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

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Hi David,

On Thu, 1 Sep 2005, David S. Miller wrote:

> From: John Heffner <jheffner@xxxxxxx>
> Date: Thu, 1 Sep 2005 22:51:48 -0400
>
> > 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.
>
> That's a strong possibility, good catch John.

That's possible, but see below.

> Although, I'm still not ruling out some box in the middle
> even though I consider it less likely than your theory.

There is no funky box in the middle, that much I can guarantee you.

I said yesterday that I don't have access to the sender. While that's true 
for the flow I had captured in those dumps, I saw the same phenomenon 
occur between two boxes I control fully. The sender is running Windows 
2000, and is separated from the receiver by a single Catalyst 6500 
switch/router (they are on different VLAN's) which doesn't do anything 
fancy (I control the switch as well).

This particular Win2k sender sends _only_ real-time data, it's not capable 
of rewinding. So it's always sending small packets, from start to finish, 
yet the problem still occurs.

Note that even real-time data can end up generating a stream of full-size 
packets occassionally. It's just very unlikely they would occur at the 
start of the flow, as market data is very thin in the pre-market open hours.

> So you're suggesting that tcp_prune_queue() should do the:
>
> 	if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
> 		tcp_clamp_window(sk, tp);
>
> check after attempting to collapse the queue.
>
> But, that window clamping should fix the problem, as we recalculate
> the window to advertise.

Patches for testing are very much welcome...

Thanks,
-Ion
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