netdev - Re: [PATCH 1/9] ipv6: implement dataplane support for rthdr type 4 (Segment Routing Header)

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Date:   Thu, 20 Oct 2016 08:47:27 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     David Lebrun <david.lebrun@...ouvain.be>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/9] ipv6: implement dataplane support for rthdr type 4
 (Segment Routing Header)

On Thu, Oct 20, 2016 at 6:04 AM, David Lebrun <david.lebrun@...ouvain.be> wrote:
> On 10/17/2016 07:01 PM, Tom Herbert wrote:
>>> > +
>>> > +       if (skb->ip_summed == CHECKSUM_COMPLETE)
>>> > +               skb->ip_summed = CHECKSUM_NONE;
>>> > +
>> Because the packet is being changed? Would it make sense to update the
>> checksum complete value based on the changes being made. Consider the
>> case that the next hop is local to the host (someone may try to
>> implement network virtualization this way).
>>
>
> Rethinking about that: even if the next hop is local, I am not sure to
> see the benefits of updating the checksum instead of setting
> CHECKSUM_NONE. For example, if the next and final hop is local and the
> packet carries a TCP payload, tcp_checksum_complete() would force the
> recomputation of the checksum anyway (unless ip_summed ==
> CHECKSUM_UNNECESSARY).
>
Or unless skb->csum_valid is set (tcp_checksum_complete calls
skb_csum_unnecessary where the check is done). If the checksum
complete value is correct then skb->csum_valid would be set from
skb_checksum_init which is called early in tcp_v4_rcv and tcp_v6_rcv.
This way if the penultimate and final hops are local and
CHECKSUM_COMPLETE is set computing the packet checksum is avoided for
a TCP packet.

Tom

> So I fail to see a path where updating the checksum would be beneficial.
>
> Am I missing something ?
>
> David
>