draft-ietf-rtgwg-segment-routing-ti-lfa-02.txt   draft-ietf-rtgwg-segment-routing-ti-lfa-03.txt 
Network Working Group S. Litkowski
Internet-Draft Cisco Network Working Group S. Litkowski
Intended status: Standards Track A. Bashandy Internet-Draft Cisco Systems
Expires: July 2020 Individual Intended status: Standards Track A. Bashandy
C. Filsfils Expires: September 5, 2020 Individual
Cisco Systems C. Filsfils
B. Decraene Cisco Systems
Orange B. Decraene
Orange
P. Francois P. Francois
INSA Lyon INSA Lyon
D. Voyer D. Voyer
Bell Canada Bell Canada
F. Clad F. Clad
P. Camarillo P. Camarillo
Cisco Systems Cisco Systems
January 18, 2020 March 4, 2020
Topology Independent Fast Reroute using Segment Routing Topology Independent Fast Reroute using Segment Routing
draft-ietf-rtgwg-segment-routing-ti-lfa-02 draft-ietf-rtgwg-segment-routing-ti-lfa-03
Abstract Abstract
This document presents Topology Independent Loop-free Alternate Fast This document presents Topology Independent Loop-free Alternate Fast
Re-route (TI-LFA), aimed at providing protection of node and Re-route (TI-LFA), aimed at providing protection of node and
adjacency segments within the Segment Routing (SR) framework. This adjacency segments within the Segment Routing (SR) framework. This
Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being
LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding
(DLFA). It extends these concepts to provide guaranteed coverage in (DLFA). It extends these concepts to provide guaranteed coverage in
any IGP network. A key aspect of TI-LFA is the FRR path selection any IGP network. A key aspect of TI-LFA is the FRR path selection
skipping to change at page 1, line 42 skipping to change at page 1, line 43
paths from the point of local repair, dramatically reducing the paths from the point of local repair, dramatically reducing the
operational need to control the tie-breaks among various FRR options. operational need to control the tie-breaks among various FRR options.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 18, 2020. This Internet-Draft will expire on September 5, 2020.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction...................................................3 1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document.........................7 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology....................................................7 2.1. Conventions used in this document . . . . . . . . . . . . 8
3. Intersecting P-Space and Q-Space with post-convergence paths...8 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. P-Space property computation for a resource X.............8 4. Intersecting P-Space and Q-Space with post-convergence paths 9
3.2. Q-Space property computation for a link S-F, over post- 4.1. P-Space property computation for a resource X . . . . . . 9
convergence paths..............................................8 4.2. Q-Space property computation for a link S-F, over post-
3.3. Q-Space property computation for a set of links adjacent to convergence paths . . . . . . . . . . . . . . . . . . . . 9
S, over post-convergence paths.................................9 4.3. Q-Space property computation for a set of links adjacent
3.4. Q-Space property computation for a node F, over post- to S, over post-convergence paths . . . . . . . . . . . 10
convergence paths..............................................9 4.4. Q-Space property computation for a node F, over post-
3.5. Scaling considerations when computing Q-Space.............9 convergence paths . . . . . . . . . . . . . . . . . . . . 10
4. TI-LFA Repair Tunnel...........................................9 4.5. Scaling considerations when computing Q-Space . . . . . . 10
4.1. FRR path using a direct neighbor.........................10 5. TI-LFA Repair path . . . . . . . . . . . . . . . . . . . . . 10
4.2. FRR path using a PQ node.................................10 5.1. FRR path using a direct neighbor . . . . . . . . . . . . 11
4.3. FRR path using a P node and Q node that are adjacent.....10 5.2. FRR path using a PQ node . . . . . . . . . . . . . . . . 11
4.4. Connecting distant P and Q nodes along post-convergence paths 5.3. FRR path using a P node and Q node that are adjacent . . 11
..............................................................10 5.4. Connecting distant P and Q nodes along post-convergence
5. Protecting segments...........................................10 paths . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. The active segment is a node segment.....................11 6. Building TI-LFA repair lists . . . . . . . . . . . . . . . . 11
5.2. The active segment is an adjacency segment...............11 6.1. Link protection . . . . . . . . . . . . . . . . . . . . . 12
5.2.1. Protecting [Adjacency, Adjacency] segment lists.....11 6.1.1. The active segment is a node segment . . . . . . . . 12
5.2.2. Protecting [Adjacency, Node] segment lists..........12 6.1.2. The active segment is an adjacency segment . . . . . 12
5.3. Protecting SR policy midpoints against node failure......13 6.2. Protecting SR policy midpoints against node failure . . . 13
5.3.1. Protecting {F, T, D} or {S->F, T, D}................13 6.2.1. Protecting {F, T, D} or {S->F, T, D} . . . . . . . . 14
5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D}..........14 6.2.2. Protecting {F, F->T, D} or {S->F, F->T, D} . . . . . 14
6. TI-LFA and SR Algorithms......................................15 6.3. Dataplane specific considerations . . . . . . . . . . . . 15
7. Usage of Adjacency segments in the repair list................15 6.3.1. MPLS dataplane considerations . . . . . . . . . . . . 15
8. Measurements on Real Networks.................................16 6.3.2. SRv6 dataplane considerations . . . . . . . . . . . . 16
9. Security Considerations.......................................21
10. IANA Considerations..........................................21
11. Conclusions..................................................21
12. Acknowledgments..............................................22
13. References...................................................22
13.1. Normative References....................................22
13.2. Informative References..................................22
1. Introduction 7. TI-LFA and SR algorithms . . . . . . . . . . . . . . . . . . 16
8. Usage of Adjacency segments in the repair list . . . . . . . 17
9. Measurements on Real Networks . . . . . . . . . . . . . . . . 17
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
12. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 22
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
14.1. Normative References . . . . . . . . . . . . . . . . . . 23
14.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Acronyms
o DLFA: Remote LFA with Directed forwarding.
o FRR: Fast Re-route.
o IGP: Interior Gateway Protocol.
o LFA: Loop-Free Alternate.
o LSDB: Link State DataBase.
o PLR: Point of Local Repair.
o RL: Repair list.
o RLFA: Remote LFA.
o SID: Segment Identifier.
o SLA: Service Level Agreement.
o SPF: Shortest Path First.
o SPT: Shortest Path Tree.
o SR: Segment Routing.
o SRLG: Shared Risk Link Group.
o TI-LFA: Topology Independant LFA.
2. Introduction
Segment Routing aims at supporting services with tight SLA guarantees Segment Routing aims at supporting services with tight SLA guarantees
[RFC8402]. By relying on SR this document provides a local repair [RFC8402]. By relying on SR this document provides a local repair
mechanism for standard IGP shortest path capable of restoring end-to- mechanism for standard IGP shortest path capable of restoring end-to-
end connectivity in the case of a sudden directly connected failure end connectivity in the case of a sudden directly connected failure
of a network component. Non-SR mechanisms for local repair are beyond of a network component. Non-SR mechanisms for local repair are
the scope of this document. Non-local failures are addressed in a beyond the scope of this document. Non-local failures are addressed
separate document [I-D.bashandy-rtgwg-segment-routing-uloop]. in a separate document [I-D.bashandy-rtgwg-segment-routing-uloop].
The term topology independent (TI) refers to the ability to provide a The term topology independent (TI) refers to the ability to provide a
loop free backup path irrespective of the topologies used in the loop free backup path irrespective of the topologies used in the
network. This provides a major improvement compared to LFA network. This provides a major improvement compared to LFA [RFC5286]
([RFC5286]) and remote LFA ([RFC7490]) which cannot be applicable in and remote LFA [RFC7490] which cannot provide a complete protection
some topologies ([RFC6571]). coverage in some topologies as described in [RFC6571].
For each destination in the network, TI-LFA pre-installs a backup For each destination in the network, TI-LFA pre-installs a backup
forwarding entry for each protected destination ready to be activated forwarding entry for each protected destination ready to be activated
upon detection of the failure of a link used to reach the upon detection of the failure of a link used to reach the
destination. TI-LFA provides protection in the event of any one of destination. TI-LFA provides protection in the event of any one of
the following: single link failure, single node failure, or single the following: single link failure, single node failure, or single
SRLG failure. In link failure mode, the destination is protected SRLG failure. In link failure mode, the destination is protected
assuming the failure of the link. In node protection mode, the assuming the failure of the link. In node protection mode, the
destination is protected assuming that the neighbor connected to the destination is protected assuming that the neighbor connected to the
primary link has failed. In SRLG protecting mode, the destination is primary link has failed. In SRLG protecting mode, the destination is
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sharing a common transmission pipe). 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 routing domain. protecting links, nodes, and SRLGs that are within a routing domain.
Protecting domain exit routers and/or links attached to another Protecting domain exit routers and/or links attached to another
routing domains are beyond the scope of this document routing domains are beyond the scope of this document
Thanks to SR, TI-LFA does not require the establishment of TLDP Thanks to SR, TI-LFA does not require the establishment of TLDP
sessions with remote nodes in order to take advantage of the sessions with remote nodes in order to take advantage of the
applicability of remote LFAs (RLFA) [RFC7490][RFC7916] or remote LFAs applicability of remote LFAs (RLFA) [RFC7490][RFC7916] or remote LFAs
with directed forwarding (DLFA)[RFC5714]. All the Segment Identifiers with directed forwarding (DLFA)[RFC5714]. All the Segment
(SIDs) are available in the link state database (LSDB) of the IGP. As Identifiers (SIDs) are available in the link state database (LSDB) of
a result, preferring LFAs over RLFAs or DLFAs, as well as minimizing the IGP. As a result, preferring LFAs over RLFAs or DLFAs, as well
the number of RLFA or DLFA repair nodes is not required anymore. as minimizing the number of RLFA or DLFA repair nodes is not required
anymore.
Thanks to SR, there is no need to create state in the network in Thanks to SR, there is no need to create state in the network in
order to enforce an explicit FRR path. This relieves the nodes order to enforce an explicit FRR path. This relieves the nodes
themselves from having to maintain extra state, and it relieves the themselves from having to maintain extra state, and it relieves the
operator from having to deploy an extra protocol or extra protocol operator from having to deploy an extra protocol or extra protocol
sessions just to enhance the protection coverage. sessions just to enhance the protection coverage.
[RFC7916] raised several operational considerations when using LFA or [RFC7916] raised several operational considerations when using LFA or
remote LFA. [RFC7916] Section 3 presents a case where a high remote LFA. [RFC7916] Section 3 presents a case where a high
bandwidth link between two core routers is protected through a PE bandwidth link between two core routers is protected through a PE
router connected with low bandwidth links. In such a case, 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
that if a link L fails on a particular node X, the bandwidth consumed 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 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 remaining links to be used are determined by the IGP routing
considering that the link L has failed (we assume that the traffic considering that the link L has failed (we assume that the traffic
uses the post-convergence path starting from the node X). In Figure uses the post-convergence path starting from the node X). In
1, we consider a network with all metrics equal to 1 except the Figure 1, we consider a network with all metrics equal to 1 except
metrics on links used by PE1, PE2 and PE3 which are 1000. An easy the metrics on links used by PE1, PE2 and PE3 which are 1000. An
network capacity planning method is to consider that if the link L easy network capacity planning method is to consider that if the link
(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 only be used in reality to carry the metrics, only X-H and X-D can only be used in reality to carry the
traffic flowing through the link L. As a consequence, the bandwidth 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 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 observe that this capacity planning policy works, however it is not
fully accurate. fully accurate.
In Figure 1, considering that the source of traffic is only from PE1 In Figure 1, 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. The
capacity planning rule presented previously has the drawback of 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 to prevent any transient
congestion (when for example X reroutes traffic before PE1 does). congestion (when for example 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 1 Figure 1
Based on this assumption, in order to facilitate the operation of Based on this assumption, in order to facilitate the operation of
FRR, and limit the implementation of local FRR policies, it looks FRR, and limit the implementation of local FRR policies, it looks
interesting to steer the traffic onto the post-convergence path from interesting to steer the traffic onto the post-convergence path from
the PLR point of view during the FRR phase. In our example, when the PLR point of view during the FRR phase. In our example, when
link L fails, X switches the traffic destined to PE3 and PE2 on the link L fails, X switches the traffic destined to PE3 and PE2 on the
post-convergence paths. This is perfectly inline with the capacity post-convergence paths. This is perfectly inline with the capacity
planning rule that was presented before and also inline with the fact planning rule that was presented before and also inline with the fact
X may converge before PE1 (or any other upstream router) and may 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. spread the X-B traffic onto the post-convergence paths rooted at X.
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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,
SRLG). When using the post-convergence path as FRR backup path, the SRLG). When using the post-convergence path as FRR backup path, the
computed post-convergence path is the one considering the failure we computed post-convergence path is the one considering the failure we
are protecting against. This means that FRR is using an expected 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: while using the
expected post-convergence path for SR traffic using node segments expected post-convergence path for SR traffic using node segments
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policies are used and the operator wants to have a backup path which policies are used and the operator wants to have a backup path which
still follows the policy requirements, this backup path should be still follows the policy requirements, this backup path should be
computed as part of the SR policy in the ingress node (or central computed as part of the SR policy in the ingress node (or central
controller) and the SR policy should not rely on local protection. controller) and the SR policy should not rely on local protection.
Another option could be to use FlexAlgo ([I-D.ietf-lsr-flex-algo]) to Another option could be to use FlexAlgo ([I-D.ietf-lsr-flex-algo]) to
express the set of constraints and use a single node segment express the set of constraints and use a single node segment
associated with a FlexAlgo to reach the destination. When using a associated with a FlexAlgo to reach the destination. When using a
node segment associated with a FlexAlgo, TI-LFA keeps providing an node segment associated with a FlexAlgo, TI-LFA keeps providing an
optimal backup by applying the appropriate set of constraints. The optimal backup by applying the appropriate set of constraints. The
relationship between TI-LFA and the SR-algorithm is detailed in relationship between TI-LFA and the SR-algorithm is detailed in
Section 6. Section 7.
Thanks to SR and the combination of Adjacency segments and Node Thanks to SR and the combination of Adjacency segments and Node
segments, the expression of the expected post-convergence path rooted segments, the expression of the expected post-convergence path rooted
at the PLR is facilitated and does not create any additional state on at the PLR is facilitated and does not create any additional state on
intermediate nodes. The easiest way to express the expected post- intermediate nodes. The easiest way to express the expected post-
convergence path in a loop-free manner is to encode it as a list of convergence path in a loop-free manner is to encode it as a list of
adjacency segments. However, in an MPLS world, this may create a adjacency segments. However, in an MPLS world, this may create a
long stack of labels to be pushed that some hardware may not be able long stack of labels to be pushed that some hardware may not be able
to push. One of the challenges of TI-LFA is to encode the expected to push. One of the challenges of TI-LFA is to encode the expected
post-convergence path by combining adjacency segments and node post-convergence path by combining adjacency segments and node
segments. Each implementation will be free to have its own path segments. Each implementation will be free to have its own path
compression optimization algorithm. This document details the basic compression optimization algorithm. This document details the basic
concepts that could be used to build the SR backup path as well as concepts that could be used to build the SR backup path as well as
the associated dataplane procedures. the associated dataplane procedures.
L L ____
S----F--{____}----D S----F--{____}----D
/\ | / /\ | /
| | | _______ / | | | _______ /
|__}---Q{_______} |__}---Q{_______}
Figure 2 TI-LFA Protection Figure 2: TI-LFA Protection
We use Figure 2 to illustrate the TI-LFA approach. We use Figure 2 to illustrate the TI-LFA approach.
The Point of Local Repair (PLR), S, needs to find a node Q (a repair The Point of Local Repair (PLR), S, needs to find a node Q (a repair
node) that is capable of safely forwarding the traffic to a node) that is capable of safely forwarding the traffic to a
destination D affected by the failure of the protected link L, a set destination D affected by the failure of the protected link L, a set
of links including L (SRLG), or the node F itself. The PLR also needs of links including L (SRLG), or the node F itself. The PLR also
to find a way to reach Q without being affected by the convergence needs to find a way to reach Q without being affected by the
state of the nodes over the paths it wants to use to reach Q: the PLR convergence state of the nodes over the paths it wants to use to
needs a loop-free path to reach Q. reach Q: the PLR needs a loop-free path to reach Q.
Section 2 defines the main notations used in the document. They are Section 3 defines the main notations used in the document. They are
in line with [RFC5714]. in line with [RFC5714].
Section 3 suggests to compute the P-Space and Q-Space properties Section 4 suggests to compute the P-Space and Q-Space properties
defined in Section 2, for the specific case of nodes lying over the defined in Section 3, for the specific case of nodes lying over the
post-convergence paths towards the protected destinations. post-convergence paths towards the protected destinations.
Using the properties defined in Section 3, Section 4 describes how Using the properties defined in Section 4, Section 5 describes how to
to compute protection lists that encode a loop-free post-convergence compute protection lists that encode a loop-free post-convergence
path towards the destination. path towards the destination.
Section 5 defines the segment operations to be applied by the PLR Section 6 defines the segment operations to be applied by the PLR to
to ensure consistency with the forwarding state of the repair node. ensure consistency with the forwarding state of the repair node.
By applying the algorithms specified in this document to actual By applying the algorithms specified in this document to actual
service providers and large enterprise networks, we provide real life service providers and large enterprise networks, we provide real life
measurements for the number of SIDs used by repair paths. Section 8 measurements for the number of SIDs used by repair paths. Section 9
summarizes these measurements. summarizes these measurements.
1.1. Conventions used in this document 2.1. 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 BCP
14 [RFC2119] [RFC8174] when and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Terminology 3. Terminology
We define the main notations used in this document as the following. 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 We refer to "old" and "new" topologies as the LSDB state before and
after the considered failure. after the considered failure.
SPT_old(R) is the Shortest Path Tree rooted at node R in the initial SPT_old(R) is the Shortest Path Tree rooted at node R in the initial
state of the network. state of the network.
SPT_new(R, X) is the Shortest Path Tree rooted at node R in the state SPT_new(R, X) is the Shortest Path Tree rooted at node R in the state
skipping to change at page 8, line 14 skipping to change at page 9, line 14
PLR stands for "Point of Local Repair". It is the router that PLR stands for "Point of Local Repair". It is the router that
applies fast traffic restoration after detecting failure in a applies fast traffic restoration after detecting failure in a
directly attached link, set of links, and/or node. directly attached link, set of links, and/or node.
Similar to [RFC7490], we use the concept of P-Space and Q-Space for Similar to [RFC7490], we use the concept of P-Space and Q-Space for
TI-LFA. TI-LFA.
The P-Space P(R,X) of a node R w.r.t. a resource X (e.g. a link S-F, The P-Space P(R,X) of a node R w.r.t. a resource X (e.g. a link S-F,
a node F, or a SRLG) is the set of nodes that are reachable from R a node F, or a SRLG) is the set of nodes that are reachable from R
without passing through X. It is the set of nodes that are not without passing through X. It is the set of nodes that are not
downstream of X in SPT_old(R). downstream of X in SPT_old(R).
The Extended P-Space P'(R,X) of a node R w.r.t. a resource X is the The Extended P-Space P'(R,X) of a node R w.r.t. a resource X is the
set of nodes that are reachable from R or a neighbor of R, without set of nodes that are reachable from R or a neighbor of R, without
passing through X. passing through X.
The Q-Space Q(D,X) of a destination node D w.r.t. a resource X is the The Q-Space Q(D,X) of a destination node D w.r.t. a resource X is the
set of nodes which do not use X to reach D in the initial state of set of nodes which do not use X to reach D in the initial state of
the network. In other words, it is the set of nodes which have D in the network. In other words, it is the set of nodes which have D in
their P-Space w.r.t. S-F, F, or a set of links adjacent to S). their P-Space w.r.t. S-F, F, or a set of links adjacent to S).
A symmetric network is a network such that the IGP metric of each A symmetric network is a network such that the IGP metric of each
link is the same in both directions of the link. link is the same in both directions of the link.
3. 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-based explicit 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 path from P to Q when they are not adjacent. Such properties will be
used in Section 4 to compute the TI-LFA repair list. used in Section 5 to compute the TI-LFA repair list.
3.1. P-Space property computation for a resource X 4.1. P-Space property computation for a resource X
A node N is in P(R, X) if it is not downstream of X in SPT_old(R). X A node N is in P(R, X) if it is not downstream of X in SPT_old(R). X
can be a link, a node, or a set of links adjacent to the PLR. A node can be a link, a node, or a set of links adjacent to the PLR. A node
N is in P'(R,X) if it is not downstream of X in SPT_old(N), for at N is in P'(R,X) if it is not downstream of X in SPT_old(N), for at
least one neighbor N of R. least one neighbor N of R.
3.2. Q-Space property computation for a link S-F, over post-convergence 4.2. Q-Space property computation for a link S-F, over post-convergence
paths paths
We want to determine which nodes on the post-convergence path from We want to determine which nodes on the post-convergence path from
the PLR to the destination D are in the Q-Space of destination D the PLR to the destination D are in the Q-Space of destination D
w.r.t. link S-F. w.r.t. link S-F.
This can be found by intersecting the post-convergence path to D, This can be found by intersecting the post-convergence path to D,
assuming the failure of S-F, with Q(D, S-F). assuming the failure of S-F, with Q(D, S-F).
3.3. Q-Space property computation for a set of links adjacent to S, 4.3. Q-Space property computation for a set of links adjacent to S,
over post-convergence paths over post-convergence paths
We want to determine which nodes on the post-convergence path from We want to determine which nodes on the post-convergence path from
the PLR to the destination D are in the Q-Space of destination D the PLR to the destination D are in the Q-Space of destination D
w.r.t. a set of links adjacent to S (S being the PLR). That is, we w.r.t. a set of links adjacent to S (S being the PLR). That is, we
aim to find the set of nodes on the post-convergence path that use aim to find the set of nodes on the post-convergence path that use
none of the members of the protected set of links, to reach D. none of the members of the protected set of links, to reach D.
This can be found by intersecting the post-convergence path to D, This can be found by intersecting the post-convergence path to D,
assuming the failure of the set of links, with the intersection among assuming the failure of the set of links, with the intersection among
Q(D, S->X) for all S->X belonging to the set of links. Q(D, S->X) for all S->X belonging to the set of links.
3.4. Q-Space property computation for a node F, over post-convergence 4.4. Q-Space property computation for a node F, over post-convergence
paths paths
We want to determine which nodes on the post-convergence from the PLR We want to determine which nodes on the post-convergence from the PLR
to the destination D are in the Q-Space of destination D w.r.t. node to the destination D are in the Q-Space of destination D w.r.t. node
F. F.
This can be found by intersecting the post-convergence path to D, This can be found by intersecting the post-convergence path to D,
assuming the failure of F, with Q(D, F). assuming the failure of F, with Q(D, F).
3.5. Scaling considerations when computing Q-Space 4.5. 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 SPT for every destination implementation tries to compute a reverse SPT for every destination
in the network to determine the Q-Space. It will be up to each in the network to determine the Q-Space. It will be up to each
implementation to determine the good tradeoff between scaling and implementation to determine the good tradeoff between scaling and
accuracy of the optimization. accuracy of the optimization.
4. TI-LFA Repair Tunnel 5. TI-LFA Repair path
The TI-LFA repair tunnel consists of an outgoing interface and a list The TI-LFA repair path (RP) consists of an outgoing interface and a
of segments (repair list) to insert on the SR header. The repair list of segments (repair list (RL)) to insert on the SR header. The
list encodes the explicit post-convergence path to the destination, repair list encodes the explicit post-convergence path to the
which avoids the protected resource X and, at the same time, is destination, which avoids the protected resource X and, at the same
guaranteed to be loop-free irrespective of the state of FIBs along time, is guaranteed to be loop-free irrespective of the state of FIBs
the nodes belonging to the explicit path. Thus there is no need for along the nodes belonging to the explicit path. Thus there is no
any co-ordination or message exchange between the PLR and any other need for any co-ordination or message exchange between the PLR and
router in the network. any other router in the network.
The TI-LFA repair tunnel 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 P to Q when these nodes are not adjacent 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 along the post convergence path. The TI-LFA repair list is expressed
generally as (Node_SID(P), EP(P, Q)). generally as (Node_SID(P), EP(P, Q)).
Most often, the TI-LFA repair list has a simpler form, as described Most often, the TI-LFA repair list has a simpler form, as described
in the following sections. Section 8 provides statistics for the in the following sections. Section 9 provides statistics for the
number of SIDs in the explicit path to protect against various number of SIDs in the explicit path to protect against various
failures. failures.
4.1. 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 on the post- When a direct neighbor is in P(S,X) and Q(D,x) and on the post-
convergence path, the outgoing interface is set to that neighbor and convergence path, the outgoing interface is set to that neighbor and
the repair segment list MUST be empty. the repair segment list MUST be empty.
This is comparable to a post-convergence LFA FRR repair. This is comparable to a post-convergence LFA FRR repair.
4.2. 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 MUST be made of a single node convergence path, the repair list MUST be made of a single node
segment to R and the outgoing interface MUST be set to the outgoing segment to R and the outgoing interface MUST be set to the outgoing
interface used to reach R. interface used to reach R.
This is comparable to a post-convergence RLFA repair tunnel. This is comparable to a post-convergence RLFA repair tunnel.
4.3. 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 on
the post-convergence path and both are adjacent to each other, the the post-convergence path and both are adjacent to each other, the
repair list MUST be made of two segments: A node segment to P (to be repair list MUST be made of two segments: A node segment to P (to be
processed first), followed by an adjacency segment from P to Q. processed first), followed by an adjacency segment from P to Q.
This is comparable to a post-convergence DLFA repair tunnel. This is comparable to a post-convergence DLFA repair tunnel.
4.4. 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 post-
convergence path. However, the PLR can perform additional convergence path. However, the PLR can perform additional
computations to compute a list of segments that represent a loop-free 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 path from P to Q. How these computations are done is out of scope of
this document. this document.
5. Protecting segments 6. 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
Section 6.1 and Section 6.2 are equally applicable to both SR-MPLS
and SRv6 dataplane, while the dataplane-specific considerations are
described in Section 6.3.
6.1. Link protection
In this section, we explain how a protecting router S processes the In this section, we explain how a protecting router S processes the
active segment of a packet upon the failure of its primary outgoing active segment of a packet upon the failure of its primary outgoing
interface for the packet, S-F. interface for the packet, S-F.
The behavior depends on the type of active segment to be protected. 6.1.1. The active segment is a node segment
5.1. 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
repair list MUST be inserted at the head of the list. The active repair list MUST be inserted at the head of the list. The active
segment becomes the first segment of the inserted repair list. segment becomes the first segment of the inserted repair list.
This behavior is slightly modified when SR-MPLS is used: 6.1.2. The active segment is an adjacency segment
o If the repair list ends with an adjacency segment terminating on
the tail-end of the active segment, and if the active segment has
been signalled with penultimate hop popping, the active segment
MUST be popped before pushing the repair list.
o If the SRGB at the Q node is different from the SRGB at the PLR,
then the active segment (before the insertion of the repair list)
MUST be updated to fit the SRGB of the Q node.
In Section 5.3, we describe the node protection behavior of PLR S,
for the specific case where the active segment is a prefix segment
for the neighbor F itself.
5.2. The active segment is an adjacency segment
We define hereafter the FRR behavior applied by S for any packet We define hereafter the FRR behavior applied by S for any packet
received with an active adjacency segment S-F for which protection received with an active adjacency segment S-F for which protection
was enabled. As protection has been enabled for the segment S-F and was enabled. As protection has been enabled for the segment S-F and
signalled in the IGP, any SR policy using this segment knows that it signalled in the IGP, any SR policy using this segment knows that it
may be transiently rerouted out of S-F in case of S-F failure. may be transiently rerouted out of S-F in case of S-F failure.
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 two "PUSH" operations are performed. If the repair list,
while avoiding S-F, terminates on F, S only pushes the repair list.
Otherwise, S pushes a node segment of F, followed by by push 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 We distinguish the case where this active segment is followed by
another adjacency segment from the case where it is followed by a another adjacency segment from the case where it is followed by a
node segment. node segment.
5.2.1. Protecting [Adjacency, Adjacency] segment lists 6.1.2.1. 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(S-F) and then two To do so, S MUST apply a "NEXT" operation on Adj(S-F) and then one or
consecutive "PUSH" operations: first it pushes a node segment for F, two "PUSH" operations. If the repair list, while avoiding S-F,
and then it pushes a repair list allowing to reach F while bypassing terminates on F, S only pushes the repair list. Otherwise, S pushes
S-F. For details on the "NEXT" and "PUSH" operations, refer to a node segment of F, followed by push of the repair list.. For
[RFC8402]. details on the "NEXT" and "PUSH" operations, 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(S-F),adj(F-M),...] will thus leave S with a segment list [adj(S-F),adj(F-M),...] will thus leave S with a segment list
matching [RT(F),node(F),adj(F-M)], where RT(F) is the repair tunnel matching [RL(F),node(F),adj(F-M)], where RL(F) is the repair path for
for destination F. destination F.
This behavior is slightly modified when SR-MPLS is used:
o If the repair list ends with an adjacency segment terminating on
F, and if the node segment of F has been signalled with
penultimate hop popping, the implementation MUST pop Adj(S-F) and
then push the repair list (the node segment of F is not pushed).
The packet will leave S with a segment list matching
[RT(F),adj(F-M)].
o If the SRGB at the Q node is different from the SRGB at the PLR,
then MPLS label representing node(F) MUST be calculated as per the
SRGB of the Q node.
In Section 5.3.2, we describe the TI-LFA behavior of PLR S when In Section 6.2.2, we describe the TI-LFA behavior of PLR S when node
node protection is applied and the two first segments are Adjacency protection is applied and the two first segments are Adjacency
Segments. Segments.
5.2.2. Protecting [Adjacency, Node] segment lists 6.1.2.2. 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(S-F),node(T),...]. the segment list on the packet matches [adj(S-F),node(T),...].
A first solution would consist in steering the packet back to F while In this case, S MUST apply a "NEXT" operation on the Adjacency
avoiding S-F. To do so, S MUST apply a "NEXT" operation on Adj(S-F) segment related to S-F, followed by a "PUSH" of a repair list
and then two consecutive "PUSH" operations: first it pushes a node redirecting the traffic to a node Q, whose path to node segment T is
segment for F, and then it pushes a repair list allowing to reach F not affected by the failure.
while bypassing S-F.
Upon failure of S-F, a packet reaching S with a segment list matching
[adj(S-F),node(T),...] will thus leave S with a segment list matching
[RT(F),node(F),node(T)].
This behavior is slightly modified when SR-MPLS is used:
o If the repair list ends with an adjacency segment terminating on
F, and if the node segment of F has been signalled with
penultimate hop popping, the implementation MUST pop Adj(S-F) and
then push the repair list (the node segment of F is not pushed).
The packet will leave S with a segment list matching
[RT(F),node(T)].
o If the SRGB at the Q node is different from the SRGB at the PLR,
then MPLS label representing node(F) MUST be calculated as per the
SRGB of the Q node.
Another solution is to not steer the packet back via F but rather
follow the new shortest path to 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.
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(S-F), node(T), ...], would leave S with a segment list matching [adj(S-F), node(T), ...], would leave S with a segment list matching
[RT(Q),node(T), ...]. Note that this second behavior is the one [RL(Q),node(T), ...]. Note that this second behavior is the one
followed for node protection, as described in Section 5.3.1. followed for node protection, as described in Section 6.2.1.
This behavior is slightly modified when SR-MPLS is used:
o If the repair list ends with an adjacency segment terminating on T
(T being the Q node), and if the node segment of T has been
signalled with penultimate hop popping, the implementation MUST
pop Adj(S-F) and then push the repair list (the node segment of T
is not pushed). The packet will leave S with a segment list
matching [RT(Q=T), ...].
o If the SRGB at the Q node is different from the SRGB at the PLR,
then the MPLS label representing node(T) MUST be calculated as per
the SRGB of the Q node.
The first proposal 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 existing path. As stated in Section 1,
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.
5.3. Protecting SR policy midpoints against node failure 6.2. Protecting SR policy midpoints against node failure
In this section, we describe the behavior of a node S configured to In this section, we describe the behavior of a node S configured to
interpret the failure of link S->F as the node failure of F, in the interpret the failure of link S->F as the node failure of F, in the
specific case where the active segment of the packet received by S is specific case where the active segment of the packet received by S is
a Prefix SID of F represented as "F"), or an Adjacency SID for the a Prefix SID of F represented as "F"), or an Adjacency SID for the
link S-F (represented as "S->F"). link S-F (represented as "S->F").
5.3.1. Protecting {F, T, D} or {S->F, T, D} The description below is intended to specify the forwarding behavior
required for node protection. The description should not be
interpreted as limiting the possible implementations of this
forwarding behavior. An implementation complies with the description
below as long as the externally visible forwarding behavior produced
by the implementation is the same as that described below.
6.2.1. Protecting {F, T, D} or {S->F, T, D}
This section describes the protection behavior of S when all of the This section describes the protection behavior of S when all of the
following conditions are true: following conditions are true:
1. the active segment is a prefix SID for a neighbor F, or an 1. the active segment is a prefix SID for a neighbor F, or an
adjacency segment S->F adjacency segment S->F
2. the primary interface used to forward the packet failed 2. the primary interface used to forward the packet failed
3. the segment following the active segment is a prefix SID (for 3. the segment following the active segment is a prefix SID (for
node T) node T)
4. node protection is active for that interface. 4. node protection is active for that interface.
In such a case, the PLR MUST: In such a case, the PLR should:
1. apply a NEXT operation; the segment F or S->F is removed 1. apply a NEXT operation; the segment F or S->F is removed
2. Confirm that the next segment is in the SRGB of F, meaning that 2. Confirm that the next segment is in the SRGB of F, meaning that
the next segment is a prefix segment, e.g. for node T the next segment is a prefix segment, e.g. for node T
3. Retrieve the segment ID of T (as per the SRGB of F) 3. Retrieve the segment ID of T (as per the SRGB of F)
4. Apply a NEXT operation followed by a PUSH operation of T's segment 4. Apply a NEXT operation followed by a PUSH operation of T's
based on the SRGB of node S. segment based on the SRGB of node S.
5. Look up T's segment (based on the updated label value) and 5. Look up T's segment (based on the updated label value) and
forward accordingly. forward accordingly.
5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D} 6.2.2. Protecting {F, F->T, D} or {S->F, F->T, D}
This section describes the protection behavior of S when all of the This section describes the protection behavior of S when all of the
following conditions are true: following conditions are true:
1. the active segment is a prefix SID for a neighbor F, or an 1. the active segment is a prefix SID for a neighbor F, or an
adjacency segment S->F adjacency segment S->F
2. the primary interface used to forward the packet failed 2. the primary interface used to forward the packet failed
3. the segment following the active segment is an adjacency SID (F- 3. the segment following the active segment is an adjacency SID (F-
>T) >T)
4. node protection is active for that interface. 4. node protection is active for that interface.
In such a case, the PLR MUST: In such a case, the PLR should:
1. Apply a NEXT operation; the segment F or S->F is removed 1. Apply a NEXT operation; the segment F or S->F is removed
2. Confirm that the next segment is an adjacency SID of F, say F->T 2. Confirm that the next segment is an adjacency SID of F, say F->T
3. Retrieve the node segment ID associated to T (as per the set of 3. Retrieve the node segment ID associated to T (as per the set of
Adjacency Segments of F) Adjacency Segments of F)
4. Apply a NEXT operation on the next segment followed by a PUSH of 4. Apply a NEXT operation on the next segment followed by a PUSH of
T's segment based on the SRGB of the node S. T's segment based on the SRGB of the node S.
5. Look up T's segment (based on the updated label value) and forward 5. Look up T's segment (based on the updated label value) and
accordingly. forward accordingly.
It is noteworthy to mention that node "S" in the procedures described It is noteworthy to mention that node "S" in the procedures described
in Sections 5.3.1 and 5.3.2 can always determine whether the in Sections 5.3.1 and 5.3.2 can always determine whether the segment
segment after popping the top segment is an adjacency SID or a after popping the top segment is an adjacency SID or a prefix-SID of
prefix-SID of the next-hop "F" as follows: the next-hop "F" as follows:
1. In a link state environment, the node "S" knows the SRGB and the 1. In a link state environment, the node "S" knows the SRGB and the
adj-SIDs of the neighboring node "F" adj-SIDs of the neighboring node "F"
2. If the new segment after popping the top segment is within the 2. If the new segment after popping the top segment is within the
SRGB or the adj-SIDs of "F", then node "S" is certain that the SRGB or the adj-SIDs of "F", then node "S" is certain that the
failure of node "F" is a midpoint failure and hence node "S" failure of node "F" is a midpoint failure and hence node "S"
applies the procedures specified in Sections 5.3.1 or 5.3.2, applies the procedures specified in Sections 5.3.1 or 5.3.2,
respectively. respectively.
3. Otherwise the failure is not a midpoint failure and hence the node 3. Otherwise the failure is not a midpoint failure and hence the
"S" may apply other protection techniques that are beyond the node "S" may apply other protection techniques that are beyond
scope of this document or simply drop the packet and wait for the scope of this document or simply drop the packet and wait for
normal protocol convergence. normal protocol convergence.
6. TI-LFA and SR Algorithms 6.3. Dataplane specific considerations
6.3.1. MPLS dataplane considerations
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.
6.3.2. SRv6 dataplane considerations
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.
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.
7. 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 has 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 overriden 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 situation can
already happen today with existing mechanisms as remote LFA. already happen today with existing mechanisms as remote LFA.
When a Node-SID is associated with the SR default algorithm,
enforcing TI-LFA to use Node-SIDs associated with a strict SPF
algorithm is a definitive solution to this problem.
[I-D.ietf-lsr-flex-algo] defines a flexible algorithm (FlexAlgo) [I-D.ietf-lsr-flex-algo] defines a flexible algorithm (FlexAlgo)
framework to be associated with Prefix SIDs. FlexAlgo allows a user framework to be associated with Prefix SIDs. FlexAlgo allows a user
to associate a constrained path to a Prefix SID rather than using the to associate a constrained path to a Prefix SID rather than using the
regular IGP shortest path. An implementation MAY support TI-LFA to regular IGP shortest path. An implementation MAY support TI-LFA to
protect Node-SIDs associated to a FlexAlgo. In such a case, rather protect Node-SIDs associated to a FlexAlgo. In such a case, rather
than computing the expected post-convergence path based on the than computing the expected post-convergence path based on the
regular SPF, an implementation SHOULD use the constrained SPF regular SPF, an implementation SHOULD use the constrained SPF
algorithm bound to the FlexAlgo instead of the regular Dijkstra in algorithm bound to the FlexAlgo (using the Flex Algo Definition)
all the SPF/rSPF computations that are occurring during the TI-LFA instead of the regular Dijkstra in all the SPF/rSPF computations that
computation. This includes the computation of the P-Space and are occurring during the TI-LFA computation. This includes the
Q-Space as well as the post-convergence path. 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.
7. Usage of Adjacency segments in the repair list 8. Usage of Adjacency segments in the repair list
The repair list of segments computed by TI-LFA may contain one or The repair list of segments computed by TI-LFA may contain one or
more adjacency segments. An adjacency segment may be protected or more adjacency segments. An adjacency segment may be protected or
not protected. not protected.
S --- R2 --- R3 --- R4 --- R5 --- D S --- R2 --- R3 --- R4 --- R5 --- D
\ | \ / \ | \ /
R7 -- R8 R7 -- R8
| | | |
R9 -- R10 R9 -- R10
Figure 3 Figure 3
In Figure 3, all the metrics are equal to 1 except In Figure 3, 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(R7-R8),adj(R8-R4)] S->D, the repair list computed by R2 will be [adj(R7-R8),adj(R8-R4)]
and the outgoing interface will be to R7. If R3 fails, R2 pushes the 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 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 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 adj(R7-R8), R7 will push an additional repair list onto the packet
following the procedures defined in Section 5. following the procedures defined in Section 6.
To avoid the possibility of this double FRR, an implementation of TI- To avoid the possibility of this double FRR, an implementation of TI-
LFA MAY pick only non protected adjacency segments when building the LFA MAY pick only non protected adjacency segments when building the
repair list. repair list.
8. Measurements on Real Networks 9. Measurements on Real Networks
This section presents measurements performed on real service provider This section presents measurements performed on real service provider
and large enterprise networks. The objective of the measurements is and large enterprise networks. The objective of the measurements is
to assess the number of SIDs required in an explicit path when the to assess the number of SIDs required in an explicit path when the
mechanisms described in this document are used to protect against the mechanisms described in this document are used to protect against the
failure scenarios within the scope of this document. The number of failure scenarios within the scope of this document. The number of
segments described in this section are applicable to instantiating segments described in this section are applicable to instantiating
segment routing over the MPLS forwarding plane. segment routing over the MPLS forwarding plane.
The measurements below indicate that for link and local SRLG The measurements below indicate that for link and local SRLG
protection, a 1 SID repair path delivers more than 99% coverage. For protection, a 1 SID repair path delivers more than 99% coverage. For
node protection a 2 SIDs repair path yields 99% coverage. node protection a 2 SIDs repair path 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 measurements are carried out as follows 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:
o For each network, the algorithms described in this document are o 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
o For each prefix, the number of SIDs used by the repair path is o For each prefix, the number of SIDs used by the repair path is
recored recored
o The percentage of number of SIDs are listed in Tables 2A/B, 3A/B, o The percentage of number of SIDs are listed in Tables 2A/B, 3A/B,
and 4A/B and 4A/B
The measurements listed in the tables indicate that for link and The measurements listed in the tables indicate that for link and
local SRLG protection, 1 SID repair paths are sufficient to protect local SRLG protection, 1 SID repair paths are sufficient to protect
more than 99% of the prefix in almost all cases. For node protection more than 99% of the prefix in almost all cases. For node protection
2 SIDs repair paths yield 99% coverage. 2 SIDs repair paths yield 99% coverage.
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| Network | Nodes | Circuits |Node-to-Link| SRLG info? | | Network | Nodes | Links |Node-to-Link| SRLG info? |
| | | | Ratio | | | | | | Ratio | |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T1 | 408 | 665 | 1 : 63 | Yes | | T1 | 408 | 665 | 1.63 | Yes |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T2 | 587 | 1083 | 1 : 84 | No | | T2 | 587 | 1083 | 1.84 | No |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T3 | 93 | 401 | 4 : 31 | Yes | | T3 | 93 | 401 | 4.31 | Yes |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T4 | 247 | 393 | 1 : 59 | Yes | | T4 | 247 | 393 | 1.59 | Yes |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T5 | 34 | 96 | 2 : 82 | Yes | | T5 | 34 | 96 | 2.82 | Yes |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T6 | 50 | 78 | 1 : 56 | No | | T6 | 50 | 78 | 1.56 | No |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T7 | 82 | 293 | 3 : 57 | No | | T7 | 82 | 293 | 3.57 | No |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T8 | 35 | 41 | 1 : 17 | Yes | | T8 | 35 | 41 | 1.17 | Yes |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T9 | 177 | 1371 | 7 : 74 | Yes | | T9 | 177 | 1371 | 7.74 | Yes |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
Table 1: Data Set Definition 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 convention that we use is as follows
o 0 SIDs: the calculated repair path starts with a directly o 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 4.1. additional SIDs. This scenario is described in Section 5.1.
o 1 SIDs: the repair node is a PQ node, in which case only 1 SID is o 1 SIDs: the repair node is a PQ node, in which case only 1 SID is
needed to guarantee loop-freeness. This scenario is covered in needed to guarantee loop-freeness. This scenario is covered in
Section 4.2. Section 5.2.
o 2 or more SIDs: The repair path consists of 2 or more SIDs as o 2 or more SIDs: The repair path consists of 2 or more SIDs as
described in Sections 4.3 and 4.4. We do not cover the case for 2 described in Sections 4.3 and 4.4. We do not cover the case for 2
SIDs (Section 4.3) separately because there was no granularity in SIDs (Section 5.3) separately because there was no granularity in
the result. Also we treat the node-SID+adj-SID and node-SID + 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 node-SID the same because they do not differ from the data plane
point of view. point of view.
Table 2A and 2B below summarize the measurements on the number of Table 2A and 2B below summarize the measurements on the number of
SIDs needed for link protection SIDs needed for link protection
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | | Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+ +-------------+------------+------------+------------+------------+
| T1 | 74.227% | 25.256% | 0.517% | 0.001% | | T1 | 74.227% | 25.256% | 0.517% | 0.001% |
skipping to change at page 21, line 28 skipping to change at page 22, line 21
| T6 | 78.362% | 99.682% | 100.0% |100.0% | 100% | | T6 | 78.362% | 99.682% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+----------+----------+----------+----------+----------+
| T7 | 66.106% | 98,918% | 100.0% |100.0% | 100% | | T7 | 66.106% | 98,918% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+----------+----------+----------+----------+----------+
| T8 | 59.712% | 100.0% | 100.0% |100.0% | 100% | | T8 | 59.712% | 100.0% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+----------+----------+----------+----------+----------+
| T9 | 98.950% | 100.0% | 100.0% |100.0% | 100% | | T9 | 98.950% | 100.0% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+ +---------+----------+----------+----------+----------+----------+
Table 4B: Node protection (repair size cumulative distribution) Table 4B: Node protection (repair size cumulative distribution)
9. 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 result in the ability to guarantee functionalities to a router that result in the ability to guarantee
an upper bound on the time taken to restore traffic flow upon the an upper bound on the time taken to restore traffic flow upon the
failure of a directly connected link or node. As these techniques failure of a directly connected link or node. As these techniques
steer traffic to the post-convergence path as quickly as possible, steer traffic to the post-convergence path as quickly as possible,
this serves to minimize the disruption associated with a local this serves to minimize the disruption associated with a local
failure which can be seen as a modest security enhancement. The failure which can be seen as a modest security enhancement. The
protection mechanisms does not protect external destinations, but protection mechanisms does not protect external destinations, but
rather provides quick restoration for destination that are internal rather provides quick restoration for destination that are internal
to a routing domain. to a routing domain.
10. IANA Considerations 11. IANA Considerations
No requirements for IANA No requirements for IANA
11. Conclusions 12. Conclusions
This document proposes a mechanism that is able to pre-calculate a This document proposes a mechanism that is able to pre-calculate a
backup path for every primary path so as to be able to protect backup path for every primary path so as to be able to protect
against the failure of a directly connected link, node, or SRLG. against the failure of a directly connected link, node, or SRLG. The
The mechanism is able to calculate the backup path irrespective of mechanism is able to calculate the backup path irrespective of the
the topology as long as the topology is sufficiently redundant. topology as long as the topology is sufficiently redundant.
12. Acknowledgments 13. Acknowledgments
We would like to thank Les Ginsberg, Stewart Bryant, Alexander We would like to thank Les Ginsberg, Stewart Bryant, Alexander
Vainsthein, Chris Bowers for their valuable comments. Vainsthein, Chris Bowers, Shraddha Hedge for their valuable comments.
This document was prepared using 2-Word-v2.0.template.dot. 14. References
13. References 14.1. Normative References
13.1. Normative References [I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
draft-ietf-spring-srv6-network-programming-11 (work in
progress), March 2020.
[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, DOI Requirement Levels", BCP 14, RFC 2119,
10.17487/RFC2119, March 1997, <https://www.rfc- DOI 10.17487/RFC2119, March 1997,
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, July Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
2016, <https://www.rfc-editor.org/info/rfc7916>. July 2016, <https://www.rfc-editor.org/info/rfc7916>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
R. Shakir, "Segment Routing Architecture", RFC 8402, DOI Decraene, B., Litkowski, S., and R. Shakir, "Segment
10.17487/RFC8402 July 2018, <http://www.rfc- Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
editor.org/info/rfc8402>. July 2018, <https://www.rfc-editor.org/info/rfc8402>.
13.2. Informative References 14.2. Informative References
[I-D.bashandy-rtgwg-segment-routing-uloop] Bashandy, A., Filsfils, [I-D.bashandy-rtgwg-segment-routing-uloop]
C., Litkowski, S., Decraene, B., Francois, P., and Psenak, Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
P. " Loop avoidance using Segment Routing", draft-bashandy- Francois, P., and P. Psenak, "Loop avoidance using Segment
rtgwg-segment-routing-uloop-07, (work in progress), January Routing", draft-bashandy-rtgwg-segment-routing-uloop-08
2020 (work in progress), January 2020.
[I-D.ietf-lsr-flex-algo] Psenak, P., Hegde, S., Filsfils, C., [I-D.ietf-lsr-flex-algo]
Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
draft-ietf-lsr-flex-algo-05 (work in progress), November A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
2019. algo-06 (work in progress), February 2020.
[I-D.ietf-spring-segment-routing-policy] Filsfils, C., Sivabalan, S., [I-D.ietf-spring-segment-routing-policy]
daniel.voyer@bell.ca, d., bogdanov@google.com, b., and P. Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
Mattes, "Segment Routing Policy Architecture", draft-ietf- P. Mattes, "Segment Routing Policy Architecture", draft-
spring-segment-routing-policy-06 (work in progress), ietf-spring-segment-routing-policy-06 (work in progress),
December 2019. December 2019.
[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, DOI IP Fast Reroute: Loop-Free Alternates", RFC 5286,
10.17487/RFC5286, September 2008, <https://www.rfc- DOI 10.17487/RFC5286, September 2008,
editor.org/info/rfc5286>. <https://www.rfc-editor.org/info/rfc5286>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
5714, DOI 10.17487/RFC5714 January 2010, <http://www.rfc- RFC 5714, DOI 10.17487/RFC5714, January 2010,
editor.org/info/rfc5714>. <https://www.rfc-editor.org/info/rfc5714>.
[RFC6571] Filsfils, C., Francois, P., Shand, M., Decraene, B., [RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP) Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571 June 2012, Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<http://www.rfc-editor.org/info/rfc6571>. <https://www.rfc-editor.org/info/rfc6571>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015, RFC 7490, DOI 10.17487/RFC7490, April 2015,
<http://www.rfc-editor.org/info/rfc7490>. <https://www.rfc-editor.org/info/rfc7490>.
Authors' Addresses Authors' Addresses
Stephane Litkowski Stephane Litkowski
Cisco Cisco Systems
France
Email: slitkows.ietf@gmail.com Email: slitkows.ietf@gmail.com
Ahmed Bashandy Ahmed Bashandy
Individual Individual
Email: abashandy.ietf@gmail.com Email: abashandy.ietf@gmail.com
Clarence Filsfils Clarence Filsfils
Cisco Systems Cisco Systems
Brussels Brussels
Belgium Belgium
Email: cfilsfil@cisco.com
Email: cfilsfil@cisco.com
Bruno Decraene Bruno Decraene
Orange Orange
Issy-les-Moulineaux Issy-les-Moulineaux
France France
Email: bruno.decraene@orange.com Email: bruno.decraene@orange.com
Pierre Francois Pierre Francois
INSA Lyon INSA Lyon
Email: pierre.francois@insa-lyon.fr Email: pierre.francois@insa-lyon.fr
Daniel Voyer Daniel Voyer
Bell Canada Bell Canada
Canada Canada
Email: daniel.voyer@bell.ca Email: daniel.voyer@bell.ca
Francois Clad Francois Clad
Cisco Systems Cisco Systems
Email: fclad@cisco.com Email: fclad@cisco.com
Pablo Camarillo Pablo Camarillo
Cisco Systems Cisco Systems
Email: pcamaril@cisco.com Email: pcamaril@cisco.com
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