Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!VENERA.ISI.EDU!postel
From: postel@VENERA.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: TCP segment size -- user defined?
Message-ID: <9010051551.AA06327@bel.isi.edu>
Date: 5 Oct 90 15:51:58 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 31

Hi.

For further discussion of the TCP MSS see RFC 879.

--jon.

	Date: 5 Oct 90 01:29:20 GMT
	From: bacchus.pa.dec.com!mogul@decwrl.dec.com  (Jeffrey Mogul)
	Subject: Re: TCP segment size -- user defined?
	Sender: tcp-ip-relay@nic.ddn.mil
	To: tcp-ip@nic.ddn.mil

	In article <ID9478.D900921.T090901.DEDOUREK@UNB.CA> DEDOUREK@UNB.CA
	writes:

	>To avoid fragmentation, can TCP MSS be equal to path MTU, or must
	>it be less by some number of octets to allow for TCP and IP headers?
	>If so, what is a good value?

	The Path MTU Discovery document (now an Internet Draft, soon to be
	an RFC if all goes well) says:

		  Note: The TCP MSS is defined to be the relevant IP datagram
		  size minus 40 [see RFC879].  The default of 576 octets for
		  the maximum IP datagram size yields a default of 536 octets
		  for the TCP MSS.

	In other words, the TCP MSS should be at least 40 octets less than the
	path MTU.

	-Jeff