[Idr] Re: draft-lp-idr-bgp-ls-sr-policy-supplement - Interaction between "pure backup path" and the reverse path segment

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Hi Sue,

Thanks a lot for sending your comments to the list.
From my understanding, the basic question is, what's the relationship between a backup SID list and a reverse SID list. I think they are two standalone types, there would not be a SID list acting as both the backup SID list and the reverse SID list at the same time.

A                  B
  ----list-1---->
  ----list-2---->
  <-- list-3----
  <-- list-4----	 
Taking a candidate path CP1(from the headend A to the endpoint B) as an example, in CP1, there're SID list-1 and list-2, list-2 is the backup path of list-1. For the reverse path of list-1, there's a list-3, which could be set via the MULTIPATH-OPPDIR-PATH TLV in draft-ietf-pce-multipath or Reverse Path Segment List Sub-TLV in draft-ietf-idr-sr-policy-path-segment.
So list-2 would still be a path from A to B, while list-3 is a reverse path from B to A. They are totally different paths. 

And another consideration is, if list-2 is set as the backup for list-1 from A to B, would list-2 acting as the reverse path of a list-4 (which is a path from B to A) at the same time ?  Currently, there's no draft saying anything about that specially. But from the definition of the backup path we can see, the backup path should not carry any other traffic besides the traffic carried originally in the primary path, so it should not acting as a reverse path of a third SID list.  

Back to the B-flag in SR Segment List TLV introduced in this document, if a SID list is reported via BGP-LS with the B-flag set, it is a pure backup path, and it should not be used for other purpose, including being used as a reverse path.   

Thanks,
Yao


Original


From: SusanHares <[e-mail]>
To: idr <[e-mail]>;
Date: 2025年11月08日 01:28
Subject: [Idr] draft-lp-idr-bgp-ls-sr-policy-supplement - Interaction between "pure backup path" and the reverse path segment

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Question: 
How does the B-Flag interact on the segment list with 
the concepts of the reverse path segment from 
draft-ietf-idr-sr-policy-path-segement-14. 
 
See discussions below. 
 
Sue 
 
draft-ietf-idr-sr-policy-path-segment-14 defines 
 
This provides a forward and reverse path. 

 
 
       SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
           Attributes: Tunnel Encaps Attribute (23)
           Tunnel Type: SR Policy
               Binding SID
               Preference
               Priority
               Policy Name
               Explicit NULL Label Policy (ENLP)
               Segment List
                   Weight
                   Path Segment
                   Segment
                   Segment
                   ...
                   Reverse Segment List
                       Path Segment
                       Segment
                       Segment
                       ....
 
 
draft-lp-idr-bgp-ls-sr-policy-supplement-03
 
Defines new flags for the Segment list: 
 
                       0                   1
                         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        | | | | | | | | | |S|B|         |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
        Figure 1: New Flags in the Flag Field of SR Segment List TLV
 
   *  S-Flag: Indicates the segment list is in administrative shut state
      when set.  The segment list may be shut by the administrator via
      CLI or other methods, and it is out of the scope of this document.
 
   *  B-Flag: Indicates that the segment list is a pure backup path as
      specified in [I-D.ietf-pce-multipath] section 4.4 when set.  When
      B-Flag is clear, it indicates it is the primary path that carries
      normal traffic
 
 
From draft-ietf-pce-multipath, we have two definitions: 
MULTIPATH-BACKUP TLV and MULTIPATH-OPPDIR-PATH.  
 
Below is the text from draft-ietf-pce-multipath
 
   This functionality is not part of the SR Policy Architecture
   [RFC9256], but is something optional that may be implemented for
   certain specialized use cases.  One such use case is the P2MP SR
   Policy [I-D.draft-ietf-pce-sr-p2mp-policy].
 
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Backup Path Count       |             Flags           |B|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID 1                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID 2                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID n                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                   Figure 3: MULTIPATH-BACKUP TLV format
 
From draft-ietf-pce-multipath ha the following opposite direction TLV? 
 
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Reserved            |             Flags         |L|N|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opposite Direction Path ID                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                 Figure 4: MULTIPATH-OPPDIR-PATH TLV format