Internet-Draft LPWAN SCHC YANG module November 2021
Minaburo & Toutain Expires 28 May 2022 [Page]
Workgroup:
lpwan Working Group
Internet-Draft:
draft-ietf-lpwan-schc-yang-data-model-06
Published:
Intended Status:
Standards Track
Expires:
Authors:
A. Minaburo
Acklio
L. Toutain
Institut MINES TELECOM; IMT Atlantique

Data Model for Static Context Header Compression (SCHC)

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 and may be updated, replaced, or obsoleted by other documents at any 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 28 May 2022.

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Table of Contents

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 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:

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].

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

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.
  • 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.

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] 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].

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.

  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

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 as 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 as defined in
       for CoAP in RFC 8824 (cf. 4.5).";
  }
Figure 5: Definition of identityref for Field Length

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.";
  }
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, 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).


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

  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.

The type is "di-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. 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;
      description
        "Target Value";
    }
    leaf position {
      type uint16;
      description
        "If only one element position is 0, otherwise position is the
         the position in the matching list.";
    }
  }
Figure 9: 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 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-match-mapping {
    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 "mo-type" (cf. Figure 11)

  typedef mo-type {
    type identityref {
      base mo-base-type;
    }
    description
      "Matching Operator (MO) to compare fields values with
      target values";
  }
Figure 11: Type definition for Matching Operator

2.8.1. Matching Operator arguments

They are viewed as a list of tv-struct.

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).";
  }

    ....
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.

  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.
  • 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 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 16: type definition for RCS

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

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.

  // -- 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
      "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.

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.

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]:

  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.";
  }
}
Figure 20: 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 21). 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;
      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 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 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 "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 21: 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. 
  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 {
      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, bi directionnal is forbiden.";
    }
    leaf dtag-size {
      type uint8;
      default "0";
      description
        "Size in bit of the DTag field (T variable from RFC8724).";
    }
    leaf w-size {
      when "not(derived-from(../fragmentation-mode,
                             'fragmentation-mode-no-ack'))";
      type uint8;
      description
        "Size in bit of the window field (M variable from RFC8724).";
    }
    leaf fcn-size {
      type uint8;
      mandatory true;
      description
        "Size in bit of the FCN field (M variable from RFC8724).";
    }
    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 {
      type uint64 {
        range "1..max";
      }
      description
        "Duration in seconds of the retransmission timer.";
    }
    leaf inactivity-timer {
      type uint64;
      description
        "Duration is seconds of the inactivity timer,
        0 indicates the timer is disabled.";
    }
    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 {
          when "derived-from(../fragmentation-mode,
                             'fragmentation-mode-ack-on-error')";
          type uint8;
          description
            "Size in bit of tiles, if not specified or set to 0,
             tile fills the fragment.";
        }
        leaf tile-in-All1 {
          when "derived-from(../fragmentation-mode,
                             'fragmentation-mode-ack-on-error')";
          type schc:all1-data-type;
          description
            "When true, sender and receiver except a tile in
            All-1 frag.";
        }
        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 (Always).";
        }
      }
      description
        "RFC 8724 defines 3 fragmentation modes.";
    }
  }

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)
           |  +--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
           +--:(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 22

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@2021-11-10.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
     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).
     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 2021-11-10 {
    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

  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";
  }

  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";
  }

  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";
  }

  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";
  }

  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";
  }

  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";
  }

  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)";
  }


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

  identity mo-match-mapping {
    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)";
  }

  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.";
  }

  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";
  }

  // -- 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
      "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.";
  }

  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.";
  }

  // --------  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;
      description
        "Target Value";
    }
    leaf position {
      type uint16;
      description
        "If only one element position is 0, otherwise position is the
         the position in 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:
       - 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
         referenceid.";
    }
    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 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 "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
      "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 {
      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, bi directionnal is forbiden.";
    }
    leaf dtag-size {
      type uint8;
      default "0";
      description
        "Size in bit of the DTag field (T variable from RFC8724).";
    }
    leaf w-size {
      when "not(derived-from(../fragmentation-mode,
                             'fragmentation-mode-no-ack'))";
      type uint8;
      description
        "Size in bit of the window field (M variable from RFC8724).";
    }
    leaf fcn-size {
      type uint8;
      mandatory true;
      description
        "Size in bit of the FCN field (M variable from RFC8724).";
    }
    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 {
      type uint64 {
        range "1..max";
      }
      description
        "Duration in seconds of the retransmission timer.";
    }
    leaf inactivity-timer {
      type uint64;
      description
        "Duration is seconds of the inactivity timer,
        0 indicates the timer is disabled.";
    }
    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 {
          when "derived-from(../fragmentation-mode,
                             'fragmentation-mode-ack-on-error')";
          type uint8;
          description
            "Size in bit of tiles, if not specified or set to 0,
             tile fills the fragment.";
        }
        leaf tile-in-All1 {
          when "derived-from(../fragmentation-mode,
                             'fragmentation-mode-ack-on-error')";
          type schc:all1-data-type;
          description
            "When true, sender and receiver except a tile in
            All-1 frag.";
        }
        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 (Always).";
        }
      }
      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;
        }
        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 23

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, , <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, , <https://www.rfc-editor.org/info/rfc8824>.

Authors' Addresses

Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Laurent Toutain
Institut MINES TELECOM; IMT Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France