Subject: Re: PPPoE vs ETHERMTU
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Angelos D. Keromytis <>
List: tech-net
Date: 07/01/2001 00:21:48
In message <200107010205.WAA13643@Twig.Rodents.Montreal.QC.CA>, der Mouse write
>I find this implausible because the top four points would be the same
>if I were using the provider-provided software on Wintel - and anything
>that broke mail fetching for that crowd would be fixed approximately
>instantly.  (However, the provider's mailhost *does* appear to be
>trying to do PMTU-D; the packets I do see have DF set.)

Speaking from personal experience: the DSL providers usually expect you to
run PPPoE on your Wintel box. As such, the TCP MSS on all connections will
be correctly determined to be 1490 (or 1492, depends on your variant of
DSL apparently). I've seen Wintel boxes behind a DSL firewall not work
with exactly the same symptoms.

Having run tcpdump on both sides of such a connection, I've noticed that
my DSL provider does not in fact generate ICMPs for packets going to the
DSL network (so the server sees 100% packet loss as soon as the packet
size exceeds 149x bytes).