Documentation Conventions for Expressing YANG in XML
draft-wu-netmod-yang-xml-doc-conventions-00

Abstract

Many documents that define YANG modules also include examples presented in XML.

IETF documentation has specific limits on line length and some XML examples have to include line wraps that would not normally be allowed according to the XML representation rules of RFC7950 and RFC7952.

This document lays out documentation conventions that allow YANG examples to be presented in IETF documentation when leaf node encoding would otherwise exceed the maximum line length. There are no implications in this document for YANG parsers: this document does not change the rules for presenting YANG models or for encoding YANG in data files or in the wire.

Status of This Memo

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This Internet-Draft will expire on July 30, 2018.

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Table of Contents

• 1. Introduction
• 2. Conventions Used in this Document
• 3. Separating Components of a Leaf Example
• 4. Splitting an Example Leaf Node Value Across Lines
• 5. Mandatory Boilerplate Text
• 6. Representing XML Encodings of Metadata Annotations
• 7. Mandatory Boilerplate for Splitting Metadata Annotations
• 8. Automatic Generation of Valid XML From Examples
• 9. Security Considerations
• 10. IANA Considerations
• 11. Acknowledgements
• 12. Normative References
• Authors' Addresses

1. Introduction

YANG [RFC7950] defines four main types of data node for data modeling and describes how these are represented in XML [XML]. For list nodes and container nodes, any whitespace, carriage returns, or line feeds between the subelements is insignificant, i.e., an implementation MAY insert whitespace, carriage return, or line feed characters between subelements.

However for leaf nodes, [RFC7950] section 7.6.6 says

• The value of the leaf node is encoded to XML according to the type and is sent as character data in the element.

Thus whitespace, carriage return, and line feed characters are interpreted as part of the leaf value if the leaf is of type string and must not be included. The same applies to leaf-list nodes.

However, when documenting examples of YANG modules represented in XML encoding it is possible that the encoding of a single leaf node will exceed the available line length (73 characters).

This document describes documentation conventions that allow the presentation of such examples in a way that is easily parsed by a human reader, but which is not representative of how the XML must be presented to a software component or carried on the wire.

There are no implications in this document for YANG parsers: this document does not change the rules for presenting YANG models or for encoding YANG in data files or in the wire.

2. Conventions Used in this Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Separating Components of a Leaf Example

An example of the documentation of a leaf node is shown in Figure 1. The leaf is called "long-leaf-node-name" and is assigned the value "long-leaf-node-value". As can be seen in the example, this fits on one line. However it would only take the addition of a few more characters to the node label or value for the example to overflow the 73 character limit.

          
<long-leaf-node-label>long-leaf-node-value</long-leaf-node-label>
          
        

Figure 1: A Simple Leaf Node Example

For the sake of documentation, the representation shown in Figure 2 SHALL be considered as equivalent to that shown in Figure 1, but when a document uses this convention it MUST also include the text shown in Section 5.

          
     <long-leaf-node-label>
       long-leaf-node-value
     </long-leaf-node-label>
          
        

Figure 2: A Split Leaf Node Example

4. Splitting an Example Leaf Node Value Across Lines

When the XML representation of a leaf node value in an example would result in a line being longer than the maximum line length for an IETF document the value of the leaf node must be split and printed on more than one lines. This is most likely to happen when the example leaf node contains a string. Indeed, if this problem arises for other leaf types it may be indicative of poorly chosen leaf values, and the YANG definitions should be revised.

In this case, conventions MUST be observed:

• The broken line MUST be terminated with a backslash ("\") without the addition of any additional space before the backslash and with no further characters after the backslash.
• Any continuation lines MUST be indented with a whitespace offset of at least two characters.
• When a backslash appears in the node value, the example MUST be arranged so that the backslash is not the final character of a broken line

Furthermore, whenever a document uses this convention it MUST also include the text shown in Section 5.

Figure 3 shows an example leaf with a long value. As can be seen, the addition of a few more characters would cause the line to be too long.

Figure 4 shows a semantically equivalent representation of the example if the text from Section 5 is also present.

          
<long-leaf-string-node-label>
   Once upon a time, in a land far away, there lived a Great King.
</long-leaf-string-node-label>
          
        

Figure 3: An Example Leaf Node With a Long String Value

          
     <long-leaf-string-node-label>
       Once upon a time, \
         in a land far away, \
         there lived a Great King.
     </long-leaf-string-node-label>
          
        

Figure 4: A Long String Leaf Node Example Split Across Lines

Figure 5 and Figure 6 show a more complex example where the node value includes both line feeds and a backslash. Note how the line breaks are arranged to avoid potential confusion and to make the real characters evident.

          
<long-leaf-complex-string-node-label>
  Punctuation is important. As are line feeds.
  Some characters are special. E.g., the backslash \. Don't forget.
</long-leaf-string-node-label>
          
        

Figure 5: An Example Leaf Node With a Complex String Value

          
     <long-leaf-complex-string-node-label>
       Punctuation is important. \
         As are line feeds.
       Some characters are special. \
         E.g., the backslash \. \
         Don't forget.
     </long-leaf-string-node-label>
          
        

Figure 6: An Example Leaf Node With a Complex String Value Split Across Lines

5. Mandatory Boilerplate Text

When either of the conventions described in Section 3 or Section 4 is used for the benefit of the representation of an example of a YANG module or YANG fragment in XML, the following text MUST be included in the document presenting the example.

• The examples in this document adopt the conventions shown in BCP XX [RFCYYYY] for splitting node labels and node values onto separate lines. This convention is used to make the examples easier to read but does not change the encoding rules for the XML representation of YANG as described in [RFC7950].

RFC Editor Note: Please replace XX and YYYY with the numbers assigned for this document.

6. Representing XML Encodings of Metadata Annotations

[RFC7952] section 5.1 provides an encoding rule for metadata annotations in XML.

When an example XML representation of a leaf node element that includes metadata attributes results in a line being longer than the maximum number of characters allowed in a line of an IETF document, the value of the leaf node must be split across more than one line.

Where possible, all line breaks should be inserted between metadata attributes. Continuation lines MUST start with a whitespace offset of at least two characters. The leading and trailing whitespace of each line MUST be ignored. Figure 7 gives an example.

Whenever this documentation convention is used, the boilerplate text shown in Section 7 MUST be present in the document using the convention.

          
<error-path
   xmlns:t="http://example.com/schema/1.2/config/verylongpathname\
     thatcannotfitononeline">
   /t:top/t:interface[t:name="Ethernet0/0"]/t:mtu/t:anotherattribute
   /t:afinalattribute
</error-path>
          
        

Figure 7: An Example Leaf Node With Metadata Split Across Lines

7. Mandatory Boilerplate for Splitting Metadata Annotations

When the convention described in Section 6 is used for the benefit of the representation of an example of a YANG module or YANG fragment containing metadata annotations in XML, the following text MUST be included in the document presenting the example.

• The examples in this document adopt the conventions shown in BCP XX [RFCYYYY] for splitting metadata annotation across multiple lines. This convention is used to make the examples easier to read but does not change the encoding rules for the XML representation of YANG metadata annotations as described in [RFC7952].

RFC Editor Note: Please replace XX and YYYY with the numbers assigned for this document.

8. Automatic Generation of Valid XML From Examples

It should be noted that it is never the intention that example YANG fragment should be converted to XML that is passed to a YANG consumer. Nevertheless, there are good reasons to be able to convert an example into valid YANG in order to parse it and check its validity against the YANG model itself. This will ensure that examples in documents are accurate and useful.

When parsing a leaf or leaf-list node in an example, the following rules should be applied to generate valid XML.

• If a white space, carriage return, or line feed character is encountered between close (">") and open ("<") angle brackets it should be stripped.
• If a white space, carriage return, or line feed character is encountered within a string value of a leaf node or leaf-list node, it should generally be preserved exactly as shown except in the special case that follows.
• If a backslash character ("\") appears within the string value of a leaf node or leaf-list node and if and only if it is immediately followed by a carriage return or line feed character then all carriage return, line feed, and whitespace characters should be stripped until the next character is encountered.
• If a white space, carriage return, or line feed character is encountered within metadata annotations, but not within quotes, it should be stripped. Parsing may expect the next valid character found to indicate the start of a new metadata attribute.
• If a backslash character ("\") appears within the quoted value of a metadata attribute and if and only if it is immediately followed by a carriage return or line feed character then all carriage return, line feed, and whitespace characters should be stripped until the next character is encountered.

9. Security Considerations

There is no direct security impact related to the XML encoding documentation convention described in this document. However, attempting to provide actual XML using the documentation conventions described in this document would have unpredictable results. The risk here is that someone uses an example as a template for actual XML. The mandatory boilerplate text provides a mitigation against this risk.

10. IANA Considerations

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

11. Acknowledgements

Thanks to Kent Watsen for discussions that kept us close to being on the right track. Additional thanks to John Scudder for flagging some nits.

12. Normative References

Authors' Addresses