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RFC 7950

The YANG 1.1 Data Modeling Language

Pages: 217
Proposed Standard
Errata
Updated by:  8342
Part 9 of 9 – Pages 184 to 217
First   Prev   None

Top   ToC   RFC7950 - Page 184   prevText

14. YANG ABNF Grammar

In YANG, almost all statements are unordered. The ABNF grammar [RFC5234] [RFC7405] defines the canonical order. To improve module readability, it is RECOMMENDED that clauses be entered in this order. Within the ABNF grammar, unordered statements are marked with comments. This grammar assumes that the scanner replaces YANG comments with a single space character. <CODE BEGINS> file "yang.abnf" module-stmt = optsep module-keyword sep identifier-arg-str optsep "{" stmtsep module-header-stmts linkage-stmts meta-stmts revision-stmts body-stmts "}" optsep submodule-stmt = optsep submodule-keyword sep identifier-arg-str optsep "{" stmtsep submodule-header-stmts linkage-stmts meta-stmts revision-stmts body-stmts "}" optsep module-header-stmts = ;; these stmts can appear in any order yang-version-stmt namespace-stmt prefix-stmt submodule-header-stmts = ;; these stmts can appear in any order yang-version-stmt belongs-to-stmt
Top   ToC   RFC7950 - Page 185
   meta-stmts          = ;; these stmts can appear in any order
                         [organization-stmt]
                         [contact-stmt]
                         [description-stmt]
                         [reference-stmt]

   linkage-stmts       = ;; these stmts can appear in any order
                         *import-stmt
                         *include-stmt

   revision-stmts      = *revision-stmt

   body-stmts          = *(extension-stmt /
                           feature-stmt /
                           identity-stmt /
                           typedef-stmt /
                           grouping-stmt /
                           data-def-stmt /
                           augment-stmt /
                           rpc-stmt /
                           notification-stmt /
                           deviation-stmt)

   data-def-stmt       = container-stmt /
                         leaf-stmt /
                         leaf-list-stmt /
                         list-stmt /
                         choice-stmt /
                         anydata-stmt /
                         anyxml-stmt /
                         uses-stmt

   yang-version-stmt   = yang-version-keyword sep yang-version-arg-str
                         stmtend

   yang-version-arg-str = < a string that matches the rule >
                          < yang-version-arg >

   yang-version-arg    = "1.1"

   import-stmt         = import-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             prefix-stmt
                             [revision-date-stmt]
                             [description-stmt]
                             [reference-stmt]
                         "}" stmtsep
Top   ToC   RFC7950 - Page 186
   include-stmt        = include-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [revision-date-stmt]
                              [description-stmt]
                              [reference-stmt]
                          "}") stmtsep

   namespace-stmt      = namespace-keyword sep uri-str stmtend

   uri-str             = < a string that matches the rule >
                         < URI in RFC 3986 >

   prefix-stmt         = prefix-keyword sep prefix-arg-str stmtend

   belongs-to-stmt     = belongs-to-keyword sep identifier-arg-str
                         optsep
                         "{" stmtsep
                             prefix-stmt
                         "}" stmtsep

   organization-stmt   = organization-keyword sep string stmtend

   contact-stmt        = contact-keyword sep string stmtend

   description-stmt    = description-keyword sep string stmtend

   reference-stmt      = reference-keyword sep string stmtend

   units-stmt          = units-keyword sep string stmtend

   revision-stmt       = revision-keyword sep revision-date optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [description-stmt]
                              [reference-stmt]
                          "}") stmtsep

   revision-date       = date-arg-str

   revision-date-stmt  = revision-date-keyword sep revision-date stmtend
Top   ToC   RFC7950 - Page 187
   extension-stmt      = extension-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [argument-stmt]
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                          "}") stmtsep

   argument-stmt       = argument-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              [yin-element-stmt]
                          "}") stmtsep

   yin-element-stmt    = yin-element-keyword sep yin-element-arg-str
                         stmtend

   yin-element-arg-str = < a string that matches the rule >
                         < yin-element-arg >

   yin-element-arg     = true-keyword / false-keyword

   identity-stmt       = identity-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              *base-stmt
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                          "}") stmtsep

   base-stmt           = base-keyword sep identifier-ref-arg-str
                         stmtend

   feature-stmt        = feature-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                          "}") stmtsep
Top   ToC   RFC7950 - Page 188
   if-feature-stmt     = if-feature-keyword sep if-feature-expr-str
                         stmtend

   if-feature-expr-str = < a string that matches the rule >
                         < if-feature-expr >

   if-feature-expr     = if-feature-term
                           [sep or-keyword sep if-feature-expr]

   if-feature-term     = if-feature-factor
                           [sep and-keyword sep if-feature-term]

   if-feature-factor   = not-keyword sep if-feature-factor /
                         "(" optsep if-feature-expr optsep ")" /
                         identifier-ref-arg

   typedef-stmt        = typedef-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             type-stmt
                             [units-stmt]
                             [default-stmt]
                             [status-stmt]
                             [description-stmt]
                             [reference-stmt]
                          "}" stmtsep

   type-stmt           = type-keyword sep identifier-ref-arg-str optsep
                         (";" /
                          "{" stmtsep
                              [type-body-stmts]
                          "}") stmtsep

   type-body-stmts     = numerical-restrictions /
                         decimal64-specification /
                         string-restrictions /
                         enum-specification /
                         leafref-specification /
                         identityref-specification /
                         instance-identifier-specification /
                         bits-specification /
                         union-specification /
                         binary-specification
Top   ToC   RFC7950 - Page 189
   numerical-restrictions = [range-stmt]

   range-stmt          = range-keyword sep range-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [error-message-stmt]
                              [error-app-tag-stmt]
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep

   decimal64-specification = ;; these stmts can appear in any order
                             fraction-digits-stmt
                             [range-stmt]

   fraction-digits-stmt = fraction-digits-keyword sep
                          fraction-digits-arg-str stmtend

   fraction-digits-arg-str = < a string that matches the rule >
                             < fraction-digits-arg >

   fraction-digits-arg = ("1" ["0" / "1" / "2" / "3" / "4" /
                               "5" / "6" / "7" / "8"])
                         / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"

   string-restrictions = ;; these stmts can appear in any order
                         [length-stmt]
                         *pattern-stmt

   length-stmt         = length-keyword sep length-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [error-message-stmt]
                              [error-app-tag-stmt]
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep
Top   ToC   RFC7950 - Page 190
   pattern-stmt        = pattern-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [modifier-stmt]
                              [error-message-stmt]
                              [error-app-tag-stmt]
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep

   modifier-stmt       = modifier-keyword sep modifier-arg-str stmtend

   modifier-arg-str    = < a string that matches the rule >
                         < modifier-arg >

   modifier-arg        = invert-match-keyword

   default-stmt        = default-keyword sep string stmtend

   enum-specification  = 1*enum-stmt

   enum-stmt           = enum-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              [value-stmt]
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep

   leafref-specification =
                         ;; these stmts can appear in any order
                         path-stmt
                         [require-instance-stmt]

   path-stmt           = path-keyword sep path-arg-str stmtend

   require-instance-stmt = require-instance-keyword sep
                            require-instance-arg-str stmtend

   require-instance-arg-str = < a string that matches the rule >
                              < require-instance-arg >

   require-instance-arg = true-keyword / false-keyword
Top   ToC   RFC7950 - Page 191
   instance-identifier-specification =
                         [require-instance-stmt]

   identityref-specification =
                         1*base-stmt

   union-specification = 1*type-stmt

   binary-specification = [length-stmt]

   bits-specification  = 1*bit-stmt

   bit-stmt            = bit-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              [position-stmt]
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                          "}") stmtsep

   position-stmt       = position-keyword sep
                         position-value-arg-str stmtend

   position-value-arg-str = < a string that matches the rule >
                            < position-value-arg >

   position-value-arg  = non-negative-integer-value

   status-stmt         = status-keyword sep status-arg-str stmtend

   status-arg-str      = < a string that matches the rule >
                         < status-arg >

   status-arg          = current-keyword /
                         obsolete-keyword /
                         deprecated-keyword

   config-stmt         = config-keyword sep
                         config-arg-str stmtend

   config-arg-str      = < a string that matches the rule >
                         < config-arg >

   config-arg          = true-keyword / false-keyword
Top   ToC   RFC7950 - Page 192
   mandatory-stmt      = mandatory-keyword sep
                         mandatory-arg-str stmtend

   mandatory-arg-str   = < a string that matches the rule >
                         < mandatory-arg >

   mandatory-arg       = true-keyword / false-keyword

   presence-stmt       = presence-keyword sep string stmtend

   ordered-by-stmt     = ordered-by-keyword sep
                         ordered-by-arg-str stmtend

   ordered-by-arg-str  = < a string that matches the rule >
                         < ordered-by-arg >

   ordered-by-arg      = user-keyword / system-keyword

   must-stmt           = must-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [error-message-stmt]
                              [error-app-tag-stmt]
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep

   error-message-stmt  = error-message-keyword sep string stmtend

   error-app-tag-stmt  = error-app-tag-keyword sep string stmtend

   min-elements-stmt   = min-elements-keyword sep
                         min-value-arg-str stmtend

   min-value-arg-str   = < a string that matches the rule >
                         < min-value-arg >

   min-value-arg       = non-negative-integer-value

   max-elements-stmt   = max-elements-keyword sep
                         max-value-arg-str stmtend

   max-value-arg-str   = < a string that matches the rule >
                         < max-value-arg >

   max-value-arg       = unbounded-keyword /
                         positive-integer-value
Top   ToC   RFC7950 - Page 193
   value-stmt          = value-keyword sep integer-value-str stmtend

   integer-value-str   = < a string that matches the rule >
                         < integer-value >

   grouping-stmt       = grouping-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *(typedef-stmt / grouping-stmt)
                              *data-def-stmt
                              *action-stmt
                              *notification-stmt
                          "}") stmtsep

   container-stmt      = container-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt]
                              *if-feature-stmt
                              *must-stmt
                              [presence-stmt]
                              [config-stmt]
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *(typedef-stmt / grouping-stmt)
                              *data-def-stmt
                              *action-stmt
                              *notification-stmt
                          "}") stmtsep
Top   ToC   RFC7950 - Page 194
   leaf-stmt           = leaf-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt]
                             *if-feature-stmt
                             type-stmt
                             [units-stmt]
                             *must-stmt
                             [default-stmt]
                             [config-stmt]
                             [mandatory-stmt]
                             [status-stmt]
                             [description-stmt]
                             [reference-stmt]
                          "}" stmtsep

   leaf-list-stmt      = leaf-list-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt]
                             *if-feature-stmt
                             type-stmt stmtsep
                             [units-stmt]
                             *must-stmt
                             *default-stmt
                             [config-stmt]
                             [min-elements-stmt]
                             [max-elements-stmt]
                             [ordered-by-stmt]
                             [status-stmt]
                             [description-stmt]
                             [reference-stmt]
                          "}" stmtsep
Top   ToC   RFC7950 - Page 195
   list-stmt           = list-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt]
                             *if-feature-stmt
                             *must-stmt
                             [key-stmt]
                             *unique-stmt
                             [config-stmt]
                             [min-elements-stmt]
                             [max-elements-stmt]
                             [ordered-by-stmt]
                             [status-stmt]
                             [description-stmt]
                             [reference-stmt]
                             *(typedef-stmt / grouping-stmt)
                             1*data-def-stmt
                             *action-stmt
                             *notification-stmt
                          "}" stmtsep

   key-stmt            = key-keyword sep key-arg-str stmtend

   key-arg-str         = < a string that matches the rule >
                         < key-arg >

   key-arg             = node-identifier *(sep node-identifier)

   unique-stmt         = unique-keyword sep unique-arg-str stmtend

   unique-arg-str      = < a string that matches the rule >
                         < unique-arg >

   unique-arg          = descendant-schema-nodeid
                         *(sep descendant-schema-nodeid)
Top   ToC   RFC7950 - Page 196
   choice-stmt         = choice-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt]
                              *if-feature-stmt
                              [default-stmt]
                              [config-stmt]
                              [mandatory-stmt]
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *(short-case-stmt / case-stmt)
                          "}") stmtsep

   short-case-stmt     = choice-stmt /
                         container-stmt /
                         leaf-stmt /
                         leaf-list-stmt /
                         list-stmt /
                         anydata-stmt /
                         anyxml-stmt

   case-stmt           = case-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt]
                              *if-feature-stmt
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *data-def-stmt
                          "}") stmtsep
Top   ToC   RFC7950 - Page 197
   anydata-stmt        = anydata-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt]
                              *if-feature-stmt
                              *must-stmt
                              [config-stmt]
                              [mandatory-stmt]
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep

   anyxml-stmt         = anyxml-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt]
                              *if-feature-stmt
                              *must-stmt
                              [config-stmt]
                              [mandatory-stmt]
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep

   uses-stmt           = uses-keyword sep identifier-ref-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt]
                              *if-feature-stmt
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *refine-stmt
                              *uses-augment-stmt
                          "}") stmtsep
Top   ToC   RFC7950 - Page 198
   refine-stmt         = refine-keyword sep refine-arg-str optsep
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              *must-stmt
                              [presence-stmt]
                              *default-stmt
                              [config-stmt]
                              [mandatory-stmt]
                              [min-elements-stmt]
                              [max-elements-stmt]
                              [description-stmt]
                              [reference-stmt]
                            "}" stmtsep

   refine-arg-str      = < a string that matches the rule >
                         < refine-arg >

   refine-arg          = descendant-schema-nodeid

   uses-augment-stmt   = augment-keyword sep uses-augment-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt]
                             *if-feature-stmt
                             [status-stmt]
                             [description-stmt]
                             [reference-stmt]
                             1*(data-def-stmt / case-stmt /
                                action-stmt / notification-stmt)
                          "}" stmtsep

   uses-augment-arg-str = < a string that matches the rule >
                          < uses-augment-arg >

   uses-augment-arg    = descendant-schema-nodeid
Top   ToC   RFC7950 - Page 199
   augment-stmt        = augment-keyword sep augment-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt]
                             *if-feature-stmt
                             [status-stmt]
                             [description-stmt]
                             [reference-stmt]
                             1*(data-def-stmt / case-stmt /
                                action-stmt / notification-stmt)
                          "}" stmtsep

   augment-arg-str     = < a string that matches the rule >
                         < augment-arg >

   augment-arg         = absolute-schema-nodeid

   when-stmt           = when-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [description-stmt]
                              [reference-stmt]
                           "}") stmtsep

   rpc-stmt            = rpc-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *(typedef-stmt / grouping-stmt)
                              [input-stmt]
                              [output-stmt]
                          "}") stmtsep
Top   ToC   RFC7950 - Page 200
   action-stmt         = action-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *(typedef-stmt / grouping-stmt)
                              [input-stmt]
                              [output-stmt]
                          "}") stmtsep

   input-stmt          = input-keyword optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             *must-stmt
                             *(typedef-stmt / grouping-stmt)
                             1*data-def-stmt
                         "}" stmtsep

   output-stmt         = output-keyword optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             *must-stmt
                             *(typedef-stmt / grouping-stmt)
                             1*data-def-stmt
                         "}" stmtsep

   notification-stmt   = notification-keyword sep
                         identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *if-feature-stmt
                              *must-stmt
                              [status-stmt]
                              [description-stmt]
                              [reference-stmt]
                              *(typedef-stmt / grouping-stmt)
                              *data-def-stmt
                          "}") stmtsep
Top   ToC   RFC7950 - Page 201
   deviation-stmt      = deviation-keyword sep
                         deviation-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [description-stmt]
                             [reference-stmt]
                             (deviate-not-supported-stmt /
                               1*(deviate-add-stmt /
                                  deviate-replace-stmt /
                                  deviate-delete-stmt))
                         "}" stmtsep

   deviation-arg-str   = < a string that matches the rule >
                         < deviation-arg >

   deviation-arg       = absolute-schema-nodeid

   deviate-not-supported-stmt =
                         deviate-keyword sep
                         not-supported-keyword-str stmtend

   deviate-add-stmt    = deviate-keyword sep add-keyword-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [units-stmt]
                              *must-stmt
                              *unique-stmt
                              *default-stmt
                              [config-stmt]
                              [mandatory-stmt]
                              [min-elements-stmt]
                              [max-elements-stmt]
                          "}") stmtsep

   deviate-delete-stmt = deviate-keyword sep delete-keyword-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [units-stmt]
                              *must-stmt
                              *unique-stmt
                              *default-stmt
                          "}") stmtsep
Top   ToC   RFC7950 - Page 202
   deviate-replace-stmt = deviate-keyword sep replace-keyword-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [type-stmt]
                              [units-stmt]
                              [default-stmt]
                              [config-stmt]
                              [mandatory-stmt]
                              [min-elements-stmt]
                              [max-elements-stmt]
                          "}") stmtsep

   not-supported-keyword-str = < a string that matches the rule >
                               < not-supported-keyword >

   add-keyword-str     = < a string that matches the rule >
                         < add-keyword >

   delete-keyword-str  = < a string that matches the rule >
                         < delete-keyword >

   replace-keyword-str = < a string that matches the rule >
                         < replace-keyword >

   ;; represents the usage of an extension
   unknown-statement   = prefix ":" identifier [sep string] optsep
                         (";" /
                          "{" optsep
                              *((yang-stmt / unknown-statement) optsep)
                           "}") stmtsep

   yang-stmt           = action-stmt /
                         anydata-stmt /
                         anyxml-stmt /
                         argument-stmt /
                         augment-stmt /
                         base-stmt /
                         belongs-to-stmt /
                         bit-stmt /
                         case-stmt /
                         choice-stmt /
                         config-stmt /
                         contact-stmt /
                         container-stmt /
                         default-stmt /
                         description-stmt /
                         deviate-add-stmt /
Top   ToC   RFC7950 - Page 203
                         deviate-delete-stmt /
                         deviate-not-supported-stmt /
                         deviate-replace-stmt /
                         deviation-stmt /
                         enum-stmt /
                         error-app-tag-stmt /
                         error-message-stmt /
                         extension-stmt /
                         feature-stmt /
                         fraction-digits-stmt /
                         grouping-stmt /
                         identity-stmt /
                         if-feature-stmt /
                         import-stmt /
                         include-stmt /
                         input-stmt /
                         key-stmt /
                         leaf-list-stmt /
                         leaf-stmt /
                         length-stmt /
                         list-stmt /
                         mandatory-stmt /
                         max-elements-stmt /
                         min-elements-stmt /
                         modifier-stmt /
                         module-stmt /
                         must-stmt /
                         namespace-stmt /
                         notification-stmt /
                         ordered-by-stmt /
                         organization-stmt /
                         output-stmt /
                         path-stmt /
                         pattern-stmt /
                         position-stmt /
                         prefix-stmt /
                         presence-stmt /
                         range-stmt /
                         reference-stmt /
                         refine-stmt /
                         require-instance-stmt /
                         revision-date-stmt /
                         revision-stmt /
                         rpc-stmt /
                         status-stmt /
                         submodule-stmt /
                         typedef-stmt /
                         type-stmt /
Top   ToC   RFC7950 - Page 204
                         unique-stmt /
                         units-stmt /
                         uses-augment-stmt /
                         uses-stmt /
                         value-stmt /
                         when-stmt /
                         yang-version-stmt /
                         yin-element-stmt

   ;; Ranges

   range-arg-str       = < a string that matches the rule >
                         < range-arg >

   range-arg           = range-part *(optsep "|" optsep range-part)

   range-part          = range-boundary
                         [optsep ".." optsep range-boundary]

   range-boundary      = min-keyword / max-keyword /
                         integer-value / decimal-value

   ;; Lengths

   length-arg-str      = < a string that matches the rule >
                         < length-arg >

   length-arg          = length-part *(optsep "|" optsep length-part)

   length-part         = length-boundary
                         [optsep ".." optsep length-boundary]

   length-boundary     = min-keyword / max-keyword /
                         non-negative-integer-value

   ;; Date

   date-arg-str        = < a string that matches the rule >
                         < date-arg >

   date-arg            = 4DIGIT "-" 2DIGIT "-" 2DIGIT

   ;; Schema Node Identifiers

   schema-nodeid       = absolute-schema-nodeid /
                         descendant-schema-nodeid

   absolute-schema-nodeid = 1*("/" node-identifier)
Top   ToC   RFC7950 - Page 205
   descendant-schema-nodeid =
                         node-identifier
                         [absolute-schema-nodeid]

   node-identifier     = [prefix ":"] identifier

   ;; Instance Identifiers

   instance-identifier = 1*("/" (node-identifier
                                 [1*key-predicate /
                                  leaf-list-predicate /
                                  pos]))

   key-predicate       = "[" *WSP key-predicate-expr *WSP "]"

   key-predicate-expr  = node-identifier *WSP "=" *WSP quoted-string

   leaf-list-predicate = "[" *WSP leaf-list-predicate-expr *WSP "]"

   leaf-list-predicate-expr = "." *WSP "=" *WSP quoted-string

   pos                 = "[" *WSP positive-integer-value *WSP "]"

   quoted-string       = (DQUOTE string DQUOTE) / (SQUOTE string SQUOTE)

   ;; leafref path

   path-arg-str        = < a string that matches the rule >
                         < path-arg >

   path-arg            = absolute-path / relative-path

   absolute-path       = 1*("/" (node-identifier *path-predicate))

   relative-path       = 1*("../") descendant-path

   descendant-path     = node-identifier
                         [*path-predicate absolute-path]

   path-predicate      = "[" *WSP path-equality-expr *WSP "]"

   path-equality-expr  = node-identifier *WSP "=" *WSP path-key-expr

   path-key-expr       = current-function-invocation *WSP "/" *WSP
                         rel-path-keyexpr
Top   ToC   RFC7950 - Page 206
   rel-path-keyexpr    = 1*(".." *WSP "/" *WSP)
                         *(node-identifier *WSP "/" *WSP)
                         node-identifier

   ;;; Keywords, using the syntax for case-sensitive strings (RFC 7405)

   ;; statement keywords
   action-keyword           = %s"action"
   anydata-keyword          = %s"anydata"
   anyxml-keyword           = %s"anyxml"
   argument-keyword         = %s"argument"
   augment-keyword          = %s"augment"
   base-keyword             = %s"base"
   belongs-to-keyword       = %s"belongs-to"
   bit-keyword              = %s"bit"
   case-keyword             = %s"case"
   choice-keyword           = %s"choice"
   config-keyword           = %s"config"
   contact-keyword          = %s"contact"
   container-keyword        = %s"container"
   default-keyword          = %s"default"
   description-keyword      = %s"description"
   deviate-keyword          = %s"deviate"
   deviation-keyword        = %s"deviation"
   enum-keyword             = %s"enum"
   error-app-tag-keyword    = %s"error-app-tag"
   error-message-keyword    = %s"error-message"
   extension-keyword        = %s"extension"
   feature-keyword          = %s"feature"
   fraction-digits-keyword  = %s"fraction-digits"
   grouping-keyword         = %s"grouping"
   identity-keyword         = %s"identity"
   if-feature-keyword       = %s"if-feature"
   import-keyword           = %s"import"
   include-keyword          = %s"include"
   input-keyword            = %s"input"
   key-keyword              = %s"key"
   leaf-keyword             = %s"leaf"
   leaf-list-keyword        = %s"leaf-list"
   length-keyword           = %s"length"
   list-keyword             = %s"list"
   mandatory-keyword        = %s"mandatory"
   max-elements-keyword     = %s"max-elements"
   min-elements-keyword     = %s"min-elements"
   modifier-keyword         = %s"modifier"
   module-keyword           = %s"module"
   must-keyword             = %s"must"
   namespace-keyword        = %s"namespace"
Top   ToC   RFC7950 - Page 207
   notification-keyword     = %s"notification"
   ordered-by-keyword       = %s"ordered-by"
   organization-keyword     = %s"organization"
   output-keyword           = %s"output"
   path-keyword             = %s"path"
   pattern-keyword          = %s"pattern"
   position-keyword         = %s"position"
   prefix-keyword           = %s"prefix"
   presence-keyword         = %s"presence"
   range-keyword            = %s"range"
   reference-keyword        = %s"reference"
   refine-keyword           = %s"refine"
   require-instance-keyword = %s"require-instance"
   revision-keyword         = %s"revision"
   revision-date-keyword    = %s"revision-date"
   rpc-keyword              = %s"rpc"
   status-keyword           = %s"status"
   submodule-keyword        = %s"submodule"
   type-keyword             = %s"type"
   typedef-keyword          = %s"typedef"
   unique-keyword           = %s"unique"
   units-keyword            = %s"units"
   uses-keyword             = %s"uses"
   value-keyword            = %s"value"
   when-keyword             = %s"when"
   yang-version-keyword     = %s"yang-version"
   yin-element-keyword      = %s"yin-element"

   ;; other keywords

   add-keyword              = %s"add"
   current-keyword          = %s"current"
   delete-keyword           = %s"delete"
   deprecated-keyword       = %s"deprecated"
   false-keyword            = %s"false"
   invert-match-keyword     = %s"invert-match"
   max-keyword              = %s"max"
   min-keyword              = %s"min"
   not-supported-keyword    = %s"not-supported"
   obsolete-keyword         = %s"obsolete"
   replace-keyword          = %s"replace"
   system-keyword           = %s"system"
   true-keyword             = %s"true"
   unbounded-keyword        = %s"unbounded"
   user-keyword             = %s"user"
Top   ToC   RFC7950 - Page 208
   and-keyword              = %s"and"
   or-keyword               = %s"or"
   not-keyword              = %s"not"

   current-function-invocation = current-keyword *WSP "(" *WSP ")"

   ;;; Basic Rules

   prefix-arg-str      = < a string that matches the rule >
                         < prefix-arg >

   prefix-arg          = prefix

   prefix              = identifier

   identifier-arg-str  = < a string that matches the rule >
                         < identifier-arg >

   identifier-arg      = identifier

   identifier          = (ALPHA / "_")
                         *(ALPHA / DIGIT / "_" / "-" / ".")

   identifier-ref-arg-str = < a string that matches the rule >
                            < identifier-ref-arg >

   identifier-ref-arg  = identifier-ref

   identifier-ref      = [prefix ":"] identifier

   string              = < an unquoted string, as returned by >
                         < the scanner, that matches the rule >
                         < yang-string >

   yang-string         = *yang-char
Top   ToC   RFC7950 - Page 209
   ;; any Unicode or ISO/IEC 10646 character, including tab, carriage
   ;; return, and line feed but excluding the other C0 control
   ;; characters, the surrogate blocks, and the noncharacters
   yang-char = %x09 / %x0A / %x0D / %x20-D7FF /
                               ; exclude surrogate blocks %xD800-DFFF
              %xE000-FDCF /    ; exclude noncharacters %xFDD0-FDEF
              %xFDF0-FFFD /    ; exclude noncharacters %xFFFE-FFFF
              %x10000-1FFFD /  ; exclude noncharacters %x1FFFE-1FFFF
              %x20000-2FFFD /  ; exclude noncharacters %x2FFFE-2FFFF
              %x30000-3FFFD /  ; exclude noncharacters %x3FFFE-3FFFF
              %x40000-4FFFD /  ; exclude noncharacters %x4FFFE-4FFFF
              %x50000-5FFFD /  ; exclude noncharacters %x5FFFE-5FFFF
              %x60000-6FFFD /  ; exclude noncharacters %x6FFFE-6FFFF
              %x70000-7FFFD /  ; exclude noncharacters %x7FFFE-7FFFF
              %x80000-8FFFD /  ; exclude noncharacters %x8FFFE-8FFFF
              %x90000-9FFFD /  ; exclude noncharacters %x9FFFE-9FFFF
              %xA0000-AFFFD /  ; exclude noncharacters %xAFFFE-AFFFF
              %xB0000-BFFFD /  ; exclude noncharacters %xBFFFE-BFFFF
              %xC0000-CFFFD /  ; exclude noncharacters %xCFFFE-CFFFF
              %xD0000-DFFFD /  ; exclude noncharacters %xDFFFE-DFFFF
              %xE0000-EFFFD /  ; exclude noncharacters %xEFFFE-EFFFF
              %xF0000-FFFFD /  ; exclude noncharacters %xFFFFE-FFFFF
              %x100000-10FFFD  ; exclude noncharacters %x10FFFE-10FFFF

   integer-value       = ("-" non-negative-integer-value) /
                          non-negative-integer-value

   non-negative-integer-value = "0" / positive-integer-value

   positive-integer-value = (non-zero-digit *DIGIT)

   zero-integer-value  = 1*DIGIT

   stmtend             = optsep (";" / "{" stmtsep "}") stmtsep

   sep                 = 1*(WSP / line-break)
                         ; unconditional separator

   optsep              = *(WSP / line-break)

   stmtsep             = *(WSP / line-break / unknown-statement)

   line-break          = CRLF / LF

   non-zero-digit      = %x31-39

   decimal-value       = integer-value ("." zero-integer-value)
Top   ToC   RFC7950 - Page 210
   SQUOTE              = %x27
                         ; single quote

   ;;; core rules from RFC 5234

   ALPHA               = %x41-5A / %x61-7A
                         ; A-Z / a-z

   CR                  = %x0D
                         ; carriage return

   CRLF                = CR LF
                         ; Internet standard newline

   DIGIT               = %x30-39
                         ; 0-9

   DQUOTE              = %x22
                         ; double quote

   HTAB                = %x09
                         ; horizontal tab

   LF                  = %x0A
                         ; line feed

   SP                  = %x20
                         ; space

   WSP                 = SP / HTAB
                         ; whitespace

   <CODE ENDS>
Top   ToC   RFC7950 - Page 211

15. NETCONF Error Responses for YANG-Related Errors

A number of NETCONF error responses are defined for error cases related to the data model handling. If the relevant YANG statement has an "error-app-tag" substatement, that overrides the default value specified below.

15.1. Error Message for Data That Violates a "unique" Statement

If a NETCONF operation would result in configuration data where a "unique" constraint is invalidated, the following error MUST be returned: error-tag: operation-failed error-app-tag: data-not-unique error-info: <non-unique>: Contains an instance identifier that points to a leaf that invalidates the "unique" constraint. This element is present once for each non-unique leaf. The <non-unique> element is in the YANG namespace ("urn:ietf:params:xml:ns:yang:1").

15.2. Error Message for Data That Violates a "max-elements" Statement

If a NETCONF operation would result in configuration data where a list or a leaf-list would have too many entries, the following error MUST be returned: error-tag: operation-failed error-app-tag: too-many-elements This error is returned once, with the error-path identifying the list node, even if there is more than one extra child present.

15.3. Error Message for Data That Violates a "min-elements" Statement

If a NETCONF operation would result in configuration data where a list or a leaf-list would have too few entries, the following error MUST be returned: error-tag: operation-failed error-app-tag: too-few-elements This error is returned once, with the error-path identifying the list node, even if there is more than one child missing.
Top   ToC   RFC7950 - Page 212

15.4. Error Message for Data That Violates a "must" Statement

If a NETCONF operation would result in configuration data where the restrictions imposed by a "must" statement are violated, the following error MUST be returned, unless a specific "error-app-tag" substatement is present for the "must" statement. error-tag: operation-failed error-app-tag: must-violation

15.5. Error Message for Data That Violates a "require-instance" Statement

If a NETCONF operation would result in configuration data where a leaf of type "instance-identifier" or "leafref" marked with require-instance "true" refers to an instance that does not exist, the following error MUST be returned: error-tag: data-missing error-app-tag: instance-required error-path: Path to the instance-identifier or leafref leaf.

15.6. Error Message for Data That Violates a Mandatory "choice" Statement

If a NETCONF operation would result in configuration data where no nodes exists in a mandatory choice, the following error MUST be returned: error-tag: data-missing error-app-tag: missing-choice error-path: Path to the element with the missing choice. error-info: <missing-choice>: Contains the name of the missing mandatory choice. The <missing-choice> element is in the YANG namespace ("urn:ietf:params:xml:ns:yang:1").

15.7. Error Message for the "insert" Operation

If the "insert" and "key" or "value" attributes are used in an <edit-config> for a list or leaf-list node and the "key" or "value" refers to an instance that does not exist, the following error MUST be returned: error-tag: bad-attribute error-app-tag: missing-instance
Top   ToC   RFC7950 - Page 213

16. IANA Considerations

This document registers one capability identifier URN from the "Network Configuration Protocol (NETCONF) Capability URNs" registry: Index Capability Identifier ------------- --------------------------------------------------- :yang-library urn:ietf:params:netconf:capability:yang-library:1.0

17. Security Considerations

This document defines a language with which to write and read descriptions of management information. The language itself has no security impact on the Internet. The same considerations are relevant as those for the base NETCONF protocol (see Section 9 in [RFC6241]). Data modeled in YANG might contain sensitive information. RPCs or notifications defined in YANG might transfer sensitive information. Security issues are related to the usage of data modeled in YANG. Such issues shall be dealt with in documents describing the data models and documents about the interfaces used to manipulate the data, e.g., the NETCONF documents. Data modeled in YANG is dependent upon: o the security of the transmission infrastructure used to send sensitive information. o the security of applications that store or release such sensitive information. o adequate authentication and access control mechanisms to restrict the usage of sensitive data. YANG parsers need to be robust with respect to malformed documents. Reading malformed documents from unknown or untrusted sources could result in an attacker gaining the privileges of the user running the YANG parser. In an extreme situation, the entire machine could be compromised.
Top   ToC   RFC7950 - Page 214

18. References

18.1. Normative References

[ISO.10646] International Organization for Standardization, "Information Technology - Universal Multiple-Octet Coded Character Set (UCS)", ISO Standard 10646:2014, 2014. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <http://www.rfc-editor.org/info/rfc3629>. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc3986>. [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <http://www.rfc-editor.org/info/rfc4648>. [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc5234>. [RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008, <http://www.rfc-editor.org/info/rfc5277>. [RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011, <http://www.rfc-editor.org/info/rfc6241>. [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, <http://www.rfc-editor.org/info/rfc7405>. [RFC7895] Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module Library", RFC 7895, DOI 10.17487/RFC7895, June 2016, <http://www.rfc-editor.org/info/rfc7895>.
Top   ToC   RFC7950 - Page 215
   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", W3C Recommendation REC-xml-20081126,
              November 2008,
              <https://www.w3.org/TR/2008/REC-xml-20081126/>.

   [XML-NAMES]
              Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
              Thompson, "Namespaces in XML 1.0 (Third Edition)", World
              Wide Web Consortium Recommendation REC-xml-names-20091208,
              December 2009,
              <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

   [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", World Wide Web Consortium Recommendation
              REC-xpath-19991116, November 1999,
              <http://www.w3.org/TR/1999/REC-xpath-19991116>.

   [XSD-TYPES]
              Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
              Second Edition", World Wide Web Consortium Recommendation
              REC-xmlschema-2-20041028, October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.

18.2. Informative References

[CoMI] van der Stok, P. and A. Bierman, "CoAP Management Interface", Work in Progress, draft-vanderstok-core-comi-09, March 2016. [IEEE754-2008] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE 754-2008, DOI 10.1109/IEEESTD.2008.4610935, 2008, <http://standards.ieee.org/findstds/ standard/754-2008.html>. [RESTCONF] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", Work in Progress, draft-ietf-netconf-restconf-16, August 2016. [RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., "Structure of Management Information Version 2 (SMIv2)", STD 58, RFC 2578, DOI 10.17487/RFC2578, April 1999, <http://www.rfc-editor.org/info/rfc2578>.
Top   ToC   RFC7950 - Page 216
   [RFC2579]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Textual Conventions for SMIv2",
              STD 58, RFC 2579, DOI 10.17487/RFC2579, April 1999,
              <http://www.rfc-editor.org/info/rfc2579>.

   [RFC3780]  Strauss, F. and J. Schoenwaelder, "SMIng - Next Generation
              Structure of Management Information", RFC 3780,
              DOI 10.17487/RFC3780, May 2004,
              <http://www.rfc-editor.org/info/rfc3780>.

   [RFC4844]  Daigle, L., Ed., and Internet Architecture Board, "The RFC
              Series and RFC Editor", RFC 4844, DOI 10.17487/RFC4844,
              July 2007, <http://www.rfc-editor.org/info/rfc4844>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <http://www.rfc-editor.org/info/rfc6020>.

   [RFC6643]  Schoenwaelder, J., "Translation of Structure of Management
              Information Version 2 (SMIv2) MIB Modules to YANG
              Modules", RFC 6643, DOI 10.17487/RFC6643, July 2012,
              <http://www.rfc-editor.org/info/rfc6643>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <http://www.rfc-editor.org/info/rfc6991>.

   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,
              <http://www.rfc-editor.org/info/rfc7951>.

   [XPATH2.0]
              Berglund, A., Boag, S., Chamberlin, D., Fernandez, M.,
              Kay, M., Robie, J., and J. Simeon, "XML Path Language
              (XPath) 2.0 (Second Edition)", World Wide Web Consortium
              Recommendation REC-xpath20-20101214, December 2010,
              <http://www.w3.org/TR/2010/REC-xpath20-20101214>.

   [XSLT]     Clark, J., "XSL Transformations (XSLT) Version 1.0", World
              Wide Web Consortium Recommendation REC-xslt-19991116,
              November 1999,
              <http://www.w3.org/TR/1999/REC-xslt-19991116>.

   [YANG-Guidelines]
              Bierman, A., "Guidelines for Authors and Reviewers of YANG
              Data Model Documents", Work in Progress,
              draft-ietf-netmod-rfc6087bis-07, July 2016.
Top   ToC   RFC7950 - Page 217

Acknowledgements

The editor wishes to thank the following individuals, who all provided helpful comments on various draft versions of this document: Mehmet Ersue, Washam Fan, Joel Halpern, Per Hedeland, Leif Johansson, Ladislav Lhotka, Lionel Morand, Gerhard Muenz, Peyman Owladi, Tom Petch, Randy Presuhn, David Reid, Jernej Tuljak, Kent Watsen, Bert Wijnen, Robert Wilton, and Dale Worley.

Contributors

The following people all contributed significantly to the initial YANG document: - Andy Bierman (YumaWorks) - Balazs Lengyel (Ericsson) - David Partain (Ericsson) - Juergen Schoenwaelder (Jacobs University Bremen) - Phil Shafer (Juniper Networks)

Author's Address

Martin Bjorklund (editor) Tail-f Systems Email: mbj@tail-f.com