From: Patrick McHardy Date: Thu, 20 Mar 2008 23:11:27 +0000 (-0700) Subject: [TCP]: Fix shrinking windows with window scaling X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=607bfbf2d55dd1cfe5368b41c2a81a8c9ccf4723;p=GitHub%2Fexynos8895%2Fandroid_kernel_samsung_universal8895.git [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. Signed-off-by: Patrick McHardy Signed-off-by: David S. Miller --- diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 01578f544ad6..72b9350006fe 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -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;