[Pce] Multipath / replication segment comment

["Stone, Andrew (Nokia - CA/Ottawa)" <[e-mail]>]

Hi all,

In Singapore I made a remark about draft-koldychev-pce-multipath that it’s a helpful draft and is also applicable to the replication segment.

I received a follow up question emailed directly, asking about whether “EROs need to share same source and destination” and how/if this could be related to RFC 8623.

For openness, sending my thoughts/comments here to the WG:

There is no requirement listed in draft-koldychev-pce-multipath that I can see which requires EROs to terminate on the same source/destination, I haven’t seen that expectation anywhere, and in my opinion there should not be. For example, one of the use cases of draft-koldychev-pce-multipath is for SR Policy to support multiple SID lists, combine that with use case such as SR-EPE, you could have multiple SID lists and have weighted ECMP traffic out different egress nodes intentionally to load balance across different peer nodes. Another example, with ingress replication, is the multipath ERO can also be re-used to describe the egress downstream paths which will be going to different receiver(s), for either further replication or consumption.

My comment regarding multipath to be used for ingress replication is because there is a need in replication segment to be able to program backup paths for each egress ERO. There were comments on this in the earlier sr-replication draft in spring wg, but appears the wording has been redone / drafts are still in a state of change. None the less, the multipath backup TLV via the ERO attributes object in draft-koldychev-pce-multipath permits the relation between the normal ERO and the backup (PCE computed) ERO, something that the current RFC 8623 does not. There’s a desire to build this into replication segment and draft-hsd-pce-sr-p2mp-policy-01  is leveraging this construct (probably need further remarks on this in the drafts to describe this intention). Comparing to RFC 8623, considering all of the nuances of replication segment (p2mp-lsp-identifier-tlv, replication-sid/binding-label, backup eros) it seems reasonable to me that draft-hsd-pce-sr-p2mp-policy defines the replication segment (draft-hsd-pce-sr-p2mp-policy-01 section 3.3) while leveraging existing/other common constructs, and defining it’s behaviour, rather than trying to just use all of RFC8623 and attempt to update and squeeze in (or out) other elements of the RFC.

Cheers
Andrew