Subject: Re: Patch for timiting TCP MSS (i.e. for new PPPoE)
To: Martin Husemann <>
From: Thor Lancelot Simon <>
List: tech-net
Date: 12/06/2001 18:05:36
On Thu, Dec 06, 2001 at 09:24:37PM +0100, Martin Husemann wrote:
> > 2) Our NAT code appears to corrupt locally-generated needs-frag ICMP
> >    messages, so a NetBSD router separating, say, an MTU-1400 PPPOE link
> >    and an MTU-1500 Ethernet will create a Path MTU blackhole.
> Maybe, but the typical PPPoE router will have a 1492 byte MTU, and the need
> to fragment packet will not be send from the router but from it's pppoe peer.

That's just not right.  Let's draw a picture:

____         ----           ____
----         ----           ----

So here we have a "client" ("CL") connected by MTU-1500 Ethernet to a
router running NetBSD ("NB") connected by PPP-over-Ethernet (MTU 1492) to 
a DSL headend box ("RB").

If CL tries to do path MTU discovery, sending packets with Don't Fragment
set, it is NB that is responsible for sending back Needs Fragment messages
for anything that doesn't fit in the 1492 byte MTU.

If NB is a NetBSD system that's doing ipf and NAT and exhibiting the bug
I'm talking about, CL will ignore its Needs Fragment messages because
they will have invalid checksums.

That creates a Path MTU blackhole.

Thor Lancelot Simon	                            
    And now he couldn't remember when this passion had flown, leaving him so
  foolish and bewildered and astray: can any man?
						   William Styron