Internet DRAFT - draft-bashandy-rtgwg-segment-routing-ti-lfa
draft-bashandy-rtgwg-segment-routing-ti-lfa
Network Working Group A. Bashandy
Internet Draft Arrcus
Intended status: Standard Track C. Filsfils
Expires: April 2019 Cisco Systems
Bruno Decraene
Stephane Litkowski
Orange
Pierre Francois
INSA Lyon
D. Voyer
Bell Canada
Francois Clad
Pablo Camarillo
Cisco Systems
October 4, 2018
Topology Independent Fast Reroute using Segment Routing
draft-bashandy-rtgwg-segment-routing-ti-lfa-05
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Bashandy Expires April 4, 2019 [Page 1]
Internet-Draft SR TI-LFA October 2018
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on April 4, 2019.
Copyright Notice
Copyright (c) 2018 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
(http://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. 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.................................7
3.4. Q-Space property computation for a node F, over post-
convergence paths..............................................7
4. TI-LFA Repair Tunnel...........................................7
4.1. The repair node is a direct neighbor......................7
Bashandy Expires April 4, 2019 [Page 2]
Internet-Draft SR TI-LFA October 2018
4.2. The repair node is a PQ node..............................8
4.3. The repair is a Q node, neighbor of the last P node.......8
4.4. Connecting distant P and Q nodes along post-convergence
paths..........................................................8
5. Protecting segments............................................8
5.1. The active segment is a node segment......................8
5.2. The active segment is an adjacency segment................9
5.2.1. Protecting [Adjacency, Adjacency] segment lists......9
5.2.2. Protecting [Adjacency, Node] segment lists...........9
5.3. Protecting SR policy midpoints against node failure......10
5.3.1. Protecting {F, T, D} or {S->F, T, D}................10
5.3.2. Protecting {F, F->T, D} or {S->F, F->T, D}..........10
6. Measurements on Real Networks.................................11
7. Security Considerations.......................................17
8. IANA Considerations...........................................17
9. Conclusions...................................................17
10. References...................................................17
10.1. Normative References....................................17
10.2. Informative References..................................17
11. Acknowledgments..............................................18
1. Introduction
Segment Routing aims at supporting services with tight SLA
guarantees [1]. By relying on segment routing 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 beyond the scope of this document. Non-local
failures are addressed in a separate document [6].
The term topology independent (Ti) refers to the ability to provide
a loop free backup path irrespective of the topologies prior the
failure and after the failure.
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 in the
event of any one of the following: single link failure, single node
failure, or single local 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 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).
Protection techniques outlined in this document are limited to
protecting links, nodes, and local SRLGs that are within a routing
Bashandy Expires April 4, 2019 [Page 3]
Internet-Draft SR TI-LFA October 2018
domain. Protecting domain exit routers and/or links attached to
another routing domains are beyond the scope of this document
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) [4][5] 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 thereby relieving the nodes from the
extra state and the operator from having to deploy an extra protocol
just to enhance FRR coverage.
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.
Using the post-convergence path in TI-LFA resolves some of
operational issues with LFA selection that are mentioned in Section
3 of [5] (e.g. using PE routers to protect against core failures, or
selecting links with low BW while links with high BW are available),
because these issues presumably have been taken care of by the
network operator as part of its original network engineering. Hence
traffic that permanently uses the PLR after the failure achieves
maximum benefits. Traffic that does not use the PLR prior to and
after the failure remains unaffected. Traffic that temporarily
continues to use the PLR after the failure benefits from the quick
switching to the backup path by minimizing traffic loss until remote
node(s) reacts.
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.
Bashandy Expires April 4, 2019 [Page 4]
Internet-Draft SR TI-LFA October 2018
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, Section 4 describes
how to compute protection lists that encode a loopfree post-
convergence path towards the destination.
Section 5 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 6 summarizes these measurements.
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 shortest distance from node A to node B in
SPT_old(A).
Dist_new(A,B, X) is the shortest distance from node A to node B in
SPT_new(A,X).
Bashandy Expires April 4, 2019 [Page 5]
Internet-Draft SR TI-LFA October 2018
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 [4], 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 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
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).
Bashandy Expires April 4, 2019 [Page 6]
Internet-Draft SR TI-LFA October 2018
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.
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 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 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. Section 6 provides statistics for the
number of SIDs in the explicit path to protect against various
failures.
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.
Bashandy Expires April 4, 2019 [Page 7]
Internet-Draft SR TI-LFA October 2018
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.
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 SR-MPLS is being used and 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.
Bashandy Expires April 4, 2019 [Page 8]
Internet-Draft SR TI-LFA October 2018
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. For details on the "NEXT" and "PUSH" operations,
refer to [7].
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.
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.
Bashandy Expires April 4, 2019 [Page 9]
Internet-Draft SR TI-LFA October 2018
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
In this section, we describe the behavior of a node S configured to
interpret the failure of link S->F as the node failure of F, in the
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}
This section describes the protection behavior of S when all of the
following conditions are true:
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 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}
This section describes the protection behavior of S when all of the
following conditions are true:
1. the active segment is a prefix SID for a neighbor F, or an
adjacency segment S->F
Bashandy Expires April 4, 2019 [Page 10]
Internet-Draft SR TI-LFA October 2018
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.
It is noteworthy to mention that node "S" in the procedures
described in Sections 5.3.1 and 5.3.2 can always determine whether
the segment after popping the top segment is an adjacency SID or a
prefix-SID of the next-hop "F" as follows:
1. In a link state environment, the node "S" knows the SRGB and the
adj-SIDs of the neighboring node "F"
2. If the new segment after popping the top segment is within the
SRGB or the adj-SIDs of "F", then node "S" is certain that the
failure of node "F" is a midpoint failure and hence node "S"
applies the procedures specified in Sections 5.3.1 or 5.3.2,
respectively.
3. Otherwise the failure is not a midpoint failure and hence the
node "S" may apply other protection techniques that are beyond
the scope of this document or simply drop the packet and wait for
normal protocol conversion.
6. 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 mechanism described in this document are used to
Bashandy Expires April 4, 2019 [Page 11]
Internet-Draft SR TI-LFA October 2018
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 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
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.
Bashandy Expires April 4, 2019 [Page 12]
Internet-Draft SR TI-LFA October 2018
+-------------+------------+------------+------------+------------+
| Network | Nodes | Circuits |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 4.1.
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 4.2.
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 4.3) separately because there was no
granularity in the result. Also we treat the node-SID+adj-SID and
node-SID + node-SID the same because they do not differ from the
data plane point of view.
Table 2A and 2B below summarize the measurements on the number of
SIDs needed for link protection
Bashandy Expires April 4, 2019 [Page 13]
Internet-Draft SR TI-LFA October 2018
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.227% | 25.256% | 0.517% | 0.001% |
+-------------+------------+------------+------------+------------+
| T2 | 81.097% | 18.738% | 0.165% | 0.0% |
+-------------+------------+------------+------------+------------+
| T3 | 95.878% | 4.067% | 0.056% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.547% | 35.666% | 1.788% | 0.0% |
+-------------+------------+------------+------------+------------+
| T5 | 85.733% | 14.267% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T6 | 81.252% | 18.714% | 0.033% | 0.0% |
+-------------+------------+------------+------------+------------+
| T7 | 98,857% | 1.143% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T8 | 94,118% | 5.882% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98.950% | 1.050% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
Table 2A: Link protection (repair size distribution)
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.227% | 99.482% | 99.999% | 100.0% |
+-------------+------------+------------+------------+------------+
| T2 | 81.097% | 99.835% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T3 | 95.878% | 99.944% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62.547% | 98.212% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T5 | 85.733% | 100.000% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T6 | 81.252% | 99.967% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T7 | 98,857% | 100.000% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T8 | 94,118% | 100.000% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98,950% | 100.000% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
Table 2B: Link protection repair size cumulative distribution
Bashandy Expires April 4, 2019 [Page 14]
Internet-Draft SR TI-LFA October 2018
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 | 74.177% | 25.306% | 0.517% | 0.001% |
+-------------+------------+------------+------------+------------+
| T2 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T3 | 93.650% | 6.301% | 0.049% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62,547% | 35.666% | 1.788% | 0.0% |
+-------------+------------+------------+------------+------------+
| T5 | 83.139% | 16.861% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T6 | No SRLG Information |
+-------------+---------------------------------------------------+
| T7 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T8 | 85.185% | 14.815% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98,940% | 1.060% | 0.0% | 0.0% |
+-------------+------------+------------+------------+------------+
Table 3A: Local SRLG protection repair size distribution
+-------------+------------+------------+------------+------------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs |
+-------------+------------+------------+------------+------------+
| T1 | 74.177% | 99.482% | 99.999% | 100.001% |
+-------------+------------+------------+------------+------------+
| T2 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T3 | 93.650% | 99.951% | 100.000% | 0.0% |
+-------------+------------+------------+------------+------------+
| T4 | 62,547% | 98.212% | 100.000% | 100.0% |
+-------------+------------+------------+------------+------------+
| T5 | 83.139% | 100.000% | 100.0% | 100.0% |
+-------------+------------+------------+------------+------------+
| T6 | No SRLG Information |
+-------------+---------------------------------------------------+
| T7 | No SRLG Information |
+-------------+------------+------------+------------+------------+
| T8 | 85.185% | 100,000% | 100.000% | 100.0% |
+-------------+------------+------------+------------+------------+
| T9 | 98,940% | 100,000% | 100.000% | 100.0% |
+-------------+------------+------------+------------+------------+
Table 3B: Local SRLG protection repair size Cumulative distribution
Bashandy Expires April 4, 2019 [Page 15]
Internet-Draft SR TI-LFA October 2018
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 | 49.771% | 47.902% | 2.156% | 0.148% | 0.023% |
+---------+----------+----------+----------+----------+----------+
| T2 | 36,528% | 59.625% | 3.628% | 0.194% | 0.025% |
+---------+----------+----------+----------+----------+----------+
| T3 | 73,287% | 25,574% | 1,128% | 0.010% | 0% |
+---------+----------+----------+----------+----------+----------+
| T4 | 36.112% | 57.350% | 6.329% | 0.199% | 0.010% |
+---------+----------+----------+----------+----------+----------+
| T5 | 73.185% | 26.815% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T6 | 78.362% | 21.320% | 0.318% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T7 | 66.106% | 32.813% | 1.082% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T8 | 59.712% | 40.288% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
| T9 | 98.950% | 1.050% | 0% | 0% | 0% |
+---------+----------+----------+----------+----------+----------+
Table 4A: Node protection (repair size distribution)
+---------+----------+----------+----------+----------+----------+
| Network | 0 SIDs | 1 SID | 2 SIDs | 3 SIDs | 4 SIDs |
+---------+----------+----------+----------+----------+----------+
| T1 | 49.771% | 97.673% | 99.829% | 99.977% | 100% |
+---------+----------+----------+----------+----------+----------+
| T2 | 36,528% | 96.153% | 99.781% | 99.975% | 100% |
+---------+----------+----------+----------+----------+----------+
| T3 | 73,287% | 98.862% | 99.990% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T4 | 36.112% | 93.461% | 99.791% | 99.990% | 100% |
+---------+----------+----------+----------+----------+----------+
| T5 | 73.185% | 100.0% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T6 | 78.362% | 99.682% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T7 | 66.106% | 98,918% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T8 | 59.712% | 100.0% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
| T9 | 98.950% | 100.0% | 100.0% |100.0% | 100% |
+---------+----------+----------+----------+----------+----------+
Table 4B: Node protection (repair size cumulative distribution)
Bashandy Expires April 4, 2019 [Page 16]
Internet-Draft SR TI-LFA October 2018
7. Security Considerations
The techniques 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 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.
8. IANA Considerations
No requirements for IANA
9. 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, node, or SRLG.
The mechanism is able to calculate the backup path irrespective of
the topology as long as the topology is sufficiently redundant.
10. References
10.1. Normative References
10.2. 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.
Bashandy Expires April 4, 2019 [Page 17]
Internet-Draft SR TI-LFA October 2018
[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>.
[5] 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>.
[6] Bashandy, A., Filsfils, C., and Litkowski, S., " Loop
avoidance using Segment Routing", draft-bashandy-rtgwg-
segment-routing-uloop-00, (work in progress), May 2017
[7] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and
Shakir, R, "Segment Routing Architecture", draft-ietf-spring-
segment-routing-11 (work in progress), February 2017
11. Acknowledgments
We would like to give Les Ginsberg special thanks for the valuable
comments and contribution
This document was prepared using 2-Word-v2.0.template.dot.
Bashandy Expires April 4, 2019 [Page 18]
Internet-Draft SR TI-LFA October 2018
Authors' Addresses
Pierre Francois
INSA Lyon
Email: pierre.francois@insa-lyon.fr
Ahmed Bashandy
Arrcus
Email: abashandy.ietf@gmail.com
Clarence Filsfils
Cisco Systems
Brussels, Belgium
Email: cfilsfil@cisco.com
Bruno Decraene
Orange
Issy-les-Moulineaux
FR
Email: bruno.decraene@orange.com
Stephane Litkowski
Orange
FR
Email: stephane.litkowski@orange.com
Daniel Voyer
Bell Canada
Canada
Email: daniel.voyer@bell.ca
Pablo Camarillo
Cisco Systems
Email: pcamaril@cisco.com
Francois Clad
Cisco Systems
Email: fclad@cisco.com
Bashandy Expires April 4, 2019 [Page 19]
