re: some further musings

Craig Partridge <craig@NNSC.NSF.NET> Tue, 05 December 1989 17:06 UTC

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Cc: mtudwg
Subject: re: some further musings
Date: Tue, 05 Dec 89 12:05:04 -0500
From: Craig Partridge <craig@NNSC.NSF.NET>

> First:  is it really necessary to reprobe for MTU changes, once the
> initial negotiation is complete?  Route changes are comparatively
> infrequent, and many -- most? -- routing changes will not affect the
> MTU.


I think you are making two separate points here:

    (1) Do we need to probe very often?  Your answer (and mine)
    is no.  Once you know the MTU, you shouldn't have to check
    it very often because the path shouldn't change.

    (2) Are the MTUs very diverse?  If not, again, you shouldn't
    need to probe often.

I object to question (2) on philosophical grounds.  IP is intended
to run over diverse networks, and we should assume the MTU varies
widely, even if it doesn't in practice.  (Indeed, given FDDI MTUs
and Jacobson's small MTUs [256byte if I remember right] for his
header-compressed SLIP links I think MTU varies quite widely in
practice too).