Internet DRAFT - draft-ietf-spring-resiliency-use-cases
draft-ietf-spring-resiliency-use-cases
Network Working Group C. Filsfils, Ed.
Internet-Draft S. Previdi, Ed.
Intended status: Informational Cisco Systems, Inc.
Expires: June 22, 2018 B. Decraene
Orange
R. Shakir
Google
December 19, 2017
Resiliency use cases in SPRING networks
draft-ietf-spring-resiliency-use-cases-12
Abstract
This document identifies and describes the requirements for a set of
use cases related to network resiliency on Segment Routing (SPRING)
networks.
Requirements Language
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
this document are used to define requirements for protocol and
architecture design.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on June 22, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Path Protection . . . . . . . . . . . . . . . . . . . . . . . 4
3. Management-free Local Protection . . . . . . . . . . . . . . 5
3.1. Management-free Bypass Protection . . . . . . . . . . . . 6
3.2. Management-free Shortest Path Based Protection . . . . . 6
4. Managed Local Protection . . . . . . . . . . . . . . . . . . 7
4.1. Managed Bypass Protection . . . . . . . . . . . . . . . . 7
4.2. Managed Shortest Path Protection . . . . . . . . . . . . 8
5. Loop Avoidance . . . . . . . . . . . . . . . . . . . . . . . 8
6. Co-existence of multiple resilience techniques in the same
infrastructure . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Manageability Considerations . . . . . . . . . . . . . . . . 10
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This document reviews various use cases for the protection of
services in a SPRING network. The terminology used hereafter is in
line with [RFC5286] and [RFC5714].
The resiliency use cases described in this document can be applied
not only to traffic that is forwarded according to the SPRING
architecture but also to traffic that originally is forwarded using
other paradigms such as LDP signalling or pure IP traffic (IP routed
traffic).
Three key alternatives are described: path protection, local
protection without operator management and local protection with
operator management.
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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
([RFC5286], [RFC5714]).
In Section 3 we discuss two different approaches providing unmanaged
local protection, namely link/node bypass protection and shortest
path based protection.
Section 4 illustrates a case allowing the operator to manage the
local protection behavior in order to accommodate specific policies.
In Section 5 we discuss the opportunity for the SPRING architecture
to provide loop-avoidance mechanisms, such that transient forwarding
state inconsistencies during routing convergence do not lead into
traffic loss.
The purpose of this document is to illustrate the different use cases
and explain how an operator could combine them in the same network
(see Section 6). 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.
Following link metrics are applied:
Link metrics are bidirectional. In other words, the same metric
value is configured at both side of each link.
Links from/to A and Z are configured with a metric of 100.
CH, GD, DI and HE links are configured with a metric of 6.
All other links are configured with a metric of 5.
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2. Path Protection
As a reminder, one of the major network operator requirements is path
disjointness capability. Network operators have deployed
infrastructures with topologies that allow paths to be computed in a
complete disjoint fashion where two paths wouldn't share any
component (link or router) hence allowing an optimal protection
strategy.
A first protection strategy consists of excluding any local repair
but instead uses end-to-end path protection where each SPRING path is
protected by a second disjoint SPRING path. In this case, the local
protection is not used along the path.
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 (primary) and T2 (backup) from A to Z.
The two paths may be used:
o concurrently, where the ingress router sends the same traffic over
the primary and secondary path. This is usually known as 1+1
protection.
o concurrently, where the ingress router splits the traffic over the
primary and secondary path. This is usually known as equal cost
multi path (ECMP) or unequal cost multi path (UCMP).
o as a primary and backup path, where the secondary path is used
only when the primary failed. This is usually known as 1:1
protection.
T1 is established over path {AB, BC, CD, DE, EZ} as the primary path
and T2 is established over path {AF, FG, GH, HI, IZ} as the backup
path. The two paths MUST be disjoint in their links, nodes and
shared risk link groups (SRLGs) to satisfy the requirement of
disjointness.
In the case of primary/backup paths, when the primary path T1 is up,
the packets of the PW are sent on T1. When T1 fails, the packets of
the PW are sent on backup path 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. Typically, the
switchover from path T1 to path T2 is done in a fast reroute fashion
(e.g.: sub-50 milliseconds range) but depending on the service that
needs to be delivered, other restoration times may be used.
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It is essential that any path, primary or backup, benefit from an
end-to-end liveness monitoring/verification. The method and
mechanisms that provide such liveness check are outside the scope of
this document. An example is given by [RFC5880].
There are multiple options for liveness check, e.g., path liveness
where the path is monitored at the network level (either by the head-
end node or by a network controller/monitoring system). Another
possible approach consists of a service-based path monitored by the
service instance (verifying reachability of the endpoint). All these
options are given here as examples. While this document does express
the requirement for a liveness mechanism, it does not mandate, nor
define, any specific one.
From a SPRING viewpoint, we would like to highlight the following
requirements:
o SPRING architecture MUST provide a way to compute paths that are
not protected by local repair techniques (as illustrated in the
example of paths T1 and T2).
o SPRING architecture MUST provide a way to instantiate pairs of
disjoint paths on a topology based on a protection strategy (link,
node or SRLG protection) and allow the validation or re-
computation of these paths upon network events.
o The SPRING architecture MUST provide end-to-end liveness check of
SPRING based paths.
3. Management-free Local Protection
This section describes two alternatives providing local protection
without requiring operator management, namely bypass protection and
shortest-path based protection.
For example, a traffic 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-50 milliseconds. When the
primary path comes back up, the operator either allows for an
automated reversion of the traffic onto it or selects an operator-
driven reversion.
The backup path computation SHOULD support the following
requirements:
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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.
3.1. Management-free Bypass Protection
One way to provide local repair is to enforce a fail-over along the
shortest path around the failed component.
In case of link protection, the point of local repair will create a
repair path avoiding the protected link and merging back to primary
path at the nexthop.
In case of node protection, the repair path will avoid the protected
node and merge back to primary path at the next-nexthop.
In case of SRLG protection, the repair path will avoid members of the
same group and merge back to primary path just after.
In our example, C protects destination Z against a failure of CD link
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 CD,
is depicted in Figure 2.
B * * *C------D * * *E
*| | * / * \ / |*
* | | */ * \/ | *
A | | /* * /\ | Z
\ | | / * * / \ | /
\| |/ **/ \|/
F------G------H------I
Figure 2: Bypass protection around link CD
When the primary path comes back up, the operator either allows for
an automated reversion of the traffic onto the primary path or
selects an operator-driven reversion.
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
the shortest path to the destination.
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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 CD, is depicted
in Figure 3.
B * * *C------D------E
*| | * / | \ / |\
* | | */ | \/ | \
A | | /* | /\ | Z
\ | | / * | / \ | *
\| |/ *|/ \|*
F------G------H * * *I
Figure 3: Shortest path protection around link CD
When the primary path comes back up, the operator either allows for
an automated reversion of the traffic onto the primary path or
selects an operator-driven reversion.
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 not want to have CD and CH used to protect each other
due the BW availability in each link and that could not suffice to
absorb the other link traffic.
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. Upon local detection of the failure, the traffic is repaired
over the backup path in sub-50 milliseconds. When primary path comes
back up, the operator either allows for an automated reversion of the
traffic onto it or selects an operator-driven reversion.
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 Z, 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.
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B * * *C------D * * *E
*| * \ / * \ / |*
* | * \/ * \/ | *
A | * /\ * /\ | Z
\ | * / \ * / \ | /
\| */ \*/ \|/
F------G * * *H------I
Figure 4: Managed Bypass Protection
The computation of the repair path SHOULD be possible in an automated
fashion as well as statically expressed in the point of local repair.
4.2. Managed Shortest Path Protection
In the case of shortest path protection, the operator does not want
to use the shortest failover via link CH, but rather reach H via {CG,
GH}, for example, due to delay, BW, SRLG or other constraint.
The resulting end-to-end path upon activation of the protection is
illustrated in Figure 5.
B * * *C------D------E
*| * \ / | \ / |\
* | * \/ | \/ | \
A | * /\ | /\ | Z
\ | * / \ | / \ | *
\| */ \|/ \|*
F------G * * *H * * *I
Figure 5: Managed Shortest Path Protection
The computation of the repair path SHOULD be possible in an automated
fashion as well as statically expressed in the point of local repair.
The computation of the repair path based on a specific constraint
SHOULD be possible on a per-destination prefix base.
5. Loop Avoidance
It is part of routing protocols behavior to have what are called
"transient routing inconsistencies". This is due to the routing
convergence that happens in each node at different times and during a
different lapse of time.
These inconsistencies may cause routing loops that last the time that
it takes for the node impacted by a network event to converge. These
loops are called "microloops".
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Usually, in a normal routing protocol operations, microloops do not
last long and in general they are only noticed during the time it
takes the network to converge. However, with the emerging of fast-
convergence and fast-reroute technologies, microloops can be an issue
in networks where sub-50 millisecond convergence/reroute is required.
Therefore, the microloop problem needs to be addressed.
Networks may be affected by microloops during convergence depending
of their topologies. Detecting microloops can be done during
topology computation (e.g., SPF computation) and therefore
microloops-avoidance techniques may be applied. An example of such
technique is to compute microloop-free path that would be used during
network convergence.
The SPRING architecture SHOULD provide solutions to prevent the
occurrence of microloops during convergence following a change in the
network state. Traditionally, the lack of packet steering capability
made it difficult to apply efficient solutions to microloops. A
SPRING enabled router could take advantage of the increased packet
steering capabilities offered by SPRING in order to steer packets in
a way that packets do not enter such loops.
6. Co-existence of multiple resilience techniques in the same
infrastructure
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, CD, E}
o Flow F2 is supported over path {C, CD, I}
o Flow F3 is supported over path {C, CD, Z}
o Flow F4 is supported over path {C, CD, Z}
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 {CG, GH, Z} for F3,
and management free bypass protection for F4.
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7. Security Considerations
This document describes requirements for the SPRING architecture to
provide resiliency in SPRING networks. As such it does not introduce
any new security considerations beyond that is discussed in
[RFC7855].
8. IANA Considerations
This document does not request any IANA allocations.
9. Manageability Considerations
This document provides use cases. Solutions aimed at supporting
these use cases should provide the necessary mechanisms in order to
allow for manageability as described in [RFC7855].
Manageability concerns the computation, installation and
troubleshooting of the repair path. Also, necessary mechanisms
SHOULD be provided in order for the operator to control when a repair
path is computed, how it has been computed and if it's installed and
used.
10. Contributors
Pierre Francois contributed to the writing of the first version of
this document.
11. Acknowledgements
Authors would like to thank Stephane Litkowski and Alexander
Vainshtein for the comments and review of this document.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.
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12.2. Informative References
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
Authors' Addresses
Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
Stefano Previdi (editor)
Cisco Systems, Inc.
Via Del Serafico, 200
Rome 00142
Italy
Email: stefano@previdi.net
Bruno Decraene
Orange
FR
Email: bruno.decraene@orange.com
Rob Shakir
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
Email: robjs@google.com
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