Message-ID: <YkvPceK00WBwI3CIoI@andrew.cmu.edu>
Date: Fri,  5 Jan 1996 18:54:18 -0500 (EST)
From: John Gardiner Myers <jgm+@CMU.EDU>
To: drums@cs.utk.edu
Subject: "be liberal in what you accept" considered harmful

"Be liberal in what you accept" can actually be a quite harmful
practice.  It's use papers over problems which otherwise would be
noticed and fixed.  In fact, I consider it one of the primary causes
of the mess that is Internet mail today.  All sorts of crap floats
around in mail because the deployed software allows it.  In fact, you
can't implement a mail system by implementing the specs--you end up
having to more or less behave like sendmail does because that's what
all the other software expects.  

For example, in the case of an email message with usable
forward-paths, but a patently unusable return-path, being liberal has
the effect of making things appear to work, even though something
upstream is broken or misconfigured.  Other mail sent through the same
route with incorrect forward-paths will be lost instead of returned to
the sender.

If, on the other hand, you reject the SMTP session, the maintainer of
the misconfigured system is far more likely to be made aware of the
problem and is far more motivated to fix it.

A prime example of the success of the rejecting the "be liberal"
philosophy is in netnews.  It used to be that every few months, some
backwater site that had been down for a few months would come back up
and try to transmit months-old articles to its neighbors.  Sometimes,
these articles would have missing or unparseable Date: headers.  The
friendly neighbors would be nice and "liberal" and either fix up those
headers with the current date or just pass them on.  The result was
that the old articles would be distributed and everyone reading
netnews would have to wade through them.

Finally, Henry Spencer released a version of his news transport
software that was extremely strict.  If the Date: header did not parse
according to the specification, it dropped the article on the floor.
For a few months after this was released, there was much howling from
people with software that generated invalid Date: headers.  But since
there was motivation, the software got fixed.  These days, nobody
complains about the pickiness of the Date: header parser.  And these
days, netnews doesn't suffer from periodic episodes of regurgitated
news.

"Being liberal" buys you short-term interoperability at the cost of
long-term interoperability and "once in a blue moon" problems.

-- 
_.John G. Myers		Internet: jgm+@CMU.EDU
			LoseNet:  ...!seismo!ihnp4!wiscvm.wisc.edu!give!up