From: Arnd Hannemann <hannemann@nets•rwth-aachen.de>
To: Jasen Betts <Jasen@treshna•com>
Cc: netdev@vger•kernel.org, bugzilla-daemon@bugzilla•kernel.org,
bugme-daemon@bugzilla•kernel.org,
Andrew Morton <akpm@linux-foundation•org>
Subject: Re: [Bugme-new] [Bug 15571] New: TCP madness - some packets are shunned.
Date: Fri, 26 Mar 2010 09:41:30 +0100 [thread overview]
Message-ID: <4BAC733A.1070403@nets.rwth-aachen.de> (raw)
In-Reply-To: <20100326053151.GA23938@treshna.com>
[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
next prev parent reply other threads:[~2010-03-26 8:41 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <bug-15571-10286@http.bugzilla.kernel.org/>
2010-03-22 21:37 ` [Bugme-new] [Bug 15571] New: TCP madness - some packets are shunned Andrew Morton
2010-03-25 15:34 ` Arnd Hannemann
[not found] ` <20100326053151.GA23938@treshna.com>
2010-03-26 8:41 ` Arnd Hannemann [this message]
2010-03-28 4:26 ` Stephen Hemminger
2010-03-30 21:08 ` Jasen Betts
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