Re: Possible BUG in IPv4 TCP window handling, all recent 2.4.x/2.6.x kernels

[John Heffner <jheffner@psc.edu>]

On Sep 1, 2005, at 6:53 PM, Ion Badulescu wrote:
> A few minutes later it has finally caught up to present time and it 
> starts receiving smaller packets containing real-time data. The TCP 
> window is still 16534 at this point.
>
> [tcpdump output removed]
>
> This is where things start going bad. The window starts shrinking from 
> 15340 all the way down to 2355 over the course of 0.3 seconds. Notice 
> the many duplicate acks that serve no purpose (there are no lost 
> packets and the tcpdump is taken on the receiver so there is no 
> packets/acks crossed in flight).

I have an idea why this is going on.  Packets are pre-allocated by the 
driver to be a max packet size, so when you send small packets, it 
wastes a lot of memory.  Currently Linux uses the packets at the 
beginning of a connection to make a guess at how best to advertise its 
window so as not to overflow the socket's memory bounds.  Since you 
start out with big segments then go to small ones, this is defeating 
that mechanism.  It's actually documented in the comments in 
tcp_input.c. :)

 * The scheme does not work when sender sends good segments opening
 * window and then starts to feed us spagetti. But it should work
 * in common situations. Otherwise, we have to rely on queue collapsing.

If you overflow the socket's memory bound, it ends up calling 
tcp_clamp_window().  (I'm not sure this is really the right thing to do 
here before trying to collapse the queue.)  If the receiving 
application doesn't fall too far behind, it might help you to set a 
much larger receiver buffer.

  -John

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