INTERNET DRAFT IP Fast-reroute Framework June 2004
Network Working Group M. Shand
Internet Draft
Expiration Date: Dec 2004 Cisco Systems
June 2004
IP Fast Reroute Framework
draft-ietf-rtgwg-ipfrr-framework-00.txt
Status of this Memo
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Abstract
This document provides a framework for the development of IP fast re-
route mechanisms which provide protection against link or router
failure by invoking locally determined repair paths. Unlike MPLS
Fast-reroute, the mechanisms are applicable to a network employing
conventional IP routing and forwarding. An essential part of such
mechanisms is the prevention of packet loss caused by the loops which
normally occur during the re-convergence of the network following a
failure.
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Terminology
This section defines words, acronyms, and actions used in this draft.
1. Introduction
When a link or node failure occurs in a routed network, there is
inevitably a period of disruption to the delivery of traffic until
the network re-converges on the new topology. Packets for
destinations which were previously reached by traversing the failed
component may be dropped or may suffer looping. Traditionally such
disruptions have lasted for periods of at least several seconds, and
most applications have been constructed to tolerate such a quality of
service.
Recent advances in routers have reduced this interval to under a
second for carefully configured networks using link state IGPs.
However, new Internet services are emerging which may be sensitive to
periods of traffic loss which are orders of magnitude shorter than
this.
Addressing these issues is difficult because the distributed nature
of the network imposes an intrinsic limit on the minimum convergence
time which can be achieved.
However, there is an alternative approach, which is to compute backup
routes that allow the failure to be repaired locally by the router(s)
detecting the failure without the immediate need to inform other
routers of the failure. In this case, the disruption time can be
limited to the small time taken to detect the adjacent failure and
invoke the backup routes. This is analogous to the technique employed
by MPLS Fast Reroute [MPLSFRR], but the mechanisms employed for the
backup routes in pure IP networks are necessarily very different.
This document provides a framework for the development of this
approach.
2. Problem Analysis
The duration of the packet delivery disruption caused by a
conventional routing transition is determined by a number of factors:
1. The time taken to detect the failure. This may be of the order
of a few mS when it can be detected at the physical layer, up to
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several tens of seconds when a routing protocol hello is
employed. During this period packets will be unavoidably lost.
2. The time taken for the local router to react to the failure.
This will typically involve generating and flooding new routing
updates, and re-computing the router's FIB.
3. The time taken to pass the information about the failure to
other routers in the network. In the absence of routing protocol
packet loss, this is typically between 10mS and 100mS per hop in
a well designed router.
4. The time taken to re-compute the forwarding tables. This is
typically a few mS for a link state protocol using Dijkstra's
algorithm.
5. The time taken to load the revised forwarding tables into the
forwarding hardware. This time is very implementation dependant
and also depends on the number of prefixes affected by the
failure, but may be several hundred mS.
The disruption will last until the routers adjacent to the failure
have completed steps 1 and 2, and then all the routers in the network
whose paths are affected by the failure have completed the remaining
steps.
The initial packet loss is caused by the router(s) adjacent to the
failure continuing to attempt to transmit packets across the failure
until it is detected. This loss is unavoidable, but the detection
time can be reduced to a few tens of mS as described in section 3.1.
Subsequent packet loss is caused by the "micro-loops" which form
because of temporary inconsistencies between routers' forwarding
tables. These occur as a result of the different times at which
routers update their forwarding tables to reflect the failure. These
variable delays are caused by steps 3, 4 and 5 above and in many
routers it is step 5 which is both the largest factor and which has
the greatest variance between routers. The large variance arises from
implementation differences and from the differing impact that a
failure has on each individual router. For example, the number of
prefixes affected by the failure may vary dramatically from one
router to another.
In order to achieve packet disruption times which are commensurate
with the failure detection times it is necessary to perform two
distinct tasks:
1. Provide a mechanism for the router(s) adjacent to the failure to
rapidly invoke a repair path, which is unaffected by any
subsequent re-convergence.
2. Provide a mechanism to prevent the effects of micro loops during
subsequent re-convergence.
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Performing the first task without the second will result in the
repair path being starved of traffic and hence being redundant.
Performing the second without the first will result in traffic being
discarded by the router(s) adjacent to the failure. Both tasks are
necessary for an effective solution to the problem.
However, repair paths can be used in isolation where the failure is
short-lived. The repair paths can be kept in place until the failure
is repaired and there is no need to advertise the failure to other
routers.
Similarly, micro loop avoidance can be used in isolation to prevent
loops arising from pre-planned management action.
Note that micro-loops can also occur when a link or node is restored
to service and thus a micro-loop avoidance mechanism is required for
both link up and link down cases.
3. Mechanisms for IP Fast-route
The set of mechanisms required for an effective solution to the
problem can be broken down into the following sub-problems.
3.1. Mechanisms for fast failure detection
It is critical that the failure detection time is minimized. A number
of approaches are possible, such as:
1. Physical detection, such as loss of light.
2. The Bidirectional Failure Detection protocol [BFD]
3. Other forms of "fast hellos"
3.2. Mechanisms for repair paths
Once a failure has been detected by one of the above mechanisms,
traffic which previously traversed the failure is transmitted over
one or more repair paths. The design of the repair paths should be
such that they can be pre-calculated in anticipation of each local
failure and made available for invocation with minimal delay. There
are three basic categories of repair paths:
1. Equal cost multiple paths (ECMP). Where such paths exist, and
one or more of the alternate paths do not traverse the failure,
they may trivially be used as repair paths.
2. Downstream paths. (Also known as "loop free feasible
alternates".) Such a path exists when a direct neighbor of the
router adjacent to the failure has a path to the destination
which cannot traverse the failure.
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3. Multihop repair paths. When there is no feasible downstream path
it may still be possible to locate a router, which is more than
one hop away from the router adjacent to the failure, from which
traffic will be forwarded to the destination without traversing
the failure.
ECMP and downstream paths offer the simplest repair paths and would
normally be used when they are available. It is anticipated that
around 80% of failures can be repaired using these alone.
Multi-hop repair paths are considerably more complex, both in the
computations required to determine their existence, and in the
mechanisms required to invoke them. They can be further classified
as:
1. Mechanisms where one or more alternate FIBs are pre-computed in
all routers and the repaired packet is instructed to be
forwarded using a "repair FIB" by some method of signaling such
as detecting a "U-turn" or marking the packet.
2. Mechanisms functionally equivalent to a loose source route which
is invoked using the normal FIB. These include tunnels and label
based mechanisms.
In many cases a repair path which reaches two-hops away from the
router detecting the failure will suffice, and it is anticipated that
around 95% of failures can be repaired by this method. However, to
effect complete repair coverage some use of longer multi-hop repair
paths is generally necessary.
3.2.1. Scope of repair paths
A particular repair path may be valid for all destinations which
require repair or may only be valid for a subset of destinations. If
a repair path is valid for a node immediately downstream of the
failure, then it will be valid for all destinations previously
reachable by traversing the failure. However, in cases where such a
repair path is difficult to achieve because it requires a high order
multi-hop repair path, it may still be possible to identify lower
order repair paths (possibly even downstream paths) which allow the
majority of destinations to be repaired. When IPFRR is unable to
provide complete repair, it is desirable that the extent of the
repair coverage can be determined and reported via network
management.
There is a tradeoff to be achieved between minimizing the number of
repair paths to be computed, and minimizing the overheads incurred in
using higher order multi-hop repair paths for destinations for which
they are not strictly necessary. However, the computational cost of
determining repair paths on an individual destination basis can be
very high.
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The use of repair paths may result in excessive traffic passing over
a link, resulting in congestion discard. This reduces the
effectiveness of IPFRR. Mechanisms to influence the distribution of
repaired traffic to minimize this effect are therefore desirable.
3.2.2. Link or node repair
A repair path may be computed to protect against failure of an
adjacent link, or failure of an adjacent node. In general, link
protection is simpler to achieve. A repair which protects against
node failure will also protect against link failure for all
destinations except those for which the adjacent node is a single
point of failure.
In some cases it may be necessary to distinguish between a link or
node failure in order that the optimal repair strategy is invoked.
Methods for link/node failure determination may be based on
techniques such as BFD. This determination may be made prior to
invoking any repairs, but this will increase the period of packet
loss following a failure unless the determination can be performed as
part of the failure detection mechanism itself. Alternatively, a
subsequent determination can be used to optimise an already invoked
default strategy.
3.2.3. Multiple failures and Shared Risk Groups
Complete protection against multiple unrelated failures is out of
scope of this work. However, it is important that the occurrence of a
second failure while one failure is undergoing repair should not
result in a level of service which is significantly worse than that
which would have been achieved in the absence of any repair strategy.
Shared Risk Groups are an example of multiple related failures, and
their protection is a matter for further study.
One specific example of an SRLG which is clearly within the scope of
this work is a node failure. This causes the simultaneous failure of
multiple links, but their closely defined topological relationship
makes the problem more tractable.
3.3. Mechanisms for micro-loop prevention
Control of micro-loops is important not only because they can cause
packet loss in traffic which is affected by the failure, but because
they can also cause congestion loss of traffic which would otherwise
be unaffected by the failure.
A number of solutions to the problem of micro-loop formation have
been proposed. The following factors are significant in their
classification:
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1. Partial or complete protection against micro-loops.
2. Delay imposed upon convergence.
3. Tolerance of multiple failures (from node failures, and in
general)
4. Computational complexity (pre-computed or real time)
5. Applicability to scheduled events
6. Applicability to link/node reinstatement.
4. Scope and applicability
Link state protocols provide ubiquitous topology information, which
facilitates the computation of repairs paths. Therefore the initial
scope of this work is in the context of link state IGPs.
Provision of similar facilities in non-link state IGPs and BGP is a
matter for further study, but the correct operation of the repair
mechanisms for traffic with a destination outside the IGP domain is
an important consideration for solutions based on this framework
5. IANA considerations
There are no IANA considerations that arise from this description of
IPFRR. However there may be changes to the IGPs to support IPFRR in
which there will be IANA considerations.
6. Security Considerations
This framework document does not itself introduce any security
issues, but attention must be paid to the security implications of
any proposed solutions to the problem.
Acknowledgments
Normative References
Internet-drafts are works in progress available from
http://www.ietf.org/internet-drafts/
Informative References
Internet-drafts are works in progress available from
http://www.ietf.org/internet-drafts/
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BFD Katz, D., and Ward, D., "Bidirectional Forwarding
Detection", draft-katz-ward-bfd-01.txt, August
2003 (work in progress).
MPLSFRR Pan, P. et al, "Fast Reroute Extensions to RSVP-
TE for LSP Tunnels", draft-ietf-mpls-rsvp-lsp-
fastreroute-05.txt
Author's Address
Mike Shand
Cisco Systems,
250, Longwater Avenue,
Green Park,
Reading, RG2 6GB,
United Kingdom. Email: mshand@cisco.com
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