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From: geof@apolling.UUCP (Geof Cooper)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP Datagram sizes
Message-ID: <8705240517.AA01407@apolling.imagen.uucp>
Date: Sun, 24-May-87 01:17:05 EDT
Article-I.D.: apolling.8705240517.AA01407
Posted: Sun May 24 01:17:05 1987
Date-Received: Sun, 24-May-87 08:47:19 EDT
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I like the idea of an IP-level solution to the fragmentation problem
since it has application to UDP protocols (I know that none that exist
today could use it, but that's no excuse for ignoring UDP).

Isn't there a destination unreachable message with the reason being
"can't fragment and had to" (sorry my ICMP spec is at the office)?  If
not, we could certainly add one.

In that case, the idea is to always send TCP packets with the "don't
fragment" bit set.  Use the scheme suggested that keeps track of MTU's
in the routing cache.  Update the cache based on DU's received
(decrease the MTU a bit and try again) -- time out the entry on a long
timer to be able to detect new routes.

The obvious improvement is to have the ICMP message also include the
MTU restriction that is appropriate -- that requires changing ICMP, of
course, but it would probably be a good idea.