Linux Router: TCP slow, UDP fast

Mathias Wolkert mathias.wolkert at
Sun Feb 15 10:10:44 UTC 2009

I would turn off ethernet flow control. Maybe you already have.

It can be really mean on tcp's own flow control if the switch has an  
issue of some kind (load).


15 feb 2009 kl. 10.24 skrev Chris <chris at>:

> 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.
> Thanks,
> Chris

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