Internet Engineering Task Force (IETF) K. Watsen

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: [email protected]

Erik Auerswald
Individual Contributor

Email: [email protected]

Adrian Farrel
Old Dog Consulting

Email: [email protected]

Qin Wu
Huawei Technologies

Email: [email protected]