: Fix shrinking windows with window scaling

Please review carefully since I'm not too familiar with the TCP code.

commit adced31497ffbb3fe6ffef2d05f2b8fc74f1f138

Author: Patrick McHardy <[email protected]>
Date:   Fri Mar 14 17:15:20 2008 +0100

[TCP]: Fix shrinking windows with window scaling

When selecting a new window, tcp_select_window() tries not to shrink the offered window by using the maximum of the remaining offered window size and the newly calculated window size. The newly calculated window size is always a multiple of the window scaling factor, the remaining window size however might not be since it depends on rcv_wup/rcv_nxt. This means we're effectively shrinking the window when scaling it down.

The dump below shows the problem (scaling factor 2^7):

- Window size of 557 (71296) is advertised, up to 3111907257:

IP 172.2.2.3.33000 > 172.2.2.2.33000: . ack 3111835961 win 557 <...>

- New window size of 514 (65792) is advertised, up to 3111907217, 40 bytes below the last end:

IP 172.2.2.3.33000 > 172.2.2.2.33000: . 3113575668:3113577116(1448) ack 3111841425 win 514 <...>

The number 40 results from downscaling the remaining window:

3111907257 - 3111841425 = 65832
65832 / 2^7 = 514
65832 % 2^7 = 40

If the sender uses up the entire window before it is shrunk, this can have chaotic effects on the connection. When sending ACKs, tcp_acceptable_seq() will notice that the window has been shrunk since tcp_wnd_end() is before tp->snd_nxt, which makes it choose tcp_wnd_end() as sequence number. This will fail the receivers checks in tcp_sequence() however since it is before it's tp->rcv_wup, making it respond with a dupack.

If both sides are in this condition, this leads to a constant flood of ACKs until the connection times out.

Make sure the window is never shrunk by aligning the remaining window to the window scaling factor.

index 01578f5..72b9350 100644
@@ -255,7 +255,7 @@ static u16 tcp_select_window(struct sock *sk)
 		 *
 		 * Relax Will Robinson.
 		 */
-		new_win = cur_win;
+		new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
 	}
 	tp->rcv_wnd = new_win;
 	tp->rcv_wup = tp->rcv_nxt;

From: Patrick McHardy <[email protected]>
Date: Fri, 14 Mar 2008 18:42:38 +0100

It looks perfect, excellent catch Patrick!

Applied, and I'll queue this up for -stable.

This bug has existed forever :(