      description
        "CDA Arguments, based on TV structure to allow several arguments.
         In RFC 8724, no argument is defined for CDA";
    }
  }

  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 entry describing
         each 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 data model defines some relations between the entries:

   *  direction must be either up or down (not bidirectional).

   *  W size is only needed for Ack Always and Ack on Error modes.

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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 l2-word-size {
      type uint8;
      default "8";
      description
        "Size in bit of the layer 2 word";
    }
    leaf direction {
      must "derived-from-or-self(., 'di-up') or derived-from-or-self(., 'di-down')" {
        error-message "direction for fragmentation rules is up or down";
      }
      type schc:direction-indicator-type;
      mandatory true;
      description
        "Should be up or down, bi directionnal is forbiden.";
    }
    leaf dtag-size {
      type uint8;
      default "0";
      description
        "Size in bit of the DTag field";
    }
    leaf w-size {
      when "not(derived-from(../fragmentation-mode, 'fragmentation-mode-no-ack'))";
      type uint8;
      description
        "Size in bit of the window field";
    }
    leaf fcn-size {
      type uint8;
      mandatory true;
      description
        "Size in bit of the FCN field";
    }
    leaf rcs-algorithm {
      type rcs-algorithm-type;
      default "schc:rcs-RFC8724";
      description
        "Algorithm used for RCS";
    }
    leaf maximum-window-size {
      type uint16;
      description
        "By default 2^wsize - 1";

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    }
    leaf retransmission-timer {
      type uint64 {
        range "1..max";
      }
      description
        "Duration in seconds of the retransmission timer"; // Check the units
    }
    leaf inactivity-timer {
      type uint64;
      description
        "Duration is seconds of the inactivity timer, 0 indicates the timer is disabled"; // check units
    }
    leaf max-ack-requests {
      type uint8 {
        range "1..max";
      }
      description
        "The maximum number of retries for a specific SCHC ACK.";
    }
    leaf maximum-packet-size {
      type uint16;
      default "1280";
      description
        "When decompression is done, packet size must not strictly exceed this limit in Bytes";
    }
    leaf fragmentation-mode {
      type schc:fragmentation-mode-type;
      mandatory true;
      description
        "Which fragmentation mode is used (noAck, AckAlways, AckonError)";
    }
    choice mode {
      case no-ack;
      case ack-always;
      case ack-on-error {
        leaf tile-size {
          type uint8;
          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          description
            "Size in bit of tiles, if not specified or set to 0: tile fills the fragment.";
        }
        leaf tile-in-All1 {
          type schc:all1-data-type;
          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          description
            "When true, sender and receiver except a tile in All-1 frag";
        }

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        leaf ack-behavior {
          type schc:ack-behavior-type;
          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          description
            "Sender behavior to acknowledge, after All-0, All-1 or when the
             LPWAN allows it (Always)";
        }
        leaf bitmap-format {
          type schc:bitmap-format-type;
          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          default "schc:bitmap-RFC8724";
          description
            "How the bitmaps are included in the Ack message.";
        }
      }
      description
        "RFC 8724 defines 3 fragmentation modes";
    }
  }

3.3.  YANG Tree

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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 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-window-size?    uint16
           |  +--rw retransmission-timer?   uint64
           |  +--rw inactivity-timer?       uint64
           |  +--rw max-ack-requests?       uint8
           |  +--rw maximum-packet-size?    uint16
           |  +--rw fragmentation-mode      schc:fragmentation-mode-type
           |  +--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
           |        +--rw bitmap-format?    schc:bitmap-format-type
           +--:(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* [position]
           |     |  +--rw value?      binary
           |     |  +--rw position    uint16
           |     +--rw matching-operator           schc:mo-type
           |     +--rw matching-operator-value* [position]
           |     |  +--rw value?      binary
           |     |  +--rw position    uint16
           |     +--rw comp-decomp-action          schc:cda-type
           |     +--rw comp-decomp-action-value* [position]
           |        +--rw value?      binary
           |        +--rw position    uint16
           +--:(no-compression)

                              Figure 23

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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 on [RFC8724]

6.  Acknowledgements

   The authors would like to thank Dominique Barthel, Carsten Bormann,
   Alexander Pelov.

7.  YANG Module

<code begins> file ietf-schc@2021-08-17.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
     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

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     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 draft-ietf-lpwan-ipv6-static-context-hc-18. Include compression
     rules and fragmentation rules.

     This module is a YANG model for SCHC rules (RFc 8724).
     RFC 8724 describes a rule 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 describes some operations in fields.
     This data model applies both to compression and fragmentation.";

  revision 2021-08-17 {
    description
      "Initial version from RFC XXXX ";
    reference
      "RFC XXX: Data Model for Static Context Header Compression (SCHC)";
  }

  feature fragmentation {
    description
      "Fragmentation is usually required only at the transportation level.";
  }

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

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

  identity fid-ipv6-base-type {
    base fid-base-type;
    description
      "Field IP 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";
  }

  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";
  }

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  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
      "correspond either to the source address or the desdination
              address prefix of RFC 8200. Depending if it is respectively
              a uplink or an downklink message.";
  }

  identity fid-ipv6-deviid {
    base fid-ipv6-base-type;
    description
      "correspond either to the source address or the desdination
              address prefix of RFC 8200. Depending if it is respectively
              a uplink or an downklink message.";
  }

  identity fid-ipv6-appprefix {
    base fid-ipv6-base-type;
    description
      "correspond either to the source address or the desdination
              address prefix of RFC 768. Depending if it is respectively
              a downlink or an uplink message.";
  }

  identity fid-ipv6-appiid {
    base fid-ipv6-base-type;
    description
      "correspond either to the source address or the desdination
              address prefix of RFC 768. Depending if it is respectively
              a downlink or an uplink message.";
  }

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

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  }

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

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

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

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

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

  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";

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  }

  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;
    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";

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  }

  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;
    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";

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  }

  identity fid-coap-option-accept {
    base fid-coap-base-type;
    description
      "CoAP option Max-Age 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;
    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";

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  }

  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 draft schc coap, section 6.4)";
  }

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

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

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

  identity fid-icmpv6-base-type {
    base fid-base-type;
    description
      "Field IP base type for UDP headers described in RFC 768";
  }

  identity fid-icmpv6-type {
    base fid-icmpv6-base-type;
    description
      "ICMPv6 field (see draft OAM)";
  }

  identity fid-icmpv6-code {
    base fid-icmpv6-base-type;
    description
      "ICMPv6 field (see draft OAM)";

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  }

  identity fid-icmpv6-checksum {
    base fid-icmpv6-base-type;
    description
      "ICMPv6 field (see draft OAM)";
  }

  identity fid-icmpv6-identifier {
    base fid-icmpv6-base-type;
    description
      "ICMPv6 field (see draft OAM)";
  }

  identity fid-icmpv6-sequence {
    base fid-icmpv6-base-type;
    description
      "ICMPv6 field (see draft OAM)";
  }

  //----------------------------------
  // 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 defined in
      for CoAP in RFC 8824 (cf. 5.3)";
  }

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

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

  identity di-base-type {

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    description
      "Used to extend field length functions";
  }

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

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

  identity di-down {
    base di-base-type;
    description
      "Direction Indication of downstream 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 RFC 8724 (cf. 7.3)";
  }

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

  identity mo-msb {
    base mo-base-type;

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    description
      "MSB MO as defined RFC 8724 (cf. 7.3)";
  }

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

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

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

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

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

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

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

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

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

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

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

  // -- type definition

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

  typedef fl-type {
    type union {
      type int64; /* positive length in bits */
      type identityref { /* function */
        base fl-base-type;
      }
    }
    description
      "Field length either a positive integer giving 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.";
  }

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  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
      "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 RFC 8724.";
  }

  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";

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  }

  // -- Ack behavior

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

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

  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.";
  }

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  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 choose to send tiles or not in all1.";
  }

  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 RSC.
       The algorithm also defines the size if the RSC field.";
  }

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

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

   // --- Bitmap format

  identity bitmap-format-base-type {
    description
      "Define how the bitmap is defined in ACK messages.";
  }

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  identity bitmap-RFC8724 {
    base bitmap-format-base-type;
    description
      "Bitmap as defined in RFC8724.";
  }

  identity bitmap-compound-ack {
    base bitmap-format-base-type;
    description
      "Compound Ack.";
  }

  typedef bitmap-format-type {
    type identityref {
      base bitmap-format-base-type;
    }
    description
      "type used in rules";
  }

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

  grouping tv-struct {
    description
      "Define 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;
    }
    leaf position {
      type uint16;
      description
        "If only one element position is 0, otherwise position is the
         matching list.";
    }
  }

  grouping compression-rule-entry {
    description
      "These entries defines a compression entry (i.e. a line)
       as defined in RFC 8724 and fragmentation parameters.

       +-------+--+--+--+------------+-----------------+---------------+
       |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 YANF 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, paramters may be
       associated to that operator.
       - Comp./Decomp. Action: A YANG id giving the compression or decompression
       action, paramters 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 refenceid.";
    }
    leaf field-length {
      type schc:fl-type;
      mandatory true;
      description
        "Field Length in bit or through a function defined as a YANG referenceid";
    }
    leaf field-position {
      type uint8;
      mandatory true;
      description
        "Field position in the header is a integer. If the field is not repeated
         in the header the value is 1, and incremented for each repetition of the field. Position
         0 means that the position is not important and order may change when decompressed";
    }
    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 "position";
      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 matching-list, should be consecutive position
         values starting from 1.";
    }
    leaf matching-operator {
      type schc:mo-type;
      must "../target-value or derived-from-or-self(., 'mo-ignore')" {

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        error-message "mo-equal, mo-msb and mo-match-mapping require 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 "position";
      uses tv-struct;
      description
        "Matching Operator Arguments, based on TV structure to allow several arguments.
         In RFC 8724, only MSB define a single argument: length in bits  ";
    }
    leaf comp-decomp-action {
      type schc:cda-type;
      mandatory true;
      description
        "CDA: Compression Decompression Action";
    }
    list comp-decomp-action-value {
      key "position";
      uses tv-struct;
      description
        "CDA Arguments, based on TV structure to allow several arguments.
         In RFC 8724, no argument is defined for CDA";
    }
  }

  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 entry describing
         each 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

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      "This grouping defines the fragmentation parameters for
       all the modes (No Ack, Ack Always and Ack on Error) specified in
       RFC 8724.";
    leaf l2-word-size {
      type uint8;
      default "8";
      description
        "Size in bit of the layer 2 word";
    }
    leaf direction {
      must "derived-from-or-self(., 'di-up') or derived-from-or-self(., 'di-down')" {
        error-message "direction for fragmentation rules are up or down";
      }
      type schc:di-type;
      mandatory true;
      description
        "Should be up or down, bi directionnal is forbiden.";
    }
    leaf dtag-size {
      type uint8;
      default "0";
      description
        "Size in bit of the DTag field";
    }
    leaf w-size {
      when "not(derived-from(../fragmentation-mode, 'fragmentation-mode-no-ack'))";
      type uint8;
      description
        "Size in bit of the window field";
    }
    leaf fcn-size {
      type uint8;
      mandatory true;
      description
        "Size in bit of the FCN field";
    }
    leaf rcs-algorithm {
      type rcs-algorithm-type;
      default "schc:rcs-RFC8724";
      description
        "Algoritm used for RCS";
    }
    leaf maximum-window-size {
      type uint16;
      description
        "By default 2^wsize - 1";
    }
    leaf retransmission-timer {

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      type uint64 {
        range "1..max";
      }
      description
        "Duration in seconds of the retransmission timer"; // Check the units
    }
    leaf inactivity-timer {
      type uint64;
      description
        "Duration is seconds of the inactivity timer, 0 indicates the timer is disabled"; // check units
    }
    leaf max-ack-requests {
      type uint8 {
        range "1..max";
      }
      description
        "The maximum number of retries for a specific SCHC ACK.";
    }
    leaf maximum-packet-size {
      type uint16;
      default "1280";
      description
        "When decompression is done, packet size must not strictly exceed this limit in Bytes";
    }
    leaf fragmentation-mode {
      type schc:fragmentation-mode-type;
      mandatory true;
      description
        "which fragmentation mode is used (noAck, AckAlways, AckonError)";
    }
    choice mode {
      case no-ack;
      case ack-always;
      case ack-on-error {
        leaf tile-size {
          type uint8;
          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          description
            "Size in bit of tiles, if not specified or set to 0: tile fills the fragment.";
        }
        leaf tile-in-All1 {
          type schc:all1-data-type;
          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          description
            "When true, sender and receiver except a tile in All-1 frag";
        }
        leaf ack-behavior {
          type schc:ack-behavior-type;

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          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          description
            "Sender behavior to acknowledge, after All-0, All-1 or when the
             LPWAN allows it (Always)";
        }
        leaf bitmap-format {
          type schc:bitmap-format-type;
          when "derived-from(../fragmentation-mode, 'fragmentation-mode-ack-on-error')";
          default "schc:bitmap-RFC8724";
          description
            "How the bitmaps are included in the Ack message.";
        }
      }
      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 combined with the length";
    }
    leaf rule-id-length {
      type uint8 {
        range "0..32";
      }
      description
        "Rule ID length in bits, value 0 is for implicit rules";
    }
    description
      "A rule ID is composed of a value and a length in bit";
  }

  // 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;
        }

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        case compression {
          uses compression-content;
        }
        case no-compression {
          description
            "RFC8724 allows a rule for uncompressed headers";
        }
        description
          "A rule is either for compression, no compression or 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 rule which are either
       compression or fragmentation";
  }
}
<code ends>

                              Figure 24

8.  Normative References

   [I-D.barthel-lpwan-oam-schc]
              Barthel, D., Toutain, L., Kandasamy, A., Dujovne, D., and
              J. C. Zuniga, "OAM for LPWAN using Static Context Header
              Compression (SCHC)", Work in Progress, Internet-Draft,
              draft-barthel-lpwan-oam-schc-02, 2 November 2020,
              <https://www.ietf.org/archive/id/draft-barthel-lpwan-oam-
              schc-02.txt>.

   [I-D.ietf-lpwan-schc-compound-ack]
              Zuniga, J. C., Gomez, C., Aguilar, S., Toutain, L.,
              Cespedes, S., and D. Wistuba, "SCHC Compound ACK", Work in
              Progress, Internet-Draft, draft-ietf-lpwan-schc-compound-
              ack-00, 9 July 2021, <https://www.ietf.org/archive/id/
              draft-ietf-lpwan-schc-compound-ack-00.txt>.

   [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>.

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   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zúñiga, "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>.

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

Minaburo & Toutain        Expires 13 March 2022                [Page 51]