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Checking references for intended status: Best Current Practice ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'RFCYYYY' is mentioned on line 263, but not defined -- Possible downref: Non-RFC (?) normative reference: ref. 'XML' Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Netmod Working Group Q. Wu 3 Internet-Draft Huawei 4 Intended status: Best Current Practice A. Farrel 5 Expires: July 30, 2018 Juniper Networks 6 B. Claise 7 Cisco Systems, Inc. 8 January 26, 2018 10 Documentation Conventions for Expressing YANG in XML 11 draft-wu-netmod-yang-xml-doc-conventions-00 13 Abstract 15 Many documents that define YANG modules also include examples 16 presented in XML. 18 IETF documentation has specific limits on line length and some XML 19 examples have to include line wraps that would not normally be 20 allowed according to the XML representation rules of RFC7950 and 21 RFC7952. 23 This document lays out documentation conventions that allow YANG 24 examples to be presented in IETF documentation when leaf node 25 encoding would otherwise exceed the maximum line length. There are 26 no implications in this document for YANG parsers: this document does 27 not change the rules for presenting YANG models or for encoding YANG 28 in data files or in the wire. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at https://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on July 30, 2018. 47 Copyright Notice 49 Copyright (c) 2018 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (https://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 65 2. Conventions Used in this Document . . . . . . . . . . . . . . 3 66 3. Separating Components of a Leaf Example . . . . . . . . . . . 3 67 4. Splitting an Example Leaf Node Value Across Lines . . . . . . 4 68 5. Mandatory Boilerplate Text . . . . . . . . . . . . . . . . . 5 69 6. Representing XML Encodings of Metadata Annotations . . . . . 6 70 7. Mandatory Boilerplate for Splitting Metadata Annotations . . 6 71 8. Automatic Generation of Valid XML From Examples . . . . . . . 7 72 9. Security Considerations . . . . . . . . . . . . . . . . . . . 7 73 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 74 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 75 12. Normative References . . . . . . . . . . . . . . . . . . . . 8 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 78 1. Introduction 80 YANG [RFC7950] defines four main types of data node for data modeling 81 and describes how these are represented in XML [XML]. For list nodes 82 and container nodes, any whitespace, carriage returns, or line feeds 83 between the subelements is insignificant, i.e., an implementation MAY 84 insert whitespace, carriage return, or line feed characters between 85 subelements. 87 However for leaf nodes, [RFC7950] section 7.6.6 says 89 The value of the leaf node is encoded to XML according to the type 90 and is sent as character data in the element. 92 Thus whitespace, carriage return, and line feed characters are 93 interpreted as part of the leaf value if the leaf is of type string 94 and must not be included. The same applies to leaf-list nodes. 96 However, when documenting examples of YANG modules represented in XML 97 encoding it is possible that the encoding of a single leaf node will 98 exceed the available line length (73 characters). 100 This document describes documentation conventions that allow the 101 presentation of such examples in a way that is easily parsed by a 102 human reader, but which is not representative of how the XML must be 103 presented to a software component or carried on the wire. 105 There are no implications in this document for YANG parsers: this 106 document does not change the rules for presenting YANG models or for 107 encoding YANG in data files or in the wire. 109 2. Conventions Used in this Document 111 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 112 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 113 "OPTIONAL" in this document are to be interpreted as described in BCP 114 14 [RFC2119] [RFC8174] when, and only when, they appear in all 115 capitals, as shown here. 117 3. Separating Components of a Leaf Example 119 An example of the documentation of a leaf node is shown in Figure 1. 120 The leaf is called "long-leaf-node-name" and is assigned the value 121 "long-leaf-node-value". As can be seen in the example, this fits on 122 one line. However it would only take the addition of a few more 123 characters to the node label or value for the example to overflow the 124 73 character limit. 126 long-leaf-node-value 128 Figure 1: A Simple Leaf Node Example 130 For the sake of documentation, the representation shown in Figure 2 131 SHALL be considered as equivalent to that shown in Figure 1, but when 132 a document uses this convention it MUST also include the text shown 133 in Section 5. 135 136 long-leaf-node-value 137 139 Figure 2: A Split Leaf Node Example 141 4. Splitting an Example Leaf Node Value Across Lines 143 When the XML representation of a leaf node value in an example would 144 result in a line being longer than the maximum line length for an 145 IETF document the value of the leaf node must be split and printed on 146 more than one lines. This is most likely to happen when the example 147 leaf node contains a string. Indeed, if this problem arises for 148 other leaf types it may be indicative of poorly chosen leaf values, 149 and the YANG definitions should be revised. 151 In this case, conventions MUST be observed: 153 o The broken line MUST be terminated with a backslash ("\") without 154 the addition of any additional space before the backslash and with 155 no further characters after the backslash. 157 o Any continuation lines MUST be indented with a whitespace offset 158 of at least two characters. 160 o When a backslash appears in the node value, the example MUST be 161 arranged so that the backslash is not the final character of a 162 broken line 164 Furthermore, whenever a document uses this convention it MUST also 165 include the text shown in Section 5. 167 Figure 3 shows an example leaf with a long value. As can be seen, 168 the addition of a few more characters would cause the line to be too 169 long. 171 Figure 4 shows a semantically equivalent representation of the 172 example if the text from Section 5 is also present. 174 175 Once upon a time, in a land far away, there lived a Great King. 176 178 Figure 3: An Example Leaf Node With a Long String Value 180 181 Once upon a time, \ 182 in a land far away, \ 183 there lived a Great King. 184 186 Figure 4: A Long String Leaf Node Example Split Across Lines 188 Figure 5 and Figure 6 show a more complex example where the node 189 value includes both line feeds and a backslash. Note how the line 190 breaks are arranged to avoid potential confusion and to make the real 191 characters evident. 193 194 Punctuation is important. As are line feeds. 195 Some characters are special. E.g., the backslash \. Don't forget. 196 198 Figure 5: An Example Leaf Node With a Complex String Value 200 201 Punctuation is important. \ 202 As are line feeds. 203 Some characters are special. \ 204 E.g., the backslash \. \ 205 Don't forget. 206 208 Figure 6: An Example Leaf Node With a Complex String Value Split 209 Across Lines 211 5. Mandatory Boilerplate Text 213 When either of the conventions described in Section 3 or Section 4 is 214 used for the benefit of the representation of an example of a YANG 215 module or YANG fragment in XML, the following text MUST be included 216 in the document presenting the example. 218 The examples in this document adopt the conventions shown in BCP 219 XX [RFCYYYY] for splitting node labels and node values onto 220 separate lines. This convention is used to make the examples 221 easier to read but does not change the encoding rules for the XML 222 representation of YANG as described in [RFC7950]. 224 RFC Editor Note: Please replace XX and YYYY with the numbers assigned 225 for this document. 227 6. Representing XML Encodings of Metadata Annotations 229 [RFC7952] section 5.1 provides an encoding rule for metadata 230 annotations in XML. 232 When an example XML representation of a leaf node element that 233 includes metadata attributes results in a line being longer than the 234 maximum number of characters allowed in a line of an IETF document, 235 the value of the leaf node must be split across more than one line. 237 Where possible, all line breaks should be inserted between metadata 238 attributes. Continuation lines MUST start with a whitespace offset 239 of at least two characters. The leading and trailing whitespace of 240 each line MUST be ignored. Figure 7 gives an example. 242 Whenever this documentation convention is used, the boilerplate text 243 shown in Section 7 MUST be present in the document using the 244 convention. 246 249 /t:top/t:interface[t:name="Ethernet0/0"]/t:mtu/t:anotherattribute 250 /t:afinalattribute 251 253 Figure 7: An Example Leaf Node With Metadata Split Across Lines 255 7. Mandatory Boilerplate for Splitting Metadata Annotations 257 When the convention described in Section 6 is used for the benefit of 258 the representation of an example of a YANG module or YANG fragment 259 containing metadata annotations in XML, the following text MUST be 260 included in the document presenting the example. 262 The examples in this document adopt the conventions shown in BCP 263 XX [RFCYYYY] for splitting metadata annotation across multiple 264 lines. This convention is used to make the examples easier to 265 read but does not change the encoding rules for the XML 266 representation of YANG metadata annotations as described in 267 [RFC7952]. 269 RFC Editor Note: Please replace XX and YYYY with the numbers assigned 270 for this document. 272 8. Automatic Generation of Valid XML From Examples 274 It should be noted that it is never the intention that example YANG 275 fragment should be converted to XML that is passed to a YANG 276 consumer. Nevertheless, there are good reasons to be able to convert 277 an example into valid YANG in order to parse it and check its 278 validity against the YANG model itself. This will ensure that 279 examples in documents are accurate and useful. 281 When parsing a leaf or leaf-list node in an example, the following 282 rules should be applied to generate valid XML. 284 o If a white space, carriage return, or line feed character is 285 encountered between close (">") and open ("<") angle brackets it 286 should be stripped. 288 o If a white space, carriage return, or line feed character is 289 encountered within a string value of a leaf node or leaf-list 290 node, it should generally be preserved exactly as shown except in 291 the special case that follows. 293 o If a backslash character ("\") appears within the string value of 294 a leaf node or leaf-list node and if and only if it is immediately 295 followed by a carriage return or line feed character then all 296 carriage return, line feed, and whitespace characters should be 297 stripped until the next character is encountered. 299 o If a white space, carriage return, or line feed character is 300 encountered within metadata annotations, but not within quotes, it 301 should be stripped. Parsing may expect the next valid character 302 found to indicate the start of a new metadata attribute. 304 o If a backslash character ("\") appears within the quoted value of 305 a metadata attribute and if and only if it is immediately followed 306 by a carriage return or line feed character then all carriage 307 return, line feed, and whitespace characters should be stripped 308 until the next character is encountered. 310 9. Security Considerations 312 There is no direct security impact related to the XML encoding 313 documentation convention described in this document. However, 314 attempting to provide actual XML using the documentation conventions 315 described in this document would have unpredictable results. The 316 risk here is that someone uses an example as a template for actual 317 XML. The mandatory boilerplate text provides a mitigation against 318 this risk. 320 10. IANA Considerations 322 There are no IANA requests or assignments included in this document. 324 11. Acknowledgements 326 Thanks to Kent Watsen for discussions that kept us close to being on 327 the right track. Additional thanks to John Scudder for flagging some 328 nits. 330 12. Normative References 332 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 333 Requirement Levels", BCP 14, RFC 2119, 334 DOI 10.17487/RFC2119, March 1997, 335 . 337 [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", 338 RFC 7950, DOI 10.17487/RFC7950, August 2016, 339 . 341 [RFC7952] Lhotka, L., "Defining and Using Metadata with YANG", 342 RFC 7952, DOI 10.17487/RFC7952, August 2016, 343 . 345 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 346 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 347 May 2017, . 349 [XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and 350 F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth 351 Edition)", World Wide Web Consortium Recommendation REC- 352 xml-20081126, November 2008, 353 . 355 Authors' Addresses 357 Qin Wu 358 Huawei 359 101 Software Avenue, Yuhua District 360 Nanjing, Jiangsu 210012 361 China 363 Email: bill.wu@huawei.com 364 Adrian Farrel 365 Juniper Networks 367 Email: afarrel@juniper.net 369 Benoit Claise 370 Cisco Systems, Inc. 371 De Kleetlaan 6a b1 372 1831 Diegem 373 Belgium 375 Phone: +32 2 704 5622 376 Email: bclaise@cisco.com