kramdown is a markdown parser by Thomas Leitner, which has a number of backends for generating HTML, Latex, and markdown again.

kramdown-rfc is an additional backend to that: It allows the generation of XML2RFC XML markup (originally known as RFC 2629 compliant markup, with a newer version documented in RFC 7749; version 3 now documented in RFC 7991 etc. and v3).

Who would care? Anybody who is writing Internet-Drafts and RFCs in the IETF and prefers (or has co-authors who prefer) to do part of their work in markdown.

kramdown-rfc is documented on this page, and also on the wiki.

Start by installing the kramdown-rfc gem (this automatically installs appropriate versions of referenced gems such as kramdown as well):

gem install kramdown-rfc

(Add a sudo and a space in front of that command if you don't have all the permissions needed.)

The guts of kramdown-rfc are in one Ruby file, lib/kramdown-rfc2629.rb --- this melds nicely into the extension structure provided by kramdown. bin/kramdown-rfc started out as a simple command-line program showing how to use this, but can now do much more (see below).

To use kramdown-rfc, you'll need Ruby (at least version 2.3, but preferably a current version), and maybe XML2RFC if you want to see the fruits of your work.

kramdown-rfc mydraft.md >mydraft.xml
xml2rfc mydraft.xml

(The most popular file name extension that IETF people have for markdown is .md -- for those who tend to think about GNU machine descriptions here, any extension such as .mkd will do, too.)

A more brief interface for both calling kramdown-rfc and XML2RFC is provided by kdrfc:

kdrfc mydraft.md

kdrfc can also use a remote installation of XML2RFC if needed:

kdrfc -r mydraft.md

Since RFC 8650, RFCs are using an updated grammar as defined in RFC 7991 to 7998 and further updated informally since, colloquially called "v3". As RFC 2629 is no longer the governing standard, what was called kramdown-rfc2629 is now called kramdown-rfc. The latter command defaults to v3 processing rules; from 2022-02-22T22:02:22 on, the old kramdown-rfc2629 driver program does as well (1.6.1). (-3/--v3 and -2/--v2 select v3 and v2 explicitly; the latter should only be needed if there is a reason to to make a document look like it's 2016.)

See also v3 announcement mail.

For historical interest stupid.mkd was an early markdown version of an actual Internet-Draft (for a protocol called STuPiD [sic!]). This demonstrated some, but not all features of kramdown-rfc. Since markdown/kramdown does not cater for all the structure of an RFC 7991 style document, some of the markup is in XML, and the example switches between XML and markdown using kramdown's {::nomarkdown} and {:/nomarkdown} (this is ugly, but works well enough). stupid.xml and stupid.txt show what kramdown-rfc and xml2rfc make out of this.

stupid-s.mkd is the same document in the new sectionized format supported by kramdown-rfc. The document metadata are in a short piece of YAML at the start, and from there, abstract, middle, references (normative and informative) and back are sections delimited in the markdown file. See the example for how this works. The sections normative and informative can be populated right from the metadata, so there is never a need to write XML any more. Much less scary, and no {:/nomarkdown} etc. is needed any more. Similarly, stupid-s.xml and stupid-s.txt show what kramdown-rfc and xml2rfc make out of this.

draft-ietf-core-block-xx.mkd is a real-world example of a current Internet-Draft done this way. For RFC and Internet-Draft references, it uses document prolog entities instead of caching the references in the XML (i.e., not standalone mode, this is easier to handle when collaborating with XML-only co-authors). See the bibxml metadata.

Please consult the examples for the structure of the YAML header, this should be mostly obvious. The stand_alone attribute controls whether the RFC/I-D references are inserted into the document (yes) or entity-referenced (no), the latter leads to increased build time, but may be more palatable for a final XML conversion. The author entry can be a single hash or a list, as in:

author:
  ins: C. Bormann
  name: Carsten Bormann
  org: Universität Bremen TZI
  abbrev: TZI
  street: Bibliothekstr. 1
  city: Bremen
  code: D-28359
  country: Germany
  phone: +49-421-218-63921
  email: cabo@tzi.org

or

author:
  -
    ins: C. Bormann
    name: Carsten Bormann
    org: Universität Bremen TZI
    email: cabo@tzi.org
  -
    ins: Z. Shelby
    name: Zach Shelby
    org: Sensinode
    role: editor
    street: Kidekuja 2
    city: Vuokatti
    code: 88600
    country: Finland
    phone: "+358407796297"
    email: zach@sensinode.com
  -
    role: editor
    ins: P. Thubert
    name: Pascal Thubert
    org: Cisco Systems
    abbrev: Cisco
    street:
    - Village d'Entreprises Green Side
    - 400, Avenue de Roumanille
    - Batiment T3
    city: Biot - Sophia Antipolis
    code: '06410'
    country: FRANCE
    phone: "+33 4 97 23 26 34"
    email: pthubert@cisco.com

(the hash keys are the XML GIs from RFC 7749, with a flattened structure. As RFC 7749 requires giving both the full name and surname/initials, we use ins as an abbreviation for "initials/surname". Yes, the toolchain is Unicode-capable, even if the final RFC output is still in ASCII.)

Note that the YAML header needs to be syntactically valid YAML. Where there is a potential for triggering some further YAML feature, a string should be put in quotes (like the "+358407796297" above, which might otherwise be interpreted as a number, losing the + sign).

The references section is built from the references listed in the YAML header and from references made inline to RFCs and I-Ds in the markdown text. Since kramdown-rfc cannot know whether a reference is normative or informative, no entry is generated by default in the references section. By indicating a normative reference as in {{!RFC2119}} or an informative one as in {{?RFC1925}}, you can completely automate the referencing, without the need to write anything in the header. Alternatively, you can write something like:

informative:
  RFC1925:
normative:
  RFC2119:

and then just write {{RFC2119}} or {{RFC1925}}. (Yes, there is a colon in the YAML, because this is a hash that could provide other information.)

Since version 1.1, references imported from the BibXML databases can be supplied with a replacement label (anchor name). E.g., RFC 793 could be referenced as {{!TCP=RFC0793}}, further references then just can say {{TCP}}; both will get [TCP] as the label. In the YAML, the same replacement can be expressed as in the first example:

 normative:
   TCP: RFC0793
 informative:
   SST: DOI.10.1145/1282427.1282421

Notes about this feature:

• Thank you, Martin Thomson, for supplying an implementation and insisting this be done.
• While this feature is now available, you are not forced to use it for everything: readers of documents often benefit from not having to look up references, so continuing to use the draft names and RFC numbers as labels may be the preferable style in many cases.
• As a final caveat, renaming anchors does not work in the stand_alone: no mode (except for IANA and DOI), as there is no such mechanism in XML entity referencing; exporting to XML while maintaining live references then may require some manual editing to get rid of the custom anchors.

If your references are not in the BibXML databases and do not have a DOI (that also happens to have correct data) either, you need to spell it out like in the examples below:

informative:
  RFC1925:
  WEI:
    title: "6LoWPAN: the Wireless Embedded Internet"
    # see the quotes above?  Needed because of the embedded colon.
    author:
      -
        ins: Z. Shelby
        name: Zach Shelby
      -
        ins: C. Bormann
        name: Carsten Bormann
    date: 2009
    seriesinfo:
      ISBN: 9780470747995
    ann: This is a really good reference on 6LoWPAN.
  ASN.1:
    title: >
      Information Technology — ASN.1 encoding rules:
      Specification of Basic Encoding Rules (BER), Canonical Encoding
      Rules (CER) and Distinguished Encoding Rules (DER)
    # YAML's ">" syntax used above is a good way to write longer titles
    author:
      org: International Telecommunications Union
    date: 1994
    seriesinfo:
      ITU-T: Recommendation X.690
  REST:
    target: http://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf
    title: Architectural Styles and the Design of Network-based Software Architectures
    author:
      ins: R. Fielding
      name: Roy Thomas Fielding
      org: University of California, Irvine
    date: 2000
    seriesinfo:
      "Ph.D.": "Dissertation, University of California, Irvine"
    format:
      PDF: http://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf
  COAP:
    title: "CoAP: An Application Protocol for Billions of Tiny Internet Nodes"
    seriesinfo:
      DOI: 10.1109/MIC.2012.29
    date: 2012
    author:
      -
        ins: C. Bormann
        name: Carsten Bormann
      -
        ins: A. P. Castellani
        name: Angelo P. Castellani
      -
        ins: Z. Shelby
        name: Zach Shelby
  IPSO:
    title: IP for Smart Objects (IPSO)
    author:
    - org:
    date: false
    seriesinfo:
      Web: http://ipso-alliance.github.io/pub/
normative:
  ECMA262:
    author:
      org: European Computer Manufacturers Association
    title: ECMAScript Language Specification 5.1 Edition
    date: 2011-06
    target: http://www.ecma-international.org/publications/files/ecma-st/ECMA-262.pdf
    seriesinfo:
      ECMA: Standard ECMA-262
  RFC2119:
  RFC6690:

(as in the author list, ins is an abbreviation for "initials/surname"; note that the first title had to be put in double quotes as it contains a colon which is special syntax in YAML.) Then you can simply reference {{ASN.1}} and {{ECMA262}} in the text. (Make sure the reference keys are valid XML names, though.)

Most of the kramdown syntax is supported and does something useful; with the exception of the math syntax (math has no special support in XML2RFC), and HTML syntax of course.

A number of more esoteric features have recently been added. (The minimum required version for each full feature is indicated.)

(1.3.x) Slowly improving support for SVG generating tools for XML2RFCv3 (i.e., with -3 flag). These tools must be installed and callable from the command line.

The basic idea is to mark an input code block with one of the following labels (language types), yielding some plaintext form in the .TXT output and a graphical form in the .HTML output. The plaintext is the input in some cases (e.g., ASCII art, mscgen), or some plaintext output generated by the tool (e.g., plantuml-utxt).

Currently supported labels as of 1.3.9:

• goat, ditaa: ASCII (plaintext) art to figure conversion
• mscgen: Message Sequence Charts
• plantuml: widely used multi-purpose diagram generator
• plantuml-utxt: Like plantuml, except that a plantuml-generated plaintext form is used
• mermaid: Very experimental; the conversion to SVG is prone to generate black-on-black text in this version
• math: display math using tex2svg for HTML/PDF and utftex for plaintext

Note that this feature does not play well with the CI (continuous integration) support in Martin Thomson's I-D Template, as that may not have the tools installed in its docker instance.

More details have been collected on the wiki.

(1.2.9:) The YAML header now allows specifying kramdown_options.

This was added specifically to provide easier access to the kramdown auto_id_prefix feature, which prefixes by some distinguishing string the anchors that are auto-generated for sections, avoiding conflicts:

kramdown_options:
  auto_id_prefix: sec-

(1.2.8:) An experimental feature was added to include BCP 14 boilerplate:

{::boilerplate bcp14}

which saves some typing. Saying "bcp14+" instead of "bcp14" adds some random clarifications at the end of the standard boilerplate text that you may or may not want to have. (Do we need other boilerplate items beyond BCP14?)

(1.0.35:) An experimental command doilit has been added. It can be used to convert DOIs given on the command line into references entries for kramdown-rfc YAML, saving a lot of typing. Note that the DOI database is not of very consistent quality, so you likely have to hand-edit the result before including it into the document (use -v to see raw JSON data from the DOI database, made somewhat readable by converting it into YAML). Use -c to enable caching (requires open-uri-cached gem). Use -h=handle in front of a DOI to set a handle different from the default a, b, etc. Similarly, use -x=handle to generate XML2RFCv2 XML instead of kramdown-rfc YAML.

(1.0.31:) The kramdown smart_quotes feature can be controlled better. By default, it is on (with default kramdown settings), unless coding: us-ascii is in effect (1.3.14: or --v3 is given), in which case it is off by default. It also can be explicitly set on (true) or off (false) in the YAML header, or to a specific value (an array of four kramdown entity names or character numbers). E.g., for a German text (that is not intended to become an Internet-Draft), one might write:

smart_quotes: [sbquo, lsquo, bdquo, ldquo]
pi:
  topblock: no
  private: yes

(1.0.30:) kramdown-rfc now uses kramdown 1.10, which leads to two notable updates:

• Support for empty link texts in the standard markdown reference syntax, as in [](#RFC7744).
• Language names in fenced code blocks now support all characters except whitespace, so you can go wild with asn.1 and C#.

A heuristic generates missing initials/surname from the name entry in author information. This should save a lot of redundant typing. You'll need to continue using the ins entry as well if that heuristic fails (e.g., for Spanish names).

Also, there is some rather experimental support for markdown display math (blocks between $$ pairs) if the tex2mail tool is available.

(1.0.23:) Move up to kramdown 1.6.0. This inherits a number of fixes and one nice feature: Markdown footnote definitions that turn into crefs can have their attributes in the footnote definition:

{:cabo: source="cabo"}

(This section to be removed by the RFC editor.)[^1]

[^1]: here is my editorial comment: warble warble.
{:cabo}

Another questionable paragraph.[^2]

[^2]: so why not delete it?
{: source="observer"}

(1.0.23:) As before, IAL attributes on a codeblock go to the figure element. Language attributes on the code block now become the artwork type, and any attribute with a name that starts "artwork-" is moved over to the artwork. So this snippet now does the obvious things:

~~~ abnf
a = b / %s"foo" / %x0D.0A
~~~
{: artwork-align="center" artwork-name="syntax"}

(1.0.22:) Index entries can be created with (((item))) or (((item, subitem))); use quotes for weird entries: (((",", comma))). If the index entry is to be marked "primary", prefix an (unquoted) ! as in (((!item))).

In addition, auto-indexing is supported by hijacking the kramdown "abbrev" syntax:

*[IANA]:
*[MUST]: BCP14
*[CBOR]: (((Object Representation, Concise Binary))) (((CBOR)))

The word in square brackets (which must match exactly, case-sensitively) is entered into the index automatically for each place where it occurs. If no title is given, just the word is entered (first example). If one is given, that becomes the main item (the auto-indexed word becomes the subitem, second example). If full control is desired (e.g., for multiple entries per occurrence), just write down the full index entries instead (third example).

(1.0.20:) As an alternative referencing syntax for references with text, {{ref}} can be expressed as [text](#ref). As a special case, a simple [ref] is interpreted as [](#ref) (except that the latter syntax is not actually allowed by kramdown). This syntax does not allow for automatic entry of items as normative/informative.

(1.0.16:) Markdown footnotes are converted into crefs (XML2RFC formal comments; note that these are only visible if the pi "comments" is set to yes). The anchor is taken from the markdown footnote name. The source, if needed, can be supplied by an IAL, as in (first example also uses an ALD):

{:cabo: source="cabo"}

(This section to be removed by the RFC editor.)[^1]{:cabo}

[^1]: here is my editorial comment

Another questionable paragraph.[^2]{: source="observer"}

[^2]: so why not delete it

Note that XML2RFC v2 doesn't allow structure in crefs. If you put any, you get the escaped verbatim XML...

(1.0.11:) Allow overriding "style" attribute (via IAL = inline attribute list) in lists and spans as in:

{:req: counter="bar" style="format R(%d)"}

{: req}
* Foo
* Bar
* Bax

Text outside the list, so a new IAL is needed.

* Foof
* Barf
* Barx
{: req}

(1.0.5:) An IAL attribute "cols" can be added to tables to override the column layout. For example, cols="* 20 30c r" sets the width attributes to 20 and 30 for the middle columns and sets the right two columns to center and right alignment, respectively. The alignment from cols overrides that from the kramdown table, if present.

(1.0.2:) An IAL attribute "vspace" can be added to a definition list to break after the definition term:

{: vspace="0"}
word:
: definition

anotherword:
: another definition

(0.x:) Files can be included with the syntax {::include fn} (needs to be in column 1 since 1.0.22; can be suppressed for use in servers by setting environment variable KRAMDOWN_SAFE since 1.0.22). A typical example from a recent RFC, where the contents of a figure was machine-generated:

~~~~~~~~~~
{::include ../ghc/packets-new/p4.out}
~~~~~~~~~~
{: #example2 title="A longer RPL example"}

(0.x:) A page break can be forced by adding a horizontal rule (----, note that this creates ugly blank space in some HTML converters).

The code is not very polished, but now quite stable; it has been successfully used for hundreds of non-trivial Internet-Drafts and RFCs. You probably still need to skim v3 if you want to write an Internet-Draft, but you don't really need to understand XML very much. Knowing the basics of YAML helps with the metadata (but you'll understand it from the examples).

Occasionally, you do need to reach through to the XML arcana, e.g. by setting attribute values using kramdown's "IAL" syntax. This can for instance be used to obtain unnumbered appendices:

Acknowledgements
================
{: numbered="false"}

John Mattsson was nice enough to point out the need for this being documented.

Note that this specific example is covered by a predefined kramdown-rfc "attribute list definition" (ALD):

{:unnumbered: numbered="false"}

so the conventional way to write this example would be the somewhat simpler:

Acknowledgements
================
{:unnumbered}

John Mattsson was nice enough to point out the need for this being documented.

If you have an old RFC and want to convert it to markdown, try just using that RFC, it is 80 % there. It may be possible to automate the remaining 20 % some more, but that hasn't been done.

If you have XML, there is an experimental upconverter that does 99 % of the work. Please contact the author if you want to try it.

Actually, if the XML was generated by kramdown-rfc, you can simply extract the input markdown from that XML file (but will of course lose any edits that have been made to the XML file after generation):

kramdown-rfc-extract-markdown myfile.xml >myfile.md

Joe Hildebrand has a grunt plugin for kramdown-rfc at: https://github.com/hildjj/grunt-kramdown-rfc2629 . Get started with it at: https://github.com/hildjj/grunt-init-rfc . This provides a self-refreshing web page with the kramdown-rfc/xml2rfc rendition of the draft you are editing.

Martin Thomson has an I-D Template for github repositories that enable collaboration on draft development. This supports kramdown-rfc out of the box. Just name your draft like draft-ietf-unicorn-protocol.md and follow the installation instructions.

Moving from XML to Markdown for RFC writing apparently is a no-brainer, so I'm not the only one who has written code for this.

Miek Gieben has done a similar thing employing pandoc, now documented in RFC 7328. He uses multiple input files instead of kramdown-rfc's sectionized input format. He keeps the metadata in a separate XML file, similar to the way the previous version of kramdown-rfc stored (and still can store) the metadata in XML in the markdown document. He also uses a slightly different referencing syntax, which is closer to what markdown does elsewhere but more verbose (this syntax is now also supported in kramdown-rfc). (Miek now also has a new thing going on with mostly different syntax, see mmark and its github repository.)

Other human-oriented markup input languages that are being used for authoring RFCXML include:

• asciidoc, with the asciidoctor-rfc tool, as documented in draft-ribose-asciirfc.
• orgmode (please help supply a more specific link here).

Since kramdown version 1.0, kramdown itself is MIT licensed, which made it possible to license kramdown-rfc under the same license.