[c-nsp] Effect of simultaneous TCP sessions on bandwidth

Youssef Bengelloun-Zahr youssef at 720.fr
Sun Nov 10 17:38:12 EST 2013


Hello,

Thanks for this useful feedback.

Best regards.



Le 11 nov. 2013 à 06:32, Octavio Alvarez <alvarezp at alvarezp.ods.org> a écrit :

> On 11/10/2013 11:11 AM, Youssef Bengelloun-Zahr wrote:
>>>>    - TCP traffic reaches up to 90 Mbits/s for one way streams (both
>>>> ways),
>>>> 
>>>>    - TCP traffic hits some kind of limit and isn't able to achieve more
>>>> than 40-60 Mbits/s in average      <=== That's the problem we are facing
> 
> If you are trying to saturate the link in both directions each of the
> acknowledge packets will compete against the other stream and will have
> a hard time reaching back.
> 
> If the device has too small buffers, the ACKs for stream 1 may be
> dropped, which may cause retransmits in stream 1, aggravating the
> problem for session 2, which will retransmit too, further aggravating
> the problem for stream 1.
> 
> If the device has too large buffers, the BW*DLY product of the link will
> increase a bit which will lower the performance of the TCP session.
> 
> A good balance must be found. Try controlling your tests and see if you
> can reduce the max BW per session. See how far you can go. I'd try
> tweaking the buffers and repeating.
> 
> Also, as a measure, ping to the other end and measure RTT. Now, with
> both sessions open, repeat the RTT measurement.



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