Date:	Mon, 07 Nov 2011 17:55:05 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Chris Siebenmann <cks@...toronto.edu>
Cc:	netdev@...r.kernel.org
Subject: Re: Bug? GRE tunnel periodically won't transmit some packets

Le lundi 07 novembre 2011 à 11:21 -0500, Chris Siebenmann a écrit :
> I have a weird problem where a GRE tunnel periodically won't transmit
> some (TCP) packets, while at the same time it will transmit others just
> fine. This is happening in the current kernel.org git head kernel as
> well as earlier ones.
> 
>  The networking environment is a GRE tunnel over IPSec in tunnel mode
> ('esp/tunnel/...') over a DSL PPPoE link. What I observe is that
> periodically outbound SSH connections stall early in the protocol
> negociation, and other TCP connections can similarly stall. Sometimes
> they recover and sometimes they time out. The problem is pretty
> reproducable and regular, although not constant (sometimes the affected
> packets get through right away).
> 
>  I have tcpdump'd both the GRE tunnel device and the underlying DSL
> PPPoE device and during a stall, the GRE tcpdump will show packets being
> sent that do not appear on the DSL PPPoE link. All of the packets that
> I've seen stalling have had 500 data octets.
> 
> Typical packets are:
> 	IP 128.100.3.52.52063 > 128.100.3.51.ssh: Flags [.], seq 22:522, ack 22, win 91, options [nop,nop,TS val 143020 ecr 966040433], length 500
> 
> (here 128.100.3.52 is the GRE tunnel IP address of the machine
> experiencing problems)
> 
> or ttcp:
> 	IP 128.100.3.52.46585 > 128.100.3.51.5001: Flags [.], seq 1:501, ack 1, win 91, options [nop,nop,TS val 729200 ecr 979199256], length 500
> 
> Ttcp had a whole run of 'length 500' packets fail to go through. SSH
> will actually successfully transmit later (different-length) packets,
> eg:
> 	128.100.3.52.52063 > 128.100.3.51.ssh: Flags [P.], seq 522:926, ack 22, win 91, options [nop,nop,TS val 143037 ecr 966040450], length 404
> 
>  The DSL PPPoE link has an MTU of 1492 and the GRE tunnel has an MTU of
> 1200 (on both ends). As far as I can tell they do pass packets of this
> size. *However*, on kernels that display this problem tracepath and 'ip
> route show table cache' both report that the GRE tunnel has a path MTU
> of 854 going from 128.100.3.52 to 128.100.3.51; however, 128.100.3.51
> sees a pmtu of 1200 for the path to 128.100.3.52.
> 
>  The machine experiencing these problems is a 64-bit x86_64 Fedora 15
> machine with various kernels. The problem does not happen with the
> current Fedora 14 kernel (nominally 2.6.35.14); it does happen with the
> Fedora 15 kernel ('2.6.40.6' aka some version of 3.0.0), the Fedora 16
> kernel (some version of 3.1.0) and on the current kernel.org git head
> as of last night. I am not running NetworkManager; all networking is
> statically configured and not changing during operation, and my IPSec
> setup is statically keyed[*].
> 
>  I would be happy to run any debugging tests or give any further
> information that people want. Should I try a different kernel git
> repo than Linus's kernel.org one?
> 
>  Thanks in advance.
> 
> (While I'm reading the mailing list I'm not directly subscribed to it,
> so copying me on replies will make sure that I see them immediately.)
> 

Do you have any errors on :

ip -s -d link show dev greXXXX


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