netdev - Re: tcp timestamp issues with google servers

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Message-ID: <87zk90s0em.fsf@tucsk.pomaz.szeredi.hu>
Date:	Tue, 22 May 2012 17:25:37 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: tcp timestamp issues with google servers

Eric Dumazet <eric.dumazet@...il.com> writes:

> On Thu, 2012-05-17 at 11:39 +0200, Miklos Szeredi wrote:
>> Sometimes connection to google.com, gmail.com and other google servers
>> doesn't work or takes ages to connect.  When this hits it hits all
>> google servers at the same time and it's persistent.  It never happens
>> to anything other than google.  Rebooting helps.  Rarely it goes away
>> spontaneously.
>> 
>> Apparently google is sometimes replying with an invalid TSecr timestamp
>> value (smaller than the one sent in the last packet) and this confuses
>> the Linux TCP stack which either discards the packet or sends a Reset.
>> 
>> Network dump attached.
>> 
>> I found only a couple of references to this issue:
>> 
>> http://gotchas.livejournal.com/3028.html
>> 
>> http://groups.google.com/group/comp.os.linux.networking/browse_thread/thread/29f56feded11b42a
>> 
>> Turning tcp timestamps fixes the issue:
>> 
>>   sysctl -w net.ipv4.tcp_timestamps=0
>> 
>> Not sure why this happens only to me and a very few others.
>> 
>> It appears to be an issue with google TCP stack (is it a modified
>> stack?) but I thought about issues in my network switch (restarting it
>> doesn't help) or something in the ISP, but those look unlikely.
>> 
>> Any ideas?
>> 
>> Thanks,
>> Miklos
>> 
>> 
>> 
>>   1   0.000000 192.168.28.100 -> 74.125.232.226 TCP 51303 > http [SYN] Seq=0 Win=14600 Len=0 MSS=1460 SACK_PERM=1 TSV=35355050 TSER=0 WS=5
>>   2   0.002730 74.125.232.226 -> 192.168.28.100 TCP http > 51303 [SYN, ACK] Seq=0 Ack=1 Win=14180 Len=0 MSS=1430 SACK_PERM=1 TSV=1184565067 TSER=35325344 WS=6
>
>
> Do you really have 2730 usec RTT between you and this (Google ?)
> server ?

So it appears.  The IP address is certainly registered to Google.

Thanks,
Miklos

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