Linux Router: TCP slow, UDP fast

Allen Smith lazlor at
Sat Feb 14 20:43:15 UTC 2009

If this router is not doing some kind of proxying, tuning tcp related 
kernel bits will not impact "long fat pipe" or "long fat network" issues.

The place that needs to be tuned for larger window sizes/scaling is the 
web server.

or search for "Linux TCP tuning" or "large fat pipes"

Also make sure your firewall isn't "helping" you out by "cleaning up" 
the TCP SYN/ACK sequence and fiddling with the window scaling stuff.

Also if you have load balancers, they might break this stuff as well.

Good luck,

Chris wrote:
> Thanks for all the answers. I'm currently going down the path of looking at
> IPtables' conntrack slowing the forwarding rate.
> If I can't find any more docs then I'll boot the router with a kernel
> without any IPtables built-in and see if that's it.
> As Lee said "rollback" ! That's the last change to the box. If I can rule
> out the logging of traffic from conntrack is slowing down
> the forwarding then I can look into hardware further ;-)
> Chris

More information about the NANOG mailing list