RFC format

The RFC format is a renderer-agnostic structured document format built using ASCII C0 control codes as a superset of the TTY text format. It uses other C0 control codes not employed by printers (and thereby not known to TTY) to create rich document structure otherwise attained with advanced systems such as Troff and LaTeX. The format does this instead of providing a meta syntax like Markdown, RTF or HTML so that it can satisfy its design constraint of being legible as TTY formatted ASCII text if all superset control codes are blindly (as in, without any parsing knowledge required) stripped out.

The recommended file extension for these documents is .rfc. The magic number all RFC files should begin with is, in hexadecimal, 06 15 06 15, representing two interleaved pairs of ASCII ACK and NAK C0 control codes. The RFC format reserves all C0 control codes not used by the TTY format except for NUL (0x00), BEL (0x07), SUB (0x1A) and ESC (0x1B).

The RFC format is called as such because it is designed to mimic the production of the IETF's RFC XML. It is not technically related to any of the IETF's tools for producing or validating RFCs.

Embed encoding

Sixteen extraneous ASCII C0 control codes are hijacked as a binary encoding medium so that each 7-bit ASCII character provides 4 bits of arbitrary binary data:

Character name	Code	Enc.	Dec.
End of Text	ETX	0x03	0x0
End of Transmission	EOT	0x04	0x1
Enquiry	ENQ	0x05	0x2
Data Link Escape	DLE	0x10	0x3
Device Control 1	DC1	0x11	0x4
Device Control 2	DC2	0x12	0x5
Device Control 3	DC3	0x13	0x6
Device Control 4	DC4	0x14	0x7
Synchronous Idle	SYN	0x16	0x8
End of Transmission Block	ETB	0x17	0x9
Cancel	CAN	0x18	0xA
End of Medium	EM	0x19	0xB
File Separator	FS	0x1C	0xC
Group Separator	GS	0x1D	0xD
Record Separator	RS	0x1E	0xE
Unit Separator	US	0x1F	0xF

This 4-bit medium is then employed to harbour an MSB sentinel variable-length integer encoding format: each 7-bit ASCII character contains 3 bits of meaningful information, while the high fourth bit is used to indicate whether the control character immediately following the current one should be collated with it as a single number. As with most variable-width binary encodings, it is invalid and undefined when control sequences are not properly encoded; they should always have zero or more of the above sixteen control characters with the high bit set in direct sequence followed by one and only one such control characters where the high bit is LOW.

Rich format

The rich format is paragraph-oriented and does not recognise any semantic distinction between whitespace characters; Line Feed 0x0A, Carriage Return 0x0D and Space 0x20 are all equivalent to one another. Parsing will collapse multitudes of such spaces into one inside paragraphs and conjoin source lines spanning many physical lines into one logical line before rendering them into monotype again with 72 characters/line.

Header for metadata

If an RFC format parser encounters an ASCII Start of Heading SOH before seeing any visible characters (including spacing) in the stream, it will parse it as a heading metadata block. Fields are simple ASCII text separated by CRLF pairs, making them dumb-printable:

1. Full document title
2. Author name
3. Author address
4. Author telephone
5. Author e-mail
6. Copyright date
7. Copyright assignment
8. Licence

Omitting any of the fields is done by leaving them empty. They are parsed in order, and therefore there should always be exactly seven CRLF pairs in a heading metadata block. This block is then terminated by an ASCII Start of Text STX character, after which the normal document text and whatever command formatting it bears appears immediately until EOF. As a whole, this header is optional.

General formatting

The number encoded in this way is, once decoded and in memory, interpreted as a formatting command. Parsing at this point should be forgiving; for example, excessive closing commands should be ignored. Here is a table of commands with their encodings in octal:

Command description	Number
Begin heading, level 1	001
Begin heading, level 2	002
Begin heading, level 3	003
Begin heading, level 4	004
Begin heading, level 5	005
Begin heading, level 6	006
End heading (relative)	007
Begin code block	010
End code block	011
Begin block quote	012
End block quote	013
Begin block quote credit	014
End block quote credit	015
Begin hard shadow	016
End hard shadow	017
Begin hyperlink display text	020
End hyperlink display text	021
Begin hyperlink shadow bridge	022
End hyperlink shadow bridge	023
Begin hyperlink URI	024
End hyperlink URI	025
Begin centred text	026
Begin right-aligned text	027
Begin justified text	030
Revert to left-aligned text	031
Render table of contents	032
Begin internal link target ident	033
End internal link target ident	034
Begin internal link display text	035
End internal link display text	036
Begin internal link shadow bridge	037
End internal link shadow bridge	040
Begin internal link reference ident	041
End internal link reference ident	042
Paragraph break	043
Line break	044
Begin all capitals	045
End all capitals	046
Whole paragraph indent	047

Shadows

The RFC format employs a simple rendering hatchet called shadows. The basic action of a shadow is, from its beginning mark to its end, it hides the text encased within from rich renderings of the document; this is called a hard shadow. A softer variant of this exists which are called shadow bridges – these are used to interlink a hyperlink or internal link's display text to its target destination, providing a semantic connection and a hiding of punctuation that would be needed in plain text for legibility, such as spacing and parentheses. Shadows provide a concise and simple way to hide plain text boilerplate from rich renderings of RFC documents while still providing them for dumb formatting strippers to create plain text renditions as the authors intended them without having to do any high-level reconstructions.

Rendering

Regardless of whether the target medium is print or digital, the RFC format is still meant to be a monospaced, paper-friendly medium that never exceeds 72 columns in width. Since RFC is page-aware, unlike TTY, it sets a maximum page height of 55 rows – this leaves 2 header rows with 1 spacer row and 1 footer row with 1 spacer row for 50 rows of content per page.