< draft-francois-rtgwg-segment-routing-ti-lfa-00.txt   draft-francois-rtgwg-segment-routing-ti-lfa-01.txt >
Network Working Group Pierre Francois Network Working Group Pierre Francois
Internet-Draft Clarence Filsfils Internet-Draft Clarence Filsfils
Intended status: Standards Track Ahmed Bashandy Intended status: Standards Track Ahmed Bashandy
Expires: February 18, 2016 Cisco Systems, Inc. Expires: November 18, 2016 Cisco Systems, Inc.
Bruno Decraene Bruno Decraene
Stephane Litkowski Stephane Litkowski
Orange Orange
Aug 17, 2015 May 17, 2016
Topology Independent Fast Reroute using Segment Routing Topology Independent Fast Reroute using Segment Routing
draft-francois-rtgwg-segment-routing-ti-lfa-00 draft-francois-rtgwg-segment-routing-ti-lfa-01
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 link and node protection of Re-route (TI-LFA), aimed at providing protection of node and
node and adjacency segments within the Segment Routing (SR) adjacency segments within the Segment Routing (SR) framework. This
framework. This Fast Re-route (FRR) behavior builds on proven IP-FRR Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being
concepts being LFAs, remote LFAs (RLFA), and remote LFAs with LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding
directed forwarding (DLFA). It extends these concepts to provide (DLFA). It extends these concepts to provide guaranteed coverage in
guaranteed coverage in any IGP network. We accommodate the FRR any IGP network. A key aspect of TI-LFA is the FRR path selection
discovery and selection approaches in order to establish protection approach establising protection over post-convergence paths from the
over post-convergence paths from the point of local repair, point of local repair, dramatically reducing the operational need to
dramatically reducing the operational need to control the tie-breaks control the tie-breaks among various FRR options.
among various FRR options.
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Intersecting P-Space and Q-Space with post-convergence 3. Intersecting P-Space and Q-Space with post-convergence
paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. P-Space property computation for a resource X . . . . . . . 5 3.1. P-Space property computation for a resource X . . . . . . 5
3.2. Q-Space property computation for a link S-F, over 3.2. Q-Space property computation for a link S-F, over
post-convergence paths . . . . . . . . . . . . . . . . . . 5 post-convergence paths . . . . . . . . . . . . . . . . . . 6
3.3. Q-Space property computation for a node F, over 3.3. Q-Space property computation for a set of links
post-convergence paths . . . . . . . . . . . . . . . . . . 6 adjacent to S, over post-convergence paths . . . . . . . . 6
4. TI-LFA Repair Tunnel . . . . . . . . . . . . . . . . . . . . . 6 3.4. Q-Space property computation for a node F, over
4.1. The repair node is a direct neighbor . . . . . . . . . . . 6 post-convergence paths . . . . . . . . . . . . . . . . . . 6
4.2. The repair node is a PQ node . . . . . . . . . . . . . . . 6 4. TI-LFA Repair Tunnel . . . . . . . . . . . . . . . . . . . . . 6
4.3. The repair is a Q node, neighbor of the last P node . . . . 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 4.4. Connecting distant P and Q nodes along
post-convergence paths . . . . . . . . . . . . . . . . . . 7 post-convergence paths . . . . . . . . . . . . . . . . . . 7
5. Protecting segments . . . . . . . . . . . . . . . . . . . . . . 7 5. Protecting segments . . . . . . . . . . . . . . . . . . . . . 7
5.1. The active segment is a node segment . . . . . . . . . . . 7 5.1. The active segment is a node segment . . . . . . . . . . . 7
5.2. The active segment is an adjacency segment . . . . . . . . 7 5.2. The active segment is an adjacency segment . . . . . . . . 8
5.2.1. Protecting [Adjacency, Adjacency] segment lists . . . . 8 5.2.1. Protecting [Adjacency, Adjacency] segment lists . . . 8
5.2.2. Protecting [Adjacency, Node] segment lists . . . . . . 8 5.2.2. Protecting [Adjacency, Node] segment lists . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.3. Protecting SR policy midpoints against node failure . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 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} . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction 1. Introduction
Segment Routing aims at supporting services with tight SLA guarantees Segment Routing aims at supporting services with tight SLA guarantees
[1]. This document provides local repair mechanisms using SR, [1]. This document provides a local repair mechanism relying on SR,
capable of restoring end-to-end connectivity in the case of a sudden capable of restoring end-to-end connectivity in the case of a sudden
failure of a link or a node, with guaranteed coverage properties. 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 neighbour 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 Using segment routing, there is no need to establish TLDP sessions
with remote nodes in order to take advantage of the applicability of with remote nodes in order to take advantage of the applicability of
remote LFAs (RLFA) or remote LFAs with directed forwarding (DLFA) remote LFAs (RLFA) or remote LFAs with directed forwarding (DLFA)
[2]. As a result, preferring LFAs over RLFAs or DLFAs, as well as [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. 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 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 enforce an explicit FRR path. As a result, we can use optimized
detour paths for each specific destination and for each possible detour paths for each specific destination and for each type of
failure in the network without creating additional forwarding state. 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 Building on such an easier forwarding environment, the FRR behavior
suggested in this document tailors the repair paths over the post- suggested in this document tailors the repair paths over the post-
convergence path from the PLR to the protected destination. convergence path from the PLR to the protected destination, given the
enabled protection mode for the interface.
As the capacity of the post-convergence path is typically planned by As the capacity of the post-convergence path is typically planned by
the operator to support the post-convergence routing of the traffic the operator to support the post-convergence routing of the traffic
for any expected failure, there is much less need for the operator to for any expected failure, there is much less need for the operator to
tune the decision among which protection path to choose. The tune the decision among which protection path to choose. The
protection path will automatically follow the natural backup path protection path will automatically follow the natural backup path
that would be used after local convergence. This also helps to that would be used after local convergence. This also helps to
reduce the amount of path changes and hence service transients: one reduce the amount of path changes and hence service transients: one
transition (pre-convergence to post-convergence) instead of two (pre- transition (pre-convergence to post-convergence) instead of two (pre-
convergence to FRR and then post-convergence). convergence to FRR and then post-convergence).
We provide the TI-LFA approach that achieves guaranteed coverage We provide the TI-LFA approach that achieves guaranteed coverage
against link or node failure, in any IGP network, relying on the against link, node, and local SRLG failure, in any IGP network,
flexibility of SR. relying on the flexibility of SR.
L ____ L ____
S----------F--{____}--D S----F--{____}----D
_|_ ___________ / /\ | /
{___}--Q--{___________} | | | _______ /
|__}---Q{_______}
Figure 1: TI-LFA Protection Figure 1: TI-LFA Protection
We use Figure 1 to illustrate the TI-LFA approach. 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 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, or destination D affected by the failure of the protected link L, a set
node F. The PLR also needs to find a way to reach Q without being of adjacent links including L (local SRLG), or the node F itself.
affected by the convergence state of the nodes over the paths it The PLR also needs to find a way to reach Q without being affected by
wants to use to reach Q. 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 In Section 2 we define the main notations used in the document. They
are in line with [2]. are in line with [2].
In Section 3, we suggest to compute the P-Space and Q-Space 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 properties defined in Section 2, for the specific case of nodes lying
over the post-convergence paths towards the protected destinations. 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 Using the properties defined in Section 3, we describe how to compute
protection lists that encode a loopfree post-convergence towards the protection lists that encode a loopfree post-convergence towards the
destination, in Section 4. destination, in Section 4.
Finally, we define the segment operations to be applied by the PLR to Finally, we define the segment operations to be applied by the PLR to
ensure consistency with the forwarding state of the repair node, in ensure consistency with the forwarding state of the repair node, in
Section 5. Section 5.
2. Terminology 2. Terminology
skipping to change at page 5, line 6 skipping to change at page 5, line 23
Dist_old(A,B) is the distance from node A to node B in SPT_old(A). 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, Dist_new(A,B, X) is the distance from node A to node B in SPT_new(A,
X). X).
Similarly to [4], we rely on the concept of P-Space and Q-Space for Similarly to [4], we rely on 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,
or a node F) is the set of nodes that are reachable from R without a node F, or a local SRLG) is the set of nodes that are reachable
passing through X. It is the set of nodes that are not downstream of from R without passing through X. It is the set of nodes that are not
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 (or F). 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 3. Intersecting P-Space and Q-Space with post-convergence paths
In this section, we suggest to determine the P-Space and Q-Space 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 properties of the nodes along the post-convergence paths from the PLR
PLR to the protected destination and compute an SR-based explicit to the protected destination and compute an SR-based explicit path
path from P to Q when they are not adjacent. Such properties will be from P to Q when they are not adjacent. Such properties will be used
used in Section 4 to compute the TI-LFA repair list. in Section 4 to compute the TI-LFA repair list.
3.1. P-Space property computation for a resource X 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(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), 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. for at least one neighbor N of R.
3.2. Q-Space property computation for a link S-F, over post-convergence 3.2. Q-Space property computation for a link S-F, over post-convergence
paths paths
We want to determine which nodes on the post-convergence from the PLR We want to determine which nodes on the post-convergence path from
to the destination D are in the Q-Space of destination D w.r.t. link the PLR to the destination D are in the Q-Space of destination D
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).
The post-convergence path to D requires to compute SPT_new(S, S-F). 3.3. Q-Space property computation for a set of links adjacent to S,
over post-convergence paths
A node N is in Q(D,S-F) if it is not downstream of S-F in We want to determine which nodes on the post-convergence path from
rSPT_old(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
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.
3.3. Q-Space property computation for a node F, over post-convergence 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 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).
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. TI-LFA Repair Tunnel 4. TI-LFA Repair Tunnel
The TI-LFA repair tunnel consists of an outgoing interface and a list 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 of segments (repair list) to insert on the SR header. The repair
list encodes the explicit post-convergence path to the destination, list encodes the explicit post-convergence path to the destination,
which avoids the protected resource X. which avoids the protected resource X.
The TI-LFA repair tunnel is found by intersecting P(S,X) and Q(D,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- with the post-convergence path to D and computing the explicit SR-
skipping to change at page 6, line 40 skipping to change at page 7, line 14
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. in the following sections.
4.1. The repair node is a direct neighbor 4.1. The repair node is a direct neighbor
When the repair node is a direct neighbor, the outgoing interface is When the repair node is a direct neighbor, the outgoing interface is
set to that neighbor and the repair segment list is empty. set to that neighbor and the repair segment list is empty.
This is comparable to an LFA FRR repair. This is comparable to a post-convergence LFA FRR repair.
4.2. The repair node is a PQ node 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 When the repair node is in P(S,X), the repair list is made of a
single node segment to the repair node. single node segment to the repair node.
This is comparable to an RLFA repair tunnel. This is comparable to a post-convergence RLFA repair tunnel.
4.3. The repair is a Q node, neighbor of the last P node 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 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 of two segments: A node segment to the adjacent P node, and an
adjacency segment from that node to the repair node. adjacency segment from that node to the repair node.
This is comparable to a 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 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- 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 loopfree computations to compute a list of segments that represent a loopfree
path from P to Q. path from P to Q.
5. Protecting segments 5. Protecting segments
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. interface for the packet, S-F.
The behavior depends on the type of active segment to be protected. The behavior depends on the type of active segment to be protected.
5.1. The active segment is a node segment 5.1. The active segment is a node segment
The active segment is kept on the SR header, unchanged (1). The 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 repair list is inserted at the head of the list. The active segment
becomes the first segment of the inserted repair list. 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 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 at the PLR, then the active segment must be updated to fit the SRGB
of the repair node. of the repair node.
In section 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 neighbour F itself.
5.2. The active segment is an adjacency segment 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. We distinguish the case where this active segment is was enabled. We distinguish the case where this active segment is
followed by another adjacency segment from the case where it is followed by another adjacency segment from the case where it is
followed by a node segment. followed by a node segment.
5.2.1. Protecting [Adjacency, Adjacency] segment lists 5.2.1. Protecting [Adjacency, Adjacency] segment lists
skipping to change at page 8, line 20 skipping to change at page 8, line 36
To do so, S applies a "NEXT" operation on Adj(S-F) and then two 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, consecutive "PUSH" operations: first it pushes a node segment for F,
and then it pushes a protection list allowing to reach F while and then it pushes a protection list allowing to reach F while
bypassing S-F. bypassing S-F.
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(M),...] will thus leave 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 [RT(F),node(F),adj(M)], where RT(F) is the repair tunnel for
destination F. destination F.
In section 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 5.2.2. 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 packet segment list matches [adj(S-F),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 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 avoiding S-F. To do so, S applies a "NEXT" operation on Adj(S-F) and
"NEXT" operation on Adj(S-F) and then two consecutive "PUSH" then two consecutive "PUSH" operations: first it pushes a node
operations: first it pushes a node segment for F, and then it pushes segment for F, and then it pushes a repair list allowing to reach F
a repair list allowing to reach F while bypassing S-F. while bypassing S-F.
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),node(T),...] will thus leave 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)]. [RT(F),node(F),node(T)].
Another solution is to not steer the packet back via F but rather 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 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 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 push a repair list redirecting the traffic to a node Q, whose path to
node segment T is not affected by the failure. 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(L), node(T), ...], would leave S with a segment list matching [adj(L), node(T), ...], would leave S with a segment list matching
[RT(Q),node(T), ...]. [RT(Q),node(T), ...]. Note that this second behaviour is the one
followed for node protection, as described in section 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
behaviour 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 behaviour of S when
1. the active segment is a prefix SID for a neighbour 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 a prefix SID (for
node T)
4. node protetion 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 behaviour of S when
1. the active segment is a prefix SID for a neighbour 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 protetion 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 Adjancey 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.
6. References 6. References
[1] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and R. [1] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and R.
Shakir, "Segment Routing Architecture", Shakir, "Segment Routing Architecture",
draft-ietf-spring-segment-routing-04 (work in progress), draft-ietf-spring-segment-routing-08 (work in progress),
July 2015. May 2016.
[2] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, [2] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714,
January 2010. January 2010.
[3] Filsfils, C., Francois, P., Shand, M., Decraene, B., Uttaro, J., [3] Filsfils, C., Francois, P., Shand, M., Decraene, B., Uttaro, J.,
Leymann, N., and M. Horneffer, "Loop-Free Alternate (LFA) Leymann, N., and M. Horneffer, "Loop-Free Alternate (LFA)
Applicability in Service Provider (SP) Networks", RFC 6571, Applicability in Service Provider (SP) Networks", RFC 6571,
June 2012. June 2012.
[4] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. So, [4] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. So,
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