Internet-Draft SR TI-LFA June 2024
Bashandy, et al. Expires 31 December 2024 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-ietf-rtgwg-segment-routing-ti-lfa-16
Published:
Intended Status:
Standards Track
Expires:
Authors:
A. Bashandy
Cisco Systems
S. Litkowski
Cisco Systems
C. Filsfils
Cisco Systems
P. Francois
INSA Lyon
B. Decraene
Orange
D. Voyer
Bell Canada

Topology Independent Fast Reroute using Segment Routing

Abstract

This document presents Topology Independent Loop-free Alternate Fast Reroute (TI-LFA), aimed at providing protection of node and adjacency segments within the Segment Routing (SR) framework. This Fast Reroute (FRR) behavior builds on proven IP-FRR concepts being LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding (DLFA). It extends these concepts to provide guaranteed coverage in any two-connected networks using a link-state IGP. A key aspect of TI-LFA is the FRR path selection approach establishing protection over the expected post-convergence paths from the point of local repair, reducing the operational need to control the tie-breaks among various FRR options.

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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any 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 31 December 2024.

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Table of Contents

1. Acronyms

2. 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 [I-D.bashandy-rtgwg-segment-routing-uloop].

The term topology independent (TI) describes the capability providing a loop free backup path that is effective accross all network topologies. This provides a major improvement compared to LFA [RFC5286] and remote LFA [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, such as one of those described in [RFC5715], [RFC6976], [RFC8333], or [I-D.bashandy-rtgwg-segment-routing-uloop] should be implemented.

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 Interior Gateway Protocol (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.

TI-LFA paths are applied from the moment the PLR detects failure of a local link and until IGP convergence at the PLR is completed. Therefore, 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 Section Section 6.2, Section 6.3, or Section 6.4 of the document. One of the possible ways to prevent such micro-loops is local convergence delay ([RFC8333]).

TI-LFA procedures are complementary to 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 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 Section 3 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 domains are beyond the scope of this document

By utilizing Segment Routing (SR), T-LFA eliminates the need to establish Targeted Label Distribution Protocol sessions with remote nodes for leveraging the benefits of Remote Loop-Free Alternates (RLFA) [RFC7490][RFC7916] or Directed Loop-Free Alternates (DLFA) [RFC5714]. All the Segment Identifiers (SIDs) required are present within the Link State Database (LSDB) of the Interior Gateway Protocol (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 makes the requirement unnecessary to establish additional state within the network for enforcing explicit Fast Reroute (FRR) paths. This spares the nodes from maintaining supplementary state and frees the operator from the necessity to implement additional protocols or protocol sessions solely to augment protection coverage.

Although not a TI-LFA requirement or constraint, 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 dataplane procedures.

Section 3 defines the main notations used in the document. They are in line with [RFC5714].

Section 4 defines the main principles of TI-LFA backup path computation.

Section 5 suggests to compute the P-Space and Q-Space properties defined in Section 3, for the specific case of nodes lying over the post-convergence paths towards the protected destinations.

Using the properties defined in Section 5, Section 6 describes how to compute protection lists that encode a loop-free post-convergence path towards the destination.

Section 7 defines the segment operations to be applied by the PLR to ensure consistency with the forwarding state of the repair node.

Section 8 discusses aspects that are specific to the dataplane.

Section 9 discusses relationship between TI-LFA and the SR-algorithm.

Certain considerations are needed when adjacency segments are used in a repare list. Section 10 provides an overview of these considerations.

Section 11 discusses security considerations.

Appendix A highlights advantages of using the expected post-convergence path during FRR.

By implementing the algorithms detailed in this document within actual service provider and large enterprise network environments, real-life measurements are presented regarding the number of SIDs utilized by repair paths. These measurements are summarized in Appendix B.

2.1. Conventions used in this document

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. Terminology

The main notations used in this document are defined as follows.

The terms "old" and "new" topologies refer to the Link State Database (LSDB) state before and after the considered failure, respectively.

SPT_old(R) is the Shortest Path Tree rooted at node R in the initial state of the network.

SPT_new(R, X) is the Shortest Path Tree rooted at node R in the state of the network after the resource X has failed.

PLR stands for "Point of Local Repair". It is the router that applies fast traffic restoration after detecting failure in a directly attached link, set of links, and/or node.

Similar to [RFC7490], the concept of P-Space and Q-Space is used for TI-LFA.

The P-space P(R,X) of a router R with regard to a resource X (e.g. a link S-F, a node F, or a SRLG) is the set of routers reachable from R using the pre-convergence shortest paths without any of those paths (including equal-cost path splits) transiting through X. A P node is a node that belongs to the P-space.

Consider the set of neighbors of a router R and a resource X. Exclude from that set, the neighbors that are reachable from R using X. The Extended P-Space P'(R,X) of a node R with regard to a resource X is the union of the P-spaces of the neighbors in that reduced set of neighbors with regard to the resource X.

The Q-space Q(R,X) of a router R with regard to a resource X is the set of routers from which R can be reached without any path (including equal-cost path splits) transiting through X. A Q node is a node that belongs to the Q-space

EP(P, Q) is an explicit SR path from a node P to a node Q.

Primary Interface: Primary Outgoing Interface: One of the outgoing interfaces towards a destination according to the IGP link-state protocol

Primary Link: A link connected to the primary interface

adj-sid(S-F): Adjacency Segment from node S to node F

4. 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 MSD (Maximum SID Depth) 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 to maximize 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.

5. 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 6 to compute the TI-LFA repair list.

5.1. Extended P-Space property computation for a resource X, over post-convergence paths

The objective is to determine which nodes on the post-convergence path from the PLR R to the destination D are in the extended P-space of R with regard to resource X (where X can be a link or a set of links adjacent to the PLR, or a neighbor node of the PLR).

This can be found by:

  • Excluding neighbors which are not on the post-convergence path when computing P'(R,X)

  • Then, intersecting the set of nodes belonging to the post-convergence path from R to D, assuming the failure of X, with P'(R, X).

5.2. Q-Space property computation for a resource X, over post-convergence paths

The goal is to determine which nodes on the post-convergence path from the Point of Local Repair (PLR) R to the destination D are in the Q-Space of destination D with regard to resource X (where X can be a link or a set of links adjacent to the PLR, or a neighbor node of the PLR).

This can be found by intersecting the set of nodes belonging to the post-convergence path from R to D, assuming the failure of X, with Q(D, X).

5.3. Scaling considerations when computing Q-Space

[RFC7490] raises scaling concerns about computing a Q-Space per destination. Similar concerns may affect TI-LFA computation if an implementation tries to compute a reverse Shortest Path Tree ([RFC7490]) for every destination in the network to determine the Q-Space. It will be up to each implementation to determine the good tradeoff between scaling and accuracy of the optimization.

6. TI-LFA Repair path

The TI-LFA repair path (RP) consists of an outgoing interface and a list of segments (repair list (RL)) to insert on the SR header in accordance with the dataplane used. The repair list encodes the explicit, and possibly post-convergence, path to the destination, which avoids the protected resource X and, at the same time, 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 co-ordination 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 repair list 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
Figure 1: Sample topology with TI-LFA

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 repair list of S for destination D considering the failure of node N1 is: <Node-SID(R1), Adj-Sid(R1-R2), Adj-Sid(R20R3)>.

Most often, the TI-LFA repair list 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.

6.1. FRR path using a direct neighbor

When a direct neighbor is in P(S,X) and Q(D,x) and the link to that direct neighbor is on the post-convergence path, the outgoing interface is set to that neighbor and the repair segment list SHOULD be empty.

This is comparable to a post-convergence LFA FRR repair.

6.2. FRR path using a PQ node

When a remote node R is in P(S,X) and Q(D,x) and on the post-convergence path, the repair list SHOULD be made of a single node segment to R and the outgoing interface SHOULD be set to the outgoing interface used to reach R.

This is comparable to a post-convergence RLFA repair tunnel.

6.3. FRR path using a P node and Q node that are adjacent

When a node P is in P(S,X) and a node Q is in Q(D,x) and both are on the post-convergence path and both are adjacent to each other, the repair list SHOULD be made of two segments: A node segment to P (to be processed first), followed by an adjacency segment from P to Q.

This is comparable to a post-convergence DLFA (LFA with directed forwarding) repair tunnel.

6.4. Connecting distant P and Q nodes along post-convergence paths

In some cases, there is no adjacent P and Q node along the post- convergence path. As mentioned in Section 4, a list of adjacency SIDs can be used to encode the path between P and Q. However, the PLR can perform additional computations to compute a list of segments that represent a loop-free path from P to Q. How these computations are done is out of scope of this document and is left to implementation.

7. Building TI-LFA repair lists

The following sections describe how to build the repair lists using the terminology defined in [RFC8402]. The procedures described in this section are equally applicable to both SR-MPLS and SRv6 dataplane, while the dataplane-specific considerations are described in Section 8.

In this section, 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, is explained. The failure of the primary outgoing interface may occur due to various triggers, such as link failure, neighbor node failure, and others.

8. Dataplane specific considerations

8.1. MPLS dataplane considerations

MPLS dataplane for Segment Routing is described in [RFC8660].

The following dataplane behaviors apply when creating a repair list using an MPLS dataplane:

  1. If the active segment is a node segment that has been signaled with penultimate hop popping and the repair list ends with an adjacency segment terminating on a node that advertised NEXT operation [RFC8402] of the active segment, then the active segment MUST be popped before pushing the repair list.

  2. If the active segment is a node segment but the other conditions in 1. are not met, the active segment MUST be popped then pushed again with a label value computed according to the Segment Routing Global Block of Q, where Q is the endpoint of the repair list. Finally, the repair list MUST be pushed.

8.2. SRv6 dataplane considerations

SRv6 dataplane and programming instructions are described respectively in [RFC8754] and [RFC8986].

The TI-LFA path computation algorithm is the same as in the SR-MPLS dataplane. Note however that the Adjacency SIDs are typically globally routed. In such case, there is no need for preceding an adjacency SID with a Prefix-SID [RFC8402] and the resulting repair list is likely shorter.

If the traffic is protected at a Transit Node, then an SRv6 SID list is added on the packet to apply the repair list. The addition of the repair list follows the headend behaviors as specified in section 5 of [RFC8986].

If the traffic is protected at an SR Segment Endpoint Node, first the Segment Endpoint packet processing is executed. Then the packet is protected as if its were a transit packet.

9. 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 has 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 overriden 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 situation can already happen today with existing mechanisms as remote LFA.

[RFC9350] defines a flexible algorithm (FlexAlgo) framework to be associated with Prefix-SIDs. FlexAlgo 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 to a FlexAlgo. 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 FlexAlgo (using the Flex Algo Definition) instead of the regular Dijkstra in all the SPF/rSPF 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. An implementation MUST only use Node-SIDs bound to the FlexAlgo and/or Adj-SIDs that are unprotected or, in case of SRv6, adj-SIDs that are bound to the FlexAlgo to build the repair list.

10. Usage of Adjacency segments in the repair list

The repair list 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
Figure 2

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 repair list 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 repair list 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 repair list onto the packet following the procedures defined in Section 7.

To avoid the possibility of this double FRR activation, an implementation of TI-LFA MAY pick only non protected adjacency segments when building the repair list. However, this 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.

11. 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 mechanisms does not protect external destinations, but rather provides quick restoration for destination that are internal to a routing domain.

Security considerations described in [RFC5286] and [RFC7490] apply to this document. Similarly, as the solution described in the document is based on Segment Routing technology, reader should be aware of the security considerations related to this technology ([RFC8402]) and its dataplane instantiations ([RFC8660], [RFC8754] and [RFC8986]). However, this document does not introduce additional security concern.

12. IANA Considerations

No requirements for IANA

13. Contributors

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

14. 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.

15. References

15.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC7916]
Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K., Horneffer, M., and P. Sarkar, "Operational Management of Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916, , <https://www.rfc-editor.org/info/rfc7916>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[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, , <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660]
Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing with the MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, , <https://www.rfc-editor.org/info/rfc8660>.
[RFC8754]
Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header (SRH)", RFC 8754, DOI 10.17487/RFC8754, , <https://www.rfc-editor.org/info/rfc8754>.
[RFC8986]
Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, DOI 10.17487/RFC8986, , <https://www.rfc-editor.org/info/rfc8986>.

15.2. Informative References

[I-D.bashandy-rtgwg-segment-routing-uloop]
Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B., Francois, P., and P. Psenak, "Loop avoidance using Segment Routing", Work in Progress, Internet-Draft, draft-bashandy-rtgwg-segment-routing-uloop-16, , <https://datatracker.ietf.org/doc/html/draft-bashandy-rtgwg-segment-routing-uloop-16>.
[RFC5286]
Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI 10.17487/RFC5286, , <https://www.rfc-editor.org/info/rfc5286>.
[RFC5714]
Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, DOI 10.17487/RFC5714, , <https://www.rfc-editor.org/info/rfc5714>.
[RFC5715]
Shand, M. and S. Bryant, "A Framework for Loop-Free Convergence", RFC 5715, DOI 10.17487/RFC5715, , <https://www.rfc-editor.org/info/rfc5715>.
[RFC6571]
Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene, B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free Alternate (LFA) Applicability in Service Provider (SP) Networks", RFC 6571, DOI 10.17487/RFC6571, , <https://www.rfc-editor.org/info/rfc6571>.
[RFC6976]
Shand, M., Bryant, S., Previdi, S., Filsfils, C., Francois, P., and O. Bonaventure, "Framework for Loop-Free Convergence Using the Ordered Forwarding Information Base (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, , <https://www.rfc-editor.org/info/rfc6976>.
[RFC7490]
Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", RFC 7490, DOI 10.17487/RFC7490, , <https://www.rfc-editor.org/info/rfc7490>.
[RFC8333]
Litkowski, S., Decraene, B., Filsfils, C., and P. Francois, "Micro-loop Prevention by Introducing a Local Convergence Delay", RFC 8333, DOI 10.17487/RFC8333, , <https://www.rfc-editor.org/info/rfc8333>.
[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, , <https://www.rfc-editor.org/info/rfc8665>.
[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, , <https://www.rfc-editor.org/info/rfc8667>.
[RFC9256]
Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, , <https://www.rfc-editor.org/info/rfc9256>.
[RFC9350]
Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, , <https://www.rfc-editor.org/info/rfc9350>.

Appendix A. Advantages of using the expected post-convergence path during FRR

[RFC7916] raised several operational considerations when using LFA or remote LFA. [RFC7916] Section 3 presents a case where a high bandwidth link between two core routers is protected through a 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 to prevent any transient congestion (when for example X reroutes traffic before PE1 does).


           H --- I --- J
           |           | \
PE4        |           |  PE3
   \       | (L)       | /
     A --- X --- B --- G
    /      |           | \
 PE1       |           |  PE2
    \      |           | /
     C --- D --- E --- F
Figure 3

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 inline with the capacity planning rule that was presented before and also inline with the fact 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, SRLG). When using the post-convergence path as 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: while using the expected post-convergence path for SR traffic using node segments only (for instance, PE to PE traffic using 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 segments in that segment list to obey those constraints. When SR policies are used and the operator wants to have a backup path which 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 FlexAlgo ([RFC9350]) to express the set of constraints and use a single node segment associated with a FlexAlgo to reach the destination. When using a node segment associated with a FlexAlgo, 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 9.

Appendix B. Analysis based on real network topologies

This section presents 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 segment routing over the MPLS forwarding plane.

The measurement below indicate that for link and local SRLG protection, a 1 SID repair path delivers more than 99% coverage. For node protection a 2 SIDs repair path 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 measurements listed in the tables indicate that for link and local SRLG protection, 1 SID repair path is sufficient to protect more than 99% of the prefix in almost all cases. For node protection 2 SIDs repair paths yield 99% coverage.

+-------------+------------+------------+------------+------------+
|   Network   |    Nodes   |  Links     |Node-to-Link| SRLG info? |
|             |            |            |    Ratio   |            |
+-------------+------------+------------+------------+------------+
|    T1       |    408     |      665   |    1.63    |    Yes     |
+-------------+------------+------------+------------+------------+
|    T2       |    587     |     1083   |    1.84    |     No     |
+-------------+------------+------------+------------+------------+
|    T3       |    93      |      401   |    4.31    |    Yes     |
+-------------+------------+------------+------------+------------+
|    T4       |    247     |      393   |    1.59    |    Yes     |
+-------------+------------+------------+------------+------------+
|    T5       |    34      |      96    |    2.82    |    Yes     |
+-------------+------------+------------+------------+------------+
|    T6       |    50      |      78    |    1.56    |     No     |
+-------------+------------+------------+------------+------------+
|    T7       |    82      |      293   |    3.57    |     No     |
+-------------+------------+------------+------------+------------+
|    T8       |    35      |      41    |    1.17    |    Yes     |
+-------------+------------+------------+------------+------------+
|    T9       |    177     |     1371   |    7.74    |    Yes     |
+-------------+------------+------------+------------+------------+
                    Table 1: Data Set Definition

The rest of this section presents the measurements done on the actual topologies. The convention that we use is as follows

Table 2A and 2B below summarize the measurements on the number of SIDs needed for link protection

+-------------+------------+------------+------------+------------+
|   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
+-------------+------------+------------+------------+------------+
|    T1       |  74.3%     |   25.3%    |   0.5%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T2       |  81.1%     |   18.7%    |   0.2%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T3       |  95.9%     |    4.1%    |   0.1%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T4       |  62.5%     |   35.7%    |   1.8%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T5       |  85.7%     |   14.3%    |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T6       |  81.2%     |   18.7%    |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T7       |  98.9%     |   1.1%     |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T8       |  94.1%     |   5.9%     |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T9       |  98.9%     |   1.0%     |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
        Table 2A: Link protection (repair size distribution)

+-------------+------------+------------+------------+------------+
|   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
+-------------+------------+------------+------------+------------+
|    T1       |  74.2%     |   99.5%    |    99.9%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T2       |  81.1%     |   99.8%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T3       |  95.9%     |   99.9%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T4       |  62.5%     |   98.2%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T5       |  85.7%     |  100.0%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T6       |  81.2%     |   99.9%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T7       |  98,8%     |  100.0%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T8       |  94,1%     |  100.0%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
|    T9       |  98,9%     |  100.0%    |   100.0%   |   100.0%   |
+-------------+------------+------------+------------+------------+
    Table 2B: Link protection repair size cumulative distribution
Table 3A and 3B summarize the measurements on the number of SIDs
needed for local SRLG protection.

+-------------+------------+------------+------------+------------+
|   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
+-------------+------------+------------+------------+------------+
|    T1       |  74.2%     |   25.3%    |   0.5%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T2       |                No SRLG Information                |
+-------------+------------+------------+------------+------------+
|    T3       |  93.6%     |    6.3%    |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T4       |  62.5%     |   35.6%    |   1.8%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T5       |  83.1%     |   16.8%    |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T6       |                No SRLG Information                |
+-------------+---------------------------------------------------+
|    T7       |                No SRLG Information                |
+-------------+------------+------------+------------+------------+
|    T8       |  85.2%     |   14.8%    |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T9       |  98,9%     |    1.1%    |   0.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
      Table 3A: Local SRLG protection repair size distribution

+-------------+------------+------------+------------+------------+
|   Network   |    0 SIDs  |    1 SID   |   2 SIDs   |   3 SIDs   |
+-------------+------------+------------+------------+------------+
|    T1       |  74.2%     |   99.5%    |  99.9%     | 100.0%     |
+-------------+------------+------------+------------+------------+
|    T2       |                No SRLG Information                |
+-------------+------------+------------+------------+------------+
|    T3       |  93.6%     |   99.9%    | 100.0%     |   0.0%     |
+-------------+------------+------------+------------+------------+
|    T4       |  62.5%     |   98.2%    | 100.0%     | 100.0%     |
+-------------+------------+------------+------------+------------+
|    T5       |  83.1%     |  100.0%    | 100.0%     | 100.0%     |
+-------------+------------+------------+------------+------------+
|    T6       |                No SRLG Information                |
+-------------+---------------------------------------------------+
|    T7       |                No SRLG Information                |
+-------------+------------+------------+------------+------------+
|    T8       |  85.2%     |   100.0%   | 100.0%     | 100.0%     |
+-------------+------------+------------+------------+------------+
|    T9       |  98.9%     |   100.0%   | 100.0%     | 100.0%     |
+-------------+------------+------------+------------+------------+
 Table 3B: Local SRLG protection repair size Cumulative distribution
The remaining two tables summarize the measurements on the number of
SIDs needed for node protection.

+---------+----------+----------+----------+----------+----------+
| Network |  0 SIDs  |   1 SID  | 2 SIDs   |  3 SIDs  |  4 SIDs  |
+---------+----------+----------+----------+----------+----------+
|    T1   |  49.8%   | 47.9%    | 2.1%     |  0.1%    |  0.0%    |
+---------+----------+----------+----------+----------+----------+
|    T2   |  36,5%   | 59.6%    | 3.6%     |  0.2%    |  0.0%    |
+---------+----------+----------+----------+----------+----------+
|    T3   |  73.3%   | 25.6%    | 1.1%     |  0.0%    |  0.0%    |
+---------+----------+----------+----------+----------+----------+
|    T4   |  36.1%   | 57.3%    | 6.3%     |  0.2%    |  0.0%    |
+---------+----------+----------+----------+----------+----------+
|    T5   |  73.2%   | 26.8%    | 0%       |  0%      |  0%      |
+---------+----------+----------+----------+----------+----------+
|    T6   |  78.3%   | 21.3%    | 0.3%     |  0%      |  0%      |
+---------+----------+----------+----------+----------+----------+
|    T7   |  66.1%   | 32.8%    | 1.1%     |  0%      |  0%      |
+---------+----------+----------+----------+----------+----------+
|    T8   |  59.7%   | 40.2%    | 0%       |  0%      |  0%      |
+---------+----------+----------+----------+----------+----------+
|    T9   |  98.9%   | 1.0%     | 0%       |  0%      |  0%      |
+---------+----------+----------+----------+----------+----------+
        Table 4A: Node protection (repair size distribution)

+---------+----------+----------+----------+----------+----------+
| Network |  0 SIDs  |   1 SID  | 2 SIDs   |  3 SIDs  |  4 SIDs  |
+---------+----------+----------+----------+----------+----------+
|    T1   |  49.7%   |  97.6%   |  99.8%   | 99.9%    |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T2   |  36.5%   |  96.1%   |  99.7%   | 99.9%    |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T3   |  73.3%   |  98.9%   |  99.9%   | 100.0%   |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T4   |  36.1%   |  93.4%   |  99.8%   | 99.9%    |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T5   |  73.2%   | 100.0%   | 100.0%   | 100.0%   |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T6   |  78.4%   | 99.7%    | 100.0%   | 100.0%   |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T7   |  66.1%   | 98.9%    | 100.0%   | 100.0%   |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T8   |  59.7%   | 100.0%   | 100.0%   | 100.0%   |  100%    |
+---------+----------+----------+----------+----------+----------+
|    T9   |  98.9%   | 100.0%   | 100.0%   | 100.0%   |  100%    |
+---------+----------+----------+----------+----------+----------+
   Table 4B: Node protection (repair size cumulative distribution)

Authors' Addresses

Ahmed Bashandy
Cisco Systems
Stephane Litkowski
Cisco Systems
France
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Pierre Francois
INSA Lyon
Bruno Decraene
Orange
Issy-les-Moulineaux
France
Daniel Voyer
Bell Canada
Canada