Subject: Re: PPPoE vs ETHERMTU
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Jun-ichiro itojun Hagino <>
List: tech-net
Date: 07/01/2001 10:38:29
>I've got a SPARC IPX that I'm using as my gateway to the world.  It
>runs PPPoE to a DSL (or at least DSLish) box from my connectivity
>This mostly works fine.  But when I try to poll my provider-provided
>mailbox (most of my traffic is tunneled to my own boxen), I get a
>connection, but when I try to download a messages, the connection
>stalls.  Snooping the traffic makes me think something odd is going on.
>It looks a bit like the won't-frag disease, the problem outlined in
>RFC2923 section 2.1, but it's not that simple.  I see small packets
>going back and forth, but when it tries to send me bulk data, I get
>600-byte packets with 1448-byte gaps between them.

	this is exactly the symptom with path MTU discovery problem
	(though it seems that your provider's mail server uses a bit strange
	TCP retransmission strategy i guess).  ask your provider to do either
	of the following:
	- turn off provider mailbox's path MTU discovery
	- let ICMPv4 need fragment message through over some of the routers
	  they have between DSL entrypoint and the provider mailbox


provider mailbox
some router	<--- bad guy
DSL entrypoint
  | smaller MTU
DSL exitpoint
your client