Subject: Re: TCP funny-ness when over-driving a 1Gbps link (and wifi)

From: Chris Friesen <[email-masked]>
Date: Thu, 26 May 2011 09:28:16 -0600

On 05/20/2011 03:33 PM, Ben Greear wrote:

> I tried a different test today: 3 TCP connections between two
> wifi station interfaces (using ath9k). Each connection is
> endpoint configured to send 100Mbps of traffic to the peer.
>
> With a single connection, it does OK (maybe 250ms round-trip time max).
> With 3 of them running, round-trip user-space to user-space latency
> often goes above 3 seconds.

<snip>

> So, seems a general issue with over-driving links with multiple TCP
> connections. Doesn't seem like a regression, and probably not really
> a bug, but maybe the buffer-bloat project will help this sort of
> thing...

Given that one rule of thumb for the send buffer size is twice the 
bandwidth delay product, it seems clear that on a wifi connection 3 
seconds worth of buffering is excessive.  I think I'd classify that as a 
bug.

Chris

-- 
Chris Friesen
Software Developer
GENBAND
[email-masked]
www.genband.com