INTERNET DRAFT IP Fast-reroute May 2004
Network Working Group S. Bryant
Internet Draft C. Filsfils
Expiration Date: Nov 2004 S. Previdi
M. Shand
Cisco Systems
May 2004
IP Fast Reroute using tunnels
draft-bryant-ipfrr-tunnels-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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Abstract
This draft describes an IP fast re-route mechanism that provides
backup connectivity in the event of a link or router failure. In the
absence of single points of failure and asymmetric costs, the
mechanism provides complete protection against any single failure. If
perfect repair is not possible, the identity of all the unprotected
links and routers is known in advance. The draft also describes the
mechanisms needed to prevent the packet loss caused by loops which
normally occur during the reconvergence of the network following a
failure.
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Table of Contents
1. Introduction......................................................5
2. Goals, non-goals, limitations and constraints.....................5
2.1. Goals.........................................................5
2.2. Non-Goals.....................................................6
2.3. Limitations...................................................6
2.4. Constraints...................................................6
3. Repair Paths......................................................7
3.1. Tunnels as Repair Paths.......................................7
3.2. Tunnel Requirements..........................................10
3.2.1. Setup....................................................10
3.2.2. Multipoint...............................................10
3.2.3. Directed forwarding......................................10
3.2.4. Security.................................................10
4. Construction of Repair Paths.....................................10
4.1. Identifying Repair Path Targets..............................10
4.2. Determining Tunneled Repair Paths............................11
4.2.1. Computing Repair Paths...................................12
4.2.2. Extended P-space.........................................13
4.2.3. Downstream Paths.........................................13
4.2.4. Selecting Repair Paths...................................13
4.3. Assigning Traffic to Repair Paths............................14
4.4. When no Repair Path is Possible..............................14
4.4.1. Unreachable Target.......................................15
4.4.2. Asymmetric Link Costs....................................15
4.4.3. Interference Between Potential Node Repair Paths.........15
4.5. Multi-homed Prefixes.........................................18
4.6. Equal Cost Path Splits.......................................19
4.6.1. Equal Cost Path Splits as Link Repair Paths..............19
4.6.2. Equal Cost Path Splits and Node Failure..................20
4.7. LANs and pseudonodes.........................................20
4.7.1. The Link between Routers A and B is a LAN................21
4.7.1.1. Case 1...............................................21
4.7.1.2. Case 2...............................................21
4.7.1.3. Simplified LAN repair................................22
4.7.2. A LAN exists at the release point........................22
4.7.3. A LAN between B and its neighbors........................22
4.7.4. The LAN is a Transit Subnet..............................23
5. Failure Detection and Repair Path Activation.....................23
5.1. Failure Detection............................................23
5.2. Repair Path Activation.......................................23
5.3. Node Failure Detection Mechanism.............................23
6. Loop Free Transition.............................................24
6.1. Incremental Cost Advertisement...............................24
6.2. Single Tunnel Per Router.....................................25
6.3. Distributed Tunnels..........................................25
6.4. Ordered SPFs.................................................26
7. Restoring Failed Components to Service...........................26
8. Implications for Network Management..............................26
9. IPFRR Capability.................................................27
10. Enhancements to routing protocols...............................27
11. IANA considerations.............................................27
12. Security Considerations.........................................27
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Terminology
This section defines words, acronyms, and actions used in this draft.
A Frequently used to denote a router that is
the source of a repair path computed in
anticipation of the failure of a neighboring
router denoted as B.
B Frequently used to denote a router whose
anticipated failure is the subject of repair
path computations.
Directed The ability of the repairing router (A) to
forwarding specify the next hop (Q) on exit from a
tunnel end-point (P)
Extended The union of the p-space of the neighbors of
P-space a specific router with respect to a common
component.
Extended p-space does not include the
additional space reachable though directed
forwarding.
FIB Forwarding Information Base. The database
used by the packet forwarder to determine
what actions to perform on a packet
IPFRR IP fast re-route
P The router in P-space to which a packet is
tunneled for repair.
PQ A router that is in both P and Q space and
hence does not need directed forwarding.
P-space P-space is the set of routers reachable from
a specific router without any path
(including equal cost path splits)
transiting a specified component.
For example, the P-space of A, is the set of
routers that A can reach without using B
(router failure case) or the A-B link
failure case).
Q The router in Q space, to which the packet
is directed by router P on exit from the
repair tunnel. Q will always be adjacent to
P, or P itself.
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Q-space Q-space is the set of routers from which a
specific router can be reached without any
path (including equal cost path splits)
transiting a specified component.
Routing The process whereby routers converge on a
transition new topology. In conventional networks this
process frequently causes some disruption to
packet delivery.
RPF Reverse Path Forwarding. I.e. checking that
a packet is received over the interface
which would be used to send packets
addressed to the source address of the
packet.
SPF Shortest Path First, e.g. Dijkstra's
algorithm.
SPT Shortest path tree
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1. Introduction
When the topology of a network changes (due to link or router
failure, recovery or management action), the routers need to converge
on a common view of the new topology. 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 plus 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 are lost due
to the continuing attempts to use of the failed component, and 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.
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. The
solution described here uses pre-computed backup routes and
controlled notification of network changes. A set of repair paths
temporarily provides substitute connectivity in place of a link, or
router that has failed. Once the set of repair paths has been
activated, there should be no further packet loss as a result of the
associated failure. To achieve the maximum benefit from repair paths,
they must be activated immediately a failure has been detected, and a
controlled transition to normal operation invoked to prevent packet
loss due to micro-looping. The packet loss attributable to the
failure will then be confined to the unavoidable loss that occurs as
a result of the latency of the failure detection mechanism itself.
The mechanisms described here have been designed for use with any
link-state routing protocol.
2. Goals, non-goals, limitations and constraints
2.1. Goals
The following are the goals of IPFRR:
o Protect against any link or router failure in the network.
o No constraints on the network topology or link costs.
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o Never worse than the existing routing convergence mechanism.
o Co-existence with non-IP fast-reroute capable routers in the
network.
2.2. Non-Goals
The following are non-goals of IPFRR:
o Protection of a single point of failure.
o To provide protection in the presence of multiple concurrent
failures other than those that occur due to the failure of a
single router.
o Shared risk group protection.
o Complete fault coverage in networks that make use of
asymmetric costs.
2.3. Limitations
The following limitations apply to IPFRR:
o Because the mechanisms described here rely on complete
topological information from the link state routing protocol,
they will only work within a single link state flooding
domain.
o Reverse Path Forwarding (RPF) checks cannot be used in
conjunction with IPFRR. This is because the use of tunnels
may result in packets arriving over different interfaces than
expected.
2.4. Constraints
The following constraints are assumed:
o Following a failure, only the routers adjacent to the failure
have any knowledge of the failure.
o There is insufficient time following a failure to compute a
repair strategy based on knowledge of the specific failure
that has occurred.
o Multiple concurrent failures may not be protected.
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3. Repair Paths
When a router detects an adjacent failure, it uses a set of repair
paths in place of the failed component, and continues to use this
until the completion of the routing transition. Only routers adjacent
to the failed component are aware of the nature of the failure. Once
the routing transition has been completed, the router will have no
further use for the repair paths since all routers in the network
will have revised their forwarding data and the failed link will have
been eliminated from this computation.
Repair paths are pre-computed in anticipation of later failures so
they can be promptly activated when a failure is detected.
Three types of repair path are considered here.
1. Equal cost path-split.
Where a link is being used as a member of an equal cost path-split
set for some destination, the other members of the set may be used
to provide an alternative path, provided that they avoid the
network component being protected.
2. Downstream Path.
A 'downstream path' is a next hop that will get a packet nearer to
its destination. It does not necessarily represent the shortest
path to the destination but has the property that a packet sent on
it will not loop back because, having traversed this hop, it is
then closer to its destination.
3. Tunnel.
A tunneled repair path tunnels traffic to some staging point from
which it will travel to its destination using normal forwarding
without looping back. The repair path can be thought of as
providing a virtual link, originating at a router adjacent to a
failure, and diverting traffic around the failure.
3.1. Tunnels as Repair Paths
The repair strategies described in this draft operate on the basis
that if a packet can somehow be sent to the other side of the
failure, it will subsequently proceed towards its destination exactly
as if it had traversed the failed component. See Figure 1.
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Repair Path from A to B
+-----------+
| |
| v
---->[A]---//----[B]----->
Figure 1 Simple Link Repair
Creating a repair path from A to B may require a packet to traverse
an unnatural route. If a suitable natural path starts at a neighbor
(i.e. it is a downstream path), then A can force the packet directly
there. If this is not the case, then A must use a tunnel to force the
packet down the repair path. Note that the tunnel does not have to go
from A to B. The tunnel can terminate at any router in the network,
provided that A can be sure that the packet will proceed correctly to
its destination from that router.
A repair path computed for a link failure may not however work
satisfactorily when the neighboring router has, itself, failed. This
is illustrated in Figure 2.
Repair path from A to B
+-------------------------+
| |
| <------------+
--->[A]---//----[B]----//-----[C]-->
+----------> |
| |
+-------------------------+
Repair Path from C to B
Figure 2 Looping Link Repair when Router Fails
Consider the case of a router B with just two neighbors A and C. When
router B fails, both A and C will observe the failure of their local
link to B, but will have no immediate knowledge that B itself has
failed. If they were both to attempt to repair traffic around their
local link, they would invoke mutual repairs which would loop.
Since it is not easy for a router to immediately distinguish between
a link failure and the failure of its neighbor, repair paths are
calculated in anticipation of adjacent router failure. Thus, for each
of its protected links, router A (Figure 3) pre-computes a set of
tunneled repair paths, one for each of the neighbors (C,D,E) of its
neighbor B on the A-B link. The set of destinations that are normally
assigned to link A-B will be assigned to a repair path based on the
neighbor of B through which router B would have forwarded traffic to
them.
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Repair AC
+----------->[C]
| |
| |
| |
----->[A]----//-----[B]---------[D]
|| | ^
|| | |
|| Repair AE | |
|+---------->[E] |
| |
+-------------------------+
Repair AD
Figure 3: Repair paths in anticipation of a router failure
The set of repair paths in Figure 3 will function correctly in the
case of link and router failure. However, in some network topologies
they may not provide a means for traffic to reach router B itself.
This is important in cases where B is a single point of failure and B
is still functional (i.e. the failure was actually a failure of the
A-B link). Hence, in addition to computing repair paths for the
neighbors of its neighbor on a protected link, a router also
calculates a repair path for the neighbor itself. This is illustrated
in Figure 4.
Repair AB
+----------------+
| |
| Repair AC |
|+---------->[C] |
|| | /
|| | /
|| |/
----->[A]----//-----[B]---------[D]
|| | ^
|| | |
|| Repair AE | |
|+---------->[E] |
| |
+-------------------------+
Repair AD
Figure 4 The full set of A-B repair paths.
In the event of a failure, the only traffic that is assigned to the
link repair path (the AB repair) is that traffic which has no other
path to its destination except via B. As we have already seen, there
is a danger that traffic assigned to this link repair path may loop
if B has failed, therefore, when the repair paths are invoked, a loop
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detection mechanism is used which promptly detects the loop and, upon
detection, withdraws the link (A-B) repair path from service.
3.2. Tunnel Requirements
The specific tunneling mechanism used to provide a repair path is
outside the scope of this document. However the following sections
describe the requirements for the tunneling mechanism.
3.2.1. Setup.
When a failure is detected, it is necessary to immediately redirect
traffic to the repair paths. Consequently, the tunnels used must be
provisioned beforehand in anticipation of the failure. IP fast re-
route will determine which tunnels it requires. It must therefore be
possible to establish tunnels automatically, without management
action, and without the need to manually establish context at the
tunnel endpoint.
3.2.2. Multipoint
To reduce the number of tunnel endpoints in the network the tunnels
should be be multi-point tunnels capable of receiving repair traffic
from any IPFRR router in the network.
3.2.3. Directed forwarding.
Directed forwarding must be supported such that the router at the
tunnel endpoint (P) can be directed by the router at the tunnel
source (A) to forward the packet directly to a specific neighbor.
Specification of the directed forwarding mechanism is outside the
scope of this document.
3.2.4. Security
A lightweight security mechanism should be supported to prevent the
abuse of the repair tunnels by an attacker. This is discussed in more
detail in Section 12.
4. Construction of Repair Paths
4.1. Identifying Repair Path Targets
To establish protection for a link or node it is necessary to
determine which neighbors of the neighboring node should be targets
of repair paths. Normally all neighbors will be used as repair path
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targets. However, in some topologies, not all neighbors will be
needed as targets because, prior to the failure, no traffic was being
forwarded through them by the repairing router. This can determined
by examining the normal spanning tree computed by the repairing
router.
In addition, the neighboring router B will also be the target of a
repair path for any destinations for which B is a single point of
failure.
4.2. Determining Tunneled Repair Paths
The objective of each tunneled repair path is to deliver traffic to a
target router when a link is observed to have failed. However, it is
seldom possible to use the target router itself as the tunnel
endpoint because other routers on the repair path, that have not
learned of the failure, will forward traffic addressed to it using
their least cost path which may be via the failed link. This is
illustrated in Figure 5 in which all link costs are one in both
directions. Router A's intended repair path for traffic to D when
link A-B fails is the path W-X-Y-Z-D. However, if router A makes D be
the tunnel endpoint and forwards the packet to router W, router W
will immediately return it to A because its least cost path to D is
A-B-D (cost 3 versus cost 4) and has no knowledge of the failure of
link A-B.
[A]--//--[B]------[D]
| |
| |
[W]---[X]---[Y]---[Z]
Figure 5. Repair path to target router D.
Thus the tunnel endpoint needs to be somewhere on the repair path
such that packets addressed to the tunnel end point will not loop
back towards router A. In addition, the release point needs to be
somewhere such that when packets are released from the tunnel they
will flow towards the target router (or their actual destination)
without being attracted back to the failed link. By inspection, in
Figure 5, suitable tunnel endpoints are routers X, Y, and Z.
Note that it is not essential that traffic assigned to a repair path
actually traverse the target router for which the repair path was
created. If, for example, in Figure 5, a packet's destination were
normally reached via the path A-B-D-Z-?-?-?, once released at any of
the possible tunnel endpoints, it would arrive at its destination by
the best available route without traversing D.
In general, the properties that are required of tunnel endpoints are:
o the end point must be reachable from the tunnel source
without traversing the failed link; and
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o once released, tunneled packets will proceed towards their
destination without being attracted back over the failed link
or node.
Provided both of these conditions are met, packets forwarded on the
repair path will not loop.
In some topologies it will not be possible to find a tunnel endpoint
that exhibits both the required properties. For example, in Figure 5,
if the cost of link X-Y were increased from one to four in both
directions, there is no longer a viable endpoint within the fragment
of the topology shown.
To solve this problem we introduce the concept of directed forwarding
from the tunnel endpoint. Directed forwarding allows the originator
of a tunneled packet to instruct that, when it is de-capsulated at
the end of the tunnel, it be forwarded via a specific adjacency, and
not be subjected to the normal forwarding decision process. This
effectively allows the tunnel to be extended by one hop. So, for
example, in Figure 5 with the cost of link X-Y set to four, it would
be possible to select X as the tunnel endpoint with the directive
that X always forward the packets it decapsulates via the
adjacency to Y. Thus, router X is reached from A using normal
forwarding, and directed forwarding is then used to force packets to
router Y, from where D can be reached using normal forwarding.
Provided link costs are symmetrical, it can be proved that it is
always possible to compute a tunneled repair path (possibly using
directed forwarding) around a link failure.
The tunnel endpoint (P) and the release point (Q) may be coincident,
or may be separated by at most one hop.
4.2.1. Computing Repair Paths
For a router A, determining tunneled repair paths around a
neighboring router B, the set of potential tunnel end points includes
all the routers that can be reached from A using normal forwarding
without traversing the failed link A-B. This is termed the "P-space"
of A with respect to the failure of B. Any router that is on an equal
cost path split via the failed link is excluded from this set.
The resulting set defines all the possible tunnel end points that
could be used in repair paths originating at router A for the failure
of link A-B. This set can be obtained by computing a spanning tree
rooted at A and excising the subtree reached via the A-B link.
The set of possible release points can be determined by computing the
set of routers that can reach the repair path target without
traversing the failed link. This is termed the "Q-space" of the
target with respect to the failure. The Q-space can be obtained by
computing a reverse spanning tree rooted at the repair path target,
with the subtree which traverses the failed link (or node) excised.
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The reverse spanning tree uses the cost towards the root rather than
from it and yields the best paths towards the root from other nodes
in the network.
The intersection of the target's Q-space with A's P-space includes
all the possible release points for any repair path not employing
directed forwarding. Where there is no intersection, but there exist
a pair of routers, P in A's P-space and Q in the target's Q-space,
router P can be used as the tunnel endpoint with directed forwarding
to the release point Q.
4.2.2. Extended P-space
The description in section 4.2.1 calculated router A's P-space rooted
at A itself. However, since router A will only use a repair path when
it has detected the failure of the link A-B, the initial hop of the
repair path need not be subject to A's normal forwarding decision
process. Thus we introduce the concept of extended P-space. Router
A's extended P-space is the union of the P-spaces of each of A's
neighbors. The use of extended P-space may allow router A to repair
to targets that were otherwise unreachable.
4.2.3. Downstream Paths
Under certain circumstances, the target's Q-space will include a
router that is a neighbor of A. This is traditionally referred to as
a downstream path and has the property that a packet sent on it will
not loop back because, having traversed this hop, it is then closer
to its destination. A trivial example of this is shown in Figure 6.
[A]--//---[B]
| |
2 |
| |
[D]-------[C]
Figure 6. A topology that will permit a single-hop release point
When a downstream path exists, no tunneling is required.
4.2.4. Selecting Repair Paths
The mechanism described in section 4.2 will identify all the possible
release points that can be used to reach each particular target. (The
circumstances when no release points exist are described in
section 4.4.) In a well-connected network there are likely to be
multiple possible release points for each target, and all will work
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correctly. For simplicity, one release point per target is chosen.
All will deliver the packets correctly so, arguably, it does not
matter which is chosen. However, one release point may be preferred
over the others on the basis of path cost or some other criteria. It
is an implementation matter as to how the release point is selected.
4.3. Assigning Traffic to Repair Paths
Once the repair path for each target has been selected, it is
necessary to determine which of the destinations normally reached via
the protected link should be assigned to which of the repair paths
when the link fails.
This is achieved by recording which neighbor of B would be used to
reach each destination reachable over A-B when running the original
SPF. Traffic assignment is then simply a matter of assigning the
traffic which B would have forwarded via each neighbor to the repair
path which has that neighbor as its target.
Although the repair paths are calculated based on traffic addressed
to specific targets, it can be proved that the traffic assignment
algorithm guarantees that the repair path can be used for any traffic
assigned to it.
Where B would normally split the traffic to a particular destination
via two or more of its neighbors, it is an implementation decision
whether the repaired traffic should be split across the corresponding
set of repair paths.
The repair path to B itself is normally used just for traffic
destined for B and any prefixes advertised by B. However, under some
circumstances, it may be impossible to compute a repair path to one
or more of B's neighbors, for example, because B is a single point of
failure. In this case traffic for the destinations served by the
otherwise irreparable targets is assigned to the repair path with B
as its target, in the optimistic assumption that router B is still
functioning. If router B is indeed still functioning, this will
ensure delivery of the traffic. If, however, router B has failed, the
traffic on this repair path will loop as previously shown in
section 3.1. The way this is detected, and the course of action when
it is detected, are described in section 5.3.
4.4. When no Repair Path is Possible
Under some circumstances, it will not be possible to identify a
repair path to one or more of the targets. This can occur for the
following reasons:
o The neighboring router that is presumed to have failed
constitutes a single point of failure in the network.
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o Severely asymmetric link costs may cause an otherwise viable
physical repair path to be unusable.
o Interference may occur between the repair paths of individual
targets.
In practice, these cases are unlikely to be encountered frequently.
Networks that will benefit from the mechanisms described here will
usually exhibit considerable redundancy and are normally operated
with largely symmetric link costs. Note that a router's inability to
compute a full set of repair paths for one of its links does not
necessarily affect its ability to do so for its other links.
Example topologies illustrating each of the three cases above are
described in the following subsections.
4.4.1. Unreachable Target
If the failure of a neighboring router makes one or more of its
neighbors genuinely unreachable, clearly it will not be possible to
establish a repair path to such targets. Such single points of
failure are not expected to be encountered frequently in properly
designed networks, and will probably occur only when the network has
previously suffered other failures that have reduced its
connectivity.
4.4.2. Asymmetric Link Costs
When link costs have been set asymmetrically, it is possible that a
repair path cannot be constructed even using directed forwarding.
Although it is trivial to construct a network fragment with this
property, this should not be regarded as a major problem. Firstly,
asymmetric link costs are seldom used deliberately. And, secondly,
even when an asymmetric link cost prevents one potential repair path
being used, there will normally be other ones available.
4.4.3. Interference Between Potential Node Repair Paths
Under some circumstances the existence of one neighbor may interfere
with a potential repair path to another. Consider the topology shown
in Figure 7 in which all links have a symmetrical cost of one, with
the exception of that between H and G, which has a cost of 3. In this
example, the fact that router F is a neighbor of B prevents the
discovery of a repair path from router A to router C despite the
existence of an apparently suitable path.
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[A]---//---[B]------ [C]
| | |
| | |
[H]-3-[G]--[F]--[E]--[D]
Figure 7. Interference between repair paths
A repair path from router A to F can use F itself as the release
point by employing directed forwarding from G. However, it is not
possible to identify a suitable release point for a repair path to
router C within the topology shown since there is nowhere that
router A can reach that will subsequently forward traffic to router C
except via the forbidden link B-C (F's least cost path to C is
F-B-C). This is because the extended P-space of router A is separated
by more than one hop from the Q-space of router C.
Since the topology shown in Figure 7 will typically form part of a
much larger topology, a different, and possibly more circuitous
repair path from A to C, that does not go via F, may be discovered.
This is illustrated in Figure 8. In this enhanced topology, a repair
path to C using Y as the release point can be used.
[A]---//---[B]-------[C]
| | |
| | |
[H]-3-[G]--[F]--[E]--[D]
| |
| |
[X]--[Y]--[Z]
Figure 8. Resolving interference in a larger network
Note that, in Figure 8, if the traffic for C were assigned to the
repair path for F, it would correctly reach C because F would assign
it to its repair path to C. That is, packets from A to C would travel
via two successive tunnels. Consequently, this is referred to as a
"secondary repair path". However, it is not always the case that
interference can be handled in this fashion and it is possible to
create looping repair paths.
One possibility of looping repair paths is illustrated in Figure 9.
All links have a symmetrical cost of one with the exception of HG,
which is cost 3 in either direction, and ED and DC which are cost 5
in the indicated direction and cost 1 in the other.
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[A]---//---[B]--------[C]
| | |^
| | |5
[H]-3-[G]--[F]--[E]---[D]
5>
Figure 9 Looping secondary repair paths
In this topology, A can establish a repair path to F, but cannot
establish a repair path to C because of interference. Router A might
assign traffic intended for C onto its repair path to F expecting it
to undergo a secondary repair towards C. However, because of the
asymmetrical link costs, F is unable to establish a repair path to C.
It is only able to establish a repair path to A. If F, like A,
elected to forward repaired traffic to C using its (only) repair path
to A, similarly expecting a secondary repair to get it to its
destination, traffic for C would loop between A and F. Thus when
interference occurs, the possibility of a secondary repair path
cannot be relied upon to ensure that traffic reaches its destination.
In order to determine the viability of secondary repair paths, it is
necessary for each router to take into account the repair paths which
the other neighbors of router B can achieve. These can be computed
locally by running the repair path computation algorithms rooted at
each of those neighbors. It is only necessary to compute the repair
paths from the routers to which router A can establish repair paths,
with targets of those routers to which repair paths have not yet been
established.
It is then possible to determine whether all routers can now be
reached by invoking secondary (or if necessary tertiary, etc.) repair
paths, and if so, to which primary repair path traffic for each
target should be assigned.
There is another, more subtle, possibility of loops arising when
secondary repair paths are used. This is illustrated in Figure 10,
where all links are cost 1 with the exception of JI which has a cost
5 in that direction and cost 1 in the direction IJ.
[A]---//---[B]--------[C]
| | |
| | |
[J] | [D]
5| | |
v| | |
[I]---[H]--[G]---[F]--[E]
Figure 10 Example of an apparently non-looping secondary repair path
which results in a loop.
Router A has a primary repair path to G (with a release point of I),
and G has a primary repair path to C (with a release point of E). It
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would appear that these form a non-looping secondary repair path from
A to C. As usual, the primary repair path from A to G has been
computed on the basis of destinations normally reachable through BG.
However, when making use of the secondary repair path, the traffic
inserted in the repair path from A to G will be destined not for one
of the routers normally reachable via BG, but for C. Hence this
repair path is not necessary valid for such traffic, and in this
example it will have a 50% probability of being forwarded back along
the path IJABC, and hence looping.
This problem can in general be avoided by choosing a release point
for the initial primary repair with the property that traffic for the
secondary target (C) is guaranteed to traverse the primary target
(G). This can be achieved by computing the reverse SPF rooted at the
secondary target (C) and examining the sub-tree which traverses the
primary target. It can be proved that in the absence of asymmetric
link costs, such a release point will always exist. Where asymmetric
link costs prevent this, the traffic can be encapsulated to the
intermediate router (G), which may require the use of double
encapsulation. On reaching router G, the traffic for C is
decapsulated and then forwarded in G's primary repair path to C (via
router E, in the example).
4.5. Multi-homed Prefixes
Up to this point, it has been assumed that any particular prefix is
"attached" to exactly one router in the network, and consequently
only the routers in the network need be considered when constructing
repair paths, etc. However, in many cases the same prefix will be
attached to two or more routers. Common cases are: -
o The subnet present on a link is advertised from both ends of
the link.
o Prefixes are propagated from one routing domain to another by
multiple routers.
o Prefixes are advertised from multiple routers to provide
resilience in the event of the failure of one of the routers.
In general, this causes no particular problems, and the shortest
route to each prefix (and hence which of the routers to which it is
attached should be used to reach it) is resolved by the normal SPF
process. However, in the particular case where one of the instances
of a prefix is attached to router B, or to a router for which router
B is a single point of failure, the situation is more complicated.
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P
|
|
[A]---//---[B]--------[C]
| | P
| | |
[W]-----[X]----[Y]----[Z]-[G]-[H]-[I]-[J]-[K]-[L]-[M]-[N]
Figure 11 A multi-homed prefix p
Consider a prefix p, which is attached to router B and some other
router N as illustrated in Figure 11. Before the failure of the link
A-B, p is reachable from A via A-B. After the failure it cannot be
assumed that B is still reachable. If traffic to p is assigned to a
link repair path to B (as it would be if p were attached only to B),
and router B has failed, then it would loop and subsequently be
dropped. Traffic for p cannot simply be assigned to whatever repair
path would be used for traffic to N, because other routers, which are
not yet aware of any failure, may direct the traffic back towards B,
since the instance of p attached to B is closer.
A solution is to treat p itself as a neighbor of B, and compute a
repair path with p as a target. However, although correct, this
solution may be infeasible where there are a very large number of
such prefixes, which would result in an unacceptably large
computational overhead.
Some simplification is possible where there exist a large number of
multi-homed prefixes which all share the same connectivity and
metrics. These may be treated as a single router and a single repair
path computed for the entire set of prefixes.
An alternative solution is to tunnel the traffic for a multi-homed
prefix to the router N where it is also attached (see Figure 11). If
this involves a repair path that was already tunneled, then this
requires double encapsulation.
4.6. Equal Cost Path Splits
Equal cost path splits may be used as a repair mechanism, but link
and node repairs need to be considered separately.
4.6.1. Equal Cost Path Splits as Link Repair Paths
When a link is used as a member of one or more path-split sets, by
definition, the destinations served could be equally well served by
any other member of the path-split set. Therefore, when the link
fails, any destinations that use the link as a path-split may be
immediately assigned to another member of the set. Clearly, if
traffic to some destinations can be repaired using a path split, it
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should not also be subject to repair by tunneling. Such destinations
should be identified before performing traffic assignment to tunneled
repair paths.
4.6.2. Equal Cost Path Splits and Node Failure
An equal cost path split may traverse the failed node (router B). In
this case, the path split may not be an appropriate repair path.
There are two cases: -
o the path split is a parallel link, having router B as a
direct neighbor, and
o the path split does not have router B as a direct neighbor,
but the route traverses router B at some point further
downstream.
These are illustrated in Figure 12 and Figure 13 respectively.
+---//---+
[A] [B]-------[D]
+--------+
Figure 12 A parallel link path split
+-2-//---+
[A] [B]-------[D]
+--[C]---+
Figure 13 A path split via an intermediate node
In both cases it must be assumed that router B has failed and some
other repair path, diverse with respect to router B, must be used.
4.7. LANs and pseudonodes
In link state protocols a LAN is represented by a construct known as
a pseudonode in IS-IS and a network LSA in OSPF.
In order to deal correctly with this representation of LANs, the
algorithms described in this draft require certain modifications.
There are four cases which require consideration. These are described
in the following subsections.
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4.7.1. The Link between Routers A and B is a LAN
In this case, the link which is being protected is a LAN, and the
router B which has potentially failed is reachable over the LAN. This
is illustrated in Figure 14.
[A]
|
=====================
| | | |
[B] [C] [D] [E]
Figure 14 The link between routers A and B is a LAN
There are two possible failure modes in this case.
4.7.1.1. Case 1
Router B or its interface to the LAN may have failed independently of
the rest of the LAN. In this case the remaining routers on the LAN
(routers C, D and E) will remain reachable from router A. These
routers do not appear as direct neighbors of router B in the link
state database and are not treated as neighbors of router B for the
purposes of this specification because no traffic from router A would
be directed through router B to any of these routers. However, each
of these neighboring routers will have router B as a neighbor and
they will initiate their own repair paths in the event of the failure
of router B or its LAN interface.
Repair paths are computed with the non-LAN neighbors of B as targets,
and also B itself (the "link-failure" repair path). Note that since
the remaining neighbors of A on the LAN are assumed to be still
reachable when the link to B has failed, these repair paths may
traverse the LAN.
A separate set of repair paths is required in anticipation of the
potential failure of each router on the LAN.
4.7.1.2. Case 2
Router A's interface to the LAN may have failed (or the entire LAN
may have failed). In either event, simultaneous failures will be
observed from router A to all the remaining routers on the LAN
(routers B, C, D and E). In this case, the pseudonode itself can be
treated as the "adjacent" router (i.e. the router normally referred
to as "router B"), and repairs constructed using the normal
mechanisms with all the neighbors of the pseudonode (routers B, C, D
and E) as repair path targets. If one or more of the routers had
failed in addition to the LAN connectivity, treating it as a repair
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path target would not be viable, but this would be a case of multiple
simultaneous failures which is out of scope of this specification.
The entire sub-tree over A's LAN interface is the failed component
and is excised from the spanning tree when computing A's extended P-
space. For the Q-spaces of the targets, the sub-tree over the LAN
interface of the target is excised.
4.7.1.3. Simplified LAN repair
A simpler alternative strategy is to always consider the LAN and all
routers attached to it as failing as a single unit. In this case, a
single set of repair paths is computed with targets being the entire
set of non-LAN neighbors of all the routers on the LAN, together with
"link-repair" paths with all the routers on the LAN as targets. Any
failure of one or more LAN adjacencies results in these repair paths
being invoked for all neighbors on the LAN. These repair paths must
not traverse the LAN, and so must be computed by excising the entire
sub-tree reachable over A's LAN interface from A's spanning tree
(i.e. the entire LAN is the failed component). The Q-spaces are
computed as normal, with the LAN neighbors or their interface to the
LAN being excised as appropriate. This is simpler than the approach
proposed above, but will fail to make use of possible repair paths
(or even path splits) over the LAN. In particular, if the only viable
repair paths involve the LAN, it will prevent any repair being
possible.
4.7.2. A LAN exists at the release point
When computing the viable release points, it may be that one or more
of the leaf nodes are actually pseudonodes. In this case, the release
point is deemed to be any of the parent nodes on the LAN by which the
pseudonode had been reached, and when computing the extended set of
release points (reachable by directed forwarding), all the remaining
routers on the LAN may be included.
4.7.3. A LAN between B and its neighbors
If there is a LAN between router B and one or more of B's neighbors
(other than router A), then rather than treating each of those
neighbors as a separate target to which a repair path must be
computed, the pseudonode itself can be treated as a single target for
which a repair path can be computed. If there are other neighbors of
B which are directly attached to B, including those which may also be
attached to the LAN, they must still be treated as an individual
repair path target.
Normally a repair path with the pseudonode as its target will have a
release point before the pseudonode. However it is possible that the
release point would be computed as the pseudonode itself. This will
occur if the reverse spanning tree rooted at the pseudonode includes
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no routers other than itself. In this case a single repair with the
pseudonode as target is not possible, and it is necessary to compute
individual repair paths whose target are each of the neighbors of B
on the LAN.
4.7.4. The LAN is a Transit Subnet.
This is the most common case, where a LAN is traversed by a repair
path, but is not in any of the special positions described above. In
this case no special treatment is required, and the normal SPF
mechanisms are applicable.
5. Failure Detection and Repair Path Activation
The details of repair path activation are inherently implementation-
dependent and must be addressed by individual design specifications.
This section describes the implementation independent aspects of the
failover to the repair path.
5.1. Failure Detection
The failure detection mechanism must provide timely detection of the
failure and activation of the repair paths. The failure detection
mechanisms may be media specific (for example loss of light), or may
be generic (for example BFD). Multiple detection mechanisms may be
used in order to improve detection latency. Note that in the case of
a LAN it may be necessary to monitor connectivity to all of the
adjacent routers on the LAN.
5.2. Repair Path Activation
The mechanism used by the router to activate the repair path
following failure will be implementation specific.
An implementation that is capable of withdrawing the repair may delay
the start of network convergence in order to minimize network
disruption in the event that the failure was a transient.
5.3. Node Failure Detection Mechanism
When router A detects a failure of the A-B link, it will invoke the
link repair path from itself to router B. This A-B link repair is
always invoked because even if all other traffic can be re-routed, B
is always a single point of failure to itself. If router B has
failed, the A-B link repair can result in a forwarding loop. A node
failure detection mechanism is therefore needed. A suitable mechanism
might be to run BFD [BFD] between A and B, over the A-B link repair
path.
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When the node failure detection mechanism has determined that router
B has failed it withdraws the A-B link repair path. The node failure
detection and revocation of the A-B link repair needs to be
expedited, in order to minimize the duration of collateral damage to
the network cause by packets looping around the A-B link repair path.
If B is a single point of failure to some destinations, then
withdrawing the A-B link repair has no impact on network
connectivity, because those destinations will have been rendered
unreachable by the failure of router B.
If B is not a single point of failure, but traffic to some
destinations is being repaired via the A-B link because of the
inability to provide suitable repair paths, then there are
destinations that are rendered temporarily unreachable by IPFRR. The
IPFRR loop free convergence mechanism delays normal convergence of
the network. Consideration therefore has to be given to the relative
importance of the traffic being protected and the traffic being
black-holed. Depending on the outcome of that consideration, the
IPFRR loop-free strategy may need to be abandoned.
6. Loop Free Transition
Once the repair paths have been activated, data will again be
forwarded correctly. At this stage only the routers directly adjacent
to the failure will be aware of the failure because no routing
information concerning the failure has yet been propagated to other
routers. The network now has to be transitioned to normal operation
using the available components.
During network transition inconsistent state may lead to the
formation of micro-loops. During this period, packets may be
prevented from reaching the repair path, may expire due to transiting
an excessive number of hops, may be subject to excessive delay, and
the resultant congestion may disrupt the passage of other packets
through the network. The use of a loop free transition technique
allows the network to re-converge without packet loss or disruption.
Four loop free transition strategies are described:
o Incremental cost advertisement
o Single Tunnel
o Distributed Tunnels
o Ordered SPF
6.1. Incremental Cost Advertisement
When a link fails, the cost of the link is normally changed from its
assigned metric to "infinity". However it can be proved that: if the
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link cost is increased in suitable increments, and the network is
allowed to stabilize before the next cost increment is advertised,
then no micro-loops will form.
This approach has the advantage that it requires no change to the
routing protocol, and will work with non-IPFRR capable routers.
However the loop-free transition is slow, particularly if large
metrics are used, and during this time the network is vulnerable to a
second failure.
6.2. Single Tunnel Per Router
When a failure is detected, the routers adjacent to the failure issue
a "covert" announcement of the failure, which is propagated through
the network by all routers, but which is understood only by IPFRR
capable routers. These routers each build a tunnel to the closest
IPFRR router adjacent to the failure. They then determine which of
their traffic would transit the failure and place that traffic in the
tunnel. When all of these tunnels are in place, the failure is then
announced as normal. Because the tunnel will be unaffected by the
transition, and because the IPFRR router at the tunnel endpoint will
continue the repair, no traffic will be disrupted by the failure.
When the network has converged, the IPFRR routers can withdraw the
tunnels. The order of tunnel insertion and withdrawal is not
important, provided the tunnels are all in place before the normal
announcement.
This technique has the disadvantage that it requires traffic to be
tunneled during the transition.
A further disadvantage of this method is that it requires co-
operation from all the routers within the routing domain to fully
protect the network against micro-loops. However it can be shown that
micro-loops will be confined to contiguous groups of non-IPFRR
capable routers, and will only affect traffic arriving at the network
through one of those routers.
6.3. Distributed Tunnels
This is similar to the single tunnel per router approach except that
all IPFRR capable routers calculate a set of repair paths using the
same algorithms as for traffic that will be affected by the failure.
This reduces the load on the tunnel endpoints, but the length of time
taken to calculate the repairs increases the convergence time.
This method suffers from the same disadvantages as the single tunnel
method.
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6.4. Ordered SPFs
Micro loops occur when a router closer to the failed component
revises its routes to take account of the failure before a router
which is further away. By analyzing the reverse spanning tree over
which traffic is directed to the failed component, it is possible to
determine a strict ordering which ensures that routers closer to the
root always process the failure after any routers further away, and
hence micro loops are prevented.
When the failure has been announced, each router waits a multiple of
some time delay value. The multiple is determined by the router's
position in the reverse spanning tree, and the delay value is chosen
to guarantee that a router can complete its processing within this
time. The convergence time may be reduced by employing a signaling
mechanism to notify the parent when all the children have completed
their processing, and hence when it was safe for the parent to
instantiate its new routes.
The property of this approach is therefore that it imposes a delay
which is bounded by the network diameter although in most cases it
will be much less.
It requires all routers in the domain to operate according to these
procedures, and the presence of non co-operating routers can give
rise to loops for any traffic which traverses them (not just traffic
which is originated through them).
7. Restoring Failed Components to Service
When a neighbor or failed link is restored to service, it will be
detected according to the normal operation of the routing protocols
by the formation of an adjacency. Normally this would result in the
information about the link being included in newly generated routing
information. However, just as in the case with increasing costs, the
sudden decrease in cost from "infinity" to the configured value of
the link cost may give rise to loops. Each of the loop-free
transition mechanism described above has a corresponding mechanism
that can be used to add a link to the network without the formation
of micro-loops.
8. Implications for Network Management
It will be clear from the above that topology changes introduced by
management action, such as enabling or disabling a link or router, or
changing the cost metric of a link may result in disruption of
traffic due to the formation of micro-loops. It will equally be clear
that the loop-free convergence strategies described above can equally
be applied to the prevention of such micro-loops.
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9. IPFRR Capability
In the previous sections it has been assumed that all routers in the
network are capable of acting as IPFRR routers, performing such tasks
as tunnel termination and directed forwarding. In practice this is
unlikely to be the case, partially because of the heterogeneous
nature of a practical network, and partially because of the need to
progressively deploy such capability. IPFRR therefore needs to
support some form of capability announcement, and the algorithms need
to take these capabilities into account when calculating their path
repair strategies. For example, the ability of routers to function as
tunnel end points and perform directed forwarding will influence the
choice of repair path. However, routers which are simply traversed by
repair paths (tunneled or not) do not need to be IPFRR capable in
order to guarantee correct operation of the repair paths.
10. Enhancements to routing protocols
It will be seen from the above that a number of enhancements to the
appropriate routing protocols are needed to support IPFRR. The
following possible enhancements have been identified:
o The ability to advertise IPFRR capability
o The ability to advertise tunnel endpoint capability
o The ability to advertise directed forwarding identifiers
o The ability to announce the start of a loop-free transition,
and to abort a loop-free transition.
o The ability to signal transition completion status to
neighbors.
o The ability to advertise that a link is protected.
Capability advertisement should make use of existing capability
mechanisms in the routing protocols. The exact set of enhancements
will depend on specific IPFRR design choices.
11. IANA considerations
There are no IANA considerations that arise from this architectural
description of IPFRR. However there will be changes to the IGPs to
support IPFRR in which there will be IANA considerations.
12. Security Considerations
Changes to the IGPs to support IPFRR do not introduce any additional
security risks.
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The security implications of the increased convergence time due to
the loop avoidance strategy depend on the approach to multiple
failures. If the presence of multiple failures results in the network
aborting the loop free strategy, then the convergence time will be
similar to that of a conventional network. On the other hand, an
attacker in a position to disrupt part of a network might use this to
disrupt the repair of a critical path.
The tunnel endpoints need to be secured to prevent their use as a
facility by an attacker. Performance considerations indicate that
tunnels cannot be secured by IPsec [IPSEC]. A system of packet
address policing, both at the tunnel endpoints and at the edges of
the network would prevent an attacker's packet arriving at a tunnel
endpoint and would seem to be the best strategy.
When a fast re-route is in progress, there may be an unacceptable
increase in traffic load over the repair path. Network operators need
to examine the computed repair paths and ensure that they have
sufficient capacity.
Acknowledgments
The authors acknowledge the significant technical contributions made
to this work by their colleagues: John Harper and Kevin Miles.
IPR Disclosure Acknowledgement
By submitting this Internet-Draft, we certify that any applicable
patent or other IPR claims of which we are aware have been disclosed,
and any of which we become aware will be disclosed, in accordance
with RFC 3668.
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/
BFD Katz, D., and Ward, D., "Bidirectional Forwarding
Detection", draft-katz-ward-bfd-01.txt, August
2003 (work in progress).
IPSEC Kent, S., Atkinson, R., "Security Architecture
for the Internet Protocol", RFC 2401
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Authors' Addresses
Stewart Bryant
Cisco Systems,
250, Longwater Avenue,
Green Park,
Reading, RG2 6GB,
United Kingdom. Email: stbryant@cisco.com
Clarence Filsfils
Cisco Systems,
De Kleetlaan 6a,
1831 Diegem,
Belgium Email: cfilsfil@cisco.com
Stefano Previdi
Cisco Systems,
Via Del Serafico 200
00142 Roma,
Italy Email: sprevidi@cisco.com
Mike Shand
Cisco Systems,
250, Longwater Avenue,
Green Park,
Reading, RG2 6GB,
United Kingdom. Email: mshand@cisco.com
Full Copyright statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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