From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Hannemann Subject: Re: [Bugme-new] [Bug 15571] New: TCP madness - some packets are shunned. Date: Fri, 26 Mar 2010 09:41:30 +0100 Message-ID: <4BAC733A.1070403@nets.rwth-aachen.de> References: <20100322143711.b78c7a93.akpm@linux-foundation.org> <4BAB8281.8000905@nets.rwth-aachen.de> <20100326053151.GA23938@treshna.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7BIT Cc: netdev@vger.kernel.org, bugzilla-daemon@bugzilla.kernel.org, bugme-daemon@bugzilla.kernel.org, Andrew Morton To: Jasen Betts Return-path: Received: from mta-1.ms.rz.RWTH-Aachen.DE ([134.130.7.72]:42615 "EHLO mta-1.ms.rz.rwth-aachen.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751578Ab0CZIly (ORCPT ); Fri, 26 Mar 2010 04:41:54 -0400 Received: from ironport-out-1.rz.rwth-aachen.de ([134.130.5.40]) by mta-1.ms.rz.RWTH-Aachen.de (Sun Java(tm) System Messaging Server 6.3-7.04 (built Sep 26 2008)) with ESMTP id <0KZV003FQS5T71D0@mta-1.ms.rz.RWTH-Aachen.de> for netdev@vger.kernel.org; Fri, 26 Mar 2010 09:41:53 +0100 (CET) In-reply-to: <20100326053151.GA23938@treshna.com> Sender: netdev-owner@vger.kernel.org List-ID: [re-adding CCs] please reply to all Am 26.03.2010 06:31, schrieb Jasen Betts: > On Thu, Mar 25, 2010 at 04:34:25PM +0100, Arnd Hannemann wrote: >> Am 22.03.2010 22:37, schrieb Andrew Morton: >>> >>> (switched to email. Please respond via emailed reply-to-all, not via the >>> bugzilla web interface). >>> >>> On Thu, 18 Mar 2010 02:46:29 GMT >>> bugzilla-daemon@bugzilla.kernel.org wrote: >>> >>>> http://bugzilla.kernel.org/show_bug.cgi?id=15571 >>>> >>>> URL: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=53646 >>>> 2 >>>> Summary: TCP madness - some packets are shunned. >>>> Product: Networking >>>> Version: 2.5 >>>> Kernel Version: 2.6.30 >>>> Platform: All >>>> OS/Version: Linux >>>> Tree: Mainline >>>> Status: NEW >>>> Severity: normal >>>> Priority: P1 >>>> Component: Other >>>> AssignedTo: acme@ghostprotocols.net >>>> ReportedBy: jasen@treshna.com >>>> Regression: No >>>> >>>> >>>> The host http://www.cv-it.com is virtually unreachable with kernel 2.6.26 >>>> (and later) slow with kernel 2.6.18 and just fine with windows XP. >>>> >>>> I used telnet to port 80 for testing. >>>> >>>> it seems to be a TCP issue, as the having the XP machine behind a linux based >>>> iptables firewall pc causes no problems, but telnet from the firewall pc itself >>>> to port 80 on www.cv-it.com does not work >> >> For me it seems to be the host is messing up with the window scale option. >> Although it claims to support window scaling: >> 16:23:17.466592 IP x.x.x.x.51151 > 121.199.32.220.80: Flags [S], seq 2159265664, win 5840, options [mss 1460,sackOK,TS val 8382141 ecr 0,nop,wscale 7], length 0 >> 16:23:17.761697 IP 121.199.32.220.80 > x.x.x.x.51151: Flags [S.], seq 3910885479, ack 2159265665, win 65535, options [mss 1448,sackOK,nop,nop,nop,nop,nop,nop,nop,nop,nop,nop,nop,wscale 8], length 0 >> >> My host (linux 2.6.32) is offering a window of 5888 (46<<7): >> 16:23:17.761740 IP x.x.x.x.51151 > 121.199.32.220.80: Flags [.], ack 1, win 46, length 0 >> >> And cv-it.com seems to think there is only a window of 46 ignoring the previously negotiated window scaling: >> 16:23:23.066318 IP 121.199.32.220.80 > x.x.x.x.51151: Flags [.], seq 1:47, ack 112, win 65160, length 46 >> >> You can disable window scaling with: >> sysctl -w "net.ipv4.tcp_window_scaling=0" >> > > yeah, that works for me. > > I don't know a lot about this stuff. wikipedia says windows XP does window > scaling also, yet it's not a problem with XP only with linux. This may be pure coincidence that XP "works". For instance if XP is only using a window scale of 1 or 2, the effect of ignoring the window scale may not be so drastic. However, the problem is: once you negotiated the window scale for a connection you must not change it and you may actually need a big window for performance reasons. So you have to pick a window scale value, so you can express the largest window you are going to use. In Linux the maximum tcp receive window can be manipulated with the "net.ipv4.tcp_rmem" sysctl. (The max is the third value) Recent linux kernels use the amount of ram your machine has to calculate the default value for this. > >> My host (linux 2.6.32) is offering a window of 5888 (46<<7): >> 16:23:17.761740 IP x.x.x.x.51151 > 121.199.32.220.80: Flags [.], ack 1, win 46, length 0 > > so you mean it seems to see '46' as '46' instead of 46<<7 == 5888 Exactly. Best regards, Arnd