Internet Engineering Task Force M. Veillette, Ed.
Internet-Draft Trilliant Networks Inc.
Intended status: Standards Track January 31, 2017
Expires: August 4, 2017

Constrained YANG Module Library


This document describes a YANG library that provides information about all the YANG modules used by a constrained network management server (e.g., a CoAP Management Interface (CoMI) server). Simple caching mechanisms are provided to allow clients to minimize retrieval of this information.

Status of This Memo

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

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This Internet-Draft will expire on August 4, 2017.

Copyright Notice

Copyright (c) 2017 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 ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must 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.

Table of Contents

1. Introduction

The YANG library specified in this document is available to clients of a given server to discover the YANG modules supported by this constrained network management server. A CoMI server provides a link to this library in the /c/mod.uri resource. The following YANG module information is provided to client applications to fully utilize the YANG data modeling language:

1.1. Major differences between ietf-constrained-yang-library and ietf-yang-library

YANG module ietf-constrained-yang-library targets the same functionality and shares the same approach as YANG module ietf-yang-library. The following changes with respect to ietf-yang-library are specified to make ietf-constrained-yang-library compatible with SID [I-D.ietf-core-yang-cbor] used by CoMI [I-D.vanderstok-core-comi] and to improve its applicability to constrained devices and networks.

o To minimize their size, each revision date is encoded in binary.

2. Terminology and Notation

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

The following terms are defined in [RFC7950]:

The following terms are defined in [I-D.ietf-core-yang-cbor]:

The following terms are defined in [I-D.vanderstok-core-comi]:

The following terms are used within this document:

3. Overview

The “ietf-constrained-yang-library” module provides information about the YANG library used by a given server. This module is defined using YANG version 1 as defined by [RFC7950], but it supports the description of YANG modules written in any revision of YANG.

3.1. Tree diagram

A simplified graphical representation of the YANG module specified in this document (ietf-constrained-yang-library) is provided below. The meaning of the symbols in this diagram is as follows:

module: ietf-constrained-yang-library
   +--ro modules-state
      +--ro module-set-id    union
      +--ro module* [sid revision]
         +--ro sid                 sid
         +--ro revision            revision
         +--ro feature*            sid
         +--ro deviation* [sid revision]
         |  +--ro sid         sid
         |  +--ro revision    revision
         +--ro conformance-type    enumeration
         +--ro submodule* [sid revision]
            +--ro sid         sid
            +--ro revision    revision
   +---n yang-library-change
      +--ro module-set-id    -> /modules-state/module-set-id

3.2. Description

3.2.1. modules-state

This mandatory container specifies the module set identifier and the list of modules supported by the server.

3.2.2. modules-state/module-set-id

This mandatory leaf contains an identifier representing the current set of modules and submodules used by a server. This identifier is server-specific when implemented as unit32 or can be used by multiple servers when implemented as identityref. The value of this leaf MUST change whenever the set of modules and submodules in the library changes. There is no requirement that the same set always results in the same module-set-id value.

This leaf allows a client to fetch the module list once, cache it, and only re-fetch it if the value of this leaf has been changed.

If the value of this leaf changes, the server also generates a “yang-library-change” notification, with the new value of “module-set-id”.

3.2.3. modules-state/module

This mandatory list contains one entry for each YANG module supported by the server. There MUST be an entry in this list for each revision of each YANG module that is used by the server. It is possible for multiple revisions of the same module to be imported, in addition to an entry for the revision that is implemented by the server.

4. YANG Module “ietf-constrained-yang-library”

RFC Ed.: update the date below with the date of RFC publication and remove this note.

<CODE BEGINS> file "ietf-constrained-yang-library@2017-01-20.yang"
module ietf-constrained-yang-library {
  namespace "urn:ietf:params:xml:ns:yang:ietf-constrained-yang-library";
  prefix "lib";

    "IETF CORE (Constrained RESTful Environments) Working Group";

    "WG Web:   <>
     WG List:  <>

     WG Chair: Carsten Bormann 

     WG Chair: Jaime Jimenez 

     Editor:   Michel Veillette

    "This module contains the list of YANG modules and submodules
    implemented by a server.

     Copyright (c) 2016 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

     This version of this YANG module is part of RFC XXXX; see
     the RFC itself for full legal notices.";

  // RFC Ed.: replace XXXX with actual RFC number and remove
  // this note.

  // RFC Ed.: update the date below with the date of the RFC
  // publication and remove this note.

  revision 2017-01-20 {
      "Initial revision.";
      "RFC XXXX: Constrained YANG Module Library.";

   * Typedefs

  typedef revision {
    type binary {
      length "4";
      "Revision date encoded as a binary string as follow:
      - First byte = Year divided by 100
      - Second byte = Year modulo 100 (0 to 99)
      - Third byte = Month (1 = January to 12 = december)
      - Forth byte = Day (1 to 31)";

  typedef sid {
    type uint64;
      "Identifier assigned to different YANG items such as
      data nodes, RPCs and actions, notifications, modules,
      sub-modules, features and deviations.";
   * Groupings

  grouping identification-info {
      "YANG modules and submodules identification information.";

    leaf sid {
      type sid;
      mandatory true;
        "SID assigned to this module or submodule.";
    leaf revision {
      type revision;
        "Revision date assigned to this module or submodule.
        A zero-length binary string is used if no revision statement
        is present in the YANG module or submodule.";
  identity module-set {
        "Base identity from which shared module-set identifiers
        are derived.";
   * Operational state data nodes

  container modules-state {
    config false;
      "Contains information about the different data models
      implemented by the server.";
    leaf module-set-id {
      type union {
        type uint32;
        type identityref {
          base "lib:module-set";
      mandatory true;
        "Identifier representing the current set of modules
        and submodules listed in the 'module' list. This
        identifier is server-specific when implemented as
        unit32 or shared between multiple servers when
        implemented as identityref. The server MUST change
        the value of this leaf each time the content of the
        'module' list instance change.";

    list module {
      key "sid revision";
        "Each entry represents one revision of one module
         currently supported by the server.";

      uses identification-info;
      leaf-list feature {
        type sid;
          "List of YANG features from this module that are
          supported by the server, regardless whether
          they are defined in the module or in any included
      list deviation {
        key "sid revision";
          "List of YANG deviation modules used by this server
          to modify the conformance of the module associated
          with this entry.  Note that the same module can be
          used for deviations for multiple modules, so the
          same entry MAY appear within multiple 'module' entries.

          The deviation module MUST be present in the 'module'
          list, with the same sid and revision values.
          The 'conformance-type' value will be 'implement' for
          the deviation module.";
        uses identification-info;
      leaf conformance-type {
        type enumeration {
          enum implement {
            value 0;
              "Indicates that the server implements one or more
              protocol-accessible objects defined in the YANG
              module identified in this entry.  This includes
              deviation statements defined in the module.

              For YANG version 1.1 modules, there is at most one
              module entry with conformance type 'implement' for a
              particular module, since YANG 1.1 requires that
              at most one revision of a module is implemented.

              For YANG version 1 modules, there SHOULD NOT be more
              than one module entry for a particular module.";
          enum import {
            value 1;
              "Indicates that the server imports reusable definitions
              from the specified revision of the module, but does
              not implement any protocol accessible objects from
              this revision.

              Multiple module entries for the same module MAY
              exist. This can occur if multiple modules import the
              same module, but specify different revision-dates in
              the import statements.";
        mandatory true;
          "Indicates the type of conformance the server is claiming
          for the YANG module identified by this entry.";
      list submodule {
        key "sid revision";
          "Each entry represents one submodule within the
           parent module.";
        uses identification-info;

   * Notifications

  notification yang-library-change {
      "Generated when the set of modules and submodules supported
      by the server has changed.";
    leaf module-set-id {
      type leafref {
        path "/lib:modules-state/lib:module-set-id";
      mandatory true;
        "Contains the module-set-id value representing the
        set of modules and submodules supported by the server
        at the time the notification is generated.";

5. IANA Considerations

5.1. YANG Module Registry

This document registers one YANG module in the YANG Module Names registry [RFC7950].

name: ietf-constrained-yang-library

namespace: urn:ietf:params:xml:ns:yang:ietf-constrained-yang-library

prefix: lib

reference: RFC XXXX

// RFC Ed.: replace XXXX with RFC number and remove this note

6. Security Considerations

This YANG module is designed to be accessed via the CoMI protocol [I-D.vanderstok-core-comi]. Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access to these data nodes.

Specifically, the ‘module’ list may help an attacker to identify the server capabilities and server implementations with known bugs. Server vulnerabilities may be specific to particular modules, module revisions, module features, or even module deviations. This information is included in each module entry. For example, if a particular operation on a particular data node is known to cause a server to crash or significantly degrade device performance, then the module list information will help an attacker identify server implementations with such a defect, in order to launch a denial of service attack on the device.

7. Acknowledgments

The YANG module defined by this memo have been derived from an already existing YANG module, ietf-yang-library [RFC7895], we will like to thanks to the authors of this YANG module. A special thank Andy Bierman for his initial recommendations for the creation of this YANG module.

8. References

8.1. Normative References

[I-D.ietf-core-yang-cbor] Veillette, M., Pelov, A., Somaraju, A., Turner, R. and A. Minaburo, "CBOR Encoding of Data Modeled with YANG", Internet-Draft draft-ietf-core-yang-cbor-03, October 2016.
[I-D.vanderstok-core-comi] Stok, P., Bierman, A., Veillette, M. and A. Pelov, "CoAP Management Interface", Internet-Draft draft-vanderstok-core-comi-11, January 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016.

8.2. Informative References

[I-D.ietf-netconf-restconf] Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", Internet-Draft draft-ietf-netconf-restconf-18, October 2016.
[RFC7895] Bierman, A., Bjorklund, M. and K. Watsen, "YANG Module Library", RFC 7895, DOI 10.17487/RFC7895, June 2016.

Author's Address

Michel Veillette (editor) Trilliant Networks Inc. 610 Rue du Luxembourg Granby, Quebec J2J 2V2 Canada Phone: +14503750556 EMail: