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

[Alexander Vainshtein <[e-mail]>]

Rishabh,
Lots of thanks for a prompt and very encouraging response.
Please see more inline below marked [[Sasha]].

Regards,
Sasha

From: Rishabh Parekh <[e-mail]>
Sent: Sunday, December 11, 2022 9:04 PM
To: Alexander Vainshtein <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; SPRING WG <[e-mail]>; James Guichard <[e-mail]>; Nitsan Dolev <[e-mail]>; Michael Gorokhovsky <[e-mail]>; Ron Sdayoor <[e-mail]>
Subject: [EXTERNAL] Re: [spring] WGLC for draft-ietf-spring-sr-replication-segment

Sasha,
Inline @ [RP]

On Sun, Dec 11, 2022 at 3:43 AM Alexander Vainshtein <[e-mail]<mailto:[e-mail]>> wrote:
Hi all,
More questions regarding the definition of the Replication State of a Replication Segment:

The draft states that:

If a Downstream Node is an egress (aka leaf) of the multi-point service, i.e. no further replication is needed, then that leaf node's Replication segment will not have any Replication State…

IMHO this is not aligned with the definition of the elements of the Replication Segment (already quoted below) and Replication State. It would be more appropriate to express the behavior of a Leaf node by stating that its Replication state (defined as quoted below) is an empty list of branches.

With this approach a Replication SID of a bud node of a service could be defined as  an SID with the Replication state including,  as one of the branches – but not the only branch in the list - a Replication SID with itself as the Node-ID and with an empty list of branches.

[RP] The text " leaf node's Replication segment will not have any Replication State…" effectively means an empty list of branches with an indicator of Leaf role in a Replication Segment.
[[Sasha]] My reading of the above is that we agree on the actual meaning of the text of the draft. From my POV, explicitly stating this meaning would help the readers, but this is a matter of taste.
A bud node is a combination
My question is:

Can the same "Downstream Replication ID in a given "Downstream Node"  be included/ in the Replication state of multiple Replication segments, especially of Replication segments identified by different Node IDs?

[RP] I assume you meant to ask  "Can the same Downstream Replication SID in a given ...." .
[[Sasha]] Yes, of course I meant SID.


I have not found any answer to this question in the text of the draft. At the same time I think that a positive answer to my question would contradict the definitions in Section 3.1 of the MVPN and EVPN with SR P2MP and Ingress Replication draft<https://clicktime.symantec.com/15t5ZsGQy3PKVC5rnKF2J?h=0xFR-Lib2WgWUxY77NS9dntWdLj3ZVPPXNXYgwA0hG0=&u=https://datatracker.ietf.org/doc/html/draft-ietf-bess-mvpn-evpn-sr-p2mp-06%23name-mpls-label> because these definitions assume that the Replication SID can be used as the "context label" for resolving the context label space space for looking up the upstream allocated label advertised in the PTA attribute of the suitable Mutlicast VPN route (in the case of aggregated P-tunnels).

IMHO and FWIW an explicit and unambiguous answer to my question should be provided by the authors in order to advance the draft.

[RP] The answer is theoretically yes, but that would be like trying to share a Replication Segment across different SR-P2MP trees (described in PIM WG SR P2MP draft). This requires a complex set of conditions that have to be met to make it work and hence we have kept it out of scope of this draft, PIM SR P2MP draft and the BESS MVPN/EVPN draft.

Just to clarify, Section 3.1 of BESS MVPN/EVPN covers the case where a single SR-P2MP tree is shared across two or more MPVNs and hence an upstream (or centrally) assigned "context" is required in packet encoding to resolve the MVPN in which payload is processed. When a SR-P2MP tree is associated with only one MPVN, this upstream "context" not required in packet; the Replication SID at Leaf/Bud is sufficient to provide the MVPN context in which payload is processed.

[[Sasha]] My reading of your response is as following:
In principle It is possible to create MP2MP SR Policies by including Downstream Replication SID in the Replication State of multiple Replication SIDs defined in different upstream nodes.  However, such usage is intentionally left out of scope of this draft as well as the PIM SR P2MP draft and the BESS MVPN/EVPN draft.

If this reading is correct, I suggest adding a corresponding clarification in the text of the corresponding drafts, especially in the text of the BESS MVPN/EVPN one because at least MVPN deals with MP2MP P-tunnels from Day 1 (not sure about EVPN).

I hope I have answered your questions,
-Rishabh



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