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For this RFC, original HTML is available from the RFC-Editor: RFC8792



Internet Engineering Task Force (IETF)                         K. Watsen
Request for Comments: 8792                               Watsen Networks
Category: Informational                                     E. Auerswald
ISSN: 2070-1721                                   Individual Contributor
                                                               A. Farrel
                                                      Old Dog Consulting
                                                                   Q. Wu
                                                     Huawei Technologies
                                                               June 2020


       Handling Long Lines in Content of Internet-Drafts and RFCs

Abstract

   This document defines two strategies for handling long lines in
   width-bounded text content.  One strategy, called the "single
   backslash" strategy, is based on the historical use of a single
   backslash ('\') character to indicate where line-folding has
   occurred, with the continuation occurring with the first character
   that is not a space character (' ') on the next line.  The second
   strategy, called the "double backslash" strategy, extends the first
   strategy by adding a second backslash character to identify where the
   continuation begins and is thereby able to handle cases not supported
   by the first strategy.  Both strategies use a self-describing header
   enabling automated reconstitution of the original content.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8792.

Copyright Notice

   Copyright (c) 2020 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
   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.  Applicability Statement
   3.  Requirements Language
   4.  Goals
     4.1.  Automated Folding of Long Lines in Text Content
     4.2.  Automated Reconstitution of the Original Text Content
   5.  Limitations
     5.1.  Not Recommended for Graphical Artwork
     5.2.  Doesn't Work as Well as Format-Specific Options
   6.  Two Folding Strategies
     6.1.  Comparison
     6.2.  Recommendation
   7.  The Single Backslash Strategy ('\')
     7.1.  Folded Structure
       7.1.1.  Header
       7.1.2.  Body
     7.2.  Algorithm
       7.2.1.  Folding
       7.2.2.  Unfolding
   8.  The Double Backslash Strategy ('\\')
     8.1.  Folded Structure
       8.1.1.  Header
       8.1.2.  Body
     8.2.  Algorithm
       8.2.1.  Folding
       8.2.2.  Unfolding
   9.  Examples
     9.1.  Example Showing Boundary Conditions
       9.1.1.  Using '\'
       9.1.2.  Using '\\'
     9.2.  Example Showing Multiple Wraps of a Single Line
       9.2.1.  Using '\'
       9.2.2.  Using '\\'
     9.3.  Example Showing "Smart" Folding
       9.3.1.  Using '\'
       9.3.2.  Using '\\'
     9.4.  Example Showing "Forced" Folding
       9.4.1.  Using '\'
       9.4.2.  Using '\\'
   10. Security Considerations
   11. IANA Considerations
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Appendix A.  Bash Shell Script: rfcfold
   Acknowledgements
   Authors' Addresses

1.  Introduction

   [RFC7994] sets out the requirements for plain-text RFCs and states
   that each line of an RFC (and hence of an Internet-Draft) must be
   limited to 72 characters followed by the character sequence that
   denotes an end-of-line (EOL).

   Internet-Drafts and RFCs often include example text or code
   fragments.  Many times, the example text or code exceeds the
   72-character line-length limit.  The 'xml2rfc' utility [xml2rfc], at
   the time of this document's publication, does not attempt to wrap the
   content of such inclusions, simply issuing a warning whenever lines
   exceed 69 characters.  Historically, there has been no convention
   recommended by the RFC Editor in place for how to handle long lines
   in such inclusions, other than advising authors to clearly indicate
   what manipulation has occurred.

   This document defines two strategies for handling long lines in
   width-bounded text content.  One strategy, called the "single
   backslash" strategy, is based on the historical use of a single
   backslash ('\') character to indicate where line-folding has
   occurred, with the continuation occurring with the first character
   that is not a space character (' ') on the next line.  The second
   strategy, called the "double backslash" strategy, extends the first
   strategy by adding a second backslash character to identify where the
   continuation begins and is thereby able to handle cases not supported
   by the first strategy.  Both strategies use a self-describing header
   enabling automated reconstitution of the original content.

   The strategies defined in this document work on any text content but
   are primarily intended for a structured sequence of lines, such as
   would be referenced by the <sourcecode> element defined in
   Section 2.48 of [RFC7991], rather than for two-dimensional imagery,
   such as would be referenced by the <artwork> element defined in
   Section 2.5 of [RFC7991].

   Note that text files are represented as lines having their first
   character in column 1, and a line length of N where the last
   character is in the Nth column and is immediately followed by an end-
   of-line character sequence.

2.  Applicability Statement

   The formats and algorithms defined in this document may be used in
   any context, whether for IETF documents or in other situations where
   structured folding is desired.

   Within the IETF, this work primarily targets the xml2rfc v3
   <sourcecode> element (Section 2.48 of [RFC7991]) and the xml2rfc v2
   <artwork> element (Section 2.5 of [RFC7749]), which, for lack of a
   better option, is used in xml2rfc v2 for both source code and
   artwork.  This work may also be used for the xml2rfc v3 <artwork>
   element (Section 2.5 of [RFC7991]), but as described in Section 5.1,
   it is generally not recommended.

3.  Requirements Language

   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.

4.  Goals

4.1.  Automated Folding of Long Lines in Text Content

   Automated folding of long lines is needed in order to support
   documents that are dynamically compiled to include content with
   potentially unconstrained line lengths.  For instance, the build
   process may wish to include content from other local files or content
   that is dynamically generated by some external process.  Both of
   these cases are discussed next.

   Many documents need to include the content from local files (e.g.,
   XML, JSON, ABNF, ASN.1).  Prior to including a file's content, the
   build process SHOULD first validate these source files using format-
   specific validators.  In order for such tooling to be able to process
   the files, the files must be in their original/natural state, which
   may entail them having some long lines.  Thus, these source files
   need to be folded before inclusion into the XML document, in order to
   satisfy 'xml2rfc' line-length limits.

   Similarly, documents sometimes contain dynamically generated output,
   typically from an external process operating on the same source files
   discussed in the previous paragraph.  For instance, such processes
   may translate the input format to another format, or they may render
   a report on, or a view of, the input file.  In some cases, the
   dynamically generated output may contain lines exceeding the
   'xml2rfc' line-length limits.

   In both cases, folding is required and SHOULD be automated to reduce
   effort and errors resulting from manual processing.

4.2.  Automated Reconstitution of the Original Text Content

   Automated reconstitution of the exact original text content is needed
   to support validation of text-based content extracted from documents.

   For instance, YANG modules [RFC7950] are already extracted from
   Internet-Drafts and validated as part of the submission process.
   Additionally, the desire to validate instance examples (i.e., XML/
   JSON documents) contained within Internet-Drafts has been discussed
   [yang-doctors-thread].

5.  Limitations

5.1.  Not Recommended for Graphical Artwork

   While the solution presented in this document works on any kind of
   text-based content, it is most useful on content that represents
   source code (XML, JSON, etc.) or, more generally, on content that has
   not been laid out in two dimensions (e.g., diagrams).

   Fundamentally, the issue is whether the text content remains readable
   once folded.  Text content that is unpredictable is especially
   susceptible to looking bad when folded; falling into this category
   are most Unified Modeling Language (UML) diagrams, YANG tree
   diagrams, and ASCII art in general.

   It is NOT RECOMMENDED to use the solution presented in this document
   on graphical artwork.

5.2.  Doesn't Work as Well as Format-Specific Options

   The solution presented in this document works generically for all
   text-based content, as it only views content as plain text.  However,
   various formats sometimes have built-in mechanisms that are better
   suited to prevent long lines.

   For instance, both the 'pyang' and 'yanglint' utilities [pyang]
   [yanglint] have the command-line option "tree-line-length", which can
   be used to indicate a desired maximum line length when generating
   YANG tree diagrams [RFC8340].

   In another example, some source formats (e.g., YANG [RFC7950]) allow
   any quoted string to be broken up into substrings separated by a
   concatenation character (e.g., '+'), any of which can be on a
   different line.

   It is RECOMMENDED that authors do as much as possible within the
   selected format to avoid long lines.

6.  Two Folding Strategies

   This document defines two nearly identical strategies for folding
   text-based content.

   The Single Backslash Strategy ('\'):
      Uses a backslash ('\') character at the end of the line where
      folding occurs, and assumes that the continuation begins at the
      first character that is not a space character (' ') on the
      following line.

   The Double Backslash Strategy ('\\'):
      Uses a backslash ('\') character at the end of the line where
      folding occurs, and assumes that the continuation begins after a
      second backslash ('\') character on the following line.

6.1.  Comparison

   The first strategy produces output that is more readable.  However,
   (1) it is significantly more likely to encounter unfoldable input
   (e.g., a long line containing only space characters), and (2) for
   long lines that can be folded, automation implementations may
   encounter scenarios that, without special care, will produce errors.

   The second strategy produces output that is less readable, but it is
   unlikely to encounter unfoldable input, there are no long lines that
   cannot be folded, and no special care is required when folding a long
   line.

6.2.  Recommendation

   It is RECOMMENDED that implementations first attempt to fold content
   using the single backslash strategy and, only in the unlikely event
   that it cannot fold the input or the folding logic is unable to cope
   with a contingency occurring on the desired folding column, then fall
   back to the double backslash strategy.

7.  The Single Backslash Strategy ('\')

7.1.  Folded Structure

   Text content that has been folded as specified by this strategy MUST
   adhere to the following structure.

7.1.1.  Header

   The header is two lines long.

   The first line is the following 36-character string; this string MAY
   be surrounded by any number of printable characters.  This first line
   cannot itself be folded.

   NOTE: '\' line wrapping per RFC 8792

   The second line is an empty line, containing only the end-of-line
   character sequence.  This line provides visual separation for
   readability.

7.1.2.  Body

   The character encoding is the same as the encoding described in
   Section 2 of [RFC7994], except that, per [RFC7991], tab characters
   are prohibited.

   Lines that have a backslash ('\') occurring as the last character in
   a line are considered "folded".

   Exceptionally long lines MAY be folded multiple times.

7.2.  Algorithm

   This section describes a process for folding and unfolding long lines
   when they are encountered in text content.

   The steps are complete, but implementations MAY achieve the same
   result in other ways.

   When a larger document contains multiple instances of text content
   that may need to be folded or unfolded, another process must
   insert/extract the individual text content instances to/from the
   larger document prior to utilizing the algorithms described in this
   section.  For example, the 'xiax' utility [xiax] does this.

7.2.1.  Folding

   Determine the desired maximum line length from input to the line-
   wrapping process, such as from a command-line parameter.  If no value
   is explicitly specified, the value "69" SHOULD be used.

   Ensure that the desired maximum line length is not less than the
   minimum header, which is 36 characters.  If the desired maximum line
   length is less than this minimum, exit (this text-based content
   cannot be folded).

   Scan the text content for horizontal tab characters.  If any
   horizontal tab characters appear, either resolve them to space
   characters or exit, forcing the input provider to convert them to
   space characters themselves first.

   Scan the text content to ensure that at least one line exceeds the
   desired maximum.  If no line exceeds the desired maximum, exit (this
   text content does not need to be folded).

   Scan the text content to ensure that no existing lines already end
   with a backslash ('\') character, as this could lead to an ambiguous
   result.  If such a line is found, and its width is less than the
   desired maximum, then it SHOULD be flagged for "forced" folding
   (folding even though unnecessary).  If the folding implementation
   doesn't support forced foldings, it MUST exit.

   If this text content needs to, and can, be folded, insert the header
   described in Section 7.1.1, ensuring that any additional printable
   characters surrounding the header do not result in a line exceeding
   the desired maximum.

   For each line in the text content, from top to bottom, if the line
   exceeds the desired maximum or requires a forced folding, then fold
   the line by performing the following steps:

   1.  Determine where the fold will occur.  This location MUST be
       before or at the desired maximum column and MUST NOT be chosen
       such that the character immediately after the fold is a space
       (' ') character.  For forced foldings, the location is between
       the '\' and the end-of-line sequence.  If no such location can be
       found, then exit (this text content cannot be folded).

   2.  At the location where the fold is to occur, insert a backslash
       ('\') character followed by the end-of-line character sequence.

   3.  On the following line, insert any number of space (' ')
       characters, provided that the resulting line does not exceed the
       desired maximum.

   The result of the previous operation is that the next line starts
   with an arbitrary number of space (' ') characters, followed by the
   character that was previously occupying the position where the fold
   occurred.

   Continue in this manner until reaching the end of the text content.
   Note that this algorithm naturally addresses the case where the
   remainder of a folded line is still longer than the desired maximum
   and, hence, needs to be folded again, ad infinitum.

   The process described in this section is illustrated by the
   "fold_it_1()" function in Appendix A.

7.2.2.  Unfolding

   Scan the beginning of the text content for the header described in
   Section 7.1.1.  If the header is not present, exit (this text content
   does not need to be unfolded).

   Remove the two-line header from the text content.

   For each line in the text content, from top to bottom, if the line
   has a backslash ('\') character immediately followed by the end-of-
   line character sequence, then the line can be unfolded.  Remove the
   backslash ('\') character, the end-of-line character sequence, and
   any leading space (' ') characters, which will bring up the next
   line.  Then continue to scan each line in the text content starting
   with the current line (in case it was multiply folded).

   Continue in this manner until reaching the end of the text content.

   The process described in this section is illustrated by the
   "unfold_it_1()" function in Appendix A.

8.  The Double Backslash Strategy ('\\')

8.1.  Folded Structure

   Text content that has been folded as specified by this strategy MUST
   adhere to the following structure.

8.1.1.  Header

   The header is two lines long.

   The first line is the following 37-character string; this string MAY
   be surrounded by any number of printable characters.  This first line
   cannot itself be folded.

   NOTE: '\\' line wrapping per RFC 8792

   The second line is an empty line, containing only the end-of-line
   character sequence.  This line provides visual separation for
   readability.

8.1.2.  Body

   The character encoding is the same as the encoding described in
   Section 2 of [RFC7994], except that, per [RFC7991], tab characters
   are prohibited.

   Lines that have a backslash ('\') occurring as the last character in
   a line immediately followed by the end-of-line character sequence,
   when the subsequent line starts with a backslash ('\') as the first
   character that is not a space character (' '), are considered
   "folded".

   Exceptionally long lines MAY be folded multiple times.

8.2.  Algorithm

   This section describes a process for folding and unfolding long lines
   when they are encountered in text content.

   The steps are complete, but implementations MAY achieve the same
   result in other ways.

   When a larger document contains multiple instances of text content
   that may need to be folded or unfolded, another process must
   insert/extract the individual text content instances to/from the
   larger document prior to utilizing the algorithms described in this
   section.  For example, the 'xiax' utility [xiax] does this.

8.2.1.  Folding

   Determine the desired maximum line length from input to the line-
   wrapping process, such as from a command-line parameter.  If no value
   is explicitly specified, the value "69" SHOULD be used.

   Ensure that the desired maximum line length is not less than the
   minimum header, which is 37 characters.  If the desired maximum line
   length is less than this minimum, exit (this text-based content
   cannot be folded).

   Scan the text content for horizontal tab characters.  If any
   horizontal tab characters appear, either resolve them to space
   characters or exit, forcing the input provider to convert them to
   space characters themselves first.

   Scan the text content to see if any line exceeds the desired maximum.
   If no line exceeds the desired maximum, exit (this text content does
   not need to be folded).

   Scan the text content to ensure that no existing lines already end
   with a backslash ('\') character while the subsequent line starts
   with a backslash ('\') character as the first character that is not a
   space character (' '), as this could lead to an ambiguous result.  If
   such a line is found and its width is less than the desired maximum,
   then it SHOULD be flagged for forced folding (folding even though
   unnecessary).  If the folding implementation doesn't support forced
   foldings, it MUST exit.

   If this text content needs to, and can, be folded, insert the header
   described in Section 8.1.1, ensuring that any additional printable
   characters surrounding the header do not result in a line exceeding
   the desired maximum.

   For each line in the text content, from top to bottom, if the line
   exceeds the desired maximum or requires a forced folding, then fold
   the line by performing the following steps:

   1.  Determine where the fold will occur.  This location MUST be
       before or at the desired maximum column.  For forced foldings,
       the location is between the '\' and the end-of-line sequence on
       the first line.

   2.  At the location where the fold is to occur, insert a first
       backslash ('\') character followed by the end-of-line character
       sequence.

   3.  On the following line, insert any number of space (' ')
       characters, provided that the resulting line does not exceed the
       desired maximum, followed by a second backslash ('\') character.

   The result of the previous operation is that the next line starts
   with an arbitrary number of space (' ') characters, followed by a
   backslash ('\') character, immediately followed by the character that
   was previously occupying the position where the fold occurred.

   Continue in this manner until reaching the end of the text content.
   Note that this algorithm naturally addresses the case where the
   remainder of a folded line is still longer than the desired maximum
   and, hence, needs to be folded again, ad infinitum.

   The process described in this section is illustrated by the
   "fold_it_2()" function in Appendix A.

8.2.2.  Unfolding

   Scan the beginning of the text content for the header described in
   Section 8.1.1.  If the header is not present, exit (this text content
   does not need to be unfolded).

   Remove the two-line header from the text content.

   For each line in the text content, from top to bottom, if the line
   has a backslash ('\') character immediately followed by the end-of-
   line character sequence and if the next line has a backslash ('\')
   character as the first character that is not a space character (' '),
   then the lines can be unfolded.  Remove the first backslash ('\')
   character, the end-of-line character sequence, any leading space
   (' ') characters, and the second backslash ('\') character, which
   will bring up the next line.  Then, continue to scan each line in the
   text content starting with the current line (in case it was multiply
   folded).

   Continue in this manner until reaching the end of the text content.

   The process described in this section is illustrated by the
   "unfold_it_2()" function in Appendix A.

9.  Examples

   The following self-documenting examples illustrate folded text-based
   content.

   The source text content cannot be presented here, as it would again
   be folded.  Alas, only the results can be provided.

9.1.  Example Showing Boundary Conditions

   This example illustrates boundary conditions.  The input contains
   seven lines, each line one character longer than the previous line.
   Numbers are used for counting purposes.  The default desired maximum
   column value "69" is used.

9.1.1.  Using '\'

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   123456789012345678901234567890123456789012345678901234567890123456
   1234567890123456789012345678901234567890123456789012345678901234567
   12345678901234567890123456789012345678901234567890123456789012345678
   123456789012345678901234567890123456789012345678901234567890123456789
   12345678901234567890123456789012345678901234567890123456789012345678\
   90
   12345678901234567890123456789012345678901234567890123456789012345678\
   901
   12345678901234567890123456789012345678901234567890123456789012345678\
   9012

9.1.2.  Using '\\'

   ========== NOTE: '\\' line wrapping per RFC 8792 ==========

   123456789012345678901234567890123456789012345678901234567890123456
   1234567890123456789012345678901234567890123456789012345678901234567
   12345678901234567890123456789012345678901234567890123456789012345678
   123456789012345678901234567890123456789012345678901234567890123456789
   12345678901234567890123456789012345678901234567890123456789012345678\
   \90
   12345678901234567890123456789012345678901234567890123456789012345678\
   \901
   12345678901234567890123456789012345678901234567890123456789012345678\
   \9012

9.2.  Example Showing Multiple Wraps of a Single Line

   This example illustrates what happens when a very long line needs to
   be folded multiple times.  The input contains one line containing 280
   characters.  Numbers are used for counting purposes.  The default
   desired maximum column value "69" is used.

9.2.1.  Using '\'

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   12345678901234567890123456789012345678901234567890123456789012345678\
   90123456789012345678901234567890123456789012345678901234567890123456\
   78901234567890123456789012345678901234567890123456789012345678901234\
   56789012345678901234567890123456789012345678901234567890123456789012\
   34567890

9.2.2.  Using '\\'

   ========== NOTE: '\\' line wrapping per RFC 8792 ==========

   12345678901234567890123456789012345678901234567890123456789012345678\
   \9012345678901234567890123456789012345678901234567890123456789012345\
   \6789012345678901234567890123456789012345678901234567890123456789012\
   \3456789012345678901234567890123456789012345678901234567890123456789\
   \01234567890

9.3.  Example Showing "Smart" Folding

   This example illustrates how readability can be improved via "smart"
   folding, whereby folding occurs at format-specific locations and
   format-specific indentations are used.

   The text content was manually folded, since the script in Appendix A
   does not implement smart folding.

   Note that the headers are surrounded by different printable
   characters than those shown in the script-generated examples.

9.3.1.  Using '\'

   [NOTE: '\' line wrapping per RFC 8792]

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>\
           urn:ietf:params:xml:ns:yang:ietf-interfaces\
         </namespace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

   Below is the equivalent of the above, but it was folded using the
   script in Appendix A.

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>urn:ietf:params:xml:ns:yang:ietf-interfaces</namesp\
   ace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

9.3.2.  Using '\\'

   [NOTE: '\\' line wrapping per RFC 8792]

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>\
           \urn:ietf:params:xml:ns:yang:ietf-interfaces\
         \</namespace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

   Below is the equivalent of the above, but it was folded using the
   script in Appendix A.

   ========== NOTE: '\\' line wrapping per RFC 8792 ==========

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>urn:ietf:params:xml:ns:yang:ietf-interfaces</namesp\
   \ace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

9.4.  Example Showing "Forced" Folding

   This example illustrates how invalid sequences in lines that do not
   have to be folded can be handled via forced folding, whereby the
   folding occurs even though unnecessary.

   The following line exceeds a 68-char max and, thus, demands folding:
   123456789012345678901234567890123456789012345678901234567890123456789

   This line ends with a backslash \

   This line ends with a backslash \
   \ This line begins with a backslash

   The following is an indented 3x3 block of backslashes:
      \\\
      \\\
      \\\

   The samples below were manually folded, since the script in the
   appendix does not implement forced folding.

   Note that the headers are prefixed by a pound ('#') character, rather
   than surrounded by 'equals' ('=') characters as shown in the script-
   generated examples.

9.4.1.  Using '\'

   # NOTE: '\' line wrapping per RFC 8792

   The following line exceeds a 68-char max and, thus, demands folding:
   1234567890123456789012345678901234567890123456789012345678901234567\
   89

   This line ends with a backslash \\


   This line ends with a backslash \\

   \ This line begins with a backslash

   The following is an indented 3x3 block of backslashes:
      \\\\

      \\\\

      \\\

9.4.2.  Using '\\'

   # NOTE: '\\' line wrapping per RFC 8792

   The following line exceeds a 68-char max and, thus, demands folding:
   1234567890123456789012345678901234567890123456789012345678901234567\
   \89

   This line ends with a backslash \

   This line ends with a backslash \\
   \
   \ This line begins with a backslash

   The following is an indented 3x3 block of backslashes:
      \\\\
      \
      \\\\
      \
      \\\

10.  Security Considerations

   This document has no security considerations.

11.  IANA Considerations

   This document has no IANA actions.

12.  References

12.1.  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>.

   [RFC7991]  Hoffman, P., "The "xml2rfc" Version 3 Vocabulary",
              RFC 7991, DOI 10.17487/RFC7991, December 2016,
              <https://www.rfc-editor.org/info/rfc7991>.

   [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>.

12.2.  Informative References

   [bash]     "GNU Bash Manual",
              <https://www.gnu.org/software/bash/manual>.

   [pyang]    "pyang", <https://pypi.org/project/pyang/>.

   [RFC7749]  Reschke, J., "The "xml2rfc" Version 2 Vocabulary",
              RFC 7749, DOI 10.17487/RFC7749, February 2016,
              <https://www.rfc-editor.org/info/rfc7749>.

   [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>.

   [RFC7994]  Flanagan, H., "Requirements for Plain-Text RFCs",
              RFC 7994, DOI 10.17487/RFC7994, December 2016,
              <https://www.rfc-editor.org/info/rfc7994>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [xiax]     "The 'xiax' Python Package",
              <https://pypi.org/project/xiax/>.

   [xml2rfc]  "xml2rfc", <https://pypi.org/project/xml2rfc/>.

   [yang-doctors-thread]
              Watsen, K., "[yang-doctors] automating yang doctor
              reviews", message to the yang-doctors mailing list, 18
              April 2018, <https://mailarchive.ietf.org/arch/msg/yang-
              doctors/DCfBqgfZPAD7afzeDFlQ1Xm2X3g>.

   [yanglint] "yanglint", commit 1b7d73d, February 2020,
              <https://github.com/CESNET/libyang#yanglint>.

Appendix A.  Bash Shell Script: rfcfold

   This non-normative appendix includes a Bash shell script [bash] that
   can both fold and unfold text content using both the single and
   double backslash strategies described in Sections 7 and 8,
   respectively.  This shell script, called 'rfcfold', is maintained at
   <https://github.com/ietf-tools/rfcfold>.

   This script is intended to be applied to a single text content
   instance.  If it is desired to fold or unfold text content instances
   within a larger document (e.g., an Internet-Draft or RFC), then
   another tool must be used to extract the content from the larger
   document before utilizing this script.

   For readability purposes, this script forces the minimum supported
   line length to be eight characters longer than the raw header text
   defined in Sections 7.1.1 and 8.1.1 so as to ensure that the header
   can be wrapped by a space (' ') character and three 'equals' ('=')
   characters on each side of the raw header text.

   When a tab character is detected in the input file, this script exits
   with the following error message:

      Error: infile contains a tab character, which is not allowed.

   This script tests for the availability of GNU awk (gawk), in order to
   test for ASCII-based control characters and non-ASCII characters in
   the input file (see below).  Note that testing revealed flaws in the
   default version of 'awk' on some platforms.  As this script uses
   'gawk' only to issue warning messages, if 'gawk' is not found, this
   script issues the following debug message:

      Debug: no GNU awk; skipping checks for special characters.

   When 'gawk' is available (see above) and ASCII-based control
   characters are detected in the input file, this script issues the
   following warning message:

      Warning: infile contains ASCII control characters (unsupported).

   When 'gawk' is available (see above) and non-ASCII characters are
   detected in the input file, this script issues the following warning
   message:

      Warning: infile contains non-ASCII characters (unsupported).

   This script does not implement the whitespace-avoidance logic
   described in Section 7.2.1.  In such a case, the script will exit
   with the following error message:

      Error: infile has a space character occurring on the folding
      column.  This file cannot be folded using the '\' strategy.

   While this script can unfold input that contains forced foldings, it
   is unable to fold files that would require forced foldings.  Forced
   folding is described in Sections 7.2.1 and 8.2.1.  When being asked
   to fold a file that would require forced folding, the script will
   instead exit with one of the following error messages:

   For '\':

      Error: infile has a line ending with a '\' character.  This file
      cannot be folded using the '\' strategy without there being false
      positives produced in the unfolding (i.e., this script does not
      force-fold such lines, as described in RFC 8792).

   For '\\':

      Error: infile has a line ending with a '\' character followed by a
      '\' character as the first non-space character on the next line.
      This script cannot fold this file using the '\\' strategy without
      there being false positives produced in the unfolding (i.e., this
      script does not force-fold such lines, as described in RFC 8792).

   Shell-level end-of-line backslash ('\') characters have been
   purposely added to the script so as to ensure that the script is
   itself not folded in this document, thus simplifying the ability to
   copy/paste the script for local use.  As should be evident by the
   lack of the mandatory header described in Section 7.1.1, these
   backslashes do not designate a folded line (e.g., as described in
   Section 7).

   <CODE BEGINS> file "rfcfold"
   #!/bin/bash --posix

   # This script may need some adjustments to work on a given system.
   # For instance, the utility 'gsed' may need to be installed.
   # Also, please be advised that 'bash' (not 'sh') must be used.

   # Copyright (c) 2020 IETF Trust, Kent Watsen, and Erik Auerswald.
   # All rights reserved.
   #
   # Redistribution and use in source and binary forms, with or without
   # modification, are permitted provided that the following conditions
   # are met:
   #
   #   * Redistributions of source code must retain the above copyright
   #     notice, this list of conditions and the following disclaimer.
   #
   #   * Redistributions in binary form must reproduce the above
   #     copyright notice, this list of conditions and the following
   #     disclaimer in the documentation and/or other materials
   #     provided with the distribution.
   #
   #   * Neither the name of Internet Society, IETF or IETF Trust, nor
   #     the names of specific contributors, may be used to endorse or
   #     promote products derived from this software without specific
   #     prior written permission.
   #
   # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   # FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
   # COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
   # (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
   # SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   # HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
   # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   # ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
   # ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

   print_usage() {
     printf "\n"
     printf "Folds or unfolds the input text file according to"
     printf " RFC 8792.\n"
     printf "\n"
     printf "Usage: rfcfold [-h] [-d] [-q] [-s <strategy>] [-c <col>]"
     printf " [-r] -i <infile> -o <outfile>\n"
     printf "\n"
     printf "  -s: strategy to use, '1' or '2' (default: try 1,"
     printf " else 2)\n"
     printf "  -c: column to fold on (default: 69)\n"
     printf "  -r: reverses the operation\n"
     printf "  -i: the input filename\n"
     printf "  -o: the output filename\n"
     printf "  -d: show debug messages (unless -q is given)\n"
     printf "  -q: quiet (suppress error and debug messages)\n"
     printf "  -h: show this message\n"
     printf "\n"
     printf "Exit status code: 1 on error, 0 on success, 255 on no-op."
     printf "\n\n"
   }

   # global vars, do not edit
   strategy=0 # auto
   debug=0
   quiet=0
   reversed=0
   infile=""
   outfile=""
   maxcol=69  # default, may be overridden by param
   col_gvn=0  # maxcol overridden?
   hdr_txt_1="NOTE: '\\' line wrapping per RFC 8792"
   hdr_txt_2="NOTE: '\\\\' line wrapping per RFC 8792"
   equal_chars="======================================================="
   space_chars="                                                       "
   temp_dir=""
   prog_name='rfcfold'

   # functions for diagnostic messages
   prog_msg() {
     if [[ "$quiet" -eq 0 ]]; then
       format_string="${prog_name}: $1: %s\n"
       shift
       printf -- "$format_string" "$*" >&2
     fi
   }

   err() {
     prog_msg 'Error' "$@"
   }

   warn() {
     prog_msg 'Warning' "$@"
   }

   dbg() {
     if [[ "$debug" -eq 1 ]]; then
       prog_msg 'Debug' "$@"
     fi
   }

   # determine name of [g]sed binary
   type gsed > /dev/null 2>&1 && SED=gsed || SED=sed

   # warn if a non-GNU sed utility is used
   "$SED" --version < /dev/null 2> /dev/null | grep -q GNU || \
   warn 'not using GNU `sed` (likely cause if an error occurs).'

   cleanup() {
     rm -rf "$temp_dir"
   }
   trap 'cleanup' EXIT

   fold_it_1() {
     # ensure input file doesn't contain the fold-sequence already
     if [[ -n "$("$SED" -n '/\\$/p' "$infile")" ]]; then
       err "infile '$infile' has a line ending with a '\\' character."\
           "This script cannot fold this file using the '\\' strategy"\
           "without there being false positives produced in the"\
           "unfolding."
       return 1
     fi

     # where to fold
     foldcol=$(expr "$maxcol" - 1) # for the inserted '\' char

     # ensure input file doesn't contain whitespace on the fold column
     grep -q "^\(.\{$foldcol\}\)\{1,\} " "$infile"
     if [[ $? -eq 0 ]]; then
       err "infile '$infile' has a space character occurring on the"\
           "folding column.  This file cannot be folded using the"\
           "'\\' strategy."
       return 1
     fi

     # center header text
     length=$(expr ${#hdr_txt_1} + 2)
     left_sp=$(expr \( "$maxcol" - "$length" \) / 2)
     right_sp=$(expr "$maxcol" - "$length" - "$left_sp")
     header=$(printf "%.*s %s %.*s" "$left_sp" "$equal_chars"\
                      "$hdr_txt_1" "$right_sp" "$equal_chars")

     # generate outfile
     echo "$header" > "$outfile"
     echo "" >> "$outfile"
     "$SED" 's/\(.\{'"$foldcol"'\}\)\(..\)/\1\\\n\2/;t M;b;:M;P;D;'\
       < "$infile" >> "$outfile" 2> /dev/null
     if [[ $? -ne 0 ]]; then
       return 1
     fi
     return 0
   }

   fold_it_2() {
     # where to fold
     foldcol=$(expr "$maxcol" - 1) # for the inserted '\' char

     # ensure input file doesn't contain the fold-sequence already
     if [[ -n "$("$SED" -n '/\\$/{N;s/\\\n[ ]*\\/&/p;D}' "$infile")" ]]
     then
       err "infile '$infile' has a line ending with a '\\' character"\
           "followed by a '\\' character as the first non-space"\
           "character on the next line.  This script cannot fold"\
           "this file using the '\\\\' strategy without there being"\
           "false positives produced in the unfolding."
       return 1
     fi

     # center header text
     length=$(expr ${#hdr_txt_2} + 2)
     left_sp=$(expr \( "$maxcol" - "$length" \) / 2)
     right_sp=$(expr "$maxcol" - "$length" - "$left_sp")
     header=$(printf "%.*s %s %.*s" "$left_sp" "$equal_chars"\
                      "$hdr_txt_2" "$right_sp" "$equal_chars")

     # generate outfile
     echo "$header" > "$outfile"
     echo "" >> "$outfile"
     "$SED" 's/\(.\{'"$foldcol"'\}\)\(..\)/\1\\\n\\\2/;t M;b;:M;P;D;'\
       < "$infile" >> "$outfile" 2> /dev/null
     if [[ $? -ne 0 ]]; then
       return 1
     fi
     return 0
   }

   fold_it() {
     # ensure input file doesn't contain a tab
     grep -q $'\t' "$infile"
     if [[ $? -eq 0 ]]; then
       err "infile '$infile' contains a tab character, which is not"\
           "allowed."
       return 1
     fi

     # folding of input containing ASCII control or non-ASCII characters
     # may result in a wrong folding column and is not supported
     if type gawk > /dev/null 2>&1; then
       env LC_ALL=C gawk '/[\000-\014\016-\037\177]/{exit 1}' "$infile"\
       || warn "infile '$infile' contains ASCII control characters"\
               "(unsupported)."
       env LC_ALL=C gawk '/[^\000-\177]/{exit 1}' "$infile"\
       || warn "infile '$infile' contains non-ASCII characters"\
               "(unsupported)."
     else
       dbg "no GNU awk; skipping checks for special characters."
     fi

     # check if file needs folding
     testcol=$(expr "$maxcol" + 1)
     grep -q ".\{$testcol\}" "$infile"
     if [[ $? -ne 0 ]]; then
       dbg "nothing to do; copying infile to outfile."
       cp "$infile" "$outfile"
       return 255
     fi

     if [[ "$strategy" -eq 1 ]]; then
       fold_it_1
       return $?
     fi
     if [[ "$strategy" -eq 2 ]]; then
       fold_it_2
       return $?
     fi
     quiet_sav="$quiet"
     quiet=1
     fold_it_1
     result=$?
     quiet="$quiet_sav"
     if [[ "$result" -ne 0 ]]; then
       dbg "Folding strategy '1' didn't succeed; trying strategy '2'..."
       fold_it_2
       return $?
     fi
     return 0
   }

   unfold_it_1() {
     temp_dir=$(mktemp -d)

     # output all but the first two lines (the header) to wip file
     awk "NR>2" "$infile" > "$temp_dir/wip"

     # unfold wip file
     "$SED" '{H;$!d};x;s/^\n//;s/\\\n *//g' "$temp_dir/wip" > "$outfile"

     return 0
   }

   unfold_it_2() {
     temp_dir=$(mktemp -d)

     # output all but the first two lines (the header) to wip file
     awk "NR>2" "$infile" > "$temp_dir/wip"

     # unfold wip file
     "$SED" '{H;$!d};x;s/^\n//;s/\\\n *\\//g' "$temp_dir/wip"\
       > "$outfile"

     return 0
   }

   unfold_it() {
     # check if file needs unfolding
     line=$(head -n 1 "$infile")
     line2=$("$SED" -n '2p' "$infile")
     result=$(echo "$line" | fgrep "$hdr_txt_1")
     if [[ $? -eq 0 ]]; then
       if [[ -n "$line2" ]]; then
         err "the second line in '$infile' is not empty."
         return 1
       fi
       unfold_it_1
       return $?
     fi
     result=$(echo "$line" | fgrep "$hdr_txt_2")
     if [[ $? -eq 0 ]]; then
       if [[ -n "$line2" ]]; then
         err "the second line in '$infile' is not empty."
         return 1
       fi
       unfold_it_2
       return $?
     fi
     dbg "nothing to do; copying infile to outfile."
     cp "$infile" "$outfile"
     return 255
   }

   process_input() {
     while [[ "$1" != "" ]]; do
       if [[ "$1" == "-h" ]] || [[ "$1" == "--help" ]]; then
         print_usage
         exit 0
       elif [[ "$1" == "-d" ]]; then
         debug=1
       elif [[ "$1" == "-q" ]]; then
         quiet=1
       elif [[ "$1" == "-s" ]]; then
         if [[ "$#" -eq "1" ]]; then
           err "option '-s' needs an argument (use -h for help)."
           exit 1
         fi
         strategy="$2"
         shift
       elif [[ "$1" == "-c" ]]; then
         if [[ "$#" -eq "1" ]]; then
           err "option '-c' needs an argument (use -h for help)."
           exit 1
         fi
         col_gvn=1
         maxcol="$2"
         shift
       elif [[ "$1" == "-r" ]]; then
         reversed=1
       elif [[ "$1" == "-i" ]]; then
         if [[ "$#" -eq "1" ]]; then
           err "option '-i' needs an argument (use -h for help)."
           exit 1
         fi
         infile="$2"
         shift
       elif [[ "$1" == "-o" ]]; then
         if [[ "$#" -eq "1" ]]; then
           err "option '-o' needs an argument (use -h for help)."
           exit 1
         fi
         outfile="$2"
         shift
       else
         warn "ignoring unknown option '$1'."
       fi
       shift
     done

     if [[ -z "$infile" ]]; then
       err "infile parameter missing (use -h for help)."
       exit 1
     fi

     if [[ -z "$outfile" ]]; then
       err "outfile parameter missing (use -h for help)."
       exit 1
     fi

     if [[ ! -f "$infile" ]]; then
       err "specified file '$infile' does not exist."
       exit 1
     fi

     if [[ "$col_gvn" -eq 1 ]] && [[ "$reversed" -eq 1 ]]; then
       warn "'-c' option ignored when unfolding (option '-r')."
     fi

     if [[ "$strategy" -eq 0 ]] || [[ "$strategy" -eq 2 ]]; then
       min_supported=$(expr ${#hdr_txt_2} + 8)
     else
       min_supported=$(expr ${#hdr_txt_1} + 8)
     fi
     if [[ "$maxcol" -lt "$min_supported" ]]; then
       err "the folding column cannot be less than $min_supported."
       exit 1
     fi

     # this is only because the code otherwise runs out of equal_chars
     max_supported=$(expr ${#equal_chars} + 1 + ${#hdr_txt_1} + 1\
          + ${#equal_chars})
     if [[ "$maxcol" -gt "$max_supported" ]]; then
       err "the folding column cannot be more than $max_supported."
       exit 1
     fi
   }

   main() {
     if [[ "$#" -eq "0" ]]; then
        print_usage
        exit 1
     fi

     process_input "$@"

     if [[ "$reversed" -eq 0 ]]; then
       fold_it
       code=$?
     else
       unfold_it
       code=$?
     fi
     exit "$code"
   }

   main "$@"
   <CODE ENDS>

Acknowledgements

   The authors thank the RFC Editor for confirming that there was
   previously no set convention, at the time of this document's
   publication, for handling long lines in source code inclusions, thus
   instigating this work.

   The authors thank the following folks for their various contributions
   while producing this document (sorted by first name): Ben Kaduk,
   Benoit Claise, Gianmarco Bruno, Italo Busi, Joel Jaeggli, Jonathan
   Hansford, Lou Berger, Martin Bjorklund, and Rob Wilton.

Authors' Addresses

   Kent Watsen
   Watsen Networks

   Email: kent+ietf@watsen.net


   Erik Auerswald
   Individual Contributor

   Email: auerswal@unix-ag.uni-kl.de


   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk


   Qin Wu
   Huawei Technologies

   Email: bill.wu@huawei.com


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