Re: [spring] C-SIDs and upper layer checksums (draft-ietf-spring-srv6-srh-compression)

[Ketan Talaulikar <[e-mail]>]

Hi Alvaro,

I find some level of confusion on the discussion threads and it might
perhaps be due to the inconsistent use of terminologies. It would help to
use the correct terminologies from RFC8754 (6man WG RFC) to bring clarity
on what is within the scope and what is beyond the scope of this document.

a) SR Source Node: the node originating the packet - it may have an SRH or
may skip it (section 4.1)
b) Transit Node: node doing IPv6 forwarding
c) (Ultimate) Destination Node (from RFC8200): the final node to which the
packet is destined

The CSID document in section 6.5 does not change or update the text in
RFC8200 sec 8.1. It is simply stating what the "final destination" is going
to be when CSID is used because RFC8200 does not talk about RHs in sec 8.1.
RFC8754 covered this aspect by specifying that the last segment is the
"final destination" but this needs to be specified when using C-SID (with
or without SRH) and for all C-SID flavors/behaviors.

I find the current text in section 6.5 to be necessary and sufficient for
implementations that claim (or need to) support SR Source Node behavior for
C-SIDs.

The CSID document does not change any behavior at the Transit Node or for
the (Ultimate) Destination Node. Therefore, the discussion of Transit Nodes
is outside the scope of this document - just as it was outside the scope
for RFC8754.

Now, if some "Special Transit Node" wants to go beyond RFC8200 and do
things like upper layer checksum validation enroute then they can refer to
the same text in section 6.5 to first understand CSID processing and to do
what is necessary for their packet processing enroute. This requires such
"Special Transit Nodes" to be aware of first SRv6 and now C-SID - this is
the same for any new packet encoding  technology.

It seems like we are putting the cart before the horse when raising
concerns about existing implementations that are not SRv6 and C-SID aware
of not being able to do their processing. Let us publish the C-SID document
so implementers of those "Special Transit Nodes" (also being referred to as
middleboxes on some threads) have a reference to upgrade for C-SID support.

Finally, I’ve not heard of issues related to these "Special Transit Nodes"
from operators that have deployed SRv6. That may be a good discussion to
have (again outside the scope of this document and perhaps in srv6ops?) -
so operators who have SRv6 deployment experience can share their learnings
and best practices.

Thanks,
Ketan


On Thu, Mar 28, 2024 at 5:34 PM Alvaro Retana <[e-mail]>
wrote:

> Section 6.5 of draft-ietf-spring-srv6-srh-compression describes the
> behavior when an originating node inside an SRv6 domain creates a
> packet with a C-SID as the final destination. This description differs
> from the text in Section 8.1 of RFC8200.
>
> We plan to send the draft to the 6man WG for review and explicitly
> highlight this difference.
>
> Please comment on the text in Section 6.5. Does anything need to be
> added, deleted, changed, or clarified?
>
> We want to ask for feedback soon; please send comments on this topic
> by April 5th.
>
> Thanks!
>
> Alvaro.
> -- for spring-chairs
>
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