Re: [spring] The SPRING WG has placed draft-voyer-spring-sr-replication-segment in state "Candidate for WG Adoption"

["Jeffrey (Zhaohui) Zhang" <[e-mail]>]

Hi,

Please see some clarifications below.

-----Original Message-----
From: Alexander Vainshtein <[e-mail]> 
Sent: Wednesday, November 13, 2019 5:12 PM
To: <[e-mail]> ([e-mail]) <[e-mail]>; [e-mail]
Cc: [e-mail]
Subject: RE: [spring] The SPRING WG has placed draft-voyer-spring-sr-replication-segment in state "Candidate for WG Adoption"

Hi all,

I have a question regarding adoption of draft-voyer-sr-spring-replication-segment as a SPRING WG document.

These concerns are based on the following:

1.       This draft (both based on its title and on its content) deals with local (in the Root node) ingress replication which, in its turn, is one of the issues that could be used for delivery of multicast.

Zzh> The draft deals with replication at ANY node: an incoming packet is replicated to a few downstream nodes.
Zzh> If that is used on a root node (of a replication tree) and all the downstream nodes are leaves of the replication tree, then it is "ingress replication", e.g. the ingress replication used for MVPN/EVPN.

2.       Local ingress replication is mentioned in the SPRING WG Charter as one of the "New types of segments mapping to forwarding behavior". The charter further says that "Any of the above <Sasha: New types of segments> may require architectural extensions"

Zzh> It's not clear what "local ingress replication" means exactly. I can understand "ingress replication" as used for MVPN/EVPN, and I can understand "local replication" (I interpret it as what this draft tries to define).
Zzh> This draft is indeed about architecture extension on replication (or "local replication"), and "ingress replication" is a special kind of replication as I mentioned above.

3.       The current (and, AFAIK, the only existing) Segment Routing Architecture document (RFC 8402<https://urldefense.com/v3/__https://tools.ietf.org/html/rfc8402__;!8WoA6RjC81c!R5uaL0FFboU6ryB3g53QkJpHP6nLdJ_YH6_lnAt0_THeoVW8aF3qEAlMXaVpdJiY$ >) explicitly states in Section 6 that "Segment Routing is defined for unicast. The application of the source-route concept to Multicast is not in the scope of this document".

Zzh> That's fine; that document is about unicast, and in this document we're trying to extend the architecture to multicast/replication, focusing on basic building block "replication segment".

The combinations of observations above strongly suggests to me that a document defining multicast-related extensions of segment routing architecture should be very useful (if not mandatory) for progressing the Replication Segment draft. From my POV the Replication Segment draft is not (and is not intended to be) such a document.

I wonder if there is an intention to produce such a document in the timeframe that could be relevant for discussion of the Replication Segment draft

Zzh> I see that people have different interpretation about "multicast". I suppose we don't have different interpretation about "replication". This document is about "replication segment" as the basic building block for replication trees (whether it is IP multicast, mLDP/RSVP-TE P2MP tunnel or anything equivalent, or ingress replication).

Zzh> There is indeed a separate document "draft-voyer-pim-sr-p2mp-policy" about how to use the basic building block to build replication trees. In fact, the two documents, "draft-voyer-spring-sr-replication-segment" and "draft-voyer-pim-sr-p2mp-policy" were split from "draft-voyer-spring-sr-p2mp-policy-03" based on the input we received, so that in Spring WG we focus on the basic building block "replication segment", while leaving tree building to other WGs.
Zzh>
Zzh> Thanks.
Zzh> Jeffrey