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

From: Arnd Hannemann <[email-masked]>
Date: Fri, 26 Mar 2010 09:41:30 +0100

[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
>>> [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"
>>
> 
> 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