Re: TCP maximum segment size determination


Jeffrey Mogul (mogul@decwrl.dec.com)
13 Nov 1987 1829-PST (Friday)


ORCHARD/BRUC@SCARECROW.WAISMAN.WISC.EDU (Bruce Orchard) writes:
    I propose adding a new option to the IP header. This option would
    give the minimum of the maximum transmission units of any network
    that handled the packet. The originating end would set it to a
    large value. Each node that transmitted the packet would compare
    the value given in the option to the maximum transmission unit of
    the outgoing network. If the network value were less, the value
    in the option would be reduced to the network value.

This is one of the several options proposed in "Fragmentation
Considered Harmful", by Chris Kent and myself, presented at the
SIGCOMM '87 Workshop this past August. I understood that the proceedings
would be distributed to members of SIGCOMM, but so far I have not
seen anything except the unpaginated version distributed at the
workshop. Chris and I are preparing a slightly expanded version
which should be available as a tech report sometime in the next few
months.

Although I think this is a great idea (and some day we'll take the
proposal given in the paper and turn it into an RFC) it's not real
likely that it is practical in the IP Internet, given the low likelihood
of changing enough hosts and gateways to make it work. Instead, we
recommend simply biting the bullet and using 576 bytes whenever you're
not absolutely sure where a packet is going. 576 bytes isn't always
fragmentation-proof, but it's a reasonable compromise.

    P.S. Is the MTU of SATNET really 256 bytes, as given in IEN 192?

It's probably slightly less. I've had a hard time discovering the
exact value.

-Jeff



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