Subject: Re: perhaps time to check our TCP against spec?
To: Jonathan Stone <>
From: Jason Thorpe <>
List: tech-net
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                             
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