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Path: utzoo!watmath!clyde!cbatt!ucbvax!THINK.COM!bruce
From: bruce@THINK.COM.UUCP
Newsgroups: mod.protocols.tcp-ip
Subject: PSN MTU problem?
Message-ID: <8702211216.AA02949@erato.Think.COM>
Date: Sat, 21-Feb-87 07:16:50 EST
Article-I.D.: erato.8702211216.AA02949
Posted: Sat Feb 21 07:16:50 1987
Date-Received: Sat, 21-Feb-87 19:37:43 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 19
Approved: tcp-ip@sri-nic.arpa

Oh, I forgot to mention another problem which I first noticed last weekend.

We are unable to send segments to SRI-NIC.ARPA of size MTU.  This time
NIC is the only host with which I have observed the problem.  If I kludge
our tcp to negotiate a slightly smaller MSS, everything works fine.

On this end, we appear to be getting RFMNs from the long packet, but
NIC evidently isn't seeing it.  The connection stays alive: we
retransmit the long packet, and they retransmit the previous ack.

This is not the standard 4.2/4.3 driver bug of not allocating buffers
at least one greater than MTU.  That was fixed long ago.  But the
symptoms are exactly the same.  This is a NEW problem, and we haven't
changed any tcp/ip code here recently.

Evidently we can RECEIVE mtu-sized packets from the NIC.

--Bruce Nemnich, Thinking Machines Corporation, Cambridge, MA
  bruce@think.com, seismo!think!bruce, bjn@mitvma.bitnet; +1 617 876 1111