Subject: Re: perhaps time to check our TCP against spec?
To: Jonathan Stone <firstname.lastname@example.org>
From: Jason Thorpe <email@example.com>
Date: 04/06/1998 23:33:30
On Mon, 06 Apr 1998 23:31:21 -0700
Jonathan Stone <jonathan@DSG.Stanford.EDU> wrote:
> >No, doing it the "traditional" way would depend on the setting of
> >subnetsarelocal, and the IP addresses of the hosts in question.
> >If in_localaddr -> TRUE, advertise:
> > MTU of interface we're currently using to get to peer, minus
> > size of TCP + IP headers.
> >...which means you could encounter fragmentation at the router.
> No, not as long as the router's MTU is at least as big as the smaller
> MTU of the end-stations in the connection.
Ah yes, because the Ethernet-connected host is going to be bounded by
ETHERMTU. In any case, the FDDI-connected hsot is still going to advertise
FDDIMTU MSS to the peer. Sorry, post-meal brain-mellow while I digest
the tasty mushrooms that were on my dinner salad.
> Are you genuinuely unaware this is a commonly-used setup? On which
> people depend? Using or talking to boxes which don't do PMTU?
No, we have it all over the place here (where I work).
> Have you asked, say, someone using a netBSD box as a PPP router for a
> home ethernet what they think of the implications of this, for traffic
> from `dumb' outside boxes talking to their router?
Well, considering that I _am_ someone who uses a NetBSD box as a PPP router
for a home Ethernet, and have hosts attached to that Ethernet which are
incapable of doing Path MTU Discovery (including, I might add, a host
running 4.2BSD), I have thought about the implications, yes. And I
often communicate with hosts on my home Ethernet from systems which do
not have Path MTU Discovery (and I happen to have the MTU on my PPP link
turned down to 576).
> Looks to me like Jason's idea does in fact break some existing setups,
> but Jason is now trying to claim that those setups are really broken
> in the first place.
Well, if they rely on a quirk of a particular implementation of TCP,
then I would assert that THEY ARE!
However, unless I missed something, I don't see how "my idea" actually
breaks the A->Ether->Router->FDDI->B scenario....
Jason R. Thorpe firstname.lastname@example.org
NASA Ames Research Center Home: +1 408 866 1912
NAS: M/S 258-5 Work: +1 650 604 0935
Moffett Field, CA 94035 Pager: +1 415 428 6939