netdev - Re: When a TCP segment is split up (to be sent through a TUN device with a small MTU) who should recalculate the checksum?

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Message-ID: <5286305F.5060903@atlantech.com>
Date:	Fri, 15 Nov 2013 15:31:59 +0100
From:	John Hughes <john@...antech.com>
To:	Vlad Yasevich <vyasevich@...il.com>
CC:	John Hughes <john@...antech.com>, netdev@...r.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?

On 15/11/13 15:20, Vlad Yasevich wrote:
>
> Can you check to see if you have the following patch in your kernel
> commit: 1cdbcb7957cf9e5f841dbcde9b38fd18a804208b
> Author: Simon Horman <horms@...ge.net.au>
> Date:   Sun May 19 15:46:49 2013 +0000
>
>     net: Loosen constraints for recalculating checksum in skb_segment()
>
>
> This commit help if the forwarding system has to re-segment the data
> before transition.  Especially if the receiving interface had GRO
> enabled with checksum offloading and the transmitting interface does
> not support checksum offloading.

No, the Debian 3.10 kernel doesn't seem to have that commit:

Around line 2859 in skbuff.c I see:

                 if (fskb != skb_shinfo(skb)->frag_list)
                         continue;

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