Internet DRAFT - draft-ietf-rtgwg-segment-routing-ti-lfa
draft-ietf-rtgwg-segment-routing-ti-lfa
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 July 2024.
Bashandy, et al. Expires 19 July 2024 [Page 1]
Internet-Draft SR TI-LFA January 2024
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
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
Bashandy, et al. Expires 19 July 2024 [Page 2]
Internet-Draft SR TI-LFA January 2024
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.
Bashandy, et al. Expires 19 July 2024 [Page 3]
Internet-Draft SR TI-LFA January 2024
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
Bashandy, et al. Expires 19 July 2024 [Page 4]
Internet-Draft SR TI-LFA January 2024
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.
Bashandy, et al. Expires 19 July 2024 [Page 5]
Internet-Draft SR TI-LFA January 2024
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 ([RFC7916]). The easiest way to express the expected
post-convergence path in a loop-free manner is to encode it as a list
of adjacency segments. However, this may create a long SID list that
some hardware may not be able to push. One of the challenges of TI-
LFA is to encode the expected post-convergence path by combining
adjacency segments and node segments. Each implementation will be
free to have its own SID list optimization algorithm. This document
details the basic concepts that could be used to build the SR backup
path as well as the associated dataplane procedures.
Section 3 defines the main notations used in the document. They are
in line with [RFC5714].
Section 4 defines the main principles of TI-LFA backup path
computation.
Section 5 suggests to compute the P-Space and Q-Space properties
defined in Section 3, for the specific case of nodes lying over the
post-convergence paths towards the protected destinations.
Using the properties defined in Section 5, Section 6 describes how to
compute protection lists that encode a loop-free post-convergence
path towards the destination.
Section 7 defines the segment operations to be applied by the PLR to
ensure consistency with the forwarding state of the repair node.
Section 11 highlights advantages of using the expected post-
convergence path during FRR.
By applying the algorithms specified in this document to actual
service providers and large enterprise networks, we provide real life
measurements for the number of SIDs used by repair paths. Section 12
summarizes these measurements.
2.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Bashandy, et al. Expires 19 July 2024 [Page 6]
Internet-Draft SR TI-LFA January 2024
3. Terminology
We define the main notations used in this document as the following.
We refer to "old" and "new" topologies as the LSDB state before and
after the considered failure.
SPT_old(R) is the Shortest Path Tree rooted at node R in the initial
state of the network.
SPT_new(R, X) is the Shortest Path Tree rooted at node R in the state
of the network after the resource X has failed.
PLR stands for "Point of Local Repair". It is the router that
applies fast traffic restoration after detecting failure in a
directly attached link, set of links, and/or node.
Similar to [RFC7490], we use the concept of P-Space and Q-Space for
TI-LFA.
The P-space P(R,X) of a router R with regard to a resource X (e.g. a
link S-F, a node F, or a SRLG) is the set of routers reachable from R
using the pre-convergence shortest paths without any of those paths
(including equal-cost path splits) transiting through X.
Consider the set of neighbors of a router R and a resource X.
Exclude from that set of 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.
EP(P, Q) is an explicit SR-based path from a node P to a node Q.
A symmetric network is a network such that the IGP metric of each
link is the same in both directions of the link.
4. Base principle
The basic algorithm to compute the repair path is to pre-compute
SPT_new(R,X) and for each destination, encode the repair path as a
loop-free SID list. One way to provide a loop-free SID list is to
use adjacency SIDs only. However, this approach may create very long
SID lists that hardware may not be able to handle due to MSD (Maximum
SID Depth) limitations.
Bashandy, et al. Expires 19 July 2024 [Page 7]
Internet-Draft SR TI-LFA January 2024
An implementation is free to use any local optimization to provide
smaller SID lists by combining Node SIDs and Adjacency SIDs. In
addition, the usage of Node-SIDs allow to maximize ECMPs over the
backup path. These optimizations are out of scope of this document,
however the subsequent sections provide some guidance on how to
leverage P-Spaces and Q-Spaces to optimize the size of the SID list.
5. Intersecting P-Space and Q-Space with post-convergence paths
One of the challenges of defining an SR path following the expected
post-convergence path is to reduce the size of the segment list. In
order to reduce this segment list, an implementation MAY determine
the P-Space/Extended P-Space and Q-Space properties (defined in
[RFC7490]) of the nodes along the expected post-convergence path from
the PLR to the protected destination and compute an SR-based explicit
path from P to Q when they are not adjacent. Such properties will be
used in Section 6 to compute the TI-LFA repair list.
5.1. Extended P-Space property computation for a resource X, over post-
convergence paths
We want to determine which nodes on the post-convergence path from
the PLR R to the destination D are in the extended P-space of R with
regard to resource X (X can be a link or a set of links adjacent to
the PLR, or a neighbor node of the PLR).
This can be found by:
* Excluding neighbors which are not on the post-convergence path
when computing P'(R,X)
* Then, intersecting the set of nodes belonging to the post-
convergence path from R to D, assuming the failure of X, with
P'(R, X).
5.2. Q-Space property computation for a resource X, over post-
convergence paths
We want to determine which nodes on the post-convergence path from
the PLR R to the destination D are in the Q-Space of destination D
with regard to resource X (X can be a link or a set of links adjacent
to the PLR, or a neighbor node of the PLR).
This can be found by intersecting the set of nodes belonging to the
post-convergence path from R to D, assuming the failure of X, with
Q(D, X).
Bashandy, et al. Expires 19 July 2024 [Page 8]
Internet-Draft SR TI-LFA January 2024
5.3. Scaling considerations when computing Q-Space
[RFC7490] raises scaling concerns about computing a Q-Space per
destination. Similar concerns may affect TI-LFA computation if an
implementation tries to compute a reverse Shortest Path Tree
([RFC7490]) for every destination in the network to determine the
Q-Space. It will be up to each implementation to determine the good
tradeoff between scaling and accuracy of the optimization.
6. TI-LFA Repair path
The TI-LFA repair path (RP) consists of an outgoing interface and a
list of segments (repair list (RL)) to insert on the SR header in
accordance with the dataplane used. The repair list encodes the
explicit post-convergence path to the destination, which avoids the
protected resource X and, at the same time, is guaranteed to be loop-
free irrespective of the state of FIBs along the nodes belonging to
the explicit path. Thus, there is no need for any co-ordination or
message exchange between the PLR and any other router in the network.
The TI-LFA repair path is found by intersecting P(S,X) and Q(D,X)
with the post-convergence path to D and computing the explicit SR-
based path EP(P, Q) from P to Q when these nodes are not adjacent
along the post convergence path. The TI-LFA repair list is expressed
generally as (Node_SID(P), EP(P, Q)).
S ------- N1 ----------- D
*\ | \ |
* \ | \ |
* \ | \ |
* N2-----R1****R2 *** R3
* *
N3 *********
***** : link with high metric (1k)
----- : link with metric 1
Figure 1: Sample topology with TI-LFA
As an example, in Figure 1, we are interested by the TI-LFA backup
from S to D considering the failure of node N1.
* First, P(S, N1) is computed and results in [N3, N2, R1].
* Then, Q(D, N1) is computed and results in [R3].
Bashandy, et al. Expires 19 July 2024 [Page 9]
Internet-Draft SR TI-LFA January 2024
* The expected post-convergence path from S to D considering the
failure of N1 is <N2 -> R1 -> R2 -> R3 -> D> (we are naming it
PCPath in this example).
* P(S, N1) intersection with PCPath is [N2, R1], R1 being the deeper
downstream node in PCP, it can be assumed to be used as P node
(this is an example and an implementation could use a different
strategy to choose the P node).
* Q(D, N1) intersection with PCPath is [R3], so R3 is picked as Q
node. An SR explicit path is then computed from R1 (P node) to R3
(Q node) following PCPath (R1 -> R2 -> R3): <AdjSID_R1R2,
AdjSID_R2R3>.
As a result, the TI-LFA repair list of S for destination D
considering the failure of node N1 is: <NodeSID(R1), AdjSID_R1R2,
AdjSID_R2R3>.
Most often, the TI-LFA repair list has a simpler form, as described
in the following sections. Section 12 provides statistics for the
number of SIDs in the explicit path to protect against various
failures.
6.1. FRR path using a direct neighbor
When a direct neighbor is in P(S,X) and Q(D,x) and the link to that
direct neighbor is on the post-convergence path, the outgoing
interface is set to that neighbor and the repair segment list SHOULD
be empty.
This is comparable to a post-convergence LFA FRR repair.
6.2. FRR path using a PQ node
When a remote node R is in P(S,X) and Q(D,x) and on the post-
convergence path, the repair list SHOULD be made of a single node
segment to R and the outgoing interface SHOULD be set to the outgoing
interface used to reach R.
This is comparable to a post-convergence RLFA repair tunnel.
6.3. FRR path using a P node and Q node that are adjacent
When a node P is in P(S,X) and a node Q is in Q(D,x) and both are on
the post-convergence path and both are adjacent to each other, the
repair list SHOULD be made of two segments: A node segment to P (to
be processed first), followed by an adjacency segment from P to Q.
Bashandy, et al. Expires 19 July 2024 [Page 10]
Internet-Draft SR TI-LFA January 2024
This is comparable to a post-convergence DLFA (LFA with directed
forwarding) repair tunnel.
6.4. Connecting distant P and Q nodes along post-convergence paths
In some cases, there is no adjacent P and Q node along the post-
convergence path. As mentioned in Section 4, a list of adjacency
SIDs can be used to encode the path between P and Q. However, the
PLR can perform additional computations to compute a list of segments
that represent a loop-free path from P to Q. How these computations
are done is out of scope of this document and is left to
implementation.
7. Building TI-LFA repair lists
The following sections describe how to build the repair lists using
the terminology defined in [RFC8402]. The procedures described in
this section are equally applicable to both SR-MPLS and SRv6
dataplane, while the dataplane-specific considerations are described
in Section 8.
In this section, we explain how a protecting router S processes the
active segment of a packet upon the failure of its primary outgoing
interface for the packet, S-F. The failure of the primary outgoing
interface may happen due to different triggers (e.g.: link failure,
neighbor node failure...)
7.1. The active segment is a node segment
The active segment MUST be kept on the SR header unchanged and the
repair list MUST be added. The active segment becomes the first
segment of the repair list. The way the repair list is added depends
on the dataplane used (see Section 8).
7.2. The active segment is an adjacency segment
We define hereafter the FRR behavior applied by S for any packet
received with an active adjacency segment S-F for which protection
was enabled. As protection has been enabled for the segment S-F and
signaled in the IGP (for instance using protocol extensions from
[RFC8667] and [RFC8665]), any SR policy using this segment knows that
it may be transiently rerouted out of S-F in case of S-F failure.
Bashandy, et al. Expires 19 July 2024 [Page 11]
Internet-Draft SR TI-LFA January 2024
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
so, one or more “PUSH” operations are performed. If the repair list,
while avoiding S-F, terminates on F, S only pushes segments of the
repair list. Otherwise, S pushes a node segment of F, followed by
the segments of the repair list. For details on the "NEXT" and
"PUSH" operations, refer to [RFC8402].
This method which merges back the traffic at the remote end of the
adjacency segment has the advantage of keeping as much as possible
the traffic on the pre-failure path. As stated in Section 2, when SR
policies are involved and a strict compliance of the policy is
required, an end-to-end protection should be preferred over a local
repair mechanism. However, this method may not provide the expected
post-convergence path to the final destination as the expected post-
convergence path may not go through F. Another method requires to
look to the next segment in the segment list.
We distinguish the case where this active segment is followed by
another adjacency segment from the case where it is followed by a
node segment.
7.2.1. Protecting [Adjacency, Adjacency] segment lists
If the next segment in the list is an Adjacency segment, then the
packet has to be conveyed to F.
To do so, S MUST apply a "NEXT" operation on Adj(S-F) and then one or
more “PUSH” operations. If the repair list, while avoiding S-F,
terminates on F, S only pushes the segments of the repair list.
Otherwise, S pushes a node segment of F, followed by the segments of
the repair list. For details on the "NEXT" and "PUSH" operations,
refer to [RFC8402].
Upon failure of S-F, a packet reaching S with a segment list matching
[adj(S-F),adj(F-M),...] will thus leave S with a segment list
matching [RL(F),node(F),adj(F-M),...], where RL(F) is the repair path
for destination F.
7.2.2. Protecting [Adjacency, Node] segment lists
If the next segment in the stack is a node segment, say for node T,
the segment list on the packet matches [adj(S-F),node(T),...].
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
redirecting the traffic to a node Q, whose path to node segment T is
not affected by the failure.
Bashandy, et al. Expires 19 July 2024 [Page 12]
Internet-Draft SR TI-LFA January 2024
Upon failure of S-F, packets reaching S with a segment list matching
[adj(S-F), node(T), ...], would leave S with a segment list matching
[RL(Q),node(T), ...].
8. Dataplane specific considerations
8.1. MPLS dataplane considerations
MPLS dataplane for Segment Routing is described in [RFC8660].
The following dataplane behaviors apply when creating a repair list
using an MPLS dataplane:
1. If the active segment is a node segment that has been signaled
with penultimate hop popping and the repair list ends with an
adjacency segment terminating on the tail-end of the active
segment, then the active segment MUST be popped before pushing
the repair list.
2. If the active segment is a node segment but the other conditions
in 1. are not met, the active segment MUST be popped then pushed
again with a label value computed according to the SRGB of Q,
where Q is the endpoint of the repair list. Finally, the repair
list MUST be pushed.
8.2. SRv6 dataplane considerations
SRv6 dataplane and programming instructions are described
respectively in [RFC8754] and [RFC8986].
The TI-LFA path computation algorithm is the same as in the SR-MPLS
dataplane. Note however that the Adjacency SIDs are typically
globally routed. In such case, there is no need for a preceding
Prefix SID and the resulting repair list is likely shorter.
If the traffic is protected at a Transit Node, then an SRv6 SID list
is added on the packet to apply the repair list. The addition of the
repair list follows the headend behaviors as specified in section 5
of [RFC8986].
If the traffic is protected at an SR Segment Endpoint Node, first the
Segment Endpoint packet processing is executed. Then the packet is
protected as if its were a transit packet.
Bashandy, et al. Expires 19 July 2024 [Page 13]
Internet-Draft SR TI-LFA January 2024
9. TI-LFA and SR algorithms
SR allows an operator to bind an algorithm to a prefix SID (as
defined in [RFC8402]. The algorithm value dictates how the path to
the prefix is computed. The SR default algorithm is known has the
"Shortest Path" algorithm. The SR default algorithm allows an
operator to override the IGP shortest path by using local policies.
When TI-LFA uses Node-SIDs associated with the default algorithm,
there is no guarantee that the path will be loop-free as a local
policy may have overriden the expected IGP path. As the local
policies are defined by the operator, it becomes the responsibility
of this operator to ensure that the deployed policies do not affect
the TI-LFA deployment. It should be noted that such situation can
already happen today with existing mechanisms as remote LFA.
[RFC9350] defines a flexible algorithm (FlexAlgo) framework to be
associated with Prefix SIDs. FlexAlgo allows a user to associate a
constrained path to a Prefix SID rather than using the regular IGP
shortest path. An implementation MAY support TI-LFA to protect Node-
SIDs associated to a FlexAlgo. In such a case, rather than computing
the expected post-convergence path based on the regular SPF, an
implementation SHOULD use the constrained SPF algorithm bound to the
FlexAlgo (using the Flex Algo Definition) instead of the regular
Dijkstra in all the SPF/rSPF computations that are occurring during
the TI-LFA computation. This includes the computation of the P-Space
and Q-Space as well as the post-convergence path. An implementation
MUST only use Node-SIDs bound to the FlexAlgo and/or Adj-SIDs that
are unprotected to build the repair list.
10. Usage of Adjacency segments in the repair list
The repair list of segments computed by TI-LFA may contain one or
more adjacency segments. An adjacency segment may be protected or
not protected.
S --- R2 --- R3 --- R4 --- R5 --- D
\ | \ /
R7 -- R8
| |
R9 -- R10
Figure 2
In Figure 2, all the metrics are equal to 1 except
R2-R7,R7-R8,R8-R4,R7-R9 which have a metric of 1000. Considering R2
as a PLR to protect against the failure of node R3 for the traffic
S->D, the repair list computed by R2 will be [adj(R7-R8),adj(R8-R4)]
Bashandy, et al. Expires 19 July 2024 [Page 14]
Internet-Draft SR TI-LFA January 2024
and the outgoing interface will be to R7. If R3 fails, R2 pushes the
repair list onto the incoming packet to D. During the FRR, if R7-R8
fails and if TI-LFA has picked a protected adjacency segment for
adj(R7-R8), R7 will push an additional repair list onto the packet
following the procedures defined in Section 7.
To avoid the possibility of this double FRR activation, an
implementation of TI-LFA MAY pick only non protected adjacency
segments when building the repair list. However, this is important
to note that FRR in general is intended to protect for a single pre-
planned failure. If the failure that happens is worse than expected
or multiple failures happen, FRR is not guaranteed to work. In such
a case, fast IGP convergence remains important to restore traffic as
quickly as possible. Implementations of TI-LFA should deal with the
occurence of multiple unrelated failures in accordance to the IP Fast
Reroute Framework [RFC5714].
11. Advantages of using the expected post-convergence path during FRR
[RFC7916] raised several operational considerations when using LFA or
remote LFA. [RFC7916] Section 3 presents a case where a high
bandwidth link between two core routers is protected through a PE
router connected with low bandwidth links. In such a case,
congestion may happen when the FRR backup path is activated.
[RFC7916] introduces a local policy framework to let the operator
tuning manually the best alternate election based on its own
requirements.
From a network capacity planning point of view, it is often assumed
for simplicity that if a link L fails on a particular node X, the
bandwidth consumed on L will be spread over some of the remaining
links of X. The remaining links to be used are determined by the IGP
routing considering that the link L has failed (we assume that the
traffic uses the post-convergence path starting from the node X). In
Figure 3, we consider a network with all metrics equal to 1 except
the metrics on links used by PE1, PE2 and PE3 which are 1000. An
easy network capacity planning method is to consider that if the link
L (X-B) fails, the traffic actually flowing through L will be spread
over the remaining links of X (X-H, X-D, X-A). Considering the IGP
metrics, only X-H and X-D can be used in reality to carry the traffic
flowing through the link L. As a consequence, the bandwidth of links
X-H and X-D is sized according to this rule. We should observe that
this capacity planning policy works, however it is not fully
accurate.
In Figure 3, considering that the source of traffic is only from PE1
and PE4, when the link L fails, depending on the convergence speed of
the nodes, X may reroute its forwarding entries to the remote PEs
Bashandy, et al. Expires 19 July 2024 [Page 15]
Internet-Draft SR TI-LFA January 2024
onto X-H or X-D; however in a similar timeframe, PE1 will also
reroute a subset of its traffic (the subset destined to PE2) out of
its nominal path reducing the quantity of traffic received by X. The
capacity planning rule presented previously has the drawback of
oversizing the network, however it allows to prevent any transient
congestion (when for example X reroutes traffic before PE1 does).
H --- I --- J
| | \
PE4 | | PE3
\ | (L) | /
A --- X --- B --- G
/ | | \
PE1 | | PE2
\ | | /
C --- D --- E --- F
Figure 3
Based on this assumption, in order to facilitate the operation of
FRR, and limit the implementation of local FRR policies, traffic can
be steered by the PLR onto its expected post-convergence path during
the FRR phase. In our example, when link L fails, X switches the
traffic destined to PE3 and PE2 on the post-convergence paths. This
is perfectly inline with the capacity planning rule that was
presented before and also inline with the fact X may converge before
PE1 (or any other upstream router) and may spread the X-B traffic
onto the post-convergence paths rooted at X.
It should be noted, that some networks may have a different capacity
planning rule, leading to an allocation of less bandwidth on X-H and
X-D links. In such a case, using the post-convergence paths rooted
at X during FRR may introduce some congestion on X-H and X-D links.
However it is important to note, that a transient congestion may
possibly happen, even without FRR activated, for instance when X
converges before the upstream routers. Operators are still free to
use the policy framework defined in [RFC7916] if the usage of the
post-convergence paths rooted at the PLR is not suitable.
Readers should be aware that FRR protection is pre-computing a backup
path to protect against a particular type of failure (link, node,
SRLG). When using the post-convergence path as FRR backup path, the
computed post-convergence path is the one considering the failure we
are protecting against. This means that FRR is using an expected
post-convergence path, and this expected post-convergence path may be
actually different from the post-convergence path used if the failure
that happened is different from the failure FRR was protecting
Bashandy, et al. Expires 19 July 2024 [Page 16]
Internet-Draft SR TI-LFA January 2024
against. As an example, if the operator has implemented a protection
against a node failure, the expected post-convergence path used
during FRR will be the one considering that the node has failed.
However, even if a single link is failing or a set of links is
failing (instead of the full node), the node-protecting post-
convergence path will be used. The consequence is that the path used
during FRR is not optimal with respect to the failure that has
actually occurred.
Another consideration to take into account is: while using the
expected post-convergence path for SR traffic using node segments
only (for instance, PE to PE traffic using shortest path) has some
advantages, these advantages reduce when SR policies ([RFC9256]) are
involved. A segment-list used in an SR policy is computed to obey a
set of path constraints defined locally at the head-end or centrally
in a controller. TI-LFA cannot be aware of such path constraints and
there is no reason to expect the TI-LFA backup path protecting one
segments in that segment list to obey those constraints. When SR
policies are used and the operator wants to have a backup path which
still follows the policy requirements, this backup path should be
computed as part of the SR policy in the ingress node (or central
controller) and the SR policy should not rely on local protection.
Another option could be to use FlexAlgo ([RFC9350]) to express the
set of constraints and use a single node segment associated with a
FlexAlgo to reach the destination. When using a node segment
associated with a FlexAlgo, TI-LFA keeps providing an optimal backup
by applying the appropriate set of constraints. The relationship
between TI-LFA and the SR-algorithm is detailed in Section 9.
12. Analysis based on real network topologies
This section presents analysis performed on real service provider and
large enterprise network topologies. The objective of the analysis
is to assess the number of SIDs required in an explicit path when the
mechanisms described in this document are used to protect against the
failure scenarios within the scope of this document. The number of
segments described in this section are applicable to instantiating
segment routing over the MPLS forwarding plane.
The measurement below indicate that for link and local SRLG
protection, a 1 SID repair path delivers more than 99% coverage. For
node protection a 2 SIDs repair path yields 99% coverage.
Table 1 below lists the characteristics of the networks used in our
measurements. The number of links refers to the number of
"bidirectional" links (not directed edges of the graph). The
measurements are carried out as follows:
Bashandy, et al. Expires 19 July 2024 [Page 17]
Internet-Draft SR TI-LFA January 2024
* For each network, the algorithms described in this document are
applied to protect all prefixes against link, node, and local SRLG
failure
* For each prefix, the number of SIDs used by the repair path is
recorded
* The percentage of number of SIDs are listed in Tables 2A/B, 3A/B,
and 4A/B
The measurements listed in the tables indicate that for link and
local SRLG protection, 1 SID repair path is sufficient to protect
more than 99% of the prefix in almost all cases. For node protection
2 SIDs repair paths yield 99% coverage.
+-------------+------------+------------+------------+------------+
| Network | Nodes | Links |Node-to-Link| SRLG info? |
| | | | Ratio | |
+-------------+------------+------------+------------+------------+
| T1 | 408 | 665 | 1.63 | Yes |
+-------------+------------+------------+------------+------------+
| T2 | 587 | 1083 | 1.84 | No |
+-------------+------------+------------+------------+------------+
| T3 | 93 | 401 | 4.31 | Yes |
+-------------+------------+------------+------------+------------+
| T4 | 247 | 393 | 1.59 | Yes |
+-------------+------------+------------+------------+------------+
| T5 | 34 | 96 | 2.82 | Yes |
+-------------+------------+------------+------------+------------+
| T6 | 50 | 78 | 1.56 | No |
+-------------+------------+------------+------------+------------+
| T7 | 82 | 293 | 3.57 | No |
+-------------+------------+------------+------------+------------+
| T8 | 35 | 41 | 1.17 | Yes |
+-------------+------------+------------+------------+------------+
| T9 | 177 | 1371 | 7.74 | Yes |
+-------------+------------+------------+------------+------------+
Table 1: Data Set Definition
The rest of this section presents the measurements done on the actual
topologies. The convention that we use is as follows
* 0 SIDs: the calculated repair path starts with a directly
connected neighbor that is also a loop free alternate, in which
case there is no need to explicitly route the traffic using
additional SIDs. This scenario is described in Section 6.1.
Bashandy, et al. Expires 19 July 2024 [Page 18]
Internet-Draft SR TI-LFA January 2024
* 1 SIDs: 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
Section 6.2.
* 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
case for 2 SIDs (Section 6.3) separately because there was no
granularity in the result. Also we treat the node-SID+adj-SID and
node-SID + node-SID the same because they do not differ from the
data plane point of view.
Table 2A and 2B below summarize the measurements on the number of
SIDs needed for link protection
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.3% | 25.3% | 0.5% | 0.0% |
+-------------+------------+------------+------------+------------+
| T2 | 81.1% | 18.7% | 0.2% | 0.0% |
+-------------+------------+------------+------------+------------+
| T3 | 95.9% | 4.1% | 0.1% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.5% | 35.7% | 1.8% | 0.0% |
+-------------+------------+------------+------------+------------+
| T5 | 85.7% | 14.3% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T6 | 81.2% | 18.7% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T7 | 98.9% | 1.1% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T8 | 94.1% | 5.9% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98.9% | 1.0% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
Table 2A: Link protection (repair size distribution)
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.2% | 99.5% | 99.9% | 100.0% |
+-------------+------------+------------+------------+------------+
| T2 | 81.1% | 99.8% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T3 | 95.9% | 99.9% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.5% | 98.2% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Bashandy, et al. Expires 19 July 2024 [Page 19]
Internet-Draft SR TI-LFA January 2024
| T5 | 85.7% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T6 | 81.2% | 99.9% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T7 | 98,8% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T8 | 94,1% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98,9% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Table 2B: Link protection repair size cumulative distribution
Table 3A and 3B summarize the measurements on the number of SIDs
needed for local SRLG protection.
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.2% | 25.3% | 0.5% | 0.0% |
+-------------+------------+------------+------------+------------+
| T2 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T3 | 93.6% | 6.3% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.5% | 35.6% | 1.8% | 0.0% |
+-------------+------------+------------+------------+------------+
| T5 | 83.1% | 16.8% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T6 | No SRLG Information |
+-------------+---------------------------------------------------+
| T7 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T8 | 85.2% | 14.8% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98,9% | 1.1% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
Table 3A: Local SRLG protection repair size distribution
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.2% | 99.5% | 99.9% | 100.0% |
+-------------+------------+------------+------------+------------+
| T2 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T3 | 93.6% | 99.9% | 100.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.5% | 98.2% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Bashandy, et al. Expires 19 July 2024 [Page 20]
Internet-Draft SR TI-LFA January 2024
| T5 | 83.1% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T6 | No SRLG Information |
+-------------+---------------------------------------------------+
| T7 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T8 | 85.2% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98.9% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Table 3B: Local SRLG protection repair size Cumulative distribution
The remaining two tables summarize the measurements on the number of
SIDs needed for node protection.
+---------+----------+----------+----------+----------+----------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
+---------+----------+----------+----------+----------+----------+
| T1 | 49.8% | 47.9% | 2.1% | 0.1% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T2 | 36,5% | 59.6% | 3.6% | 0.2% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T3 | 73.3% | 25.6% | 1.1% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T4 | 36.1% | 57.3% | 6.3% | 0.2% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T5 | 73.2% | 26.8% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T6 | 78.3% | 21.3% | 0.3% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T7 | 66.1% | 32.8% | 1.1% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T8 | 59.7% | 40.2% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T9 | 98.9% | 1.0% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
Table 4A: Node protection (repair size distribution)
+---------+----------+----------+----------+----------+----------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
+---------+----------+----------+----------+----------+----------+
| T1 | 49.7% | 97.6% | 99.8% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+
| T2 | 36.5% | 96.1% | 99.7% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+
| T3 | 73.3% | 98.9% | 99.9% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T4 | 36.1% | 93.4% | 99.8% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+
Bashandy, et al. Expires 19 July 2024 [Page 21]
Internet-Draft SR TI-LFA January 2024
| T5 | 73.2% | 100.0% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T6 | 78.4% | 99.7% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T7 | 66.1% | 98.9% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T8 | 59.7% | 100.0% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T9 | 98.9% | 100.0% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
Table 4B: Node protection (repair size cumulative distribution)
13. Security Considerations
The techniques described in this document are internal
functionalities to a router that result in the ability to guarantee
an upper bound on the time taken to restore traffic flow upon the
failure of a directly connected link or node. As these techniques
steer traffic to the post-convergence path as quickly as possible,
this serves to minimize the disruption associated with a local
failure which can be seen as a modest security enhancement. The
protection mechanisms does not protect external destinations, but
rather provides quick restoration for destination that are internal
to a routing domain.
Security considerations described in [RFC5286] and [RFC7490] apply to
this document. Similarly, as the solution described in the document
is based on Segment Routing technology, reader should be aware of the
security considerations related to this technology ([RFC8402]) and
its dataplane instantiations ([RFC8660], [RFC8754] and [RFC8986]).
However, this document does not introduce additional security
concern.
14. IANA Considerations
No requirements for IANA
15. 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
Bashandy, et al. Expires 19 July 2024 [Page 22]
Internet-Draft SR TI-LFA January 2024
16. Acknowledgments
We would like to thank Les Ginsberg, Stewart Bryant, Alexander
Vainsthein, Chris Bowers, Shraddha Hedge, Wes Hardaker for their
valuable comments.
17. References
17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and P. Sarkar, "Operational Management of
Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
July 2016, <https://www.rfc-editor.org/info/rfc7916>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
17.2. Informative References
Bashandy, et al. Expires 19 July 2024 [Page 23]
Internet-Draft SR TI-LFA January 2024
[I-D.bashandy-rtgwg-segment-routing-uloop]
Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
Francois, P., and P. Psenak, "Loop avoidance using Segment
Routing", Work in Progress, Internet-Draft, draft-
bashandy-rtgwg-segment-routing-uloop-16, 17 December 2023,
<https://datatracker.ietf.org/doc/html/draft-bashandy-
rtgwg-segment-routing-uloop-16>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <https://www.rfc-editor.org/info/rfc5715>.
[RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<https://www.rfc-editor.org/info/rfc6571>.
[RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
2013, <https://www.rfc-editor.org/info/rfc6976>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<https://www.rfc-editor.org/info/rfc7490>.
[RFC8333] Litkowski, S., Decraene, B., Filsfils, C., and P.
Francois, "Micro-loop Prevention by Introducing a Local
Convergence Delay", RFC 8333, DOI 10.17487/RFC8333, March
2018, <https://www.rfc-editor.org/info/rfc8333>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
Bashandy, et al. Expires 19 July 2024 [Page 24]
Internet-Draft SR TI-LFA January 2024
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.
Authors' Addresses
Ahmed Bashandy
Cisco Systems
Email: abashandy.ietf@gmail.com
Stephane Litkowski
Cisco Systems
France
Email: slitkows@cisco.com
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
Pierre Francois
INSA Lyon
Email: pierre.francois@insa-lyon.fr
Bruno Decraene
Orange
Issy-les-Moulineaux
France
Email: bruno.decraene@orange.com
Bashandy, et al. Expires 19 July 2024 [Page 25]
Internet-Draft SR TI-LFA January 2024
Daniel Voyer
Bell Canada
Canada
Email: daniel.voyer@bell.ca
Bashandy, et al. Expires 19 July 2024 [Page 26]
