Internet Engineering Task Force (IETF) M. Shand
Request for Comments: 5714 S. Bryant
Category: Informational Cisco Systems
ISSN: 2070-1721 January 2010
IP Fast Reroute Framework
Abstract
This document provides a framework for the development of IP fast-
reroute mechanisms that 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.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5714.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scope and Applicability . . . . . . . . . . . . . . . . . . . 5
4. Problem Analysis . . . . . . . . . . . . . . . . . . . . . . . 5
5. Mechanisms for IP Fast-Reroute . . . . . . . . . . . . . . . . 7
5.1. Mechanisms for Fast Failure Detection . . . . . . . . . . 7
5.2. Mechanisms for Repair Paths . . . . . . . . . . . . . . . 8
5.2.1. Scope of Repair Paths . . . . . . . . . . . . . . . . 9
5.2.2. Analysis of Repair Coverage . . . . . . . . . . . . . 9
5.2.3. Link or Node Repair . . . . . . . . . . . . . . . . . 10
5.2.4. Maintenance of Repair Paths . . . . . . . . . . . . . 10
5.2.5. Local Area Networks . . . . . . . . . . . . . . . . . 11
5.2.6. Multiple Failures and Shared Risk Link Groups . . . . 11
5.3. Mechanisms for Micro-Loop Prevention . . . . . . . . . . . 12
6. Management Considerations . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. Informative References . . . . . . . . . . . . . . . . . . . . 14
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 that 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 that may be sensitive to
periods of traffic loss that 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 that 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
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employed by MPLS fast-reroute [RFC4090], 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.
Note that in order to further minimize the impact on user
applications, it may be necessary to design the network such that
backup paths with suitable characteristics (for example, capacity
and/or delay) are available for the algorithms to select. Such
considerations are outside the scope of this document.
2. Terminology
This section defines words and acronyms used in this document and
other documents discussing IP fast-reroute.
D Used to denote the destination router under
discussion.
Distance_opt(A,B) The metric sum of the shortest path from A to B.
Downstream Path This is a subset of the loop-free alternates
where the neighbor N meets the following
condition:
Distance_opt(N, D) < Distance_opt(S,D)
E Used to denote the router that is the primary
neighbor to get from S to the destination D.
Where there is an ECMP set for the shortest path
from S to D, these are referred to as E_1, E_2,
etc.
ECMP Equal cost multi-path: Where, for a particular
destination D, multiple primary next-hops are
used to forward traffic because there exist
multiple shortest paths from S via different
output layer-3 interfaces.
FIB Forwarding Information Base. The database used
by the packet forwarder to determine what actions
to perform on a packet.
IPFRR IP fast-reroute.
Link(A->B) A link connecting router A to router B.
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LFA Loop-Free Alternate. A neighbor N, that is not a
primary neighbor E, whose shortest path to the
destination D does not go back through the router
S. The neighbor N must meet the following
condition:
Distance_opt(N, D) < Distance_opt(N, S) +
Distance_opt(S, D)
Loop-Free Neighbor A neighbor N_i, which is not the particular
primary neighbor E_k under discussion, and whose
shortest path to D does not traverse S. For
example, if there are two primary neighbors E_1
and E_2, E_1 is a loop-free neighbor with regard
to E_2, and vice versa.
Loop-Free Link-Protecting Alternate
A path via a Loop-Free Neighbor N_i that reaches
destination D without going through the
particular link of S that is being protected. In
some cases, the path to D may go through the
primary neighbor E.
Loop-Free Node-Protecting Alternate
A path via a Loop-Free Neighbor N_i that reaches
destination D without going through the
particular primary neighbor (E) of S that is
being protected.
N_i The ith neighbor of S.
Primary Neighbor A neighbor N_i of S which is one of the next hops
for destination D in S's FIB prior to any
failure.
R_i_j The jth neighbor of N_i.
Repair Path The path used by a repairing node to send traffic
that it is unable to send via the normal path
owing to a failure.
Routing Transition The process whereby routers converge on a new
topology. In conventional networks, this process
frequently causes some disruption to packet
delivery.
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RPF Reverse Path Forwarding, i.e., checking that a
packet is received over the interface that would
be used to send packets addressed to the source
address of the packet.
S Used to denote a router that is the source of a
repair that is computed in anticipation of the
failure of a neighboring router denoted as E, or
of the link between S and E. It is the viewpoint
from which IP fast-reroute is described.
SPF Shortest Path First, e.g., Dijkstra's algorithm.
SPT Shortest path tree
Upstream Forwarding Loop
A forwarding loop that involves a set of routers,
none of which is directly connected to the link
that has caused the topology change that
triggered a new SPF in any of the routers.
3. Scope and Applicability
The initial scope of this work is in the context of link state IGPs.
Link state protocols provide ubiquitous topology information, which
facilitates the computation of repairs paths.
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.
Complete protection against multiple unrelated failures is out of
scope of this work.
4. 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 milliseconds when it can be detected at the physical
layer, up to several tens of seconds when a routing protocol
Hello is employed. During this period, packets will be
unavoidably lost.
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2. The time taken for the local router to react to the failure.
This will typically involve generating and flooding new routing
updates, perhaps after some hold-down delay, 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 10 milliseconds and 100
milliseconds per hop.
4. The time taken to re-compute the forwarding tables. This is
typically a few milliseconds 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 dependent
and also depends on the number of prefixes affected by the
failure, but may be several hundred milliseconds.
The disruption will last until the routers adjacent to the failure
have completed steps 1 and 2, and until 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 milliseconds as described in
Section 5.1.
In some topologies, subsequent packet loss may be caused by the
"micro-loops" which may form as a result of temporary inconsistencies
between routers' forwarding tables [RFC5715]. These inconsistencies
are caused by steps 3, 4, and 5 above, and in many routers it is step
5 that is both the largest factor and that 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 reduce packet disruption times to a duration commensurate
with the failure detection times, two mechanisms may be required:
a. A mechanism for the router(s) adjacent to the failure to rapidly
invoke a repair path, which is unaffected by any subsequent re-
convergence.
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b. In topologies that are susceptible to micro-loops, a micro-loop
control mechanism may be required [RFC5715].
Performing the first task without the second may 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.
Repair paths may always be used in isolation where the failure is
short-lived. In this case, the repair paths can be kept in place
until the failure is repaired, therefore there is no need to
advertise the failure to other routers.
Similarly, micro-loop avoidance may be used in isolation to prevent
loops arising from pre-planned management action. In which case the
link or node being shut down can remain in service for a short time
after its removal has been announced into the network, and hence it
can function as its own "repair path".
Note that micro-loops may also occur when a link or node is restored
to service, and thus a micro-loop avoidance mechanism may be required
for both link up and link down cases.
5. Mechanisms for IP Fast-Reroute
The set of mechanisms required for an effective solution to the
problem can be broken down into the sub-problems described in this
section.
5.1. Mechanisms for Fast Failure Detection
It is critical that the failure detection time is minimized. A
number of well-documented approaches are possible, such as:
1. Physical detection; for example, loss of light.
2. Protocol detection that is routing protocol independent; for
example, the Bidirectional Failure Detection protocol [BFD].
3. Routing protocol detection; for example, use of "fast Hellos".
When configuring packet-based failure detection mechanisms it is
important that consideration be given to the likelihood and
consequences of false indications of failure. The incidence of false
indication of failure may be minimized by appropriately prioritizing
the transmission, reception, and processing of the packets used to
detect link or node failure. Note that this is not an issue that is
specific to IPFRR.
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5.2. Mechanisms for Repair Paths
Once a failure has been detected by one of the above mechanisms,
traffic that 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 multi-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. Loop-free alternate paths. Such a path exists when a direct
neighbor of the router adjacent to the failure has a path to the
destination that can be guaranteed not to traverse the failure.
3. Multi-hop repair paths. When there is no feasible loop-free
alternate 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 loop-free alternate paths (as described in [RFC5286]) offer
the simplest repair paths and would normally be used when they are
available. It is anticipated that around 80% of failures (see
Section 5.2.2) can be repaired using these basic methods alone.
Multi-hop repair paths are 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:
a. 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 per-packet
signaling such as detecting a "U-turn" [UTURN], [FIFR] or by
marking the packet [SIMULA].
b. Mechanisms functionally equivalent to a loose source route that
is invoked using the normal FIB. These include tunnels
[TUNNELS], alternative shortest paths [ALT-SP], and label-based
mechanisms.
c. Mechanisms employing special addresses or labels that are
installed in the FIBs of all routers with routes pre-computed to
avoid certain components of the network. For example, see
[NOTVIA].
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In many cases, a repair path that reaches two hops away from the
router detecting the failure will suffice, and it is anticipated that
around 98% of failures (see Section 5.2.2) can be repaired by this
method. However, to provide complete repair coverage, some use of
longer multi-hop repair paths is generally necessary.
5.2.1. 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 loop-free alternate paths) that
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 trade-off 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.
It will frequently be the case that the majority of destinations may
be repaired using only the "basic" repair mechanism, leaving a
smaller subset of the destinations to be repaired using one of the
more complex multi-hop methods. Such a hybrid approach may go some
way to resolving the conflict between completeness and complexity.
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.
5.2.2. Analysis of Repair Coverage
The repair coverage obtained is dependent on the repair strategy and
highly dependent on the detailed topology and metrics. Estimates of
the repair coverage quoted in this document are for illustrative
purposes only and may not be always be achievable.
In some cases the repair strategy will permit the repair of all
single link or node failures in the network for all possible
destinations. This can be defined as 100% coverage. However, where
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the coverage is less than 100%, it is important for the purposes of
comparisons between different proposed repair strategies to define
what is meant by such a percentage. There are four possibilities:
1. The percentage of links (or nodes) that can be fully protected
(i.e., for all destinations). This is appropriate where the
requirement is to protect all traffic, but some percentage of the
possible failures may be identified as being un-protectable.
2. The percentage of destinations that can be protected for all link
(or node) failures. This is appropriate where the requirement is
to protect against all possible failures, but some percentage of
destinations may be identified as being un-protectable.
3. For all destinations (d) and for all failures (f), the percentage
of the total potential failure cases (d*f) that are protected.
This is appropriate where the requirement is an overall "best-
effort" protection.
4. The percentage of packets normally passing though the network
that will continue to reach their destination. This requires a
traffic matrix for the network as part of the analysis.
5.2.3. 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 [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 optimize an already invoked
default strategy.
5.2.4. Maintenance of Repair Paths
In order to meet the response-time goals, it is expected (though not
required) that repair paths, and their associated FIB entries, will
be pre-computed and installed ready for invocation when a failure is
detected. Following invocation, the repair paths remain in effect
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until they are no longer required. This will normally be when the
routing protocol has re-converged on the new topology taking into
account the failure, and traffic will no longer be using the repair
paths.
The repair paths have the property that they are unaffected by any
topology changes resulting from the failure that caused their
instantiation. Therefore, there is no need to re-compute them during
the convergence period. They may be affected by an unrelated
simultaneous topology change, but such events are out of scope of
this work (see Section 5.2.6).
Once the routing protocol has re-converged, it is necessary for all
repair paths to take account of the new topology. Various
optimizations may permit the efficient identification of repair paths
that are unaffected by the change, and hence do not require full re-
computation. Since the new repair paths will not be required until
the next failure occurs, the re-computation may be performed as a
background task and be subject to a hold-down, but excessive delay in
completing this operation will increase the risk of a new failure
occurring before the repair paths are in place.
5.2.5. Local Area Networks
Protection against partial or complete failure of LANs is more
complex than the point-to-point case. In general, there is a trade-
off between the simplicity of the repair and the ability to provide
complete and optimal repair coverage.
5.2.6. Multiple Failures and Shared Risk Link 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 Link Groups (SRLGs) are an example of multiple related
failures, and the more complex aspects of their protection are a
matter for further study.
One specific example of an SRLG that 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.
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5.3. Mechanisms for Micro-Loop Prevention
Ensuring the absence of micro-loops is important not only because
they can cause packet loss in traffic that is affected by the
failure, but because by saturating a link with looping packets micro-
loops can cause congestion. This congestion can then lead to routers
discarding traffic that would otherwise be unaffected by the failure.
A number of solutions to the problem of micro-loop formation have
been proposed and are summarized in [RFC5715]. The following factors
are significant in their classification:
1. Partial or complete protection against micro-loops.
2. Convergence delay.
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.
7. Topological constraints.
6. Management Considerations
While many of the management requirements will be specific to
particular IPFRR solutions, the following general aspects need to be
addressed:
1. Configuration
A. Enabling/disabling IPFRR support.
B. Enabling/disabling protection on a per-link or per-node
basis.
C. Expressing preferences regarding the links/nodes used for
repair paths.
D. Configuration of failure detection mechanisms.
E. Configuration of loop-avoidance strategies
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2. Monitoring and operational support
A. Notification of links/nodes/destinations that cannot be
protected.
B. Notification of pre-computed repair paths, and anticipated
traffic patterns.
C. Counts of failure detections, protection invocations, and
packets forwarded over repair paths.
D. Testing repairs.
7. 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.
Where the chosen solution uses tunnels it is necessary to ensure that
the tunnel is not used as an attack vector. One method of addressing
this is to use a set of tunnel endpoint addresses that are excluded
from use by user traffic.
There is a compatibility issue between IPFRR and reverse path
forwarding (RPF) checking. Many of the solutions described in this
document result in traffic arriving from a direction inconsistent
with a standard RPF check. When a network relies on RPF checking for
security purposes, an alternative security mechanism will need to be
deployed in order to permit IPFRR to used.
Because the repair path will often be of a different length than the
pre-failure path, security mechanisms that rely on specific Time to
Live (TTL) values will be adversely affected.
8. Acknowledgements
The authors would like to acknowledge contributions made by Alia
Atlas, Clarence Filsfils, Pierre Francois, Joel Halpern, Stefano
Previdi, and Alex Zinin.
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9. Informative References
[ALT-SP] Tian, A., "Fast Reroute using Alternative Shortest Paths",
Work in Progress, July 2004.
[BFD] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", Work in Progress, January 2010.
[FIFR] Nelakuditi, S., Lee, S., Lu, Y., Zhang, Z., and C. Chuah,
"Fast Local Rerouting for Handling Transient Link
Failures", IEEE/ACM Transactions on Networking, Vol. 15,
No. 2, DOI 10.1109/TNET.2007.892851, available
from http://www.ieeexplore.ieee.org, April 2007.
[NOTVIA] Shand, M., Bryant, S., and S. Previdi, "IP Fast Reroute
Using Not-via Addresses", Work in Progress, July 2009.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[SIMULA] Kvalbein, A., Hansen, A., Cicic, T., Gjessing, S., and O.
Lysne, "Fast IP Network Recovery using Multiple Routing
Configurations", Infocom 10.1109/INFOCOM.2006.227,
available from http://www.ieeexplore.ieee.org, April 2006.
[TUNNELS] Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
Fast Reroute using tunnels", Work in Progress,
November 2007.
[UTURN] Atlas, A., "U-turn Alternates for IP/LDP Fast-Reroute",
Work in Progress, February 2006.
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Authors' Addresses
Mike Shand
Cisco Systems
250, Longwater Avenue.
Reading, Berks RG2 6GB
UK
EMail: mshand@cisco.com
Stewart Bryant
Cisco Systems
250, Longwater Avenue.
Reading, Berks RG2 6GB
UK
EMail: stbryant@cisco.com
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RFC, FYI, BCP