Network Working Group Pierre Francois
Internet Draft Individual Contributor
Intended status: Informational A. Bashandy
Expires: May 2017 C. Filsfils
Cisco Systems
November 16, 2016
Topology Independent Fast Reroute using Segment Routing
draft-francois-rtgwg-segment-routing-ti-lfa-02
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 post-convergence paths from the
point of local repair, dramatically reducing the operational need to
control the tie-breaks among various FRR options.
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Table of Contents
1. Introduction...................................................3
1.1. Conventions used in this document.........................5
2. Terminology....................................................5
3. Intersecting P-Space and Q-Space with post-convergence paths...6
3.1. P-Space property computation for a resource X.............6
3.2. Q-Space property computation for a link S-F, over post-
convergence paths..............................................6
3.3. Q-Space property computation for a set of links adjacent to
S, over post-convergence paths.................................6
3.4. Q-Space property computation for a node F, over post-
convergence paths..............................................6
4. TI-LFA Repair Tunnel...........................................7
4.1. The repair node is a direct neighbor......................7
4.2. The repair node is a PQ node..............................7
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............................................8
5.1. The active segment is a node segment......................8
5.2. The active segment is an adjacency segment................8
5.2.1. Protecting [Adjacency, Adjacency] segment lists......8
5.2.2. Protecting [Adjacency, Node] segment lists...........9
5.3. Protecting SR policy midpoints against node failure.......9
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5.3.1. Protecting {F, T, D} or {S->F, T, D}.................9
5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D}..........10
6. Security Considerations.......................................11
7. IANA Considerations...........................................11
8. Conclusions...................................................11
9. References....................................................11
9.1. Normative References.....................................11
9.2. Informative References...................................11
10. Acknowledgments..............................................11
1. Introduction
Segment Routing aims at supporting services with tight SLA
guarantees [1]. This document provides a local repair mechanism
relying on SR-capable of restoring end-to-end connectivity in the
case of a sudden failure of a network component.
For each destination in the network, TI-LFA prepares a data-plane
switch-over to be activated upon detection of the failure of a
link used to reach the destination. TI-LFA provides protection
against link failure, node failure, and local SRLG failures. 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 local 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).
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. This allows for a protection path selection approach
meeting operational needs rather than a topologically constrained
one.
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
type of failure without creating additional forwarding state.
Also, the mode of protection (link, node, SRLG) is not constrained
to be network wide or node wide, but can be managed on a per
interface basis.
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, given the enabled protection mode for the interface.
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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 the TI-LFA approach that achieves guaranteed coverage
against link, node, and local SRLG failure, in any IGP network,
relying on the flexibility of SR.
L ____
S----F--{____}----D
/\ | /
| | | _______ /
|__}---Q{_______}
Figure 1 TI-LFA Protection
We use Figure 1 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 adjacent links including L (local 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.
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.
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.
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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.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in RFC-2119
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to
be interpreted as carrying RFC-2119 significance.
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).
Similarly to [4], we rely on 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 local 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.
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
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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.
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 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 TI-LFA 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).
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.
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 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).
3.3. 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.
3.4. 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.
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This can be found by intersecting the post-convergence path to D,
assuming the failure of F, with Q(D, F).
4. TI-LFA Repair Tunnel
The TI-LFA 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 TI-LFA 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 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.
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 a post-convergence 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 a post-convergence RLFA repair tunnel.
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 post-convergence 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.
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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 for the packet, S-F.
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.
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.
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
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.
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.
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In Section 5.3.2, we describe the TI-LFA behavior of PLR S when
node protection is applied and the two first segments are Adjacency
Segments.
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. 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), ...]. Note that this second behavior is the one
followed for node protection, as described in Section 5.3.1.
5.3. Protecting SR policy midpoints against node failure
As planned in the previous version of this document, 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 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 link S-F (represented as "S-
>F").
5.3.1. Protecting {F, T, D} or {S->F, T, D}
We describe the protection behavior of S when
1. the active segment is a prefix SID for a neighbor F, or an
adjacency segment S->F
2. the primary interface used to forward the packet failed
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3. the segment following the active segment is a prefix SID (for
node T)
4. node protection is active for that interface.
The TILFA Node FRR behavior becomes equivalent to:
1. Pop; the segment F or S->F is removed
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
3. Identify T (as per the SRGB of F)
4. Pop the next segment and push T's segment based on the local SRGB
5. forward the packet according to T.
5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D}
We describe the protection behavior of S when
1. the active segment is a prefix SID for a neighbor F, or an
adjacency segment S->F
2. the primary interface used to forward the packet failed
3. the segment following the active segment is an adjacency SID (F-
>T)
4. node protection is active for that interface.
The TILFA Node FRR behavior becomes equivalent to:
1. Pop; the segment F or S->F is removed
2. Confirm that the next segment is an adjacency SID of F, say F->T
3. Identify T (as per the set of Adjacency Segments of F)
4. Pop the next segment and push T's segment based on the local SRGB
5. forward the packet according to T.
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6. Security Considerations
The behavior described in this document is internal functionality
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 such no additional security
risk is introduced by using the mechanisms proposed in this
document.
7. IANA Considerations
No requirements for IANA
8. Conclusions
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
against the failure of a directly connected link or node. The
mechanism is able to calculate the backup path irrespective of the
topology as long as the topology is sufficiently redundant.
9. References
9.1. Normative References
9.2. Informative References
[1] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and R.
Shakir, "Segment Routing Architecture", draft-ietf-spring-
segment-routing-08 (work in progress), May 2016.
[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 N. So,
"Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", RFC
7490, DOI 10.17487/RFC7490, April 2015, <http://www.rfc-
editor.org/info/rfc7490>.
10. Acknowledgments
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This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Pierre Francois
pfrpfr@gmail.com
Ahmed Bashandy
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134, USA
Email: bashandy@cisco.com
Clarence Filsfils
Cisco Systems
Brussels, Belgium
Email: cfilsfil@cisco.com
Prodosh Mohapatra
Sproute Networks
Email: mpradosh@yahoo.com
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