Re: Possible BUG in IPv4 TCP window handling, all recent2.4.x/2.6.x kernels

[David S. Miller]

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.

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

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.
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