RFC 9855: Topology Independent Fast Reroute Using Segment Routing

Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9855.¶

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1. Introduction

This document outlines a local repair mechanism that leverages Segment Routing (SR) to restore end-to-end connectivity in the event of a failure involving a directly connected network component. This mechanism is designed for standard link-state Interior Gateway Protocol (IGP) shortest path scenarios. Non-SR mechanisms for local repair are beyond the scope of this document. Non-local failures are addressed in a separate document [SR-LOOP].¶

The term Topology Independent (TI) describes the capability providing a loop-free backup path that is effective across all network topologies. This provides a major improvement compared to LFA [RFC5286] and RLFA [RFC7490], which cannot provide a complete protection coverage in some topologies as described in [RFC6571].¶

When the network reconverges after failure, micro-loops [RFC5715] can form due to transient inconsistencies in the forwarding tables of different routers. If it is determined that micro-loops are a significant issue in the deployment, then a suitable loop-free convergence method should be implemented, such as one of those described in [RFC5715], [RFC6976], [RFC8333], or [SR-LOOP].¶

TI-LFA operates locally at the Point of Local Repair (PLR) upon detecting a failure in one of its direct links. Consequently, this local operation does not influence:¶

TI-LFA paths are activated from the instant the PLR detects a failure in a local link and remain in effect until the IGP convergence at the PLR is fully achieved. Consequently, they are not susceptible to micro-loops that may arise due to variations in the IGP convergence times across different nodes through which these paths traverse. This ensures a stable and predictable routing environment, minimizing disruptions typically associated with asynchronous network behavior. However, an early (relative to the other nodes) IGP convergence at the PLR and the consecutive "early" release of TI-LFA paths may cause micro-loops, especially if these paths have been computed using the methods described in Sections 5.2, 5.3, or 5.4 of this document. One of the possible ways to prevent such micro-loops is local convergence delay [RFC8333].¶

TI-LFA procedures are complementary to the application of any micro-loop avoidance procedures in the case of link or node failure:¶

For each destination (as specified by the IGP) in the network, TI-LFA pre-installs a backup forwarding entry for each protected destination ready to be activated upon detection of the failure of a link used to reach the destination. TI-LFA provides protection in the event of any one of the following: single link failure, single node failure, or single Shared Risk Link Group (SRLG) failure. In link failure mode, the destination is protected assuming the failure of the link. In node protection mode, the destination is protected assuming that the neighbor connected to the primary link (see Section 2) has failed. In SRLG protecting mode, the destination is protected assuming that a configured set of links sharing fate with the primary link has failed (e.g., a linecard or a set of links sharing a common transmission pipe).¶

Protection techniques outlined in this document are limited to protecting links, nodes, and SRLGs that are within a link-state IGP area. Protecting domain exit routers and/or links attached to another routing domain is beyond the scope of this document.¶

By utilizing SR, TI-LFA eliminates the need to establish Targeted Label Distribution Protocol sessions with remote nodes for leveraging the benefits of Remote Loop-Free Alternates (RLFAs) [RFC7490] [RFC7916] or Directed Loop-Free Alternates (DLFAs) [IPFRR-TUNNELS]. All the Segment Identifiers (SIDs) required are present within the Link State Database (LSDB) of the IGP. Consequently, there is no longer a necessity to prefer LFAs over RLFAs or DLFAs, nor is there a need to minimize the number of RLFA or DLFA repair nodes.¶

Utilizing SR also eliminates the need to establish an additional state within the network for enforcing explicit Fast Reroute (FRR) paths. This spares the nodes from maintaining a supplementary state and frees the operator from the necessity to implement additional protocols or protocol sessions solely to augment protection coverage.¶

TI-LFA also brings the benefit of the ability to provide a backup path that follows the expected post-convergence path considering a particular failure, which reduces the need of locally configured policies that influence the backup path selection [RFC7916]. The easiest way to express the expected post-convergence path in a loop-free manner is to encode it as a list of Adjacency segments. However, this may create a long segment list that some hardware may not be able to program. One of the challenges of TI-LFA is to encode the expected post-convergence path by combining Adjacency segments and node segments. Each implementation may independently develop its own algorithm for optimizing the ordered segment list. This document provides an outline of the fundamental concepts applicable to constructing the SR backup path, along with the related data plane procedures. Appendix A contains a more detailed description of some of the aspects of TI-LFA related to post-convergence path.¶

This document is structured as follows:¶

2. Terminology

3. Base Principle

The basic algorithm to compute the repair path is to pre-compute SPT_new(R, X) and, for each destination, encode the repair path as a loop-free segment list. One way to provide a loop-free segment list is to use Adjacency SIDs only. However, this approach may create very long SID lists that hardware may not be able to handle due to Maximum SID Depth (MSD) limitations.¶

An implementation is free to use any local optimization to provide smaller segment lists by combining Node-SIDs and Adjacency SIDs. In addition, the usage of Node-SIDs allow for maximizing ECMPs over the backup path. These optimizations are out of scope of this document; however, the subsequent sections provide some guidance on how to leverage P-spaces and Q-spaces to optimize the size of the segment list.¶

4. Intersecting P-space and Q-space with Post-Convergence Paths

One of the challenges of defining an SR path following the expected post-convergence path is to reduce the size of the segment list. In order to reduce this segment list, an implementation MAY determine the P-space / extended P-space and Q-space properties (defined in [RFC7490]) of the nodes along the expected post-convergence path from the PLR to the protected destination and compute an SR explicit path from P to Q when they are not adjacent. Such properties will be used in Section 5 to compute the TI-LFA RL.¶

5. TI-LFA Repair Path

The TI-LFA repair path consists of an outgoing interface and a list of segments (a Repair List (RL)) to insert on the SR header in accordance with the data plane used. The RL encodes the explicit (and possibly post-convergence) path to the destination, which avoids the protected resource X. At the same time, the RL is guaranteed to be loop-free, irrespective of the state of FIBs along the nodes belonging to the explicit path, as long as the states of the FIBs are programmed according to a link-state IGP. Thus, there is no need for any coordination or message exchange between the PLR and any other router in the network.¶

The TI-LFA repair path is found by intersecting P(S, X) and Q(D, X) with the post-convergence path to D and computing the explicit SR-based path EP(P, Q) from a node P in P(S, X) to a node Q in Q(D, X) when these nodes are not adjacent along the post-convergence path. The TI-LFA RL is expressed generally as (Node-SID(P), EP(P, Q)).¶

  S --------- N1 --------- D
  *\          | \          |
  * \         |  \         |
  *  \        |   \        |
  *   N2----- R1***R2 *** R3
  *           *
  N3 **********

    ***** : link with high metric (1k)
    ----- : link with metric 1

As an example, in Figure 1, the focus is on the TI-LFA backup from S to D, considering the failure of node N1.¶

As a result, the TI-LFA RL of S for destination D considering the failure of node N1 is: <Node-SID(R1), Adj-SID(R1-R2), Adj-SID(R2-R3)>.¶

Most often, the TI-LFA RL has a simpler form, as described in the following sections. Appendix B provides statistics for the number of SIDs in the explicit path to protect against various failures.¶

6. Building TI-LFA Repair Lists for SR Segments

The following sections describe how to build the RLs using the terminology defined in [RFC8402]. The procedures described in this section are equally applicable to both the Segment Routing over MPLS (SR-MPLS) and the Segment Routing over IPv6 (SRv6) data plane, while the data plane-specific considerations are described in Section 7.¶

This section explains the process by which a protecting router S handles the active segment of a packet upon the failure of its primary outgoing interface for the packet S-F. The failure of the primary outgoing interface may occur due to various triggers, such as link failure, neighbor node failure, and others.¶

7. Data Plane-Specific Considerations

8. TI-LFA and SR Algorithms

SR allows an operator to bind an algorithm to a Prefix-SID (as defined in [RFC8402]). The algorithm value dictates how the path to the prefix is computed. The SR default algorithm is known as the "Shortest Path" algorithm. The SR default algorithm allows an operator to override the IGP shortest path by using local policies. When TI-LFA uses Node-SIDs associated with the default algorithm, there is no guarantee that the path will be loop-free, as a local policy may have overridden the expected IGP path. As the local policies are defined by the operator, it becomes the responsibility of this operator to ensure that the deployed policies do not affect the TI-LFA deployment. It should be noted that such a situation can already happen today with existing mechanisms such as RLFA.¶

[RFC9350] defines a Flexible Algorithm framework to be associated with Prefix-SIDs. A Flexible Algorithm allows a user to associate a constrained path to a Prefix-SID rather than using the regular IGP shortest path. An implementation MAY support TI-LFA to protect Node-SIDs associated with a Flexible Algorithm. In such a case, rather than computing the expected post-convergence path based on the regular SPF, an implementation SHOULD use the constrained SPF algorithm bound to the Flexible Algorithm (using the Flexible Algorithm Definition) instead of the regular Dijkstra in all the SPF/reverse SPF computations that are occurring during the TI-LFA computation. This includes the computation of the P-space and Q-space as well as the post-convergence path. Furthermore, the implementation SHOULD only use Node-SIDs/Adj-SIDs bound to the Flexible Algorithm and/or unprotected Adj-SIDs of the regular SPF to build the RL. The use of regular Dijkstra for the TI-LFA computation or for building the repair path using SIDs other than those recommended does not ensure that the traffic going over the TI-LFA repair path during the FRR period is honoring the Flexible Algorithm constraints.¶

9. Usage of Adjacency Segments in the Repair List

The RL of segments computed by TI-LFA may contain one or more Adjacency segments. An Adjacency segment may be protected or not protected.¶

        S --- R2 --- R3 ---- R4 --- R5 --- D
                 *   |  \   *
                   * |   \ *
                    R7 ** R8
                     *    |
                     *    |
                    R9 -- R10

In Figure 2, all the metrics are equal to 1 except R2-R7,R7-R8,R8-R4,R7-R9, which have a metric of 1000. Considering R2 as a PLR to protect against the failure of node R3 for the traffic S->D, the RL computed by R2 will be [adj-sid(R7-R8), adj-sid(R8-R4)], and the outgoing interface will be to R7. If R3 fails, R2 pushes the RL onto the incoming packet to D. During the FRR, if R7-R8 fails and if TI-LFA has picked a protected Adjacency segment for Adj-SID(R7-R8), R7 will push an additional RL onto the packet following the procedures defined in Section 6.¶

To avoid the possibility of this double FRR activation, an implementation of TI-LFA MAY pick only unprotected Adjacency segments when building the RL. However, it is important to note that FRR in general is intended to protect for a single pre-planned failure. If the failure that happens is worse than expected or multiple failures happen, FRR is not guaranteed to work. In such a case, fast IGP convergence remains important to restore traffic as quickly as possible.¶

10. Security Considerations

The techniques described in this document are internal functionalities to a router that can guarantee an upper bound on the time taken to restore traffic flow upon the failure of a directly connected link or node. As these techniques steer traffic to the post-convergence path as quickly as possible, this serves to minimize the disruption associated with a local failure, which can be seen as a modest security enhancement. The protection mechanism does not protect external destinations, but rather provides quick restoration for destinations that are internal to a routing domain.¶

The security considerations described in [RFC5286] and [RFC7490] apply to this document. Similarly, as the solution described in this document is based on SR technology, the reader should be aware of the security considerations related to this technology (see [RFC8402]) and its data plane instantiations (see [RFC8660], [RFC8754], and [RFC8986]). However, this document does not introduce additional security concerns.¶

11. IANA Considerations

This document has no IANA actions.¶

Appendix A. Advantages of Using the Expected Post-Convergence Path During FRR

[RFC7916] raises several operational considerations when using LFA or RLFA. Section 3 of [RFC7916] presents a case where a high bandwidth link between two core routers is protected through a Provider Edge (PE) router connected with low bandwidth links. In such a case, congestion may happen when the FRR backup path is activated. [RFC7916] introduces a local policy framework to let the operator tuning manually the best alternate election based on its own requirements.¶

From a network capacity planning point of view, it is often assumed for simplicity that if a link L fails on a particular node X, the bandwidth consumed on L will be spread over some of the remaining links of X. The remaining links to be used are determined by the IGP routing considering that the link L has failed (we assume that the traffic uses the post-convergence path starting from the node X). In Figure 3, we consider a network with all metrics equal to 1 except the metrics on links used by PE1, PE2, and PE3, which are 1000. An easy network capacity planning method is to consider that if the link L (X-B) fails, the traffic actually flowing through L will be spread over the remaining links of X (X-H, X-D, X-A). Considering the IGP metrics, only X-H and X-D can be used in reality to carry the traffic flowing through the link L. As a consequence, the bandwidth of links X-H and X-D is sized according to this rule. We should observe that this capacity planning policy works; however, it is not fully accurate.¶

In Figure 3, considering that the source of traffic is only from PE1 and PE4, when the link L fails, depending on the convergence speed of the nodes, X may reroute its forwarding entries to the remote PEs onto X-H or X-D; however, in a similar timeframe, PE1 will also reroute a subset of its traffic (the subset destined to PE2) out of its nominal path, reducing the quantity of traffic received by X. The capacity planning rule presented previously has the drawback of oversizing the network; however, it allows for preventing any transient congestion (for example, when X reroutes traffic before PE1 does).¶

              H --- I --- J *
              |           |  *
   PE4        |           |   PE3
      \       | (L)       |  *
      * A --- X --- B --- G *
     *        |           |  *
   PE1        |           |   PE2
     *        |           |  *
      * C --- D --- E --- F *

Based on this assumption, in order to facilitate the operation of FRR and limit the implementation of local FRR policies, traffic can be steered by the PLR onto its expected post-convergence path during the FRR phase. In our example, when link L fails, X switches the traffic destined to PE3 and PE2 on the post-convergence paths. This is perfectly in line with the capacity planning rule that was presented before and also in line with the fact that X may converge before PE1 (or any other upstream router) and may spread the X-B traffic onto the post-convergence paths rooted at X.¶

It should be noted that some networks may have a different capacity planning rule, leading to an allocation of less bandwidth on X-H and X-D links. In such a case, using the post-convergence paths rooted at X during FRR may introduce some congestion on X-H and X-D links. However, it is important to note that a transient congestion may possibly happen even without FRR activated, for instance, when X converges before the upstream routers. Operators are still free to use the policy framework defined in [RFC7916] if the usage of the post-convergence paths rooted at the PLR is not suitable.¶

Readers should be aware that FRR protection is pre-computing a backup path to protect against a particular type of failure (link, node, or SRLG). When using the post-convergence path as an FRR backup path, the computed post-convergence path is the one considering the failure we are protecting against. This means that FRR is using an expected post-convergence path, and this expected post-convergence path may be actually different from the post-convergence path used if the failure that happened is different from the failure FRR was protecting against. As an example, if the operator has implemented a protection against a node failure, the expected post-convergence path used during FRR will be the one considering that the node has failed. However, even if a single link is failing or a set of links is failing (instead of the full node), the node-protecting post-convergence path will be used. The consequence is that the path used during FRR is not optimal with respect to the failure that has actually occurred.¶

Another consideration to take into account is as follows: while using the expected post-convergence path for SR traffic using node segments only (for instance, PE to PE traffic using the shortest path) has some advantages, these advantages reduce when SR policies [RFC9256] are involved. A segment list used in an SR policy is computed to obey a set of path constraints defined locally at the head-end or centrally in a controller. TI-LFA cannot be aware of such path constraints, and there is no reason to expect the TI-LFA backup path protecting one segment in that segment list to obey those constraints. When SR policies are used and the operator wants to have a backup path that still follows the policy requirements, this backup path should be computed as part of the SR policy in the ingress node (or central controller), and the SR policy should not rely on local protection. Another option could be to use a Flexible Algorithm [RFC9350] to express the set of constraints and use a single node segment associated with a Flexible Algorithm to reach the destination. When using a node segment associated with a Flexible Algorithm, TI-LFA keeps providing an optimal backup by applying the appropriate set of constraints. The relationship between TI-LFA and the SR algorithm is detailed in Section 8.¶

Appendix B. Analysis Based on Real Network Topologies

This section presents an analysis performed on real service provider and large enterprise network topologies. The objective of the analysis is to assess the number of SIDs required in an explicit path when the mechanisms described in this document are used to protect against the failure scenarios within the scope of this document. The number of segments described in this section are applicable to instantiating SR over the MPLS forwarding plane.¶

The measurement below indicates that, for link and local SRLG protection, a repair path of 1 SID or less delivers more than 99% coverage. For node protection, a repair path of 2 SIDs or less yields 99% coverage.¶

Table 1 below lists the characteristics of the networks used in our measurements. The number of links refers to the number of "bidirectional" links (not directed edges of the graph). The measurements are carried out as follows:¶

The rest of this section presents the measurements done on the actual topologies. The conventions that we use are as follows:¶

Tables 2 and 3 below summarize the measurements on the number of SIDs needed for link protection.¶

Tables 4 and 5 summarize the measurements on the number of SIDs needed for local SRLG protection.¶

The remaining two tables summarize the measurements on the number of SIDs needed for node protection.¶

Acknowledgments

The authors would like to thank Les Ginsberg, Stewart Bryant, Alexander Vainsthein, Chris Bowers, Shraddha Hedge, Wes Hardaker, Gunter Van de Velde, and John Scudder for their valuable comments.¶

Contributors

In addition to the authors listed on the front page, the following co-authors have also contributed to this document:¶

Authors' Addresses