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Versions: 00 01                                                         
LSR                                                         Shaofu. Peng
Internet-Draft                                                 Ran. Chen
Intended status: Standards Track                         ZTE Corporation
Expires: April 14, 2022                                Ketan. Talaulikar
                                                           Peter. Psenak
                                                           Cisco Systems
                                                        October 11, 2021


           Algorithm Related IGP-Adjacency SID Advertisement
           draft-ietf-lsr-algorithm-related-adjacency-sid-01

Abstract

   Segment Routing architecture supports the use of multiple routing
   algorithms, i.e, different constraint-based shortest-path
   calculations can be supported.  There are two standard algorithms:
   SPF and Strict-SPF, defined in Segment Routing architecture.  There
   are also other user defined algorithms according to Flex-algo
   applicaiton.  However, an algorithm identifier is often included as
   part of a Prefix-SID advertisement, that maybe not satisfy some
   scenarios where multiple algorithm share the same link resource.
   This document complement that the algorithm identifier can be also
   included as part of a Adjacency-SID advertisement

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 14, 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Use-cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Adjacency Segment Identifier per Algorithm  . . . . . . . . .   4
     4.1.  ISIS Adjacency Segment Identifier per Algorithm . . . . .   4
       4.1.1.  ISIS Adjacency Segment Identifier (Adj-SID) per
               Algorithm Sub-TLV . . . . . . . . . . . . . . . . . .   4
       4.1.2.  ISIS Adjacency Segment Identifier (LAN-Adj-SID) per
               Algorithm Sub-TLV . . . . . . . . . . . . . . . . . .   5
     4.2.  OSPFv2 Adjacency Segment Identifier per Algorithm . . . .   6
       4.2.1.  OSPFv2 Adj-SID per Algorithm Sub-TLV  . . . . . . . .   6
       4.2.2.  OSPFv2 LAN Adj-SID per Algorithm Sub-TLV  . . . . . .   7
     4.3.  OSPFv3 Adjacency Segment Identifier per Algorithm . . . .   8
       4.3.1.  OSPFv3 Adj-SID per Algorithm Sub-TLV  . . . . . . . .   8
       4.3.2.  OSPFv3 LAN Adj-SID per Algorithm Sub-TLV  . . . . . .   9
   5.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Deployment Considerations . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  IANA ISIS Considerations  . . . . . . . . . . . . . . . .  12
     7.2.  IANA OSPFv2 Considerations  . . . . . . . . . . . . . . .  12
     7.3.  IANA OSPFv3 Considerations  . . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  13
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Segment Routing architecture [RFC8402] supports the use of multiple
   routing algorithms, i.e, different constraint-based shortest-path
   calculations can be supported.  There are two standard algorithms,
   i.e, SPF and Strict-SPF, that defined in Segment Routing
   architecture.  For SPF, the packet is forwarded along the well known
   ECMP-aware Shortest Path First (SPF) algorithm employed by the IGPs.
   However, it is explicitly allowed for a midpoint to implement another



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   forwarding based on local policy.  For Strict Shortest Path First
   (Strict-SPF), it mandates that the packet be forwarded according to
   the ECMP-aware SPF algorithm and instructs any router in the path to
   ignore any possible local policy overriding the SPF decision.

   There are also other user defined algorithms according to IGP Flex
   Algorithm [I-D.ietf-lsr-flex-algo].  IGP Flex Algorithm proposes a
   solution that allows IGPs themselves to compute constraint based
   paths over the network, and it also specifies a way of using Segment
   Routing (SR) Prefix-SIDs and SRv6 locators to steer packets along the
   constraint-based paths.  It specifies a set of extensions to ISIS,
   OSPFv2 and OSPFv3 that enable a router to send TLVs that identify (a)
   calculation-type, (b) specify a metric-type, and (c )describe a set
   of constraints on the topology, that are to be used to compute the
   best paths along the constrained topology.  A given combination of
   calculation-type, metric-type, and constraints is known as an FAD
   (Flexible Algorithm Definition).

   However, an algorithm identifier is often included as part of a
   Prefix-SID advertisement, that maybe not satisfy some scenarios where
   multiple algorithm share the same link resource.  This document
   complement that the algorithm identifier can be also included as part
   of an Adjacency-SID advertisement for SR-MPLS.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Use-cases

   The main use-case is that a TI-LFA backup path computed in Flex-algo
   plane may contain Adjacency Segments and require to contain an
   algorithm-aware Adjacency-SID, which can not only steer the traffic
   towards the link, but also distinguish traffic between different
   algorithms.  Benefit from this, for the protected Adjacency-SID which
   belongs to a TI-LFA path within specific Flex-algo plane, the backup
   path of such Adjacency-SID can continue to follow the algorithm
   specific constraints that is consistent with the primary path.  And,
   more enhancement treatments related with specific algorithm, such as
   statistics of traffic of different algorithms on the same link, etc,
   will be possible.






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4.  Adjacency Segment Identifier per Algorithm

4.1.  ISIS Adjacency Segment Identifier per Algorithm

   [RFC8667] describes the IS-IS extensions that need to be introduced
   for Segment Routing operating on an MPLS data plane.  It defined
   Adjacency Segment Identifier (Adj-SID) sub-TLV advertised with TLV-
   22/222/23/223/141, and Adjacency Segment Identifier (LAN-Adj-SID)
   Sub-TLV advetised with TLV-22/222/23/223.  Accordingly, this document
   defines two new optional Sub-TLVs, "ISIS Adjacency Segment Identifier
   (Adj-SID) per Algorithm Sub-TLV" and "ISIS Adjacency Segment
   Identifier (LAN-Adj-SID) per Algorithm Sub-TLV".

4.1.1.  ISIS Adjacency Segment Identifier (Adj-SID) per Algorithm Sub-
        TLV

   ISIS Adjacency Segment Identifier (Adj-SID) per Algorithm Sub-TLV has
   the following format:

        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    |     Flags     |     Weight    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Algorithm   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         SID/Label/Index (variable)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 1: ISIS Adjacency Segment Identifier (Adj-SID) per Algorithm
                                  Format

   where:

   Type: TBA1.

   Length: 6 or 7 depending on size of the SID.

   Flags: Refer to Adjacency Segment Identifier (Adj-SID) sub-TLV.

   Weight: Refer to Adjacency Segment Identifier (Adj-SID) sub-TLV.

   Algorithm: The Algorithm field contains the identifier of the
   algorithm the router uses to apply algorithm specific treatment
   configured on the adjacency.

   SID/Label/Index: Refer to Adjacency Segment Identifier (Adj-SID) sub-
   TLV.



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   For a P2P link, an SR-capable router MAY allocate different Adj-SID
   for different algorithm, if this link joins different algorithm
   related plane.

4.1.2.  ISIS Adjacency Segment Identifier (LAN-Adj-SID) per Algorithm
        Sub-TLV

   ISIS Adjacency Segment Identifier (LAN-Adj-SID) per Algorithm Sub-TLV
   has the following format:

       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    |      Flags    |    Weight     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Algorithm   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Neighbor System-ID (ID length octets)        |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   SID/Label/Index (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 2: ISIS Adjacency Segment Identifier (LAN-Adj-SID) per
                             Algorithm Format

   where:

   Type: TBA2.

   Length: Variable.

   Flags: Refer to Adjacency Segment Identifier (LAN-Adj-SID) Sub-TLV.

   Weight: Refer to Adjacency Segment Identifier (LAN-Adj-SID) Sub-TLV.

   Algorithm: The Algorithm field contains the identifier of the
   algorithm the router uses to apply algorithm specific treatment
   configured on the adjacency.

   SID/Label/Index: Refer to Adjacency Segment Identifier (LAN-Adj-SID)
   Sub-TLV.






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   For a broadcast link, an SR-capable router MAY allocate different
   Adj-SID for different algorithm, if this link joins different
   algorithm related plane.

4.2.  OSPFv2 Adjacency Segment Identifier per Algorithm

   [RFC8665] describes the OSPFv2 extensions that need to be introduced
   for Segment Routing operating on an MPLS data plane.  It defined Adj-
   SID Sub-TLV and LAN Adj-SID Sub-TLV advertised with Extended Link TLV
   defined in [RFC7684].  Accordingly, this document defines two new
   optional Sub-TLVs, "OSPFv2 Adj-SID per Algorithm Sub-TLV" and "OSPFv2
   LAN Adj-SID per Algorithm Sub-TLV".

4.2.1.  OSPFv2 Adj-SID per Algorithm Sub-TLV

   OSPFv2 Adj-SID per Algorithm Sub-TLV has the following format:

       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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Flags     |   Algorithm   |     MT-ID     |  Weight       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   SID/Label/Index (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 3: OSPFv2 Adj-SID per Algorithm Format

   where:

   Type: TBA3

   Length: 7 or 8 octets, depending on the V-Flag.

   Flags: Refer to OSPFv2 Adj-SID Sub-TLV.

   Algorithm: The Algorithm field contains the identifier of the
   algorithm the router uses to apply algorithm specific treatment
   configured on the adjacency.

   MT-ID: Refer to OSPFv2 Adj-SID Sub-TLV.

   Weight: Refer to OSPFv2 Adj-SID Sub-TLV.

   SID/Index/Label: Refer to OSPFv2 Adj-SID Sub-TLV.





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   For a P2P link, an SR-capable router MAY allocate different Adj-SID
   for different algorithm, if this link joins different algorithm
   related plane.

4.2.2.  OSPFv2 LAN Adj-SID per Algorithm Sub-TLV

   OSPFv2 LAN Adj-SID per Algorithm Sub-TLV has the following format:

       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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Flags     |   Algorithm   |     MT-ID     |    Weight     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Neighbor ID                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    SID/Label/Index (variable)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 4: OSPFv2 LAN Adj-SID per Algorithm Format

   where:

   Type: TBA4

   Length: 11 or 12 octets, depending on the V-Flag.

   Flags: Refer to OSPFv2 LAN Adj-SID Sub-TLV.

   Algorithm: The Algorithm field contains the identifier of the
   algorithm the router uses to apply algorithm specific treatment
   configured on the adjacency.

   MT-ID: Refer to OSPFv2 LAN Adj-SID Sub-TLV.

   Weight: Refer to OSPFv2 LAN Adj-SID Sub-TLV.

   Neighbor ID: Refer to OSPFv2 LAN Adj-SID Sub-TLV.

   SID/Index/Label: Refer to OSPFv2 LAN Adj-SID Sub-TLV.

   For a broadcast link, an SR-capable router MAY allocate different
   Adj-SID for different algorithm, if this link joins different
   algorithm related plane.






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4.3.  OSPFv3 Adjacency Segment Identifier per Algorithm

   [RFC8666] describes the OSPFv3 extensions that need to be introduced
   for Segment Routing operating on an MPLS data plane.  It defined Adj-
   SID Sub-TLV and LAN Adj-SID Sub-TLV advertised with Router-Link TLV
   as defined in [RFC8362].  Accordingly, this document defines two new
   optional Sub-TLVs, "OSPFv3 Adj-SID per Algorithm Sub-TLV" and "OSPFv3
   LAN Adj-SID per Algorithm Sub-TLV".

4.3.1.  OSPFv3 Adj-SID per Algorithm Sub-TLV

   OSPFv3 Adj-SID per Algorithm Sub-TLV has the following format:

       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           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Flags     |     Weight    |   Algorithm   |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   SID/Label/Index (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 5: OSPFv3 Adj-SID per Algorithm Format

   where:

   Type: TBA5

   Length: 7 or 8 octets, depending on the V-Flag.

   Flags: Refer to OSPFv3 Adj-SID Sub-TLV.

   Weight: Refer to OSPFv3 Adj-SID Sub-TLV.

   Algorithm: The Algorithm field contains the identifier of the
   algorithm the router uses to apply algorithm specific treatment
   configured on the adjacency.

   Reserved: SHOULD be set to 0 on transmission and MUST be ignored on
   reception.

   SID/Index/Label: Refer to OSPFv3 Adj-SID Sub-TLV.

   For a P2P link, an SR-capable router MAY allocate different Adj-SID
   for different algorithm, if this link joins different algorithm
   related plane.




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4.3.2.  OSPFv3 LAN Adj-SID per Algorithm Sub-TLV

   OSPFv3 LAN Adj-SID per Algorithm Sub-TLV has the following format:

       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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Flags     |     Weight    |   Algorithm   |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Neighbor ID                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    SID/Label/Index (variable)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 6: OSPFv3 LAN Adj-SID per Algorithm Format

   where:

   Type: TBA6

   Length: 11 or 12 octets, depending on the V-Flag.

   Flags: Refer to OSPFv3 LAN Adj-SID Sub-TLV.

   Weight: Refer to OSPFv3 LAN Adj-SID Sub-TLV.

   Algorithm: The Algorithm field contains the identifier of the
   algorithm the router uses to apply algorithm specific treatment
   configured on the adjacency.

   Reserved: SHOULD be set to 0 on transmission and MUST be ignored on
   reception.

   Neighbor ID: Refer to OSPFv3 LAN Adj-SID Sub-TLV.

   SID/Index/Label: Refer to OSPFv3 LAN Adj-SID Sub-TLV.

   For a broadcast link, an SR-capable router MAY allocate different
   Adj-SID for different algorithm, if this link joins different
   algorithm related plane.

5.  Operations

   The method introduced in this document enables the traffic of
   different flex-algo plane to be distinguished on the same link, so
   that these traffic can be applied with different local treatment



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   (such as providing different repair path, traffic statistics, etc)
   per algorithm.

   Depending on the implementation, operators can configure multiple
   Adacency-SIDs each for different algorithm on the same link.  One of
   the difficulties is that during this configuration phase it is not
   straightforward for a link to be included in an Flex-algo plane, as
   this can only be determined after all nodes in the network have
   negotiated the FAD.  A simple way is that as long as an IGP instance
   enable an algorithm for a level/area, all links joined to that level/
   area should allocate Adjacency-SIDs for that algorithm statically,
   however, this will waste SID resources.  Another way is RECOMMENDED
   to allocate and withdraw Adjacency-SID per algorithm dynamically
   according to the result of FAD negotiation, i.e., Adjacency-SID per
   algorithm is assigned only to those links that have joined the Flex-
   algo plane.

   The following figure shows an example of Adjacency-SID per algorithm.

           [S1]--------[D]--------[S2]
            |           |          |
            |           |          |
            |           |          |
           [A]---------[B]--------[C]

       Figure 7: Flex-algo LFA Path with Adjacency-SID per Algorithm

   Suppose that node S1, A, B, D and their inter-connected links belongs
   to FA-id 128 plane, and S2, B, C, D and their inter-connected links
   belongs to FA-id 129 plane.  The IGP metric of link B-D is 100, and
   all other links have IGP metric 1.  Both FA-id 128 and 129 use IGP
   default metric type for path calculation.  In FA-id 128 plane, from
   S1 to destination D, the primary path is S1-D, and the TI-LFA backup
   path is segment list {node(B), adjacency(B-D)}. Similarly, In FA-id
   129 plane, from S2 to destination D, the primary path is S2-D, and
   the TI-LFA backup path is segment list {node(B), adjacency(B-D)}. The
   above TI-LFA path of FA-id 128 plane can be translated to {node-
   SID(B)@FA-id128, adjacency-SID(B-D)@FA-id128}, and TI-LFA path of FA-
   id 129 plane will be translate to {node-SID(B)@FA-id129, adjacency-
   SID(B-D)@FA-id129}. So that node B can distinguish the flow of FA-id
   128 and FA-id 129 based on different adjacency-SID(B-D), and take
   different treatment of them when they are send to the same outgoing
   link B-D.








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6.  Deployment Considerations

   When multiple flex-algos are deployed in the network and they share
   the same link, multiple algorithm specific Adjacency-SIDs may need to
   be allocated on such a link, to distinguish the traffic of different
   algorithms and provide possible different treatment.

   Even if a link is only used by a single flex-algo, because the link
   always belongs to algorithm 0 by default, both the traditional
   Adjacency-SID (termd as adj-sid@algo-0) and the algorithm specific
   Adjacency-SID (termd as adj-sid@algo-x) may need to be allocated on
   that link, so that the potentional repair paths of the two Adjacency-
   SIDs can be distinguished.

   If the topology of multiple flex-algo planes, and physical topology,
   are isomorphic, that is, they contain the same nodes and same inter-
   connected links, but due to the differences between these FADs (such
   as different metric types), different repair paths will also be
   calculated on the same topology.  Therefore, multiple algorithm
   specific Adjacency-SIDs may still need to be provided on the same
   link.

   It is not recommended to bind a link to algorithm 1 (Strict SPF) and
   allocate adj-sid@algo-1.  Such Adjacency-SID is no useful.

   The operator may configure the policy on the node to turn off the
   algorithm specific processing capability for each algorithm, and the
   node will not allocate algorithm specific Adjacency-SIDs on the links
   those joined to the flex-algo plane, this is a local behavior.  As
   mentioned before, the algorithm specific processing capability can be
   further subdivided into repair path per algo, statistics per algo,
   etc.  Assuming that a node only wants to support the capability of
   repair path per algo, in this case, for an individual link, it is
   also controlled by the adjacency backup capability.  When adjacency
   backup is disabled, it will let the capablitiy of repair path per
   algo be also invalid, so the link does not need to allocate algorithm
   specific Adjacency-SIDs.

   In any case, when instantiate a segment list (such as a TI-LFA path)
   within a specific flex-algo plane, for each Adjacency Segment of that
   list, if it has a corresponding algorithm specific Adjacency-SID, the
   algorithm specific Adjacency-SID MUST be used to construct SID list;
   if it has not, traditional Adjacency-SID can be used.








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7.  IANA Considerations

7.1.  IANA ISIS Considerations

   This document makes the following registrations in the "Sub-TLVs for
   TLV 22, 23, 25, 141, 222, and 223" registry.

      +------+--------------------+----+----+----+-----+-----+-----+
      | Type | Description        | 22 | 23 | 25 | 141 | 222 | 223 |
      +======+====================+====+====+====+=====+=====+=====+
      |      | Adjacency Segment  |    |    |    |     |     |     |
      | TBA1 | Identifier per     | y  | y  | n  |  y  |  y  |  y  |
      |      | Algorithm          |    |    |    |     |     |     |
      +------+--------------------+----+----+----+-----+-----+-----+
      |      | LAN Adjacency      |    |    |    |     |     |     |
      | TBA2 | Segment Identifier | y  | y  | n  |  y  |  y  |  y  |
      |      | per Algorithm      |    |    |    |     |     |     |
      +------+--------------------+----+----+----+-----+-----+-----+

7.2.  IANA OSPFv2 Considerations

   This document makes the following registrations in the OSPFv2
   Extended Link TLV Sub-TLVs Registry.

   +-------+------------------------------------------+---------------+
   | Value | Description                              | Reference     |
   +=======+==========================================+===============+
   | TBA3  | OSPFv2 Adj-SID per Algorithm Sub-TLV     | This document |
   +-------+------------------------------------------+---------------+
   | TBA4  | OSPFv2 LAN Adj-SID per Algorithm Sub-TLV | This document |
   +-------+------------------------------------------+---------------+

7.3.  IANA OSPFv3 Considerations

   This document makes the following registrations in the "OSPFv3
   Extended-LSA Sub-TLVs" Registry.

   +-------+------------------------------------------+---------------+
   | Value | Description                              | Reference     |
   +=======+==========================================+===============+
   | TBA5  | OSPFv3 Adj-SID per Algorithm Sub-TLV     | This document |
   +-------+------------------------------------------+---------------+
   | TBA6  | OSPFv3 LAN Adj-SID per Algorithm Sub-TLV | This document |
   +-------+------------------------------------------+---------------+







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8.  Security Considerations

   There are no new security issues introduced by the extensions in this
   document.  Refer to [RFC8665], [RFC8666], [RFC8667] for other
   security considerations.

9.  Acknowledgements

   We would like to thank Aijun Wang, Robert Raszuk, Gyan Mishra, Jie
   Dong and Xuesong Geng for their reviews and discussions to the
   content of this document.

10.  Contributors

   The following people gave a substantial contribution to the content
   of this document.

   Les Ginsberg
   Cisco Systems, Inc.
   United States of America
   Email: ginsberg@cisco.com

11.  Normative References

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-17 (work in progress), July 2021.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.




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   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8666]  Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
              for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
              December 2019, <https://www.rfc-editor.org/info/rfc8666>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

Authors' Addresses

   Shaofu Peng
   ZTE Corporation
   China

   Email: peng.shaofu@zte.com.cn


   Ran Chen
   ZTE Corporation
   China

   Email: chen.ran@zte.com.cn


   Ketan Talaulikar
   Cisco Systems
   India

   Email: ketant@cisco.com









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   Peter Psenak
   Cisco Systems
   Slovakia

   Email: ppsenak@cisco.com














































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