Internet DRAFT - draft-francois-spring-resiliency-use-case
draft-francois-spring-resiliency-use-case
Network Working Group Pierre Francois
Internet-Draft IMDEA Networks
Intended status: Informational Clarence Filsfils
Expires: October 10, 2014 Cisco Systems, Inc.
Bruno Decraene
Orange
Rob Shakir
BT
April 8, 2014
Use-cases for Resiliency in SPRING
draft-francois-spring-resiliency-use-case-02
Abstract
This document describes the use cases for resiliency in SPRING
networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Path protection . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Management free local protection . . . . . . . . . . . . . . . 4
3.1. Management free bypass protection . . . . . . . . . . . . . 5
3.2. Management-free shortest path based protection . . . . . . 5
4. Managed local protection . . . . . . . . . . . . . . . . . . . 6
4.1. Managed bypass protection . . . . . . . . . . . . . . . . . 6
4.2. Managed shortest path protection . . . . . . . . . . . . . 6
5. Co-existence . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
SPRING aims at providing a network architecture supporting services
with tight SLA guarantees [1]. This document reviews various use
cases for the protection of services in a SPRING network. Note that
these use cases are in particular applicable to existing LDP based
and pure IP networks.
Three key alternatives are described: path protection, local
protection without operator management and local protection with
operator management.
Path protection lets the ingress node be in charge of the failure
recovery, as discussed in Section 2.
The rest of the document focuses on approaches where protection is
performed by the node adjacent to the failed component, commonly
referred to as local protection techniques or Fast Reroute
techniques.
We discuss two different approaches to provide unmanaged local
protection, namely link/node bypass protection and shortest path
based protection, in Section 3.
A case is then made to allow the operator to manage the local
protection behavior in order to accommodate specific policies, in
Section 4.
The purpose of this document is to illustrate the different
approaches and explain how an operator could combine them in the same
network (see Section 5). Solutions are not defined in this document.
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. All
link metrics are equal to 1, with the exception of the links from/to
A and Z, which are configured with a metric of 100.
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2. Path protection
A first protection strategy consists in excluding any local repair
but instead use end-to-end path protection.
For example, a Pseudo Wire (PW) from A to Z can be "path protected"
in the direction A to Z in the following manner: the operator
configures two SPRING paths T1 and T2 from A to Z. The two paths are
installed in the forwarding plane of A and hence are ready to forward
packets. The two paths are made disjoint using the SPRING
architecture.
T1 is established over path {AB, BC, CD, DE, EZ} and T2 over path
{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 T1
comes back up, the operator either allows for an automated reversion
of the traffic onto T1 or selects an operator-driven reversion. The
solution to detect the end-to-end liveness of the path is out of the
scope of this document.
From a SPRING viewpoint, we would like to highlight the following
requirement: the two configured paths T1 and T2 MUST NOT benefit from
local protection.
3. Management free local protection
This section describes two alternatives to provide local protection
without requiring operator management, namely bypass protection and
shortest-path based protection.
For example, a demand from A to Z, transported over the shortest
paths provided by the SPRING architecture, benefits from management-
free local protection by having each node along the path
automatically pre-compute and pre-install a backup path for the
destination Z. Upon local detection of the failure, 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.
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3.1. Management free bypass protection
One way to provide local repair is to enforce a failover along the
shortest path around the failed component, ending at the protected
nexthop, so as to bypass the failed component and re-join the pre-
convergence path at the nexthop. In the case of node protection,
such bypass ends at the next-nexthop.
In our example, C protects Z, that it initially reaches via CD, by
enforcing the traffic over the bypass {CH, HD}. The resulting end-
to-end path between A and Z, upon recovery against the failure of
C-D, is depicted in Figure 2.
B * * *C------D * * *E
*| | * / * * / |*
* | | */ * */ | *
A | | /* * /* | Z
\ | | / * * / * | *
\| |/ **/ *|*
F------G------H------I
Figure 2: Bypass protection around link C-D
3.2. Management-free shortest path based protection
An alternative protection strategy consists in management-free local
protection, aiming at providing a repair for the destination based on
shortest path state for that destination.
In our example, C protects Z, that it initially reaches via CD, by
enforcing the traffic over its shortest path to Z, considering the
failure of the protected component. The resulting end-to-end path
between A and Z, upon recovery against the failure of C-D, is
depicted in Figure 3.
B * * *C------D------E
*| | * / | \ * |*
* | | */ | \* | *
A | | /* | *\ | Z
\ | | / * | * \ | *
\| |/ *|* \|*
F------G------H * * *I
Figure 3: Reference topology
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4. Managed local protection
There may be cases where a management free repair does not fit the
policy of the operator. For example, in our illustration, the
operator may want to not have C-D and C-H used to protect each other,
in fear of a shared risk among the two links.
In this context, 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
path.
We discuss such aspects for both bypass and shortest path based
protection schemes.
4.1. Managed bypass protection
Let us illustrate the case using our reference example. For the
demand from A to B, the operator does not want to use the shortest
failover path to the nexthop, {CH, HD}, but rather the path
{CG,GH,HD}, as illustrated in Figure 4.
B * * *C------D * * *E
*| * \ / * * / |*
* | * \/ * */ | *
A | * /\ * /* | Z
\ | * / \ * / * | *
\| */ \*/ *|*
F------G * * *H------I
Figure 4: Managed bypass protection
4.2. Managed shortest path protection
In the case of shortest path protection, the case is the one of an
operator who does not want to use the shortest failover via link C-H,
but rather reach H via {CG, GH}.
The resulting end-to-end path upon activation of the protection is
illustrated in Figure 5.
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B * * *C------D------E
*| * \ / | \ * |*
* | * \/ | \* | *
A | * /\ | *\ | Z
\ | * / \ | * \ | *
\| */ \|* \|*
F------G * * *H * * *I
Figure 5: Managed shortest path protection
5. Co-existence
The operator may want to support several very-different services on
the same packet-switching infrastructure. As a result, the SPRING
architecture SHOULD allow for the co-existence of the different use
cases listed in this document, in the same network.
Let us illustrate this with the following example.
o Flow F1 is supported over path {C, C-D, E}
o Flow F2 is supported over path {C, C-D, I)
o Flow F3 is supported over path {C, C-D, Z)
o Flow F4 is supported over path {C, C-D, Z}
o It should be possible for the operator to configure the network to
achieve path protection for F1, management free shortest path
local protection for F2, managed protection over path {C-G, G-H,
Z} for F3, and management free bypass protection for F4.
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-01
(work in progress), October 2013.
Authors' Addresses
Pierre Francois
IMDEA Networks
Leganes
ES
Email: pierre.francois@imdea.org
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Clarence Filsfils
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
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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