Network Working Group Pierre Francois
Internet-Draft IMDEA Networks
Intended status: Standards Track Clarence Filsfils
Expires: August 4, 2014 Cisco Systems, Inc.
Bruno Decraene
Orange
Rob Shakir
BT
January 31, 2014
Use-cases for Resiliency in Segment Routing
draft-francois-sr-resiliency-use-case-00
Abstract
This document describes the use cases for resiliency in SR networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Path protection . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Management-free local protection . . . . . . . . . . . . . . . 4
4. Managed local protection . . . . . . . . . . . . . . . . . . . 4
5. Co-existence . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Segment Routing (SR) aims at supporting services with tight SLA
guarantees [1]. This document reviews alternative techniques to
provide Fast Reroute (FRR) for SR traffic. Note that these
techniques can be applied to protect LSPs created with LDP as well as
pure IP traffic.
A FRR technique involves the pre-computation and dataplane pre-
installation of backup path such as the repair traffic in 50msec upon
failure detection. The term "protection" is often used as a synonym
for FRR. Such technique supposes the existing of a sub-10msec
failure detection technique.
Three key alternatives are described: path protection, local
protection without operator management and local protection with
operator management.
The purpose of this document is to illustrate the different
techniques and explain how an operator could combine them in the same
network. Solutions are not defined in this document.
PE1
/ \
/ \
B------C------D------E
/| | \ / | \ / |\
/ | | \/ | \/ | \
A | | /\ | /\ | Z
\ | | / \ | / \ | /
\| |/ \|/ \|/
F------G------H------I
Figure 1: Reference topology
We use Figure 1 as a reference topology throughout the document. We
describe the various use-cases in the next sections. All link
metrics are equal to 1, with the exception of the links of PE1 which
are configured with a metric of 100.
2. Path protection
A first protection strategy consists in excluding any local repair
but instead use end-to-end path protection.
For example, a PW from A to Z can be "path protected" in the
direction A to Z in the following manner: the operator configures two
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SR tunnels T1 and T2 from A to Z. The two tunnels are installed in
the forwarding plane of A and hence are ready to encapsulate and
forward packets. The two tunnels are installed over disjoint paths
using adjacency segments (T1 over segment list {AB, BC, CD, DE, EZ}
and T2 over segment list {AF, FG, GH, HI, IZ}). When T1 is up, the
packets of the PW are sent on T1. When T1 fails, the packets of the
PW are sent on T2. When the tunnel T1 comes back up, the operator
either allows for an automated reversion of the traffic onto T1 or
selects an operator-driven reversion. The end-to-end liveness of a
tunnel is for example checked with BFD.
From an SR viewpoint, we would like to highlight the following
requirement: the adjacency segments used to support the tunnels T1
and T2 MUST NOT benefit from local protection. This is achieved by
resetting the B-flag in the related AdjSID's as per the IGP
extensions defined in [3].
3. Management-free local protection
An alternative protection strategy consists in management-free local
protection.
For example, a PW from C to E transported by the single segment
NodeSID(E) benefits from management-free local protection by having
each node along the path (e.g. C and D) to automatically pre-compute
and pre-install backup path for the destination E. Upon local
detection of the failure (e.g. link BFD), the traffic is repaired
over the backup path in sub-50msec.
The backup path computation should support the following
requirements:
o 100% link, node, and SRLG protection in any topology
o Automated computation by the IGP
o Selection of the backup path such as to minimize the chance for
transient congestion and/or delay during the protection period, as
reflected by the IGP metric configuration in the network.
An SR solution aimed at supporting these requirements is defined in
[2].
4. Managed local protection
A final alternative protection strategy consists in managed local
protection.
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For policy reasons, the operator may want very specific backup paths
to be used.
For example, the operator may want the backup path to end at the
next-hop (or next-next-hop for node failure) hence excluding IPFRR/
LFA types of backup path. Also, the operator might want to tightly
control the backup path to the next-hop: for the destination Z upon
the failure of link CD, the backup path CGHD might be desired while
the backup paths CGD and CHD are refused.
The protection mechanism must support the explicit configuration of
the backup path either under the form of high-level constraints (end
at the next-hop, end at the next-next-hop, minimize this metric,
avoid this SRLG...) or under the form of an explicit segment list.
5. Co-existence
The operator may want to support several very-different services on
the same packet-switching infrastructure.
The SR resiliency architecture allows the co-existence of different
FRR techniques.
Let us illustrate this for adjacency segments with the following
example.
o Node C is configured with 3 adjacency segments for the connected
interface to D: AdjSID(CD1), AdjSID(CD2) and AdjSID(CD3)
o SR Flow F1: from A to E over segment list
{NodeSID(C), AdjSID(CD1), NodeSID(E)}
o SR Flow F2: from F to I over segment list
{NodeSID(C), AdjSID(CD2), NodeSID(I)}
o SR Flow F3: from G to Z over segment list
{NodeSID(C), AdjSID(CD3), NodeSID(Z)}
o Node C is configured with a distinct protection technique for each
adjacency segment. AdjSID(CD1) is configured without protection,
AdjSID(CD2) is configured to benefit from management-free local
protection and AdjSID(CD3) is configured for managed local
protection over the path {AdjSID(CH), AdjSID(HD)}
Such a co-existence is partially supported by the SR IGP extensions
[ref tbd]
o Multiple adjacency segments can be advertised for the same
adjacency
o The non-protected property of AdjSID(CD1) is signalled by a reset
B flag.
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o The protected property of AdjSID(CD2) and AdjSID(CD3) are
signalled by a set B flag.
The SR IGP extension should be extended to discreminate between
AdjSID(CD2) and AdjSID(CD3). A single flag could be defined (managed
path vs fully automated).
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-rtgwg-segment-routing-00
(work in progress), June 2013.
[2] Francois, P., Filsfils, C., Bashandy, A., Decraene, B., and S.
Litkowski, "Topology Independent Fast Reroute using Segment
Routing", November 2013.
[3] Previdi, S., Filsfils, C., and A. Bashandy, "IS-IS Segment
Routing Extensions", October 2013.
Authors' Addresses
Pierre Francois
IMDEA Networks
Leganes
ES
Email: pierre.francois@imdea.org
Clarence Filsfils
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
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Bruno Decraene
Orange
Issy-les-Moulineaux
FR
Email: bruno.decraene@orange.com
Rob Shakir
BT
London
UK
Email: rob.shakir@bt.com
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