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RFC 8340 - YANG Tree Diagrams

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Internet Engineering Task Force (IETF)                      M. Bjorklund
Request for Comments: 8340                                Tail-f Systems
BCP: 215                                                  L. Berger, Ed.
Category: Best Current Practice                  LabN Consulting, L.L.C.
ISSN: 2070-1721                                               March 2018

                           YANG Tree Diagrams


   This document captures the current syntax used in YANG module tree
   diagrams.  The purpose of this document is to provide a single
   location for this definition.  This syntax may be updated from time
   to time based on the evolution of the YANG language.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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
   BCPs 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

Copyright Notice

   Copyright (c) 2018 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
   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 ....................................................2
   2. Tree Diagram Syntax .............................................3
      2.1. Submodules .................................................5
      2.2. Groupings ..................................................5
      2.3. yang-data ..................................................5
      2.4. Collapsed Node Representation ..............................6
      2.5. Comments ...................................................6
      2.6. Node Representation ........................................6
   3. Usage Guidelines for RFCs .......................................7
      3.1. Wrapping Long Lines ........................................8
      3.2. Groupings ..................................................8
      3.3. Long Diagrams ..............................................8
   4. YANG Schema Mount Tree Diagrams .................................9
      4.1. Representation of Mounted Schema Trees ....................10
   5. IANA Considerations ............................................12
   6. Security Considerations ........................................12
   7. Informative References .........................................12
   Authors' Addresses ................................................13

1.  Introduction

   YANG tree diagrams were first published in RFC 6536.  Such diagrams
   are used to provide a simplified graphical representation of a data
   model and can be automatically generated via tools such as "pyang"
   [PYANG].  This document describes the syntax used in YANG tree
   diagrams.  It is expected that this document will be updated or
   replaced as changes to the YANG language [RFC7950] necessitate.

   Today's common practice is to include the definition of the syntax
   used to represent a YANG module in every document that provides a
   tree diagram.  This practice has several disadvantages; therefore,
   the purpose of this document is to provide a single location for this
   definition.  It is not the intent of this document to restrict future
   changes, but rather to ensure that such changes are easily identified
   and suitably agreed upon.

   An example tree diagram can be found in Section 3 of [RFC8343]; the
   following is a portion of it:

     +--rw interfaces
        +--rw interface* [name]
           +--rw name                        string
           +--rw description?                string
           +--rw type                        identityref
           +--rw enabled?                    boolean
           +--rw link-up-down-trap-enable?   enumeration {if-mib}?

2.  Tree Diagram Syntax

   This section describes the meaning of the symbols used in YANG tree

   A full tree diagram of a module represents all elements.  It includes
   the name of the module and sections for top-level module statements
   (typically containers), augmentations, rpcs, and notifications all
   identified under a module statement.  Module trees may be included in
   a document as a whole, by one or more sections, or even by subsets of

   A module is identified by "module:" followed by the module-name.
   This is followed by one or more sections, in order:

   1.  The top-level data nodes defined in the module, offset by
       two spaces.

   2.  Augmentations, offset by two spaces and identified by the keyword
       "augment" followed by the augment target node and a colon (":")

   3.  RPCs, offset by two spaces and identified by "rpcs:".

   4.  Notifications, offset by two spaces and identified by

   5.  Groupings, offset by two spaces and identified by the keyword
       "grouping" followed by the name of the grouping and a colon (":")

   6.  yang-data, offset by two spaces and identified by the keyword
       "yang-data" followed by the name of the yang-data structure and a
       colon (":") character.

   The relative organization of each section is provided using a
   text-based format that is typical of a file system directory tree
   display command.  Each node in the tree is prefaced with "+--".
   Schema nodes that are children of another node are offset from the
   parent by three spaces.  Sibling schema nodes are listed with the
   same space offset and, when separated by lines, are linked via a
   vertical bar ("|") character.

   The full format, including spacing conventions, is:

     module: <module-name>
       |  +--<node>
       |     +--<node>

       augment <target-node>:
       augment <target-node>:

            |  +--<node>

            |  +--<node>

       grouping <grouping-name>:
            |  +--<node>
       grouping <grouping-name>:

       yang-data <yang-data-name>:
            |  +--<node>
       yang-data <yang-data-name>:

2.1.  Submodules

   Submodules are represented in the same fashion as modules but are
   identified by "submodule:" followed by the (sub)module-name.  For

     submodule: <module-name>
       |  +--<node>
       |     +--<node>

2.2.  Groupings

   Nodes within a used grouping are normally expanded as if the nodes
   were defined at the location of the "uses" statement.  However, it is
   also possible to not expand the "uses" statement but to instead print
   the name of the grouping.

   For example, the following diagram shows the "tls-transport" grouping
   from [RFC7407] unexpanded:

       +--rw tls
          +---u tls-transport

   If the grouping is expanded, it could be printed as:

       +--rw tls
          +--rw port?                 inet:port-number
          +--rw client-fingerprint?   x509c2n:tls-fingerprint
          +--rw server-fingerprint?   x509c2n:tls-fingerprint
          +--rw server-identity?      snmp:admin-string

   Groupings may optionally be present in the "groupings" section.

2.3.  yang-data

   If the module defines a "yang-data" structure [RFC8040], these
   structures may optionally be present in the "yang-data" section.

2.4.  Collapsed Node Representation

   At times when the composition of the nodes within a module schema is
   not important in the context of the presented tree, sibling nodes and
   their children can be collapsed using the notation "..." in place of
   the text lines used to represent the summarized nodes.  For example:

       |  ...


   Single line comments, starting with "//" (possibly indented) and
   ending at the end of the line, may be used in the tree notation.

2.6.  Node Representation

   Each node in a YANG module is printed as:

     <status>--<flags> <name><opts> <type> <if-features>

       <status> is one of:
         +  for current
         x  for deprecated
         o  for obsolete

       <flags> is one of:
         rw  for configuration data nodes and choice nodes
         ro  for non-configuration data nodes and choice nodes,
             output parameters to rpcs and actions, and
             notification parameters
         -w  for input parameters to rpcs and actions
         -u  for uses of a grouping
         -x  for rpcs and actions
         -n  for notifications
         mp  for nodes containing a "mount-point" extension statement

         Case nodes do not have any <flags>.

       <name> is the name of the node
         (<name>) means that the node is a choice node
        :(<name>) means that the node is a case node

         If the node is augmented into the tree from another module,
         its name is printed as <prefix>:<name>, where <prefix> is the
         prefix defined in the module where the node is defined.

         If the node is a case node, there is no space before the

       <opts> is one of:
         ?  for an optional leaf, choice, anydata, or anyxml
         !  for a presence container
         *  for a leaf-list or list
         [<keys>] for a list's keys
         /  for a top-level data node in a mounted module
         @  for a top-level data node of a module identified in a
            mount point parent reference

       <type> is the name of the type for leafs and leaf-lists

         If the type is a leafref, the type is printed as either
         (1) "-> TARGET", where TARGET is the leafref path,
         with prefixes removed if possible or (2) "leafref".

       <if-features> is the list of features this node depends on,
         printed within curly brackets and a question mark "{...}?"

   Arbitrary whitespace is allowed between any of the whitespace-
   separated fields (e.g., <opts> and <type>).  Additional whitespace
   may, for example, be used to "column align" fields (e.g., within a
   list or container) to improve readability.

3.  Usage Guidelines for RFCs

   This section provides general guidelines related to the use of tree
   diagrams in RFCs.

3.1.  Wrapping Long Lines

   Internet-Drafts and RFCs limit the number of characters that may
   appear in a line of text to 72 characters.  When the tree
   representation of a node results in a line being longer than this
   limit, the line should be broken between <opts> and <type> or between
   <type> and <if-feature>.  The new line should be indented so that it
   starts below <name> with a whitespace offset of at least two
   characters.  For example:

       +---n yang-library-change
          +--ro module-set-id
                  -> /modules-state/module-set-id

   Long paths (e.g., leafref paths or augment targets) can be split and
   printed on more than one line.  For example:

     augment /nat:nat/nat:instances/nat:instance/nat:mapping-table

   The previously mentioned "pyang" command can be helpful in producing
   such output; for example, the notification diagram above was produced

     pyang -f tree --tree-line-length 50 ietf-yang-library.yang

   When a tree diagram is included as a figure in an Internet-Draft or
   RFC, "--tree-line-length 69" works well.

3.2.  Groupings

   If the YANG module is comprised of groupings only, then the tree
   diagram should contain the groupings.  The "pyang" compiler can be
   used to produce a tree diagram with groupings using the
   "-f tree --tree-print-groupings" command-line parameters.

3.3.  Long Diagrams

   Tree diagrams can be split into sections to correspond to document
   structure.  As tree diagrams are intended to provide a simplified
   view of a module, diagrams longer than a page should generally be
   avoided.  If the complete tree diagram for a module becomes too long,
   the diagram can be split into several smaller diagrams.  For example,
   it might be possible to have one diagram with the data node and
   another with all notifications.  If the data nodes tree is too long,
   it is also possible to split the diagram into smaller diagrams for

   different subtrees.  When long diagrams are included in a document,
   authors should consider whether to include the long diagram in the
   main body of the document or in an appendix.

   An example of such a split can be found in [RFC7407], where
   Section 2.4 of that document shows the diagram for "engine

       +--rw snmp
          +--rw engine
             // more parameters from the "engine" subtree here

   Further, Section 2.5 of [RFC7407] shows the diagram for "target

       +--rw snmp
          +--rw target* [name]
             // more parameters from the "target" subtree here

   The previously mentioned "pyang" command can be helpful in producing
   such output; for example, the above example was produced using:

     pyang -f tree --tree-path /snmp/target ietf-snmp.yang

4.  YANG Schema Mount Tree Diagrams

   "YANG schema mount" is defined in [SCHEMA-MOUNT] and warrants some
   specific discussion.  Schema mount is a generic mechanism that allows
   for the mounting of one or more YANG modules at a specified location
   of another (parent) schema.  The specific location is referred to as
   a "mount point", and any container or list node in a schema may serve
   as a mount point.  Mount points are identified via the inclusion of
   the "mount-point" extension statement as a substatement under a
   container or list node.  Mount point nodes are thus directly
   identified in a module schema definition and can be identified in a
   tree diagram as indicated above using the "mp" flag.

   In the following example taken from [YANG-NIs], "vrf-root" is a
   container that includes the "mount-point" extension statement as part
   of its definition:

     module: ietf-network-instance
       +--rw network-instances
          +--rw network-instance* [name]
             +--rw name           string
             +--rw enabled?       boolean
             +--rw description?   string
             +--rw (ni-type)?
             +--rw (root-type)
                |  +--mp vrf-root

4.1.  Representation of Mounted Schema Trees

   The actual modules made available under a mount point are controlled
   by a server and are provided to clients.  This information is
   typically provided via the schema mount module
   ("ietf-yang-schema-mount") defined in [SCHEMA-MOUNT].  The schema
   mount module supports the exposure of both mounted schema and
   "parent-references".  Parent references are used for XML Path
   Language (XPath) evaluation within mounted modules and do not
   represent client-accessible paths; the referenced information is
   available to clients via the parent schema.  Schema mount also
   defines an "inline" type of mount point, where mounted modules are
   exposed via the YANG library module.

   Although the modules made available under a mount point are not
   specified in YANG modules that include mount points, the document
   defining the module will describe the intended use of the module and
   may identify both modules that will be mounted and parent modules
   that can be referenced by mounted modules.  An example of such a
   description can be found in [YANG-NIs].  A specific implementation of
   a module containing mount points will also support a specific list of
   mounted and referenced modules.  In describing both intended use and
   actual implementations, it is helpful to show how mounted modules
   would be instantiated and referenced under a mount point using tree

   In such diagrams, the mount point should be treated much like a
   container that uses a grouping.  The flags should also be set based
   on the "config" leaf mentioned above, and the mount-related options
   indicated above should be shown for the top-level nodes in a mounted
   or referenced module.  The following example, taken from [YANG-NIs],

   represents the prior example with the YANG modules "ietf-routing"
   [YANG-Routing] and "ietf-ospf" [OSPF-YANG] mounted, nodes from the
   YANG module "ietf-interfaces" [RFC8343] accessible via a
   parent-reference, and "config" indicating "true":

     module: ietf-network-instance
       +--rw network-instances
          +--rw network-instance* [name]
             +--rw name           string
             +--rw enabled?       boolean
             +--rw description?   string
             +--rw (ni-type)?
             +--rw (root-type)
                   +--mp vrf-root
                      +--ro rt:routing-state/
                      |  +--ro router-id?
                      |  +--ro control-plane-protocols
                      |     +--ro control-plane-protocol* [type name]
                      |        +--ro ospf:ospf
                      |           +--ro instance* [af]
                      |           ...
                      +--rw rt:routing/
                      |  +--rw router-id?
                      |  +--rw control-plane-protocols
                      |     +--rw control-plane-protocol* [type name]
                      |     +--rw ospf:ospf
                      |        +--rw instance* [af]
                      |           ...
                      +--ro if:interfaces@
                      |  ...
                      +--ro if:interfaces-state@
                      |  ...

   It is worth highlighting that the "ietf-ospf" module augments the
   "ietf-routing" module, and although it is listed in the schema mount
   module (or inline YANG library), there is no special mount-related
   notation in the tree diagram.

   A mount point definition alone is not sufficient to identify whether
   the mounted modules are used for configuration data or for
   non-configuration data.  This is determined by the
   "ietf-yang-schema-mount" module's "config" leaf associated with the
   specific mount point and is indicated on the top-level mounted nodes.

   For example, in the above tree, when the "config" leaf for the
   "ietf-routing" module indicates "false", the nodes in the
   "rt:routing" subtree would have different flags:

                      +--ro rt:routing/
                      |  +--ro router-id?
                      |  +--ro control-plane-protocols

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   There is no security impact related to the tree diagrams defined in
   this document.

7.  Informative References

              Yeung, D., Qu, Y., Zhang, J., Chen, I., and A. Lindem,
              "Yang Data Model for OSPF Protocol", Work in Progress,
              draft-ietf-ospf-yang-10, March 2018.

   [PYANG]    "pyang", February 2018,

   [RFC7407]  Bjorklund, M. and J. Schoenwaelder, "A YANG Data Model for
              SNMP Configuration", RFC 7407, DOI 10.17487/RFC7407,
              December 2014, <https://www.rfc-editor.org/info/rfc7407>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,

              Bjorklund, M. and L. Lhotka, "YANG Schema Mount", Work in
              Progress, draft-ietf-netmod-schema-mount-08, October 2017.

   [YANG-NIs] Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Model for Network Instances", Work in
              Progress, draft-ietf-rtgwg-ni-model-11, March 2018.

              Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for
              Routing Management (NMDA Version)", Work in Progress,
              draft-ietf-netmod-rfc8022bis-11, January 2018.

Authors' Addresses

   Martin Bjorklund
   Tail-f Systems

   Email: mbj@tail-f.com

   Lou Berger (editor)
   LabN Consulting, L.L.C.

   Email: lberger@labn.net


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