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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Andrew Rybchenko [mailto:andrew.rybchenko@oktetlabs.ru]
> Sent: Thursday, 10 November 2022 11.09
> 
> On 11/10/22 12:55, Morten Brørup wrote:
> >> From: Andrew Rybchenko [mailto:andrew.rybchenko@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."

> 
> Thanks a lot,
> Andrew.
>