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From: kre@munnari.oz.au (Robert Elz)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: minimum maximum transfer unit.
Message-ID: <34860@boulder.Colorado.EDU>
Date: 8 May 91 10:05:21 GMT
Sender: daemon@boulder.Colorado.EDU
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    Date:        Tue, 7 May 91 17:47:30 CDT
    From:        dana@thinman.cray.com (Dana Dawson)
    Message-ID:  <9105072247.AA00263@thinman.cray.com>

    and 576 for non-local destinations because that is the
    smallest "required" MTU

This is simply not true, and is a myth that deserves to
be obliterated.

576 is the size of an IP datagram that all hosts must be
able to receive, ie: its one that you know that you can
send to any other host without it being rejected because of
being too big.

The smallest required MTU is 80 octets (ie: you can fragment
anything, as long as fragments are at least 80, except
possibly the last - this allows space for maximal headers,
and at least some data, in each fragment).

Choosing 576 as the size to send (its often 512 data,
plus headers, in reality, ie: typically 556 for tcp)
is simply a matter of observation, few nets have an MTU
smaller than 576, so fragmentation isn't likely to be needed.
But there is no guarantee.

kre