Subject: Re: [Bugme-new] [Bug 15571] New: TCP madness - some packets are shunned.

From: Arnd Hannemann <[email-masked]>
Date: Thu, 25 Mar 2010 16:34:25 +0100

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
> [email-masked] 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: [email-masked]
>>         ReportedBy: [email-masked]
>>         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"

Best regards,
Arnd