Network Working Group                                       M. Bjorklund
Internet-Draft                                            Tail-f Systems
Intended status: Standards Track                          L. Berger, Ed.
Expires: December 15, 31, 2017                       LabN Consulting, L.L.C.
                                                           June 13, 29, 2017

                           YANG Tree Diagrams


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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Tree Diagram Syntax . . . . . . . . . . . . . . . . . . . . .   2   3
     2.1.  Submodules  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Groupings . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Collapsed Node Representation . . . . . . . . . . . . . .   4
     2.4.  Node Representation . . . . . . . . . . . . . . . . . . .   4
     2.5.  Extensions  . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Next Steps  Usage Guidelines For RFCs . . . . . . . . . . . . . . . . . .   5
     3.1.  Wrapping Long Lines . . . . . . .   3 . . . . . . . . . . . .   6
   4.  YANG Schema Mount Tree Diagrams . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.   7
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   4   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   4   8

1.  Introduction

   YANG Tree Diagrams were first published in [RFC7223].  Such diagrams
   are commonly used to provided a simplified graphical representation
   of a data model and can be automatically generated via tools such as
   "pyang".  (See <>).  This document
   provides 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 common practice is 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 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

   The remainder of this document contains YANG Tree Diagram syntax
   based on output from pyang version 1.7.1.

2.  Tree Diagram Syntax

   This section provides the meaning of the symbols used in YANG Tree

   Each node in

   A full tree diagram of 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
       -x represents all elements.  It includes
   the name of the module and sections for top level module statements
   (typically containers), augmentations, rpcs and actions
       -n  for notifications

     <name> is the name 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 node
       (<name>) means that module-name.  Top
   level module statements are listed immediately following, offset by 4
   spaces.  Augmentations are listed next, offset by 2 spaces and
   identified by the node is a choice node
      :(<name>) means that keyword "augment" followed by the augment target
   node is and a case node

      If the node colon (':') character.  This is followed by, RPCs which
   are identified by "rpcs:" and are also offset by 2 spaces.
   Notifications are last and are identified by "notifications:" and are
   also offset by 2 spaces.

   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.  Schema peer nodes separated are listed with the same space
   offset and, when separated by lines, linked via a pipe ('|')

   The full format, including spacing conventions is:

   module: <module-name>

     |  +--<node>
     |     +--<node>
     augment <target-node>:


          |  +--<node>

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 expanded as if the nodes were
   defined at the location of the uses statement.

2.3.  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, peer nodes and
   their children can be collapsed using the notation '...' in place of
   the text lines used to represent the summarized nodes.  For example:

     |  ...

2.4.  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
       -x  for rpcs and actions
       -n  for notifications
       mp   for schema mount points

     <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>.

     <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 mounted module
       @  for a node made available via a schema mount
          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 "-> TARGET",
       where TARGET is either the leafref path, with prefixed removed
       if possible.

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

2.5.  Extensions


3.  Next Steps

   Authors' note:  Usage Guidelines For RFCs

   This draft is currently a bit rough.  The next/future
   version is expected section provides general guidelines related to add text covering the use of tree
   diagrams in RFCs.  This section covers [Authors' note: will cover]
   different types of trees and when to use them; for example, complete
   module trees, subtrees, trees for groupings etc.  Maybe also how

3.1.  Wrapping Long Lines

   Internet Drafts and RFCs limit the number of characters that may in a
   line of text to deal with 72 characters.  When the limited tree representation of a
   node results in line
   lengths being longer than this limit the line should be
   broken between <opts> and <type>.  The type should be indented so
   that the new line 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

   The previously 'pyang' command can be helpful in RFCs. producing such
   output, for example the above example was produced using:

     pyang -f tree --tree-line-length 50 < ietf-yang-library.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 document is a
   generic mechanism that allows for mounting one data model consisting
   of any number of YANG modules at a specified location of another
   (parent) schema.  Modules containing mount points will identify mount
   points by name using the mount-point extension.  These mount-points
   should be identified, as indicated above using the 'mp' flag.  For

       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?

   Note that a mount point definition alone is not sufficient to
   identify if a mount point configuration or for non-configuration
   data.  This is determined by the yang-schema-mount module 'config'
   leaf associated with the specific mount point.

   In describing the intended use of a module containing a mount point,
   it is helpful to show how the mount point would look with mounted
   modules.  In such cases, 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 realted options
   indicated above should be shown.  For example, the following
   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/
                        |  ...
                        +--rw rt:routing/
                        |  ...
                        +--ro if:interfaces@
                        |  ...
                        +--ro if:interfaces-state@

   The with 'config' indicating false, the only change would be to the
   flag on the rt:routing node:

                        +--ro rt:routing/

5.  IANA Considerations

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


6.  Informative References

              Bjorklund, M. and L. Lhotka, "YANG Schema Mount", draft-
              ietf-netmod-schema-mount-05 (work in progress), May 2017.

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

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

Authors' Addresses

   Martin Bjorklund
   Tail-f Systems


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