Subject: PPPoE vs ETHERMTU
To: None <>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
List: tech-net
Date: 06/30/2001 20:27:04
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.

The reason it's not just won't-frag is that I sniffed the traffic at
the Ethernet level, and the PPPoE packets I would expect to see Just
Aren't There.  It's hard to believe the provider isn't sending them at
all, since mail fetching wouldn't work for anyone in that case, and
that they'd fix pronto.  So I have to think the machine isn't receiving
them for some reason.  And the most plausible reason is that they're
oversize, slightly over the Ethernet MTU.

At first I thought it was just the le driver on the SPARC (and indeed I
have a kernel building with LEBLEN set to 1700 to see if that helps).
But then I hooked the DSL box and the SPARC up via a hub, an old dumb
10baseT-only hub; to that hub I also hooked an Alpha I have, in which I
have the card that the service provider sent me and which therefore is
presumably capable of receiving oversize packets.

And sniffing with the alpha doesn't see the "missing" packets coming
out of the DSL box.

Ideas, anyone?

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