Is it correct to report checksum good when there is no checksum?

[Andrew Rybchenko]

On 11/10/22 13:29, Morten Brørup wrote:
>> From: Andrew Rybchenko [mailto:andrew.rybchenko at oktetlabs.ru]
>> Sent: Thursday, 10 November 2022 11.09
>>
>> On 11/10/22 12:55, Morten Brørup wrote:
>>>> From: Andrew Rybchenko [mailto:andrew.rybchenko at oktetlabs.ru]
>>>> Sent: Thursday, 10 November 2022 10.26
>>>>
>>>> Hi all,
>>>>
>>>> some drivers report RTE_MBUF_F_RX_IP_CKSUM_GOOD for IPv6 packets.
>>>> For me it looks strange, but I see some technical reasons behind.
>>>
>>> Please note: IPv6 packets by definition have no IP checksum.
>>>
>>>> Documentation in lib/mbuf/rte_mbuf_core.h is a bit vague.
>>>> Should UNKNOWN or NONE be used instead?
>>>
>>> Certainly not NONE. Its description says: "the IP checksum is *not*
>> correct in the packet [...]". But there is no incorrect IP checksum in
>> the packet.
>>>
>>
>> Thanks, I should read the definition of none more careful.
>>
>>> I will argue against UNKNOWN. Its description says: "no information
>> about the RX IP checksum". But we do have information about it! We know
>> that the IP checksum is not there (the value is "NULL"), and that it is
>> not supposed to be there (the value is supposed to be "NULL").
>>>
>>
>> I thought that "no checksum" => "no information" => UNKNOWN
> 
> That was my initial interpretation too, and it stuck with me for a while.
> 
> But then I tried hard to read it differently, tweaking it to support the conclusion I was looking for.
> 
>>
>>> So I consider GOOD the correct response here.
>>>
>>> GOOD also means that the application can proceed processing the
>> packet normally without further IP header checksum checking, so it's
>> good for performance.
>>>
>>
>> It is very important point and would be nice to have in GOOD
>> case definition (both IP and L4 cases). It is the right
>> motivation why GOOD makes sense for IPv6.
>>
>>> It should be added to the description of RTE_MBUF_F_RX_IP_CKSUM_GOOD
>> that IPv6 packets always return this value, because IPv6 packets have
>> no IP header checksum, and that is what is expected of them.
>>>
>>
>> Could you make a patch?
> 
> Too busy right now, but I'll put it on my todo list. :-)
> 
>>
>> Bonus question is UDP checksum 0 case. GOOD as well?
>> (just want to clarify the documentation while we're on it).
> 
> No. The UDP checksum is not optional in IPv6.
> 
> RFC 2460 section 8.1 bullet 4 says: "Unlike IPv4, when UDP packets are originated by an IPv6 node, the UDP checksum is not optional. [...] IPv6 receivers must discard UDP packets containing a zero checksum, and should log the error."
> 

Yes I know, but I'm asking about IPv4 case with UDP checksum 0.