Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!mcsun!hp4nl!tnofel!assg7
From: assg7@fel.tno.nl (Rene van den Assem)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: minimum maximum transfer unit.
Message-ID: <1991May13.090902.12327@fel.tno.nl>
Date: 13 May 91 09:09:02 GMT
References: <34860@boulder.Colorado.EDU>
Organization: TNO Physics and Electronics Laboratory
Lines: 31

kre@munnari.oz.au (Robert Elz) writes:

>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.

Yes, you're right. I've run into problems on this with a popular
PC TCP/IP implementation that claims to support fragmentation.
It actually allows only for two fragments and assumes that by setting
the TCP MSS to 536(?), it will avoid fragmentation. Alas, this doesn't
work for me.

Rene van den Assem,
TNO Physics and Electonics Lab
The Netherlands
assem@fel.tno.nl