Network Working Group                                        A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: October 23, 2019        Institut MINES TELECOM ; IMT Atlantique
                                                          April 21, 2019

        Data Model for Static Context Header Compression (SCHC)


   This document describes a YANG data model for the SCHC (Static
   Context Header Compression).  A generic module is defined, that can
   be applied for any headers and also a specific model for the IPv6 UDP
   protocol stack is also proposed.  Note that this draft is a first
   attempt to define a YANG data module for SCHC, more work is needed to
   use all the YANG facilities.

Status of This Memo

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   This Internet-Draft will expire on October 23, 2019.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   SCHC [I-D.ietf-lpwan-ipv6-static-context-hc] defines a compression
   technique for LPWAN networks based on static context.  The context
   contains a list of rules (cf.  Figure 1).  Each rule contains itself
   a list of field descriptions composed of a field identifier (FID), a
   field length (FID), a field position (FP), a field direction (DI), a
   target value (TV), a matching operator (MO) and a Compression/
   Decompression Action (CDA).

     |                      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

   The goal of this document is to provide an YANG data model to
   represent SCHC Compression and Fragmentation rules, to allow
   management over a LPWAN network.  The main constraints are:

   o  since the device may be managed through the LPWAN network,
      management messages must be compact.  COREconf offers a
      representation based on CBOR.

   o  this data model can be extended with new values, such as new field
      id, new MO or CDA.

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2.  YANG types

2.1.  Field Identifier

   The field identifier is used to identify a specific field.  It is
   viewed as an uint32.

2.2.  Target Value field

   A value may be associated for each field in a rule.  The value's type
   depends on the field.  It can be an integer, a prefix, a string, or
   any other type carried by the field.  The LPWA-types regroups all the
   possibles values.  Figure 2 gives its definition.

       typedef lpwan-types {
         type union {
           type uint8;
           type uint16;
           type uint32;
           type uint64;
           type inet:ipv6-prefix;
           type string;

                           Figure 2: Value types

   Note that as defined in [I-D.ietf-lpwan-ipv6-static-context-hc], Dev
   and App Prefixes can be of type inet:ipv6-prefix-type, but this type
   derives from ASCII characters, a binary representation such as uint64
   will be more compact.

2.3.  Matching Operators

   A matching operator is used to check the field value stored in the
   rule against the value contained in the header field.  If there is no
   matching the rule is not selected.  Two instances of matching
   operator are defined to allow the rule selection from informations
   contained either in the compressed header or the uncompressed header.
   The SCHC document [I-D.ietf-lpwan-ipv6-static-context-hc] defines
   four operators:

   o  equal: The rule's value must be equal to the packet header value
      for a specific field.

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   o  ignore: There is no check for this field.

   o  MSB(x): This operator compare the most significant bits.  The
      operator takes one argument representing the length of least
      significant bit part, which will be ignored during the matching
      but sent if the rule matches.

   o  match-mapping: From the list of values of the Target Value, This
      operator will match if one of those values is equal to the field
      value and will send the index of the list representing this value.

       /* Matching operator type         */
       typedef matching-operator-type {
           type enumeration {
               enum equal;
               enum ignore;
               enum msb;
               enum match-mapping;

                       Figure 3: Matching operators

   Figure 3 represents the Matching Operator type definition.

2.4.  Compression Decompression Actions

   The SCHC document [I-D.ietf-lpwan-ipv6-static-context-hc] defines
   some compression decompression actions (CDA).  The CDA tells how to
   compress and decompress the field.  They are defined in Figure 4.
   they are coded the same way as MO.

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           /* Compression-Decompression action type       */
           typedef compression-decompression-action-type {
                           type enumeration {
                                   enum not-sent;
                                   enum value-sent;
                                   enum lsb;
                                   enum mapping-sent;
                                   enum compute-length;
                                   enum compute-checksum;
                                   enum esiid-did;
                                   enum laiid-did;

                        Figure 4: Action functions

3.  Generic rule definition

   Each rule's row is defined by several leaves, composed of:

   o  a field key which will be used as a key,

   o  a field name that can be used for debugging purpose,

   o  a field length that containing the length of the field,

   o  a field position that gives the number of instances,

   o  a field direction indicates the packet direction,

   o  a field target value containing the value that will be compared,

   o  a matching operators for rule selection

   o  an compression/decompression action to compress/decompress the

   Figure 5 defines the format.

        grouping rule-entry {
                leaf field-id {
                        type int32;
                        description "Field ID unique value representing the Field";

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                leaf field-length {
                        type uint8;
                        description "size in bits of the field";

                leaf field-position {
                        type uint8;
                        description "For repeated fields, we need to be able to
                                     distinguish between successive occurences";

                leaf direction {
                     type direction-type;
                list target-values {
                     key tv-key;
                     leaf tv-key {
                        type int8;
                     leaf target-value {
                        type lpwan-types;
                     description "Target Values can be a list of value, for
                                  match-mapping. For other MO, only one entry is specified";
                leaf matching-operator {
                     type matching-operator-type;

                leaf matching-operator-parameter {
                     type lpwan-types;
                     description "If the matching operator requires a parameter
                                  (for example lsb or msb), the value is provided here.";

                leaf compression-decompression-action {
                     type compression-decompression-action-type;

                leaf compression-decompression-action-parameter {
                     type lpwan-types;
                     description "If the matching operator requires a parameter
                                  (for example lsb or msb), the value is provided here.";

                        Figure 5: Action functions

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4.  YANG static context model

   This lead to the generic rule definition, represented Figure 7.  It
   defines a set of rules.

  grouping compression-rule {

    leaf rule-id {
            type uint8;
            description "The number of the context rule that should be applied.";
    leaf rule-id-length {
            type uint8;

    list rule-fields {
      key "field-id field-position direction";
      uses rule-entry;


              Figure 6: YANG definition of the generic module

 module: ietf-lpwan-schc-rule
   +--rw rule-id?          uint8
   +--rw rule-id-length?   uint8
   +--rw rule-fields* [field-id field-position direction]
      +--rw field-id                                      int32
      +--rw field-length?                                 uint8
      +--rw field-position                                uint8
      +--rw direction                                     direction-type
      +--rw target-values* [tv-key]
      |  +--rw tv-key          int8
      |  +--rw target-value?   lpwan-types
      +--rw matching-operator?                            m.-o.-type
      +--rw matching-operator-parameter?                  lpwan-types
      +--rw compression-decompression-action?             c.-d.-a.-type
      +--rw compression-decompression-action-parameter?   lpwan-types

                       Figure 7: Generic module tree

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   The YANG tree is given Figure 7.

 SID        Assigned to
 ---------  --------------------------------------------------
 60000      node /rule-fields
 60001      node /rule-fields/compression-decompression-action
 60002      node /rule-fields/compression-decompression-action-parameter
 60003      node /rule-fields/direction
 60004      node /rule-fields/field-id
 60005      node /rule-fields/field-length
 60006      node /rule-fields/field-position
 60007      node /rule-fields/matching-operator
 60008      node /rule-fields/matching-operator-parameter
 60009      node /rule-fields/target-values
 60010      node /rule-fields/target-values/target-value
 60011      node /rule-fields/target-values/tv-key
 60012      node /rule-id
 60013      node /rule-id-length

 File ietf-lpwan-schc-rule@2016-10-31.sid created
 Number of SIDs available : 100
 Number of SIDs assigned : 14

                    Figure 8: Example of SID allocation

   Figure 8 gives a simple allocation for SID value.  SID values from
   100 to 113 are used for /generic-rules/context-rules/rule-fields/
   field-compression-decompression-action.  SID value from 1009 to 1012
   are used in /generic-rules/context-rules/rule-fields/field-matching-

5.  Acknowledgement

   The authors would like to thank Michel Veillette, Alexander Pelov,
   Antoni Markovski for their help on COMI/CoOL and YANG.

6.  Normative References

              Veillette, M., Stok, P., Pelov, A., and A. Bierman, "CoAP
              Management Interface", draft-ietf-core-comi-04 (work in
              progress), November 2018.

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              Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
              Zuniga, "LPWAN Static Context Header Compression (SCHC)
              and fragmentation for IPv6 and UDP", draft-ietf-lpwan-
              ipv6-static-context-hc-18 (work in progress), December

Authors' Addresses

   Ana Minaburo
   1137A Avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex


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


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