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From: braden@ISI.EDU (Bob Braden)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP Datagram sizes
Message-ID: <8705261645.AA00187@braden.isi.edu>
Date: Tue, 26-May-87 12:45:12 EDT
Article-I.D.: braden.8705261645.AA00187
Posted: Tue May 26 12:45:12 1987
Date-Received: Thu, 28-May-87 02:22:44 EDT
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HWB:

Your comments are generally right on the mark, especially about the need
to dramatically increase packet sizes for high-speed nets in the
future.  However, I think that in the short term one cannot
always ignore the performance difference between 576 and 1006 byte MTU's
over typical WAN's.  [By the way, this question never occurred to me
before... what is the MTU of the NSFnet backbone? How about the regional
networks?] 

You suggest sticking to 576 byte packets. A better strategy may be to
adopt a larger MTU (say, 1500) and let fragmentation fall where it will.
Suppose you use 1500 across a path which has the ARPANET in the middle...
then each FTP/SMTP packet will be split into 1000 and 500 byte pieces,
for an average of 750 bytes per packet.  That is 75% efficiency, good
enough is many cases.  If a particular host has a high percentage of its
traffic across a WAN with a 1006-byte MTU, the host administrator can
adjust the effective MTU parameter of the interface down to 1006 to get
that last 25%.  A host needs intrumentation on its IP layer to detect and
report a situation of particularly bad statistics. Also, someone should
remind us which will beat up ARPANET/MILNET more... 576 packets, or
(1000,500) byte pairs. 

   Why fuss about fragmentation into small packets, in a community
   that practices single-character echoplex terminal interactions??

So, how do we go towards 20K packets?  What LAN technology will we need
to get there?  What will this imply for host interfaces?  How can we take
care of hosts that have not been converted to big buffers?  It seems that
when some parts of the Internet take very big packets while other parts
still take miserable little ones, it will be absolutely necessary for a
host to be able to learn the properties of a path it is using.  Yes,
Vint, paths do change dynamically, but as a practical matter they don't
change that fast, and we are probably willing to take a performance hit
if the new path makes our MTU choice suboptimal. 

Bob Braden