Re: [Idr] draft-chen-idr-mbinding - BSID terminating node

[Huaimo Chen <[e-mail]>]

Hi Andrew,

    Thanks much for your further comments and suggestions.
    My responses are inline below with [HC2].

Best Regards,
Huaimo
________________________________
From: Stone, Andrew (Nokia - CA/Ottawa) <[e-mail]>
Sent: Sunday, November 13, 2022 4:01 PM
To: Huaimo Chen <[e-mail]>; [e-mail] <[e-mail]>
Cc: [e-mail] <[e-mail]>
Subject: Re: draft-chen-idr-mbinding - BSID terminating node


Hi Huaimo,



Thanks for the responses. I've added additional replies below with [Andrew], basically just captures what we discussed on the mic at the PCE WG Friday session (which took place after the email exchange) - reapplying below for this thread.



As discussed in PCE WG session, seems to me there should be a solution document discussing the Node Protection for BSIDs and defining what kind of information needs to be sent down to the neighboring node independent of protocol encoding. Either SPRING or RTGWG seems appropriate. If one doc is already in the works discussing this appreciate if you can steer me to it.

[HC2]:  Draft "SR-TE Path Midpoint Restoration" below talks about the information about the Node Protection for BSIDs

https://datatracker.ietf.org/doc/draft-hu-spring-segment-routing-proxy-forwarding/

[https://www.ietf.org/lib/dt/8.17.0/ietf/images/ietf-logo-card.png]<https://datatracker.ietf.org/doc/draft-hu-spring-segment-routing-proxy-forwarding/>
draft-hu-spring-segment-routing-proxy-forwarding-20 - SR-TE Path Midpoint Restoration - Internet Engineering Task Force<https://datatracker.ietf.org/doc/draft-hu-spring-segment-routing-proxy-forwarding/>
Segment Routing Traffic Engineering (SR-TE) supports explicit paths using segment lists containing adjacency-SIDs, node-SIDs and binding- SIDs. The current SR FRR such as TI-LFA provides fast re-route protection for the failure of a node along a SR-TE path by the direct neighbor or say point of local repair (PLR) to the failure. However, once the IGP converges, the SR FRR is no longer ...
datatracker.ietf.org
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Thanks

Andrew



From: Huaimo Chen <[e-mail]>
Date: Thursday, November 10, 2022 at 10:41 PM
To: "Stone, Andrew (Nokia - CA/Ottawa)" <[e-mail]>, "[e-mail]" <[e-mail]>
Cc: "[e-mail]" <[e-mail]>
Subject: RE: draft-chen-idr-mbinding - BSID terminating node



Hi Andrew,



Thank you very much for your comments.

My responses are inline below with [HC].



Best Regards,

Huaimo

From: Idr <[e-mail]> On Behalf Of Stone, Andrew (Nokia - CA/Ottawa)
Sent: Monday, November 7, 2022 6:33 PM
To: [e-mail]
Subject: [Idr] draft-chen-idr-mbinding - BSID terminating node



Hi IDR, Authors



Following up with my comment at the mic in the IDR meeting earlier today. The below would also apply to the sibling draft in PCE-WG. During the meeting it was confirmed the BSID SL is not provided to the neighbor node, however:



[HC]: A BSID is associated with a list of SIDs for a path segment. When BGP sends the

BSID with the list of SIDs to a node named N, BGP also sends the BSID with the list of SIDs

plus the ID of node N to the neighbors of N.



[Andrew] ACK. Thanks for clarifying the intention. Recommend the document clarifying that a SID list is indeed pushed down to the neighbor nodes and not just the node id + bsid value.



[HC2]: Will add some text into the document accordingly.





  1.  How does a node providing protection, know where the BSID tunnel terminates on?



[HC]: If the first SID in the list is a node SID, when node N failed, the neighbor node of N will

replace the BSID with the list of SIDs in the packet after the SID for node N is removed/popped

and sends the packet  to the first SID in the path segment bond to BSID.



[Andrew] ACK. Will discuss more below….





For example, given the following network path: [A]--100--[B]--200--[C]--300--[D]--400--[E]--500--[F]



Where A,B,C,D,E,F are nodes and 100, 200, 300, 400, 500 are local adjacency-sids.



BSID 1000 is deployed to node C with SL: 300, 400.



Therefore a headend tunnel is programmed on A with SL: 100, 200, 1000, 500.



As per the draft, we want node protection for node C thus protect BSID 1000. When programming its neighbor [B] the document says to only inform node B:  {C, 1000}



[HC]: If the first SID in the list is an adjacency SID (e.g., adjacency 300 for the adjacency from C to D),

when node N (e.g., node C here)  failed, the neighbor node(e.g., node B)  of N (e.g., node C) will

get the node SID of next hop node  (i.e., node D) using the adjacency SID 300, and replace the BSID

1000 with the node SID of next hop node (i.e., node D) and the rest of the SIDs in the list,

and sends the packet  to the first SID (i.e., the node SID of node D) in the path segment bond to BSID.



[Andrew] I think this creates a few implied assumptions which may not always be feasible. The first is an assumption that the neighbor node can and must resolve the next-node for first SID within the BSID SL. If this a simple same instance, same area neighbor, then yes the node can peak into IGP to resolve it as you describe. For non-igp flooded and received SIDs then it's not feasible. The second assumption is that the neighboring node must be able to push at least the same number of SIDs that were embedded within the BSID itself, which might not be feasible. Third, it assumes no micro loops may form during reconvergence by using the node sid of the next hop node. To work around that, we may need to push yet-another SID to reach that next node loop free, thus requiring an MSD capability equal to the BSID SL length + 1. It's likely worth asking: Is the goal to follow 100% the same encoding beyond the first hop, or simply replace the BSID with an instruction list that gets the delivery of the traffic to the BSID terminating node so that packet forwarding can continue?  Given all of the above, since PCE/Controller is programming the BSID and notifying the neighbors of the BSID, why not have PCE/Controller a compute the SID list for the neighboring node to replace the BSID with upon node failure?



[HC2]: There are a few ways in which a network is configured. When a network is configured to run IGP, the neighbor node can resolve the next-node for the first SID within the BSID segment list. When a network is configured to run BGP or PCE, BGP or PCE can help the neighbor node resolve the next-node for the first SID within the BSID segment list.

Regarding to the MSD capability, the neighbor node should do as much as possible. If it can push the segment list for the BSID, it should push the list.  If it can only push part of the segment list (i.e., the last few SIDs in the segment list), it should push them. Note that the top SID is the node SID. At an extreme or worst case, it should push one SID, which is the last SID in the segment list (if the last SID is adjacent SID, the node SID of the next hop node for the adjacency needs to be found and pushed).

It seems that there is no loop. The node protection we are talking about is for the period after a node failed and the IGP has converged. After the IGP has converged, the path (repairing path) getting around the failed node is the shortest path after the convergence, there should not be any loop if there is not another new failure.



How will B know BSID 1000 terminates on E?  If B only peaks into the next-sid in the stack (500), that value is of local significance to E only thus does not actually know it should send to E.



[HC]: When node D receives the packet, node D can process its adjacency SID 400 (for adjacency

From D to E), and sends the packet to node E.



  1.  It could be possible for BSID 1000 SL to also contain BSIDs in a nested manner, thus further masking where the BSID actually terminates. As well, the next SID in the path (ex: 500) could also be a local BSID too.

[HC]: BSID 500 is for node E, right? If so, when node E failed, its neighbor node D will execute the

Procedure similar to that executed by node B for node C’s failure.



  1.  I did not not mentioned at the mic: the BSID terminating node may be in another domain/IGP instance not within view of the protecting node as a valuable use case is BSIDs on borders.

[HC]: For multiple domains and BSID on the border of the domains, this case is an egress protection

(the border node is the egress of the upstream domain) for protecting the failure of the egress (i.e.,

the border node).  A backup egress node is selected or configured to protect the (primary) egress

node. The backup egress node can get the information about the path segment associated with BSID,

and sends the packet along the path segment associated with the BSID.



[Andrew] As discussed in PCE WG, I think the egress protection case is quite different than protecting a BSID which happens to be on a border node. In the egress protection case, the packet is being de-encapsulated from the end to end tunnel. In the BSID case on border case, the packet is still encapsulated and in flight within the tunnel. While it's egressing the domain, it's not actually egressing the tunnel, thus there’s not really a primary / backup node scenario.



[HC2]: One border node B, which is adjacent to the possible failed border node N with BSID and in the same domain or area as N, can provide protection for N regarding to BSID. This border node B can replace the BSID with the segment list after N failed.



It seems to me at minimum: the node identifier of where the BSID terminates (E in example above) is required, but that would still not be sufficient to solve [3].

[HC]: As my explanation above, it seems that these are resolved.



Thanks

Andrew