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.
Data Link Escape	DLE	0x10	0x0
Device Control 1	DC1	0x11	0x1
Device Control 2	DC2	0x12	0x2
Device Control 3	DC3	0x13	0x3
Device Control 4	DC4	0x14	0x4
Enquiry	ENQ	0x05	0x5
Synchronous Idle	SYN	0x16	0x6
End of Transmission Block	ETB	0x17	0x7
Cancel	CAN	0x18	0x8
End of Medium	EM	0x19	0x9
End of Text	ETX	0x03	0xA
End of Transmission	EOT	0x04	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
No-op	0000
Begin heading, level 1	0001
Begin heading, level 2	0002
Begin heading, level 3	0003
Begin heading, level 4	0004
Begin heading, level 5	0005
Begin heading, level 6	0006
End heading (relative)	0007
Begin code block	0010
End code block	0011
Begin block quote	0012
End block quote	0013
Begin block quote credit	0014
End block quote credit	0015
Begin hard shadow	0016
End hard shadow	0017
Begin hyperlink display text	0020
End hyperlink display text	0021
Begin hyperlink shadow bridge	0022
End hyperlink shadow bridge	0023
Begin hyperlink URI	0024
End hyperlink URI	0025
Begin centred text	0026
Begin right-aligned text	0027
Begin justified text	0030
Revert to left-aligned text	0031
Render table of contents	0032
Begin internal link target ident	0033
End internal link target ident	0034
Begin internal link display text	0035
End internal link display text	0036
Begin internal link shadow bridge	0037
End internal link shadow bridge	0040
Begin internal link reference ident	0041
End internal link reference ident	0042
Paragraph break	0043
Line break	0044
Begin all capitals	0045
End all capitals	0046
Whole paragraph indent	0047
Page break to even	0050
Page break to odd	0051
0052–0077 are undefined
Figure 5 lines, ¼ width	0100
Figure 10 lines, ¼ width	0101
Figure 15 lines, ¼ width	0102
Figure 20 lines, ¼ width	0103
Figure 25 lines, ¼ width	0104
Figure 30 lines, ¼ width	0105
Figure 40 lines, ¼ width	0106
Figure 50 lines, ¼ width	0107
Figure 5 lines, ½ width	0110
Figure 10 lines, ½ width	0111
Figure 15 lines, ½ width	0112
Figure 20 lines, ½ width	0113
Figure 25 lines, ½ width	0114
Figure 30 lines, ½ width	0115
Figure 40 lines, ½ width	0116
Figure 50 lines, ½ width	0117
Figure 5 lines, ¾ width	0120
Figure 10 lines, ¾ width	0121
Figure 15 lines, ¾ width	0122
Figure 20 lines, ¾ width	0123
Figure 25 lines, ¾ width	0124
Figure 30 lines, ¾ width	0125
Figure 40 lines, ¾ width	0126
Figure 50 lines, ¾ width	0127
Figure 5 lines, full width	0130
Figure 10 lines, full width	0131
Figure 15 lines, full width	0132
Figure 20 lines, full width	0133
Figure 25 lines, full width	0134
Figure 30 lines, full width	0135
Figure 40 lines, full width	0136
Figure 50 lines, full width	0137

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.

Figures

The RFC format does not provide any direct means for embedding figure data (beyond the aforementioned ASCII tables which are not true 'figures' anyway). However, it does provide a comprehensive command set for accommodating figures into the rigid and portable geometry of RFC documents. True figures could then be emplaced into this 'saved space' in the course of the rendering process, or if such rendering is not practical, the space could be left empty in a visually acceptable way. Commands 0100-0137 are provisioned for this purpose and provide a simple matrix of figure sizes: line height may be 5, 10, 15, 20, 25, 30, 40 or 50 high and the figure may be ¼ width (17 columns), ½ width (35 columns), ¾ width (51 columns) or full width (68 columns). Figures are always centred, and in the case of ¼ and ¾ width figures will have their remainder column on the right, and their top is the line on which the figure command first appears. Rendering of the figure involves an implicit carriage return before centring and as usual must respect the overwriting behaviour of the TTY format, making it possible to overlay text on top of figures directly in the document. Document authors should navigate as necessary around their inserted figure according to its prescribed size.

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.