Algorithm Related IGP-Adjacency SID Advertisement
draft-ietf-lsr-algorithm-related-adjacency-sid-08

LSR                                                            Ran. Chen
Internet-Draft                                              Shaofu. Peng
Intended status: Standards Track                         ZTE Corporation
Expires: 19 April 2026                                 Ketan. Talaulikar
                                                           Peter. Psenak
                                                           Cisco Systems
                                                         16 October 2025

           Algorithm Related IGP-Adjacency SID Advertisement
           draft-ietf-lsr-algorithm-related-adjacency-sid-08

Abstract

   The SR policy architecture defines the Prefix-SID algorithm, with an
   algorithm identifier included in the Prefix-SID advertisement.
   However, the Prefix-SID algorithm does not address scenarios where
   multiple algorithms share the same link resources.  This document
   proposes adding the algorithm identifier in an Adjacency-SID
   advertisement.

Status of This Memo

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   This Internet-Draft will expire on 19 April 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components

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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Adjacency Segment Identifier(SID) per Algorithm . . . . . . .   3
     4.1.  ISIS Adjacency Segment Identifier per Algorithm . . . . .   4
       4.1.1.  ISIS Adjacency-SID per Algorithm Sub-TLV  . . . . . .   4
       4.1.2.  ISIS LAN Adjacency-SID per Algorithm Sub-TLV  . . . .   5
     4.2.  OSPFv2 Adjacency Segment Identifier per Algorithm . . . .   6
       4.2.1.  OSPFv2 Adjacency-SID per Algorithm Sub-TLV  . . . . .   6
       4.2.2.  OSPFv2 LAN Adjacency-SID per Algorithm Sub-TLV  . . .   7
     4.3.  OSPFv3 Adjacency Segment Identifier per Algorithm . . . .   8
       4.3.1.  OSPFv3 Adjacency-SID per Algorithm Sub-TLV  . . . . .   8
       4.3.2.  OSPFv3 LAN Adjacency-SID per Algorithm Sub-TLV  . . .   9
   5.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Examples of Algorithm Specific Adjacency-SID  . . . . . .  11
   6.  Deployment Considerations . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  IANA ISIS Considerations  . . . . . . . . . . . . . . . .  13
     7.2.  IANA OSPFv2 Considerations  . . . . . . . . . . . . . . .  13
     7.3.  IANA OSPFv3 Considerations  . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   Segment Routing (SR) architecture [RFC8402] defines the Prefix-SID
   algorithm, with an algorithm identifier included in the Prefix-SID
   advertisement.

   IGP Flex Algorithm [RFC9350] 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).

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   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.  In addition to
   Prefix-SID, this document complement that the algorithm identifier
   can be also included as part of an Adjacency-SID advertisement for
   SR-MPLS, so that each Flex-algo plane corresponding to different
   algorithm types can be allocated with a dedicated segment ID related
   to the corresponding algorithm type.

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

   Typical application scenarios of the algorithm related SID are as
   follows:

   *  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.

   *  Help enhance PHBs (per hop behavior) related to specific algorithm
      types on the data plane.  Generally, QoS policies related to the
      algorithm type of each Flex-algo plane can be configured and
      installed, and the packets can be forwarded based one the
      algorithm related SID and QoS policy.

   *  Help enhance OAM related to specific algorithm types on the
      control plane and data plane, such as statistics of traffic of
      different algorithms on the same link, ping/traceroute detection
      (or other tools) for specific algorithm.

4.  Adjacency Segment Identifier(SID) per Algorithm

   This section describe that the algorithm related SID is flooded
   through the IGP protocol.

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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-SID per Algorithm
   Sub-TLV" and "ISIS LAN Adjacency-SID per Algorithm Sub-TLV", which
   contain the algorithm type fields.

4.1.1.  ISIS Adjacency-SID per Algorithm Sub-TLV

   ISIS Adjacency-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-SID per Algorithm Format

   where:

   Type: TBD1.

   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.

   For a P2P link, an SR-capable router MAY allocate different
   Adjacency-SIDs for different algorithms, if this link participates in
   the plane related to different algorithms.

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4.1.2.  ISIS LAN Adjacency-SID per Algorithm Sub-TLV

   ISIS LAN Adjacency-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 LAN Adjacency-SID per Algorithm Format

   where:

   Type: TBD2.

   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.

   For a broadcast link, an SR-capable router MAY allocate different
   Adjacency-SIDs for different algorithms, if this link participates in
   the plane related to different algorithms.

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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 Adjacency-SID per Algorithm Sub-TLV" and
   "OSPFv2 LAN Adjacency-SID per Algorithm Sub-TLV", which contain the
   algorithm type fields.

4.2.1.  OSPFv2 Adjacency-SID per Algorithm Sub-TLV

   OSPFv2 Adjacency-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 Adjacency-SID per Algorithm Format

   where:

   Type: TBD3

   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.

   For a P2P link, an SR-capable router MAY allocate different
   Adjacency-SIDs for different algorithms, if this link participates in
   the plane related to different algorithms.

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

   OSPFv2 LAN Adjacency-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 Adjacency-SID per Algorithm Format

   where:

   Type: TBD4

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

   Flags: Refer to OSPFv2 LAN Adjacency-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
   Adjacency-SIDs for different algorithms, if this link participates in
   the plane related to different algorithms.

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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 Adjacency-SID per Algorithm Sub-TLV" and
   "OSPFv3 LAN Adjacency-SID per Algorithm Sub-TLV", which contain the
   algorithm type fields.

4.3.1.  OSPFv3 Adjacency-SID per Algorithm Sub-TLV

   OSPFv3 Adjacency-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 Adjacency-SID per Algorithm Format

   where:

   Type: TBD5

   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.

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   For a P2P link, an SR-capable router MAY allocate different
   Adjacency-SIDs for different algorithms, if this link participates in
   the plane related to different algorithms.

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

   OSPFv3 LAN Adjacency-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 Adjacency-SID per Algorithm Format

   where:

   Type: TBD6

   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
   Adjacency-SIDs for different algorithms, if this link participates in
   the plane related to different algorithms.

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5.  Procedures

   The method introduced in this document enables the traffic of
   different flex-algo plane to be distinguished when they are routed on
   the same link, and can be applied with different local treatment
   (such as providing different repair paths, traffic statistics, QoS
   policies, etc) per algorithm.

   A node may, according to each flex-algo plane (corresponding to the
   specific algorithm types) which it participated in, allocate segment
   identifiers corresponding to each algorithm types, and these segment
   identifiers need to be flooded through IGP.  As defined in Section 4,
   algorithm field must be encoded in the flooded packet.

   Depending on implementation, SIDs allocation is generally triggered
   by configuration.  For algorithm specific Adjacency-SID, 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.  Note that Node-SID per algorithm may also face
   similar difficulties (considering the abnormal situation where nodes
   have to stop participating in the flex-algo plane after FAD
   negotiation, referring to section 5.3 of [RFC9350]).

   Developers can flexibly refer to any of the following implementation
   choices.

   *  One choice is that as long as an IGP instance with an algorithm
      enabled for a level/area is configured on the node, the node may
      allocate Adjacency-SIDs for that algorithm statically for all
      links joined to that level/area.  Similar way may be also applied
      to node-SID per algorithm.  That is, algorithm specific SID can be
      allocated regardless of the felx-algo participation and wining
      FAD.  If the router stops participating, or the link is excluded
      from the flex-algo, the advertised algorithm specific SID does not
      cause any issue, but is just not used.

   *  Another choice is to allocate and withdraw algorithm specific
      Adjacency-SID dynamically according to the result of FAD
      negotiation, i.e., algorithm specific Adjacency-SID is allocated
      and advertised only for those links that have joined the Flex-algo
      plane.  Similar choice may be also applied to node-SID per
      algorithm.  This choice is RECOMMENDED.

   The RECOMMENDED implementation choice also make sense for other type
   of states per algorithm.  A node may, according to each flex-algo
   plane (corresponding to the specific algorithm types) which it
   participated in, config local treatments (such as repair paths,

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   traffic statistics counters, QoS policies, etc) corresponding to each
   algorithm types and apply them to the links that participated in the
   corresponding flex-algo plane.  In this case, the node may
   dynamically create or delete these local treatments according to the
   result of FAD negotiation.

   The (LAN) Adjacency-SID per Algorithm Sub-TLV MUST NOT be used to
   advertise algorithm 0 specific SIDs.

   Note that the advertisement specification defined in Section 4 does
   not have any requirements for the SID allocation rules.  Some
   particular advertisement method based on particular allocation rules
   are not within the scope of this document.

   Once the node originates an algorithm specific Adjacency-SID and
   sends it to the network, the coresponding local SID entry (i.e., an
   MPLS label forwarding entry) must be installed on the fowarding
   plane.  The local SID entry, combined with local treatments (such as
   QoS polices), are used to continue to forward data packets in the
   context of the specific algorithm.

   A node may receive different algo-SIDs (corresponding to different
   algorithm types with the related flex-algo plane) originated from
   other nodes and flooded by IGP.  As defined in Section 4, the
   algorithm field can be gotten from the flooded packet to indicate
   algorithm specific SIDs.  Then, algo-SIDs, with other SIDs, are
   maintained in the link state database.

   If the received algorithm is not within the range (128,255), the
   related (LAN) Adjacency-SID per Algorithm Sub-TLV MUST be ingored.

   When a node receives a forwarding data packet whose active segment is
   an algorithm specific Adjacency-SID and matches the coresponding
   local SID entry, the node forwards the data packet to the
   corresponding outgoing port and applies algorithm related local
   treatments (such as QoS policies) to the packet.  The local
   treatments may also be applied for the case of algorithm specific
   Node-SID.

5.1.  Examples of Algorithm Specific Adjacency-SID

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

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

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

   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 translated to {node-SID(B)@FA-id129, adjacency-
   SID(B-D)@FA-id129}. So that node B can distinguish the flows of FA-id
   128 and FA-id 129 based on different adjacency-SID(B-D) and their
   related label forwarding entries, and take different treatments of
   them when they are forwarded to the same outgoing link B-D.

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.

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   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 algorithm, statistics per
   algorithm, QoS policy per algorithm, etc.  Assuming that a node wants
   to support the capability of repair path per algorithm, 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 algorithm 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.

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-SID per  |    |    |    |     |     |     |
      | TBD1 | Algorithm          | y  | y  | n  |  y  |  y  |  y  |
      +------+--------------------+----+----+----+-----+-----+-----+
      |      | LAN Adjacency-SID  |    |    |    |     |     |     |
      | TBD2 | per Algorithm      | y  | y  | n  |  y  |  y  |  y  |
      +------+--------------------+----+----+----+-----+-----+-----+

7.2.  IANA OSPFv2 Considerations

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

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      +-------+------------------------------------+---------------+
      | Value | Description                        | Reference     |
      +=======+====================================+===============+
      | TBD3  | OSPFv2 Adjacency-SID               | This document |
      |       | per Algorithm Sub-TLV              |               |
      +-------+------------------------------------+---------------+
      | TBD4  | OSPFv2 LAN Adjacency-SID           | This document |
      |       | per Algorithm Sub-TLV              |               |
      +-------+------------------------------------+---------------+

7.3.  IANA OSPFv3 Considerations

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

      +-------+------------------------------------+---------------+
      | Value | Description                        | Reference     |
      +=======+====================================+===============+
      | TBD5  | OSPFv3 Adjacency-SID               | This document |
      |       | per Algorithm Sub-TLV              |               |
      +-------+------------------------------------+---------------+
      | TBD6  | OSPFv3 LAN Adjacency-SID           | This document |
      |       | per Algorithm Sub-TLV              |               |
      +-------+------------------------------------+---------------+

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

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11.  Normative References

   [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>.

   [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>.

   [RFC9350]  Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
              and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
              DOI 10.17487/RFC9350, February 2023,
              <https://www.rfc-editor.org/info/rfc9350>.

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Authors' Addresses

   Ran Chen
   ZTE Corporation
   China
   Email: chen.ran@zte.com.cn

   Shaofu Peng
   ZTE Corporation
   China
   Email: peng.shaofu@zte.com.cn

   Ketan Talaulikar
   Cisco Systems
   India
   Email: ketant.ietf@gmail.com

   Peter Psenak
   Cisco Systems
   Slovakia
   Email: ppsenak@cisco.com

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