netdev - Re: [PATCH net-next 6/8] net: gre: Implement LCO for GRE over IPv4

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Date:	Wed, 20 Jan 2016 11:58:57 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	"Rustad, Mark D" <mark.d.rustad@...el.com>,
	Edward Cree <ecree@...arflare.com>,
	David Laight <David.Laight@...lab.com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-net-drivers@...arflare.com" <linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net-next 6/8] net: gre: Implement LCO for GRE over IPv4

On Wed, Jan 20, 2016 at 11:35 AM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Wed, Jan 20, 2016 at 11:11 AM, Rustad, Mark D
> <mark.d.rustad@...el.com> wrote:
>> Alexander Duyck <alexander.duyck@...il.com> wrote:
>>
>>> Actually you may want to go the other way on that.  If they weren't
>>> flipping the checksum value for GRE before why should we start doing
>>> that now?  I'm pretty sure the checksum mangling is a very UDP centric
>>> thing.  There is no need to introduce it to other protocols.
>>
>>
>> If different checksum representations are needed, then there really should
>> be an explicit indication of whether it is a UDP-style checksum or other in
>> the skb I would think rather than guessing it based on the offset. Of course
>> it would be convenient if all the protocols that use a one's complement
>> checksum would tolerate the UDP representation. I have a long (and now old)
>> history working with real one's complement machines, and so I would want to
>> believe that any correct implementation would tolerate it, but I don't know
>> for a fact that they do.
>
> The only reason why UDP does the bit flip is because it has reserved a
> checksum of 0 as a special value.  For the checksum math itself either
> 0xFFFF or 0 are interchangeable.  The only time they would make any
> difference would be if we had a value of 0 that we were checksumming,
> but since that is not the case the values will always end up
> converging back onto 0xFFFF as the final result in the case of a
> correct checksum.
>
0xffff is mathematically equivalent to 0x0 for checksum. I would
rather always flip 0 to 0xffff in LCO rather than adding an explicit
indication (i.e. another flag) in SKB that it has a UDP checksum.

Tom

> - Alex