Using Markup in YANG Text Arguments
draft-lhotka-netmod-yang-markup-00

Abstract

This document defines a YANG extension that allows for specifying a media type for text in the arguments of these YANG statements: "contact", "description", "error-message", "organization" and "reference."

Status of This Memo

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This Internet-Draft will expire on September 10, 2017.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Declaring the Media Type of Text Arguments
• 4. YANG Module
• 5. IANA Considerations
• 6. Security Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

Descriptions play an important role in YANG data models. They may specify normative semantic constraints that cannot be expressed formally, instructions for implementors of the data model, and other vital information. That is why descriptions often comprise several paragraphs of text and sometimes also other structures such all numbered or bulleted lists.

So far, YANG modules have mostly used only plain text in the argument of the "description" statement, i.e., formatted text structures using just whitespace. This is generally sufficient for human readers and also for including YANG modules in RFC documents, but less so for tools that are designed to process or use YANG modules in other ways, for example:

• converting YANG compact syntax to other formats (YIN, HTML)
• including YANG modules in standards and other documents whose source form in not plain text
• including description text in user interface elements.

The above considerations also apply, albeit to a lesser extent, to other YANG statements with more structured arguments: "contact", "error-message", "organization" and "reference".

In principle, it is possible to use any kind of markup such as markdown [RFC7764] but doing so would not be not very effective and tool-friendly unless the markup format is either standardized or declared in the module source.

This document defines a YANG extension statement, "text-media-type", that can be placed as a substatement of the "module" or "submodule" statement to indicate the media type that is used throughout the (sub)module for markup in the arguments of the five YANG statements mentioned above.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Declaring the Media Type of Text Arguments

Section 4 below contains the "ietf-yang-text-media-type" module that defines a YANG language extension: "text-media-type". Its purpose is to declare media type that is used for markup in the arguments of the following YANG statements: [RFC7950]).

• contact
• description
• error-message
• organization
• reference

The common characteristic of these five statements is that their argument is represented as the <text> element in YIN format (Section 13 of

The "text-media-type" extension is intended to be used at the top level of a YANG module or submodule, i.e., as a substatement of either "module" or "submodule" statement. The declared media type applies to arguments of the above YANG statements throughout the module or submodule.

The "text-media-type" extension MAY be ignored, and a module in which it appears MUST be valid YANG.

The argument of the "text-media-type" extension is a string specifying the media type in the format defined in [RFC6838]. The media type SHOULD be registered by IANA in the "text" registry [IANA-MEDIA-TYPES]. Media type parameters MAY be used.

It is RECOMMENDED to use only media types representing "lightweight" markup that is easy to read even in the unprocessed source form, such as "text/markdown".

YANG modules have to be use the UTF-8 character encoding, see Section 6 of [RFC7950]. This implies the following rules:

• Whenever the "charset" parameter is present, its value MUST be "UTF-8".
• If no default value is defined for the "charset" parameter of a given media type, or if a default value is defined but is different from "UTF-8", then the "charset" parameter MUST be present with the value of "UTF-8".

Even if the "text-media-type" extension statement is present and markup used in text arguments, all YANG and YIN syntax rules still apply.

The "ietf-yang-text-media-type" module not only defines but also uses the "text-media-type" extension statement, which illustrates its typical use.

4. YANG Module

RFC Editor: In this section, replace all occurrences of 'XXXX' with the actual RFC number and all occurrences of the revision date below with the date of RFC publication (and remove this note).

<CODE BEGINS> file "ietf-yang-text-media-type@2017-03-09.yang"

module ietf-yang-text-media-type {

  namespace "urn:ietf:params:xml:ns:yang:ietf-yang-text-media-type";

  prefix ymt;

  ymt:text-media-type "text/markdown; charset=UTF-8";

  organization
    "IETF NETMOD (NETCONF Data Modeling Language) Working Group";

  contact
    "WG Web:   <https://tools.ietf.org/wg/netmod/>
     WG List:  <mailto:netmod@ietf.org>

     WG Chair: Lou Berger
               <mailto:lberger@labn.net>

     WG Chair: Kent Watsen
               <mailto:kwatsen@juniper.net>

     Editor:   Ladislav Lhotka
               <mailto:lhotka@nic.cz>";

  description
    "This module defines the *text-media-type* extension that allows
     for specifying media-type for markup that is used in arguments
     of these YANG statements: contact, description, error-message,
     organization, and reference.

     Copyright (c) 2016 IETF Trust and the persons identified as
     authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject to
     the license terms contained in, the Simplified BSD License set
     forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
     NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and
     'OPTIONAL' in the module text are to be interpreted as described
     in RFC 2119 (https://tools.ietf.org/html/rfc2119).

     This version of this YANG module is part of RFC XXXX
     (https://tools.ietf.org/html/rfcXXXX); see the RFC itself for
     full legal notices.";

  revision 2017-03-09 {
    description
      "Initial revision.";
    reference
      "RFC XXXX: Using Markup in YANG Text Arguments";
  }

  extension text-media-type {
    argument type;
    description
      "This extension allows for specifying a media type that is used
       for markup in the arguments of the following YANG statements:

       - contact

       - description

       - error-message

       - organization

       - reference

       The *text-media-type* extension statement MAY be used at the
       top level of a module or submodule, i.e., as a substatement of
       *module* or *submodule*, and no more than once. The declared
       media type applies throughout the module or submodule.

       The argument SHOULD be a media type registered by IANA in the
       *text* registry. Media type parameters MAY be present.

       This YANG extension is only indicative and optional to
       implement. Tools MAY ignore it completely or support just a
       subset of markup directives that are available for a given
       media type.";
    reference
      "- IANA: Media Types, 2016-12-21. Available [online]
         (http://www.iana.org/assignments/media-types/media-types.xhtml)

       - [RFC 6838](https://tools.ietf.org/html/rfc6838): Media Type
         Specifications and Registration Procedures";
  }
}

<CODE ENDS>

5. IANA Considerations

RFC Editor: In this section, replace all occurrences of 'XXXX' with the actual RFC number and all occurrences of the revision date below with the date of RFC publication (and remove this note).

This document registers the following namespace URI in the "IETF XML registry" [RFC3688]:

URI: urn:ietf:params:xml:ns:yang:ietf-yang-text-media-type
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
        

This document registers the following YANG module in the "YANG Module Names" registry [RFC6020]:

Name:         ietf-yang-text-media-type
Namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-text-media-type
Prefix:       ymt
Reference:    RFC XXXX
        

6. Security Considerations

The "text-media-type" extension defined in this document provides information that is completely optional. YANG modules and submodules in which it is present have to satisfy all rules of the YANG language. The extension therefore doen't introduce any new threats.

7. References

7.1. Normative References

7.2. Informative References

Author's Address