Re: [ietf-822] "--" first in lines to indicate start of signature

[Russ Allbery <[e-mail]>]

Jacob Palme <[e-mail]> writes:

> A common convention is that e-mail messages contain a short description
> of the author, called "signature" and the start of this is indicated by
> a line only containing the two characters "--".

The full, "correct" convention is actually the three characters "-- " with
the trailing space.  Obviously, this causes all sorts of problems in
practice, and a lot of software is tolerant of the space being removed.
But that was originally the standard to try to avoid issues with the other
uses of -- on a line by itself.

> Is this convention recommended or discused in some IETF standard? I
> cannot find it in rfc5322.

I believe, although am not certain, that this convention originated in the
netnews world.  It is documented in RFC 1849 (aka "Son of 1036"), a highly
influential draft that was never formally published as an RFC until the
USEFOR work, during which it was published directly as historic.

   Early difficulties in inferring return addresses from article headers
   led to "signatures": short closing texts, automatically added to the
   end of articles by posting agents, identifying the poster and giving
   his network addresses, etc.  If a poster or posting agent does append
   a signature to an article, the signature SHOULD be preceded with a
   delimiter line containing (only) two hyphens (ASCII 45) followed by
   one blank (ASCII 32).  Posting agents SHOULD limit the length of
   signatures, since verbose excess bordering on abuse is common if no
   restraint is imposed; 4 lines is a common limit.

      NOTE: While signatures are arguably a blemish, they are a well-
      understood convention, and conveying the same information in
      headers exposes it to mangling and makes it rather less
      conspicuous.  A standard delimiter line makes it possible for
      reading agents to handle signatures specially if desired.
      (This is unfortunately hampered by extensive misunderstanding of,
      and misuse of, the delimiter.)

      NOTE: The choice of delimiter is somewhat unfortunate, since it
      relies on preservation of trailing white space, but it is too
      well-established to change.  There is work underway to define a
      more sophisticated signature scheme as part of MIME, and this will
      presumably supersede the current convention in due time.

      NOTE: Four 75-column lines of signature text is 300 characters,
      which is ample to convey name and mail-address information in all
      but the most bizarre situations.

So far as I know, it was never documented as part of a standards-track
RFC.  It probably would have been mentioned in the USEAGE document that
USEFOR considered taking up (draft-ietf-usefor-useage-01), but the working
group ran out of steam before that work got very far.

-- 
Russ Allbery ([e-mail])              <http://www.eyrie.org/~eagle/>