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From: JNC@XX.LCS.MIT.EDU ("J. Noel Chiappa")
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP Datagram sizes
Message-ID: <12304502322.33.JNC@XX.LCS.MIT.EDU>
Date: Fri, 22-May-87 18:49:58 EDT
Article-I.D.: XX.12304502322.33.JNC
Posted: Fri May 22 18:49:58 1987
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	This or a close variant of it sounds like a good idea to me.
It's been clear for a time that the TCP MaxSegSize negotiation only
gives you part of what you want.

	I'd suggest two minor changes. First, the message gives 'the
maximum datagram sise for sending to the destination *host + TOS*'.
(It has to work if the destination net is subnetted, we don't need to
messages, blah, blah, blah standard JNC flame.)
	It's also not clear whether you'd make it an ICMP message that
was returned every time a message was fragmented. (In any case, you
can simulate that using the existing Don't Fragment flag.) Such a
message makes using fragmentation for real almost impossible; the
extra network load every time a packet was fragmented would be
significant (like hosts that ignore Redirects). I think you'd want a
special mechanism which the user has to invoke, sort of like record
route, where it goes along the path; it is initialized to the MTU of
the outbound link from the host, and each node in the path resets the
value to the min of that and of the MTU on the next hop link. I don't
think you want a a special ICMP type, since then all switches would
have to examine all packets going though to see if they were an ICMP
packet of that type; extra overhead. I think the right thing is an IP
option, 'record minimum MTU'.
	In general, I think this is a good idea though.

	Noel
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