Subject: Limiting the advertised MSS, again.
To: None <>
From: Rick Byers <>
List: tech-net
Date: 12/06/2001 22:02:11
Someone mentioned that lowering the MTU on a route should cause the
TCP MSS to get lowered correspondingly.  I thought it was working, but I
must have been drunk or something, because the per-route MTU doesn't seem
to affect the advertised MSS at all (which I should have known since I've
was staring at the tcp_mss_to_advertise code).

So, other than the patch I recently posted, there doesn't appear to be
any way to convince NetBSD to lower the advertised MSS without lowering
the interface MTU.  I consider being able to lower the advertised MSS a
handy feature, especially considering all the broken networks.  Its
conceivable that someone might want to use NetBSD behind a reduced-mtu
gateway which they have no control over, and which doesn't implement TCP
MSS clamping.  In that scenario, the only way to cope with blackholed
sites is to convince NetBSD to lower the MSS it advertises.

Anyway, I know some people belive we shouldn't provide an option to cope
with broken networks, but I believe the situation is already desperate
enough that there is nothing to be gained by holding this feature back.
Does anyone else think there should be a sysctl for lowering the MSS, or
is there some better way?  Perhaps a per-route MRU or something to that