Linux Router: TCP slow, UDP fast

Mikael Abrahamsson swmike at
Sat Feb 14 17:57:33 UTC 2009

On Sat, 14 Feb 2009, Chris wrote:

> The RTTs vary massively because the router is forwarding from websites 
> on the LAN to visitors worldwide. Is that what you meant ?

And your TCP speed when doing testing is always 300-600 kilobyte/s 
regardless of RTT between the boxes with which you're testing?

Without TCP window scaling turned on on the boxes doing TCP with each 
others, you're always limited to 1/RTT*64k bytes/s of transfer speed.

Changing window scaling on the linux router will of course not change the 
behaviour of the traffic going thru it, only TCP sessions that itself 

Mikael Abrahamsson    email: swmike at

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