From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765757AbYDQBQh (ORCPT ); Wed, 16 Apr 2008 21:16:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754397AbYDQBHt (ORCPT ); Wed, 16 Apr 2008 21:07:49 -0400 Received: from sous-sol.org ([216.99.217.87]:58879 "EHLO sous-sol.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759213AbYDQBHs (ORCPT ); Wed, 16 Apr 2008 21:07:48 -0400 Message-Id: <20080417010348.192847503@sous-sol.org> References: <20080417010122.148289106@sous-sol.org> User-Agent: quilt/0.46-1 Date: Wed, 16 Apr 2008 18:02:02 -0700 From: Chris Wright To: linux-kernel@vger.kernel.org, stable@kernel.org Cc: Justin Forbes , Zwane Mwaikambo , "Theodore Ts'o" , Randy Dunlap , Dave Jones , Chuck Wolber , Chris Wedgwood , Michael Krufky , Chuck Ebbert , Domenico Andreoli , torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Patrick McHardy , David S Miller Subject: TCP: Fix shrinking windows with window scaling Content-Disposition: inline; filename=tcp-fix-shrinking-windows-with-window-scaling.patch Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org -stable review patch. If anyone has any objections, please let us know. --------------------- From: Patrick McHardy Upstream commit: 607bfbf2d55dd1cfe5368b41c2a81a8c9ccf4723 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. Signed-off-by: Patrick McHardy Signed-off-by: David S. Miller Signed-off-by: Chris Wright --- net/ipv4/tcp_output.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -258,7 +258,7 @@ static u16 tcp_select_window(struct sock * * 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; --