Re: [IPv6] Subject: Mandating SRH when using C-SIDs (draft-ietf-spring-srv6-srh-compression)

[Robert Raszuk <[e-mail]>]

Hi Tom,

Not really.

RFC8200 defines an exception which is tunneling and says:

         As an exception to the default behavior, protocols that use UDP
         as a tunnel encapsulation may enable zero-checksum mode for a
         specific port (or set of ports) for sending and/or receiving.
         Any node implementing zero-checksum mode must follow the
         requirements specified in "Applicability Statement for the Use
         of IPv6 UDP Datagrams with Zero Checksums" [RFC6936
<https://datatracker.ietf.org/doc/html/rfc6936>].


So in practice if we always tunnel SRv6 there is no issue.

Even Andrew agreed with that :)

Cheers,
Robert

On Thu, Mar 28, 2024 at 4:36 PM Tom Herbert <[e-mail]> wrote:

> On Thu, Mar 28, 2024 at 7:46 AM Robert Raszuk <[e-mail]> wrote:
> >
> > Hi Tom,
> >
> > > because of SRH
> >
> > Ok I buy this that there are devices which do check checksum and are not
> final destination of the packets  ... I was more talking about plain
> forwarding devices (aka P routers). Then I doubt firewalls would be sitting
> in the core of the networks.
> >
> > But let me come black to what I believe is the main disconnect.
> >
> > Why SRH would cause an issue ? I think there is claimed issue *ONLY*
> with SRv6 packets which are not encapsulated - call it raw - sent by the
> hosts which talk SRv6 and sent with more then one SID/uSID which may get
> swapped on the way.
> >
> > Because only in those cases the destination address will be changing
> while checksum of the tunnel header will not be zero.
> >
> > So what we should I think discuss are really B.1 and B.2.2 cases.
>
> Robert,
>
> The scenario that I'm talking about is really simple, and it's not
> specific to segment routing.  If someone sends a TCP in an IPv6 packet
> with no routing header then the convention is that the TCP checksum is
> valid end to end. So if the addresses are changed in flight, like in
> NAT, then we expect that some part of the packet covered by the
> checksum is adjusted to offset the change. If a packet is sent in
> segment routing without an SRH with EtherType 0x86DD then it IS an
> IPv6 packet to the network so all the conventions and requirements of
> IPv6 should be applied. IMO, if SRv6 can't maintain these conventions
> and requirements then it should fork from IPv6 and use a different
> EtherType.
>
> Tom
>
> >
> > Francois, Pablo - could you comment on this how often do we see those
> type of SRv6 deployments ? And also could you comment if operator who
> enables SRv6 in the first place sees those checksum errors how difficult is
> to address it ?
> >
> > Thx,
> > Robert
> >
> >
> > On Thu, Mar 28, 2024 at 3:29 PM Tom Herbert <[e-mail]> wrote:
> >>
> >> On Thu, Mar 28, 2024 at 6:26 AM Robert Raszuk <[e-mail]>
> wrote:
> >> >
> >> > Hi Alvaro,
> >> >
> >> > On this specific topic I think you have flatted it a bit too much.
> >> >
> >> > These are apparently the options on the table:
> >> >
> >> > A) Original packet get's encapsulated with IPv6 header
> >> >
> >> >       A.1 SHR is added to it
> >> >
> >> >              A.1.1. Regular SIDs are used
> >> >              A.1.2  Compresses SIDs are used
> >> >
> >> >       A.2 SRH is not added to it
> >> >
> >> >              A.2.1. Regular SID is used as destination
> >> >              A.2.2  Compresses SIDs are used in a container
> >> >              A.2.3  Compresses SID is used
> >> >
> >> > B) Original packet get's send from SRv6 host (without encapsulation)
> >> >
> >> >     B.1 SHR is added to it
> >> >
> >> >              B.1.1. Regular SIDs are used
> >> >              B.1.2  Compresses SIDs are used
> >> >
> >> >       B.2 SRH is not added to it
> >> >
> >> >              B.2.1. Regular SID is used as destination
> >> >              B.2.2  Compresses SIDs are used in a container
> >> >              B.2.3  Compresses SID is used
> >> >
> >> > So within all checksum related discussions so far it seems that the
> only concern is about B.2.2 and perhaps B.1 however folks did state that if
> there is SRH added there is no issue so I am not sure how the presence of
> SRH fixes it.
> >> >
> >> > Maybe there was some assumption that presence of SRH mandates
> encapsulation, but I do not believe this is the case for native SRv6 hosts.
> >> >
> >> > All in all I think it should be no business for transit nodes to
> verify packet's upper layer checksum. I do not know if there is any RFC
> which would describe what is an expected behavior for transit nodes or even
> say that they MAY do it.
> >>
> >> Robert,
> >>
> >> I can go further than that. I believe that intermediate nodes have no
> >> business parsing into the transport layer, and yet firewalls do that
> >> all the time even though there is no standard RFC on it (I've asked
> >> for someone to formalize the requirements of firewalls, but to no
> >> avail). Validating the checksum in flight is an instance of this, and
> >> there are devices that commonly do this in deployment. Protocol
> >> specific checksum offload in NICs is one example. Also, if someone is
> >> seeing checksum failures in their network, an obvious action is to
> >> sample packets from routers in the path and look at the traces. If the
> >> checksum is incorrect on the wire because of SRH then the operator
> >> sees a whole bunch of checksum errors at the router, but has no way to
> >> distinguish those packets that are actually good from those that are
> >> bad.
> >>
> >> It's a long established convention in IP that the transport checksum
> >> is maintained to be correct on the wire-- this is done in NAT by
> >> adjusting the checksum directly, there's also checksum neutral NAT
> >> that adjusts another part of the IPv6 header to keep the transport
> >> layer checksum correct. IMO, deviating from this convention is risky,
> >> not just to SRH packets but that can have collateral damage like
> >> breaking the user's ability to debug bad links as I described above.
> >>
> >> Tom
> >>
> >> >
> >> > Kind regards,
> >> > Robert
> >> >
> >> >
> >> >
> >> > On Thu, Mar 28, 2024 at 1:06 PM Alvaro Retana <[e-mail]>
> wrote:
> >> >>
> >> >> Focusing on the C-SID draft, some have suggested requiring the
> >> >> presence of the SRH whenever C-SIDs are used. Please discuss whether
> >> >> that is the desired behavior (or not) -- please be specific when
> >> >> debating the benefits or consequences of either behavior.
> >> >>
> >> >> Please keep the related (but independent) discussion of requiring the
> >> >> SRH whenever SRv6 is used separate. This larger topic may impact
> >> >> several documents and is better handled in a different thread (with
> >> >> 6man and spring included).
> >> >>
> >> >> Thanks!
> >> >>
> >> >> Alvaro
> >> >> -- for spring-chairs
> >> >>
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