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IETF RFC 7895
Last modified on Thursday, June 30th, 2016
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Internet Engineering Task Force (IETF) A. Bierman
Request for Comments: 7895 YumaWorks
Category: Standards Track M. Bjorklund
ISSN: 2070-1721 Tail-f Systems
K. Watsen
Juniper Networks
June 2016
YANG Module Library
 Abstract
This document describes a YANG library that provides information
about all the YANG modules used by a network management server (e.g.,
a Network Configuration Protocol (NETCONF) server). Simple caching
mechanisms are provided to allow clients to minimize retrieval of
this information.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7895.
Copyright Notice
Copyright (c) 2016 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
(http://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
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.
Bierman, et al. Standards Track PAGE 1 
RFC 7895 YANG Library June 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Tree Diagrams . . . . . . . . . . . . . . . . . . . . . . 3
2. YANG Module Library . . . . . . . . . . . . . . . . . . . . . 4
2.1. modules-state . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. modules-state/module-set-id . . . . . . . . . . . . . 4
2.1.2. modules-state/module . . . . . . . . . . . . . . . . 5
2.2. YANG Library Module . . . . . . . . . . . . . . . . . . . 5
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
3.1. YANG Module Registry . . . . . . . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Normative References . . . . . . . . . . . . . . . . . . 12
5.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
There is a need for standard mechanisms to identify the YANG modules
and submodules that are in use by a server that implements YANG data
models. If a large number of YANG modules are utilized by the
server, then the YANG library contents needed can be relatively
large. This information changes very infrequently, so it is
important that clients be able to cache the YANG library contents and
easily identify whether their cache is out of date.
YANG library information can be different on every server and can
change at runtime or across a server reboot.
If the server implements multiple protocols to access the YANG-
defined data, each such protocol has its own conceptual instantiation
of the YANG library.
The following information is needed by a client application (for each
YANG module in the library) to fully utilize the YANG data modeling
language:
o name: The name of the YANG module.
o revision: Each YANG module and submodule within the library has a
revision. This is derived from the most recent revision statement
within the module or submodule. If no such revision statement
exists, the module's or submodule's revision is the zero-length
string.
Bierman, et al. Standards Track PAGE 2
RFC 7895 YANG Library June 2016
o submodule list: The name and revision of each submodule used by
the module MUST be identified.
o feature list: The name of each YANG feature supported by the
server MUST be identified.
o deviation list: The name of each YANG module used for deviation
statements MUST be identified.
1.1. Terminology
The keywords "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].
The following terms are defined in [RFC 6241]:
o client
o server
The following terms are defined in [YANG1.1]:
o module
o submodule
The following terms are used within this document:
o YANG library: A collection of YANG modules and submodules used by
a server.
1.2. Tree Diagrams
A simplified graphical representation of the data model is used in
this document. The meaning of the symbols in these diagrams is as
follows:
o Brackets "[" and "]" enclose list keys.
o Abbreviations before data node names: "rw" means configuration
data (read-write) and "ro" state data (read-only).
o Symbols after data node names: "?" means an optional node, "!"
means a presence container, and "*" denotes a list and leaf-list.
Bierman, et al. Standards Track PAGE 3
RFC 7895 YANG Library June 2016
o Parentheses enclose choice and case nodes, and case nodes are also
marked with a colon (":").
o Ellipsis ("...") stands for contents of subtrees that are not
shown.
2. YANG Module Library
The "ietf-yang-library" module provides information about the YANG
library used by a server. This module is defined using YANG version
1, but it supports the description of YANG modules written in any
revision of YANG.
Following is the YANG Tree Diagram for the "ietf-yang-library"
module:
+--ro modules-state
+--ro module-set-id string
+--ro module* [name revision]
+--ro name yang:yang-identifier
+--ro revision union
+--ro schema? inet:uri
+--ro namespace inet:uri
+--ro feature* yang:yang-identifier
+--ro deviation* [name revision]
| +--ro name yang:yang-identifier
| +--ro revision union
+--ro conformance-type enumeration
+--ro submodule* [name revision]
+--ro name yang:yang-identifier
+--ro revision union
+--ro schema? inet:uri
2.1. modules-state
This mandatory container holds the identifiers for the YANG data
model modules supported by the server.
2.1.1. modules-state/module-set-id
This mandatory leaf contains a unique implementation-specific
identifier representing the current set of modules and submodules on
a specific server. The value of this leaf MUST change whenever the
set of modules and submodules in the YANG library changes. There is
no requirement that the same set always results in the same "module-
set-id" value.
Bierman, et al. Standards Track PAGE 4
RFC 7895 YANG Library June 2016
This leaf allows a client to fetch the module list once, cache it,
and only refetch 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".
Note that for a NETCONF server that implements YANG 1.1 [YANG1.1], a
change of the "module-set-id" value results in a new value for the
:yang-library capability defined in [YANG1.1]. Thus, if such a
server implements NETCONF notifications [RFC 5277], and the
notification "netconf-capability-change" [RFC 6470], a
"netconf-capability-change" notification is generated whenever the
"module-set-id" changes.
2.1.2. modules-state/module
This mandatory list contains one entry for each YANG data model
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.
2.2. YANG Library Module
The "ietf-yang-library" module defines monitoring information for the
YANG modules used by a server.
The "ietf-yang-types" and "ietf-inet-types" modules from [RFC 6991]
are used by this module for some type definitions.
<CODE BEGINS> file "ietf-yang-library@2016-06-21.yang"
module ietf-yang-library {
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-library";
prefix "yanglib";
import ietf-yang-types {
prefix yang;
}
import ietf-inet-types {
prefix inet;
}
Bierman, et al. Standards Track PAGE 5
RFC 7895 YANG Library June 2016
organization
"IETF NETCONF (Network Configuration) Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/netconf/>
WG List: <mailto:netconf@ietf.org>
WG Chair: Mehmet Ersue
<mailto:mehmet.ersue@nsn.com>
WG Chair: Mahesh Jethanandani
<mailto:mjethanandani@gmail.com>
Editor: Andy Bierman
<mailto:andy@yumaworks.com>
Editor: Martin Bjorklund
<mailto:mbj@tail-f.com>
Editor: Kent Watsen
<mailto:kwatsen@juniper.net>";
description
"This module contains monitoring information about the YANG
modules and submodules that are used within a YANG-based
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
(http://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 7895; see
the RFC itself for full legal notices.";
revision 2016-06-21 {
description
"Initial revision.";
reference
"RFC 7895: YANG Module Library.";
}
Bierman, et al. Standards Track PAGE 6
RFC 7895 YANG Library June 2016
/*
* Typedefs
*/
typedef revision-identifier {
type string {
pattern '\d{4}-\d{2}-\d{2}';
}
description
"Represents a specific date in YYYY-MM-DD format.";
}
/*
* Groupings
*/
grouping module-list {
description
"The module data structure is represented as a grouping
so it can be reused in configuration or another monitoring
data structure.";
grouping common-leafs {
description
"Common parameters for YANG modules and submodules.";
leaf name {
type yang:yang-identifier;
description
"The YANG module or submodule name.";
}
leaf revision {
type union {
type revision-identifier;
type string { length 0; }
}
description
"The YANG module or submodule revision date.
A zero-length string is used if no revision statement
is present in the YANG module or submodule.";
}
}
grouping schema-leaf {
description
"Common schema leaf parameter for modules and submodules.";
Bierman, et al. Standards Track PAGE 7
RFC 7895 YANG Library June 2016
leaf schema {
type inet:uri;
description
"Contains a URL that represents the YANG schema
resource for this module or submodule.
This leaf will only be present if there is a URL
available for retrieval of the schema for this entry.";
}
}
list module {
key "name revision";
description
"Each entry represents one revision of one module
currently supported by the server.";
uses common-leafs;
uses schema-leaf;
leaf namespace {
type inet:uri;
mandatory true;
description
"The XML namespace identifier for this module.";
}
leaf-list feature {
type yang:yang-identifier;
description
"List of YANG feature names from this module that are
supported by the server, regardless of whether they are
defined in the module or any included submodule.";
}
list deviation {
key "name revision";
description
"List of YANG deviation module names and revisions
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 name and revision values.
The 'conformance-type' value will be 'implement' for
the deviation module.";
uses common-leafs;
Bierman, et al. Standards Track PAGE 8
RFC 7895 YANG Library June 2016
}
leaf conformance-type {
type enumeration {
enum implement {
description
"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 name, 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 name.";
}
enum import {
description
"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 name MAY
exist. This can occur if multiple modules import the
same module but specify different revision dates in
the import statements.";
}
}
mandatory true;
description
"Indicates the type of conformance the server is claiming
for the YANG module identified by this entry.";
}
list submodule {
key "name revision";
description
"Each entry represents one submodule within the
parent module.";
uses common-leafs;
uses schema-leaf;
}
}
}
Bierman, et al. Standards Track PAGE 9
RFC 7895 YANG Library June 2016
/*
* Operational state data nodes
*/
container modules-state {
config false;
description
"Contains YANG module monitoring information.";
leaf module-set-id {
type string;
mandatory true;
description
"Contains a server-specific identifier representing
the current set of modules and submodules. The
server MUST change the value of this leaf if the
information represented by the 'module' list instances
has changed.";
}
uses module-list;
}
/*
* Notifications
*/
notification yang-library-change {
description
"Generated when the set of modules and submodules supported
by the server has changed.";
leaf module-set-id {
type leafref {
path "/yanglib:modules-state/yanglib:module-set-id";
}
mandatory true;
description
"Contains the module-set-id value representing the
set of modules and submodules supported at the server at
the time the notification is generated.";
}
}
}
<CODE ENDS>
Bierman, et al. Standards Track PAGE 10
RFC 7895 YANG Library June 2016
3. IANA Considerations
3.1. YANG Module Registry
This document registers one URI in the "IETF XML Registry" [RFC 3688].
Following the format in RFC 3688, the following registration has been
made.
URI: urn:ietf:params:xml:ns:yang:ietf-yang-library
Registrant Contact: The NETCONF WG of the IETF.
XML: N/A, the requested URI is an XML namespace.
This document registers one YANG module in the "YANG Module Names"
registry [RFC 6020].
name: ietf-yang-library
namespace: urn:ietf:params:xml:ns:yang:ietf-yang-library
prefix: yanglib
reference: RFC 7895
4. Security Considerations
The YANG module defined in this memo is designed to be accessed via
the NETCONF protocol [RFC 6241]. The lowest NETCONF layer is the
secure transport layer and the mandatory-to-implement secure
transport is SSH [RFC 6242]. The NETCONF access control model
[RFC 6536] provides the means to restrict access for particular
NETCONF users to a pre-configured subset of all available NETCONF
protocol operations and content.
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 (e.g., via get, get-config, or
notification) to these data nodes. These are the subtrees and data
nodes and their sensitivity/vulnerability:
o /modules-state/module: The module list used in a server
implementation may help an attacker identify the server
capabilities and server implementations with known bugs. Although
some of this information may be available to all users via the
NETCONF <hello> message (or similar messages in other management
protocols), this YANG module potentially exposes additional
details that could be of some assistance to an attacker. 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
Bierman, et al. Standards Track PAGE 11
RFC 7895 YANG Library June 2016
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.
5. References
5.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC 3688, January 2004,
<http://www.rfc-editor.org/info/RFC 3688>.
[RFC 6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC 6020, October 2010,
<http://www.rfc-editor.org/info/RFC 6020>.
[RFC 6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC 6241, June 2011,
<http://www.rfc-editor.org/info/RFC 6241>.
[RFC 6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC 6242, June 2011,
<http://www.rfc-editor.org/info/RFC 6242>.
[RFC 6536] Bierman, A. and M. Bjorklund, "Network Configuration
Protocol (NETCONF) Access Control Model", RFC 6536,
DOI 10.17487/RFC 6536, March 2012,
<http://www.rfc-editor.org/info/RFC 6536>.
[RFC 6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC 6991, July 2013,
<http://www.rfc-editor.org/info/RFC 6991>.
5.2. Informative References
[RFC 5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, DOI 10.17487/RFC 5277, July 2008,
<http://www.rfc-editor.org/info/RFC 5277>.
Bierman, et al. Standards Track PAGE 12
RFC 7895 YANG Library June 2016
[RFC 6470] Bierman, A., "Network Configuration Protocol (NETCONF)
Base Notifications", RFC 6470, DOI 10.17487/RFC 6470,
February 2012, <http://www.rfc-editor.org/info/RFC 6470>.
[YANG1.1] Bjorklund, M., "The YANG 1.1 Data Modeling Language", Work
in Progress, draft-ietf-netmod-rfc6020bis-12, April 2016.
Acknowledgements
Contributions to this material by Andy Bierman are based upon work
supported by the Space & Terrestrial Communications Directorate
(S&TCD) under Contract No. W15P7T-13-C-A616. Any opinions, findings,
conclusions, or recommendations expressed in this material are those
of the author(s) and do not necessarily reflect the views of the
Space & Terrestrial Communications Directorate (S&TCD).
Authors' Addresses
Andy Bierman
YumaWorks
Email: andy@yumaworks.com
Martin Bjorklund
Tail-f Systems
Email: mbj@tail-f.com
Kent Watsen
Juniper Networks
Email: kwatsen@juniper.net
Bierman, et al. Standards Track PAGE 13
RFC TOTAL SIZE: 24164 bytes
PUBLICATION DATE: Thursday, June 30th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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