[spring] Re: [EXTERNAL] Re: WG Adoption Call for draft-dong-spring-srv6-inter-layer-programming

[Alexander Vainshtein <[e-mail]>]

Hi all,
I’d like to bring your attention the following text fragment from Section 3 of draft-dong that complements the definition of End.IL procedures in Section 3 of draft-dong<https://datatracker.ietf.org/doc/html/draft-dong-spring-srv6-inter-layer-programming-11#section-3> :

When forwarding packets through the underlay network connection towards the remote endpoint node, depending on the type of connection, layer-2 encapsulation may be required. The information needed for layer-2 encapsulation may be provisioned via mechanisms such as static Neighbor Discovery (ND) cache or using specific MAC address.

I assume that the  “ND cache” mentioned in this fragment  is the Neighbor Cache as defined in Section 5.1 of RFC 4861<https://datatracker.ietf.org/doc/html/rfc4861#section-5.1> and copied here for your convenience.

      Neighbor Cache
                   - A set of entries about individual neighbors to
                     which traffic has been sent recently.  Entries are
                     keyed on the neighbor's on-link unicast IP address
                     and contain such information as its link-layer
                     address, a flag indicating whether the neighbor is
                     a router or a host (called IsRouter in this
                     document), a pointer to any queued packets waiting
                     for address resolution to complete, etc.  A
                     Neighbor Cache entry also contains information used
                     by the Neighbor Unreachability Detection algorithm,
                     including the reachability state, the number of
                     unanswered probes, and the time the next Neighbor
                     Unreachability Detection event is scheduled to take
                     place.

Please note the reference to the neighbor’s on-link unicast IPv6 address in the quoted text.

If my assumption above is correct, then the local endpoint of the so-called “network-connection” becomes undistinguishable from any other IPv6-capable interface, and any difference between End.X and End.IL disappears.


My 2c,
Sasha

From: [e-mail] <[e-mail]>
Sent: Friday, April 18, 2025 8:58 PM
To: [e-mail]
Cc: [e-mail]; [e-mail]; Zafar Ali (zali) <[e-mail]>; Alvaro Retana <[e-mail]>; SPRING WG <[e-mail]>
Subject: [EXTERNAL] [spring] Re: WG Adoption Call for draft-dong-spring-srv6-inter-layer-programming

Hi Minxue,

I have clarification questions.

Looking at the specification of End.IL and End.X, the only difference seems to be

End.IL:
   S15.   Send the packet through the underlay network connection
          identified by S.

End.X
   S15.   Submit the packet to the IPv6 module for transmission
          to the new destination via a member of J


Is that a correct understanding?

If so, are those different words to express the same forwarding behavior?
Otherwise, could you point out the technical difference in term of data plane behavior?
Please distinguish the difference which are required to the signaled to the source (i.e., why the source would have an incorrect behavior if End.X  was signaled instead of End.IL)

Since some of the discussion focuses on the term “layer 3 adjacency” from End.X, would it work if instead of defining End.IL, your document would extend End.X as below:

OLD:  Any SID instance of this behavior is associated with a set, J, of one or more L3 adjacencies.
NEW: Any SID instance of this behavior is associated with a set, J, of one or more L3 adjacencies or network connections.

Thanks,
Best regards,
--Bruno

From: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Sent: Thursday, April 10, 2025 8:16 AM
To: Zafar Ali (zali) <[e-mail]<mailto:[e-mail]>>; Alvaro Retana <[e-mail]<mailto:[e-mail]>>; SPRING WG <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; Zafar Ali (zali) <[e-mail]<mailto:[e-mail]>>
Subject: Re: Re: [spring] WG Adoption Call for draft-dong-spring-srv6-inter-layer-programming

Hi Zafar,

Thanks for your interests and comments on this draft.

Regarding your question on whether existing SRv6 behaviors can be used, section 2 of this draft has shown the challenges in establishing L3 adjacency between the two endpoints of the underlay connection. If it is not an L3 adjacency, then SRv6 End.X behavior is not applicable, something new is needed for indicating the forwarding instruction to an non-L3 underlay connection.

Regarding your question on the implementation, section 3 of this draft provides specifications on how the layer-2 encapsulation information can be obtained. With that, S15 can be implemented.  S14 is executed on the sending side of the underlay connection, which is capable of processing IPv6 header and SRH. The egress of the underlay connection should also be capable of L3 processing. It is just the connection between them is not L3. Actually there are already implementations which proved the feasibility of this function.

Regarding your suggestion of using BSID, the binding SID (H.Encaps or End.B6.Encaps in SRv6) was used to instruct a node to encapsulate a new IPv6 header and SRH to the packet, which is quite different from the expected behavior in this inter-layer case, as no new IPv6 header or SRH should be added.

Your question on IP side debugging is not quite clear to me, you may want to elaborate on it. To me the OAM of the inter-layer paths can be something discussed in a separate document.

As a network operator who owns multi-layered networks, this function is needed for efficient inter-layer path integration, and your contribution is welcome.

Best regards,
Minxue
________________________________
-------------------------------------
王敏学/ Wang Minxue
中国移动通信研究院 基础网络技术研究所 / China Mobile Research Institute
地址: 北京市西城区宣武门西大街32号创新大厦,100053
电话: [phone]-33202
传真：[phone]
Email: [e-mail]<mailto:[e-mail]>
-------------------------------------

From: Zafar Ali (zali)<mailto:[e-mail]>
Date: 2025-04-09 07:02
To: Alvaro Retana<mailto:[e-mail]>; SPRING WG<mailto:[e-mail]>
CC: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; Zafar Ali (zali)<mailto:[e-mail]>
Subject: Re: [spring] WG Adoption Call for draft-dong-spring-srv6-inter-layer-programming
Dear author and the WG,


There was a lot of discussion on this draft, especially on the need for defining "End.IL", which is the basis of the draft.

As far as I know the discussion was not closed and authors have not established the need for defining "End.IL".

To keep myself honest, I will also respond to one of the original emails in that thread.

I am happy to be corrected if a closure was obtained.



Comments from that discussion++;



Why a locally instantiated static adjacency SID cannot be used?



The reason given was this is a non-IP link but then the question is how I will implement the following code in the (IPv6) packet path



   S14.   Update IPv6 DA with Segment List[Segments Left]

   S15.   Send the packet through the underlay network connection

          identified by S.

   S16.   }



How would I implement S15.

To implement S15, I need some local construct to forward the digitally encoded packet on the optical link S.

That local construct can very well be a locally instantiated static adjacency SID.



It is also not clear how the receiving side processes the “optical signal” to continue processing of the IPv6 packet (i.e., how to implement the receive side of S14). Again, you need a packet termination endpoint for it to work.



•         There was discussion on the packet termination part does not have IP address associated with it.

o   Use of unnumbered interface was suggested.



If the true need to “hide” optical interfaces behind “S” – use of BSID provides much better construct for "abstraction" of optical network/ interfaces to packet network was done here, as suggested in the following draft.

https://datatracker.ietf.org/doc/html/draft-anand-spring-poi-sr-08#section-5<https://datatracker.ietf.org/doc/html/draft-anand-spring-poi-sr-08#section-5>



The way the draft tries to hide optical interface looks like a Layer violation.

•         How do I debug IP side if the END.IL is mis-forwarding – assume I can implement it.



As the authors have not established the need for END.IL and hence the draft, I respectfully object to the adoption call.

•         For the reason mentioned above, I do not know how to implement End.IL as it is defined or if it is at all needed (see comment above)

•         I am happy to participate in the closure of any gap but in its current state the draft is not ready for adoption.

Thanks

Regards … Zafar

From: Alvaro Retana <[e-mail]<mailto:[e-mail]>>
Date: Wednesday, April 2, 2025 at 3:06 PM
To: SPRING WG <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>, [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Subject: [spring] WG Adoption Call for draft-dong-spring-srv6-inter-layer-programming

Dear WG:

This message starts a two-week adoption call for draft-dong-spring-srv6-inter-layer-programming, ending on April/16. From the Abstract:

   Following the SRv6 Network Programming concept, this document defines
   SRv6 based mechanisms for inter-layer network programming, which can
   help to integrate the packet network layer with its underlying layers
   efficiently.


   https://datatracker.ietf.org/doc/draft-dong-spring-srv6-inter-layer-programming/<https://datatracker.ietf.org/doc/draft-dong-spring-srv6-inter-layer-programming/>


Please review the draft and consider whether you support its adoption by the WG. Please share any thoughts with the list to indicate support or opposition -- this is not a vote.

If you are willing to provide a more in-depth review, please state it explicitly to give the chairs an indication of the energy level in the working group willing to work on the document.

WG adoption is the start of the process. The fundamental question is whether you agree the proposal is worth the WG's time to work on and whether this draft represents a good starting point. The chairs are particularly interested in hearing the opinions of people who are not authors of the document.


Thanks!

Alvaro (for the Chairs)

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