[spring] Re: My question at the mike about draft-dong-spring-srv6-inter-layer-programming

["Dongjie (Jimmy)" <[e-mail]>]

Hi Sasha,

Thanks a lot for your comments during the SPRING session and in the follow-up mails. Please see some replies inline with [Jie]:

From: Alexander Vainshtein [mailto:Alexander.Vainshtein=[e-mail]]
Sent: Sunday, July 28, 2024 1:05 AM
To: Dongjie (Jimmy) <[e-mail]>; [e-mail]
Cc: [e-mail]
Subject: Re: My question at the mike about draft-dong-spring-srv6-inter-layer-programming

Jie,
Lots of thanks for your email.

First, I would like to apologize for not responding to your emails earlier.

[Jie] No problem, it is always good to know your opinion no matter early or late:)


I think that there is a certain mismatch of terminology that have resulted in misunderstanding.

[Jie] I also realized this, and have checked with my colleague who is closer to the implementation.


In my (and, AFAIK, relatively common) terminology frames received from a Layer 2 logical interface are disposed based solely in their L2 header. (In the case of Ethernet this would include Destination and Source MAC addresses and zero, one or two VLAN tags - but not the "true" Ethertype that follows these tags. Other L2 media uses similar arrangements). I.e., the node that receives Ethernet frames from what it considers a L2 interface cannot differentiate between, say, IPv4, ARP, IPv6 and MPLS.

[Jie] Agreed, although to my understanding MPLS sits between layer 2 and layer 3.


It is the ability to differentiate between different protocols based on the "true" Ethertype and, say, look up the Destination IPv6 address in the appropriate FIB (which is required for SRv6) that makes an interface a L3 one from my POV.

[Jie] Agree that for an interface to process SRv6 packets, it needs to be associated with some L3 functionality. While it does not need to be a full L3 interface which is visible in the routing topology. In other word, the L3 adjacency is better to be avoided. We can add some clarification in next update.


Regarding your concern about L3 interfaces involving adjacencies, I also think this is unfounded. It is quite easy to make an IGP adjacency unusable for normal IP forwarding by assigning maximum cost to the corresponding link -but using Adj-SIDs allocated and advertised for such a link in SR-TE policies that are set up by the appropriate controller.

[Jie] The primary goal here is not about how to make an IGP adjacency unusable in IP routing, it is about how to avoid the challenge and cost of establishing IGP adjacency between two remote endpoints (they can even belong to different IGP domains). As since the underlay interface and path can be created/deleted dynamically, adding such link using Adj-SIDs to IGP would also impact the protocol stability.

Thus it would be beneficial to distinguish it from the L3 Adj-SIDs/End.X SIDs in SPRING, and its advertisement can also be separate from the control plane mechanisms for Adj-SIDs/End.X SIDs.

Hope this helps to clarify the intent of this effort.

Best regards,
Jie


Hopefully these notes will be useful


Regards,
Sasha



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________________________________
From: Dongjie (Jimmy) <jie.dong=[e-mail]<mailto:jie.dong=[e-mail]>>
Sent: Saturday, July 27, 2024 7:59:30 AM
To: Alexander Vainshtein <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Subject: [EXTERNAL] Re: My question at the mike about draft-dong-spring-srv6-inter-layer-programming

Hi Sasha,

Thanks for your question at the mic. Please see some replies inline:

________________________________________
From: Alexander Vainshtein <Alexander.Vainshtein=[e-mail]<mailto:Alexander.Vainshtein=[e-mail]>>
Sent: Saturday, July 27, 2024 5:27
To: [e-mail]<mailto:[e-mail]>
Cc: [e-mail]<mailto:[e-mail]>
Subject: [spring] My question at the mike about draft-dong-spring-srv6-inter-layer-programming

Hi all,
Just repeating the question about the draft<https://datatracker.ietf.org/doc/html/draft-dong-spring-srv6-inter-layer-programming-08> I've asked at he mike at the SPRING WG session today.


* Suppose that there is an underlay link between a pair of IP nodes that is not "visible in he L3 topology". To me this means that there no P-capable (logical) interfaces associated with the endpoints of this underlay link

[Jie] The interface of the underlay link is not L3-capable, while it still can have some packet processing capability. You may consider it as a layer-2 logical interface.


* Suppose further that one of these nodes (the upstream one) allocates and advertises an SID with End.XU behavior for this underlay link

* The upstream node receives an IPv6 packets with the tops SRv6 SID on it being the End.XU. It strips this SID (this the common behavior of all End-like SIDs) and send the resulting IPv6 packet across the link to the downstream node/\.
Now the question: How should the downstream node process the received packet if its local endpoint of the undelay link s not associated with an IP-capable logical interface?

[Jie] Similar to what I said above, the receiving interface is not L3-capable, while it can receive and process the packet properly in layer-2, the inner L3 packet header can be processed by the node.


If the endpoints of the underlay ink are associated with L3 interfaces in both nodes, the link becomes visible in L3 topology, and a regular End.X SID can be allocated and advertised for it.

[Jie] As described in the draft, making it an L3 adjacency between the two endpoints is both challenging and unnecessary. And operator does not want this link to be visible in L3 topology. Thus regular End.X SID does not meet the requirement here.


Hope this help to answer your question.

Best regards,
Jie


Hopefully this clarifies my question.

Regards,
Sasha




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