From: John Hughes <john@calva.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: John Hughes <john@atlantech.com>, netdev@vger.kernel.org
Subject: Re: When a TCP segment is split up (to be sent through a TUN device with a small MTU) who should recalculate the checksum?
Date: Fri, 15 Nov 2013 15:02:08 +0100 [thread overview]
Message-ID: <52862960.6090408@calva.com> (raw)
In-Reply-To: <1384520815.28716.63.camel@edumazet-glaptop2.roam.corp.google.com>
On 15/11/13 14:06, Eric Dumazet wrote:
> On Fri, 2013-11-15 at 09:52 +0100, John Hughes wrote:
>> I have two offices, joined by a OpenVPN tunnel. I've upgraded the
>> kernels in the machines running the tunnel to 3.10. All of a sudden I'm
>> getting horrible transmission delays between the two offices.
>>
>> office1 LAN--------office1 tunnel machine
>> |
>> | openvpn tunnel
>> |
>> office2 tunnel machine------office2 LAN
>>
>>
>> What seems to be happening is that packets are arriving at the LAN
>> interface of the machine running the tunnel and being combined by
>> generic-receive-offload. These packets then have to be split up again
>> as they are too big for the tunnels MTU.
>>
>> But when the packets are split the TCP checksum doesn't seem to be being
>> recalculated, so the systems on the other end of the tunnel ignore them,
>> forcing many retries and the observed delays.
>>
>>
> Thanks for the report
>
> It depends on the offload capabilities of the NIC forwarding packets
The NIC is a kernel tun device.
>
> ethtool -k eth0
# ethtool -k tun1
Features for tun1:
rx-checksumming: off [fixed]
tx-checksumming: off
tx-checksum-ipv4: off [fixed]
tx-checksum-ip-generic: off [requested on]
tx-checksum-ipv6: off [fixed]
tx-checksum-fcoe-crc: off [fixed]
tx-checksum-sctp: off [fixed]
...
It seems someone has asked the tunnel to compute checksums
(tx-checksum-ip-generic), but it can't do that.
>
> TCP checksums can be recomputed by tcp_gso_segment() (it was named
> tcp_tso_segment() in linux 3.10), unless the NIC told it was doing the
> checksum computation itself.
>
> ethtool -K eth0 tx off
>
> Should request stack to perform the cheksums.
>
> What is the NIC doing the transmits ?
>
>
>
next prev parent reply other threads:[~2013-11-15 14:16 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-15 8:52 When a TCP segment is split up (to be sent through a TUN device with a small MTU) who should recalculate the checksum? John Hughes
2013-11-15 13:06 ` Eric Dumazet
2013-11-15 14:02 ` John Hughes [this message]
2013-11-15 14:20 ` Vlad Yasevich
2013-11-15 14:31 ` John Hughes
2013-11-15 14:41 ` Vlad Yasevich
2013-11-15 14:55 ` John Hughes
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