Subject: Re: perhaps time to check our TCP against spec?
To: Jason Thorpe <>
From: Jonathan Stone <jonathan@DSG.Stanford.EDU>
List: tech-net
Date: 04/06/1998 23:31:21
Jason Thorpe <> writes:

>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.

Are you genuinuely unaware this is a commonly-used setup?  On which
people depend?  Using or talking to boxes which don't do PMTU?

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?

>If in_localaddr -> FALSE, advertise tcp_mssdflt (512).
>...which means you're sending smaller segments than you could be.
>Seems equally, if not more, broken as not using Path MTU Discovery and
>advertising MSS the way we currently do.

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.