Algorithm Related IGP-Adjacency SID Advertisement

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Table of Contents

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).¶

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.¶

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.¶

     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)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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, 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.¶

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

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 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

   +------+--------------------+----+----+----+-----+-----+-----+
   | 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  |
   +------+--------------------+----+----+----+-----+-----+-----+

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

   +-------+------------------------------------+---------------+
   | 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.¶

Authors' Addresses