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Versions: (draft-toutain-lpwan-schc-yang-data-model) 00 01 02 03

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: January 11, 2021         Institut MINES TELECOM; IMT Atlantique
                                                           July 10, 2020


        Data Model for Static Context Header Compression (SCHC)
                draft-ietf-lpwan-schc-yang-data-model-03

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 January 11, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are 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.  Field Identifier  . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Field length  . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Field position  . . . . . . . . . . . . . . . . . . . . .   6
     2.5.  Direction Indicator . . . . . . . . . . . . . . . . . . .   6
     2.6.  Target Value  . . . . . . . . . . . . . . . . . . . . . .   7
     2.7.  Matching Operator . . . . . . . . . . . . . . . . . . . .   8
       2.7.1.  Matching Operator arguments . . . . . . . . . . . . .   9
     2.8.  Compression Decompression Actions . . . . . . . . . . . .  10
       2.8.1.  Compression Decompression Action arguments  . . . . .  12
   3.  Rule definition . . . . . . . . . . . . . . . . . . . . . . .  12
     3.1.  Compression rule  . . . . . . . . . . . . . . . . . . . .  14
       3.1.1.  Compression context representation. . . . . . . . . .  14
       3.1.2.  Rule definition . . . . . . . . . . . . . . . . . . .  15
     3.2.  Fragmentation rule  . . . . . . . . . . . . . . . . . . .  16
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   5.  Security considerations . . . . . . . . . . . . . . . . . . .  24
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   7.  YANG Module . . . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

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 an abstract 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:

   o  the same definition on both ends, even if the internal
      representation is different.

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

   o  ...






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

   SCHC compression is generic, the main mechanism do no refers to a
   specific fields.  A field is abstracted through an ID, a position, a
   direction and a value that can be a numerical value or a string.
   [RFC8724] and [I-D.ietf-lpwan-coap-static-context-hc] specifies
   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 an abstract 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.  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



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   the field ID value can be constructed.  In the examples, it was given
   through a string indexed by the protocol name (e.g.  IPv6.version,
   CoAP.version,...).

   Using the YANG model, each field MUST be identified through a global
   YANG identityref.  A YANG field ID derives from the field-id-base-
   type.  Figure 2 gives some field ID definitions.  Note that some
   field IDs can be splitted is smaller pieces.  This is the case for
   "fid-ipv6-trafficclass-ds" and "fid-ipv6-trafficclass-ecn" which are
   a subset of "fid-ipv6-trafficclass-ds".

      identity field-id-base-type  {
              description "Field ID with SID";
       }

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

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

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

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

     ...


             Figure 2: Definition of identityref for field IDs

   Figure 2 gives an example of field ID identityref definitions.  The
   base identity is field-id-base-type, and field id are derived for it.
   The naming convention is "fid" followed by the protocol name and the
   field name.





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   The yang model in annex (see Section 7) gives the full definition of
   the field ID for [RFC8724], [I-D.ietf-lpwan-coap-static-context-hc],
   and [I-D.barthel-lpwan-oam-schc].

   The type associated to this identity is field-id-type (cf.  Figure 3)

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

                  Figure 3: Type definition for field IDs

2.3.  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
   [I-D.ietf-lpwan-coap-static-context-hc] defines the "tkl" function
   for managing the CoAP Token length field.

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

     identity fl-variable {
              base field-length-base-type;
              description "residue length in Byte is sent";
     }

     identity fl-token-length {
              base field-length-base-type;
              description "residue length in Byte is sent";
     }


           Figure 4: Definition of identityref for field ILength

   As for field ID, field length function can be defined as a
   identityref as shown in Figure 4.

   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 5).





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

                Figure 5: Type definition for field Length

   The naming convention is fl followed by the function name as defined
   in SCHC specifications.

2.4.  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 taken into account during the rule selection
   process.

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

2.5.  Direction Indicator

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





















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     identity direction-indicator-base-type {
           description "used to extend field length functions";
     }

     identity di-bidirectional {
              base direction-indicator-base-type;
              description "Direction Indication of bi directionality";
     }

     identity di-up {
              base direction-indicator-base-type;
              description "Direction Indication of upstream";
     }

     identity di-down {
              base direction-indicator-base-type;
              description "Direction Indication of downstream";
     }


       Figure 6: Definition of identityref for direction indicators

   Figure 6 gives the identityref for Direction Indicators.

   The type is "direction-indicator-type" (cf.  Figure 7).

    typedef direction-indicator-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.";
        type identityref {
            base direction-indicator-base-type;
        }
    }

            Figure 7: Type definition for direction indicators

2.6.  Target Value

   Target Value may be either a string or binary sequence.  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.  By default the position is set to
   0.

   The leaf "value" is not mandatory to represent a non existing value
   in a TV.





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    grouping target-values-struct {
        description "defines the target value element. Can be either an arbitrary
        binary or ascii element. All target values are considered as a matching lists.
        Position is used to order values, by default position 0 is used when containing
        a single element.";

        leaf value {
            type union {
                type binary;
                type string;
            }
        }
        leaf position {
            description "If only one element position is 0, otherwise position is the
            matching list.";
            type uint16;
        }
    }

                   Figure 8: Definition of target value

   Figure 8 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.

2.7.  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.




















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     identity matching-operator-base-type {
        description "used to extend Matching Operators with SID values";
     }

     identity mo-equal {
        base matching-operator-base-type;
        description "RFC 8724";
     }

     identity mo-ignore {
        base matching-operator-base-type;
        description "RFC 8724";
     }

     identity mo-msb {
        base matching-operator-base-type;
        description "RFC 8724";
     }

     identity mo-matching {
        base matching-operator-base-type;
        description "RFC 8724";
     }

         Figure 9: Definition of identityref for Matching Operator

   the type is "matching-operator-type" (cf.  Figure 10)

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

             Figure 10: Type definition for Matching Operator

2.7.1.  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.








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2.8.  Compression Decompression Actions

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















































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        identity compression-decompression-action-base-type;

       identity cda-not-sent {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

       identity cda-value-sent {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

       identity cda-lsb {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

       identity cda-mapping-sent {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

       identity cda-compute-length {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

       identity cda-compute-checksum {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

       identity cda-deviid {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

      identity cda-appiid {
           base compression-decompression-action-base-type;
               description "RFC 8724";
       }

     Figure 11: Definition of identityref for Compresion Decompression
                                  Action

   The type is "comp-decomp-action-type" (cf.  Figure 12)





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


      Figure 12: Type definition for Compresion Decompression Action

2.8.1.  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.

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.





























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

    grouping rule-id-type {
        leaf rule-id {
            type uint32;
            description "rule ID value, this value must be unique combined with the length";
        }
        leaf rule-length {
            type uint8 {
                range 0..32;
            }
            description "rule ID length in bits, value 0 is for implicit rules";
        }
    }

// SCHC table for a specific device.

    container schc {
        leaf version{
            type uint64;
            mandatory false;
            description "used as an indication for versioning";
        }
        list rule {
            key "rule-id rule-length";
            uses rule-id-type;
            choice nature {
                case fragmentation {
                    uses fragmentation-content;
                }
                case compression {
                    uses compression-content;
                }
            }
        }
    }



                  Figure 13: 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 identify though a version id.






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3.1.  Compression rule

   A compression rule is composed of entries describing its processing
   (cf.  Figure 14).  An entry contains all the information defined in
   Figure 1 with the types defined above.

3.1.1.  Compression context representation.

   The compression rule described Figure 1 is associated to a rule ID.
   The compression rule entry is defined in Figure 14.  Each column in
   the table is either represented by a leaf or a list.  Note that
   Matching Operators and Compression Decompression actions can have
   arguments.  They are viewed a ordered list of strings and numbers as
   in target values.

    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 YANG id.
        - Field Position: a positive (and possibly equal to 0) integer.
        - Direction Indicator: a YANG identifier giving the direction.
        - Target value: a value against which the header Field is compared.
        - Matching Operator: a YANG id giving the operation, parameters may be
        associated to that operator.
        - Comp./Decomp. Action: A YANG id giving the compression or decompression
        action, parameters may be associated to that action.
        ";

        leaf field-id {
            description "Field ID, identify a field in the header with a YANG identityref.";
            mandatory true;
            type schc:field-id-type;
        }
        leaf field-length {
            description "Field Length in bit or through a function defined as a YANG identityref";
            mandatory true;
            type schc:field-length-type;
        }
        leaf field-position {
            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



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            0 means that the position is not important and order may change when decompressed";
            mandatory true;
            type uint8;
        }
        leaf direction-indicator {
            description "Direction Indicator, a YANG identityref to say if the packet is bidirectionnal,
            up or down";
            mandatory true;
            type schc:direction-indicator-type;
        }
        list target-values {
            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.";
            key position;
            uses target-values-struct;
        }
        leaf matching-operator {
            mandatory true;
            type schc:matching-operator-type;
        }
        list matching-operator-value {
            key position;
            uses target-values-struct;
        }
        leaf comp-decomp-action {
            mandatory true;
            type schc:comp-decomp-action-type;
        }
        list comp-decomp-action-value {
            key position;
            uses target-values-struct;
        }
    }

               Figure 14: Definition of a compression entry

3.1.2.  Rule definition

   A compression rule is a list of entries.











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    grouping compression-content {
        description "define a compression rule composed of a list of entries.";
        list entry {
            key "field-id field-position direction-indicator";
            uses compression-rule-entry;
        }
    }

                Figure 15: Definition of a compression rule

   To identify a specific entry Field ID, position and direction are
   needed.

3.2.  Fragmentation rule

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

   Figure 16 gives the first elements found in this structure.  It
   starts with a direction.  Since fragmentation rules are
   unidirectional, they contain a mandatory direction.  The type is the
   same as the one used in compression entries, but the use of
   bidirectionnal is forbidden.

   The next elements describe size of SCHC fragmentation header fields.
   Only the FCN size is mandatory and value must be higher or equal to
   1.

























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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 direction {
            type schc:direction-indicator-type;
            description "should be up or down, bi directionnal is forbidden.";
            mandatory true;
        }
        leaf dtagsize {
            type uint8;
            description "size in bit of the DTag field";

        }
        leaf wsize {
            type uint8;
            description "size in bit of the window field";
        }
        leaf fcnsize {
            type uint8 {
                range 1..max;
            }
            description "size in bit of the FCN field";
            mandatory true;
        }
...

     Figure 16: Definition of a fragmentation parameters, SCHC header

   RCS algorithm is defined (Figure 17), by default with the CRC
   computation proposed in [RFC8724].  The algorithms are identified
   through an identityref specified in the SCHC Data Model and with the
   type RCS-algorithm-type (Figure 18).

   ...
           leaf RCS-algorithm {
               type RCS-algorithm-type;
               default schc:RFC8724-RCS;
               description "Algoritm used for RCS";
           }
   ...

    Figure 17: Definition of a fragmentation parameters, RCS algorithm







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

       typedef RCS-algorithm-type {
           type identityref {
               base RCS-algorithm-base-type;
           }
       }

          Figure 18: Definition of identityref for RCS Algorithm

   Figure 19 gives the parameters used by the state machine to handle
   fragmentation:

   o  maximum-window-size contains the maximum FCN value that can be
      used.

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

   o  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.

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

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














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...
        leaf maximum-window-size {
            type uint16;
            description "by default 2^wsize - 2";
        }

        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";
        }
...

     Figure 19: Definition of a fragmentation state machine parameters

   Figure 20 gives information related to a specific compression mode:
   fragmentation-mode MUST be set with a specific behavior.  Identityref
   are given Figure 21.

   For Ack on Error some specific information may be provided:

   o  tile-size gives in bits the size of the tile; If set to 0 a single
      tile is inserted inside a fragment.

   o  tile-in All1 indicates if All1 contains only the RCS (all1-data-
      no) or may contain a single tile (all1-data-yes).  Since the
      reassembly process may detect this behavior, the choice can be
      left to the fragmentation process.  In that case identityref all1-




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      data-sender-choice as to be specified.  All possible values are
      given Figure 21.

   o  ack-behavior tells when the fragmentation process may send
      acknowledgments.  When ack-behavior-after-All0 is specified, the
      ack may be sent after the reception of All-0 fragment.  When ack-
      behavior-after-All1 is specified, the ack may be sent after the
      reception of All-1 fragment at the end of the fragmentation
      process.  ack-behavior-always do not impose a limitation at the
      SCHC level.  The constraint may come from the LPWAN technology.
      All possible values are given Figure 21.

...
        leaf fragmentation-mode {
            type schc:fragmentation-mode-type;
            description "which fragmentation mode is used (noAck, AckAlways, AckonError)";
            mandatory true;
        }

        choice mode {
            case no-ack;
            case ack-always;
            case ack-on-error {
                leaf tile-size {
                    type uint8;
                    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;
                    description "When true, sender and receiver except a tile in All-1 frag";
                }
                leaf ack-behavior {
                    type schc:ack-behavior-type;
                    description "Sender behavior to acknowledge, after All-0, All-1 or when the
                    LPWAN allows it (Always)";
                }
            }
       }
...

       Figure 20: Definition of a fragmentation specific information

// -- FRAGMENTATION TYPE

// -- fragmentation modes

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



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    }

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

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

    typedef fragmentation-mode-type {
        type identityref {
            base fragmentation-mode-base-type;
        }
    }

// -- Ack behavior

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

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

    identity ack-behavior-after-All1 {
        description "fragmentation expects Ack after sending All1 fragment.";
        base ack-behavior-base-type;
    }

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

    typedef ack-behavior-type {
        type identityref {
            base ack-behavior-base-type;
        }
    }



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// -- All1 with data types

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

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

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

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

    typedef all1-data-type {
        type identityref {
            base all1-data-base-type;
        }
    }



              Figure 21: Specific types for Ack On Error mode

   ## YANG Tree



















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module: schc
  +--rw schc
     +--rw version?   uint64
     +--rw rule* [rule-id rule-length]
        +--rw rule-id                       uint32
        +--rw rule-length                   uint8
        +--rw (nature)?
           +--:(fragmentation)
           |  +--rw direction               schc:direction-indicator-type
           |  +--rw dtagsize?               uint8
           |  +--rw wsize?                  uint8
           |  +--rw fcnsize                 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:field-id-type
                 +--rw field-length                schc:field-length-type
                 +--rw field-position              uint8
                 +--rw direction-indicator         schc:direction-indicator-type
                 +--rw target-values* [position]
                 |  +--rw value?      union
                 |  +--rw position    uint16
                 +--rw matching-operator           schc:matching-operator-type
                 +--rw matching-operator-value* [position]
                 |  +--rw value?      union
                 |  +--rw position    uint16
                 +--rw comp-decomp-action          schc:comp-decomp-action-type
                 +--rw comp-decomp-action-value* [position]
                    +--rw value?      union
                    +--rw position    uint16


                                 Figure 22





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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 schc@2020-02-28.yang
module schc{
    yang-version "1";
    namespace "urn:ietf:lpwan:schc:rules-description";
    prefix "schc";

    description
    "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.";




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    revision 2020-06-15 {
        description "clean up and add descriptions, merge schc-id to this file";
    }

    revision 2020-02-28 {
        description "Add Fragmentation parameters";
    }

    revision 2020-01-23 {
        description "Modified TV with binary and union";
    }

    revision 2020-01-07 {
        description "First version of the YANG model";
    }

// -------------------------
//  Field ID type definition
//--------------------------

// generic value TV definition

    identity field-id-base-type  {
           description "Field ID with SID";
    }

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

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

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

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




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

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

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

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

    identity fid-ipv6-devprefix {
        base field-id-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 field-id-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 field-id-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 field-id-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.";
    }




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

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

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

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

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

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

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

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

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

    identity fid-coap-code-detail {
        base field-id-base-type;
        description "CoAP code detail from RFC 7252";



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    }

    identity fid-coap-mid {
        base field-id-base-type;
        description "CoAP message ID from RFC 7252";
    }

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

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

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

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

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

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

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

    identity fid-coap-option-location-path {
        base field-id-base-type;
        description "CoAP option Location-Path from RFC 7252";
    }

    identity fid-coap-option-uri-path {



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        base field-id-base-type;
        description "CoAP option Uri-Path from RFC 7252";
    }

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

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

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

    identity fid-coap-option-accept {
        base field-id-base-type;
        description "CoAP option Max-Age from RFC 7252";
    }

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

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

    identity fid-coap-option-block1 {
        base field-id-base-type;
        description "CoAP option Block1 from RFC 7959";
    }

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

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



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

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

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

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

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

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

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

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

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

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



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    }

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

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


/// !!!!!!! See future CoAP extentions

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

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

  identity fl-variable {
           base field-length-base-type;
           description "residue length in Byte is sent";
  }

  identity fl-token-length {
           base field-length-base-type;
           description "residue length in Byte is sent";
  }

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

  identity direction-indicator-base-type {
        description "used to extend field length functions";
  }

  identity di-bidirectional {
           base direction-indicator-base-type;
           description "Direction Indication of bi directionality";
  }

  identity di-up {
           base direction-indicator-base-type;



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           description "Direction Indication of upstream";
  }

  identity di-down {
           base direction-indicator-base-type;
           description "Direction Indication of downstream";
  }

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

   identity matching-operator-base-type {
      description "used to extend Matching Operators with SID values";
   }

   identity mo-equal {
      base matching-operator-base-type;
      description "RFC 8724";
   }

   identity mo-ignore {
      base matching-operator-base-type;
      description "RFC 8724";
   }

   identity mo-msb {
      base matching-operator-base-type;
      description "RFC 8724";
   }

   identity mo-matching {
      base matching-operator-base-type;
      description "RFC 8724";
   }

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

    identity compression-decompression-action-base-type;

    identity cda-not-sent {
        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

    identity cda-value-sent {



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        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

    identity cda-lsb {
        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

    identity cda-mapping-sent {
        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

    identity cda-compute-length {
        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

    identity cda-compute-checksum {
        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

    identity cda-deviid {
        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

   identity cda-appiid {
        base compression-decompression-action-base-type;
            description "RFC 8724";
    }

// -- type definition

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

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



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            type identityref { /* function */
                base field-length-base-type;
            }
        }
    }

    typedef direction-indicator-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.";
        type identityref {
            base direction-indicator-base-type;
        }
    }

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

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

// -- FRAGMENTATION TYPE

// -- fragmentation modes

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

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

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



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    }

    typedef fragmentation-mode-type {
        type identityref {
            base fragmentation-mode-base-type;
        }
    }

// -- Ack behavior

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

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

    identity ack-behavior-after-All1 {
        description "fragmentation expects Ack after sending All1 fragment.";
        base ack-behavior-base-type;
    }

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

    typedef ack-behavior-type {
        type identityref {
            base ack-behavior-base-type;
        }
    }

// -- All1 with data types

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

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

    identity all1-data-yes {
        description "All1 MUST contain a tile";



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        base all1-data-base-type;
    }

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

    typedef all1-data-type {
        type identityref {
            base all1-data-base-type;
        }
    }

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

    typedef RCS-algorithm-type {
        type identityref {
            base RCS-algorithm-base-type;
        }
    }



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

    grouping target-values-struct {
        description "defines the target value element. Can be either an arbitrary
        binary or ascii element. All target values are considered as a matching lists.
        Position is used to order values, by default position 0 is used when containing
        a single element.";

        leaf value {
            type union {
                type binary;
                type string;
            }
        }



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

    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 {
            description "Field ID, identify a field in the header with a YANG refenceid.";
            mandatory true;
            type schc:field-id-type;
        }
        leaf field-length {
            description "Field Length in bit or through a function defined as a YANG referenceid";
            mandatory true;
            type schc:field-length-type;
        }
        leaf field-position {
            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";
            mandatory true;
            type uint8;
        }
        leaf direction-indicator {
            description "Direction Indicator, a YANG referenceid to say if the packet is bidirectionnal,
            up or down";
            mandatory true;



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            type schc:direction-indicator-type;
        }
        list target-values {
            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.";
            key position;
            uses target-values-struct;
        }
        leaf matching-operator {
            mandatory true;
            type schc:matching-operator-type;
        }
        list matching-operator-value {
            key position;
            uses target-values-struct;
        }
        leaf comp-decomp-action {
            mandatory true;
            type schc:comp-decomp-action-type;
        }
        list comp-decomp-action-value {
            key position;
            uses target-values-struct;
        }
    }

    grouping compression-content {
        description "define a compression rule composed of a list of entries.";
        list entry {
            key "field-id field-position direction-indicator";
            uses compression-rule-entry;
        }
    }

    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 direction {
            type schc:direction-indicator-type;
            description "should be up or down, bi directionnal is forbiden.";
            mandatory true;
        }
        leaf dtagsize {
            type uint8;
            description "size in bit of the DTag field";



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        }
        leaf wsize {
            type uint8;
            description "size in bit of the window field";
        }
        leaf fcnsize {
            type uint8;
            description "size in bit of the FCN field";
            mandatory true;
        }
        leaf RCS-algorithm {
            type RCS-algorithm-type;
            default schc:RFC8724-RCS;
            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"; // 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;
            description "which fragmentation mode is used (noAck, AckAlways, AckonError)";



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            mandatory true;
        }

        choice mode {
            case no-ack;
            case ack-always;
            case ack-on-error {
                leaf tile-size {
                    type uint8;
                    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;
                    description "When true, sender and receiver except a tile in All-1 frag";
                }
                leaf ack-behavior {
                    type schc:ack-behavior-type;
                    description "Sender behavior to acknowledge, after All-0, All-1 or when the
                    LPWAN allows it (Always)";
                }
            }
       }
    }



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

    grouping rule-id-type {
        leaf rule-id {
            type uint32;
            description "rule ID value, this value must be unique combined with the length";
        }
        leaf rule-length {
            type uint8 {
                range 0..32;
            }
            description "rule ID length in bits, value 0 is for implicit rules";
        }
    }

// SCHC table for a specific device.

    container schc {
        leaf version{
            type uint64;
            mandatory false;
            description "used as an indication for versioning";



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        }
        list rule {
            key "rule-id rule-length";
            uses rule-id-type;
            choice nature {
                case fragmentation {
                    uses fragmentation-content;
                }
                case compression {
                    uses compression-content;
                }
            }
        }
    }


}

<code ends>

                                 Figure 23

8.  Normative References

   [I-D.barthel-lpwan-oam-schc]
              Barthel, D., Toutain, L., Kandasamy, A., Dujovne, D., and
              J. Zuniga, "OAM for LPWAN using Static Context Header
              Compression (SCHC)", draft-barthel-lpwan-oam-schc-01 (work
              in progress), March 2020.

   [I-D.ietf-lpwan-coap-static-context-hc]
              Minaburo, A., Toutain, L., and R. Andreasen, "LPWAN Static
              Context Header Compression (SCHC) for CoAP", draft-ietf-
              lpwan-coap-static-context-hc-15 (work in progress), July
              2020.

   [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>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/info/rfc8724>.





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Authors' Addresses

   Ana Minaburo
   Acklio
   1137A avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex
   France

   Email: ana@ackl.io


   Laurent Toutain
   Institut MINES TELECOM; IMT Atlantique
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   France

   Email: Laurent.Toutain@imt-atlantique.fr
































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