Internet DRAFT - draft-francois-spring-ti-lfa
draft-francois-spring-ti-lfa
Network Working Group Pierre Francois
Internet-Draft Institute IMDEA Networks
Intended status: Standards Track Clarence Filsfils
Expires: November 14, 2014 Ahmed Bashandy
Cisco Systems, Inc.
Bruno Decraene
Stephane Litkowski
Orange
May 13, 2014
Topology Independent Fast Reroute using Segment Routing
draft-francois-spring-ti-lfa-00
Abstract
This document presents a Fast Reroute (FRR) approach aimed at
providing link and node protection of node and adjacency segments
within the Segment Routing (SR) framework. This 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. We accommodate the
FRR discovery and selection approaches in order to establish
protection over post-convergence paths from the point of local
repair, dramatically reducing the operator's need to control the tie-
breaks among various FRR options.
Status of this Memo
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Intersecting P-Space and Q-Space with post-convergence
paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. P-Space property computation for a resource X . . . . . . . 5
3.2. Q-Space property computation for a link S-F, over
post-convergence paths . . . . . . . . . . . . . . . . . . 5
3.3. Q-Space property computation for a node F, over
post-convergence paths . . . . . . . . . . . . . . . . . . 6
4. EPC Repair Tunnel . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. The repair node is a direct neighbor . . . . . . . . . . . 6
4.2. The repair node is a PQ node . . . . . . . . . . . . . . . 6
4.3. The repair is a Q node, neighbor of the last P node . . . . 7
4.4. Connecting distant P and Q nodes along
post-convergence paths . . . . . . . . . . . . . . . . . . 7
5. Protecting segments . . . . . . . . . . . . . . . . . . . . . . 7
5.1. The active segment is a node segment . . . . . . . . . . . 7
5.2. The active segment is an adjacency segment . . . . . . . . 7
5.2.1. Protecting [Adjacency, Adjacency] segment lists . . . . 8
5.2.2. Protecting [Adjacency, Node] segment lists . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Segment Routing aims at supporting services with tight SLA guarantees
[1]. This document provides local repair mechanisms using SR,
capable of restoring end-to-end connectivity in the case of a sudden
failure of a link or a node, with guaranteed coverage properties.
Using segment routing, there is no need to establish TLDP sessions
with remote nodes in order to take advantage of the applicability of
remote LFAs (RLFA) or remote LFAs with directed forwarding (DLFA)
[2]. As a result, preferring LFAs over RLFAs or DLFAs, as well as
minimizing the number of RLFA or DLFA repair nodes is not required.
Using SR, there is no need to create state in the network in order to
enforce an explicit FRR path. As a result, we can use optimized
detour paths for each specific destination and for each possible
failure in the network without creating additional forwarding state.
Building on such an easier forwarding environment, the FRR behavior
suggested in this document tailors the repair paths over the post-
convergence path from the PLR to the protected destination.
As the capacity of the post-convergence path is typically planned by
the operator to support the post-convergence routing of the traffic
for any expected failure, there is much less need for the operator to
tune the decision among which protection path to choose. The
protection path will automatically follow the natural backup path
that would be used after local convergence. This also helps to
reduce the amount of path changes and hence service transients: one
transition (pre-convergence to post-convergence) instead of two (pre-
convergence to FRR and then post-convergence).
We provide an EPC-FRR approach that achieves guaranteed coverage
against link or node failure, in any IGP network, relying on the
flexibility of SR.
L ____
S----------F--{____}--D
_|_ ___________ /
{___}--Q--{___________}
Figure 1: EPC Protection
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We use Figure 1 to illustrate the EPC-FRR 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, or
node F. 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.
In Section 2 we define the main notations used in the document. They
are in line with [2].
In Section 3, we suggest to compute the P-Space and Q-Space
properties defined in Section 2, for the specific case of nodes lying
over the post-convergence paths towards the protected destinations.
The failure of a link S-F as well as the failure of a neighbor F is
discussed.
Using the properties defined in Section 3, we describe how to compute
protection lists that encode a loopfree post-convergence towards the
destination, in Section 4.
Finally, we define the segment operations to be applied by the PLR to
ensure consistency with the forwarding state of the repair node, in
Section 5.
2. 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.
Dist_old(A,B) is the distance from node A to node B in SPT_old(A).
Dist_new(A,B, X) is the distance from node A to node B in SPT_new(A,
X).
The P-Space P(R,X) of a node R w.r.t. a resource X (e.g. a link S-F,
or a node F) 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
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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.
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 (or F).
A symmetric network is a network such that the IGP metric of each
link is the same in both directions of the link.
3. Intersecting P-Space and Q-Space with post-convergence paths
In this section, we suggest to determine the P-Space and Q-Space
properties of the nodes along on the post-convergence paths 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 4 to compute the EPC-FRR repair list.
3.1. 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).
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.
3.2. Q-Space property computation for a link S-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. 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).
The post-convergence path to D requires to compute SPT_new(S, S-F).
A node N is in Q(D,S-F) if it is not downstream of S-F in
rSPT_old(D).
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3.3. 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).
The post-convergence path to D requires to compute SPT_new(S, F).
A node N is in Q(D,F) if it is not downstream of F in rSPT_old(D).
4. EPC Repair Tunnel
The EPC repair tunnel consists of an outgoing interface and a list of
segments (repair list) to insert on the SR header. The repair list
encodes the explicit post-convergence path to the destination, which
avoids the protected resource X.
The EPC repair tunnel 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 EPC repair list is expressed generally as
(Node_SID(P), EP(P, Q)).
Most often, the EPC repair list has a simpler form, as described in
the following sections.
4.1. The repair node is a direct neighbor
When the repair node is a direct neighbor, the outgoing interface is
set to that neighbor and the repair segment list is empty.
This is comparable to an LFA FRR repair.
4.2. The repair node is a PQ node
When the repair node is in P(S,X), the repair list is made of a
single node segment to the repair node.
This is comparable to an RLFA repair tunnel.
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4.3. The repair is a Q node, neighbor of the last P node
When the repair node is adjacent to P(S,X), the repair list is made
of two segments: A node segment to the adjacent P node, and an
adjacency segment from that node to the repair node.
This is comparable to a DLFA repair tunnel.
4.4. 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 loopfree
path from P to Q.
5. Protecting segments
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.
The behavior depends on the type of active segment to be protected.
5.1. The active segment is a node segment
The active segment is kept on the SR header, unchanged (1). The
repair list is inserted at the head of the list. The active segment
becomes the first segment of the inserted repair list.
A future version of the document will describe the FRR behavior when
the active segment is a node segment destined to F, and F has failed.
Note (1): If the SRGB at the repair node is different from the SRGB
at the PLR, then the active segment must be updated to fit the SRGB
of the repair node.
5.2. 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. 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.
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5.2.1. 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 applies a "NEXT" operation on Adj(S-F) and then two
consecutive "PUSH" operations: first it pushes a node segment for F,
and then it pushes a protection list allowing to reach F while
bypassing S-F.
Upon failure of S-F, a packet reaching S with a segment list matching
[adj(S-F),adj(M),...] will thus leave S with a segment list matching
[RT(F),node(F),adj(M)], where RT(F) is the repair tunnel for
destination F.
5.2.2. Protecting [Adjacency, Node] segment lists
If the next segment in the stack is a node segment, say for node T,
the packet segment list matches [adj(S-F),node(T),...].
A first solution would consist in steering the packet back to F while
avoiding S-F, similarly to the previous case. To do so, S applies a
"NEXT" operation on Adj(S-F) and then two consecutive "PUSH"
operations: first it pushes a node segment for F, and then it pushes
a repair list allowing to reach F 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)].
Another solution is to not steer the packet back via F but rather
follow the new shortest path to T. In this case, S just needs to
apply a "NEXT" operation on the Adjacency segment related to S-F, and
push 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(L), node(T), ...], would leave S with a segment list matching
[RT(Q),node(T), ...].
6. References
[1] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti,
S., Henderickx, W., Tantsura, J., and E. Crabbe, "Segment
Routing Architecture", draft-filsfils-spring-segment-routing-01
(work in progress), May 2014.
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[2] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714,
January 2010.
[3] Filsfils, C., Francois, P., Shand, M., Decraene, B., Uttaro, J.,
Leymann, N., and M. Horneffer, "Loop-Free Alternate (LFA)
Applicability in Service Provider (SP) Networks", RFC 6571,
June 2012.
[4] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S. Ning,
"Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-02 (work in
progress), May 2013.
[5] Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP Fast
Reroute using tunnels", draft-bryant-ipfrr-tunnels-03 (work in
progress), November 2007.
Authors' Addresses
Pierre Francois
Institute IMDEA Networks
Leganes
ES
Email: pierre.francois@imdea.org
Clarence Filsfils
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
Ahmed Bashandy
Cisco Systems, Inc.
San Jose
US
Email: bashandy@cisco.com
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Bruno Decraene
Orange
Issy-les-Moulineaux
FR
Email: bruno.decraene@orange.com
Stephane Litkowski
Orange
FR
Email: bruno.decraene@orange.com
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