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Versions: 00 01                                                         
Network Working Group                                     J. Kiesewalter
Internet-Draft                                        Universität Bremen
Intended status: Informational                           C. Bormann, Ed.
Expires: 11 May 2022                              Universität Bremen TZI
7 November 2021

Mapping between YANG and SDF
draft-kiesewalter-asdf-yang-sdf-01

Abstract

YANG and SDF are two languages for modelling the interaction with and
the data interchanged with devices in the network.  As their areas of
application (network management, IoT, resp.) overlap, it is useful to
be able to translate between the two.

The present specification provides information about how models in
one of the two languages can be translated into the other.  This
specification is not intended to be normative, but to help with
creating translators.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 11 May 2022.

Copyright (c) 2021 IETF Trust and the persons identified as the

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.

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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.

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
2.  Pairing SDF and YANG features . . . . . . . . . . . . . . . .   3
3.  Mapping from YANG to SDF  . . . . . . . . . . . . . . . . . .  11
3.1.  Module  . . . . . . . . . . . . . . . . . . . . . . . . .  11
3.2.  Submodule . . . . . . . . . . . . . . . . . . . . . . . .  13
3.3.  Container Statement . . . . . . . . . . . . . . . . . . .  13
3.4.  Leaf Statement  . . . . . . . . . . . . . . . . . . . . .  15
3.5.  Leaf-List Statement . . . . . . . . . . . . . . . . . . .  17
3.6.  List Statement  . . . . . . . . . . . . . . . . . . . . .  18
3.7.  Grouping Statement  . . . . . . . . . . . . . . . . . . .  19
3.8.  Uses Statement  . . . . . . . . . . . . . . . . . . . . .  20
3.9.  Choice Statement  . . . . . . . . . . . . . . . . . . . .  21
3.10. RPC Statement . . . . . . . . . . . . . . . . . . . . . .  24
3.11. Action Statement  . . . . . . . . . . . . . . . . . . . .  24
3.12. Notification Statement  . . . . . . . . . . . . . . . . .  26
3.13. Augment Statement . . . . . . . . . . . . . . . . . . . .  27
3.14. Anydata and Anyxml Statements . . . . . . . . . . . . . .  28
3.15. Type Statement  . . . . . . . . . . . . . . . . . . . . .  28
3.16. String Built-In Type  . . . . . . . . . . . . . . . . . .  29
3.17. Decimal64 Built-In Type . . . . . . . . . . . . . . . . .  31
3.18. Integer Built-In Types  . . . . . . . . . . . . . . . . .  33
3.19. Boolean Built-In Type . . . . . . . . . . . . . . . . . .  34
3.20. Binary Built-In Type  . . . . . . . . . . . . . . . . . .  34
3.21. Enumeration Built-In Type . . . . . . . . . . . . . . . .  35
3.22. Bits Built-In Type  . . . . . . . . . . . . . . . . . . .  35
3.23. Union Built-In Type . . . . . . . . . . . . . . . . . . .  36
3.24. Leafref and Identityref Built-In Types  . . . . . . . . .  37
3.25. Empty Built-In Type . . . . . . . . . . . . . . . . . . .  37
3.26. Instance-Identifier Built-In Type . . . . . . . . . . . .  37
3.27. Typedef Statement . . . . . . . . . . . . . . . . . . . .  38
3.28. Identity Statement  . . . . . . . . . . . . . . . . . . .  38
3.29. Config Statement  . . . . . . . . . . . . . . . . . . . .  38
3.30. Status Statement  . . . . . . . . . . . . . . . . . . . .  39
3.31. Reference Statement . . . . . . . . . . . . . . . . . . .  39
3.32. When and Must Statements  . . . . . . . . . . . . . . . .  39
3.33. Extension Statement . . . . . . . . . . . . . . . . . . .  40
4.  Mapping from SDF to YANG  . . . . . . . . . . . . . . . . . .  40
4.1.  Information Block . . . . . . . . . . . . . . . . . . . .  40
4.2.  Namespace Section . . . . . . . . . . . . . . . . . . . .  41
4.3.  SdfThing Quality  . . . . . . . . . . . . . . . . . . . .  41
4.4.  SdfObject Quality . . . . . . . . . . . . . . . . . . . .  41

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4.5.  Common Qualities  . . . . . . . . . . . . . . . . . . . .  42
4.6.  Data Qualities  . . . . . . . . . . . . . . . . . . . . .  50
4.7.  SdfData Quality . . . . . . . . . . . . . . . . . . . . .  57
4.8.  SdfProperty Quality . . . . . . . . . . . . . . . . . . .  59
4.9.  SdfAction Quality . . . . . . . . . . . . . . . . . . . .  62
4.10. SdfEvent Quality  . . . . . . . . . . . . . . . . . . . .  63
5.  Challenges  . . . . . . . . . . . . . . . . . . . . . . . . .  64
5.1.  Differences in Expressiveness of SDF and YANG . . . . . .  64
5.2.  Round Trips . . . . . . . . . . . . . . . . . . . . . . .  66
5.3.  Type References . . . . . . . . . . . . . . . . . . . . .  67
6.  Implementation Considerations . . . . . . . . . . . . . . . .  70
7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  70
8.  Security considerations . . . . . . . . . . . . . . . . . . .  70
9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  70
9.1.  Normative References  . . . . . . . . . . . . . . . . . .  70
9.2.  Informative References  . . . . . . . . . . . . . . . . .  70
Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  71
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  71

1.  Introduction

YANG [RFC7950] and SDF [I-D.ietf-asdf-sdf] are two languages for
modelling the interaction with and the data interchanged with devices
in the network.  As their areas of application (network management,
IoT, resp.) overlap, it is useful to be able to translate between the
two.

The present specification provides information about how models in
one of the two languages can be translated into the other.  This
specification is not intended to be normative, but to help with
creating translators.

2.  Pairing SDF and YANG features

Table 1 gives an overview over how language features of YANG can be
mapped to SDF features.  In many cases, several translations are
possible, and the right choice depends on the context.  The mappings
in this draft often accommodate the use of the YANG parser Libyang
[LIBYANG].

For YANG statements that are not mentioned in the table no conversion
to SDF was found that preserves the statement's semantics.

For possible conversions of YANG's built-in types please refer to
Section 3.  Please note that a 'type object' is not the same as an
sdfObject but refers to SDF's built-in type 'object', also called
compound-type.  This built-in type makes use of the 'properties'
quality which is not to be confused with the sdfProperty class.  The

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data types number/decimal64, integer, boolean, string are also
referred to as simple (data) types.  In turn, the types array and
object are sometimes referred to as complex (data) types.  Concerning
YANG, the expression 'schema tree' refers to the model's tree whereas
'data tree' describes the tree of an instance of the model.

+=================+==============+================================+
| YANG statement  | remark on    | converted to SDF               |
|                 | YANG         |                                |
|                 | statement    |                                |
+=================+==============+================================+
| module          |              | SDF model (i.e., info block,   |
|                 |              | namespace section &            |
|                 |              | definitions)                   |
+-----------------+--------------+--------------------------------+
| submodule       | included in  | integrated into SDF model of   |
|                 | supermodule  | supermodule                    |
+-----------------+--------------+--------------------------------+
|                 | on its own   | SDF model                      |
+-----------------+--------------+--------------------------------+
| container       | top-level    | sdfObject                      |
+-----------------+--------------+--------------------------------+
|                 | one level    | sdfProperty of type object     |
|                 | below top-   | (compound-type)                |
|                 | level        |                                |
+-----------------+--------------+--------------------------------+
|                 | on any other | property (type object) of the  |
|                 | level        | 'parent' definition of type    |
|                 |              | object (compound-type)         |
+-----------------+--------------+--------------------------------+
| leaf            | on top-level | sdfProperty (type              |
|                 | and one      | integer/number/boolean/string) |
|                 | level below  |                                |
+-----------------+--------------+--------------------------------+
|                 | on any other | property (type                 |
|                 | level        | integer/number/boolean/string) |
|                 |              | of the 'parent' definition of  |
|                 |              | type object (compound-type)    |
+-----------------+--------------+--------------------------------+
| leaflist        | on top-level | sdfProperty of type array      |
|                 | and one      |                                |
|                 | level below  |                                |
+-----------------+--------------+--------------------------------+
|                 | on any other | property (type array) of the   |
|                 | level        | 'parent' definition of type    |
|                 |              | object (compound-type)         |
+-----------------+--------------+--------------------------------+
| list            | on top-level | sdfProperty of type array with |

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|                 | and one      | items of type object           |
|                 | level below  | (compound-type)                |
+-----------------+--------------+--------------------------------+
|                 | on any other | property (type array with      |
|                 | level        | items of type object           |
|                 |              | (compound-type)) of the        |
|                 |              | 'parent' definition of type    |
|                 |              | object* (compound-type)        |
+-----------------+--------------+--------------------------------+
| choice          |              | sdfChoice                      |
+-----------------+--------------+--------------------------------+
| case            | belonging to | element of the sdfChoice       |
|                 | choice       |                                |
+-----------------+--------------+--------------------------------+
| grouping        |              | sdfData of compound-type (type |
|                 |              | object) at the top level which |
|                 |              | can then be referenced         |
+-----------------+--------------+--------------------------------+
| uses            | referencing  | sdfRef to the SDF definition   |
|                 | a grouping   | corresponding to the           |
|                 |              | referenced grouping            |
+-----------------+--------------+--------------------------------+
| rpc             |              | sdfAction at the top-level of  |
|                 |              | the SDF model                  |
+-----------------+--------------+--------------------------------+
| action          |              | sdfAction of the sdfObject     |
|                 |              | corresponding to a container   |
|                 |              | the action is a descendant     |
|                 |              | node to                        |
+-----------------+--------------+--------------------------------+
| notification    |              | sdfEvent                       |
+-----------------+--------------+--------------------------------+
| anydata         |              | not converted                  |
+-----------------+--------------+--------------------------------+
| anyxml          |              | not converted                  |
+-----------------+--------------+--------------------------------+
| augment         |              | augment's target is converted  |
|                 |              | with the augmentation already  |
|                 |              | applied, mentioned in the      |
|                 |              | description                    |
+-----------------+--------------+--------------------------------+
| type            | referring to | type with other data qualities |
|                 | a built-in   | (e.g., default) if necessary   |
|                 | type         |                                |
+-----------------+--------------+--------------------------------+
| type            | referring to | sdfRef to the corresponding    |
|                 | a typedef    | sdfData element                |
+-----------------+--------------+--------------------------------+

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| base            |              | sdfRef to the sdfData          |
|                 |              | definition corresponding to    |
|                 |              | the base                       |
+-----------------+--------------+--------------------------------+
| bit             |              | 'parent' definition is of      |
|                 |              | compound-type and gets one     |
|                 |              | entry in the properties        |
|                 |              | quality of type boolean for    |
|                 |              | each bit                       |
+-----------------+--------------+--------------------------------+
| enum            |              | each enum statement's argument |
|                 |              | is added as an element to the  |
|                 |              | SDF enum quality's string      |
|                 |              | array                          |
+-----------------+--------------+--------------------------------+
| fraction-digits |              | multipleOf quality             |
+-----------------+--------------+--------------------------------+
| length          | single       | minLength/maxLength qualities  |
|                 | length range |                                |
+-----------------+--------------+--------------------------------+
|                 | single value | minLength and maxLength        |
|                 |              | qualities set to the same      |
|                 |              | value                          |
+-----------------+--------------+--------------------------------+
|                 | contains     | sdfChoice with alternatives    |
|                 | alternatives | for minLength/maxLength        |
|                 |              | qualities                      |
+-----------------+--------------+--------------------------------+
| path            |              | sdfRef to the corresponding    |
|                 |              | SDF definition                 |
+-----------------+--------------+--------------------------------+
| pattern         | single       | pattern quality                |
|                 | pattern      |                                |
+-----------------+--------------+--------------------------------+
|                 | multiple     | pattern quality, the regular   |
|                 | patterns     | expressions are combined using |
|                 |              | positive lookahead             |
+-----------------+--------------+--------------------------------+
|                 | invert-match | pattern quality, the regular   |
|                 |              | expression is modified using   |
|                 |              | negative lookahead             |
+-----------------+--------------+--------------------------------+
| range           | single range | minimum/maximum qualities      |
+-----------------+--------------+--------------------------------+
|                 | single value | const quality                  |
|                 | (constant)   |                                |
+-----------------+--------------+--------------------------------+
|                 | contains     | sdfChoice with either minimum/ |

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|                 | alternatives | maximum or const quality as    |
|                 |              | alternatives                   |
+-----------------+--------------+--------------------------------+
| typedef         |              | sdfData definition, sdfRef     |
|                 |              | where it is used               |
+-----------------+--------------+--------------------------------+
| identity        |              | sdfData definition, sdfRef     |
|                 |              | where it is used               |
+-----------------+--------------+--------------------------------+
| config          | of a         | set writable for all elements  |
|                 | container    | in the sdfObject that can be   |
|                 | that became  | marked as writable (i.e., that |
|                 | an sdfObject | use the data qualities)        |
+-----------------+--------------+--------------------------------+
|                 | of any other | set writable                   |
|                 | YANG element |                                |
+-----------------+--------------+--------------------------------+
| import          |              | the module that the import     |
|                 |              | references is converted        |
|                 |              | (elements can now be           |
|                 |              | referenced by sdfRef) and its  |
|                 |              | prefix and namespace are added |
|                 |              | the to namespace section       |
+-----------------+--------------+--------------------------------+
| revisions       |              | first revision date becomes    |
|                 |              | version in information block   |
+-----------------+--------------+--------------------------------+
| namespace       |              | added to namespace section     |
+-----------------+--------------+--------------------------------+
| prefix          |              | added to namespace section     |
+-----------------+--------------+--------------------------------+

Table 1: Mapping YANG to SDF

Table 2 provides the inverse mapping.

+=============+=========================+===========================+
| SDF quality | remark on SDF quality   | converted to YANG         |
+=============+=========================+===========================+
| sdfThing    |                         | container node            |
+-------------+-------------------------+---------------------------+
| sdfObject   |                         | container node            |
+-------------+-------------------------+---------------------------+
| sdfProperty | type                    | leaf node                 |
|             | integer/number/boolean/ |                           |
|             | string                  |                           |
+-------------+-------------------------+---------------------------+
|             | type array with items   | leaf-list node            |

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|             | of type                 |                           |
|             | integer/number/boolean/ |                           |
|             | string                  |                           |
+-------------+-------------------------+---------------------------+
|             | type array with items   | list node                 |
|             | of type object          |                           |
|             | (compound-type)         |                           |
+-------------+-------------------------+---------------------------+
|             | type object (compound-  | container node            |
|             | type)                   |                           |
+-------------+-------------------------+---------------------------+
| sdfAction   | at the top-level, *not* | rpc node                  |
|             | part of an sdfObject    |                           |
+-------------+-------------------------+---------------------------+
|             | inside of an sdfObject  | action node as child      |
|             |                         | node to the container     |
|             |                         | corresponding to the      |
|             |                         | sdfObject                 |
+-------------+-------------------------+---------------------------+
| sdfEvent    |                         | notification node with    |
|             |                         | child nodes that were     |
|             |                         | translated like           |
|             |                         | sdfProperty               |
+-------------+-------------------------+---------------------------+
| sdfData     | type                    | typedef                   |
|             | integer/number/boolean/ |                           |
|             | string                  |                           |
+-------------+-------------------------+---------------------------+
|             | type array with items   | grouping node with        |
|             | of type                 | leaf-list child node      |
|             | integer/number/boolean/ |                           |
|             | string                  |                           |
+-------------+-------------------------+---------------------------+
|             | type array with items   | grouping node with list   |
|             | of type object          | child node                |
|             | (compound-type)         |                           |
+-------------+-------------------------+---------------------------+
|             | type object (compound-  | grouping node             |
|             | type)                   |                           |
+-------------+-------------------------+---------------------------+
| sdfRef      | referenced definition   | type is set to the        |
|             | was converted to        | typedef corresponding     |
|             | typedef                 | to the sdfData            |
|             |                         | definition                |
+-------------+-------------------------+---------------------------+
|             | referenced definition   | leafref                   |
|             | was converted to leaf   |                           |
|             | or leaf-list node       |                           |

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+-------------+-------------------------+---------------------------+
|             | referenced definition   | "uses" node that          |
|             | was converted to        | references                |
|             | grouping node           | corresponding grouping    |
|             |                         | (and refine if            |
|             |                         | necessary)                |
+-------------+-------------------------+---------------------------+
| sdfRequired | referenced definition   | set the mandatory         |
|             | was converted to a leaf | statement of the          |
|             | or choice node          | corresponding leaf/       |
|             |                         | choice node to true       |
+-------------+-------------------------+---------------------------+
|             |                         | find the first            |
|             |                         | descendant node that is   |
|             |                         | either a leaf/choice      |
|             |                         | node and set their        |
|             |                         | mandatory statement to    |
|             |                         | true or that is a leaf-   |
|             |                         | list/list node and set    |
|             |                         | their min-elements        |
|             |                         | statement to 1 (if not    |
|             |                         | already >= 0)             |
+-------------+-------------------------+---------------------------+
| sdfChoice   |                         | choice node with one      |
|             |                         | case node for each        |
|             |                         | alternative of the        |
|             |                         | sdfChoice, each           |
|             |                         | alternative is            |
|             |                         | converted like            |
|             |                         | sdfProperty               |
+-------------+-------------------------+---------------------------+
| type        |                         |                           |
+-------------+-------------------------+---------------------------+
| const       | corresponding YANG      | range statement with a    |
|             | element has empty range | single value              |
+-------------+-------------------------+---------------------------+
|             | range not empty         | add single value          |
|             |                         | alternative to range      |
|             |                         | statement (must be        |
|             |                         | disjunct)                 |
+-------------+-------------------------+---------------------------+
| default     | type is one of          | default statement of      |
|             | integer/number/boolean/ | leaf/leaf-list nodes      |
|             | string or array with    |                           |
|             | items of these types    |                           |
+-------------+-------------------------+---------------------------+
| minimum/    | corresponding YANG      | range statement           |
| maximum     | element has empty range |                           |

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+-------------+-------------------------+---------------------------+
|             | range not empty         | add range alternative     |
|             |                         | to range statement        |
|             |                         | (must be disjunct)        |
+-------------+-------------------------+---------------------------+
| multipleOf  |                         | fraction-digits           |
|             |                         | statement                 |
+-------------+-------------------------+---------------------------+
| minLength/  |                         | length statement          |
| maxLength   |                         |                           |
+-------------+-------------------------+---------------------------+
| pattern     |                         | pattern statement         |
+-------------+-------------------------+---------------------------+
| minItems/   |                         | min-elements/max-         |
| maxItems    |                         | elements statements       |
+-------------+-------------------------+---------------------------+
| uniqueItems | if the 'parent' SDF     | unique statement          |
| set to true | definition is converted | mentioning all of the     |
|             | to a list node          | leaf/leaf-list nodes in   |
|             |                         | the list node's sub-      |
|             |                         | tree                      |
+-------------+-------------------------+---------------------------+
| items       |                         | sub-statements of list/   |
|             |                         | leaf-list node            |
|             |                         | corresponding to the      |
|             |                         | item quality's 'parent'   |
|             |                         | definition                |
+-------------+-------------------------+---------------------------+
| properties  |                         | child nodes of            |
|             |                         | container/grouping node   |
|             |                         | corresponding to the      |
|             |                         | properties quality's      |
|             |                         | 'parent' definition       |
+-------------+-------------------------+---------------------------+
| unit        |                         | units statement           |
+-------------+-------------------------+---------------------------+
| enum        |                         | type enumeration with     |
|             |                         | enum statements for       |
|             |                         | each string in the SDF    |
|             |                         | enum quality              |
+-------------+-------------------------+---------------------------+
| sdfType     | has value 'byte-string' | built-in type 'binary'    |
+-------------+-------------------------+---------------------------+
| writable    |                         | config statement          |
+-------------+-------------------------+---------------------------+

Table 2: Mapping SDF to YANG

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3.  Mapping from YANG to SDF

This section specifies one possible mapping for each of the YANG
statements to SDF in detail.  For reference on the individual YANG
statements see [RFC7950] and [I-D.ietf-asdf-sdf] for SDF.  Examples
have been included where they serve to assist the reader's
understanding of the conversion.

3.1.  Module

*  YANG: Section 7.1 (module) of [RFC7950]

*  SDF:

-  Section 3.1 (information block) of [I-D.ietf-asdf-sdf]

-  Sections 3.2 and 4 (namespaces section) of [I-D.ietf-asdf-sdf]

The module statement in YANG subsumes all other statements included
in a module.  After conversion the SDF model as a whole corresponds
to the YANG module.  The argument of the namespace statement of the
YANG module is added to the SDF namespace quality together with the
argument of the prefix statement of the YANG module which also
becomes the entry of the defaultNamespace quality in the SDF model.
Additionally, the namespaces and prefixes of each of the modules
mentioned in the import statements are added to the namespace quality
of the SDF model.  Libyang loads the imported modules automatically
and in the correct version.  These modules are then also converted
and stored so their definitions can be referenced via the sdfRef
common quality when necessary.  Figure 2 and Figure 1 illustrate
these mappings.

The contents of the organization, contact and yang-version statements
are stored alongside the description of the YANG module in a special
sdfData definition designated to hold information on the module that
does not fit into the SDF information block.  This is done in with a
conversion note to facilitate round trips in the future as described
in Section 5.2.  To illustrate this conversion, Figure 2 contains a
converted model with an sdfData definition called ietf-foo-info.  The
original YANG module can be found in Figure 1.  The description of
the module is scanned for information regarding copyright and
qualities of the information block in the SDF model.  The version
quality of the information block is set to the first revision date
given in the YANG revision statement.  All other revision dates are
ignored as of now.

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YANG modules can define features via the feature statement to make
parts of the module conditional.  The abilities of a server are
checked against the features stated in the module.  Nodes reference
features as an argument to the if-feature statement.  If a server
does not support a certain feature, nodes that reference that feature
are ignored by the server.  Since this functionality cannot be
represented in SDF yet, YANG features are stored in the description
of the sdfData definition designated to hold information on the
module.  The conversion note that is added to the descriptions looks
as described in Section 5.2.

If the deviation statement (introducing a deviation from the original
YANG module) is present in the YANG module, Libyang applies the
deviation directly and the converter converts the module that way.
The presence of the deviation in the original YANG module is not
indicated in the resulting SDF model as of now which might cause
inconsistencies after round trips.  This is not believed to be of
great importance because deviations are supposed to only occur in
unpublished modules.

module ietf-foo {
namespace "urn:ietf:params:xml:ns:yang:ietf-foo";
prefix "foo";
organization "Foo Inc.";
contact "foo@mail.com";
description
"This is an example module

revision 2016-03-20;
feature bar;
feature baz;

// ... more statements
}

Figure 1: Example YANG module

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{
"defaultNamespace": "foo",
"info": {
"title": "ietf-foo",
"version": "2016-03-20"
},
"namespace": { "foo": "urn:ietf:params:xml:ns:yang:ietf-foo" },
"sdfData": {
"ietf-foo-info": {
"description": "This is an example module\n\nCopyright Foo Inc.\n\nLicense XY\n!Conversion note: revision 2016-03-20!\n\n!Conversion note: organization Foo Inc.!\n\n!Conversion note: contact foo@mail.com!\n!Conversion note: feature bar!\n\n!Conversion note: feature baz!\n"
}
}
}

Figure 2: SDF conversion of YANG module from the last figure

3.2.  Submodule

*  YANG: Section 7.2 (submodule) of [RFC7950]

If a complex YANG module is composed of several components, the
single components can be represented via the submodule statement.
For conversion, the nodes of a submodule that is included into its
super-module with the include statement are integrated into the
super-module and converted that way.  This is due to the way Libyang
represents included submodules.  Submodules on their own cannot be
converted since Libyang does not parse files that solely contain a
submodule.

3.3.  Container Statement

*  YANG: Section 7.5 (container) of [RFC7950]

*  SDF:

-  Sections 2.2.1 and 5.1 (sdfObject) of [I-D.ietf-asdf-sdf]

-  Sections 2.2.6 and 6.3 (sdfThing) of [I-D.ietf-asdf-sdf]

YANG uses container nodes to group together other nodes.  Containers
on the top-level of a module are converted to sdfObject definitions.
This is illustrated in the definition called level0 in Figure 3 and
Figure 4.  A container that is a direct child node to a top-level
container is converted to a compound-type sdfProperty definition
inside an sdfObject, as illustrated in the definition called level1
in Figure 3 and Figure 4.  Any other container becomes an entry to

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the properties quality of the compound-type definition corresponding
to the parent node of the container.  An example of this mapping can
be found in Figure 3 and Figure 4 in the definition called level2.

Since the first SDF Internet-Draft did not contain the compound-type
as a possible argument to the type quality, containers used to be
translated to sdfThing definitions.  This, was not a very suitable
conversion semantically, however.  At that time, sdfThings were the
only elements that could contain elements of the same class, that is
sdfThings could contain other sdfThings.  This ability is required to
represent the tree structure of YANG where, for example, containers
can contain other containers.  In the second SDF Internet-Draft the
compound-type was introduced.  This feature effectively makes it
possible for elements of the sdfData and sdfProperty classes to
contain elements that share the same qualities.

A sub-statement to the container statement that cannot be represented
in SDF as of now is the optional presence statement.  The argument of
the presence statement assigns a meaning to the presence or absence
of a container node in an instance of the module.  This concept is
expressed in the description of the SDF definition in question as
shown in Section 5.2.  This is also illustrated in the definition
level2 in Figure 3 and Figure 4.

module container-example {
// [...]
container level0 {
container level1 {
container level2 {
presence "Enables SSH";
// [...]
}
}
}
}

Figure 3: YANG module with multiple nested container statements

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{
; [...]
"sdfObject": {
"level0": {
"sdfProperty": {
"level1": {
"properties": {
"level2": {
"properties": {
"description": "!Conversion note: presence Enables SSH!\n",
; [...]
},
"type": "object"
}
},
"type": "object"
}
}
}
}
}

Figure 4: SDF conversion of the YANG module from the last figure

3.4.  Leaf Statement

*  YANG: Section 7.6 (leaf) of [RFC7950]

*  SDF:

-  Sections 2.2.2 and 5.2 (sdfProperty) of [I-D.ietf-asdf-sdf]

-  Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

Leaf nodes in YANG represent scalar variables.  If a leaf statement
occurs at the top-level of the module or as a direct child node of a
top-level container (which is converted to sdfObject) it becomes an
sdfProperty.  On any other level a leaf is mapped to an entry of the
properties quality of the compound-type definition corresponding to
the parent node of the leaf.  In both cases the SDF type quality is
set to one of the simple data types because leaf nodes can only have
simple data types.  Leaf nodes can be assigned default values which
are used in case the node does not exist in an instance of the YANG
module.  The default value of a leaf is converted to SDF through the
quality default.  The units sub-statement of a leaf node in YANG
becomes the SDF quality unit.  An example of such a conversion can be
found in the level0 element in Figure 5 and Figure 6.  The SDF
quality unit is constrained to the SenML unit names.  Although it

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could cause conformance issues, the content of the YANG units
statement is not processed to fit the SenML unit names as of now.
This is due to the low probability that a unit from a YANG module is
not listed in the SenML unit names in comparison to the time required
to implement a mechanism to check conformance and convert non-
conforming units.  This feature might be added in later versions of
the converter.  YANG leaf nodes can be marked as mandatory to occur
in an instance of the module by the mandatory statement.  The
statement takes true and false as arguments.  This can easily be
mapped to SDF through the sdfRequired quality.  A reference to the
SDF equivalent of the mandatory YANG leaf node is added to the
sdfRequired quality of the containing sdfObject.  If a mandatory leaf
is transformed to an entry in the properties quality of a compound-
type definition in SDF, said entry is mentioned in the required
quality.  If the sdfRequired or required quality does not already
exist it is added at this point.  The latter is demonstrated in the
level2 element in Figure 5 and Figure 6.

module leaf-example {
// [...]
leaf level0 {
type int32;
units "kg";
default 14;
}
container dummy0 {
leaf level1 { type string; }
container dummy1 {
leaf level2 {
type string;
mandatory true;
}
}
}
}

Figure 5: YANG module containing multiple leaf statements

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{
; [...]
"sdfObject": {
"dummy0": {
"sdfProperty": {
"dummy1": {
"properties": {
"level2": { "type": "string" }
},
"required": [ "level2" ],
"type": "object"
},
"level1": { "type": "string" }
}
}
},
"sdfProperty": {
"level0": {
"default": 14,
; [...]
"type": "integer",
"unit": "kg"
}
}
}

Figure 6: SDF conversion of the YANG module from the last figure

3.5.  Leaf-List Statement

*  YANG: Section 7.7 (leaf-list) of [RFC7950]

*  SDF:

-  Sections 2.2.2 and 5.2 (sdfProperty) of [I-D.ietf-asdf-sdf]

-  Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

Similarly to leaf nodes, leaf-list nodes hold data of simple types in
YANG but as items in an array.  As such, leaf-lists are converted to
sdfProperties if they occur on the top-level or one level below in a
module.  On any other level a leaf-list becomes an entry to the
properties quality of the compound-type definition corresponding to
the parent node of the leaf-list.  In both cases the type is set to
array.  The items of the array are of simple data types since leaf-
list nodes can only have simple data types as well.  The minimal and
maximal number of elements in a YANG leaf-list can be specified by
the min-elements and max-elements sub-statements.  This is analogue

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to the minItems and maxItems qualities of SDF which are set
accordingly by the converter.  A leaf-list can specify whether the
system or the user is responsible for ordering the entries of the
leaf-list.  This information is stored in the ordered-by statement in
YANG which is represented in SDF by a remark in the description (as
shown in Section 5.2) of the SDF equivalent to the leaf-list node in
question.  Since leaf-list nodes are just leaf nodes that can occur
multiple times, the units and default statements of leaf-list nodes
are converted as described for leaf nodes in Section 3.4.

3.6.  List Statement

*  YANG: Section 7.8 (list) of [RFC7950]

*  SDF:

-  Sections 2.2.2 and 5.2 (sdfProperty) of [I-D.ietf-asdf-sdf]

-  Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

The list statement of YANG is similar to the leaf-list statement.
The only difference is that, opposed to leaf-lists, lists represent
an assortment of _nodes_ that can occur multiple times.  Therefore,
YANG lists are mapped to SDF similarly to leaf-lists.  List nodes on
the top-level or one level below become sdfProperties.  On any other
level a list is converted to an entry to the properties quality of
the compound-type definition corresponding to the parent node of the
list.  The type is set to array for both alternatives.  Since lists
contain a set of nodes, the items of the corresponding array are of
type object.  The minimal and maximal number of elements in a list
can be specified by the min-elements and max-elements sub-statements.
This is analogue to the minItems and maxItems qualities of SDF which
are set accordingly by the converter.  List nodes in YANG can define
one or multiple keys leafs of the list via the key statement.  There
is no SDF quality that could represent this feature.  To preserve the
information the names of the list keys are stored in the description
of the SDF definition in question as described in section
Section 5.2.  The unique sub-statement of the YANG list defines a
number of descendant leaf nodes of the list that must have a unique
combination of values in a module instance.  This concept can be
partly represented through the uniqueItems quality of SDF.  However,
the boolean-typed uniqueItems quality only specifies that the items
of an SDF array have to be unique with _all_ of their values
combined.  The YANG statement unique specifies a _selection_ of leaf
node values in the list that must be unique when combined.  Thus, in
addition to setting the uniqueItems quality of the SDF equivalent of
the YANG list to true, a conversion note is added to the SDF
equivalents of all leafs that are mentioned in the unique statement.

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This is done as shown in Section Section 5.2.  The ordered-by
statement of a list is also preserved in a conversion note.  An
example conversion of a list node with the mentioned sub-statements
to SDF can be found in Figure 7 and Figure 8.

list server {
key "name";
unique "ip";
ordered-by user;
min-elements 1;
max-elements 100;
leaf name { type string; }
leaf ip { type string; }
}

Figure 7: YANG list node

"sdfProperty": {
"server": {
"description": "!Conversion note: key name!\n!Conversion note: ordered-by user!\n",
"items": {
"properties": {
"ip": {
"description": "!Conversion note: unique!\n",
"type": "string"
},
"name": { "type": "string" }
},
"type": "object"
},
"maxItems": 100.0,
"minItems": 1.0,
"type": "array",
"uniqueItems": true
}
}

Figure 8: SDF conversion of the YANG list node from the last figure

3.7.  Grouping Statement

*  YANG: Section 7.12 (grouping) of [RFC7950]

*  SDF: Section 5.5 (sdfData) of [I-D.ietf-asdf-sdf]

Grouping nodes are very similar to container nodes with the
difference that the set of nodes contained in a grouping does not
occur in the data tree unless the grouping has been referenced at

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least once by a uses node.  Thus, a grouping node is converted to a
compound-type sdfData definition which defines a reusable definition
that is not a declaration as well.  The nodes inside the grouping are
converted as entries to the properties quality in SDF.  Figure 9 and
Figure 10 contain an example conversion of a grouping.

3.8.  Uses Statement

*  YANG: Section 7.13 (uses) of [RFC7950]

*  SDF: Section 4.4 (sdfRef) of [I-D.ietf-asdf-sdf]

A uses node has the purpose of referencing a grouping node.  The set
of child nodes of the referenced grouping are copied to wherever the
uses node is featured.  Some of the sub-statements of the referenced
grouping can be altered via the refine statement of the uses node.
In SDF a uses node is represented by the sdfRef quality which is
added to the definition in question.  As an argument the sdfRef
contains a reference to the sdfData definition corresponding to the
grouping referenced by the uses node.  If the uses node contains a
refine statement, the specified refinements are also applied in the
target SDF definition.  An example for such a conversion is
illustrated in Figure 9 and Figure 10.

module restaurant {
// [...]
grouping dish {
leaf name { type string; }
leaf price { type int32; }
}
// [...]
uses dish {
refine name { mandatory true; }
}
}
}

Figure 9: YANG module with uses and grouping statements

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{
; [...]
"sdfData": {
"dish": {
"properties": {
"name": { "type": "string" },
"price": {
; [...]
"type": "integer"
}
},
"type": "object"
}
},
"sdfProperty": {
"items": {
"properties": {
"dish": {
"sdfRef": "#/sdfData/dish",
"required": [ "name" ],
}
}
"type": "object"
},
"type": "array"
}
}
}

Figure 10: SDF conversion of the YANG module from the last figure

3.9.  Choice Statement

*  YANG: Section 7.9 (choice) of [RFC7950]

*  SDF: Section 4.7.2 (sdfChoice) of [I-D.ietf-asdf-sdf]

Conversion of the choice statement from YANG is simple since it is
similar to the sdfChoice quality.  The choice statement is used to
define alternative sub-trees for the node the choice occurs in.  Only
one of the alternatives is present in the data tree.  A YANG choice
is converted to an sdfProperty if it occurs on top-level or one level
below, like the snack definition in Figure 11 and Figure 12.  On any
other level a choice is mapped to an entry of the properties quality
of the compound-type definition corresponding to the parent node of
the choice.  The food-level2 definition in Figure 11 and Figure 12 is
an example of this kind of mapping.  The SDF equivalent of the choice

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contains the sdfChoice quality.  Case or other child nodes of the
choice are mapped to SDF as one of the named alternatives of the
sdfChoice each.  What cannot be represented is the default sub-
statement of the YANG choice that defines which of the alternatives
is considered the default one.  This information is preserved in a
conversion note as described in Section 5.2.

container food {
container food-level2 {
choice dinner {
default home-cooked;
case restaurant {
leaf steak { type boolean; }
leaf pizza { type boolean; }
}
case home-cooked {
leaf pasta { type boolean; }
}
}
}
choice snack {
case sports-arena {
leaf pretzel { type boolean; }
leaf beer { type boolean; }
}
case late-night {
leaf chocolate { type boolean; }
}
}
}

Figure 11: YANG container using the choice statement

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"sdfObject": {
"food": {
"sdfProperty": {
"food-level2": {
"properties": {
"dinner": {
"description": "!Conversion note: default home-cooked!\n",
"sdfChoice": {
"home-cooked": {
"properties": {
"pasta": { "type": "boolean" }
},
"type": "object"
},
"restaurant": {
"properties": {
"pizza": { "type": "boolean" },
"steak": { "type": "boolean" }
},
"type": "object"
}
}
}
},
"type": "object"
},
"snack": {
"description": "!Conversion note: default late-night!\n",
"sdfChoice": {
"late-night": {
"properties": {
"chocolate": { "type": "boolean" }
},
"type": "object"
},
"sports-arena": {
"properties": {
"beer": { "type": "boolean" },
"pretzel": { "type": "boolean" }
},
"type": "object"
}
}
}
}
}
}

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Figure 12: SDF conversion of the YANG container from the last figure

3.10.  RPC Statement

*  YANG: Section 7.14 (rpc) of [RFC7950]

*  SDF: Sections 2.2.3 and 5.3 (sdfAction) of [I-D.ietf-asdf-sdf]

Remote procedure calls (RPCs) can be modeled in YANG with rpc nodes
which have up to one input child node holding the commands input data
and up to one output node for the output data.  In YANG RPCs can only
occur on the top-level because in contrast to actions in YANG they do
not belong to a container.  This can easily be represented by
sdfActions.  The corresponding sdfAction is not placed inside an
sdfObject or sdfThing but at the top-level of the SDF model to
represent independence from a container.  The input node of the RPC
is converted to the sdfInputData quality of the sdfAction which is of
type object.  Equivalently, the output node of the RPC becomes the
sdfOutputData of the sdfAction, which is also of type object.
Groupings and typedefs in the RPC are converted to sdfData
definitions inside the sdfAction.

3.11.  Action Statement

*  YANG: Section 7.15 (action) of [RFC7950]

*  SDF: Sections 2.2.3 and 5.3 (sdfAction) of [I-D.ietf-asdf-sdf]

Action nodes in YANG work similarly to rpc nodes in the way that they
are used to model operations that can be invoked in the module and
also have up to one input and output child node respectively.  As
mentioned before, YANG actions are affiliated to a container.  The
representation of this affiliation is not quite trivial because YANG
containers are not translated to sdfObjects in all cases.  Only
sdfObjects can have sdfActions, however.  If an action occurs in a
container that is a below-top-level container (and thus not converted
to sdfObject), as illustrated in Figure 13, the affiliation cannot be
represented directly in SDF as of now.  Figure 14 shows how an XML
instance of calling the action in Figure 13 and the reply would look
like.  As an input, the action specifies the container server it is
affiliated to and its name.  The actual action, reset and the value
of its input, reset-at are specified inside the container instance.
The result after converting the container from Figure 13 to SDF can
be found in Figure 15: To ensure equivalence of model instances a
copy of the contents of the converted container is set as the
sdfInputData of the sdfAction.  The sdfInputData is of type object.
The conversion of the actual action along with its input is added to
the copy of the container conversion as another entry to its

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properties quality.  Furthermore, a conversion note is added as
described in Section 5.2.  Equivalently, the output nodes of the
action become the sdfOutputData of the sdfAction which is also of
type object.  Groupings and typedefs in the action node are converted
to sdfData definitions inside the sdfAction.

container example-container {}
container server {
leaf name { type string; }
action reset {
input {
leaf reset-at { type string; }
}
output {
leaf reset-finished-at { type string; }
}
}
}
}

Figure 13: YANG container using the action statement

<rpc message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<action xmlns="urn:ietf:params:xml:ns:yang:1">
<server xmlns="urn:example:server-farm">
<name>apache-1</name>
<reset>
<reset-at>2014-07-29T13:42:00Z</reset-at>
</reset>
</server>
</action>
</rpc>

<reset-finished-at xmlns="urn:example:server-farm">
2014-07-29T13:42:12Z
</reset-finished-at>

Figure 14: XML instance of the action from the last figure

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"sdfObject": {
"example-container": {
"sdfAction": {
"reset": {
"description": "Action connected to server\n\n",
"sdfInputData": {
"properties": {
"server": {
"properties": {
"name": { "type": "string"  },
"reset": {
"properties": {
"reset-at": { "type": "string" }
},
"type": "object"
}
},
"type": "object"
}
},
"required": [ "server" ],
"type": "object"
},
"sdfOutputData": {
"properties": {
"reset-finished-at": { "type": "string" }
},
"type": "object"
}
}
},
"sdfProperty": {
"server": {
"properties": {
"name": { "type": "string" }
},
"type": "object"
}
}
}
}

Figure 15: SDF conversion of the YANG container from Figure 13

*  YANG: Section 7.16 (notification) of [RFC7950]

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*  SDF: Sections 2.2.4 and 5.4 (sdfEvent) of [I-D.ietf-asdf-sdf]

Notification nodes are converted to sdfEvent definitions.  Their
child nodes are converted to the sdfOutputData of the sdfEvent which
is of type object.  Groupings and typedefs in the notification node
are converted to sdfData definitions inside the sdfEvent.

3.13.  Augment Statement

*  YANG: Section 7.17 (augment) of [RFC7950]

*  SDF: Section 4.6. (common qualities) of [I-D.ietf-asdf-sdf]

The augment statement can either occur at the top-level of a module
to add nodes to an existing target module or sub-module, or in a uses
statement to augment the targeted and thus integrated grouping.  The
conversion of the augment statement to SDF is not trivial because SDF
does not feature this mechanism.

The tool used to deserialize YANG modules, Libyang, adds the nodes
into the target of the augment statement automatically for targets
that are modules or sub-modules.  This is adopted in the mapping: The
SDF model that corresponds to target of the the augment statement is
converted with the augmentation already applied.  A conversion note
is added to the description as described in Section 5.2 to preserve
where the augmentation was issued from.  This mapping is illustrated
in Figure 16, Figure 17 and Figure 18.  If the resulting SDF model
has to be converted back to YANG, definitions that are marked as
augmentations are converted back accordingly.  This way of mapping
the augment statement to SDF causes problems if the augmentation
target lies within a module whose converted version is already
available and should not be replaced.  Because, as of now, SDF does
not offer means to extend already existing models retroactively these
augmentations cannot be converted to SDF.

When the target of the augment is a grouping the augmentation cannot
be represented in SDF, either.  The reason for this is that grouping
nodes are converted to SDF definitions with the type object.  The
nodes inside the grouping are converted with the help of the
properties quality.  It is currently not possible to add properties
to the properties quality, it can only be overridden as a whole.

module example-module {
// [...]
leaf leaf1 { type string; }
}

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Figure 16: YANG module that serves as an augmentation target

module augmenting-module {
// [...]
augment "/example" {
leaf additional-leaf { type string; }
}
}

Figure 17: YANG module using the augment statement on the module
from the last figure

{
; [...]
"sdfProperty": {
"leaf1": { "type": "string" },
"description": "!Conversion note: augmented-by augmenting-module!\n",
"type": "string"
}
}
}

Figure 18: SDF conversion of the YANG module from Figure 16 after
conversion of the YANG module from Figure 17

3.14.  Anydata and Anyxml Statements

*  YANG: Sections 7.10 and 7.11 (augment) of [RFC7950]

*  SDF: Section 4.6 (common qualities) of [I-D.ietf-asdf-sdf]

The anydata and anyxml statements are designated for nodes in the
schema tree whose structure is unknown at the design time of the
module or in general.  Since this is not a concept that can be
represented in SDF as of now, anydata and anyxml nodes are not
converted.  Instead, to preserve the information a conversion note is
added to the SDF element corresponding to the parent node of the
anydata or anyxml node as described in Section 5.2.

3.15.  Type Statement

*  YANG: Section 7.4 (type) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

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The type statement of YANG is used to specify the built-in or derived
type used by a leaf or typedef node.  Mapping this statement to YANG
is trivial if the argument is a simple data type because the SDF data
qualities also contain a type quality.  A derived type used as an
argument to the YANG type statement is converted via the sdfRef
quality.  As an argument, the sdfRef quality contains a reference to
the sdfData definition corresponding to the derived type.  If the
derived type is restricted, for example with the length statement,
the restrictions are converted as they would be for the base type and
added to the SDF definition containing the type in question.

There are multiple sub-statements to the type statement that depend
on its value.  The conversion of those sub-statements is discussed in
the section of the built-in type the sub-statement belongs to.

3.16.  String Built-In Type

*  YANG: Section 9.4 (string) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

The YANG built-in type string is converted to the SDF built-in type
string.  Strings in YANG can be restricted in length and by regular
expressions.

The length statement can specify either a constant length, a lower
inclusive length, an upper inclusive length or both a lower and upper
inclusive length.  A length statement can also specify more than one
disjoint constant length or length ranges.  The values min and max in
a length statement represent the minimum and maximum lengths accepted
for strings.  If the length statement in YANG does not contain a
constant value but a length range it is converted to the minLength
and maxLength SDF qualities.  This is illustrated in Figure 19 and
Figure 20.  If a constant value is defined through the YANG length
statement the minLength and maxLength qualities are set to the same
value.  If the length statement specifies multiple length ranges or
constant values the sdfChoice quality is used for conversion.  The
named alternatives of the sdfChoice contain the single converted
length ranges or constant values each.  If the min and max values are
present in the YANG length statement they are converted to the
respective minimum and maximum lengths accepted for strings.

The YANG pattern statement can be used to hold regular expressions
that the affiliated string has to match.  To patterns from YANG in
SDF the pattern quality can be used.  One problem in the conversion
of patterns is that YANG strings can be restricted by multiple
patterns but SDF strings can have at most one pattern.  To represent
multiple patterns from YANG in SDF the patterns are combined into one

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regular expression with the help of positive look-ahead.  Figure 19
contains an example leaf of type string with multiple defined
patterns which is converted as shown in Figure 20.  This does not
always convey the meaning of the original regular expression.
Another issue is the possibility to declare invert-match patterns in
YANG.  These types of patterns are converted to SDF by adding
negative look-ahead to the regular expression, as illustrated in
Figure 21 and Figure 22.  To preserve the original patterns and to
facilitate round trips, the original patterns are stored with a
conversion note in the description of the containing definition as
described in section Section 5.2.

leaf example {
type string {
length "1..4";
pattern "[0-9]*";
pattern "[a-z]*";
}
}

Figure 19: YANG leaf node with type string, multiple pattern
statements and a length statement

"sdfProperty": {
"example": {
"description": "!Conversion note: pattern [0-9]*!\n!Conversion note: pattern [a-z]*!\n",
"maxLength": 4.0,
"minLength": 1.0,
"pattern": "(?=[0-9]*)[a-z]*",
"type": "string"
}
}

Figure 20: SDF conversion of the YANG leaf from the last figure

leaf example {
type string {
pattern "[0-9]*" { modifier invert-match; }
}
}

Figure 21: YANG leaf definition with type string and an invert-
match pattern

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"sdfProperty": {
"example": {
"description": "!Conversion note: pattern [0-9]*!\n",
"pattern": "((?!([0-9]*)).)*",
"type": "string"
}
}

Figure 22: SDF conversion of the YANG leaf from the last figure

Another, more general problem regarding the conversion of regular
expressions from YANG to SDF is the fact that YANG uses a regular
expression language as defined by W3C Schema while SDF adopts
ECMAscript regular expressions.  Both regular expression languages
share most of their features.  Since this does not cause problems in
most cases and regarding the time constraints of this thesis, this
issue is not given any further attention beyond what was stated in
this paragraph.  There is, however, a project of the IETF Network
Working Group to create an interoperable regular expression format.
Once the work on the draft has progressed the format might be adopted
by the SDF/YANG converter.

3.17.  Decimal64 Built-In Type

*  YANG: Section 9.3 (decimal64) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

The decimal64 built-in type of YANG is converted to the number type
in SDF.  A decimal64 type in YANG has a mandatory fraction-digits
sub-statement that specifies the possible number of digits after the
decimal separator.  The value of the fraction-digits statement is
converted to the multipleOf quality of SDF which states the
resolution of a number, that is the size of the minimal distance
between number values.  Figure 23 and Figure 24 contain examples for
the conversion of the decimal64 built-in type.

A YANG decimal64 type can be restricted by means of the range
statement specifying either a constant value, a lower inclusive
bound, an upper inclusive bound or both a lower and upper inclusive
value.  The range statement can also be used to specify multiple
disjoint constant values or ranges.  The min and max key words in a
range statement represent the minimum and maximum values of the type
in question.  If the range statement in YANG contains a range and not
a constant value it is converted to the minimum and maximum data
qualities in SDF.  This is illustrated in the definition called my-
sensor-value in the example.  If a constant value is defined through
the YANG range the SDF const quality is set accordingly, as shown for

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the definition room-temperature in the example.  If the range
specifies multiple ranges or constant values the sdfChoice quality is
used for conversion.  The named alternatives of the sdfChoice contain
the single converted ranges or constant values each.  An example for
this conversion can be found in the my-sensor-value3 example
definition.  If the min and max values are present in the YANG range
they are converted to the respective minimum and maximum values for
the type in question, as shown for the max value in the example
definition my-sensor-value2.

module decimal64-example {
// [...]
leaf my-sensor-value {
type decimal64 {
fraction-digits 2;
range "-50.0..150.0";
}
}
leaf my-sensor-value2 {
type decimal64 {
fraction-digits 4;
range "0..max";
}
}
leaf my-sensor-value3 {
type decimal64 {
fraction-digits 6;
range "0.0..1.0 | 5.0";
}
}
leaf room-temperature {
type decimal64 {
fraction-digits 1;
range "21.5";
}
}
}

Figure 23: YANG module using the decimal64 built-in type

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{
; [...]
"sdfProperty": {
"my-sensor-value": {
"maximum": 150.0,
"minimum": -50.0,
"multipleOf": 0.01,
"type": "number"
},
"my-sensor-value2": {
"maximum": 3.3999999521443642e+38,
"minimum": 0.0,
"multipleOf": 0.0001,
"type": "number"
},
"my-sensor-value3": {
"sdfChoice": {
"range_option_1": {
"maximum": 0.0,
"minimum": 1.0,
"multipleOf": 0.000001,
"type": "number"
},
"range_option_2": {
"const": 5.0,
"multipleOf": 0.000001,
"type": "number"
}
}
},
"room-temperature": {
"const": 21.5,
"multipleOf": 0.1,
"type": "number"
}
}
}

Figure 24: SDF conversion of the YANG module from the last figure

3.18.  Integer Built-In Types

*  YANG: Section 9.2 (integer) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

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In YANG there are 8 different integer types: int8, uint8, int16,
uint16, int32, uint32, int64 and uint64.  Each of them is converted
to type integer in SDF.  A conversion note specifying the exact type
maximum qualities of the SDF definition that the converted type
belongs to are set to the respective minimum and maximum values of
the integer type in question.  If the YANG type also specifies a
range, the minimum and maximum SDF qualities are altered accordingly.
Like the decimal64 YANG built-in type, the YANG integer types can
also be restricted by a range statement.  The integer range statement
is converted as described in Section 3.17.

leaf example {
type int32;
}

Figure 25: YANG leaf with the int32 built-in type

"sdfProperty": {
"example": {
"description": "!Conversion note: type int32!\n",
"maximum": 2147483647,
"minimum": -2147483648,
"type": "integer"
}
}

Figure 26: SDF conversion of the YANG leaf from the last figure

3.19.  Boolean Built-In Type

*  YANG: Section 9.5 (boolean) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

The YANG boolean built-in type holds a boolean value, that is one of
either true or false.  It is converted to the SDF boolean type.
There are no further sub-statements to this type in YANG.

3.20.  Binary Built-In Type

*  YANG: Section 9.8 (binary) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

To represent binary data, the YANG built-in type binary can be used.
If the argument of the YANG type statement is binary the SDF type
quality is set to string.  In addition, the sdfType quality is set to

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byte-string.  A YANG binary can have a sub-statement restricting its
length.  This is converted to SDF via the minLength and maxLength
qualities.  Like the string YANG built-in type, the binary type can
also be restricted by a length statement.  This length statement is
converted as described in Section 3.16.

3.21.  Enumeration Built-In Type

*  YANG: Section 9.6 (enumeration) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

The YANG built-in type enumeration is used to map string-valued
alternatives to integer values.  Additionally each string can have a
description and other sub-statements.  SDF also specifies an enum
quality which is used to represent YANG enumerations.  The SDF enum
quality only holds an array of strings.  All other information is
stored in conversion notes in the description of the SDF definition
the enum belongs to, as specified in Section 5.2.

3.22.  Bits Built-In Type

*  YANG: Section 9.8 (bits) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

SDF does not specify a built-in type to represent a set of named bits
and their positions like YANG does with its built-in type bits.
Therefore, this built-in type has to be converted to SDF type object
with one entry to the properties quality of type boolean for each
bit.  The property is named after the name of the bit.  The position
of the bit is stored in a conversion note as described in
Section 5.2.  An example conversion of a leaf with type bits to SDF
can be found in Figure 27 and Figure 28.

leaf example {
type bits {
description "1 if automatic adaption is enabled, 0 otherwise";
position 1;
}
bit battery-only { position 2; }
bit disable-sensor { position 0; }
}
}

Figure 27: YANG leaf that is of the built-in type bits

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"sdfProperty": {
"example": {
"description": "!Conversion note: type bits!\n",
"properties": {
"description": "Bit at position 1: 1 if automatic adaption is enabled, 0 otherwise",
"type": "boolean"
},
"battery-only": {
"description": "Bit at position 2",
"type": "boolean"
},
"disable-sensor": {
"description": "Bit at position 0",
"type": "boolean"
}
},
"type": "object"
}
}

Figure 28: SDF conversion of the YANG leaf from the last figure

3.23.  Union Built-In Type

*  YANG: Section 9.12 (union) of [RFC7950]

*  SDF: Section 4.7.2 (sdfChoice) of [I-D.ietf-asdf-sdf]

YANG unions hold a set of alternatives for the type statement.
Although the union built-in type of YANG does not exist as a built-in
type in SDF, its meaning can be easily represented by the sdfChoice
quality.  The sdfChoice corresponding to the union contains a set of
named alternatives each named after the respective type in the YANG
union and each containing nothing but the SDF type quality set to the
SDF equivalent of the respective type.  Figure 29 and Figure 30
illustrate this mapping.

leaf example {
type union {
type string;
type boolean;
}
}

Figure 29: YANG leaf that uses the union built-in type

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"sdfProperty": {
"example": {
"description": "!Conversion note: type union!\n",
"sdfChoice": {
"boolean": {
"type": "boolean"
},
"string": {
"type": "string"
}
}
}
}

Figure 30: SDF conversion of the YANG leaf from the last figure

3.24.  Leafref and Identityref Built-In Types

*  YANG: Section 9.9 (leafref) of [RFC7950] Section 9.10
(identityref) of [RFC7950]

*  SDF: Section 4.4 (sdfRef) of [I-D.ietf-asdf-sdf]

The YANG built-in types leafref and identityref are used to reference
a leaf node or identity definition respectively.  They are
represented in SDF by the sdfRef quality.  As an argument said sdfRef
quality contains a reference to the SDF element corresponding to the
target of the leafref or identityref statement.

3.25.  Empty Built-In Type

*  YANG: Section 9.11 (empty) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

Another concept that is not contained in SDF directly is that of the
YANG built-in type empty.  YANG elements with this type convey
meaning by their mere existence or non-existence.  This is
represented in SDF using the compound-type with an empty set of
properties.

3.26.  Instance-Identifier Built-In Type

*  YANG: Section 9.13 (instance-identifier) of [RFC7950]

The instance-identifier built-in type of YANG is used to refer to a
particular instance of a node in the data tree.  As of now, it cannot
be represented functionally in SDF because there is currently no

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possibility to refer to specific instances of SDF definitions.  This
feature might be added to SDF in the future.  For now, this type is
represented by the string built-in type of SDF.  Furthermore, a
conversion note is added to the resulting SDF definition as specified
in Section 5.2. .

3.27.  Typedef Statement

*  YANG: Section 9.3 (typedef) of [RFC7950]

*  SDF: Section 4.4 (sdfRef) of [I-D.ietf-asdf-sdf]

The typedef statement has the purpose to define derived types in
YANG.  The SDF class sdfData is used to represent typedefs after
conversion.  The usage of a derived type via the type statement is
converted to an sdfRef to the corresponding sdfData definition.  If a
derived type is restricted according to its base type, for example
with a range statement, the restrictions are converted as they would
be for the base type and added to the sdfData definition.

3.28.  Identity Statement

*  YANG: Section 7.18 (identity) of [RFC7950]

*  SDF: Section 5.5 (sdfData) of [I-D.ietf-asdf-sdf]

The YANG identity statement is used to denote the name and existence
of an identity.  Identities can be based on one or more other
identities.  They are referenced with the identityref statement.
This concept is converted to SDF by sdfData definitions for each
identity.  If an identity is based on one other identity this is
represented by an sdfRef reference to the sdfData definition
corresponding to the base identity.  If an identity has multiple base
identities it is converted to a compound-type sdfData definition with
one property for each base identity.  Each property contains an
sdfRef reference to the sdfData definition corresponding to one of
the base identities.

3.29.  Config Statement

*  YANG: Section 7.21.1 (config) of [RFC7950]

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

The config statement of YANG can have the boolean values true or
false as arguments.  If config is set to true the element containing
the config statement represents readable and writable configuration
data.  If the config statement is set to false the element containing

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the statement represents read-only state data.  This is transferred
to SDF via the readable and writable qualities.  If the config
statement is set to true it is mapped to the readable and writable
qualities both being set to true.  If the config statement is set to
false it is converted by setting the readable quality to true and the
writable quality to false.  There are, however, cases in which the
SDF definition corresponding to the YANG element containing the
config statement is not one that can use data qualities.  This is the
case, for example, if a top-level container, which is converted to
sdfObject, holds a config statement.  In this case, all definitions
inside the sdfObject that can use data qualities have the readable
and writable qualities set as described above.

3.30.  Status Statement

*  YANG: Section 7.21.2 (status) of [RFC7950]

*  SDF: Section 4.6 (common qualities) of [I-D.ietf-asdf-sdf]

The status statement of YANG is used to express whether a definition
is either current, deprecated or obsolete.  In SDF there is no
quality with a similar meaning.  Thus, the YANG status statement is
represented by a conversion note in the description of the SDF
definition corresponding to the YANG element the status statement
occurred in as described in Section 5.2.

3.31.  Reference Statement

*  YANG: Section 7.21.4 (reference) of [RFC7950]

*  SDF: Section 4.6 (common qualities) of [I-D.ietf-asdf-sdf]

In YANG the reference statement holds a human-readable reference to
an external document related to its containing YANG definition.  This
information is preserved through a conversion note in the description
of the SDF definition equivalent to the node containing the reference
statement as described in Section 5.2.

3.32.  When and Must Statements

*  YANG: Section 7.5.3 (must) of [RFC7950] Section 7.21.5 (when) of
[RFC7950]

*  SDF: Section 4.6 (common qualities) of [I-D.ietf-asdf-sdf]

As mentioned before, YANG provides means to impose conditions on its
definitions.  If a node in the data tree has an unfulfilled must or
when condition it is invalidated.  Must and when conditions use XML

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Path Language expressions to indicate dependencies.  This feature is
not realizable in SDF as of now and is thus preserved through
conversion notes as described in Section 5.2.

There is a query language similar to XML Path Language for JSON
called JSONPath.  If SDF adopts JSONPath or something similar in the
future the converter can be extended to process the functionality of
must and when statements.

3.33.  Extension Statement

*  YANG: Section 7.19 (extension) of [RFC7950]

*  SDF: Section 4.6 (common qualities) of [I-D.ietf-asdf-sdf]

The extension statement in YANG has the purpose of defining new
statements for the YANG language.  This is not a concept that can be
transferred to SDF yet.  When an extension is used, this fact has to
be stored in a conversion note in the description of the SDF
definition that is analogue to the YANG definition containing the
extension statement, as described in Section 5.2.  The definition of
the extension is not converted.

4.  Mapping from SDF to YANG

In this section the conversion of each element of SDF to YANG is
explained in detail.  For reference on the individual YANG statements
see [RFC7950] and [I-D.ietf-asdf-sdf] for SDF.  Examples have been
inserted where they are necessary to understand the mapping.

4.1.  Information Block

*  SDF: Section 3.1 (information block) of [I-D.ietf-asdf-sdf]

*  YANG: Section 7.1 (module) of [RFC7950]

At the top of an SDF model the information block holds meta data,
the model.  When mapping an SDF model to YANG, the content of the
title quality is used as the name for the YANG module.  For this, the
title string has to be modified to only contain lower case letters,
digits and the characters "_", "-" and ".".  If the version quality
contains a date in the format _month-day-year_ it is analogue to the
revision statement of YANG and converted as such.  The strings from
the resulting YANG module since there are no dedicated YANG
statements equivalent to these qualities.

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4.2.  Namespace Section

*  SDF: Sections 3.2 and 4 (namespaces section) of
[I-D.ietf-asdf-sdf]

*  YANG: Section 7.1.3 (namespace) of [RFC7950] Section 7.1.5
(import) of [RFC7950]

The purpose of the namespace section in an SDF model is to specify
its (optional) namespace and the namespaces of external models whose
definitions are referenced.  The namespace section has a namespace
quality mapping namespace URIs to a shortened name for that URI.  The
shortened name is also used as a prefix when referring to external
definitions.  If an SDF model is supposed to contribute globally
available definitions, a value is given to the defaultNamespace
quality and mapped to a namespace URI in the namespace quality.  To
map this to YANG, three of its statements are necessary: the import,
the prefix and the namespace statement.  To be able to use
definitions from external modules in YANG, their names have to be
declared by one import statement each.  As a first step, each
external SDF model that is mentioned in the namespace map also has to
be converted to a YANG module.  The default namespaces of the
external SDF models are represented in the prefix sub-statement of
the respective import statement.  To represent the namespace and
short name of the model, if present, the YANG namespace and prefix
statements that are set accordingly.  Both are top-level statements.

4.3.  SdfThing Quality

*  SDF: Sections 2.2.6 and 6.3 (sdfThing) of [I-D.ietf-asdf-sdf]

*  YANG: Section 7.5 (container) of [RFC7950]

An sdfThing definition holds the definition of a complex device that
can be made up of multiple sdfObjects and multiple other sdfThings.
SdfThings are converted to YANG container nodes.  The sdf-spec
extension is inserted to inform about the origin of the container as
an sdfThing.  This is necessary to facilitate round-trips because the
container could also originate from an sdfObject.

4.4.  SdfObject Quality

*  SDF: Sections 2.2.1 and 5.1 (sdfObject) of [I-D.ietf-asdf-sdf]

*  YANG: Section 7.5 (container) of [RFC7950]

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SdfObject definitions are the main building blocks of an SDF model,
grouping together definitions of the classes sdfProperty, sdfData,
sdfAction and sdfEvent.  They can also be used as arrays via their
minItems and maxItems qualities.  An sdfObject is mapped to a YANG
container node if it is not defined as an array.  Otherwise the
sdfObject can be converted to a list node with the min-elements and
max-elements statements set analogous to the minItems and maxItems
qualities.  This feature was only recently added to SDF and is thus
not yet implemented neither in the SDF serializer/deserializer nor in
the SDF/YANG converter.

4.5.  Common Qualities

*  SDF: Section 4.6 (common qualities) of [I-D.ietf-asdf-sdf]

*  YANG:

-  Section 7.21.3 (description) of [RFC7950]

-  Section 7.3 (typedef) of [RFC7950]

-  Section 9.9 (leafref) of [RFC7950]

-  Section 7.13 (uses) of [RFC7950]

-  Section 3 (terminology for mandatory) of [RFC7950]

The set of qualities that is grouped under the name of _common
qualities_ can be used to provide meta data for SDF definitions.

The description quality is converted to the YANG description
statement.  The label quality is ignored because it is identical to
the identifier of the definition in most cases.

The sdfRef quality is supposed to hold references to other
definitions whose qualities are then copied into the referencing
definition.  Qualities of the referenced definition can also be
overridden by defining them again in the referencing definition.  The
conversion of an sdfRef depends on what is referenced by it and what
that is converted to.  Figure 31 and Figure 32, as well as Figure 33
and Figure 34 illustrate different conversions of the sdfRef quality.
If the referenced definition is converted to a typedef the sdfRef is
analogous to the type statement in YANG which points to the typedef.
Overridden qualities can be represented by the respective sub-
statements of the type which in turn override the sub-statements of
the type of the typedef.  This is the case for simpleDataRef in
Figure 31 and Figure 32.  If the referenced definition is mapped to a
leaf or leaf-list node it can be referenced by the leafref built-in

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type in YANG.  This is the case for simplePropertyRef and
simpleArrayPropertyRef in Figure 33 and Figure 34.  In this case
overridden qualities cannot be represented in SDF.  If the YANG
equivalent of the referenced definition is a grouping node the sdfRef
is converted to a uses node which points to said grouping.  The uses
node is placed inside an additional container to preserve the name of
the referencing SDF definition and to avoid sibling nodes with
identical names (which is invalid in YANG).  This is what is done for
compoundDataRef, simpleArrayDataRef and compoundArrayDataRef in
Figure 31 and Figure 32.  In all other cases the YANG equivalent of
the referenced SDF definition cannot be referenced directly but has
first to be packaged in a grouping node.  This is done by first
creating a grouping on the top-level of the module in order for the
grouping to be available globally (in case it is also referenced in
another model).  The YANG node that is equivalent to the referenced
SDF definition is copied into the new grouping and afterwards
replaced with a uses node referencing the grouping.  This is done to
avoid redundancy.  Lastly, the actual sdfRef is represented by
another uses node referencing the newly created grouping.  The uses
node is placed inside a container node that represents the SDF
definition that contains the sdfRef to preserve the name of the SDF
definition.  Furthermore, there cannot be two sibling nodes with the
same name in YANG.  The definitions compoundPropertyRef and
compoundArrayPropertyRef in Figure 33 and Figure 34 are examples of
such conversions.  If SDF qualities of the referenced definition are
overridden in the referencing definition this is represented with the
refine statement which can be a sub-statement to uses node (see
compoundArrayPropertyRef in Figure 33 and Figure 34).

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{
; [...]
"sdfObject": {
"ExampleObject": {
"sdfData": {
"simpleData": { "type": "string" },
"compoundData": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
},
"simpleArrayData": {
"type": "array",
"items": { "type": "string" }
},
"compoundArrayData": {
"type": "array",
"items": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
}
}
},
"sdfProperty": {
"simpleDataRef": {
"sdfRef": "#/sdfObject/ExampleObject/sdfData/simpleData",
"pattern": "[a-z]*"
},
"compoundDataRef": { "sdfRef": "#/sdfObject/ExampleObject/sdfData/compoundData" },
"simpleArrayDataRef": { "sdfRef": "#/sdfObject/ExampleObject/sdfData/simpleArrayData" },
"compoundArrayDataRef": { "sdfRef": "#/sdfObject/ExampleObject/sdfData/compoundArrayData" }
}
}
}
}

Figure 31: SDF model that uses the sdfRef with different sdfData
definitions

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module exampleModel {
// [...]
typedef simpleData { type string; }
grouping compoundArrayData {
helper:sdf-spec "sdfData";
list compoundArrayData {
config false;
leaf A { type string; }
leaf B { type string; }
}
}
grouping compoundData {
helper:sdf-spec "sdfData";
leaf A { type string; }
leaf B { type string; }
}
grouping simpleArrayData {
helper:sdf-spec "sdfData";
leaf-list simpleArrayData { type string; }
}
container ExampleObject {
helper:sdf-spec "sdfObject";
container compoundArrayDataRef {
helper:sdf-spec "sdfProperty";
uses compoundArrayData;
}
container compoundDataRef {
helper:sdf-spec "sdfProperty";
uses compoundData;
}
container simpleArrayDataRef {
helper:sdf-spec "sdfProperty";
uses simpleArrayData;
}
leaf simpleDataRef {
type simpleData { pattern "[a-z]*"; }
}
}
}

Figure 32: YANG conversion of the SDF model from the last figure

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{
; [...]
"sdfObject": {
"ExampleObject2": {
"sdfProperty": {
"simpleProperty": { "type": "string" },
"compoundProperty": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
},
"simpleArrayProperty": {
"type": "array",
"items": { "type": "string" }
},
"compoundArrayProperty": {
"type": "array",
"items": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
}
},
"simplePropertyRef": { "sdfRef": "#/sdfObject/ExampleObject2/sdfProperty/simpleProperty" },
"compoundPropertyRef": { "sdfRef": "#/sdfObject/ExampleObject2/sdfProperty/compoundProperty" },
"simpleArrayPropertyRef": { "sdfRef": "#/sdfObject/ExampleObject2/sdfProperty/simpleArrayProperty" },
"compoundArrayPropertyRef": {
"sdfRef": "#/sdfObject/ExampleObject2/sdfProperty/compoundArrayProperty",
"minItems": 4
}
}
}
}
}

Figure 33: SDF model that uses the sdfRef with sdfProperty
definitions

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module exampleModel {
// [...]
grouping compoundArrayProperty {
list compoundArrayProperty {
helper:sdf-spec "sdfProperty";
key "A";
leaf A { type string; }
leaf B { type string; }
}
}
grouping compoundProperty {
helper:sdf-spec "sdfProperty";
leaf A { type string; }
leaf B { type string; }
}
container ExampleObject2 {
helper:sdf-spec "sdfObject";
container compoundPropertyRef {
helper:sdf-spec "sdfProperty";
uses compoundProperty;
}
uses compoundProperty;
leaf-list simpleArrayProperty { type string; }

leaf-list simpleArrayPropertyRef {
type leafref { path "/ExampleObject2/simpleArrayProperty"; }
}
leaf simpleProperty { type string; }
leaf simplePropertyRef {
type leafref { path "/ExampleObject2/simpleProperty"; }
}
container compoundArrayPropertyRef {
uses compoundArrayProperty {
refine compoundArrayProperty { min-elements 4; }
}
}
uses compoundArrayProperty;
}
}

Figure 34: YANG conversion of the SDF model from the last figure

The common quality sdfRequired contains a list of SDF declarations
that are mandatory to be present in an instance of the SDF model.
The issue with the conversion of this quality is that in YANG not all
nodes can be marked with the mandatory statement while in SDF all
declarations (that means sdfProperties, sdfActions and sdfEvents that
occur in an sdfObject) can be mentioned in the sdfRequired list.  In

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YANG only leaf and choice nodes (and anyxml and anydata nodes but
these are not used for conversion) can be directly labeled as
mandatory.  List and leaf-list nodes can indirectly be made mandatory
through the min-elements statement.  Furthermore, container nodes
without a presence statement that have at least one mandatory node as
a child are also mandatory themselves.  Not all SDF declarations are
always converted to YANG leaf, choice, list or leaf-list nodes,
however.  Thus, if the YANG node equivalent to the mandatory SDF
declaration is a non-presence container, its sub-tree is traversed
until a leaf or choice node is found.  This leaf or choice node is
labeled as mandatory, now making its parent container mandatory as
well because one of its child nodes is mandatory.  An example for
such a conversion is illustrated in the compoundProperty definition
in Figure 35 and Figure 36.  Consequently, if the parent node of the
now mandatory container would be a container it would now be
mandatory as well.  Alternatively, if a list or leaf-list node is
found first, the min-elements statement of the node is set to 1 if it
is not already set to a value greater than zero, which also makes a
node mandatory.  This is illustrated in the simpleArrayProperty and
compoundArrayProperty definitions in Figure 35 and Figure 36.  To
prevent loss of information and to facilitate round trips, the
declaration originally listed in the sdfRequired quality is preserved
in the sdf-spec extension as described in Section 5.2.

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"sdfObject": {
"ExampleObject": {
"sdfRequired": [
"#/sdfObject/ExampleObject/sdfProperty/simpleProperty",
"#/sdfObject/ExampleObject/sdfEvent/compoundProperty",
"#/sdfObject/ExampleObject/sdfEvent/simpleArrayProperty",
"#/sdfObject/ExampleObject/sdfEvent/compoundArrayProperty"
],
"sdfProperty": {
"simpleProperty": { "type": "string" },
"compoundProperty": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
},
"simpleArrayProperty": {
"type": "array",
"items": { "type": "string" }
},
"compoundArrayProperty": {
"type": "array",
"items": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
}
}
}
}
}

Figure 35: SDF model that contains the sdfRequired quality

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container ExampleObject {
helper:sdf-spec "sdfObject";

list compoundArrayProperty {
helper:sdf-spec "sdfProperty";
helper:sdf-spec "sdfRequired";
config false;
min-elements 1;
leaf A { type string; }
leaf B { type string; }
}

container compoundProperty {
helper:sdf-spec "sdfProperty";
helper:sdf-spec "sdfRequired";
leaf A {
type string;
mandatory true;
}
leaf B { type string; }
}

leaf-list simpleArrayProperty {
helper:sdf-spec "sdfProperty";
helper:sdf-spec "sdfRequired";
type string;
min-elements 1;
}

leaf simpleProperty {
helper:sdf-spec "sdfRequired";
type string;
mandatory true;
}
}

Figure 36: YANG conversion of the last figure

4.6.  Data Qualities

*  SDF: Section 4.7 (data qualities) of [I-D.ietf-asdf-sdf]

*  YANG:

-  Section 7.4.1 (type) of [RFC7950]

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The set of qualities labeled as _data qualities_ contains qualities
inspired by the json-schema.org specifications that SDF adopted as
well as qualities specifically defined for SDF.  In the first group
there is a total of 18 qualities out of which some are
interdependent.

The quality that a lot of the other qualities presence or absence
depends on is the type quality.  The type can be one of number,
string, boolean, integer, array or object.  This quality is directly
converted to the YANG type statement for all simple type.  The type
number becomes decimal64, integer becomes int64.  The types string
and boolean have built-in type equivalents in YANG.  The types array
and object cannot be converted to a YANG built-in type directly.
Instead SDF definitions with these types are converted as described
in Section 4.8 and Section 4.7, that is type array is mapped to
leaflist or list nodes and type object is mapped to container nodes.

If a constant value is defined in an SDF definition, the data quality
const is used to hold it.  If the value of the type quality is number
or integer the const quality is mapped to the range sub-statement of
the type statement of YANG, which can also contain a single value.
An example of such a conversion is illustrated in displayWidth in
Figure 37 and Figure 38.  For constant string values the YANG pattern
statement containing the constant string is used, as shown in the
displayText definition in Figure 37 and Figure 38.  Unfortunately,
constant values of types boolean and array can only be preserved in
YANG through the sdf-spec extension.

"sdfObject": {
"Display": {
"sdfProperty": {
"displayText": {
"type": "string",
"const": "Hello World!"
},
"displayWidth": {
"type": "integer",
"const": 300
}
}
}
}

Figure 37: SdfObject that contains the const quality

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container Display {
helper:sdf-spec "sdfObject";
leaf displayText {
type string { pattern "Hello World!"; }
}
leaf displayWidth {
type int64 { range "300"; }
}
}

Figure 38: YANG conversion of the sdfObject from the last figure

The default data quality in SDF holds the default value for its
definition.  Since YANG leaf and leaf-list nodes have a default sub-
statement, SDF default values of simple types or of type array with
items of simple types can easily be represented.

The data qualities minimum, maximum, exclusiveMinimum and
exclusiveMaximum which are only valid for the types number and
integer are converted using the YANG range statement again.  For
exclusive boundaries the range is reduced accordingly in YANG.  This
is only possible for integer types or if the multipleOf quality
specifies the size by which the number limit has to be reduced.
Alternatives in the YANG range have to be disjoint, however.  This
poses a problem when the range statement is already used to map a
constant value.  Thus, if both minimum or maximum and constant values
are defined, this is represented through the YANG union built-in
type, instead.  As illustrated in Figure 39 and Figure 40, in the
YANG conversion of the definition the union contains the same type
twice, but with different ranges.

"sdfProperty": {
"displayWidth": {
"type": "integer",
"const": 300,
"minimum": 100,
"maximum": 1000
}
}

Figure 39: SdfProperty that uses the minimum and maximum qualities in
conjunction with the const quality

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leaf displayWidth {
type union {
type int64 { range "300"; }
type int64 { range "100..1000"; }
}
}

Figure 40: YANG conversion of the sdfProperty from the last figure

The multipleOf data quality is one that can only be used in
conjunction with the number type in SDF and states the resolution of
the decimal value, that is, of which decimal number the value is a
multiple of.  This quality is converted to the fraction-digits sub-
statement to the type statement in YANG by counting the digits after
the decimal separator of the value of the multipleOf quality.  Since
the fraction-digits statement is mandatory in YANG, it is set to 6 by
default.  This is done because six is also the default decimal
resolution of the std::to_string() method of the C++ standard
library.  This method is used for transferring data from the C++
objects that represent SDF definitions into JSON.

The minLength and maxLength data qualities of SDF are used to hold
the minimal and maximal length of strings.  This concept can be
transferred to YANG by using the length sub-statement of the type
statement that specifies a length range.

The SDF pattern data quality holds regular expressions for string
typed definitions.  This can be converted directly to the pattern
sub-statement of the type statement in YANG.  As already mentioned in
Section 3.16 regular expressions cannot be converted directly between
SDF and YANG in theory, due to the differing languages used for
regular expressions.  Because of the time limitations of this thesis
no further measures are taken to insure the conformance of converted
regular expressions.

The string type in SDF can be supplemented by the format quality.
This quality can specify one of the formats found on json-schema.org.
This could be translated to YANG referencing typedefs from the widely
used ietf-yang-types module.  To not rely on external modules, the
format is only preserved through an addition of the sdf-spec
extension to the YANG equivalent of the SDF definition the format
quality is contained in.

The length of an array in SDF can be restricted by the minItems and
maxItems qualities.  In YANG, both list and leaf-list nodes use the
sub-statements min-elements and max-elements to express the same
concept.  They are therefore used to convert the SDF array length
qualities.

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Another restriction for SDF arrays is the uniqueItems quality that
can be set to either true or false.  If it is set to true all items
of an array are required to be different.  For this purpose, YANG
specifies the key and the unique sub-statements for list nodes.  The
combined values of the mentioned nodes have to be unique.  These
statements can only be applied to leaf nodes in the sub-tree.  This
does not pose a problem, however, because the uniqueness of a
definition can only be measured by the uniqueness of its scalar
values anyway.  Thus, if an SDF array is converted to a YANG list
node and the uniqueItems SDF quality is set to true, the key
statement of the list states the first descendant leaf node of the
list as the key, as illustrated in the compoundArrayProperty
definition in Figure 41 and Figure 42.  The key statement is chosen
over the unique statement because it must be present in all writable
lists anyway.  It is not possible to explicitly represent the
uniqueItems quality in leaf-list nodes.  However, the values of leaf-
list nodes that represent configuration data, and are therefore
writable, must be unique.  The writable quality is set to true by
default.  Thus, to represent an SDF array with unique items, in YANG
the config statement is set to true whenever the writable quality in
SDF is not set to false.  An example of such a conversion can be
found in the simpleArrayProperty definition in Figure 41 and
Figure 42.  Non-writable arrays with unique items cannot be
represented as YANG leaf-lists.

"sdfObject": {
"ExampleObject": {
"sdfProperty": {
"simpleArrayProperty": {
"type": "array",
"uniqueItems": true,
"items": { "type": "string" }
},
"compoundArrayProperty": {
"type": "array",
"uniqueItems": true,
"items": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
}
}
}
}
}

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Figure 41: SdfObject containing the uniqueItems quality

container ExampleObject {
helper:sdf-spec "sdfObject";

list compoundArrayProperty {
helper:sdf-spec "sdfProperty";
key "A";
leaf A { type string; }
leaf B { type string; }
}

leaf-list simpleArrayProperty {
type string;
config true;
}
}

Figure 42: YANG conversion of the sdfObject from the last figure

The items data quality of SDF is a quality that specifies item
constraints for the items of an array-typed SDF definition using a
subset of the common and data qualities.  SDF definitions with the
type array are converted to list or leaf-list nodes.  These node
types in themselves indicate that a node represents an array.  Thus,
the qualities defined in the item constraints of an array are
converted to the sub-statements of the equivalent list or leaf-list
node as described in this section.  Figure 41 and Figure 42 contain
an illustration of this mapping.

Another SDF data quality is the properties quality.  Properties
defined through this quality are different from sdfProperties.  The
properties quality is used in conjunction with the object type and
contains a set of named definitions made up of data qualities
themselves.  SDF definitions of type object are converted to
container or grouping nodes.  Thus, the named entries in the
properties quality are each transformed to the child nodes of the
container or grouping in question.  This is illustrated in
Section 4.8 in the compoundProperty definition of Figure 45 and
Figure 46.  To label the properties as mandatory the required quality
is used.  Since it is resembling the sdfRequired quality, it is
translated in the same way.  The SDF type object was first introduced
in SDF version 1.1 and made conversion of SDF models to YANG
significantly more complicated.  On the other hand, it is crucial to
represent the tree structure of YANG.

The second group of qualities that is part of the data qualities
includes 11 qualities that are defined specifically for SDF.

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The unit quality can be set to any of the SenML unit names to
represent the unit of an SDF definition.  There is also a similar
statement that can be defined as a sub-statement to typedef
definitions, leaf nodes and leaf-list nodes.  The units statement in
YANG can contain any string and thus is simply set to the SenML unit
name from the SDF definition.

An important data quality is the sdfChoice quality.  It represents
the choice between several sets of named definitions made up of data
qualities themselves.  YANG provides a very similar statement, the
choice statement.  An sdfChoice is turned into a YANG choice node.
Each of the alternatives of the sdfChoice is converted like an
sdfProperty (see Section 4.8) and added to the choice node inside its
own case node.  SdfChoice definitions that give the choice between
the type quality could also be mapped to the YANG type union.  This
is omitted for reasons of simplicity.  An example conversion of the
sdfChoice quality can be found in Figure 43 and Figure 44.

"sdfObject": {
"ExampleObject": {
"sdfProperty": {
"choiceProperty": {
"sdfChoice": {
"foo": { "type": "string" },
"bar": { "type": "boolean" },
"baz": { "type": "integer" }
}
}
}
}
}

Figure 43: SdfObject with an sdfChoice quality

container ExampleObject {
helper:sdf-spec "sdfObject";
choice choiceProperty {
case bar {
leaf bar { type boolean; }
}
case baz {
leaf baz { type int64; }
}
case foo {
leaf foo { type string; }
}
}
}

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Figure 44: YANG conversion of the sdfObject from the last figure

SDF also offers the possibility to define the choice between string
values by means of the enum data quality.  It consists of an array of
strings.  This concept also exists in YANG with the enumeration type
and the corresponding enum sub-statement to the type statement.  For
an SDF definition that contains the enum quality the YANG type of its
equivalent is set to enumeration.  Each of the strings in the array
of the enum SDF quality is converted to an enum entry in the type
statement in YANG.  The enum entries are also assigned an associated
value.

The scaleMinimum and scaleMaximum qualities represent limits in units
as specified by the unit quality.  They are not mapped to YANG
because they will not be included in future versions of SDF.  They
are to be replaced in the future, therefore a mapping will have to be
developed for their replacement.

The contentFormat quality of SDF can provide an additional IANA
content type.  This information is preserved with the help of sdf-
spec extension in the YANG equivalent of the SDF definition.

Another way to complement the type quality is the sdfType quality
that can either be set to byte-string or unix-time.  A byte string is
converted to the YANG type binary.  There is no built-in YANG type
corresponding to unix time it is thus converted through the YANG
units statement.  The unit of the YANG conversion mentions unix-time
as an argument.

or write operations are allowed on definitions.  Read operations are
always allowed in YANG modules so a readable quality that is set to
false cannot be represented in YANG.  The config YANG statement can
be used to represent the value of the writable quality, however.  If
an SDF definition is explicitly marked as writable config is set to
true.  Otherwise, it is set to false.

The observable and nullable qualities in SDF cannot be represented in
YANG but are preserved by adding an sdf-spec extension to the YANG
equivalent of their containing SDF definition.

4.7.  SdfData Quality

*  SDF: Sections 2.2.5 and 5.5 (sdfData) of [I-D.ietf-asdf-sdf]

*  YANG:

-  Section 7.13 (uses) of [RFC7950]

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-  Section 7.12 (grouping) of [RFC7950]

Elements of the sdfData class are meant to hold data type definitions
to be shared by sdfProperty, sdfAction and sdfEvent definitions.
SdfData definitions can make use of the data qualities and the common
qualities described in Section 4.6 and Section 4.5 respectively.
Because an sdfData definition embodies a data type definition the
YANG statements typedef and grouping have to be used for conversion.
Which of the two is used depends on the value of the type quality of
the sdfData definition.  If the type is one of the simple data types,
that is integer, number, boolean or string, the sdfData definition is
converted to a YANG typedef.  If the type is object the sdfData
definition is mapped to a grouping node with each of the entries of
the properties quality of the compound-type being mapped to a child
node of the grouping.  When mapping sdfData definitions with type
array to YANG, the type mentioned in the type quality of the items
quality is essential as well.  If an array has items of any of the
simple types the resulting YANG element is a grouping node containing
a single leaf-list node.  Otherwise, if the array items are compound-
types the sdfData definition is converted into a grouping node
containing a single list node.  The child nodes of the list node are
equivalent to the entries of the properties quality that is contained
in the item quality.

One issue with converting sdfData definitions of type array is the
added grouping node that is necessary to hold the equivalent leaf-
list or list node.  If the grouping is used in the schema tree the
added level will cause model instances of the original and converted
model to be in-equivalent.  If the sdfData definition is referenced
in the SDF model via the sdfRef common quality this is represented in
YANG with the uses statement pointing to the grouping equivalent to
the sdfData definition.  The sdfRef quality can occur at most once in
each definition while there can be multiple uses statements in a
single container, list or grouping.  Thus, instead of representing
definitions containing an sdfRef by a parent node containing a uses
node, the aforementioned issue with array-typed sdfData definitions
could be solved by replacing the parent node with the uses node
itself, effectively removing the excess level.  This, however, gives
rise to other issues because the name of the superordinate definition
of the sdfRef is lost this way.  An example for this issue is
illustrated in Figure 53 and Figure 54.  If the sdfData definition is
converted to a typedef no such issues arise.  The typedef in question
is inserted as an argument to the YANG type quality wherever the
original sdfData definition was referenced by an sdfRef.

Another issue is a different view on global accessibility of data
type definitions in YANG and SDF.  In SDF, all definitions are
globally available as long as a default namespace is defined in the

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SDF model.  In YANG on the other hand, only data type definitions,
that is groupings and typedefs, that occur on the top-level of a YANG
module are globally accessible.  Thus, to represent the global
accessibility of all data type definitions in SDF, all converted
sdfData definition equivalents in YANG are added to the top-level of
the created module.

Since these issues are also discussed in Section 4.5, examples
conversion can be found there in Figure 31 and Figure 32.

4.8.  SdfProperty Quality

*  SDF: Sections 2.2.2 and 5.2 (sdfProperty) of [I-D.ietf-asdf-sdf]

*  YANG:

-  Section 7.6 (leaf) of [RFC7950]

-  Section 7.7 (leaf-list) of [RFC7950]

-  Section 7.8 (list) of [RFC7950]

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SdfProperty definitions represent elements of state as suggested by
their name.  SdfProperty definitions can make use of the data
qualities and the common qualities described in Section 4.6 and
Section 4.5.  The mapping of an sdfProperty definition to YANG
depends on the value of the type quality.  SdfProperties with simple
types are mapped to leaf nodes in YANG, as illustrated in the
simpleProperty definition in Figure 45 and Figure 46.  If the type is
complex, that is type object, conversion results in a container node
with each of the entries in the properties quality being mapped to a
child node of the container.  An example of such a conversion is the
compoundProperty definition in Figure 45 and Figure 46.  If the
sdfProperty is of type array the deciding factor is the type quality
inside the items quality.  If an array has items of a simple type, it
is converted to a leaf-list node.  This is demonstrated by the
simpleArrayProperty definition in Figure 45 and Figure 46.
Otherwise, if the items are of compound-type the sdfProperty becomes
a list node in YANG.  The child nodes of the list node are equivalent
to the entries of the properties quality in the compound-type, as
illustrated in Figure 45 and Figure 46 through the
compoundArrayProperty definition.  List nodes that represent
configuration data, that means data that is writable, must specify at
least one of its descendant leaf nodes as a key identifier.  In SDF
definitions that use the data qualities, such as sdfProperties, the
writable quality is set to true by default.  Therefore, the key
statement of the list node is set to the first descendant leaf node
of the list by default by the converter to comply with this rule.
For round trips, this work-around is noted through the sdf-spec
extension.

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"sdfObject": {
"ExampleObject": {
"sdfProperty": {
"simpleProperty": { "type": "string" },
"compoundProperty": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
},
"simpleArrayProperty": {
"type": "array",
"items": { "type": "string" }
},
"compoundArrayProperty": {
"type": "array",
"items": {
"type": "object",
"properties": {
"A": { "type": "string" },
"B": { "type": "string" }
}
}
}
}
}
}

Figure 45: SdfObject with an sdfProperty definition

container ExampleObject {
helper:sdf-spec "sdfObject";
list compoundArrayProperty {
key "A";
leaf A { type string; }
leaf B { type string; }
}
container compoundProperty {
leaf A { type string; }
leaf B { type string; }
}
leaf-list simpleArrayProperty { type string; }
leaf simpleProperty { type string; }
}

Figure 46: YANG conversion of the sdfObject from the last figure

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4.9.  SdfAction Quality

*  SDF: Sections 2.2.3 and 5.3 (sdfAction) of [I-D.ietf-asdf-sdf]

*  YANG:

-  Section 7.14 (rpc) of [RFC7950]

-  Section 7.15 (action) of [RFC7950]

To represent operations that can be invoked in a model the sdfAction
class is used.  Since operations can have input and output data the
sdfAction class is equipped with the sdfInputData and sdfOutputData
qualities that can both make use of the data qualities and the common
qualities described in Section 4.6 and Section 4.5.  An sdfAction can
also define its own set of data types in the form of sdfData
definitions.  Whether an sdfAction is converted to an rpc node (which
can only occur at the top-level of a module) or an action node (which
is always tied to a container node) depends on its location inside
the SDF model.  SdfActions that are not part of an sdfObject but can
be found independently at the top of an SDF model are converted to
rpc nodes.  All other sdfActions occurring inside an sdfObject become
action nodes inside the YANG container equivalent to the sdfObject,
as illustrated in Figure 47 and Figure 48 . The sdfInputData and
sdfOutputData of an sdfAction are converted like sdfProperties (see
Section 4.8) and added as the input and output node of the YANG RPC/
action respectively.

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"sdfObject": {
"ExampleObject": {
"sdfAction": {
"printString": {
"sdfInputData": {
"type": "object",
"properties": {
"content": { "type": "string" },
"colour": { "type": "string" }
}
},
"sdfOutputData": {
"type": "object",
"properties": {
"success": { "type": "boolean" }
}
}
}
}
}
}

Figure 47: SdfObject definition that contains an sdfAction definition

container ExampleObject {
helper:sdf-spec "sdfObject";
action printString {
input {
leaf colour { type string; }
leaf content { type string; }
}
output {
leaf success { type boolean; }
}
}
}

Figure 48: YANG conversion of the sdfObject from the last figure

4.10.  SdfEvent Quality

*  SDF: Sections 2.2.4 and 5.4 (sdfEvent) of [I-D.ietf-asdf-sdf]

*  YANG: Section 7.16 (notification) of [RFC7950]

The purpose of the sdfEvent class is to model signals that inform
about occurrences or events in an sdfObject.  To represent the
emitted output data, sdfEvents can make use of the sdfOutputData

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quality which in turn uses the data qualities.  An sdfEvent is
converted to a notification node with one child node representing the
sdfOutputData definition.  The sdfOutputData definition is converted
like an sdfProperty (see Section 4.8).  Figure 49 and Figure 50
contain the SDF and YANG representations of a warning notification
which communicates the device and reason of the warning.

"sdfEvent": {
"warning": {
"sdfOutputData": {
"type": "object",
"properties": {
"warningDevice": { "type": "string" },
"warningReason": { "type": "string" }
}
}
}
}

Figure 49: SdfEvent definition

leaf warningDevice { type string; }
leaf warningReason { type string; }
}

Figure 50: YANG conversion of the sdfEvent from the last figure

5.  Challenges

Since conversion between SDF and YANG is not always trivial this
section takes a look at the various challenges that arose in the
process of finding an adequate mapping for each of the language's
features to one another.

5.1.  Differences in Expressiveness of SDF and YANG

SDF and YANG differ in their expressiveness in different areas.
Compared to the other format, both are stronger in some areas and
weaker in others.

Areas in which YANG is more expressive are regular expressions,
operations, some of the built-in types (bits and empty) and the
retrospective augmentation of existing definitions.  In YANG,
multiple regular expressions to be matched can be defined and they
can also be labeled as invert-match expressions.  Both features are
difficult to express in SDF as of now.  Furthermore, YANG and SDF use
slightly different regular expression languages.  YANG uses a regular

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expression language as defined by W3C Schema while SDF adopts
ECMAscript regular expressions.  Operations in YANG can be defined on
their own or with an affiliation to a YANG container.  This
affiliation is not always trivial to represent in SDF.  The YANG
built-in types bits and empty do not have equivalents in SDF.  The
semantics of those types can, however, easily be mapped to SDF.  A
YANG statement whose semantics cannot be fully mapped to SDF is the
augment statement.  The augmentation can be applied and then
converted but cannot be represented as a retrospective addition to an
SDF definition or model.  Another Language feature of YANG that SDF
does not offer is the option to place constraints on valid data via
XPath expressions and the option to make sections of the model
conditional with the feature statement.  YANG, furthermore, puts no
constraints on the value of its units statement, whereas SDF does
only allow SenML unit names in the unit quality.

SDF offers more possibilities to define default and constant values,
the latter especially in conjunction with minimum and maximum values.
YANG uses a single statement, the range statement, for constant,
minimum and maximum values.  Although there can be multiple values or
ranges in one range statement that are interpreted as alternatives
they all need to be disjoint.  This imposes a strict limit on what
can be expressed through the statement.  An example for a conversion
where this is a problem would be an sdfData definition with values
for the minimum and maximum qualities but also a given constant value
that fits inside the given minimum and maximum range, like the
example in Figure 51.  Such a definition could be converted to a YANG
typedef with a range that states the minimum and maximum value as one
range and the constant as an alternative, like the example conversion
in Figure 52.  This example does not validate in YANG because the
range alternatives are not disjoint.  This problem is solved through
use of the union built-in type.  Furthermore, labeling definitions as
readable, observable and nullable, as possible in SDF, is foreign to
YANG.  SDF is also more expressive in the way it labels definitions
that must obligatorily occur in model instances.  Basically all
definitions can be labeled as such through the sdfRequired and
required qualities.  In YANG, only leaf, choice, anydata and anyxml
nodes can be marked with the mandatory statement directly.
Container, list and leaf-list nodes can only be made mandatory
indirectly and there is no general mechanism in YANG for all kinds of
nodes.

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"sdfData": {
"someValue": {
"type": "integer",
"minimum": 1,
"maximum": 5,
"const": 3
}
}

Figure 51: SdfData definition with the qualities minimum, maximum
and const

typedef someValue {
type int32;
range "1 .. 5 | 3" // invalid in YANG
}

Figure 52: YANG conversion of the SDF definition in the last figure

5.2.  Round Trips

One of the bigger issues in building the converter is the
facilitation of round trips, i.e. converting a model from one format
to the other and in a next step back to the original.  This issue is
tightly linked to the differences in expressiveness between the two
formats which makes mapping between them non-injective and thus non-

To be able to track the origins of an SDF element after conversion
from YANG, currently, a so-called \textit{conversion note} is added
to the description of the element.  The note specifies a statement
and optionally an argument to the statement.  An example for a note
stating that the original argument to the type statement was bits is:
!Conversion note: type bits!. This approach is not able to preserve
all information from the YANG module without exceptions since sub-
statements cannot be specified.  It is, however, sufficient in the
majority of cases.

This issue was also discussed in one of the meetings of the ASDF
working group.  The possibility to introduce a new mechanism for
information that adds no functionality, the possibility to preserve
information from the original model in a separate mapping file for
each model was proposed.  Mapping files for SDF models could contain
selectors that assign additional information to the selected SDF
element or element group.  No decision has been made yet on the
definite structure of such mapping files.  Therefore, some
requirements from the perspective of the SDF/YANG converter are

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listed here.  Generally speaking, the information attached to an SDF
element should have at least the same information content in the
mapping file as in the previously mentioned conversion note, that is
a statement and optionally an argument.  To also cater to statements
with further sub-statements, nesting should be possible, that is
defining further statements as arguments should be possible.  It is
also necessary to be able to specify multiple statements to attach to
a selected SDF elements.  Another solution to round trips with
mapping files would be to reference the associated YANG element of
the selected SDF element.  This way, all information would be
preserved.  Round trips would be easy because the original YANG
definition would stay attached to the converted SDF definition.
Opposing to that, if the SDF conversion of the YANG model is used to
be converted further into other languages, the supplementary
information of the original YANG element would still have to be
extracted.  This defeats the purpose of SDF to reduce the number of
necessary mappings between languages.  Thus, to attach statements
with arguments to SDF definitions in mapping files is the better
solution, in our opinion.

To preserve the original SDF language element after conversion to
YANG a new sdf-spec extension is defined in YANG.  The extension
states the original SDF quality and optionally an argument, similarly
to the conversion note used to preserve information from YANG.

The eventuality that round trips occur in model conversion makes
building the converter significantly more complex because all
features of the target format have to be accounted for.  Features of
the target format that would otherwise not be used for conversion
must now be considered in the case of a round trip.

5.3.  Type References

Both SDF and YANG offer the possibility to reference predefined
types.  SDF uses only a single quality for this purpose (sdfRef)
whereas YANG has several statements that are all used in different
referencing contexts (leafref, identityref, type, uses).  The way the
uses statement and the sdfRef quality are converted regularly leads
the compound-type) in SDF that make instances of the same model in
SDF and YANG in-equivalent and complicate round trips.  If no
additional elements are inserted, information, for example the name
of an element, is lost.

Both the uses statement and the sdfRef quality embed the content of
the referenced expression where they are located.  Issues arise
because YANG provides only groupings to be embedded via the uses
statement.  Groupings are the non-declaration-equivalent to

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containers.  There is no non-declaration-equivalent to YANG lists,
however.  This means that list definitions in YANG need to be
packaged in a grouping.  If such a grouping with a single list inside
is transcribed from YANG to SDF there will be an extra layer that
looks redundant but otherwise does no harm.  For the reasons stated
above, an sdfData definition of type array with items of compound-
type is converted to a list node inside a grouping in YANG.  Problems
arise when said sdfData definition is embedded via sdfRef because
this cannot be converted directly to YANG.  Such a scenario is
illustrated in Figure 53 and Figure 54.  The sdfData definition menu
Referencing the menu via sdfRef in the sdfProperty definitions
from said grouping is copied into the containers.  The containers are
because there cannot be two sibling uses nodes with the same target
grouping (because no two sibling nodes must have the same name).

Another issue with the mapping of type references is the
accessibility of elements.  Only typedefs and groupings that appear
on the top-level of the tree can be reused globally.  If these nodes
appear within a sub-tree they are only available in the scope of the
sub-tree.  Since there is no such restriction in SDF, mapping sdfData
definitions directly would cause accessibility problems in the
resulting YANG module.  Thus, mapped sdfData definitions have to be
moved to the top-level.  In YANG it is furthermore assumed that every
type of node in the tree is addressable, while SDF focuses on
sdfProperties, sdfActions and sdfEvents as addressable affordances.

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; [...]
"sdfData": {
"dish": {
"type": "object",
"properties": {
"name": { "type": "string" },
"price": { "type": "number" }
}
},
"type": "array",
"items": { "sdfRef": "#/sdfData/dish" }
}
},
"sdfObject": {
"restaurant" : {
"sdfProperty": {
"dish_of_the_day": { "sdfRef": "#/sdfData/dish" }
}
}
}
}

Figure 53: SDF model with type definitions of types object and array

module restaurant {
// [...]
grouping dish {
leaf name { type string; }
leaf price {
type decimal64 { fraction-digits 6; }
}
}
key "name";
uses dish;
}
}
container restaurant {
container dish_of_the_day { uses dish; }
}
}

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Figure 54: YANG conversion of the SDF model in the last figure

6.  Implementation Considerations

An implementation of an initial converter between SDF and YANG can be
found at [SDF-YANG-CONVERTER]; the source code can be found at
[SDF-YANG-CONVERTER-IMPL].

7.  IANA Considerations

This document makes no requests of IANA.

8.  Security considerations

The security considerations of [RFC7950] and [I-D.ietf-asdf-sdf]
apply.

9.  References

9.1.  Normative References

[I-D.ietf-asdf-sdf]
Koster, M. and C. Bormann, "Semantic Definition Format
(SDF) for Data and Interactions of Things", Work in
Progress, Internet-Draft, draft-ietf-asdf-sdf-08, 25
October 2021, <https://www.ietf.org/archive/id/draft-ietf-
asdf-sdf-08.txt>.

[RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.

9.2.  Informative References

[LIBYANG]  Vasko, M., Sedlák, D., and more contributors, "libyang",
<https://github.com/CESNET/libyang>.

[SDF-YANG-CONVERTER]
Kiesewalter, J., "SDF YANG converter playground", n.d.,
<sdf-yang-converter.org>.

[SDF-YANG-CONVERTER-IMPL]
Kiesewalter, J., "SDF YANG converter", n.d.,
<https://github.com/jkiesewalter/sdf-yang-converter>.

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Acknowledgements

TBD.

Jana Kiesewalter
Universität Bremen

Email: jankie@uni-bremen.de

Carsten Bormann (editor)
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany

Phone: +49-421-218-63921
Email: cabo@tzi.org

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