A definition of what constitutes a header is probably
necessary in order for a header registry to be
optimally useful. (note that I did not say "message
header" as RFC 2046 distinguishes messages from MIME
entities, and the latter also have headers) It
should be clear what is to be registered as well as
what does not qualify for registration. For the
moment, let's consider headers in text messages and
ignore HTTP "headers".
Certainly RFC 2822 is clear with respect to message
headers, but it does not explicitly cover MIME
entities. The MIME RFCs were reasonably clear when
originally written, but some of the extension RFCs
have muddied the waters a bit.
Some of the murky areas:
1. RFC 1894 defines the message/delivery-status media
type whose ''body [...] consists of one or more "fields"
formatted according to the ABNF of RFC 822 header
"fields"''. So, for the purpose of the registry,
are the per-message fields defined in 1894 considered
header fields or not? If so, there's the added
complication of the per-recipient fields, which
begin following a blank line after the per-message
fields. Normally a blank line separates headers
and body; are the per-recipient fields then body text
or should they also be considered header fields? If
headers, then the message/delivery-status media type
has no body as such; only headers separated into
groups by blank lines.
RFC 2298's message/disposition-notification is
similar.
If the DSN and MDN fields are to be considered header
fields, a definition based on the location of the
fields within a message is likely to be troublesome
or convoluted.
2. RFC 2425 defines a media type text/directory which
also uses structured fields. However, the structure
differs somewhat from RFC 822/2822 and one would not
normally expect to find headers in a discrete text media
type -- were it not for RFC 1892's text/rfc822-headers
media type.
Crafting a suitable definition that takes these cases
into consideration is also challenging.
3. RFC 2156 defines a number of header fields for X.400
gateway use and cautions against general use. Nevertheless,
RFC 2421 uses Importance and Sensitivity. Supersedes is
in widespread use in usenet news article headers and
appears in the usefor draft, but with a subset of the 2156
syntax.
Most of the 2156 header fields, and the 1894 and 2298
fields (and 822's "Encrypted") are not listed in Jacob Palme's
draft-palme-mailext-headers-06.txt document. [incidentally,
reference 41 points to an older version]