Subject: Re: Patch for timiting TCP MSS (i.e. for new PPPoE)
To: None <>
From: Bill Sommerfeld <>
List: tech-net
Date: 12/06/2001 21:57:58
[a previous attempt to reply escaped early..]

> > 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:
> ____         ----           ____
> |CL|----E----|NB|----POE----|RB|
> ----         ----           ----
> 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").

here's another picture:

____         ----           ____                ______        ______
|CL|----E----|NB|----POE----|RB|----Internet----| FW |--------| WS |
----         ----           ----                ------        ------

FW = "Firewall"; "WS" = "Web server".

"WS" sends out 1500-byte packets with DF set.

1500 byte packet gets to "RB"; "RB" sends ICMP "fragmentation needed"
back to WS.

"FW", configurated in violation of RFC2979, blocks all ICMP packets,
including ICMP "fragmentation needed" messages, so WS never knows to
shrink its MTU to CL.

Unfortunately, the as-built internet contains many of combinations
like FW and WS, and, as a result, path MTU discovery does not work
very well in the as-built internet.

Because the owner of CL has no way of getting the owner of FW and WS
to fix things, CL will think that PPPoE routers which whack the
outgoing MSS options work better than PPPoE routers which don't.

						- Bill