INTERNET DRAFT IP Fast-reroute Using Tunnels Oct 2004
Network Working Group S. Bryant
Internet Draft C. Filsfils
Expiration Date: Apr 2005 S. Previdi
M. Shand
Cisco Systems
Oct 2004
IP Fast Reroute using tunnels
draft-bryant-ipfrr-tunnels-01.txt
Status of this Memo
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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.
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Table of Contents
1. Introduction......................................................4
2. Goals, non-goals, limitations and constraints.....................4
2.1. Goals.........................................................4
2.2. Non-Goals.....................................................5
2.3. Limitations...................................................5
2.4. Constraints...................................................5
3. Repair Paths......................................................6
3.1. Tunnels as Repair Paths.......................................6
3.2. Tunnel Requirements...........................................9
3.2.1. Setup.....................................................9
3.2.2. Multipoint................................................9
3.2.3. Directed forwarding.......................................9
3.2.4. Security..................................................9
4. Construction of Repair Paths.....................................10
4.1. Identifying Repair Path Targets..............................10
4.2. Determining Tunneled Repair Paths............................10
4.2.1. Computing Repair Paths...................................11
4.2.2. Extended F-space.........................................12
4.2.3. Loop-free Alternates.....................................12
4.2.4. Selecting Repair Paths...................................12
4.3. Assigning Traffic to Repair Paths............................13
4.4. When no Repair Path is Possible..............................13
4.4.1. Unreachable Target.......................................14
4.4.2. Asymmetric Link Costs....................................14
4.4.3. Interference Between Potential Node Repair Paths.........14
4.5. Multi-homed Prefixes.........................................17
4.6. LANs and pseudo-nodes........................................18
4.6.1. The Link between Routers S and E is a LAN................19
4.6.1.1. Case 1...............................................19
4.6.1.2. Case 2...............................................19
4.6.1.3. Simplified LAN repair................................20
4.6.2. A LAN exists at the release point........................20
4.6.3. A LAN between E and its neighbors........................20
4.6.4. The LAN is a Transit Subnet..............................21
5. Failure Detection and Repair Path Activation.....................21
5.1. Failure Detection............................................21
5.2. Repair Path Activation.......................................21
5.3. Node Failure Detection Mechanism.............................21
6. Loop Free Transition.............................................22
7. IPFRR Capability.................................................22
8. Enhancements to routing protocols................................23
9. IANA considerations..............................................23
10. Security Considerations.........................................23
Terminology
This draft uses the terms defined in [FRMWK]. This section defines
additional words, acronyms, and actions used in this draft.
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Directed The ability of the repairing router (S) to
forwarding specify the next hop (G) on exit from a
tunnel end-point (F)
Extended F- The union of the F-space of the neighbors of
space a specific router with respect to a common
component.
Extended F-space does not include the
additional space reachable though directed
forwarding.
F The router in F-space to which a packet is
tunneled for repair.
FG A router that is in both F and G space and
hence does not need directed forwarding.
F-space F-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 F-space of S, is the set of
routers that S can reach without using E
(router failure case) or the S-E link
failure case).
G The router in G space, to which the packet
is directed by router F on exit from the
repair tunnel. G will always be adjacent to
F, or F itself.
G-space G-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.
Interference The condition where the network costs are
such that a repairing router cannot tunnel a
packet sufficiently far from a failed node
such that it is not attracted back to the
failed node via another of that node's
neighbors.
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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.
A framework for IP Fast Reroute [IPFRR] provides a summary of the
proposed IPFRR solutions, and a partial solution using equal cost
multi-path and loop-free alternate case is described in [BASIC].
This draft describes extensions to the basic repair mechanism in
which we propose the use of tunnels to provide additional logical
downstream paths. These mechanisms provide almost 100% repair
connectivity in practical networks.
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 used to achieve the repair:
1. Equal cost path-split.
2. Loop-free Alternate.
3. Tunnel.
The operation of equal cost path-split and loop-free alternate is
described in [BASIC]. 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.
This is equivalent to providing a virtual loop-free alternate to
supplement the physical loop-free alternates.
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.
Repair Path from S to E
+-----------+
| |
| v
---->[S]---//----[E]----->
Figure 1 Simple Link Repair
Creating a repair path from S to E may require a packet to traverse
an unnatural route. If a suitable natural path starts at a neighbor
(i.e. it is a loop-free alternate), then S can force the packet
directly there. If this is not the case, then S may create one by
using a tunnel to carry the packet to a point in the network where
there is a real loop-free alternate. Note that the tunnel does not
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have to go from S to E. The tunnel can terminate at any router in
the network, provided that S 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 S to E
+-------------------------+
| |
| <------------+
--->[S]---//----[E]----//-----[S1]-->
+----------> |
| |
+-------------------------+
Repair Path from S1 to E
Figure 2 Looping Link Repair when Router Fails
Consider the case of a router E with just two neighbors S and S1.
When router E fails, both S and S1 will observe the failure of
their local link to E, but will have no immediate knowledge that E
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 S (Figure 3)
pre-computes a set of tunneled repair paths, one for each of the
neighbors (S1, S2 and S3) of its neighbor E on the S-E link. The
set of destinations that are normally assigned to link S-E will be
assigned to a repair path based on the neighbor of E through which
router E would have forwarded traffic to them.
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Repair S-S1
+----------->[S1]
| |
| |
| |
----->[S]----//-----[E]---------[S2]
|| | ^
|| | |
||Repair S-S3 | |
|+---------->[S3] |
| |
+-------------------------+
Repair S-S2
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
E itself. This is important in cases where E is a single point of
failure and E is still functional (i.e. the failure was actually a
failure of the S-E 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 S-E
+----------------+
| |
| Repair S-S1 |
|+---------->[S1]|
|| | /
|| | /
|| |/
----->[S]----//-----[E]---------[S2]
|| | ^
|| | |
||Repair S-S3 | |
|+---------->[S3] |
| |
+-------------------------+
Repair S-S2
Figure 4 The full set of S-E repair paths.
In the event of a failure, the only traffic that is assigned to the
link repair path (the S-E repair) is that traffic which has no
other path to its destination except via E. As we have already
seen, there is a danger that traffic assigned to this link repair
path may loop if E has failed, therefore, when the repair paths are
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invoked, a loop detection mechanism is used which promptly detects
the loop and, upon detection, withdraws the link (S-E) repair path
from service.
3.2. Tunnel Requirements
There are a number of IP in IP tunnel mechanisms that may be used
to fulfill the requirements of this design. Suitable candidates
include IP-in-IP [RFC1853], GRE [RFC1701] and L2TPv3 [L2TPv3]. The
selection of the specific tunneling mechanism (and any necessary
enhancements) 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 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 (F) can be directed by the router at the tunnel
source (S) to forward the packet directly to a specific neighbor.
Specification of the directed forwarding mechanism is outside the
scope of this document. Directed forwarding might be provided using
an enhancement to the IP tunneling encapsulation, or it might be
provided through the use of a single MPLS label stack entry
[RFC3032] interposed between the IP tunnel header and the packet
being repaired.
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 10.
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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
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 be
determined by examining the normal shortest path tree (SPT)
computed by the repairing router.
In addition, the neighboring router E will also be the target of a
repair path for any destinations for which E 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 S's intended repair path for traffic to D when
link S-E fails is the path W-X-Y-Z-S1. However, if router S makes
S1 be the tunnel endpoint and forwards the packet to router W,
router W will immediately return it to S because its least cost
path to S1 is S-E-S1 (cost 3 versus cost 4) and has no knowledge of
the failure of link S-E.
[S]--//--[E]-----[S1]
| |
| |
[W]---[X]---[Y]---[Z]
Figure 5. Repair path to target router S1.
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 S. 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
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was created. If, for example, in Figure 5, if a packet's
destination were normally reached via the path S-E-S1-Z-?-?-?, once
it was released at any of the possible tunnel endpoints, it would
arrive at its destination by the best available route without
traversing S1.
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
o when 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
decapsulated 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 S using normal forwarding, and directed forwarding is then
used to force packets to router Y, from where S1 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, and that the tunnel
endpoint (F) and the release point (G) will be coincident, or may
be separated by at most one hop.
4.2.1. Computing Repair Paths
For a router S, determining tunneled repair paths around a
neighboring router E, the set of potential tunnel end points
includes all the routers that can be reached from S using normal
forwarding without traversing the failed link S-E. This is termed
the "F-space" of S with respect to the failure of E. Any router
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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 S for the
failure of link S-E. This set can be obtained by computing an SPT
rooted at S and excising the sub-tree reached via the S-E 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 "G-space" of the
target with respect to the failure. The G-space can be obtained by
computing a reverse shortest path tree (rSPT) rooted at the repair
path target, with the sub-tree which traverses the failed link (or
node) excised. The rSPT 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 G-space with S's F-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, F in S's F-space and G in the target's
G-space, router F can be used as the tunnel endpoint with directed
forwarding to the release point G.
4.2.2. Extended F-space
The description in section 4.2.1 calculated router S's F-space
rooted at S itself. However, since router S will only use a repair
path when it has detected the failure of the link S-E, the initial
hop of the repair path need not be subject to S's normal forwarding
decision process. Thus we introduce the concept of extended F-
space. Router S's extended F-space is the union of the F-spaces of
each of S's neighbors. The use of extended F-space may allow router
S to repair to targets that were otherwise unreachable.
4.2.3. Loop-free Alternates
When a loop-free alternate exists, no tunneling is required.
4.2.4. Selecting Repair Paths
The mechanisms described above 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
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.
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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 E would be used to
reach each destination reachable over S-E when running the original
SPF. Traffic assignment is then simply a matter of assigning the
traffic which E 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 E 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 E itself is normally used just for traffic
destined for E and any prefixes advertised by E. However, under
some circumstances, it may be impossible to compute a repair path
to one or more of E's neighbors, for example, because E 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 E as its target, in the optimistic assumption that router E is
still functioning. If router E is indeed still functioning, this
will ensure delivery of the traffic. If, however, router E 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, is 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.
o Severely asymmetric link costs may cause an otherwise
viable physical repair path to be unusable.
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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 6, in which all links have a symmetrical
cost of one, with the exception of that between H and I, which has
a cost of 3. In this example, the fact that router J is a neighbor
of E prevents the discovery of a repair path from router S to
router S1 despite the existence of an apparently suitable path.
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[S]---//---[E]-------[S1]
| | |
| | |
[H]-3-[I]--[J]--[K]--[L]
Figure 6. Interference between repair paths
A repair path from router S to J can use J itself as the release
point by employing directed forwarding from I. However, it is not
possible to identify a suitable release point for a repair path to
router S1 within the topology shown since there is nowhere that
router S can reach that will subsequently forward traffic to
router S1 except via the forbidden link E-S1 (J's least cost path
to S1 is J-E-S1). This is because the extended F-space of router S
is separated by more than one hop from the G-space of router S1.
Since the topology shown in Figure 6 will typically form part of a
much larger topology, a different, and possibly more circuitous
repair path from S to S1, that does not go via J, may be
discovered. This is illustrated in Figure 7. In this enhanced
topology, a repair path to S1 using Y as the release point can be
used.
[S]---//---[B]-------[S1]
| | |
| | |
[H]-3-[I]--[J]--[K]--[L]
| |
| |
[X]--[Y]--[Z]
Figure 7. Resolving interference in a larger network
Note that, in Figure 6, if the traffic for S1 were assigned to the
repair path for J, it would correctly reach S1 because J would
assign it to its repair path to S1. That is, packets from S to S1
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 8.
All links have a symmetrical cost of one with the exception of H-I,
which is cost 3 in both directions, and K-L and L-S1 which are cost
5 in the indicated direction and cost 1 in the other.
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[S]---//---[E]--------[S1]
| | |^
| | |5
[H]-3-[I]--[J]--[K]---[L]
5>
Figure 8 Looping secondary repair paths
In this topology, S can establish a repair path to J, but cannot
establish a repair path to S1 because of interference. Router S
might assign traffic intended for S1 onto its repair path to J
expecting it to undergo a secondary repair towards S1. However,
because of the asymmetrical link costs, J is unable to establish a
repair path to S1. It is only able to establish a repair path to S.
If J, like S, elected to forward repaired traffic to S1 using its
(only) repair path to S, similarly expecting a secondary repair to
get it to its destination, traffic for S1 would loop between S
and J. 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 E 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 S 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 9,
where all links are cost 1 with the exception of L-K which has a
cost 5 in that direction and cost 1 in the direction K-L.
[S]---//---[E]--------[S1]
| | |
| | |
[L] | [D]
5| | |
v| | |
[K]---[J]--[I]---[H]--[E]
Figure 9 Example of an apparently non-looping secondary repair path
which results in a loop.
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Router S has a primary repair path to I (with a release point
of K), and I has a primary repair path to S1 (with a release point
of E). It would appear that these form a non-looping secondary
repair path from S to S1. As usual, the primary repair path from S
to I has been computed on the basis of destinations normally
reachable through E-I. However, when making use of the secondary
repair path, the traffic inserted in the repair path from S to I
will be destined not for one of the routers normally reachable via
E-I, but for S1. 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 K-L-S-E-S1, 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 (S1) is guaranteed to traverse the primary
target (I). This can be achieved by computing the rSTF rooted at
the secondary target (S1) 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 (I), which may require the
use of double encapsulation. On reaching router I, the traffic for
S1 is decapsulated and then forwarded in I's primary repair path to
S1 (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 E, or to a router for which
router E is a single point of failure, the situation is more
complicated.
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P
|
|
[S]---//---[E]--------[S1]
| | p
| | |
[W]-----[X]----[Y]----[Z]-[I]-[J]-[K]-[L]-[M]-[N]
Figure 10 A multi-homed prefix p
Consider a prefix p, which is attached to router E and some other
router N as illustrated in Figure 10. Before the failure of the
link S-E, p is reachable from S via S-E. After the failure it
cannot be assumed that E is still reachable. If traffic to p is
assigned to a link repair path to E (as it would be if p were
attached only to E), and router E 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 E, since the instance of p attached
to E is closer.
A solution is to treat p itself as a neighbor of E, 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 10).
If this involves a repair path that was already tunneled, then this
requires double encapsulation.
4.6. LANs and pseudo-nodes
In link state protocols a LAN is represented by a construct known
as a pseudo-node 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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4.6.1. The Link between Routers S and E is a LAN
In this case, the link which is being protected is a LAN, and the
router E which has potentially failed is reachable over the LAN.
This is illustrated in Figure 11.
[S]
|
=====================
| | | |
[E] [X] [Y] [Z]
Figure 11 The link between routers S and E is a LAN
There are two possible failure modes in this case.
4.6.1.1. Case 1
Router E 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 X, Y and Z) will remain reachable from router S. These
routers do not appear as direct neighbors of router E in the link
state database and are not treated as neighbors of router E for the
purposes of this specification because no traffic from router S
would be directed through router E to any of these routers.
However, each of these neighboring routers will have router E as a
neighbor and they will initiate their own repair paths in the event
of the failure of router E or its LAN interface.
Repair paths are computed with the non-LAN neighbors of E as
targets, and also E itself (the "link-failure" repair path). Note
that since the remaining neighbors of S on the LAN are assumed to
be still reachable when the link to E 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.
4.6.1.2. Case 2
Router S'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 S to all the remaining routers on the LAN
(routers E, X, Y and Z). In this case, the pseudo-node itself can
be treated as the "adjacent" router (i.e. the router normally
referred to as "router E"), and repairs constructed using the
normal mechanisms with all the neighbors of the pseudo-node
(routers E, X, Y and Z) as repair path targets. If one or more of
the routers had failed in addition to the LAN connectivity,
treating it as a repair path target would not be viable, but this
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would be a case of multiple simultaneous failures which is out of
scope of this specification.
The entire sub-tree over S's LAN interface is the failed component
and is excised from the SPT when computing S's extended F-space.
For the G-spaces of the targets, the sub-tree over the LAN
interface of the target is excised.
4.6.1.3. 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 S's LAN
interface from S's SPT (i.e. the entire LAN is the failed
component). The G-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.
4.6.2. 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 pseudo-nodes. In this case, the
release point is deemed to be any of the parent nodes on the LAN by
which the pseudo-node 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.
4.6.3. A LAN between E and its neighbors
If there is a LAN between router E and one or more of E's neighbors
(other than router S), then rather than treating each of those
neighbors as a separate target to which a repair path must be
computed, the pseudo-node itself can be treated as a single target
for which a repair path can be computed. If there are other
neighbors of E which are directly attached to E, 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 pseudo-node as its target will have
a release point before the pseudo-node. However it is possible that
the release point would be computed as the pseudo-node itself. This
will occur if the rSPT rooted at the pseudo-node includes no
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routers other than itself. In this case a single repair with the
pseudo-node as target is not possible, and it is necessary to
compute individual repair paths whose target are each of the
neighbors of E on the LAN.
4.6.4. 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 S detects a failure of the S-E link, it will invoke the
link repair path from itself to router S. This S-E link repair is
always invoked because even if all other traffic can be re-routed,
E is always a single point of failure to itself. If router E has
failed, the S-E 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 S and E, over the S-E
link repair path.
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When the node failure detection mechanism has determined that
router E has failed it withdraws the S-E link repair path. The node
failure detection and revocation of the S-E link repair needs to be
expedited, in order to minimize the duration of collateral damage
to the network cause by packets looping around the S-E link repair
path.
If E is a single point of failure to some destinations, then
withdrawing the S-E link repair has no impact on network
connectivity, because those destinations will have been rendered
unreachable by the failure of router E.
If E is not a single point of failure, but traffic to some
destinations is being repaired via the S-E 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. A loop free transition technique
which allows the network to re-converge without packet loss or
disruption is therefore required.
A number of suitable loop-free convergence techniques are described
in [LVCONV].
7. 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,
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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.
8. 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.
9. 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.
10. Security Considerations
Changes to the IGPs to support IPFRR do not introduce any
additional security risks.
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.
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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.
Intellectual Property Statement
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this standard. Please address the information to the IETF at
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Full copyright statement
Copyright (C) The Internet Society (2004). 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
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REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
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EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
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ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
Normative References
There are no normative references.
Informative References
Internet-drafts are works in progress available from
http://www.ietf.org/internet-drafts/
[BASIC] Alia Atlas, Ed., et al., "Basic Specification
for IP Fast-Reroute: Loop-free Alternates",
<draft-ietf-rtgwg-ipfrr-spec-base-01.txt>,
October 2004, (work in progress).
[BFD] Katz, D., and Ward, D., "Bidirectional
Forwarding Detection", <draft-katz-ward-bfd-
01.txt>, August 2003 (work in progress).
[IPFRR] Shand, M., "IP Fast-reroute Framework",
<draft-ietf-rtgwg-ipfrr-framework-02.txt>,
October 2004, (work in progress).
[IPSEC] Kent, S., Atkinson, R., "Security Architecture
for the Internet Protocol", RFC 2401
[L2TPv3] J. et al., "Layer Two Tunneling Protocol
(Version 3)", <draft-ietf-l2tpext-l2tp-base-
14.txt>, June 2004, (work in progress).
[LFCONV] Bryant, S., Shand, M., "A Framework for Loop-
free Convergence", <draft-bryant-shand-lf-conv-
frmwk-00.txt>, October 2004,(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> (work in
progress).
[RFC1701] S. Hanks. et al.,RFC-1701,"Generic Routing
Encapsulation (GRE)". October 1994.
[RFC1853] Simpson, W., RFC-1853, "IP in IP Tunneling".
October 1995.
[RFC3032] Rosen E., et al., RFC-3032, "MPLS Label Stack
Encoding", January 2001.
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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
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