Network Working Group                                       M. Bjorklund
Internet-Draft                                            Tail-f Systems
Intended status: Best Current Practice                    L. Berger, Ed.
Expires: July 27, August 12, 2018                         LabN Consulting, L.L.C.
                                                        January 23,
                                                        February 8, 2018

                           YANG Tree Diagrams


   This document captures the current syntax used in YANG module Tree
   Diagrams.  The purpose of the 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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 27, August 12, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Tree Diagram Syntax . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Submodules  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Groupings . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  yang-data . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Collapsed Node Representation . . . . . . . . . . . . . .   6
     2.5.  Comments  . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.6.  Node Representation . . . . . . . . . . . . . . . . . . .   7
   3.  Usage Guidelines For RFCs . . . . . . . . . . . . . . . . . .   8
     3.1.  Wrapping Long Lines . . . . . . . . . . . . . . . . . . .   8
     3.2.  Groupings . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Long Diagrams . . . . . . . . . . . . . . . . . . . . . .   9
   4.  YANG Schema Mount Tree Diagrams . . . . . . . . . . . . . . .  10
     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 [RFC6536].  Such diagrams
   are used to provided a simplified graphical representation of a data
   model and can be automatically generated via tools such as "pyang".
   (See <>).  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,
   see [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 and the
   purpose of the 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 such changes are easily identified and
   suitably agreed upon.

   An example tree diagram can be found in [RFC7223] Section 3.  A
   portion of which follows:

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

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 subsets of

   A module is identified by "module:" followed 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 2

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

   3.  RPCs, offset by 2 spaces and identified by "rpcs:".

   4.  Notifications, offset by 2 spaces and identified by

   5.  Groupings, offset by 2 spaces, and identified by the keyword
       "grouping" followed by the name of the grouping and a colon (":")

   6.  yang-data, offset by 2 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 prefaces with "+--".  Schema nodes
   that are children of another node are offset from the parent by 3
   spaces.  Sibling schema nodes are listed with the same space offset
   and, when separated by lines, linked via a vertical bar ("|")

   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 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 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 are
   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
         ro  for non-configuration data, 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

       <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.

       <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 in a parent referenced module

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

         If the type is a leafref, the type is either printed as
         "-> TARGET", where TARGET is the leafref path, with prefixes
         removed if possible, or printed as "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 in a
   line of text to 72 characters.  When the tree representation of a
   node results in 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
   white space 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 shows the diagram for "engine configuration":

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

   Further, section 2.5 shows the diagram for "target configuration":

       +--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 [I-D.ietf-netmod-schema-mount] and
   warrants some specific discussion.  Schema mount is a generic
   mechanism that allows for 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 [I-D.ietf-rtgwg-ni-model],
   "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 is controlled
   by a server and is provided to clients.  This information is
   typically provided via the Schema Mount module defined in
   [I-D.ietf-netmod-schema-mount].  The Schema Mount module supports
   exposure of both mounted schema and "parent-references".  Parent
   references are used for 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 mount point where mounted modules are
   exposed via the YANG library module.

   While 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 [I-D.ietf-rtgwg-ni-model].  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 diagrams.

   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
   [I-D.ietf-rtgwg-ni-model], represents the prior example with YANG
   Routing and OSPF modules mounted, YANG Interface module nodes
   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 OSPF module augments the Routing
   module, and while it is listed in the Schema Mount module (or inline
   YANG library) there is no special mount-related notation in the tree

   A mount point definition alone is not sufficient to identify if the
   mounted modules are used for configuration 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" for the 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

   There are no IANA requests or assignments included in this document.

6.  Security Considerations

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

7.  Informative References

              Bjorklund, M. and L. Lhotka, "YANG Schema Mount", draft-
              ietf-netmod-schema-mount-08 (work in progress), October

              Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Network Instances", draft-ietf-rtgwg-ni-
              model-05 (work in progress), December 2017.

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536,
              DOI 10.17487/RFC6536, March 2012, <https://www.rfc-

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,

   [RFC7407]  Bjorklund, M. and J. Schoenwaelder, "A YANG Data Model for
              SNMP Configuration", RFC 7407, DOI 10.17487/RFC7407,
              December 2014, <>.

   [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,

Authors' Addresses

   Martin Bjorklund
   Tail-f Systems


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