Network Working Group S. Litkowski
Internet-Draft Cisco Systems
Intended status: Standards Track A. Bashandy
Expires: December 31, 2021 Individual
C. Filsfils
Cisco Systems
P. Francois
INSA Lyon
B. Decraene
Orange
D. Voyer
Bell Canada
June 29, 2021
Topology Independent Fast Reroute using Segment Routing
draft-ietf-rtgwg-segment-routing-ti-lfa-07
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 IGP network. 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, dramatically 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 December 31, 2021.
Litkowski, et al. Expires December 31, 2021 [Page 1]
Internet-Draft SR TI-LFA June 2021
Copyright Notice
Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Conventions used in this document . . . . . . . . . . . . 8
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Intersecting P-Space and Q-Space with post-convergence paths 9
4.1. P-Space property computation for a resource X . . . . . . 9
4.2. Q-Space property computation for a link S-F, over post-
convergence paths . . . . . . . . . . . . . . . . . . . . 9
4.3. Q-Space property computation for a set of links adjacent
to S, over post-convergence paths . . . . . . . . . . . 9
4.4. Q-Space property computation for a node F, over post-
convergence paths . . . . . . . . . . . . . . . . . . . . 10
4.5. Scaling considerations when computing Q-Space . . . . . . 10
5. TI-LFA Repair path . . . . . . . . . . . . . . . . . . . . . 10
5.1. FRR path using a direct neighbor . . . . . . . . . . . . 10
5.2. FRR path using a PQ node . . . . . . . . . . . . . . . . 11
5.3. FRR path using a P node and Q node that are adjacent . . 11
5.4. Connecting distant P and Q nodes along post-convergence
paths . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Building TI-LFA repair lists . . . . . . . . . . . . . . . . 11
6.1. Link protection . . . . . . . . . . . . . . . . . . . . . 11
6.1.1. The active segment is a node segment . . . . . . . . 11
6.1.2. The active segment is an adjacency segment . . . . . 12
6.2. Dataplane specific considerations . . . . . . . . . . . . 13
6.2.1. MPLS dataplane considerations . . . . . . . . . . . . 13
6.2.2. SRv6 dataplane considerations . . . . . . . . . . . . 13
7. TI-LFA and SR algorithms . . . . . . . . . . . . . . . . . . 14
8. Usage of Adjacency segments in the repair list . . . . . . . 14
9. Measurements on Real Networks . . . . . . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
Litkowski, et al. Expires December 31, 2021 [Page 2]
Internet-Draft SR TI-LFA June 2021
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
14.1. Normative References . . . . . . . . . . . . . . . . . . 21
14.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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 SRGB: Segment Routing Global Block.
o SRLG: Shared Risk Link Group.
o TI-LFA: Topology Independant LFA.
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 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
Litkowski, et al. Expires December 31, 2021 [Page 3]
Internet-Draft SR TI-LFA June 2021
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].
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 routing domain.
Protecting domain exit routers and/or links attached to another
routing domains are beyond the scope of this document
Thanks to SR, TI-LFA does not require the establishment of TLDP
sessions 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.
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
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.
[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
Litkowski, et al. Expires December 31, 2021 [Page 4]
Internet-Draft SR TI-LFA June 2021
tuning manually the best alternate election based on its own
requirements.
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
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 1, 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 only 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 1, considering that the source of traffic is only from PE1
and PE4, when the link L fails, depending on the convergence speed of
the nodes, X may reroute its forwarding entries to the remote PEs
onto X-H or X-D; however in a similar timeframe, PE1 will also
reroute a subset of its traffic (the subset destined to PE2) out of
its nominal path reducing the quantity of traffic received by X. The
capacity planning rule presented previously has the drawback of
oversizing the network, however it allows to prevent any transient
congestion (when for example X reroutes traffic before PE1 does).
H --- I --- J
| | \
PE4 | | PE3
\ | (L) | /
A --- X --- B --- G
/ | | \
PE1 | | PE2
\ | | /
C --- D --- E --- F
Figure 1
Based on this assumption, in order to facilitate the operation of
FRR, and limit the implementation of local FRR policies, it looks
interesting to steer the traffic onto the post-convergence path from
the PLR point of view during the FRR phase. In our example, when
Litkowski, et al. Expires December 31, 2021 [Page 5]
Internet-Draft SR TI-LFA June 2021
link L fails, X switches the traffic destined to PE3 and PE2 on the
post-convergence paths. This is perfectly inline with the capacity
planning rule that was presented before and also inline with the fact
X may converge before PE1 (or any other upstream router) and may
spread the X-B traffic onto the post-convergence paths rooted at X.
It should be noted, that some networks may have a different capacity
planning rule, leading to an allocation of less bandwidth on X-H and
X-D links. In such a case, using the post-convergence paths rooted
at X during FRR may introduce some congestion on X-H and X-D links.
However it is important to note, that a transient congestion may
possibly happen, even without FRR activated, for instance when X
converges before the upstream routers. Operators are still free to
use the policy framework defined in [RFC7916] if the usage of the
post-convergence paths rooted at the PLR is not suitable.
Readers should be aware that FRR protection is pre-computing a backup
path to protect against a particular type of failure (link, node,
SRLG). When using the post-convergence path as FRR backup path, the
computed post-convergence path is the one considering the failure we
are protecting against. This means that FRR is using an expected
post-convergence path, and this expected post-convergence path may be
actually different from the post-convergence path used if the failure
that happened is different from the failure FRR was protecting
against. As an example, if the operator has implemented a protection
against a node failure, the expected post-convergence path used
during FRR will be the one considering that the node has failed.
However, even if a single link is failing or a set of links is
failing (instead of the full node), the node-protecting post-
convergence path will be used. The consequence is that the path used
during FRR is not optimal with respect to the failure that has
actually occurred.
Another consideration to take into account is: while using the
expected post-convergence path for SR traffic using node segments
only (for instance, PE to PE traffic using shortest path) has some
advantages, these advantages reduce when SR policies
([I-D.ietf-spring-segment-routing-policy]) 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
the 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 ([I-D.ietf-lsr-flex-algo]) to
Litkowski, et al. Expires December 31, 2021 [Page 6]
Internet-Draft SR TI-LFA June 2021
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 7.
Thanks to SR and the combination of Adjacency segments and Node
segments, the expression of the expected post-convergence path rooted
at the PLR is facilitated and does not create any additional state on
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
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
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 path
compression 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.
L ____
S----F--{____}----D
/\ | /
| | | _______ /
|__}---Q{_______}
Figure 2: TI-LFA Protection
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
node) that is capable of safely forwarding the traffic to a
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 to find a way to reach Q without being affected by the
convergence state of the nodes over the paths it wants to use to
reach Q: the PLR needs a loop-free path to reach Q.
Section 3 defines the main notations used in the document. They are
in line with [RFC5714].
Section 4 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.
Litkowski, et al. Expires December 31, 2021 [Page 7]
Internet-Draft SR TI-LFA June 2021
Using the properties defined in Section 4, Section 5 describes how to
compute protection lists that encode a loop-free post-convergence
path towards the destination.
Section 6 defines the segment operations to be applied by the PLR to
ensure consistency with the forwarding state of the repair node.
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 9
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.
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 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
without passing through X. It is the set of nodes that are not
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
set of nodes that are reachable from R or a neighbor of R, without
passing through X.
Litkowski, et al. Expires December 31, 2021 [Page 8]
Internet-Draft SR TI-LFA June 2021
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
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).
A symmetric network is a network such that the IGP metric of each
link is the same in both directions of the link.
4. 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 5 to compute the TI-LFA repair list.
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
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
least one neighbor N of R.
4.2. Q-Space property computation for a link S-F, over post-convergence
paths
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
w.r.t. link S-F.
This can be found by intersecting the post-convergence path to D,
assuming the failure of S-F, with Q(D, S-F).
4.3. Q-Space property computation for a set of links adjacent to S,
over post-convergence paths
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
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
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,
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.
Litkowski, et al. Expires December 31, 2021 [Page 9]
Internet-Draft SR TI-LFA June 2021
4.4. Q-Space property computation for a node F, over post-convergence
paths
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
F.
This can be found by intersecting the post-convergence path to D,
assuming the failure of F, with Q(D, F).
4.5. 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 SPT 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.
5. 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. 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)).
Most often, the TI-LFA repair list has a simpler form, as described
in the following sections. Section 9 provides statistics for the
number of SIDs in the explicit path to protect against various
failures.
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-
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.
Litkowski, et al. Expires December 31, 2021 [Page 10]
Internet-Draft SR TI-LFA June 2021
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-
convergence path, the repair list MUST 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.
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
the post-convergence path and both are adjacent to each other, the
repair list SHOULD be made of two segments: A node segment to P (to
be processed first), followed by an adjacency segment from P to Q.
This is comparable to a post-convergence DLFA repair tunnel.
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-
convergence path. 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.
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 are equally applicable to both SR-MPLS and SRv6
dataplane, while the dataplane-specific considerations are described
in Section 6.2.
6.1. Link protection
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.
6.1.1. 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 6.2).
Litkowski, et al. Expires December 31, 2021 [Page 11]
Internet-Draft SR TI-LFA June 2021
6.1.2. 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
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.
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
another adjacency segment from the case where it is followed by a
node segment.
6.1.2.1. 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
two "PUSH" operations. 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 push 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.
Litkowski, et al. Expires December 31, 2021 [Page 12]
Internet-Draft SR TI-LFA June 2021
6.1.2.2. 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.
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), ...].
6.2. Dataplane specific considerations
6.2.1. 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.
6.2.2. 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
Litkowski, et al. Expires December 31, 2021 [Page 13]
Internet-Draft SR TI-LFA June 2021
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.
7. 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.
[I-D.ietf-lsr-flex-algo] 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.
8. 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.
Litkowski, et al. Expires December 31, 2021 [Page 14]
Internet-Draft SR TI-LFA June 2021
S --- R2 --- R3 --- R4 --- R5 --- D
\ | \ /
R7 -- R8
| |
R9 -- R10
Figure 3
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
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)]
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 6.
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.
9. Measurements on Real Networks
This section presents measurements performed on real service provider
and large enterprise networks. The objective of the measurements 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 measurements 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:
o For each network, the algorithms described in this document are
applied to protect all prefixes against link, node, and local SRLG
failure
o For each prefix, the number of SIDs used by the repair path is
recored
Litkowski, et al. Expires December 31, 2021 [Page 15]
Internet-Draft SR TI-LFA June 2021
o 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 paths are 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
o 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 5.1.
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
Section 5.2.
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
SIDs (Section 5.3) separately because there was no granularity in
the result. Also we treat the node-SID+adj-SID and node-SID +
Litkowski, et al. Expires December 31, 2021 [Page 16]
Internet-Draft SR TI-LFA June 2021
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% |
+-------------+------------+------------+------------+------------+
| 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% |
+-------------+------------+------------+------------+------------+
Litkowski, et al. Expires December 31, 2021 [Page 17]
Internet-Draft SR TI-LFA June 2021
| T9 | 98,9% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Table 2B: Link protection repair size cumulative distribution
Table 3A and 3B summarize the measurements on the number of SIDs
needed for local SRLG protection.
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.2% | 25.3% | 0.5% | 0.0% |
+-------------+------------+------------+------------+------------+
| T2 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T3 | 93.6% | 6.3% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.5% | 35.6% | 1.8% | 0.0% |
+-------------+------------+------------+------------+------------+
| T5 | 83.1% | 16.8% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T6 | No SRLG Information |
+-------------+---------------------------------------------------+
| T7 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T8 | 85.2% | 14.8% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98,9% | 1.1% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
Table 3A: Local SRLG protection repair size distribution
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.2% | 99.5% | 99.9% | 100.0% |
+-------------+------------+------------+------------+------------+
| T2 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T3 | 93.6% | 99.9% | 100.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.5% | 98.2% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T5 | 83.1% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T6 | No SRLG Information |
+-------------+---------------------------------------------------+
| T7 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T8 | 85.2% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Litkowski, et al. Expires December 31, 2021 [Page 18]
Internet-Draft SR TI-LFA June 2021
| T9 | 98.9% | 100.0% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Table 3B: Local SRLG protection repair size Cumulative distribution
The remaining two tables summarize the measurements on the number of
SIDs needed for node protection.
+---------+----------+----------+----------+----------+----------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
+---------+----------+----------+----------+----------+----------+
| T1 | 49.8% | 47.9% | 2.1% | 0.1% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T2 | 36,5% | 59.6% | 3.6% | 0.2% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T3 | 73.3% | 25.6% | 1.1% | 0.0% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T4 | 36.1% | 57.3% | 6.3% | 0.2% | 0.0% |
+---------+----------+----------+----------+----------+----------+
| T5 | 73.2% | 26.8% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T6 | 78.3% | 21.3% | 0.3% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T7 | 66.1% | 32.8% | 1.1% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T8 | 59.7% | 40.2% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T9 | 98.9% | 1.0% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
Table 4A: Node protection (repair size distribution)
+---------+----------+----------+----------+----------+----------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
+---------+----------+----------+----------+----------+----------+
| T1 | 49.7% | 97.6% | 99.8% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+
| T2 | 36.5% | 96.1% | 99.7% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+
| T3 | 73.3% | 98.9% | 99.9% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T4 | 36.1% | 93.4% | 99.8% | 99.9% | 100% |
+---------+----------+----------+----------+----------+----------+
| T5 | 73.2% | 100.0% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T6 | 78.4% | 99.7% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T7 | 66.1% | 98.9% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T8 | 59.7% | 100.0% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
Litkowski, et al. Expires December 31, 2021 [Page 19]
Internet-Draft SR TI-LFA June 2021
| T9 | 98.9% | 100.0% | 100.0% | 100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
Table 4B: Node protection (repair size cumulative distribution)
10. 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.
11. IANA Considerations
No requirements for IANA
12. 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
13. Acknowledgments
We would like to thank Les Ginsberg, Stewart Bryant, Alexander
Vainsthein, Chris Bowers, Shraddha Hedge for their valuable comments.
14. References
Litkowski, et al. Expires December 31, 2021 [Page 20]
Internet-Draft SR TI-LFA June 2021
14.1. 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>.
14.2. Informative References
[I-D.bashandy-rtgwg-segment-routing-uloop]
Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
Francois, P., and P. Psenak, "Loop avoidance using Segment
Routing", draft-bashandy-rtgwg-segment-routing-uloop-10
(work in progress), December 2020.
Litkowski, et al. Expires December 31, 2021 [Page 21]
Internet-Draft SR TI-LFA June 2021
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-15 (work in progress), April 2021.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-11 (work in progress),
April 2021.
[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>.
[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>.
[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>.
Authors' Addresses
Stephane Litkowski
Cisco Systems
France
Email: slitkows@cisco.com
Ahmed Bashandy
Individual
Email: abashandy.ietf@gmail.com
Litkowski, et al. Expires December 31, 2021 [Page 22]
Internet-Draft SR TI-LFA June 2021
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
Daniel Voyer
Bell Canada
Canada
Email: daniel.voyer@bell.ca
Litkowski, et al. Expires December 31, 2021 [Page 23]
