Re: [spring] WGLC for draft-ietf-spring-sr-replication-segment

[Rishabh Parekh <[e-mail]>]

James,
Replies inline @ [RP]

On Wed, Feb 15, 2023 at 6:58 AM James Guichard <
[e-mail]> wrote:

> Hi Rishabh, Authors, & WG:
>
>
>
> Having reviewed the latest version of
> https://datatracker.ietf.org/doc/draft-ietf-spring-sr-replication-segment/
> <https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fdraft-ietf-spring-sr-replication-segment%2F&data=05%7C01%7Cjames.n.guichard%40futurewei.com%7C60f2bcc04ef24f5ec5b908db0ee6f572%7C0fee8ff2a3b240189c753a1d5591fedc%7C1%7C0%7C638120157363761147%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=uCFEzdCq5sYQCRvqLbpb%2Bcd6DJK5WXyNBHLutSDLPXc%3D&reserved=0>
> I would appreciate some clarification from the authors on the specifics of
> packet replication and forwarding between the replication point and
> downstream nodes. The draft as I read it bases forwarding at a replication
> point on the combination of a replication SID which triggers and selects
> the behavior and the replication state held at that node. The replication
> state indicates which downstream nodes the packet should be replicated to
> and those nodes may or may not be adjacent to the replication node. In the
> non-adjacent case my understanding is that the replication state may
> include an additional segment-list and this seems to be what the text in
> section 2.2. is saying by referencing H.Encaps.Red to re-encapsulate the
> packet with a new SRH and outer IPv6 header. If this is correct could it be
> made more explicit; at a minimum I would expect to see a reference to RFC
> 8986 section 5.2.
>

[RP] Your understanding is correct. We can add a reference to RFC 8986
Section 5.2 as you suggest, but you say "... could it be made more explicit
..". Do you mean the current text is not clear about this?

>
>
> In addition to this I would like to clarify the case where
> re-encapsulation is not needed i.e. when an explicit path to a downstream
> node is not necessary and best path forwarding suffices. The text says that
> in this case the outer IPv6 header is re-used and the downstream
> replication SID is written into the IPv6 header destination address. This
> address is most likely NOT contained within the SRH which is a detachment
> from the normal SRv6 forwarding case and I would like to hear the authors
> and WGs opinions on this.
>

[RP] Yes, an encapsulation is not needed when a Downstream node is adjacent
or best path forwarding to a non-adjacent node is sufficient. The
downstream node's Replication SID (from Replication State) is written in
outer IPv6 DA and packet is forwarded based on the locator of the
downstream node. Our (i.e. authors) opinion is that is permissible within
the SRv6 architecture by new End.Replication behavior (associated with
incoming local Replication SID) defined in the draft. Furthermore, there is
already precedence in SRv6 architecture to process an incoming packet based
on local state and forward the modified packet. RFC 8986 defines
End.B6.Encaps and End.B6.Encaps.Red (and End.BM) functions that rely on
local SR policy state to modify an incoming packet.

Thanks,
-Rishabh