INTERNET DRAFT IP Fast Reroute Using Not-via Addresses March 2006
Network Working Group S. Bryant
Internet Draft M. Shand
Expiration Date: September 2006 S. Previdi
Cisco Systems
March 2006
IP Fast Reroute Using Not-via Addresses
<draft-bryant-shand-ipfrr-notvia-addresses-02.txt>
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Abstract
This draft describes a mechanism that provides fast reroute in an
IP network through encapsulation to "not-via" addresses. A single
level of encapsulation is used. The mechanism protects unicast,
multicast and LDP traffic against link, router and shared risk
group failure, regardless of network topology and metrics.
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Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119 [RFC2119].
Table of Contents
1. Introduction........................................................3
2. Overview of Not-via Repairs.........................................3
3. Repair Computation..................................................5
4. How a repairing node repairs........................................6
4.1 Node Failure.................................................. ...6
4.2 Link Failure.................................................. ...6
4.3 Multi-homed Prefix............................................. ..7
4.4 Shared Risk Link Groups........................................ ..8
4.5 Multicast...................................................... .12
4.6 Fast Reroute in an MPLS LDP Network............................ .12
5. Local Area Networks................................................12
5.1 Simple LAN Repair.............................................. .13
5.2 LAN Component Repair........................................... .14
5.3 LAN Repair Using Diagnostics................................... .15
6. Loop Free Alternates...............................................15
6.1 Optimizing not-via computations using LFAs..................... .16
6.2 Use of LFAs with SRLGs.......................................... 17
7. Equal Cost Multi-Path..............................................17
8. Multiple Simultaneous Failures.....................................17
9. Encapsulation......................................................17
10. Routing Extensions................................................17
11. Incremental Deployment............................................18
12. IANA considerations...............................................18
13. Security Considerations...........................................18
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1. Introduction
When a link or a router fails, only the neighbors of the failure
are initially aware that the failure has occurred. In a network
operating IP fast reroute (IPFRR), the routers that are the
neighbors of the failure repair the failure. These repairing
routers have to steer packets to their destinations despite the
fact that most other routers in the network are unaware of the
nature and location of the failure.
A common limitation in most IPFRR mechanisms is an inability to
steer the repaired packet round an identified failure. The extent
to which this limitation affects the repair coverage is topology
dependent. The mechanism proposed here is to encapsulate the
packet to an address that explicitly identifies the network
component that the repair must avoid. This produces a repair
mechanism, which, provided the network in not partitioned by the
failure, will always achieve a repair.
2. Overview of Not-via Repairs
The purpose of a repair is to deliver packets to their
destination without traversing a known failure in the network,
i.e. to deliver the packet not via the failure. A special address
is assigned to each protected component. This address is called
the not-via address. The semantics of a not-via address are that
a packet addressed to a not-via address must be delivered to the
router advertising that address, not via the protected component
(link, node, SRLG etc.) with which that address is associated.
A simple example would be node repair in which an additional
address is assigned to each interface in the network. To repair a
failure, the repairing router encapsulates the packet to the not-
via address of the router interface on the far side of the
failure. The routers on the repair path then know to which router
they must deliver the packet, and which network component they
must avoid. The network fragment shown in Figure 1 illustrates a
not-via repair for the case of a router failure.
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A
| Bp is the address to use to get
| a packet to B not-via P
|
S----------P----------B. . . . . . . . . .D
\ | Bp^
\ | |
\ | |
\ C |
\ |
----------------+
Repair to Bp
Figure 1: Not-via repair of router failure
Assume that S has a packet for some destination D that it would
normally send via P and B, and that S suspects that P has failed.
S encapsulates the packet to Bp. The path from S to Bp is the
shortest path from S to B not going via P. If the network
contains a path from S to B that does not transit router P, i.e.
the network is not partitioned by the failure of P, then the
packet will be successfully delivered to B. When the packet
addressed to Bp arrives at B, B removes the encapsulation and
forwards the repaired packet towards its final destination.
Note that if the path from B to the final destination includes
one or more nodes that are included in the repair path, a packet
may back track after the encapsulation is removed. However,
because the decapsulating router is always closer to the packet
destination than the encapsulating router, the packet will not
loop.
For complete protection, all of P's neighbors will require a not-
via address that allows traffic to be directed to them without
traversing P. This is shown in Figure 2.
A
|Ap
|
Sp Pa|Pb
S----------P----------B
Ps|Pc Bp
|
Cp|
C
Figure 2: The set of Not-via P Addresses
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3. Repair Computation
The not-via repair mechanism requires that all routers on the
path from S to B (Figure 1) have a route to Bp. They can
calculate this by failing node P, running an SPF, and finding the
shortest route to B.
A router has no simple way of knowing whether it is on the
shortest path for any particular repair. It is therefore
necessary for every router to calculate the path it would use in
the event of any possible router failure. Each router therefore
fails every router in the network, one at a time, and calculates
its own best route to each of the neighbors of that router. In
other words, with reference to Figure 2, some router X will
consider each router in turn to be P, fail P, and then calculate
its own route to each of the not-via P addresses advertised by
the neighbors of P. i.e. X calculates its route to Sp, Ap, Bp,
and Cp, in each case, not via P.
To calculate the repair paths a router has to calculate n-1 SPFs
where n is the number of routers in the network. This is
expensive to compute. However, the problem is amenable to a
solution in which each router (X) proceeds as follows. X first
calculates the base topology with all routers functional and
determines its normal path to all not-via addresses. This can be
performed as part of the normal SPF computation. For each router
P in the topology, X then performs the following actions:-
1. Removes router P from the topology.
2. Performs an incremental SPF on the modified topology. This
incremental calculation is terminated when all of the not-
via P addresses are attached to the SPT.
3. Reverts to the base topology.
This algorithm is significantly less expensive than a set of full
SPFs. Thus, although a router has to calculate the repair paths
for n-1 failures, the computational effort is much less than n-1
SPFs.
Experiments on a selection of real world network topologies with
between 40 and 400 nodes suggest that the worst-case
computational complexity using the above optimizations is
equivalent to performing between 5 and 13 full SPFs. Further
optimizations are described in section 6.1
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4. How a repairing node repairs
This section explains the operation of each type of repair
necessary in the network.
4.1 Node Failure
When router P fails (Figure 2) S encapsulates any packet that it
would send to B via P to Bp, and then sends the encapsulated
packet on the shortest path to Bp. S follows the same procedure
for routers A, and C in Figure 2. The packet is decapsulated at
the repair target (A, B or C) and then forwarded normally to its
destination. The repair target can be determined as part of the
normal SPF by recording the "next-next-hop" for each destination
in addition to the normal next-hop.
Notice that with this technique only one level of encapsulation
is needed, and that it is possible to repair ANY failure
regardless of link metrics and any asymmetry that may be present
in the network. The only exception to this is where the failure
was a single point of failure that partitioned the network, in
which case ANY repair is clearly impossible.
4.2 Link Failure
The normal mode of operation of the network would be to assume
router failure. However, where some destinations are only
reachable through the failed router, it is desirable that an
attempt be made to repair to those destinations by assuming that
only a link failure has occurred.
To perform a link repair, S encapsulates to Ps (i.e. it instructs
the network to deliver the packet to P not-via S). All of the
neighbors of S will have calculated a path to Ps in case S itself
had failed. S could therefore give the packet to any of its
neighbors (except, of course, P). However, S should preferably
send the encapsulated packet on the shortest available path to P.
This path is calculated by running an SPF with the link SP
failed. Note that this may again be an incremental calculation,
which can terminate when address Ps has been reattached.
It is necessary to consider the behavior of IPFRR solutions when
a link repair is attempted in the presence of node failure. In
its simplest form the not-via IPFRR solution prevents the
formation of loops forming as a result of mutual repair, by never
providing a repair path for a not-via address. Referring to
Figure 2, if A was the neighbor of P that was on the link repair
path from S to P, and P itself had failed, the repaired packet
from S would arrive at A encapsulated to Ps. A would have
detected that the AP link had failed and would normally attempt
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to repair the packet. However, no repair path is provided for any
not-via address, and so A would be forced to drop the packet,
thus preventing the formation of loop.
4.3 Multi-homed Prefix
A multi-homed Prefix (MHP) is reachable via more than one router
in the network. When IPFRR router S (Figure 3) discovers that P
has failed, it needs to send MHP packets addressed to X, which
are normally reachable through P, to an alternate router, which
is still able to reach X.
X X X
| | |
| | |
| Sp |Pb |
Z...............S----------P----------B...............Y
Ps|Pc Bp
|
Cp|
C
Figure 3: Multi-home Prefixes
S should choose the closest router that can reach X during the
failure as the alternate router. S determines which router to use
as the alternate while running the SPF with P failed. This is
accomplished by continuing to run the incremental SPF with P
failed until all of P's not-via addresses and its MHPs (X) are
attached.
First, consider the case where the shortest alternate path to X
is via Z. S can reach Z without using the failed router P.
However, S cannot just send the packet towards Z, because the
other routers in the network will not be aware of the failure of
P, and may loop the packet back to S. S therefore encapsulates
the packet to Z (using a normal address for Z). When Z receives
the encapsulated packet it removes the encapsulation and forwards
the packet to X.
Now consider the case where the shortest alternate path to X is
via Y, which S reaches via P and B. To reach Y, S must first
repair the packet to B using the normal not-via repair mechanism.
To do this S encapsulates the packet for X to Bp. When B receives
the packet it removes the encapsulation and discovers that the
packet is intended for MHP X. The situation now reverts to the
previous case, in which the shortest alternate path does not
require traversal of the failure. B therefore follows the
algorithm above and encapsulates the packet to Y (using a normal
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address for Y). Y removes the encapsulation and forwards the
packet to X.
It may be that the cost of reaching X using local delivery from
the alternate router is greater than the cost of reaching X via
P. Under those circumstances, the alternate router would normally
forward to X via P, which would cause the IPFRR repair to loop.
To prevent the repair from looping the alternate router must
locally deliver a packet received via a repair encapsulation.
Notice that using the not-via approach, only one level of
encapsulation was needed to repair MHPs to the alternate router.
4.4 Shared Risk Link Groups
A Shared Risk Link Group (SRLG) is a set of links whose failure
can be caused by a single action such as a conduit cut or line
card failure. When repairing the failure of a link that is a
member of an SRLG, it must be assumed that all the other links
that are also members of the SRLG have also failed. Consequently,
any repair path must be computed to avoid not just the adjacent
link, but also all the links which are members of the same SRLG.
In Figure 4 below, the links S-P and A-B are both members of
SRLG "a". The semantics of the not-via address Ps changes from
simply "P not-via the link S-P" to be "P not-via the link S-P or
any other link with which S-P shares an SRLG" In Figure 4 this is
the links that are members of SRLG "a". I.e. links S-P and A-B.
Since the information about SRLG membership of all links is
available in the Link State Database, all nodes computing routes
to the not-via address Ps can infer these semantics, and perform
the computation by failing all the links in the SRLG when running
the iSPF.
Note that it is not necessary for S to consider repairs to any
other nodes attached to members of the SRLG (such as B). It is
sufficient for S to repair to the other end of the adjacent link
(P in this case).
a Ps
S----------P---------D
| |
| a |
A----------B
| |
| |
C----------E
Figure 4: Shared Risk Link Group
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In some cases, it may be that the links comprising the SRLG occur
in series on the path from S to the destination D, as shown
in Figure 5. In this case, multiple consecutive repairs may be
necessary. S will first repair to Ps, then P will repair to Dp.
In both cases, because the links concerned are members of SRLG
"a" the paths are computed to avoid all members of SRLG "a".
a Ps a Dp
S----------P---------D
| | |
| a | |
A----------B |
| | |
| | |
C----------E---------F
Figure 5: Shared Risk Link Group members in series
While the use of multiple repairs in series introduces some
additional overhead, these semantics avoid the potential
combinatorial explosion of not-via addresses that could otherwise
occur.
Note that although multiple repairs are used, only a single level
of encapsulation is required. This is because the first repair
packet is de-capsulated before the packet is re-encapsulated
using the not-via address corresponding to the far side of the
next link which is a member of the same SRLG. In some cases the
de-capsulation and re-encapsulation takes place (at least
notionally) at a single node, while in other cases, these
functions may be performed by different nodes. This scenario is
illustrated in Figure 6 below.
a Ps a Dg
S----------P---------G--------D
| | | |
| a | | |
A----------B | |
| | | |
| | | |
C----------E---------F--------H
Figure 6: Shared Risk Link Group members in series
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In this case, S first encapsulates to Ps, and node P decapsulates
the packet and forwards it "native" to G using its normal FIB
entry for destination D. G then repairs the packet to Dg.
It can be shown that such multiple repairs can never form a loop
because each repair causes the packet to move closer to its
destination.
It is often the case that a single link may be a member of
multiple SRLGs, and those SRLG may not be isomorphic. This is
illustrated in Figure 7 below.
ab Ps a Dg
S----------P---------G--------D
| | | |
| a | | |
A----------B | |
| | | |
| b | | b |
C----------E---------F--------H
| |
| |
J----------K
Figure 7: Multiple Shared Risk Link Groups
The link SP is a member of SRLGs "a" and "b". When a failure of
the link SP is detected, it must be assumed that BOTH SRLGs have
failed. Therefore the not-via path to Ps must be computed by
failing all links which are members of SRLG "a" or SRLG "b". I.e.
the semantics of Ps is now "P not-via any links which are members
of any of the SRLGs of which link SP is a member". This is
illustrated in Figure 8 below.
ab Ps a Dg
S----/-----P---------G---/----D
| | | |
| a | | |
A----/-----B | |
| | | |
| b | | b |
C----/-----E---------F---/----H
| |
| |
J----------K
Figure 8: Topology used for repair computation for link S-P
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In this case, the repair path to Ps will be S-A-C-J-K-E-B-P. It
may appear that there is no path to D because GD is a member of
SRLG "a" and FH is a member of SRLG "b". This is true if BOTH
SRLGs "a" and "b" have in fact failed. But that would be an
instance of multiple uncorrelated failures which are out of scope
for this design. It practice it is likely that either SRLG "a" or
SRLG "b" has failed, but these were indistinguishable from the
point of view of S. However, each link repair is considered
independently. So, when the packet arrives at G, if only SRLG "b"
has failed it will be delivered across the link GD, while if only
SRLG "a" has failed it will be repaired around the path G-F-H-D.
This is illustrated in Figure 9 below.
ab Ps a Dg
S----/-----P---------G---/----D
| | | |
| a | | |
A----/-----B | |
| | | |
| b | | b |
C----------E---------F--------H
| |
| |
J----------K
Figure 9: Topology used for repair computation for link G-D
A repair strategy that assumes the worst-case failure for each
link can often result in longer repair paths than necessary. In
cases where only a single link fails, rather than the full SRLG,
this strategy may occasionally fail to identify a repair even
though a viable repair path exists in the network. The use of
sub-optimal repair paths is an inevitable consequence of this
compromise approach. The failure to identify any repair is a
serious deficiency, but is a rare occurrence in a robustly
designed network. This problem can be addressed by:-
1. Reporting that the link in question is irreparable, so that
the network designer can take appropriate action.
2. Modifying the design of the network to avoid this
possibility.
3. Using some form of SRLG diagnostic (for example, by running
BFD over alternate repair paths) to determine which SRLG
member(s) has actually failed and using this information to
select an appropriate pre-computed repair path. However,
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aside from the complexity of performing the diagnostics,
this requires multiple not-via addresses per interface,
which has poor scaling properties.
4.5 Multicast
Multicast traffic is repaired in a similar way to unicast,
however the multicast forwarder is able to use the not-via
address to which the multicast packet was addressed as an
indication of the expected receive interface and hence to
correctly run the required RPF check.
A more complete description of multicast operation will be
provided in a future version of this draft.
4.6 Fast Reroute in an MPLS LDP Network.
Not-via addresses are IP addresses and LDP will distribute labels
for them in the usual way. The not-via repair mechanism may
therefore be used to provide fast re-route in an MPLS network by
first pushing the label which the repair endpoint uses to forward
the packet, and then pushing the label corresponding to the not-
via address needed to effect the repair. Referring once again to
Figure 1, if S has a packet destined for D that it must reach via
P and B, S first pushes B's label for D. S then pushes the label
that its next hop to Bp needs to reach Bp.
Note that in an MPLS LDP network it is necessary for S to have
the repair endpoint's label for the destination. When S is
effecting a link repair it already has this. In the case of a
node repair, S either needs to set up a directed LDP session with
each of its neighbor's neighbors, or it needs to use the next-
next hop label distribution mechanism proposed in [NNHL]. Where
an extended SRLG is being repaired, S must determine which
routers its traffic would traverse on egress from the SRLG, and
then establish directed LDP sessions with each of those routers.
5. Local Area Networks
LANs are a special type of SRLG and are solved using the SRLG
mechanisms outlined above. With all SRLGs there is a trade-off
between the sophistication of the fault detection and the size of
the SRLG. Protecting against link failure of the LAN link(s) is
relatively straightforward, but as with all fast reroute
mechanisms, the problem becomes more complex when it is desired
to protect against the possibility of failure of the nodes
attached to the LAN as well as the LAN itself.
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+--------------Q------C
|
|
|
A--------S-------(N)-------------P------B
|
|
|
+--------------R------D
Figure 10: Local Area Networks
Consider the LAN shown in Figure 10. For connectivity purposes,
we consider that the LAN is represented by the pseudonode (N). To
provide IPFRR protection, S must run a connectivity check to each
of its protected LAN adjacencies P, Q, and R, using, for example
BFD [BFD].
When S discovers that it has lost connectivity to P, it is unsure
whether the failure is:
. its own interface to the LAN,
. the LAN itself,
. the LAN interface of P,
. the node P.
5.1 Simple LAN Repair
A simple approach to LAN repair is to consider the LAN and all of
its connected routers as a single SRLG. Thus, the address P not
via the LAN (Pl) would require P to be reached not-via any router
connected to the LAN. This is shown in Figure 11.
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Ql Cl
+-------------Q--------C
| Qc
|
As Sl | Pl Bl
A--------S-------(N)------------P--------B
Sa | Pb
|
| Rl Dl
+-------------R--------D
Rd
Figure 11: Local Area Networks - LAN SRLG
In this case, when S detected that P had failed it would send
traffic reached via P and B to B not-via the LAN or any router
attached to the LAN (i.e. to Bl). Any destination only reachable
through P would be addressed to P not-via the LAN or any router
attached to the LAN (except of course P).
Whilst this approach is simple, it assumes that a large portion
of the network adjacent to the failure has also failed. This will
result in the use of sub-optimal repair paths and in some cases
the inability to identify a viable repair.
5.2 LAN Component Repair
In this approach, possible failures are considered at a finer
granularity, but without the use of diagnostics to identify the
specific component that has failed. Because S is unable to
diagnose the failure it must repair traffic sent through P and B,
to B not-via P,N (i.e. not via P and not via N), on the
conservative assumption that both the entire LAN and P have
failed. Destinations for which P is a single point of failure
must as usual be sent to P using an address that avoids the
interface by which P is reached from S, i.e. to P not-via N.
Similarly for routers Q and R.
Notice that each router that is connected to a LAN must, as
usual, advertise one not-via address for each neighbor. In
addition, each router on the LAN must advertise an extra address
not via the pseudonode (N).
Notice also that each neighbor of a router connected to a LAN
must advertise two not-via addresses, the usual one not via the
neighbor and an additional one, not via either the neighbor or
the pseudonode. The required set of LAN address assignments is
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shown in Figure 12 below. Each router on the LAN, and each of its
neighbors, is advertising exactly one address more than it would
otherwise have advertised if this degree of connectivity had been
achieved using point-to-point links.
Qs Qp Qc Cqn
+--------------Q---------C
| Qr Qn Cq
|
Asn Sa Sp Sq | Ps Pq Pb Bpn
A--------S-------(N)-------------P---------B
As Sr Sn | Pr Pn Bp
|
| Rs Rp Pd Drn
+--------------R---------D
Rq Rn Dr
Figure 12: Local Area Networks
5.3 LAN Repair Using Diagnostics
A more specific LAN repair can be undertaken by using
diagnostics. In order to explicitly diagnose the failed network
component, S correlates the connectivity reports from P and one
or more of the other routers on the LAN, in this case, Q and R.
If it lost connectivity to P alone, it could deduce that the LAN
was still functioning and that the fault lay with either P, or
the interface connecting P to the LAN. It would then repair to B
not via P (and P not-via N for destinations for which P is a
single point of failure) in the usual way. If S lost connectivity
to more than one router on the LAN, it could conclude that the
fault lay only with the LAN, and could repair to P, Q and R not-
via N, again in the usual way.
6. Loop Free Alternates
The use of loop free alternates (LFA) as a repair mechanism has
been studied [LFA]. Where an LFA exists, S may use this in place
of the not-via repair mechanism for unicast packets (including
MHP alternate routers). Multicast traffic requires the use of a
repair encapsulation so that the packets are delivered to the
router at the repair endpoint in order to correctly re-join the
multicast tree and so that the necessary RPF check can be made.
LFAs are computed on a per destination basis and in general, only
a subset of the destinations requiring repair will have a
suitable LFA repair. In this case, those destinations which are
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repairable by LFAs are so repaired and the remainder of the
destinations are repaired using the not-via encapsulation. This
has the advantage of reducing the volume of traffic that requires
encapsulation.
In some cases, all the destinations, including the repair
endpoint, are repairable by an LFA. In this case, all unicast
traffic may be repaired without encapsulation. Multicast traffic
still requires encapsulation, but for the nodes on the LFA repair
path the computation of the not-via forwarding entry is
unnecessary since, by definition, their normal path to the repair
endpoint is not via the failure.
6.1 Optimizing not-via computations using LFAs
The above observation permits an optimization to the not-via
computations. If repairing node S has an LFA to the repair
endpoint it is not necessary for any router to perform the
incremental SPF with the link SP removed in order to compute the
route to the not-via address Ps. This is because the correct
routes will already have been computed as a result of the SPF on
the base topology. Node S can signal this condition to all other
routers by including a bit in its LSP or LSA associated with each
LFA protected link. Routers computing not-via routes can then
omit the running of the iSPF for links with this bit set.
When running the iSPF for a particular link AB, the calculating
router first checks whether the link AB is present in the
existing SPT. If the link is not present in the SPT, no further
work is required. This check is a normal part of the iSPF
computation.
If the link is present in the SPT, this optimization introduces a
further check to determine whether the link is marked as
protected by an LFA in the direction in which the link appears in
the SPT. If so the iSPF need not be performed. For example, if
the link appears in the SPT in the direction A->B and A has
indicated that the link AB is protected by an LFA no further
action is required for this link.
If the receipt of this information is delayed, the correct
operation of the protocol is not compromised provided that the
not-via computation is performed on the latest available
information.
This optimization is not particularly beneficial to nodes close
to the repair since, as has been observed above, the computation
for nodes on the LFA path is trivial. However, for nodes upstream
of the link SP for which S-P is in the path to P, there is a
significant reduction in the computation required.
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6.2 Use of LFAs with SRLGs
Section 4.4 above describes the repair of links which are members
of one or more SRLGs. LFAs can be used for the repair of such
links provided that any other link with which S-P shares an SRLG
is avoided when computing the LFA. This is described for the
simple case of "local-SRLGs" in [LFA].
7. Equal Cost Multi-Path
A router can use an equal cost multi-path (ECMP) repair in place
of a not-via repair for unicast packets.
A router computing a not-via repair path MAY subject the repair
to ECMP.
8. Multiple Simultaneous Failures
The failure of a node or an SRLG can result in multiple
correlated failures, which may be repaired using the mechanisms
described in this design. This design will not correctly repair a
set of unanticipated multiple failures. Such failures are out of
scope of this design.
It is important that the routers in the network are able to
discriminate between these two classes of failure, and take
appropriate action.
9. Encapsulation
Any IETF specified IP in IP encapsulation may be used to carry a
not-via repair. IP in IP [IPIP], GRE [RFC1701] and L2TPv3
[L2TPv3], all have the necessary and sufficient properties. The
requirement is that both the encapsulating router and the router
to which the encapsulated packet is addressed have a common
ability to process the chose encapsulation type.
When an MPLS LDP network is being protected, the encapsulation
would normally be an additional MPLS label. In an MPLS enabled IP
network an MPLS label may be used in place of an IP in IP
encapsulation in the case above.
10. Routing Extensions
IPFRR requires IGP extensions. Each IPFRR router that is directly
connected to a protected network component must advertise a not-
via address for that component. This must be advertised in such a
way that the association between the protected component (link,
router or SRLG) and the not-via address can be determined by the
other routers in the network.
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It is necessary that not-via capable routers advertise in the IGP
that they will calculate not-via routes.
It is necessary for routers to advertise the type of
encapsulation that they support (MPLS, GRE [RFC1701], L2TPv3
etc). However, the deployment of mixed IP encapsulation types
within a network is deprecated.
11. Incremental Deployment
Incremental deployment is supported by excluding routers that are
not calculating not-via routes from the base topology. In that
way repairs may be steered around island of routers that are not
IPFRR capable.
Routers that are protecting a network component need to have the
capability to encapsulate and decapsulate packets. However,
routers that are on the repair path only need to be capable of
calculating not-via paths and including the not-via addresses in
their FIB i.e. these routers do not need any changes to their
forwarding mechanism.
12. IANA considerations
There are no IANA considerations that arise from this draft.
13. Security Considerations
The repair endpoints present vulnerability in that they might be
used as a method of disguising the delivery of a packet to a
point in the network. The primary method of protection should be
through the use of a private address space for the not-via
addresses. These addresses MUST NOT be advertised outside the
area, and SHOULD be filtered at the network entry points. In
addition, a mechanism might be developed that allowed the use of
the mild security available through the use of a key [RFC1701]
[L2TPv3]. With the deployment of such mechanisms, the repair
endpoints would not increase the security risk beyond that of
existing IP tunnel mechanisms.
An attacker may attempt to overload a router by addressing an
excessive traffic load to the decapsulation endpoint. Typically,
routers take a 50% performance penalty in decapsulating a packet.
The attacker could not be certain that the router would be
impacted, and the extremely high volume of traffic needed, would
easily be detected as an anomaly.
If an attacker were able to influence the availability of a link,
they could cause the network to invoke the not-via repair
mechanism. A network protected by not-via IPFRR is less
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vulnerable to such an attack than a network that undertook a full
convergence in response to a link up/down event.
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be
claimed to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights. Information on the procedures with respect to
rights in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the
use of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR
repository at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to
the IETF at ietf-ipr@ietf.org.
Full copyright statement
Copyright (C) The Internet Society (2006). 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.
Normative References
There are no normative references.
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Informative References
Internet-drafts are works in progress available from
<http://www.ietf.org/internet-drafts/>
[BFD] Katz, D., Ward, D., "Bidirectional Forwarding
Detection", < draft-ietf-bfd-base-04.txt>, July
2005, (work in progress).
RFC1701 RFC 1701, Generic Routing Encapsulation (GRE).
S. Hanks, T. Li, D. Farinacci, P. Traina.
October 1994.
[IPFRR] Shand, M., Bryant, S., "IP Fast-reroute
Framework",
<draft-ietf-rtgwg-ipfrr-framework-05.txt>,
October 2005, (work in progress).
[l2TPV3] J. Lau, Ed., et al., "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March
2005.
[LFA] A. Atlas, Ed, A. Zinin, Ed, "Basic
Specification for IP Fast-Reroute: Loop-free
Alternates", <draft-ietf-rtgwg-ipfrr-spec-base-
04.txt>, July 2005, (work in progress).
[LDP] Andersson, L., Doolan, P., Feldman, N.,
Fredette, A. and B. Thomas, "LDP
Specification", RFC 3036,
January 2001.
[NNHL] Shen, N., et al "Discovering LDP Next-Nexthop
Labels", <draft-shen-mpls-ldp-nnhop-label-
02.txt>, May 2005, (work in progress)
[MPLS-TE] Ping Pan, et al, "Fast Reroute Extensions to
RSVP-TE for LSP Tunnels", RFC 4090, May 2005.
[TUNNEL] Bryant, S., et al, "IP Fast Reroute using
tunnels", <draft-bryant-ipfrr-tunnels-02.txt>,
April 2005 (work in progress).
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Authors' Addresses
Stewart Bryant
Cisco Systems,
250, Longwater Avenue,
Green Park,
Reading, RG2 6GB,
United Kingdom. Email: stbryant@cisco.com
Stefano Previdi
Cisco Systems
Via Del Serafico, 200
00142 Rome,
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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