Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!sdd.hp.com!elroy.jpl.nasa.gov!ncar!csn!ub!uhura.cc.rochester.edu!rochester!pt.cs.cmu.edu!g.gp.cs.cmu.edu!tgl
From: tgl@g.gp.cs.cmu.edu (Tom Lane)
Newsgroups: comp.compression
Subject: Re: COMPRESSING of binary data into mailable ASCII
Message-ID: <12496@pt.cs.cmu.edu>
Date: 27 Mar 91 15:41:59 GMT
References: <D88-JWA.91Mar24223223@byse.nada.kth.se> <12485@pt.cs.cmu.edu> <1991Mar26.233839.24835@nntp-server.caltech.edu>
Organization: Carnegie-Mellon University, CS/RI
Lines: 40

I said:
> One thing that might be good to avoid is putting '.' at the start
> of a line, which btoa will do quite happily.
and Mark Adler said:
> Why?

Sorry to be obscure.  NNTP, the standard protocol for shipping netnews
messages across Internet, uses '.' alone on a line as end-of-message
marker.  To ensure that a data line like this can't be mistaken as
end-of-message, any '.' appearing at the start of a data line is doubled
(sent as '..') and then undoubled at the receiver.  Thus, '.' at the
start of a line is sent correctly, but you pay extra to send it across
the network link.

I believe NNTP is derived from the network mail transfer protocol, so
the same effect probably holds for ordinary mail messages.

Thus, it would be marginally more efficient if we could ensure '.'
didn't appear at the start of a line.  The only simple way I can see to
do this is not to use '.' at all in the encoding character set.  If we
can get 85 safe characters without '.', then it'd be a win.

> Are the any other things to avoid that people out
> there knows messes up mailers?  I already know about an indented first
> line getting chewed up sometimes.

I think it's pretty clear that we don't want to use space as one of the
encoding characters: it's likely to get lost if it is the first or last
character in a line, some overenthusiastic programs may translate runs
of spaces into tabs, etc etc.

The '.' business and EBCDIC compatibility are the only other constraints
I know about.

Mark, would you mind posting the character set you use for 'ship'?
I wasn't paying attention at the time...

-- 
			tom lane
Internet: tgl@cs.cmu.edu	BITNET: tgl%cs.cmu.edu@cmuccvma