Network Working Group S. Bryant
Internet-Draft M. Shand
Intended status: Informational Cisco Systems
Expires: November 2, 2007 May 2007
Applicability of Loop-free Convergence
draft-bryant-shand-lf-applicability-03
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Copyright (C) The IETF Trust (2007).
Abstract
This draft describes the applicability of loop free convergence
technologies to a number of network applications.
Requirements Language
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 RFC2119 [RFC2119].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Component Failure . . . . . . . . . . . . . . . . . . . . . 4
2.2. Component Repair . . . . . . . . . . . . . . . . . . . . . 4
2.3. Managing withdrawal of a component . . . . . . . . . . . . 5
2.4. Managing Insertion of a Component . . . . . . . . . . . . . 5
2.5. Managing Change of a Link Cost . . . . . . . . . . . . . . 5
2.6. External Cost Change . . . . . . . . . . . . . . . . . . . 6
2.7. MPLS Applicability . . . . . . . . . . . . . . . . . . . . 6
2.8. Routing Vector and Path Vector Convergence . . . . . . . . 6
3. IANA considerations . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
5. Informative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
When there is a change to the network topology (due to the failure or
restoration of a link or router, or as a result of management action)
the routers need to converge on a common view of the new topology,
and the paths to be used for forwarding traffic to each destination.
During this process, referred to as a routing transition, packet
delivery between certain source/destination pairs may be disrupted.
This occurs due to the time it takes for the topology change to be
propagated around the network together with the time it takes each
individual router to determine and then update the forwarding
information base (FIB) for the affected destinations. During this
transition, packets may be lost due to the continuing attempts to use
the failed component, and/or due to forwarding loops. Forwarding
loops arise due to the inconsistent FIBs that occur as a result of
the difference in time taken by routers to execute the transition
process. This is a problem that occurs in both IP networks and MPLS
networks that use LDP [RFC3036] as the label switched path (LSP)
signaling protocol.
The service failures caused by routing transitions are largely hidden
by higher-level protocols that retransmit the lost data. However new
Internet services are emerging which are more sensitive to the packet
disruption that occurs during a transition. To make the transition
transparent to their users, these services require a short routing
transition. Ideally, routing transitions would be completed in zero
time with no packet loss.
Regardless of how optimally the mechanisms involved have been
designed and implemented, it is inevitable that a routing transition
will take some minimum interval that is greater than zero. This has
led to the development of a TE fast-reroute mechanism for MPLS
[RFC4090]. Alternative mechanisms that might be deployed in an MPLS
network and mechanisms that may be used in an IP network are work in
progress in the IETF [I-D.ietf-rtgwg-ipfrr-framework] . Any repair
mechanism may however be disrupted by the formation of micro-loops
during the period between the time when the failure is announced, and
the time when all FIBs have been updated to reflect the new topology.
This disruptive effect of micro-loops led the IP fast re-route
designers to develop mechanisms to control the re-convergence of
networks in order to prevent disruption of the repair and collateral
damage to other traffic in the network
[I-D.bryant-shand-lf-conv-frmwk],
[I-D.ietf-rtgwg-microloop-analysis].
The purpose of this note is to draw the attention of the IETF
community to the more general nature of the micro-looping problem,
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and the wider applicability of loop-free convergence technology.
2. Applicability
Loop free convergence strategies are applicable to any problem in
which inconsistency in the FIB causes the formation of micro-loops.
For example, the convergence of a network following:
1) Component failure.
2) Component repair.
3) Managing withdrawal of a component.
4) Managing insertion or a component.
5) Management change of link cost (either positive or negative).
6) External cost change, for example change of external gateway as
a result of a BGP change.
7) A shared risk link group (SRLG) failure.
In each case, a component may be a link or a router.
2.1. Component Failure
When fast-reroute is used to provide the temporary repair of a failed
component, the use of a loop-free convergence mechanism enables the
re-convergence of the network to be performed without additional
packet loss caused by starvation or micro-looping.
The need for loop-free convergence was first appreciated during the
design of IP fast reroute. However the mechanism is also applicable
to the case where an MPLS-TE tunnel is used to provide a link or node
repair within an MPLS network where LDP is used to distribute labels.
Except in special circumstances, controlled convergence in the
presence of component failure should only be used when a temporary
repair is available. This is because controlled convergence is
always slower than uncontrolled (traditional) convergence, and would
result in an extended period of traffic lost as a result of the
failure if there were no other means of delivering the traffic.
2.2. Component Repair
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Micro-loops may form when a component is (re)introduced into a
network. All of the known loop-free convergence methods are capable
of avoiding such micro-loops. It is not necessary to employ any
repair mechanism to take advantage of this facility, because the new
component may be used to provide connectivity before its presence is
made known to the rest of the network.
2.3. Managing withdrawal of a component
From the perspective of the routing protocol, management withdrawal
of a component is indistinguishable from an unexpected component
failure, and will be subject to the same micro-loops. The network
will therefore benefit from the use of a micro-loop prevention
mechanism.
Unlike the failure case, the component being withdrawn may be used to
forward packets during the transition, and therefore no repair
mechanism is needed.
Unlike the case of component failure or repair, management withdrawal
of a component is normally not time critical. Consideration may
therefore be given to the use of the incremental cost change loop-
free convergence mechanism. This mechanism was discarded as a
candidate in the case of fast re-route because of its slow time to
converge, however it is a mechanism that is backwards compatible with
existing routers and may therefore be of use in this application.
Note that unlike any of the other mechanism described here, this
technique can be used without modification to ANY router in the
network.
2.4. Managing Insertion of a Component
From the perspective of the routing protocol, management insertion of
a component is indistinguishable from component repair, and will be
subject to the same micro-loops. The network will therefore benefit
from the use of a micro-loop prevention mechanism. No repair
mechanism is needed and it is not normally time critical.
2.5. Managing Change of a Link Cost
Component failure and component repair are extreme examples of cost
change. Micro-loops may also form when a link cost is changed (in
either direction) during the process of network re-configuration.
The use of a loop-free convergence technique prevents the formation
of micro-loops during this otherwise benign process. No repair
mechanism is needed in this case, because the link is still available
for use.
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2.6. External Cost Change
An external cost change can result in a change to the preferred
external route to a destination. Micro-loops may form during the
process of switching from the old border router to the new one. The
loop-free control of this change will prevent the loss of packets
during this network transition.
2.7. MPLS Applicability
Where the network is an MPLS enabled network using the LDP protocol
to learn labels, and fast re-route is provided through the use of
single hop MPLS-TE tunnels protected by MPLS-TE fast reroute, micro
loops may form during convergence. Loop free convergence is
therefore applicable to this network configuration.
2.8. Routing Vector and Path Vector Convergence
The work to date on controlled convergence has focused on link state
IGPs. The ability to control the convergence of routing vector and
path vector routing protocols would also be useful tools in the
management of the Internet.
Routing vector convergence may be controlled by incrementally
changing the link cost one unit at a time and waiting for the network
to converge. In link state routing protocols employing wide metrics
such a solution would normally be considered as too slow to deploy,
although recent work on optimizing the number of increments has
significantly improved the convergence time. In the case of distance
vector routing protocols the much smaller maximum metric makes this
more tractable, provided that is, the per metric change convergence
time is considered acceptable.
Similarly with path vector routing protocols the path length can be
incrementally padded. Since practical path vector routing protocols
which use path length as an input to the routing decision are
equivalent to using the hop count as a metric (i.e. the maximum per
hop metric is one ,or in special cases a very small number) the
number of increments needed is limited to the length of the path
around the failure. This property may make this a tractable
approach. [I-D.bryant-shand-lf-conv-frmwk] (Section 5.1), although
the per change convergence time may still be a significant concern.
3. IANA considerations
There are no IANA considerations that arise from this draft.
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4. Security Considerations
All micro-loop control mechanisms raise significant security issues
which must be addressed in their detailed technical description.
5. Informative References
[I-D.bryant-shand-lf-conv-frmwk]
Bryant, S. and M. Shand, "A Framework for Loop-free
Convergence", draft-bryant-shand-lf-conv-frmwk-03 (work in
progress), October 2006.
[I-D.ietf-rtgwg-ipfrr-framework]
Shand, M. and S. Bryant, "IP Fast Reroute Framework",
draft-ietf-rtgwg-ipfrr-framework-06 (work in progress),
October 2006.
[I-D.ietf-rtgwg-microloop-analysis]
Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols",
draft-ietf-rtgwg-microloop-analysis-01 (work in progress),
October 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
B. Thomas, "LDP Specification", RFC 3036, January 2001.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
Authors' Addresses
Stewart Bryant
Cisco Systems
250, Longwater, Green Park,
Reading RG2 6GB, UK
UK
Email: stbryant@cisco.com
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Mike Shand
Cisco Systems
250, Longwater, Green Park,
Reading RG2 6GB, UK
UK
Email: mshand@cisco.com
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