Documentation Conventions for Expressing XML and JSON encoded YANG Data Node Instance
draft-wu-netmod-yang-xml-doc-conventions-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Authors | Qin Wu , Adrian Farrel , Benoît Claise | ||
| Last updated | 2018-05-30 | ||
| Replaced by | draft-kwatsen-netmod-artwork-folding, draft-kwatsen-netmod-artwork-folding | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
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draft-wu-netmod-yang-xml-doc-conventions-02
Netmod Working Group Q. Wu
Internet-Draft Huawei
Intended status: Best Current Practice A. Farrel
Expires: December 1, 2018 Juniper Networks
B. Claise
Cisco Systems, Inc.
May 30, 2018
Documentation Conventions for Expressing XML and JSON encoded YANG Data
Node Instance
draft-wu-netmod-yang-xml-doc-conventions-02
Abstract
Many documents that define YANG modules also include YANG data node
Instance examples presented in XML/JSON.
IETF documentation has specific limits on line length and some YANG
data node Instance examples such as XML examples have to include line
wraps that would not normally be allowed according to the XML
representation rules of RFC7950 and RFC7952.The same applies to JSON.
This document lays out documentation conventions that allow YANG
examples to be presented in IETF documentation when XML/JSON encoded
YANG data instance would otherwise exceed the maximum line length and
provide consistent XML/JSON encoded YANG data node instance example
within an Internet-Draft or RFC. 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
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 1, 2018.
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Copyright Notice
Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in this Document . . . . . . . . . . . . . . 3
2.1. Glossary of New Terms . . . . . . . . . . . . . . . . . . 4
3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Separating Components of a Leaf Example . . . . . . . . . . . 5
5. Splitting an Example Leaf Node Value Across Lines . . . . . . 6
6. Representing XML and JSON Encodings of Metadata Annotations . 7
7. Mandatory Boilerplate for Splitting Leaf node value and
Metadata Annotations . . . . . . . . . . . . . . . . . . . . 8
8. Automatic decoding From YANG data node Instance Examples with
split line . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12. Normative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Backslash appears in the node value not used for
split line . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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
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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
interpreted as part of the leaf value when the leaf is of type string
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 YANG
examples to be presented in IETF documentation when XML/JSON encoded
YANG data instance would otherwise exceed the maximum line length and
provide consistent XML/JSON encoded YANG data node instance example
within an Internet-Draft or RFC. These document conventions also
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.
The following terms are defined in [RFC7950] and are not redefined
here:
o data node
o leaf
o leaf-list
o instance
The following term is defined in [RFC7951] and [RFC7952] and are not
redefined here:
o data node Instance
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o data node identifier
2.1. Glossary of New Terms
Encoding function: Convert XML/JSON encoded YANG data node instance
Example with long lines not fitting into an Internet-Draft or RFC
into XML/JSON encoded YANG data node instance Example with split
line fitting into an Internet-Draft or RFC.
Decoding function: Re-Convert XML/JSON encoded YANG data node
instance Example with split line fitting into an Internet-Draft or
RFC back to valid YANG representation in data file or on the wire
without split line.
3. Objectives
To reduce complexity of XML/JSON encoded YANG data node instance
Example representation, the following assumptions are made:
o If the problem comes from the poorly chosen length of YANG data
node instance encoded by XML or JSON,it is recommended to reduce
the length of YANG data node instance name or YANG data node
instance value to the minimum before applying document convention
defined in this document.
o If the problem comes from the poorly chosen length of the YANG
data node instance containing metadata annotation as attribute, it
is recommended to reduce the length of each attribute to the
minimum before applying the document convention defined in this
document.
In order to provide consistent XML/JSON encoded YANG data node
instance example within an Internet-Draft or RFC, the following
design criteria are used:
o Split long line YANG data node instance(e.g.,leaf node instance)
exceeding 73 characters limits with the backslash ("\") and use
backslash ("\")to indicate wrapping at the end of the line.
o Split long line YANG data node instance containing metadata
annotation attributes with the backslash ("\")and use backslash
("\")to indicate wrapping at the end of the line.
o When a backslash appears in the node value not used for split
line, the example MUST be arranged so that this backslash is not
the final character of a broken line.
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o Allow the representation of such examples, if possible,in a way
that is easily parsed by a human reader instead of machine or
software component:
* The first child node within each parent node of the XML/JSON
encoded YANG data node instance should be indented with a
whitespace offset of two characters.
* Where possible, all line breaks should be inserted between
metadata attributes.
* Where possible, all line breaks should not be inserted between
close (">") and open ("<") angle brackets within XML encoded
YANG data instance.
* Where possible, all line breaks should be inserted between
double-quote strings or between data node identifier and data
node separated by colon character within JSON encoded YANG data
instance.
o Allow automatic generation of YANG representation without split
line in data files or in the wire from representation of XML/JSON
encoded Example with split line within an Internet-Draft or RFC
presented based on document convention defined in this
document.(See section 8)
4. 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 purpose, 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 7. Note that the first example representation
in figure 2 is more easily parsed by a human reader than the second
example in figure 2.
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<long-leaf-node-label> \
long-leaf-node-value \
</long-leaf-node-label>
or
<long-leaf-node-label> long-le \
af-node-value </long-leaf-nod \
e-label>
Figure 2: A Split Leaf Node Example
5. 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:
o 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. The backslash at the
end of the line indicates wrapping.
o Any continuation lines MUST align with the first line and not be
indented with any whitespace offset.
These conventions also apply to JSON representation of a leaf node
value [RFC7951].
Furthermore, whenever a document uses this convention it MUST also
include the text shown in Section 7.
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 7 is also present.
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<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
6. Representing XML and JSON Encodings of Metadata Annotations
[RFC7952] section 5.1 and section 5.2 provide an encoding rule for
metadata annotations in XML and JSON respectively.
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 align with the first line and
not be indented with any whitespace. The leading and trailing
whitespace of each line MUST be ignored. Figure 5 gives a XML
example.
When an example JSON representation of a leaf node element that
includes metadata attributes starting with the "@" character 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. Continuation lines MUST align with
the first line and indented with one whitespace character. The
leading and trailing whitespace of each line MUST be ignored.
Figure 6 gives a JSON example.
Whenever this documentation convention is used, the boilerplate text
shown in Section 7 MUST be present in the document using the
convention.
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<foo xmlns:elm=http://example.org/example-last-modified\
elm:last-modified="2015-09-16T10:27:35+02:00">
...
</foo>
Figure 5: An XML Example Leaf Node With Metadata Split Across Lines
"cask": {
"@": {
"example-org-example-last-modified:last-modified":\
"2015-09-16T10:27:35+02:00"
},
...
}
Figure 6: A JSON Example Leaf Node With Metadata Split Across Lines
7. Mandatory Boilerplate for Splitting Leaf node value and Metadata
Annotations
When the conventions described in , (Section 4), (Section 5)Section 6
are used for the benefit of the representation of an example of a
YANG module or YANG fragment in XML/JSON or YANG frament containing
metadata annotations in XML/JSON, 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 or 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/JSON 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 decoding From YANG data node Instance Examples with split
line
It should be noted that it is never the intention that example YANG
data node Instance fragment generated by encoding function should be
converted to YANG representation in data file that is passed to a
YANG consumer. Nevertheless, there are good reasons to be able to
convert an YANG data node instance example with split line into valid
YANG representation without split line in data file using decoding
function 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.
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When parsing a leaf or leaf-list node in an example, the following
rules should be applied to generate valid XML.
o If a white space, carriage return, or line feed character is
encountered between close (">") and open ("<") angle brackets it
should be stripped.
o 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.
o 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.
o 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.
o 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/JSON encoding
documentation convention described in this document. However,
attempting to provide actual XML/JSON 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/JSON. 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
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nits. Also thanks Martin Bjorklund, Charles Eckel, Robert Wilton and
many others for valuable comments and review.
12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG",
RFC 7951, DOI 10.17487/RFC7951, August 2016,
<https://www.rfc-editor.org/info/rfc7951>.
[RFC7952] Lhotka, L., "Defining and Using Metadata with YANG",
RFC 7952, DOI 10.17487/RFC7952, August 2016,
<https://www.rfc-editor.org/info/rfc7952>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
<https://www.w3.org/TR/2008/REC-xml-20081126/>.
Appendix A. Backslash appears in the node value not used for split line
Figure 7 and Figure 8 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., //Same line as the first row
the backslash \. Don't forget. //Same line as the firs row
</long-leaf-string-node-label>
Figure 7: An Example Leaf Node With a Complex String Value
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<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 8: An Example Leaf Node With a Complex String Value Split
Across Lines
Authors' Addresses
Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: bill.wu@huawei.com
Adrian Farrel
Juniper Networks
Email: afarrel@juniper.net
Benoit Claise
Cisco Systems, Inc.
De Kleetlaan 6a b1
1831 Diegem
Belgium
Phone: +32 2 704 5622
Email: bclaise@cisco.com
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