[plug] Strange TCP problem talking to Westnet pop3

[Cameron Patrick]

Hi all,

I've been bashing my head against the wall intermittently with this
one for a few days.  I have a machine which won't download mail
properly from mail.westnet.com.au; however it doesn't seem to be a
Westnet problem because I can fetch the messages for this account from
my DSL at home (also on Westnet) and from UCC.  The Linux box in
question is running Debian sarge with a 2.6.8 kernel.

TCP connections to mail.westnet are stalling after 20--25kb of data
has been received.  This only seems to affect mail.westnet as I can
download large files from elsewhere with no problem, and fetching mail
from mail.westnet via an SSH tunnel works fine too.  Looking at a
tcpdump, it doesn't seem to be an MTU issue; lowering my MTU didn't
help; and receiving lots of small packets (eg. by sending a pop3 LIST
command 200 times) also stalled the connection.  Some googling
suggested that TCP ECN might be my problem but it was already disabled
via sysctl.

Looking at the tcpdump in ethereal shows something very odd going on:
the final packet containing 'real' data is shorter than the others
(1010 byte ethernet frame vs. 1474 bytes).  After the ACK, the next
packet I receive is a "TCP Out-of-order Continuation" (sequence number
is 16613 rather than the 22725 that I would expect) and despite
several repeated ACKs for the previous packet, mail.westnet keeps
retransmitting the 'out-of-order' packet.  (NB the above may be
a bit confused, my knowledge of the internals of TCP is not that
great.)

I'm at my wit's end here... any suggestions on what to try next?

Cameron.