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From: mogul@wrl.dec.com (Jeffrey Mogul)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: TCP segment size -- user defined?
Message-ID: <1990Oct5.012920.4846@wrl.dec.com>
Date: 5 Oct 90 01:29:20 GMT
References: <BILLW@MATHOM.CISCO.COM> <ID9478.D900921.T090901.DEDOUREK@UNB.CA>
Sender: news@wrl.dec.com (News)
Organization: DEC Western Research
Lines: 17

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