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From: chris@GYRE.UMD.EDU.UUCP
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  TCP maximum segment size determination
Message-ID: <8711120215.AA02549@gyre.umd.edu>
Date: Wed, 11-Nov-87 21:15:25 EST
Article-I.D.: gyre.8711120215.AA02549
Posted: Wed Nov 11 21:15:25 1987
Date-Received: Thu, 19-Nov-87 06:32:47 EST
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There is a good standard argument against setting the MSS via an IP
option, and that is that the route the SYN packet takes is not
necessarily the same as the route that other packets will take.  (In
practise, I think we see a fair number of routes that,
diagrammatically, look like this:

			net1
	 /------>f1------------>f2-----\
	|				v
	X				Y
	^				|
	 \-------g1<------------g2<----/
			net2

where the return path is consistently different from the originating
path.  And of course, since the Internet does not rely on virtual
circuits, it can reroute dynamically, invalidating MSSes on the fly.)

4.3BSD sets the MSS to 576 (which becomes 536 data bytes) when
crossing a gateway.  This is not necessarily ideal but is the
official recommended practise.

Chris