Network Working Group A. Bashandy
Internet-Draft S. Litkowski
Intended status: Standards Track C. Filsfils
Expires: 19 July 2024 Cisco Systems
P. Francois
INSA Lyon
B. Decraene
Orange
D. Voyer
Bell Canada
16 January 2024
Topology Independent Fast Reroute using Segment Routing
draft-ietf-rtgwg-segment-routing-ti-lfa-13
Abstract
This document presents Topology Independent Loop-free Alternate Fast
Re-route (TI-LFA), aimed at providing protection of node and
adjacency segments within the Segment Routing (SR) framework. This
Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being
LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding
(DLFA). It extends these concepts to provide guaranteed coverage in
any two connected networks using a link-state IGP. A key aspect of
TI-LFA is the FRR path selection approach establishing protection
over the expected post-convergence paths from the point of local
repair, reducing the operational need to control the tie-breaks among
various FRR options.
Status of This Memo
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This Internet-Draft will expire on 19 July 2024.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Conventions used in this document . . . . . . . . . . . . 6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Base principle . . . . . . . . . . . . . . . . . . . . . . . 7
5. Intersecting P-Space and Q-Space with post-convergence
paths . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Extended P-Space property computation for a resource X,
over post-convergence paths . . . . . . . . . . . . . . . 8
5.2. Q-Space property computation for a resource X, over
post-convergence paths . . . . . . . . . . . . . . . . . 8
5.3. Scaling considerations when computing Q-Space . . . . . . 9
6. TI-LFA Repair path . . . . . . . . . . . . . . . . . . . . . 9
6.1. FRR path using a direct neighbor . . . . . . . . . . . . 10
6.2. FRR path using a PQ node . . . . . . . . . . . . . . . . 10
6.3. FRR path using a P node and Q node that are adjacent . . 10
6.4. Connecting distant P and Q nodes along post-convergence
paths . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Building TI-LFA repair lists . . . . . . . . . . . . . . . . 11
7.1. The active segment is a node segment . . . . . . . . . . 11
7.2. The active segment is an adjacency segment . . . . . . . 11
7.2.1. Protecting [Adjacency, Adjacency] segment lists . . . 12
7.2.2. Protecting [Adjacency, Node] segment lists . . . . . 12
8. Dataplane specific considerations . . . . . . . . . . . . . . 13
8.1. MPLS dataplane considerations . . . . . . . . . . . . . . 13
8.2. SRv6 dataplane considerations . . . . . . . . . . . . . . 13
9. TI-LFA and SR algorithms . . . . . . . . . . . . . . . . . . 14
10. Usage of Adjacency segments in the repair list . . . . . . . 14
11. Advantages of using the expected post-convergence path during
FRR . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
12. Analysis based on real network topologies . . . . . . . . . . 17
13. Security Considerations . . . . . . . . . . . . . . . . . . . 22
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14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
17.1. Normative References . . . . . . . . . . . . . . . . . . 23
17.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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.
* RSPT: Reverse Shortest Path Tree.
* SID: Segment Identifier.
* SLA: Service Level Agreement.
* SPF: Shortest Path First.
* SPT: Shortest Path Tree.
* SR: Segment Routing.
* SRGB: Segment Routing Global Block.
* SRLG: Shared Risk Link Group.
* TI-LFA: Topology Independant LFA.
* TLDP: Target Label Distribution Protocol.
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2. Introduction
Segment Routing aims at supporting services with tight SLA guarantees
[RFC8402]. By relying on SR this document provides a local repair
mechanism for standard link-state IGP shortest path capable of
restoring end-to-end connectivity in the case of a sudden directly
connected failure of a network component. Non-SR mechanisms for
local repair are beyond the scope of this document. Non-local
failures are addressed in a separate document
[I-D.bashandy-rtgwg-segment-routing-uloop].
The term topology independent (TI) refers to the ability to provide a
loop free backup path irrespective of the topologies used in the
network. This provides a major improvement compared to LFA [RFC5286]
and remote LFA [RFC7490] which cannot provide a complete protection
coverage in some topologies as described in [RFC6571].
When the network reconverges, micro-loops [RFC5715] can form due to
transient inconsistencies in the forwarding tables of different
routers. If it is determined that micro-loops are a significant
issue in the deployment, then a suitable loop-free convergence
method, such as one of those described in [RFC5715], [RFC6976],
[RFC8333], or [I-D.bashandy-rtgwg-segment-routing-uloop] should be
implemented.
TI-LFA is a local operation applied by the PLR when it detects
failure of one of its local links. As such, it does not affect:
* Micro-loops that appear - or do not appear – as part of the
distributed IGP convergence [RFC5715] on the paths to the
destination that do not pass thru TI-LFA paths:
- As explained in [RFC5714], such micro-loops may result in the
traffic not reaching the PLR and therefore not following TI-LFA
paths.
* Micro-loops that appear – or do not appear - when the failed link
is repaired.
TI-LFA paths are loop-free. What’s more, they follow the post-
convergence paths, and, therefore, not subject to micro-loops due to
difference in the IGP convergence times of the nodes thru which they
pass.
TI-LFA paths are applied from the moment the PLR detects failure of a
local link and until IGP convergence at the PLR is completed.
Therefore, early (relative to the other nodes) IGP convergence at the
PLR and the consecutive ”early” release of TI-LFA paths may cause
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micro-loops, especially if these paths have been computed using the
methods described in Section Section 6.2, Section 6.3, or Section 6.4
of the draft. One of the possible ways to prevent such micro-loops
is local convergence delay ([RFC8333]).
TI-LFA procedures are complementary to application of any micro-loop
avoidance procedures in the case of link or node failure:
* Link or node failure requires some urgent action to restore the
traffic that passed thru the failed resource. TI-LFA paths are
pre-computed and pre-installed and therefore suitable for urgent
recovery
* The paths used in the micro-loop avoidance procedures typically
cannot be pre-computed.
For each destination in the network, TI-LFA pre-installs a backup
forwarding entry for each protected destination ready to be activated
upon detection of the failure of a link used to reach the
destination. TI-LFA provides protection in the event of any one of
the following: single link failure, single node failure, or single
SRLG failure. In link failure mode, the destination is protected
assuming the failure of the link. In node protection mode, the
destination is protected assuming that the neighbor connected to the
primary link has failed. In SRLG protecting mode, the destination is
protected assuming that a configured set of links sharing fate with
the primary link has failed (e.g. a linecard or a set of links
sharing a common transmission pipe).
Protection techniques outlined in this document are limited to
protecting links, nodes, and SRLGs that are within a link-state IGP
area. Protecting domain exit routers and/or links attached to
another routing domains are beyond the scope of this document
By using SR, TI-LFA does not require the establishment of TLDP
sessions (Targeted Label Distribution Protocol) with remote nodes in
order to take advantage of the applicability of remote LFAs (RLFA)
[RFC7490][RFC7916] or remote LFAs with directed forwarding
(DLFA)[RFC5714]. All the Segment Identifiers (SIDs) are available in
the link state database (LSDB) of the IGP. As a result, preferring
LFAs over RLFAs or DLFAs, as well as minimizing the number of RLFA or
DLFA repair nodes is not required anymore.
By using SR, there is no need to create state in the network in order
to enforce an explicit FRR path. This relieves the nodes themselves
from having to maintain extra state, and it relieves the operator
from having to deploy an extra protocol or extra protocol sessions
just to enhance the protection coverage.
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Although not a Ti-LFA requirement or constraint, TI-LFA also brings
the benefit of the ability to provide a backup path that follows the
expected post-convergence path considering a particular failure which
reduces the need of locally configured policies that drive the backup
path selection ([