Linux Router: TCP slow, UDP fast

Chris chris at
Sun Feb 15 09:24:50 UTC 2009

Thanks, Karl, Allen and Nickola.
I failed-over to another router last night and briefly had full expected
throughput but this morning despite dropping providers and moving between
routers again for trial and error I still see _outbound_ TCP at about the
same 300 - 600kbps per session.

I eliminated conntract modules firstly, then iptables as a whole. I've
eliminated TSO and checksumming (which caused very sticky connections) on
the e1000 NIC.

The failover router has a slightly older kernel and was working before
Christmas so it's not most likely not kernel versions. I've also tried
removing FIB_TRIE as a stab in the dark with no success. And the failover
router connects using FE not GE so I've eliminated NICs and connection
speeds to a front-facing switch.

The only constant is the front-facing switch (it's negotiating perfectly at
FD though) so all I can think of is removing that from the equation.

It's definitely only _outbound_ TCP getting buffered though ! I've pushed
92Mbps on a FE link with UDP and uploaded at 16Mbps on a 16Mbps link.

Any last ideas appreciated before causing headaches removing switches would
be appreciated.



More information about the NANOG mailing list