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

[Tom Herbert <[e-mail]>]

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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