rfc9855.original   rfc9855.txt 
Network Working Group A. Bashandy Internet Engineering Task Force (IETF) A. Bashandy
Internet-Draft Individual Request for Comments: 9855 HPE
Intended status: Standards Track S. Litkowski Category: Standards Track S. Litkowski
Expires: 16 August 2025 C. Filsfils ISSN: 2070-1721 C. Filsfils
Cisco Systems Cisco Systems
P. Francois P. Francois
INSA Lyon INSA Lyon
B. Decraene B. Decraene
Orange Orange
D. Voyer D. Voyer
Bell Canada Bell Canada
12 February 2025 October 2025
Topology Independent Fast Reroute using Segment Routing Topology Independent Fast Reroute Using Segment Routing
draft-ietf-rtgwg-segment-routing-ti-lfa-21
Abstract Abstract
This document presents Topology Independent Loop-free Alternate Fast This document presents Topology Independent Loop-Free Alternate (TI-
Reroute (TI-LFA), aimed at providing protection of node and adjacency LFA) Fast Reroute (FRR), which is aimed at providing protection of
segments within the Segment Routing (SR) framework. This Fast node and Adjacency segments within the Segment Routing (SR)
Reroute (FRR) behavior builds on proven IP Fast Reroute concepts framework. This FRR behavior builds on proven IP FRR concepts being
being LFAs, remote LFAs (RLFA), and remote LFAs with directed LFAs, Remote LFAs (RLFAs), and Directed Loop-Free Alternates (DLFAs).
forwarding (DLFA). It extends these concepts to provide guaranteed It extends these concepts to provide guaranteed coverage in any two-
coverage in any two-connected networks using a link-state IGP. An connected networks using a link-state IGP. An important aspect of
important aspect of TI-LFA is the FRR path selection approach TI-LFA is the FRR path selection approach establishing protection
establishing protection over the expected post-convergence paths from over the expected post-convergence paths from the Point of Local
the point of local repair, reducing the operational need to control Repair (PLR), reducing the operational need to control the tie-breaks
the tie-breaks among various FRR options. among various FRR options.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Abbreviations and Notations
3.1. Conventions used in this document . . . . . . . . . . . . 8 2.2. Conventions Used in This Document
4. Base principle . . . . . . . . . . . . . . . . . . . . . . . 8 3. Base Principle
5. Intersecting P-Space and Q-Space with post-convergence 4. Intersecting P-space and Q-space with Post-Convergence Paths
paths . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Extended P-space Property Computation for a Resource X over
5.1. Extended P-Space property computation for a resource X, Post-Convergence Paths
over post-convergence paths . . . . . . . . . . . . . . . 8 4.2. Q-space Property Computation for a Resource X over
5.2. Q-Space property computation for a resource X, over Post-Convergence Paths
post-convergence paths . . . . . . . . . . . . . . . . . 9 4.3. Scaling Considerations When Computing Q-space
5.3. Scaling considerations when computing Q-Space . . . . . . 9 5. TI-LFA Repair Path
6. TI-LFA Repair path . . . . . . . . . . . . . . . . . . . . . 9 5.1. FRR Path Using a Direct Neighbor
6.1. FRR path using a direct neighbor . . . . . . . . . . . . 11 5.2. FRR Path Using a PQ Node
6.2. FRR path using a PQ node . . . . . . . . . . . . . . . . 11 5.3. FRR Path Using a P Node and Q Node That Are Adjacent
6.3. FRR path using a P node and Q node that are adjacent . . 11 5.4. Connecting Distant P and Q Nodes Along Post-Convergence
6.4. Connecting distant P and Q nodes along post-convergence Paths
paths . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6. Building TI-LFA Repair Lists for SR Segments
7. Building TI-LFA repair lists for SR Segments . . . . . . . . 11 6.1. The Active Segment Is a Node Segment
7.1. The active segment is a node segment . . . . . . . . . . 12 6.2. The Active Segment Is an Adjacency Segment
7.2. The active segment is an adjacency segment . . . . . . . 12 6.2.1. Protecting [Adjacency, Adjacency] Segment Lists
7.2.1. Protecting [Adjacency, Adjacency] segment lists . . . 13 6.2.2. Protecting [Adjacency, Node] Segment Lists
7.2.2. Protecting [Adjacency, Node] segment lists . . . . . 13 7. Data Plane-Specific Considerations
8. Dataplane specific considerations . . . . . . . . . . . . . . 13 7.1. MPLS Data Plane Considerations
8.1. MPLS dataplane considerations . . . . . . . . . . . . . . 13 7.2. SRv6 Data Plane Considerations
8.2. SRv6 dataplane considerations . . . . . . . . . . . . . . 14 8. TI-LFA and SR Algorithms
9. TI-LFA and SR algorithms . . . . . . . . . . . . . . . . . . 14 9. Usage of Adjacency Segments in the Repair List
10. Usage of Adjacency segments in the repair list . . . . . . . 15 10. Security Considerations
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16 11. IANA Considerations
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 12. References
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 16 12.1. Normative References
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 12.2. Informative References
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 Appendix A. Advantages of Using the Expected Post-Convergence Path
15.1. Normative References . . . . . . . . . . . . . . . . . . 17 During FRR
15.2. Informative References . . . . . . . . . . . . . . . . . 17 Appendix B. Analysis Based on Real Network Topologies
Appendix A. Advantages of using the expected post-convergence path Acknowledgments
during FRR . . . . . . . . . . . . . . . . . . . . . . . 19 Contributors
Appendix B. Analysis based on real network topologies . . . . . 21 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Acronyms
* DLFA: Remote LFA with Directed forwarding.
* FRR: Fast Re-route.
* IGP: Interior Gateway Protocol.
* LFA: Loop-Free Alternate.
* LSDB: Link State DataBase.
* PLR: Point of Local Repair.
* RL: Repair list.
* RLFA: Remote LFA.
* SID: Segment Identifier.
* SPF: Shortest Path First.
* SR: Segment Routing.
* SRLG: Shared Risk Link Group.
* TI-LFA: Topology Independent LFA.
2. Introduction 1. Introduction
This document outlines a local repair mechanism that leverages This document outlines a local repair mechanism that leverages
Segment Routing (SR) to restore end-to-end connectivity in the event Segment Routing (SR) to restore end-to-end connectivity in the event
of a failure involving a directly connected network component. This of a failure involving a directly connected network component. This
mechanism is designed for standard link-state Interior Gateway mechanism is designed for standard link-state Interior Gateway
Protocol (IGP) shortest path scenarios. Non-SR mechanisms for local Protocol (IGP) shortest path scenarios. Non-SR mechanisms for local
repair are beyond the scope of this document. Non-local failures are repair are beyond the scope of this document. Non-local failures are
addressed in a separate document addressed in a separate document [SR-LOOP].
[I-D.bashandy-rtgwg-segment-routing-uloop].
The term topology independent (TI) describes the capability providing The term Topology Independent (TI) describes the capability providing
a loop free backup path that is effective accross all network a loop-free backup path that is effective across all network
topologies. This provides a major improvement compared to LFA topologies. This provides a major improvement compared to LFA
[RFC5286] and remote LFA [RFC7490] which cannot provide a complete [RFC5286] and RLFA [RFC7490], which cannot provide a complete
protection coverage in some topologies as described in [RFC6571]. protection coverage in some topologies as described in [RFC6571].
When the network reconverges after failure, micro-loops [RFC5715] can When the network reconverges after failure, micro-loops [RFC5715] can
form due to transient inconsistencies in the forwarding tables of form due to transient inconsistencies in the forwarding tables of
different routers. If it is determined that micro-loops are a different routers. If it is determined that micro-loops are a
significant issue in the deployment, then a suitable loop-free significant issue in the deployment, then a suitable loop-free
convergence method, such as one of those described in [RFC5715], convergence method should be implemented, such as one of those
[RFC6976], [RFC8333], or [I-D.bashandy-rtgwg-segment-routing-uloop] described in [RFC5715], [RFC6976], [RFC8333], or [SR-LOOP].
should be implemented.
TI-LFA operates locally at the Point of Local Repair (PLR) upon TI-LFA operates locally at the Point of Local Repair (PLR) upon
detecting a failure in one of its direct links. Consequently, this detecting a failure in one of its direct links. Consequently, this
local operation does not influence: local operation does not influence:
* Micro-loops that may or may not form during the distributed * Micro-loops that may or may not form during the distributed IGP
Interior Gateway Protocol (IGP) convergence as delineated in convergence as delineated in [RFC5715]:
[RFC5715]:
- These micro-loops occur on routes directed towards the - These micro-loops occur on routes directed towards the
destination that do not traverse TI-LFA-configured paths. destination that do not traverse paths configured for TI-LFA.
According to [RFC5714], the formation of such micro-loops can According to [RFC5714], the formation of such micro-loops can
prevent traffic from reaching the PLR, thereby bypassing the prevent traffic from reaching the PLR, thereby bypassing the
TI-LFA paths established for rerouting. TI-LFA paths established for rerouting.
* Micro-loops that may or may not develop when the previously failed * Micro-loops that may or may not develop when the previously failed
link is restored to functionality. link is restored to functionality.
TI-LFA paths are activated from the instant the PLR detects a failure 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 in a local link and remain in effect until the IGP convergence at the
Protocol (IGP) convergence at the PLR is fully achieved. PLR is fully achieved. Consequently, they are not susceptible to
Consequently, they are not susceptible to micro-loops that may arise micro-loops that may arise due to variations in the IGP convergence
due to variations in the IGP convergence times across different nodes times across different nodes through which these paths traverse.
through which these paths traverse. This ensures a stable and This ensures a stable and predictable routing environment, minimizing
predictable routing environment, minimizing disruptions typically disruptions typically associated with asynchronous network behavior.
associated with asynchronous network behavior. However, an early However, an early (relative to the other nodes) IGP convergence at
(relative to the other nodes) IGP convergence at the PLR and the the PLR and the consecutive "early" release of TI-LFA paths may cause
consecutive ”early” release of TI-LFA paths may cause micro-loops, micro-loops, especially if these paths have been computed using the
especially if these paths have been computed using the methods methods described in Sections 5.2, 5.3, or 5.4 of this document. One
described in Section Section 6.2, Section 6.3, or Section 6.4 of the of the possible ways to prevent such micro-loops is local convergence
document. One of the possible ways to prevent such micro-loops is delay [RFC8333].
local convergence delay ([RFC8333]).
TI-LFA procedures are complementary to application of any micro-loop TI-LFA procedures are complementary to the application of any micro-
avoidance procedures in the case of link or node failure: loop avoidance procedures in the case of link or node failure:
* Link or node failure requires some urgent action to restore the * Link or node failure requires some urgent action to restore the
traffic that passed thru the failed resource. TI-LFA paths are traffic that passed through the failed resource. TI-LFA paths are
pre-computed and pre-installed and therefore suitable for urgent pre-computed and pre-installed; therefore, they are suitable for
recovery urgent recovery.
* The paths used in the micro-loop avoidance procedures typically * The paths used in the micro-loop avoidance procedures typically
cannot be pre-computed. cannot be pre-computed.
For each destination (as specified by the IGP) in the network, TI-LFA For each destination (as specified by the IGP) in the network, TI-LFA
pre-installs a backup forwarding entry for each protected destination pre-installs a backup forwarding entry for each protected destination
ready to be activated upon detection of the failure of a link used to 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 reach the destination. TI-LFA provides protection in the event of
any one of the following: single link failure, single node failure, any one of the following: single link failure, single node failure,
or single SRLG failure. In link failure mode, the destination is or single Shared Risk Link Group (SRLG) failure. In link failure
protected assuming the failure of the link. In node protection mode, mode, the destination is protected assuming the failure of the link.
the destination is protected assuming that the neighbor connected to In node protection mode, the destination is protected assuming that
the primary link Section 3 has failed. In SRLG protecting mode, the the neighbor connected to the primary link (see Section 2) has
destination is protected assuming that a configured set of links failed. In SRLG protecting mode, the destination is protected
sharing fate with the primary link has failed (e.g. a linecard or a assuming that a configured set of links sharing fate with the primary
set of links sharing a common transmission pipe). 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 Protection techniques outlined in this document are limited to
protecting links, nodes, and SRLGs that are within a link-state IGP protecting links, nodes, and SRLGs that are within a link-state IGP
area. Protecting domain exit routers and/or links attached to area. Protecting domain exit routers and/or links attached to
another routing domains are beyond the scope of this document another routing domain is beyond the scope of this document.
By utilizing Segment Routing (SR), TI-LFA eliminates the need to By utilizing SR, TI-LFA eliminates the need to establish Targeted
establish Targeted Label Distribution Protocol sessions with remote Label Distribution Protocol sessions with remote nodes for leveraging
nodes for leveraging the benefits of Remote Loop-Free Alternates the benefits of Remote Loop-Free Alternates (RLFAs) [RFC7490]
(RLFA) [RFC7490][RFC7916] or Directed Loop-Free Alternates (DLFA) [RFC7916] or Directed Loop-Free Alternates (DLFAs) [IPFRR-TUNNELS].
[RFC5714]. All the Segment Identifiers (SIDs) required are present All the Segment Identifiers (SIDs) required are present within the
within the Link State Database (LSDB) of the Interior Gateway Link State Database (LSDB) of the IGP. Consequently, there is no
Protocol (IGP). Consequently, there is no longer a necessity to longer a necessity to prefer LFAs over RLFAs or DLFAs, nor is there a
prefer LFAs over RLFAs or DLFAs, nor is there a need to minimize the need to minimize the number of RLFA or DLFA repair nodes.
number of RLFA or DLFA repair nodes.
Utilizing SR makes the requirement unnecessary to establish Utilizing SR also eliminates the need to establish an additional
additional state within the network for enforcing explicit Fast state within the network for enforcing explicit Fast Reroute (FRR)
Reroute (FRR) paths. This spares the nodes from maintaining paths. This spares the nodes from maintaining a supplementary state
supplementary state and frees the operator from the necessity to and frees the operator from the necessity to implement additional
implement additional protocols or protocol sessions solely to augment protocols or protocol sessions solely to augment protection coverage.
protection coverage.
TI-LFA also brings the benefit of the ability to provide a backup TI-LFA also brings the benefit of the ability to provide a backup
path that follows the expected post-convergence path considering a path that follows the expected post-convergence path considering a
particular failure which reduces the need of locally configured particular failure, which reduces the need of locally configured
policies that influence the backup path selection ([RFC7916]). The policies that influence the backup path selection [RFC7916]. The
easiest way to express the expected post-convergence path in a loop- easiest way to express the expected post-convergence path in a loop-
free manner is to encode it as a list of adjacency segments. free manner is to encode it as a list of Adjacency segments.
However, this may create a long segment list that some hardware may 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 not be able to program. One of the challenges of TI-LFA is to encode
the expected post-convergence path by combining adjacency segments the expected post-convergence path by combining Adjacency segments
and node segments. Each implementation may independently develop its and node segments. Each implementation may independently develop its
own algorithm for optimizing the ordered segment list. This document own algorithm for optimizing the ordered segment list. This document
provides an outline of the fundamental concepts applicable to provides an outline of the fundamental concepts applicable to
constructing the SR backup path, along with the related dataplane constructing the SR backup path, along with the related data plane
procedures. Appendix A describes some of the post-convergence path procedures. Appendix A contains a more detailed description of some
related aspects of TI-LFA in more detail. of the aspects of TI-LFA related to post-convergence path.
Section 3 defines the main notations used in the document. They are This document is structured as follows:
in line with [RFC5714].
Section 4 defines the main principles of TI-LFA backup path * Section 2 defines the main notations used in the document. They
computation. are in line with [RFC5714].
Section 5 suggests to compute the P-Space and Q-Space properties * Section 3 defines the main principles of TI-LFA backup path
defined in Section 3, for the specific case of nodes lying over the computation.
post-convergence paths towards the protected destinations.
Using the properties defined in Section 5, Section 6 describes how to * Section 4 suggests to compute the P-space and Q-space properties
compute protection lists that encode a loop-free post-convergence defined in Section 2 for the specific case of nodes lying over the
path towards the destination. post-convergence paths towards the protected destinations.
Section 7 defines the segment operations to be applied by the PLR to * Section 5 describes how to compute protection lists that encode a
ensure consistency with the forwarding state of the repair node. loop-free post-convergence path towards the destination using the
properties defined in Section 4.
Section 8 discusses aspects that are specific to the dataplane. * Section 6 defines the segment operations to be applied by the PLR
to ensure consistency with the forwarding state of the repair
node.
Section 9 discusses relationship between TI-LFA and the SR-algorithm. * Section 7 discusses aspects that are specific to the data plane.
Certain considerations are needed when adjacency segments are used in * Section 8 discusses the relationship between TI-LFA and the SR
a repare list. Section 10 provides an overview of these algorithm.
considerations.
Section 11 discusses security considerations. * Section 9 provides an overview of the certain considerations that
are needed when Adjacency segments are used in a Repair List (RL).
Appendix A highlights advantages of using the expected post- * Section 10 discusses security considerations.
convergence path during FRR.
By implementing the algorithms detailed in this document within * Appendix A highlights advantages of using the expected post-
actual service provider and large enterprise network environments, convergence path during FRR.
real-life measurements are presented regarding the number of SIDs
utilized by repair paths. These measurements are summarized in
Appendix B.
3. Terminology * Appendix B summarizes the measurements from 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.
The main notations used in this document are defined as follows. 2. Terminology
The terms "old" and "new" topologies refer to the Link State Database 2.1. Abbreviations and Notations
(LSDB) state before and after the considered failure, respectively.
SPT_old(R) is the Shortest Path Tree rooted at node R in the initial DLFA: Directed Loop-Free Alternate
state of the network.
SPT_new(R, X) is the Shortest Path Tree rooted at node R in the state FRR: Fast Reroute
of the network after the resource X has failed.
PLR stands for "Point of Local Repair". It is the router that IGP: Interior Gateway Protocol
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 LFA: Loop-Free Alternate
TI-LFA.
The P-space P(R,X) of a router R with regard to a resource X (e.g. a LSDB: Link State Database
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. PLR: Point of Local Repair
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 RL: Repair List
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. RLFA: Remote Loop-Free Alternate
Primary Interface: Primary Outgoing Interface: One of the outgoing SID: Segment Identifier
interfaces towards a destination according to the IGP link-state
protocol
Primary Link: A link connected to the primary interface SPF: Shortest Path First
adj-sid(S-F): Adjacency Segment from node S to node F SPT: Shortest Path Tree
3.1. Conventions used in this document SR: Segment Routing
SRLG: Shared Risk Link Group
TI-LFA: Topology Independent Loop-Free Alternate
The main notations used in this document are defined as follows:
* The terms "old" and "new" topologies refer to the LSDB state
before and after the considered failure, respectively.
* SPT_old(R) is the SPT rooted at node R in the initial state of the
network.
* SPT_new(R, X) is the SPT rooted at node R in the state of the
network after the resource X has failed.
* The PLR 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 an 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.
* The primary interface and primary outgoing interface are one of
the outgoing interfaces towards a destination according to the IGP
link-state protocol.
* The primary link is a link connected to the primary interface.
* The Adj-SID(S-F) is the Adjacency segment from node S to node F.
2.2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
4. Base principle 3. Base Principle
The basic algorithm to compute the repair path is to pre-compute 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 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 loop-free segment list. One way to provide a loop-free segment list
is to use adjacency SIDs only. However, this approach may create 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 very long SID lists that hardware may not be able to handle due to
MSD (Maximum SID Depth) limitations. Maximum SID Depth (MSD) limitations.
An implementation is free to use any local optimization to provide An implementation is free to use any local optimization to provide
smaller segment lists by combining Node SIDs and Adjacency SIDs. In smaller segment lists by combining Node-SIDs and Adjacency SIDs. In
addition, the usage of Node-SIDs allow to maximize ECMPs over the addition, the usage of Node-SIDs allow for maximizing ECMPs over the
backup path. These optimizations are out of scope of this document, backup path. These optimizations are out of scope of this document;
however the subsequent sections provide some guidance on how to however, the subsequent sections provide some guidance on how to
leverage P-Spaces and Q-Spaces to optimize the size of the segment leverage P-spaces and Q-spaces to optimize the size of the segment
list. list.
5. Intersecting P-Space and Q-Space with post-convergence paths 4. Intersecting P-space and Q-space with Post-Convergence Paths
One of the challenges of defining an SR path following the expected 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 post-convergence path is to reduce the size of the segment list. In
order to reduce this segment list, an implementation MAY determine order to reduce this segment list, an implementation MAY determine
the P-Space/Extended P-Space and Q-Space properties (defined in the P-space / extended P-space and Q-space properties (defined in
[RFC7490]) of the nodes along the expected post-convergence path from [RFC7490]) of the nodes along the expected post-convergence path from
the PLR to the protected destination and compute an SR explicit path 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 from P to Q when they are not adjacent. Such properties will be used
in Section 6 to compute the TI-LFA repair list. in Section 5 to compute the TI-LFA RL.
5.1. Extended P-Space property computation for a resource X, over post- 4.1. Extended P-space Property Computation for a Resource X over Post-
convergence paths Convergence Paths
The objective is to determine which nodes on the post-convergence 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 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 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). links adjacent to the PLR or a neighbor node of the PLR).
This can be found by: This can be found by:
* Excluding neighbors which are not on the post-convergence path * excluding neighbors that are not on the post-convergence path when
when computing P'(R,X) computing P'(R, X), then
* Then, intersecting the set of nodes belonging to the post- * intersecting the set of nodes belonging to the post-convergence
convergence path from R to D, assuming the failure of X, with path from R to D, assuming the failure of X, with P'(R, X).
P'(R, X).
5.2. Q-Space property computation for a resource X, over post- 4.2. Q-space Property Computation for a Resource X over Post-
convergence paths Convergence Paths
The goal is to determine which nodes on the post-convergence path 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 from the PLR R to the destination D are in the Q-space of destination
the Q-Space of destination D with regard to resource X (where X can D with regard to resource X (where X can be a link or a set of links
be a link or a set of links adjacent to the PLR, or a neighbor node adjacent to the PLR, or a neighbor node of the PLR).
of the PLR).
This can be found by intersecting the set of nodes belonging to the 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 post-convergence path from R to D, assuming the failure of X, with
Q(D, X). Q(D, X).
5.3. Scaling considerations when computing Q-Space 4.3. Scaling Considerations When Computing Q-space
[RFC7490] raises scaling concerns about computing a Q-Space per [RFC7490] raises scaling concerns about computing a Q-space per
destination. Similar concerns may affect TI-LFA computation if an destination. Similar concerns may affect TI-LFA computation if an
implementation tries to compute a reverse Shortest Path Tree implementation tries to compute a reverse Shortest Path Tree (SPT)
([RFC7490]) for every destination in the network to determine the [RFC7490] for every destination in the network to determine the
Q-Space. It will be up to each implementation to determine the good Q-space. It will be up to each implementation to determine the good
tradeoff between scaling and accuracy of the optimization. tradeoff between scaling and accuracy of the optimization.
6. TI-LFA Repair path 5. TI-LFA Repair Path
The TI-LFA repair path consists of an outgoing interface and a list The TI-LFA repair path consists of an outgoing interface and a list
of segments (repair list (RL)) to insert on the SR header in of segments (a Repair List (RL)) to insert on the SR header in
accordance with the dataplane used. The repair list encodes the accordance with the data plane used. The RL encodes the explicit
explicit, and possibly post-convergence, path to the destination, (and possibly post-convergence) path to the destination, which avoids
which avoids the protected resource X and, at the same time, is the protected resource X. At the same time, the RL is guaranteed to
guaranteed to be loop-free irrespective of the state of FIBs along be loop-free, irrespective of the state of FIBs along the nodes
the nodes belonging to the explicit path as long as the states of the belonging to the explicit path, as long as the states of the FIBs are
FIBs are programmed according to a link-state IGP. Thus, there is no programmed according to a link-state IGP. Thus, there is no need for
need for any co-ordination or message exchange between the PLR and any coordination or message exchange between the PLR and any other
any other router in the network. router in the network.
The TI-LFA repair path is found by intersecting P(S,X) and Q(D,X) 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- 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) 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. 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, The TI-LFA RL is expressed generally as (Node-SID(P), EP(P, Q)).
Q)).
S ------- N1 ----------- D S --------- N1 --------- D
*\ | \ | *\ | \ |
* \ | \ | * \ | \ |
* \ | \ | * \ | \ |
* N2-----R1****R2 *** R3 * N2----- R1***R2 *** R3
* * * *
N3 ********* N3 **********
***** : link with high metric (1k) ***** : link with high metric (1k)
----- : link with metric 1 ----- : link with metric 1
Figure 1: Sample topology with TI-LFA Figure 1: Sample Topology with TI-LFA
As an example, in Figure 1, the focus is on the TI-LFA backup from S As an example, in Figure 1, the focus is on the TI-LFA backup from S
to D, considering the failure of node N1. to D, considering the failure of node N1.
* First, P(S, N1) is computed and results in [N3, N2, R1]. * First, P(S, N1) is computed and results in [N3, N2, R1].
* Then, Q(D, N1) is computed and results in [R3]. * Then, Q(D, N1) is computed and results in [R3].
* The expected post-convergence path from S to D considering the * The expected post-convergence path from S to D considering the
failure of N1 is <N2 -> R1 -> R2 -> R3 -> D> (we are naming it failure of N1 is <N2 -> R1 -> R2 -> R3 -> D> (we are naming it
PCPath in this example). "PCPath" in this example).
* P(S, N1) intersection with PCPath is [N2, R1], R1 being the deeper * P(S, N1) intersection with PCPath is [N2, R1]. With R1 being the
downstream node in PCPath, it can be assumed to be used as P node deeper downstream node in PCPath, it can be assumed to be used as
(this is an example and an implementation could use a different a P node (this is an example, and an implementation could use a
strategy to choose the P node). different strategy to choose the P node).
* Q(D, N1) intersection with PCPath is [R3], so R3 is picked as Q * Q(D, N1) intersection with PCPath is [R3], so R3 is picked as a Q
node. An SR explicit path is then computed from R1 (P node) to R3 node. An SR-explicit path is then computed from R1 (P node) to R3
(Q node) following PCPath (R1 -> R2 -> R3): <Adj-Sid(R1-R2), Adj- (Q node) following PCPath (R1 -> R2 -> R3): <Adj-SID(R1-R2), Adj-
Sid(R2-R3)>. SID(R2-R3)>.
As a result, the TI-LFA repair list of S for destination D As a result, the TI-LFA RL of S for destination D considering the
considering the failure of node N1 is: <Node-SID(R1), Adj-Sid(R1-R2), failure of node N1 is: <Node-SID(R1), Adj-SID(R1-R2), Adj-
Adj-Sid(R20R3)>. SID(R2-R3)>.
Most often, the TI-LFA repair list has a simpler form, as described Most often, the TI-LFA RL has a simpler form, as described in the
in the following sections. Appendix B provides statistics for the following sections. Appendix B provides statistics for the number of
number of SIDs in the explicit path to protect against various SIDs in the explicit path to protect against various failures.
failures.
6.1. FRR path using a direct neighbor 5.1. FRR Path Using a Direct Neighbor
When a direct neighbor is in P(S,X) and Q(D,x) and the link to that When a direct neighbor is in P(S, X) and Q(D, x), and the link to
direct neighbor is on the post-convergence path, the outgoing that direct neighbor is on the post-convergence path, the outgoing
interface is set to that neighbor and the repair segment list is interface is set to that neighbor and the repair segment list is
empty. empty.
This is comparable to a post-convergence LFA FRR repair. This is comparable to a post-convergence LFA FRR repair.
6.2. FRR path using a PQ node 5.2. FRR Path Using a PQ Node
When a remote node R is in P(S,X) and Q(D,x) and on the post- When a remote node R is in P(S, X) and Q(D, x) and on the post-
convergence path, the repair list is made of a single node segment to convergence path, the RL is made of a single node segment to R, and
R and the outgoing interface is set to the outgoing interface used to the outgoing interface is set to the outgoing interface used to reach
reach R. R.
This is comparable to a post-convergence RLFA repair tunnel. This is comparable to a post-convergence RLFA repair tunnel.
6.3. FRR path using a P node and Q node that are adjacent 5.3. 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 When a node P is in P(S, X) and a node Q is in Q(D, x), and both are
the post-convergence path and both are adjacent to each other, the on the post-convergence path and are adjacent to each other, the RL
repair list is made of two segments: A node segment to P (to be is made of two segments: a node segment to P (to be processed first),
processed first), followed by an adjacency segment from P to Q. followed by an Adjacency segment from P to Q.
This is comparable to a post-convergence DLFA (LFA with directed This is comparable to a post-convergence Directed Loop-Free Alternate
forwarding) repair tunnel. (DLFA) repair tunnel.
6.4. Connecting distant P and Q nodes along post-convergence paths 5.4. Connecting Distant P and Q Nodes Along Post-Convergence Paths
In some cases, there is no adjacent P and Q node along the post- In some cases, there is no adjacent P and Q node along the
convergence path. As mentioned in Section 4, a list of adjacency post-convergence path. As mentioned in Section 3, a list of
SIDs can be used to encode the path between P and Q. However, the Adjacency SIDs can be used to encode the path between P and Q.
PLR can perform additional computations to compute a list of segments However, the PLR can perform additional computations to compute a
that represent a loop-free path from P to Q. How these computations shorter list of segments that represent a loop-free path from P to Q.
are done is out of scope of this document and is left to How these computations are done is out of scope of this document and
implementation. is left to implementation.
7. Building TI-LFA repair lists for SR Segments 6. Building TI-LFA Repair Lists for SR Segments
The following sections describe how to build the repair lists using The following sections describe how to build the RLs using the
the terminology defined in [RFC8402]. The procedures described in terminology defined in [RFC8402]. The procedures described in this
this section are equally applicable to both SR-MPLS and SRv6 section are equally applicable to both the Segment Routing over MPLS
dataplane, while the dataplane-specific considerations are described (SR-MPLS) and the Segment Routing over IPv6 (SRv6) data plane, while
in Section 8. the data plane-specific considerations are described in Section 7.
In this section, the process by which a protecting router S handles This section explains the process by which a protecting router S
the active segment of a packet upon the failure of its primary handles the active segment of a packet upon the failure of its
outgoing interface for the packet, S-F, is explained. The failure of primary outgoing interface for the packet S-F. The failure of the
the primary outgoing interface may occur due to various triggers, primary outgoing interface may occur due to various triggers, such as
such as link failure, neighbor node failure, and others. link failure, neighbor node failure, and others.
7.1. The active segment is a node segment 6.1. The Active Segment Is a Node Segment
The active segment MUST be kept on the SR header unchanged and the The active segment MUST be kept on the SR header unchanged and the RL
repair list MUST be added. The active segment becomes the first MUST be added. The active segment becomes the first segment after
segment after the repair list. The way the repair list is added the RL. The way the RL is added depends on the data plane used (see
depends on the dataplane used (see Section 8). Section 7).
7.2. The active segment is an adjacency segment 6.2. The Active Segment Is an Adjacency Segment
The FRR behavior applied by S for any packet received with an active This section defines the FRR behavior applied by S for any packet
adjacency segment S-F, for which protection was enabled, is defined received with an active Adjacency segment S-F for which protection
here. Since protection has been enabled for the segment S-F and was enabled. Since protection has been enabled for the segment S-F
signaled in the IGP (for instance, using protocol extensions from and signaled in the IGP (for instance, using protocol extensions from
[RFC8667] and [RFC8665]), a calculator of any SR policy utilizing [RFC8667] and [RFC8665]), a calculator of any SR policy utilizing
this segment is aware that it may be transiently rerouted out of S-F this segment is aware that it may be transiently rerouted out of S-F
in the event of an S-F failure. in the event of an S-F failure.
The simplest approach for link protection of an adjacency segment S-F The simplest approach for link protection of an Adjacency segment S-F
is to create a repair list that will carry the traffic to F. To do is to create an RL that will carry the traffic to F. To do so, one
so, one or more “PUSH” operations are performed. If the repair list, or more "PUSH" operations are performed. If the RL, while avoiding
while avoiding S-F, terminates on F, S only pushes segments of the S-F, terminates on F, S only pushes segments of the RL. Otherwise, S
repair list. Otherwise, S pushes a node segment of F, followed by pushes a node segment of F, followed by the segments of the RL. For
the segments of the repair list. For details on the "NEXT" and details on the "NEXT" and "PUSH" operations, refer to [RFC8402].
"PUSH" operations, refer to [RFC8402].
This method, which merges back the traffic at the remote end of the This method, which merges back the traffic at the remote end of the
adjacency segment, has the advantage of keeping as much as possible Adjacency segment, has the advantage of keeping as much traffic as
the traffic on the pre-failure path. When SR policies are involved possible on the pre-failure path. When SR policies are involved and
and strict compliance with the policy is required, an end-to-end strict compliance with the policy is required, an end-to-end
protection (beyond the scope of this document) should be preferred protection (beyond the scope of this document) should be preferred
over the local repair mechanism described above. over the local repair mechanism described above.
Note, however, that when the SR source node is using traffic Note, however, that when the SR source node is using Traffic
engineering (TE), it will generally not be possible for the PLR to Engineering (TE), it will generally not be possible for the PLR to
know what post-convergence path will be selected by the source node know what post-convergence path will be selected by the source node
once it detects the failure, since computation of the TE path is a once it detects the failure, since computation of the TE path is a
local matter that depends on constraints that may not be known at the local matter that depends on constraints that may not be known at the
PLR. Therefore, no method applied at the PLR can guarantee PLR. Therefore, no method applied at the PLR can guarantee
protection will follow the post-convergence path. protection will follow the post-convergence path.
The case where the active segment is followed by another adjacency The case where the active segment is followed by another Adjacency
segment is distinguished from the case where it is followed by a node segment is distinguished from the case where it is followed by a node
segment. Repair techniques for the respective cases are provided in segment. Repair techniques for the respective cases are provided in
the following subsections. the following subsections.
7.2.1. Protecting [Adjacency, Adjacency] segment lists 6.2.1. Protecting [Adjacency, Adjacency] Segment Lists
If the next segment in the list is an Adjacency segment, then the If the next segment in the list is an Adjacency segment, then the
packet has to be conveyed to F. packet has to be conveyed to F.
To do so, S MUST apply a "NEXT" operation on Adj-Sid(S-F) and then To do so, S MUST apply a "NEXT" operation on Adj-SID(S-F) and then
one or more “PUSH” operations. If the repair list, while avoiding one or more "PUSH" operations. If the RL, while avoiding S-F,
S-F, terminates on F, S only pushes the segments of the repair list. terminates on F, S only pushes the segments of the RL. Otherwise, S
Otherwise, S pushes a node segment of F, followed by the segments of pushes a node segment of F, followed by the segments of the RL. For
the repair list. For details on the "NEXT" and "PUSH" operations, details on the "NEXT" and "PUSH" operations, refer to [RFC8402].
refer to [RFC8402].
Upon failure of S-F, a packet reaching S with a segment list matching Upon failure of S-F, a packet reaching S with a segment list matching
[adj-sid(S-F),adj-sid(F-M),...] will thus leave S with a segment list [adj-sid(S-F), adj-sid(F-M), ...] will thus leave S with a segment
matching [RL(F),node(F),adj-sid(F-M),...], where RL(F) is the repair list matching [RL(F), node(F), adj-sid(F-M), ...], where RL(F) is the
list for destination F. RL for destination F.
7.2.2. Protecting [Adjacency, Node] segment lists 6.2.2. Protecting [Adjacency, Node] Segment Lists
If the next segment in the stack is a node segment, say for node T, If the next segment in the stack is a node segment, say for node T,
the segment list on the packet matches [adj-sid(S-F),node(T),...]. the segment list on the packet matches [adj-sid(S-F), node(T), ...].
In this case, S MUST apply a "NEXT" operation on the Adjacency In this case, S MUST apply a "NEXT" operation on the Adjacency
segment related to S-F, followed by a "PUSH" of a repair list segment related to S-F, followed by a "PUSH" of an RL redirecting the
redirecting the traffic to a node Q, whose path to node segment T is traffic to a node Q, whose path to node segment T is not affected by
not affected by the failure. the failure.
Upon failure of S-F, packets reaching S with a segment list matching Upon failure of S-F, packets reaching S with a segment list matching
[adj-sid(S-F), node(T), ...], would leave S with a segment list [adj-sid(S-F), node(T), ...] would leave S with a segment list
matching [RL(Q),node(T), ...]. matching [RL(Q), node(T), ...].
8. Dataplane specific considerations 7. Data Plane-Specific Considerations
8.1. MPLS dataplane considerations 7.1. MPLS Data Plane Considerations
MPLS dataplane for Segment Routing is described in [RFC8660]. The MPLS data plane for SR is described in [RFC8660].
The following dataplane behaviors apply when creating a repair list The following data plane behaviors apply when creating an RL using an
using an MPLS dataplane: MPLS data plane:
1. If the active segment is a node segment that has been signaled 1. If the active segment is a node segment that has been signaled
with penultimate hop popping and the repair list ends with an with penultimate hop popping, and the RL ends with an Adjacency
adjacency segment terminating on a node that advertised NEXT segment terminating on the penultimate node of the active
operation [RFC8402] of the active segment, then the active segment, then the active segment MUST be popped before pushing
segment MUST be popped before pushing the repair list. the RL.
2. If the active segment is a node segment but the other conditions 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 in 1. are not met, the active segment MUST be popped and then
again with a label value computed according to the Segment pushed again with a label value computed according to the Segment
Routing Global Block of Q, where Q is the endpoint of the repair Routing Global Block (SRGB) of Q, where Q is the endpoint of the
list. Finally, the repair list MUST be pushed. RL. Finally, the RL MUST be pushed.
8.2. SRv6 dataplane considerations 7.2. SRv6 Data Plane Considerations
SRv6 dataplane and programming instructions are described SRv6 data plane and programming instructions are described
respectively in [RFC8754] and [RFC8986]. respectively in [RFC8754] and [RFC8986].
The TI-LFA path computation algorithm is the same as in the SR-MPLS The TI-LFA path computation algorithm is the same as in the SR-MPLS
dataplane. Note however that the Adjacency SIDs are typically data plane. Note, however, that the Adjacency SIDs are typically
globally routed. In such case, there is no need for preceding an globally routed. In such a case, there is no need for preceding an
adjacency SID with a Prefix-SID [RFC8402] and the resulting repair Adjacency SID with a Prefix-SID [RFC8402], and the resulting RL is
list is likely shorter. likely shorter.
If the traffic is protected at a Transit Node, then an SRv6 SID list 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 is added on the packet to apply the RL. The addition of the RL
repair list follows the headend behaviors as specified in section 5 follows the head-end behaviors as specified in Section 5 of
of [RFC8986]. [RFC8986].
If the traffic is protected at an SR Segment Endpoint Node, first the If the traffic is protected at an SR Segment Endpoint Node, first the
Segment Endpoint packet processing is executed. Then the packet is Segment Endpoint packet processing is executed. Then, the packet is
protected as if its were a transit packet. protected as if it were a transit packet.
9. TI-LFA and SR algorithms 8. TI-LFA and SR Algorithms
SR allows an operator to bind an algorithm to a prefix-SID (as SR allows an operator to bind an algorithm to a Prefix-SID (as
defined in [RFC8402]. The algorithm value dictates how the path to defined in [RFC8402]). The algorithm value dictates how the path to
the prefix is computed. The SR default algorithm is known has the the prefix is computed. The SR default algorithm is known as the
"Shortest Path" algorithm. The SR default algorithm allows an "Shortest Path" algorithm. The SR default algorithm allows an
operator to override the IGP shortest path by using local policies. operator to override the IGP shortest path by using local policies.
When TI-LFA uses Node-SIDs associated with the default algorithm, 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 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 policy may have overridden the expected IGP path. As the local
policies are defined by the operator, it becomes the responsibility policies are defined by the operator, it becomes the responsibility
of this operator to ensure that the deployed policies do not affect of this operator to ensure that the deployed policies do not affect
the TI-LFA deployment. It should be noted that such situation can the TI-LFA deployment. It should be noted that such a situation can
already happen today with existing mechanisms as remote LFA. already happen today with existing mechanisms such as RLFA.
[RFC9350] defines a flexible algorithm (FlexAlgo) framework to be [RFC9350] defines a Flexible Algorithm framework to be associated
associated with Prefix-SIDs. FlexAlgo allows a user to associate a with Prefix-SIDs. A Flexible Algorithm allows a user to associate a
constrained path to a Prefix-SID rather than using the regular IGP constrained path to a Prefix-SID rather than using the regular IGP
shortest path. An implementation MAY support TI-LFA to protect Node- shortest path. An implementation MAY support TI-LFA to protect Node-
SIDs associated with a Flex Algo. In such a case, rather than SIDs associated with a Flexible Algorithm. In such a case, rather
computing the expected post-convergence path based on the regular than computing the expected post-convergence path based on the
SPF, an implementation SHOULD use the constrained SPF algorithm bound regular SPF, an implementation SHOULD use the constrained SPF
to the Flex Algo (using the Flex Algo Definition) instead of the algorithm bound to the Flexible Algorithm (using the Flexible
regular Dijkstra in all the SPF/rSPF computations that are occurring Algorithm Definition) instead of the regular Dijkstra in all the SPF/
during the TI-LFA computation. This includes the computation of the reverse SPF computations that are occurring during the TI-LFA
P-Space and Q-Space as well as the post-convergence path. computation. This includes the computation of the P-space and
Furthermore, the implementation SHOULD only use Node-SIDs/Adj-SIDs Q-space as well as the post-convergence path. Furthermore, the
bound to the Flex Algo and/or unprotected Adj-SIDs of the regular SPF implementation SHOULD only use Node-SIDs/Adj-SIDs bound to the
to build the repair list. The use of regular Dijkstra for the TI-LFA Flexible Algorithm and/or unprotected Adj-SIDs of the regular SPF to
computation or building of the repair path using SIDs other than build the RL. The use of regular Dijkstra for the TI-LFA computation
those recommended does not ensure that the traffic going over TI-LFA or for building the repair path using SIDs other than those
repair path during the fast-reroute period is honoring the Flex Algo recommended does not ensure that the traffic going over the TI-LFA
repair path during the FRR period is honoring the Flexible Algorithm
constraints. constraints.
10. Usage of Adjacency segments in the repair list 9. Usage of Adjacency Segments in the Repair List
The repair list of segments computed by TI-LFA may contain one or The RL of segments computed by TI-LFA may contain one or more
more adjacency segments. An adjacency segment may be protected or Adjacency segments. An Adjacency segment may be protected or not
not protected. protected.
S --- R2 --- R3 ---- R4 --- R5 --- D S --- R2 --- R3 ---- R4 --- R5 --- D
* | \ * * | \ *
* | \ * * | \ *
R7 ** R8 R7 ** R8
* | * |
* | * |
R9 -- R10 R9 -- R10
Figure 2 Figure 2
In Figure 2, all the metrics are equal to 1 except 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 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 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- S->D, the RL computed by R2 will be [adj-sid(R7-R8), adj-sid(R8-R4)],
sid(R8-R4)] and the outgoing interface will be to R7. If R3 fails, and the outgoing interface will be to R7. If R3 fails, R2 pushes the
R2 pushes the repair list onto the incoming packet to D. During the RL onto the incoming packet to D. During the FRR, if R7-R8 fails and
FRR, if R7-R8 fails and if TI-LFA has picked a protected adjacency if TI-LFA has picked a protected Adjacency segment for Adj-
segment for adj-sid(R7-R8), R7 will push an additional repair list SID(R7-R8), R7 will push an additional RL onto the packet following
onto the packet following the procedures defined in Section 7. the procedures defined in Section 6.
To avoid the possibility of this double FRR activation, an To avoid the possibility of this double FRR activation, an
implementation of TI-LFA MAY pick only non protected adjacency implementation of TI-LFA MAY pick only unprotected Adjacency segments
segments when building the repair list. However, this is important when building the RL. However, it is important to note that FRR in
to note that FRR in general is intended to protect for a single pre- general is intended to protect for a single pre-planned failure. If
planned failure. If the failure that happens is worse than expected the failure that happens is worse than expected or multiple failures
or multiple failures happen, FRR is not guaranteed to work. In such happen, FRR is not guaranteed to work. In such a case, fast IGP
a case, fast IGP convergence remains important to restore traffic as convergence remains important to restore traffic as quickly as
quickly as possible. possible.
11. Security Considerations 10. Security Considerations
The techniques described in this document are internal The techniques described in this document are internal
functionalities to a router that can guarantee an upper bound on the functionalities to a router that can guarantee an upper bound on the
time taken to restore traffic flow upon the failure of a directly time taken to restore traffic flow upon the failure of a directly
connected link or node. As these techniques steer traffic to the connected link or node. As these techniques steer traffic to the
post-convergence path as quickly as possible, this serves to minimize post-convergence path as quickly as possible, this serves to minimize
the disruption associated with a local failure which can be seen as a the disruption associated with a local failure, which can be seen as
modest security enhancement. The protection mechanisms does not a modest security enhancement. The protection mechanism does not
protect external destinations, but rather provides quick restoration protect external destinations, but rather provides quick restoration
for destination that are internal to a routing domain. for destinations 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:
* Francois Clad, Cisco Systems
* Pablo Camarillo, Cisco Systems 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.
14. Acknowledgments 11. IANA Considerations
The authors would like to thank Les Ginsberg, Stewart Bryant, This document has no IANA actions.
Alexander Vainsthein, Chris Bowers, Shraddha Hedge, Wes Hardaker,
Gunter Van de Velde and John Scudder for their valuable comments.
15. References 12. References
15.1. Normative References 12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K., [RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and P. Sarkar, "Operational Management of Horneffer, M., and P. Sarkar, "Operational Management of
Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916, Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
July 2016, <https://www.rfc-editor.org/info/rfc7916>. July 2016, <https://www.rfc-editor.org/info/rfc7916>.
skipping to change at page 17, line 45 skipping to change at line 766
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>. <https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986, (SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021, DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>. <https://www.rfc-editor.org/info/rfc8986>.
15.2. Informative References 12.2. Informative References
[I-D.bashandy-rtgwg-segment-routing-uloop] [IPFRR-TUNNELS]
Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B., Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
Francois, P., and P. Psenak, "Loop avoidance using Segment Fast Reroute using tunnels", Work in Progress, Internet-
Routing", Work in Progress, Internet-Draft, draft- Draft, draft-bryant-ipfrr-tunnels-03, 16 November 2007,
bashandy-rtgwg-segment-routing-uloop-17, 29 June 2024, <https://datatracker.ietf.org/doc/html/draft-bryant-ipfrr-
<https://datatracker.ietf.org/doc/html/draft-bashandy- tunnels-03>.
rtgwg-segment-routing-uloop-17>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for [RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286, IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008, DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>. <https://www.rfc-editor.org/info/rfc5286>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010, RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>. <https://www.rfc-editor.org/info/rfc5714>.
skipping to change at page 19, line 21 skipping to change at line 832
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, [RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture", A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022, RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>. <https://www.rfc-editor.org/info/rfc9256>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., [RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350, and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023, DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>. <https://www.rfc-editor.org/info/rfc9350>.
Appendix A. Advantages of using the expected post-convergence path [SR-LOOP] Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
during FRR Francois, P., and P. Psenak, "Loop avoidance using Segment
Routing", Work in Progress, Internet-Draft, draft-
bashandy-rtgwg-segment-routing-uloop-17, 29 June 2024,
<https://datatracker.ietf.org/doc/html/draft-bashandy-
rtgwg-segment-routing-uloop-17>.
[RFC7916] raised several operational considerations when using LFA or Appendix A. Advantages of Using the Expected Post-Convergence Path
remote LFA. [RFC7916] Section 3 presents a case where a high During FRR
bandwidth link between two core routers is protected through a PE
router connected with low bandwidth links. In such a case, [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. congestion may happen when the FRR backup path is activated.
[RFC7916] introduces a local policy framework to let the operator [RFC7916] introduces a local policy framework to let the operator
tuning manually the best alternate election based on its own tuning manually the best alternate election based on its own
requirements. requirements.
From a network capacity planning point of view, it is often assumed 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 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 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 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 routing considering that the link L has failed (we assume that the
traffic uses the post-convergence path starting from the node X). In 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 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 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 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 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 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 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 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 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 this capacity planning policy works; however, it is not fully
accurate. accurate.
In Figure 3, considering that the source of traffic is only from PE1 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 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 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 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 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 its nominal path, reducing the quantity of traffic received by X.
capacity planning rule presented previously has the drawback of The capacity planning rule presented previously has the drawback of
oversizing the network, however it allows to prevent any transient oversizing the network; however, it allows for preventing any
congestion (when for example X reroutes traffic before PE1 does). transient congestion (for example, when X reroutes traffic before PE1
does).
H --- I --- J H --- I --- J *
| | \ | | *
PE4 | | PE3 PE4 | | PE3
\ | (L) | / \ | (L) | *
A --- X --- B --- G * A --- X --- B --- G *
/ | | \ * | | *
PE1 | | PE2 PE1 | | PE2
\ | | / * | | *
C --- D --- E --- F * C --- D --- E --- F *
Figure 3 Figure 3
Based on this assumption, in order to facilitate the operation of Based on this assumption, in order to facilitate the operation of FRR
FRR, and limit the implementation of local FRR policies, traffic can and limit the implementation of local FRR policies, traffic can be
be steered by the PLR onto its expected post-convergence path during steered by the PLR onto its expected post-convergence path during the
the FRR phase. In our example, when link L fails, X switches the FRR phase. In our example, when link L fails, X switches the traffic
traffic destined to PE3 and PE2 on the post-convergence paths. This destined to PE3 and PE2 on the post-convergence paths. This is
is perfectly inline with the capacity planning rule that was perfectly in line with the capacity planning rule that was presented
presented before and also inline with the fact X may converge before before and also in line with the fact that X may converge before PE1
PE1 (or any other upstream router) and may spread the X-B traffic (or any other upstream router) and may spread the X-B traffic onto
onto the post-convergence paths rooted at X. the post-convergence paths rooted at X.
It should be noted, that some networks may have a different capacity 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 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 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. 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 However, it is important to note that a transient congestion may
possibly happen, even without FRR activated, for instance when X possibly happen even without FRR activated, for instance, when X
converges before the upstream routers. Operators are still free to converges before the upstream routers. Operators are still free to
use the policy framework defined in [RFC7916] if the usage of the use the policy framework defined in [RFC7916] if the usage of the
post-convergence paths rooted at the PLR is not suitable. post-convergence paths rooted at the PLR is not suitable.
Readers should be aware that FRR protection is pre-computing a backup Readers should be aware that FRR protection is pre-computing a backup
path to protect against a particular type of failure (link, node, path to protect against a particular type of failure (link, node, or
SRLG). When using the post-convergence path as FRR backup path, the SRLG). When using the post-convergence path as an FRR backup path,
computed post-convergence path is the one considering the failure we the computed post-convergence path is the one considering the failure
are protecting against. This means that FRR is using an expected we are protecting against. This means that FRR is using an expected
post-convergence path, and this expected post-convergence path may be post-convergence path, and this expected post-convergence path may be
actually different from the post-convergence path used if the failure actually different from the post-convergence path used if the failure
that happened is different from the failure FRR was protecting that happened is different from the failure FRR was protecting
against. As an example, if the operator has implemented a protection against. As an example, if the operator has implemented a protection
against a node failure, the expected post-convergence path used against a node failure, the expected post-convergence path used
during FRR will be the one considering that the node has failed. 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 However, even if a single link is failing or a set of links is
failing (instead of the full node), the node-protecting post- failing (instead of the full node), the node-protecting post-
convergence path will be used. The consequence is that the path used convergence path will be used. The consequence is that the path used
during FRR is not optimal with respect to the failure that has during FRR is not optimal with respect to the failure that has
actually occurred. actually occurred.
Another consideration to take into account is: while using the Another consideration to take into account is as follows: while using
expected post-convergence path for SR traffic using node segments the expected post-convergence path for SR traffic using node segments
only (for instance, PE to PE traffic using shortest path) has some only (for instance, PE to PE traffic using the shortest path) has
advantages, these advantages reduce when SR policies ([RFC9256]) are some advantages, these advantages reduce when SR policies [RFC9256]
involved. A segment-list used in an SR policy is computed to obey a are involved. A segment list used in an SR policy is computed to
set of path constraints defined locally at the head-end or centrally obey a set of path constraints defined locally at the head-end or
in a controller. TI-LFA cannot be aware of such path constraints and centrally in a controller. TI-LFA cannot be aware of such path
there is no reason to expect the TI-LFA backup path protecting one constraints, and there is no reason to expect the TI-LFA backup path
segments in that segment list to obey those constraints. When SR protecting one segment in that segment list to obey those
policies are used and the operator wants to have a backup path which constraints. When SR policies are used and the operator wants to
still follows the policy requirements, this backup path should be have a backup path that still follows the policy requirements, this
computed as part of the SR policy in the ingress node (or central backup path should be computed as part of the SR policy in the
controller) and the SR policy should not rely on local protection. ingress node (or central controller), and the SR policy should not
Another option could be to use FlexAlgo ([RFC9350]) to express the rely on local protection. Another option could be to use a Flexible
set of constraints and use a single node segment associated with a Algorithm [RFC9350] to express the set of constraints and use a
FlexAlgo to reach the destination. When using a node segment single node segment associated with a Flexible Algorithm to reach the
associated with a FlexAlgo, TI-LFA keeps providing an optimal backup destination. When using a node segment associated with a Flexible
by applying the appropriate set of constraints. The relationship Algorithm, TI-LFA keeps providing an optimal backup by applying the
between TI-LFA and the SR-algorithm is detailed in Section 9. 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 Appendix B. Analysis Based on Real Network Topologies
This section presents analysis performed on real service provider and This section presents an analysis performed on real service provider
large enterprise network topologies. The objective of the analysis and large enterprise network topologies. The objective of the
is to assess the number of SIDs required in an explicit path when the analysis is to assess the number of SIDs required in an explicit path
mechanisms described in this document are used to protect against the when the mechanisms described in this document are used to protect
failure scenarios within the scope of this document. The number of against the failure scenarios within the scope of this document. The
segments described in this section are applicable to instantiating number of segments described in this section are applicable to
segment routing over the MPLS forwarding plane. instantiating SR over the MPLS forwarding plane.
The measurement below indicate that for link and local SRLG The measurement below indicates that, for link and local SRLG
protection, a 1 SID repair path delivers more than 99% coverage. For protection, a repair path of 1 SID or less delivers more than 99%
node protection a 2 SIDs repair path yields 99% coverage. 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 Table 1 below lists the characteristics of the networks used in our
measurements. The number of links refers to the number of measurements. The number of links refers to the number of
"bidirectional" links (not directed edges of the graph). The "bidirectional" links (not directed edges of the graph). The
measurements are carried out as follows: measurements are carried out as follows:
* For each network, the algorithms described in this document are * For each network, the algorithms described in this document are
applied to protect all prefixes against link, node, and local SRLG applied to protect all prefixes against link, node, and local SRLG
failure failure.
* For each prefix, the number of SIDs used by the repair path is * For each prefix, the number of SIDs used by the repair path is
recorded recorded.
* The percentage of number of SIDs are listed in Tables 2A/B, 3A/B, * The percentage of number of SIDs are listed in Tables 2, 3, 4, 5,
and 4A/B 6, and 7.
The measurements listed in the tables indicate that for link and +=========+=======+=======+====================+============+
local SRLG protection, 1 SID repair path is sufficient to protect | Network | Nodes | Links | Node-to-Link Ratio | SRLG Info? |
more than 99% of the prefix in almost all cases. For node protection +=========+=======+=======+====================+============+
2 SIDs repair paths yield 99% coverage. | 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
| 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 The rest of this section presents the measurements done on the actual
topologies. The convention that we use is as follows topologies. The conventions that we use are as follows:
* 0 SIDs: the calculated repair path starts with a directly * 0 SIDs: The calculated repair path starts with a directly
connected neighbor that is also a loop free alternate, in which connected neighbor that is also a loop-free alternate; in which
case there is no need to explicitly route the traffic using case, there is no need to explicitly route the traffic using
additional SIDs. This scenario is described in Section 6.1. additional SIDs. This scenario is described in Section 5.1.
* 1 SIDs: the repair node is a PQ node, in which case only 1 SID is * 1 SID: The repair node is a PQ node; in which case, only 1 SID is
needed to guarantee a loop-free path. This scenario is covered in needed to guarantee a loop-free path. This scenario is covered in
Section 6.2. Section 5.2.
* 2 or more SIDs: The repair path consists of 2 or more SIDs as * 2 or more SIDs: The repair path consists of 2 or more SIDs as
described in Section 6.3 and Section 6.4. We do not cover the described in Sections 5.3 and 5.4. We do not cover the case for 2
case for 2 SIDs (Section 6.3) separately because there was no SIDs (Section 5.3) separately because there was no granularity in
granularity in the result. Also we treat the node-SID+adj-SID and the result. Also, we treat the Node-SID + Adj-SID and Node-SID +
node-SID + node-SID the same because they do not differ from the Node-SID the same because they do not differ from the data plane
data plane point of view. point of view.
Table 2A and 2B below summarize the measurements on the number of Tables 2 and 3 below summarize the measurements on the number of SIDs
SIDs needed for link protection needed for link protection.
+-------------+------------+------------+------------+------------+ +=========+========+=======+========+========+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+ +=========+========+=======+========+========+
| T1 | 74.3% | 25.3% | 0.5% | 0.0% | | T1 | 74.3% | 25.3% | 0.5% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T2 | 81.1% | 18.7% | 0.2% | 0.0% | | T2 | 81.1% | 18.7% | 0.2% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T3 | 95.9% | 4.1% | 0.1% | 0.0% | | T3 | 95.9% | 4.1% | 0.1% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T4 | 62.5% | 35.7% | 1.8% | 0.0% | | T4 | 62.5% | 35.7% | 1.8% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T5 | 85.7% | 14.3% | 0.0% | 0.0% | | T5 | 85.7% | 14.3% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T6 | 81.2% | 18.7% | 0.0% | 0.0% | | T6 | 81.2% | 18.7% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T7 | 98.9% | 1.1% | 0.0% | 0.0% | | T7 | 98.9% | 1.1% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T8 | 94.1% | 5.9% | 0.0% | 0.0% | | T8 | 94.1% | 5.9% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T9 | 98.9% | 1.0% | 0.0% | 0.0% | | T9 | 98.9% | 1.0% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
Table 2A: Link protection (repair size distribution)
+-------------+------------+------------+------------+------------+ Table 2: Link Protection (Repair Size
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | Distribution)
+-------------+------------+------------+------------+------------+
| T1 | 74.2% | 99.5% | 99.9% | 100.0% | +=========+========+=======+========+========+
+-------------+------------+------------+------------+------------+ | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
| T2 | 81.1% | 99.8% | 100.0% | 100.0% | +=========+========+=======+========+========+
+-------------+------------+------------+------------+------------+ | T1 | 74.2% | 99.5% | 99.9% | 100% |
| T3 | 95.9% | 99.9% | 100.0% | 100.0% | +---------+--------+-------+--------+--------+
+-------------+------------+------------+------------+------------+ | T2 | 81.1% | 99.8% | 100% | 100% |
| T4 | 62.5% | 98.2% | 100.0% | 100.0% | +---------+--------+-------+--------+--------+
+-------------+------------+------------+------------+------------+ | T3 | 95.9% | 99.9% | 100% | 100% |
| T5 | 85.7% | 100.0% | 100.0% | 100.0% | +---------+--------+-------+--------+--------+
+-------------+------------+------------+------------+------------+ | T4 | 62.5% | 98.2% | 100% | 100% |
| T6 | 81.2% | 99.9% | 100.0% | 100.0% | +---------+--------+-------+--------+--------+
+-------------+------------+------------+------------+------------+ | T5 | 85.7% | 100% | 100% | 100% |
| T7 | 98,8% | 100.0% | 100.0% | 100.0% | +---------+--------+-------+--------+--------+
+-------------+------------+------------+------------+------------+ | T6 | 81.2% | 99.9% | 100% | 100% |
| T8 | 94,1% | 100.0% | 100.0% | 100.0% | +---------+--------+-------+--------+--------+
+-------------+------------+------------+------------+------------+ | T7 | 98.8% | 100% | 100% | 100% |
| T9 | 98,9% | 100.0% | 100.0% | 100.0% | +---------+--------+-------+--------+--------+
+-------------+------------+------------+------------+------------+ | T8 | 94.1% | 100% | 100% | 100% |
Table 2B: Link protection repair size cumulative distribution +---------+--------+-------+--------+--------+
Table 3A and 3B summarize the measurements on the number of SIDs | T9 | 98.9% | 100% | 100% | 100% |
+---------+--------+-------+--------+--------+
Table 3: Link Protection (Repair Size
Cumulative Distribution)
Tables 4 and 5 summarize the measurements on the number of SIDs
needed for local SRLG protection. needed for local SRLG protection.
+-------------+------------+------------+------------+------------+ +=========+========+=======+========+========+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+ +=========+========+=======+========+========+
| T1 | 74.2% | 25.3% | 0.5% | 0.0% | | T1 | 74.2% | 25.3% | 0.5% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T2 | No SRLG Information | | T2 | No SRLG information |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T3 | 93.6% | 6.3% | 0.0% | 0.0% | | T3 | 93.6% | 6.3% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T4 | 62.5% | 35.6% | 1.8% | 0.0% | | T4 | 62.5% | 35.6% | 1.8% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T5 | 83.1% | 16.8% | 0.0% | 0.0% | | T5 | 83.1% | 16.8% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T6 | No SRLG Information | | T6 | No SRLG information |
+-------------+---------------------------------------------------+ +---------+----------------------------------+
| T7 | No SRLG Information | | T7 | No SRLG information |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T8 | 85.2% | 14.8% | 0.0% | 0.0% | | T8 | 85.2% | 14.8% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
| T9 | 98,9% | 1.1% | 0.0% | 0.0% | | T9 | 98.9% | 1.1% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+ +---------+--------+-------+--------+--------+
Table 3A: Local SRLG protection repair size distribution
Table 4: Local SRLG Protection (Repair
Size Distribution)
+=========+========+=======+========+========+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+=========+========+=======+========+========+
| T1 | 74.2% | 99.5% | 99.9% | 100% |
+---------+--------+-------+--------+--------+
| T2 | No SRLG information |
+---------+--------+-------+--------+--------+
| T3 | 93.6% | 99.9% | 100% | 0.0% |
+---------+--------+-------+--------+--------+
| T4 | 62.5% | 98.2% | 100% | 100% |
+---------+--------+-------+--------+--------+
| T5 | 83.1% | 100% | 100% | 100% |
+---------+--------+-------+--------+--------+
| T6 | No SRLG information |
+---------+----------------------------------+
| T7 | No SRLG information |
+---------+--------+-------+--------+--------+
| T8 | 85.2% | 100% | 100% | 100% |
+---------+--------+-------+--------+--------+
| T9 | 98.9% | 100% | 100% | 100% |
+---------+--------+-------+--------+--------+
Table 5: Local SRLG Protection (Repair
Size Cumulative 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 The remaining two tables summarize the measurements on the number of
SIDs needed for node protection. SIDs needed for node protection.
+---------+----------+----------+----------+----------+----------+ +=========+========+=======+========+========+========+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs | | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
+---------+----------+----------+----------+----------+----------+ +=========+========+=======+========+========+========+
| T1 | 49.8% | 47.9% | 2.1% | 0.1% | 0.0% | | T1 | 49.8% | 47.9% | 2.1% | 0.1% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T2 | 36,5% | 59.6% | 3.6% | 0.2% | 0.0% | | T2 | 36.5% | 59.6% | 3.6% | 0.2% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T3 | 73.3% | 25.6% | 1.1% | 0.0% | 0.0% | | T3 | 73.3% | 25.6% | 1.1% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T4 | 36.1% | 57.3% | 6.3% | 0.2% | 0.0% | | T4 | 36.1% | 57.3% | 6.3% | 0.2% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T5 | 73.2% | 26.8% | 0% | 0% | 0% | | T5 | 73.2% | 26.8% | 0.0% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T6 | 78.3% | 21.3% | 0.3% | 0% | 0% | | T6 | 78.3% | 21.3% | 0.3% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T7 | 66.1% | 32.8% | 1.1% | 0% | 0% | | T7 | 66.1% | 32.8% | 1.1% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T8 | 59.7% | 40.2% | 0% | 0% | 0% | | T8 | 59.7% | 40.2% | 0.0% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T9 | 98.9% | 1.0% | 0% | 0% | 0% | | T9 | 98.9% | 1.0% | 0.0% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
Table 4A: Node protection (repair size distribution)
+---------+----------+----------+----------+----------+----------+ Table 6: 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% | | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
+---------+----------+----------+----------+----------+----------+ +=========+========+=======+========+========+========+
| T2 | 36.5% | 96.1% | 99.7% | 99.9% | 100% | | T1 | 49.7% | 97.6% | 99.8% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T3 | 73.3% | 98.9% | 99.9% | 100.0% | 100% | | T2 | 36.5% | 96.1% | 99.7% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T4 | 36.1% | 93.4% | 99.8% | 99.9% | 100% | | T3 | 73.3% | 98.9% | 99.9% | 100% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T5 | 73.2% | 100.0% | 100.0% | 100.0% | 100% | | T4 | 36.1% | 93.4% | 99.8% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T6 | 78.4% | 99.7% | 100.0% | 100.0% | 100% | | T5 | 73.2% | 100% | 100% | 100% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T7 | 66.1% | 98.9% | 100.0% | 100.0% | 100% | | T6 | 78.4% | 99.7% | 100% | 100% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T8 | 59.7% | 100.0% | 100.0% | 100.0% | 100% | | T7 | 66.1% | 98.9% | 100% | 100% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
| T9 | 98.9% | 100.0% | 100.0% | 100.0% | 100% | | T8 | 59.7% | 100% | 100% | 100% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+--------+-------+--------+--------+--------+
Table 4B: Node protection (repair size cumulative distribution) | T9 | 98.9% | 100% | 100% | 100% | 100% |
+---------+--------+-------+--------+--------+--------+
Table 7: Node Protection (Repair Size Cumulative
Distribution)
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:
Francois Clad
Cisco Systems
Pablo Camarillo
Cisco Systems
Authors' Addresses Authors' Addresses
Ahmed Bashandy Ahmed Bashandy
Individual HPE
United States of America
Email: abashandy.ietf@gmail.com Email: abashandy.ietf@gmail.com
Stephane Litkowski Stephane Litkowski
Cisco Systems Cisco Systems
France France
Email: slitkows@cisco.com Email: slitkows@cisco.com
Clarence Filsfils Clarence Filsfils
Cisco Systems Cisco Systems
Brussels Brussels
Belgium Belgium
Email: cfilsfil@cisco.com Email: cfilsfil@cisco.com
Pierre Francois Pierre Francois
INSA Lyon INSA Lyon
Email: pierre.francois@insa-lyon.fr Email: pierre.francois@insa-lyon.fr
Bruno Decraene Bruno Decraene
Orange Orange
Issy-les-Moulineaux
France France
Email: bruno.decraene@orange.com Email: bruno.decraene@orange.com
Daniel Voyer Daniel Voyer
Bell Canada Bell Canada
Canada Canada
Email: daniel.voyer@bell.ca Email: daniel.voyer@bell.ca
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