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Subject: Re: path MTU discovery issue
From: Liviu Daia (Liviu.Daiaimar.ro)
Date: Wed Sep 20 2000 - 18:54:54 CDT

On 20 September 2000, Bennett Todd <betrahul.net> wrote:
> 2000-09-20-18:03:26 Liviu Daia:
[...]
> > Anyway, unless you really understand what you're doing, I'd strongly
> > recommend leaving the MTU alone. To get an idea about how bad things
> > might get, try setting the MTU to 1499 on a network segment and to
> > 1500 on another, and fire up tcpdump.
>
> My understanding (I may be wrong here) is that the 3-way handshake of
> TCP connection setup negotiates an MTU that's the minimum of the two
> endpoint MTUs, without Path MTU Discovery. If that understanding is
> correct, then reducing the MTU far enough to stop fragmentation is
> fine as long as you're doing it on an endpoint of connections, not on
> an intermediate router.
>
> Do I have that wrong?

    It's actually more involved, see Richard Stevens' excellent book
"TCP Illustrated" for a full explanation of what's going on. Basically,
fragmentation can occur at several layers, and you want the sizes of the
various fragments to be multiples to each other --- otherwise the number
of fragments involved might even double. Among other things, this tends
to hit slow links quite hard.

    Regards,

    Liviu Daia 

-- 
Dr. Liviu Daia               e-mail:   Liviu.Daiaimar.ro
Institute of Mathematics     web page: http://www.imar.ro/~daia
of the Romanian Academy      PGP key:  http://www.imar.ro/~daia/daia.asc