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From: tcp-ip@ucbvax.ARPA
Newsgroups: fa.tcp-ip
Subject: Re: Fragmentation
Message-ID: <7816@ucbvax.ARPA>
Date: Wed, 5-Jun-85 03:46:22 EDT
Article-I.D.: ucbvax.7816
Posted: Wed Jun  5 03:46:22 1985
Date-Received: Thu, 6-Jun-85 08:07:20 EDT
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Organization: University of California at Berkeley
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From: Dennis Rockwell <drockwel@CSNET-SH.ARPA>

A solution that I used in the later BBN 4.1 TCP/IP implementation was for
the IP layer to pass to TCP the size of the largest fragment which went to
make up the current IP packet.  TCP then unilaterally restricted its segment
size to twice the fragment size (less an appropriate fudge factor).  This
produces no tinygrams, although interfaces that can't handle back-to-back
packets are still in trouble.

Yes, it's a hack that violates level separation rules, but it made a big
difference in practice.

A useful facility that the SATNET folks have been good enough to provide are
the IP echo hosts on the far side of satellite connections (goonhilly-echo
for example).  These switch the IP source and destination addresses and ship
the packet out again.  Since SATNET has the smallest MTU, the longest delay,
and a penchant for dropping packets (this is not SATNET's fault; the speed
mismatch between the ARPAnet and SATNET is ferocious and the gateway clogs),
it's handy for testing TCP/IP implementations.

It has been known to crash insufficiently bulletproofed software.

Dennis Rockwell
CSNET Technical Staff