Network Working Group K. Kompella
Internet Draft Y. Rekhter
Category: Standards Track Juniper Networks
J. Vasseur
cisco Systems
T. Chung
Bell Canada
Expires: December 2003 June 2003
Multi-area MPLS Traffic Engineering
draft-kompella-mpls-multiarea-te-04.txt
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Kompella et al Standards Track [Page 1]
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Abstract
An ISIS/OSPF routing domain may consist of multiple areas. This
document postulates a set of mechanisms, and then outlines how these
mechanisms could be used to establish/maintain Traffic Engineering
Label Switching Paths (LSPs) that span multiple areas.
1. Introduction
An ISIS or OSPF routing domain may consist of multiple areas or
levels. This document postulates a set of mechanisms, and then
outlines how these mechanisms could be used to establish and maintain
Traffic Engineering Label Switching Paths (LSPs) that span multiple
areas. These mechanisms can be used both for multiple OSPF areas or
ISIS levels, or for multiple instances of routing protocols.
2. Set of mechanisms
In this section we postulate a set of mechanisms that could be used
to construct LSPs that span multiple ISIS or OSPF areas. The actual
application of these mechanisms to construct such LSPs is covered in
the next section. We assume that the mechanisms listed below are
either already available, or could be realized via extensions to the
existing signaling (RSVP) and/or routing (ISIS/OSPF) protocols used
for MPLS Traffic Engineering.
In this document we use the OSPF term "backbone area". In the context
of ISIS this term means the set of L2 routers. We also use the OSPF
term "ABR". In the context of ISIS this terms means a router that is
both L2 and L1.
2.1. Passing constraints
An LSR that acts as a head-end of an LSP should be able to pass the
constraints associated with this LSP to some other node. The other
node may, or may not be along the path taken by the LSP. Note that
the existing signalling (RSVP/CR-LDP) already provides a way to pass
some of the constraints, like resource affinity. [Vasseur] proposes
additional extensions to RSVP to pass additional constraints. A more
general way to provide this functionality is described in [Kompella].
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2.2. Loose hops
An LSR should be able to specify loose ERO hops, and let some
intermediate LSR(s) along the path to expand it to strict hops. Note
that the ability to specify loose hops is already available.
2.3. Path computation server
An LSR that originates an LSP should be able to "ask" some other node
to compute the path for the LSP. The node that computes the path may,
or may not be along the path taken by the LSP. Depending on the
amount of the topology and TE information available to the node, the
node may compute either the whole path, or a segment of the path. In
the latter case the computed path would include loose hops.
This mechanism requires the ability of the LSR that originates the
LSP to pass the constraints associated with that LSP to the node that
is going to compute the path for the LSP. It also requires the
ability of the LSR that originates the LSP to pass the address of LSR
at the tail-end of the LSP to the node that is going to compute the
path for the LSP, and for the node that is going to compute the path
for the LSP the ability to pass back to the node that originates the
LSP the ERO that contains the results of the path computation.
Once the path computation server computes the path, if the server is
not along the path, the server is no longer responsible for
maintaining the feasibility of the path (for one thing, the server
may not even know whether the LSR established the path in the first
place).
A path computation server usually has the topology and TE information
of more than one area, including the backbone area. An example of a
node that could act as as a path computation server and meet this
requirement would be an OSPF Area Border Router (ABR), as an ABR has
the topology and TE information of the backbone area, plus all the
other areas connected to that ABR. An extreme case of a path
computation server is a node that has the topology and TE information
of the whole ISIS/OSPF routing domain.
2.4. Acquiring topology/TE information for multiple areas
It could be possible for a node that originates an LSP to obtain the
topology and TE information of not just its own area, but other areas
as well. This information may be subject to filtering (e.g., the node
could obtain only the information about FAs in other areas). The node
should be able to perform Constrained SPF (CSPF) based on this
information.
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One possible scenario is when the node, in addition to the topology
and TE information of its own area, also obtains the topology and TE
information of the backbone area.
An extreme case is when a node obtains the topology and TE
information of the whole OSPF/ISIS domain.
3. Constructing multi-area TE LSPs
Consider the situation where the head-end of a TE LSP is in a
different ISIS/OSPF area than the tail-end of a TE LSP. We'll refer
to the area that the head-end is in as the head-end area. We'll
refer to the area that the tail-end is in as the tail-end area.
Given the set of the mechanisms outlined in the previous section, the
following sub-sections outline some of the possible scenarios that
use these mechanisms in order to construct TE LSPs that span multiple
OSPF/ISIS areas.
In the context of this document we define an optimal route as a route
that would be computed in the absence of areas. In other words, an
optimal route in a given network that is composed of a single
OSPF/ISIS routing domain is a route that is computed if the network
isn't partitioned into areas.
It is easy to construct examples that show that partitioning a
network into areas, and the resulting loss of routing information
may lead to sub-optimal routing, in a sense that the computed routes
would be different than optimal routes. That is true irrespective of
a particular scheme to aggregate/abstract/summarize routing
information. One corollary of this is that it is impossible to
guarantee optimal routing in the presence of routing information
aggregation/abstraction/summarization.
3.1. Scenario 1
Path computation is done on a per area basis. The head-end LSR
computes strict hops within its own area. The head-end LSR then
initiates LSP path setup. The setup includes the information about
the constraints associated with the LSP.
The ABR in the head-end area uses the topology and TE information of
the backbone area, as well as the information about the constraints
associated with the LSP (the ABR receives this information as part of
the LSP setup) to compute strict hops within the backbone area to the
ABR in the tail-end area.
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The ABR in the tail-end area uses the topology and TE information of
the tail-end area, as well as the information about the constraints
associated with the LSP (the ABR receives this information as part of
the LSP setup) to compute strict hops within the tail-end area from
itself to the tail-end LSR.
Note that since the choice of ABR in the head-end area is determined
by only the information in the head-end area, the inability to find a
route across multiple areas doesn't mean that the route doesn't exist
- it may mean that another ABR in the head-end area should be chosen,
and/or it may mean than another ABR in the tail-end area should be
chosen. Thus the failure to find a route may require to try another
ABRs. The total number of such attempts could be as large as H*T
(where H is the number of ABRs in the head-end area, and T is the
number of ABRs in the tail-end area).
To exert control over which ABRs a particular LSP can traverse, the
LSR that originates this LSP could be preconfigured with the
addresses of these ABRs.
In this scenario the only information that has to be distributed
across area boundaries are the reachability information associated
with routers' interface addresses (including loopback addresses, if
an LSP is to be terminated on a router loopback address). Both OSPF
and ISIS already provide mechanisms to accomplish this.
Handling link/node failures could be done as a two-phase approach,
where in the first phase a failure is handled within an area where
the failure occurs, and only if that fails, the head-end LSR is
notified of the failure and is expected to handle it. To direct
failure notifications to the ABR of the area where the failure occurs
(rather than to the head-end LSR), the ABR should use the Notify
Request Object.
Alternatively, link/node failures could always be handled by the
head-end LSR.
3.2. Scenario 2
The head-end LSR requests an ABR in its (head-end) area to compute
the path all the way from the LSR to the ABR in the tail-end area.
This is possible because the ABR in the head-end area maintains the
topology and TE information both for the head-end area and for the
backbone area. The head-end LSR then initiates the LSP setup using
the computed path. As part of the LSP setup, the ABR in the tail-end
area then computes the rest of the path.
Alternatively, the head-end LSR, after requesting and obtaining the
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path from the ABR in the head-end area, may request the ABR in the
tail-end area (which is known from the path computed by the ABR in
the head-end area) to compute the path to the destination. Once this
is done, the head-end LSR would have a complete path to the tail-end
LSR and could use it for the LSP setup.
Note that the ABR in the head-end area that computes the path all the
way to the ABR in the tail-end area may, or may not be on the path
taken by the LSP.
In this scenario the only information that has to be distributed
across area boundaries are the reachability information associated
with routers' interface addresses (including loopback addresses, if
an LSP is to be terminated on a router loopback address). Both OSPF
and ISIS already provide mechanisms to accomplish this.
In this scenario the head-end LSR may end up in a situation, where
the head-end LSR, after requesting and obtaining the path than spans
both the head-end area and the backbone area from the ABR in the
head-end area, may find out that there is no feasible path from the
ABR in the tail-end area to the destination. A way to handle this is
for the head-end LSR to issue another request to the ABR in the head-
end area, but indicate in the request that the ABR in the tail-end
area (the one that was selected by the previous request) should be
excluded from the path (this could be done by adding an additional
constraint to the request).
This scenario is likely to reduce the number of LSP setup failures,
relative to the scenario 1. This is because in contrast with the
first scenario, there is no need to try multiple ABRs in the head-end
area.
Handling link/node failures could be done as a two-phase approach,
where in the first phase a failure is handled within an area where
the failure occurs, and only if that fails, the head-end LSR is
notified of the failure and is expected to handle it. To direct
failure notifications to the ABR of the area where the failure occurs
(rather than to the head-end LSR), the ABR should use the Notify
Request Object. Note also, that the ABR will be informed of the
failure by the routing protocol (ISIS/OSPF).
Alternatively, link/node failures could always be handled by the
head-end LSR.
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3.3. Scenario 3
The head-end LSR obtains the TE information from the backbone area,
and uses it to compute the path all the way to the ABR in the tail-
end area. The head-end LSR then initiates the LSP setup using the
computed path. As part of the LSP setup, the ABR in the tail-end area
then computes the rest of the path.
Alternatively, the head-end LSR, after computing the path to the ABR
in the tail-end area, may request the ABR in the tail-end area the
head-end area) to compute the path to the destination. Once this is
done, the head-end LSR would have a complete path to the tail-end LSR
and could use it for the LSR setup.
A variation on the above alternative is for the head-end LSR to
request the ABR in the tail-end area to compute paths to the
destination from all/some of the ABRs in the tail-end area. Once
this is done, the head-end LSR could use this information to compute
a path through the head-end and the backbone areas. This variation
could improve path optimality and reduce the probability of the LSP
setup failure, relative to the other alternatives described in this
section. On the other hand, it deterministically (not
probabalistically) increases the computational overhead on the ABRs,
relative to the other alternatives described in this section.
If the head-end area contains LSRs that don't originate LSPs, then
these LSRs need not maintain the TE information from the backbone
area.
This scenario is similar to scenario 2, except for the entity that
computes path through the head-end and the backbone areas, and the
amount of information that the originating LSR has to maintain. With
respect to the amount of information, the originating LSR maintains
the same information as the ABR in the head-end area.
Handling link/node failures could be done as a two-phase approach,
where in the first phase a failure is handled within an area where
the failure occurs, and only if that fails, the head-end LSR is
notified of the failure and is expected to handle it. To direct
failure notifications to the ABR of the area where the failure occurs
(rather than to the head-end LSR), the ABR should use the Notify
Request Object.
If the head-end LSR is notified of a failure, and the failure is
neither at the ABR in the tail-end area, not in the tail-end area
itself, the head-end LSR should compute a (feasible) path to the same
ABR in the tail-end area, as the one that was used prior to the
failure. If the head-end LSR is notified of the failure, and the
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failure is either at the ABR in the tail-end area, or at the tail-end
area, the head-end LSR should compute a (feasible) path to some other
ABR in the tail-end area.
Alternatively, link/node failures could always be handled by the
head-end LSR.
3.4. Scenario 4
The head-end LSR requests one of the ABRs in the head-end area to
compute a path to the destination. When the ABR in the head-end area
receives the request, the ABR requests one of the ABRs in the tail-
end area to compute paths to the destination from all/some of the
ABRs in the tail-end area. Once the ABR in the head-end area obtains
this information from the ABR in the tail-end area, the ABR in the
head-end area could compute the path through the head-end and the
backbone areas, concatenates the results of this computation with the
appropriate path through the tail-end area, and then return the
result to the head-end LSR.
Note that the ABR in the head-end area that computes the path all the
way to the ABR in the tail-end area may, or may not be on the path
taken by the LSP.
Likewise, the ABR in the tail-end area that computes the path(s) in
the tail-end area may, or may not be on the path taken by the LSP.
In this scenario, just like in the Scenario 2, the only information
that has to be distributed across area boundaries are the
reachability information associated with routers' interface addresses
(including loopback addresses, if an LSP is to be terminated on a
router loopback address). Both OSPF and ISIS already provide
mechanisms to accomplish this.
While this scenario is likely to reduce the probability of the LSP
setup failure relative to the Scenario 2, it does this at the expense
of deterministically (and not probabalistically) higher computational
overhead placed on ABRs.
Note that the approach outlined in this scenario results in computing
optimal routes.
The approach described in this section could reduce the probability
of the LSP setup failure, relative to the approach outlined in
Scenario 2. On the other hand, it deterministically (not
probabalistically) increases the computational overhead on the ABRs,
relative to the approach outlined in Scenario 2.
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The approach described in this section could also facilitate
computation of node/link disjoint paths.
3.5. Scenario 5
The head-end LSR requests an entity that has the entire network (all
areas) topology to compute the whole path.
Except for the entity that has the entire network topology, in this
scenario the only information that has to be distributed across area
boundaries are the reachability information associated with routers'
interface addresses (including loopback addresses, if an LSP is to be
terminated on a router loopback address). Both OSPF and ISIS already
provide mechanisms to accomplish this.
Note that the approach outlined in this scenario results in computing
optimal routes.
Handling link/node failures could be done as a two-phase approach,
where in the first phase a failure is handled within an area where
the failure occurs, and only if that fails, the head-end LSR is
notified of the failure and is expected to handle it. To direct
failure notifications to the ABR of the area where the failure occurs
(rather than to the head-end LSR), the ABR should use the Notify
Request Object.
If the head-end LSR is notified of a failure, the head-end LSR should
request the entity that computed the whole path to compute another
path. When issuing such a request, the LSR should specify that the
path should exclude the element that caused the failure.
The approach described in this section could also facilitate
computation of node/link disjoint paths.
4. Pre-engineered backbone area
In certain cases it may be desirable to "pre-engineer" the backbone
area by constructing a set of TE LSPs that would be used as FAs by
the traffic that has to traverse the backbone area. The scenarios
outline above do not preclude this as an option.
With a pre-engineered backbone area, a variation of Scenario 3 would
be to restrict the backbone information leaked to the non-backbone
areas to only the information associated with the FAs in the backbone
area.
Note that an LSP that starts/ends in a non-backbone area, but
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traverses the backbone area may use more than one FA. That means, for
example, that even if there is no FA between any of the ABRs in the
head-end area and any of the ABRs in the tail-end area, an LSP could
still be established, as long as there are some other FAs that
combined together provide a path from the head-end to the tail-end
area.
Stability of the LSPs that are used as FAs in the backbone area is
unaffected by the establishment/tear down of the LSPs that start/end
in other areas, but traverse the backbone area.
Pre-engineering the backbone area enables aggregation of the Label
Forwarding State in the backbone area, as a TE LSP that is used as an
FA could carry/nest multiple LSPs. One possible application of this
feature would be in conjunction with GMPLS controlling OXCs, where
OXCs would be placed in the backbone area.
5. TE information summary
One possible application of summarization of TE information would be
to prune off unfeasible ABRs, as follows. An ABR in the tail-end
area would compute two paths: (a) a min hop path to the destination
ignoring all other constraints (such as b/w or resource affinity),
and (b) a max unreserved b/w path to the destination, again ignoring
all other constraints (such as resource affinity or unreserved b/w).
The ABR will then advertise the hop count from the first path and the
max unreserved b/w from the second path into the backbone area. The
ABR in the head-end area will do exactly the same, and then advertise
this information into the head-end area. An LSR in the head-end area
that needs to originate an LSP could use this information to prune
off unfeasible ABRs. For example, if the LSP requires X amount of
bandwidth, the LSR should rule out any of the ABRs in its area that
advertise less than X amount of unreserved b/w to the LSP's
destination.
While pruning off unfeasible ABRs, as outlined in the previous
paragraph, could reduce the number of LSP setup failures, it is not
without drawbacks. For one thing, it increases the amount of control
(routing) information that an LSR has to maintain, as well as the
volume of the control information that an LSR receives from other
LSRs, and has to process. In addition, it adds computational overhead
to ABRs, as they need to perform additional computation. Moreover,
establishment/tear down of inter-area LSPs increases the frequency
with which this computation would need to be performed. Finally, it
is important to realize that this approach doesn't eliminate LSP
setup failures, as for one thing, the information computed and
advertised by ABRs may be "out-of-date". Yet another reason for LSP
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setup failures is that the unreserved b/w information computed by
ABRs doesn't take into account any other constraints, while a
particular LSP may be subject not just to unreserved b/w, but to
other constraints (e.g., resource affinity) as well. So, even if from
the unreserved b/w constraint point of view a particular ABR is
feasible for a particular LSP, when taking into account all the
constraints associated with that LSP, the ABR may no longer be
feasible.
And to reiterate the point made earlier, since route computation with
this approach uses summarized/aggreated/abstracted information, this
approach doesn't guarantee optimal routing.
6. Protection in the multi-area environment
An LSP that spans multiple area could be protected in a variety of
ways. One alternative is to establish a backup (secondary) LSP, and
make sure that this LSP is node/link disjoined from the (primary) LSP
that it protects.
Another alternative is to provide protection on a per-area basis,
where LSP's segment that is within a given area is protected by
another LSP (this other LSP protects just that segment). Note that by
using the label stacking construct a single LSP could be used for
protecting segments of multiple other LSPs.
Finally, within each area one could employ the same protection
mechanisms as are currently used for LSPs that are confined to a
single area.
7. Reoptimization
Reoptimization is the set of mechanisms by which an existing (already
established) LSP is rerouted over a more optimal path. In general
reoptimization could be either periodic (timer-driven), or triggered
(event-driven). An example of an event that could trigger
reoptimization is coming up of a link that was previously down.
Periodic (time-driven) reoptimization is performed by the head-end
LSR.
Handling event-driven reoptimization could be done on a per area
basis. That is, if the event that triggers reoptimization happens in
the head-end area, it is the responsibility of the head-end LSR. If
the event happens in the backbone area, then it is the responsibility
of the ABR in the head-end area. Finally, if the event happens in the
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tail-end area, then it is the responsibility of the ABR in the tail-
end area.
Just like with a sigle area reoptimization, the multi-area
reoptimization should be done without traffic disruption (by using
the approach known as "make before break").
Normative References
Informative References
[Kompella] Kompella, K., "Carrying Constraints in RSVP", work in
progress
[Vasseur] JP Vasseur et al, "RSVP Path computation request and reply
messages", work in progress
Security Considerations
Security considerations will be discussed in a future version.
IANA Considerations
As of now, there are no demands of IANA.
Acknowledgments
We would like to thank Noel Chiappa, Tony Li, Robert Raszuk, and Alex
Zinin for helping us with the ideas presented in this document.
Authors' Addresses
Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: kireeti@juniper.net
Yakov Rekhter
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
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Email: yakov@juniper.net
Jean Philippe Vasseur
Cisco Systems, Inc.
11, rue Camille Desmoulins
92782 Issy les Moulineaux Cedex 9
France
Email: jpv@cisco.com
Ting Wo Chung
Bell Canada
181 Bay Street, Suite 350, Toronto, Ontario, Canada, M5J 2T3
Email: ting_wo.chung@bell.ca
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