Network Working Group S. Yasukawa
Request for Comments: 5671 NTT
Category: Informational A. Farrel, Ed.
Old Dog Consulting
October 2009
Applicability of the Path Computation Element (PCE) to
Point-to-Multipoint (P2MP) MPLS and GMPLS Traffic Engineering (TE)
Abstract
The Path Computation Element (PCE) provides path computation
functions in support of traffic engineering in Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) networks.
Extensions to the MPLS and GMPLS signaling and routing protocols have
been made in support of point-to-multipoint (P2MP) Traffic Engineered
(TE) Label Switched Paths (LSPs).
This document examines the applicability of PCE to path computation
for P2MP TE LSPs in MPLS and GMPLS networks. It describes the
motivation for using a PCE to compute these paths and examines which
of the PCE architectural models are appropriate.
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Table of Contents
1. Introduction ....................................................2
2. Architectural Considerations ....................................4
2.1. Offline Computation ........................................4
2.2. Online Computation .........................................4
2.2.1. LSR Loading .........................................5
2.2.2. PCE Overload ........................................6
2.2.3. PCE Capabilities ....................................6
3. Fragmenting the P2MP Tree .......................................7
4. Central Replication Points ......................................8
5. Reoptimization and Modification .................................9
6. Repair .........................................................10
7. Disjoint Paths .................................................11
8. Manageability Considerations ...................................11
8.1. Control of Function and Policy ............................11
8.2. Information and Data Models ...............................11
8.3. Liveness Detection and Monitoring .........................12
8.4. Verifying Correct Operation ...............................12
8.5. Requirements on Other Protocols and Functional
Components ................................................12
8.6. Impact on Network Operation ...............................13
9. Security Considerations ........................................13
10. Acknowledgments ...............................................13
11. References ....................................................13
11.1. Normative References .....................................13
11.2. Informative References ...................................13
1. Introduction
The Path Computation Element (PCE) defined in [RFC4655] is an entity
that is capable of computing a network path or route based on a
network graph and of applying computational constraints. The
intention is that the PCE is used to compute the path of Traffic
Engineered Label Switched Paths (TE LSPs) within Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) networks.
[RFC4655] defines various deployment models that place PCEs
differently within the network. The PCEs may be collocated with the
Label Switching Routers (LSRs), may be part of the management system
that requests the LSPs to be established, or may be positioned as one
or more computation servers within the network.
Requirements for point-to-multipoint (P2MP) MPLS TE LSPs are
documented in [RFC4461], and signaling protocol extensions for
setting up P2MP MPLS TE LSPs are defined in [RFC4875]. In this
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document, P2MP MPLS TE networks are considered in support of various
features including layer 3 multicast VPNs [RFC4834], video
distribution, etc.
Fundamental to the determination of the paths for P2MP LSPs within a
network is the selection of branch points. Not only is this
selection constrained by the network topology and available network
resources, but it is determined by the objective functions that may
be applied to path computation. For example, one standard objective
is to minimize the total cost of the tree (that is, to minimize the
sum of the costs of each link traversed by the tree) to produce what
is known as a Steiner tree. Another common objective function
requires that the cost to reach each leaf of the P2MP tree be
minimized.
The selection of branch points within the network is further
complicated by the fact that not all LSRs in the network are
necessarily capable of performing branching functions. This
information may be recorded in the Traffic Engineering Database (TED)
that the PCE uses to perform its computations, and may have been
distributed using extensions to the Interior Gateway Protocol (IGP)
operating within the network [RFC5073].
Additionally, network policies may dictate specific branching
behavior. For example, it may be decided that, for certain types of
LSPs in certain types of networks, it is important that no branch LSR
is responsible for handling more than a certain number of downstream
branches for any one LSP. This might arise because the replication
mechanism used at the LSRs is a round-robin copying process that
delays the data transmission on each downstream branch by the time
taken to replicate the data onto each previous downstream branch.
Alternatively, administrative policies may dictate that replication
should be concentrated on specific key replication nodes behaving
like IP multicast rendezvous points (perhaps to ensure appropriate
policing of receivers in the P2MP tree, or perhaps to make protection
and resiliency easier to implement).
Path computation for P2MP TE LSPs presents a significant challenge
because of the complexity of the computations described above.
Determining disjoint protection paths for P2MP TE LSPs can add
considerably to this complexity, while small modifications to a P2MP
tree (such as adding or removing just one leaf) can completely change
the optimal path. Reoptimization of a network containing multiple
P2MP TE LSPs requires considerable computational resources. All of
this means that an ingress LSR might not have sufficient processing
power to perform the necessary computations, and even if it does, the
act of path computation might interfere with the control and
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management plane operation necessary to maintain existing LSPs. The
PCE architecture offers a way to offload such path computations from
LSRs.
2. Architectural Considerations
2.1. Offline Computation
Offline path computation is performed ahead of time, before the LSP
setup is requested. That means that it is requested by, or performed
as part of, a management application. This model can be seen in
Section 5.5 of [RFC4655].
The offline model is particularly appropriate to long-lived LSPs
(such as those present in a transport network) or for planned
responses to network failures. In these scenarios, more planning is
normally a feature of LSP provisioning.
This model may also be used where the network operator wishes to
retain full manual control of the placement of LSPs, using the PCE
only as a computation tool to assist the operator, not as part of an
automated network.
Offline path computation may be applied as a background activity for
network reoptimization to determine whether and when the current LSP
placements are significantly sub-optimal. See Section 5 for further
discussions of reoptimization.
2.2. Online Computation
Online path computation is performed on-demand as LSRs in the network
determine that they need to know the paths to use for LSPs. Thus,
each computation is triggered by a request from an LSR.
As described in [RFC4655], the path computation function for online
computation may be collocated with the LSR that makes the request, or
it may be present in a computation-capable PCE server within the
network. The PCE server may be another LSR in the network, a
dedicated server, or a functional component of a Network Management
System (NMS). Furthermore, the computation is not necessarily
achieved by a single PCE operating on its own, but may be the result
of cooperation between several PCEs.
The remainder of this document makes frequent reference to these
different online models in order to indicate which is more
appropriate in different P2MP scenarios.
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2.2.1. LSR Loading
An important feature of P2MP path computation is the processing load
that it places on the network element that is determining the path.
Roughly speaking, the load to compute a least-cost-to-leaf tree is
the same as the cost to compute a single optimal path to each leaf in
turn. The load to compute a Steiner tree is approximately an order
of magnitude greater, although algorithms exist to approximate
Steiner trees in roughly the same order of magnitude of time as for a
least-cost-to-leaf tree.
Whereas many LSRs are capable of simple Constrained Shortest Path
First (CSPF) computations to determine a path for a single point-to-
point (P2P) LSP, they rapidly become swamped if called on to perform
multiple such computations, such as when recovering from a network
failure. Thus, it is reasonable to expect that an LSR would struggle
to handle a P2MP path computation for a tree with many destinations.
The result of an LSR becoming overloaded by a P2MP path computation
may be two-fold. First, the LSR may be unable to provide timely
computations of paths for P2P LSPs; this may impact LSP setup times
for simple demand-based services and could damage the LSR's ability
to recover services after network faults. Secondly, the LSR's
processing capabilities may be diverted from other important tasks,
not the least of which is maintaining the control plane protocols
that are necessary to the support of existing LSPs and forwarding
state within the network. It is obviously critically important that
existing traffic should not be disrupted by the computation of a path
for a new LSP.
It is also not reasonable to expect the ingress LSRs of P2MP LSRs to
be specially powerful and capable of P2MP computations. Although a
solution to the overloading problem would be to require that all LSRs
that form the ingresses to P2MP LSPs be sufficiently high-capacity to
perform P2MP computations, this is not an acceptable solution
because, in all other senses, the ingress to a P2MP LSP is just a
normal ingress LSR.
Thus, there is an obvious solution: off-load P2MP path computations
from LSRs to remotely located PCEs. Such PCE function can be
provided on dedicated or high-capacity network elements (such as
dedicated servers, or high-end routers that might be located as
Autonomous System Border Routers - ASBRs).
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2.2.2. PCE Overload
Since P2MP path computations are resource-intensive, it may be that
the introduction of P2MP LSPs into an established PCE network will
cause overload at the PCEs. That is, the P2MP computations may block
other P2P computations and might even overload the PCE.
Several measures can be taken within the PCE architecture to
alleviate this situation as described in [RFC4655]. For example,
path computation requests can be assigned priorities by the LSRs that
issue them. Thus, the LSRs could assign lower priority to the P2MP
requests, ensuring that P2P requests were serviced in preference.
Furthermore, the PCEs are able to apply local and network-wide policy
and this may dictate specific processing rules [RFC5394].
But ultimately, a network must possess sufficient path computation
resources for its needs and this is achieved within the PCE
architecture simply by increasing the number of PCEs.
Once there are sufficient PCEs available within the network, the LSRs
may choose between them and may use overload notification information
supplied by the PCEs to spot which PCEs are currently over-loaded.
Additionally, a PCE that is becoming over-loaded may redistribute its
queue of computation requests (using the PCE cooperation model
described in [RFC4655]) to other, less burdened PCEs within the
network.
2.2.3. PCE Capabilities
An LSR chooses between available PCEs to select the one most likely
to be able to perform the requested path computation. This selection
may be based on overload notifications from the PCEs, but could also
consider other computational capabilities.
For example, it is quite likely that only a subset of the PCEs in the
network have the ability to perform P2MP computations since this
requires advanced functionality. Some of those PCEs might have the
ability to satisfy certain objective functions (for example, least
cost to destination), but lack support for other objective functions
(for example, Steiner). Additionally, some PCEs might not be capable
of the more complex P2MP reoptimization functionality.
The PCE architecture allows an LSR to discover the capabilities of
the PCEs within the network at the same time it discovers their
existence. Further and more detailed exchanges of PCE capabilities
can be made directly between the PCEs and the LSRs. This exchange of
PCE capabilities information allows a Path Computation Client (PCC)
to select the PCE that can best answer its computation requests.
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3. Fragmenting the P2MP Tree
A way to reduce the computational burden of computing a large P2MP
tree on a single PCE is to fragment or partition the tree. This may
be particularly obvious in a multi-domain network (such as multiple
routing areas), but is equally applicable in a single domain.
Consider the network topology in Figure 1. A P2MP LSP is required
from ingress LSR A to egress LSRs s, t, u, v, w, x, y, and z. Using
a single PCE model, LSR A may request the entire path of the tree and
this may be supplied by the PCE. Alternatively, the PCE that is
consulted by LSR A may only compute the first fragment of the tree
(for example, from A to K, L, and M) and may rely on other PCEs to
compute the three smaller trees from K to t, u, and v; from L to w
and x; and from M to s, y, and z.
The LSR consulted by A may simply return the first subtree and leave
LSRs K, L, and M to invoke PCEs in their turn in order to complete
the signaling. Alternatively, the first PCE may cooperate with other
PCEs to collect the paths for the later subtrees and return them in a
single computation response to PCE A. The mechanisms for both of
these approaches are described in the PCE architecture [RFC4655].
t
/
/
n--u
/
/
e--f--h--K--o--v
/
/
A--b--c--d--g--i--L--p--w
\ \
\ \
j x
\
\
M--r--y
\ \
\ \
s z
Figure 1: A P2MP Tree with Intermediate Computation Points
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A further possibility is that LSRs at which the subtrees are stitched
together (K, L, and M in this example) are selected from a set of
potential such points using a cooperative PCE technique, such as the
Backward Recursive Path Computation (BRPC) mechanism [RFC5441].
Indeed, if LSRs K, L, and M were ASBRs or Area Border Routers (ABRs),
the BRPC technique would be particularly applicable.
Note, however, that while these mechanisms are superficially
beneficial, it is far from obvious how the first LSR selects the
transit LSRs K, L, and M, or how the leaf nodes are assigned to be
downstream of particular downstream nodes. The computation to
determine these questions may be no less intensive than the
determination of the full tree unless there is some known property of
the leaf node identifiers such as might be provided by address
aggregation.
4. Central Replication Points
A deployment model for P2MP LSPs is to use centralized, well-known
replication points. This choice may be made for administrative or
security reasons, or because of particular hardware capability
limitations within the network. Indeed, this deployment model can be
achieved using P2P LSPs between ingress and replication point as well
as between replication point and each leaf so as to achieve a P2MP
service without the use of P2MP MPLS-TE.
The motivations for this type of deployment are beyond the scope of
this document, but it is appropriate to examine how PCE might be used
to support this model.
In Figure 2, a P2MP service is required from ingress LSR a to egress
LSRs m, n, o, and p. There are four replication-capable LSRs in the
network: D, E, J, and K.
When LSR a consults a PCE, it could be given the full P2MP path from
LSR a to the leaves, but in this model, the PCE simply returns a P2P
path to the first replication point (in this case, LSR D). LSR D
will consult a PCE in its turn and determine the P2P LSPs to egress
LSRs m and p as well as the P2P LSP to the next replication point,
LSR J. Finally, LSR J will use a PCE to determine P2P LSPs to
egresses n and o.
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f--i--m
/
/
a--b--c--D--g--J--n
\ \
\ \
E h K o
\
\
l--p
Figure 2: Using Centralized Replication Points
In this model of operation, it is quite likely that the PCE function
is located at the replication points, which will be high-capacity
LSRs. One of the main features of the computation activity is the
selection of the replication points (for example, why is LSR D
selected in preference to LSR E, and why is LSR J chosen over LSR
K?). This selection may be made solely on the basis of path
optimization as it would be for a P2MP computation, but may also be
influenced by policy issues (for example, LSR D may be able to give
better security to protect against rogue leaf attachment) or network
loading concerns (for example, LSR E may already be handling a very
large amount of traffic replication for other P2MP services).
5. Reoptimization and Modification
Once established, P2MP LSPs are more sensitive to modification than
their P2P counterparts. If an egress is removed from a P2P LSP, the
whole LSP is torn down. But egresses may be added to and removed
from active P2MP LSPs as receivers come and go.
The removal of an egress from a P2MP LSP does not require any new
path computation since the tree can be automatically pruned.
The addition of a new egress to a P2MP LSP can be handled as the
computation of an appropriate branch point and the determination of a
P2P path from the branch point to the new egress. This is a
relatively simple computation and can be achieved by reverse-path
CSPF, much as in the manner of some multicast IP routing protocols.
However, repeated addition to and removal from a P2MP LSP will almost
certainly leave it in a sub-optimal state. The tree shape that was
optimal for the original set of destinations will be distorted by the
changes and will not be optimal for the new set of destinations.
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Further, as resource availability changes in the network due to other
LSPs being released or network resources being brought online, the
path of the P2MP LSP may become sub-optimal.
Computing a new optimal path for the P2MP LSP is as simple as
computing any optimal P2MP path, but selecting a path that can be
applied within the network as a migration from the existing LSP may
be more complex. Additional constraints may be applied by the
network administrator so that only subsets of the egresses (or
subtrees of the P2MP tree) are optimized at any time. In these
cases, the computational load of reoptimization may be considerable,
but fortunately reoptimization computations may be performed as
background activities. Splitting the P2MP tree into subtrees, as
described in Section 3, may further reduce the computation load and
may assist with administrative preferences for partial tree
reoptimization.
Network-wide reoptimization of multiple LSPs [RFC5557] can achieve
far greater improvements in optimality within overloaded networks
than can be achieved by reoptimizing LSPs sequentially. Such
computation would typically be performed offline and would usually
require a dedicated processor such as a PCE invoked by the NMS.
6. Repair
LSP repair is necessary when a network fault disrupts the ability of
the LSP to deliver data to an egress. For a P2MP LSP, a network
fault is (statistically) likely to only impact a small subset of the
total set of egresses. Repair activity, therefore, does not need to
recompute the path of the entire P2MP tree. Rather, it needs to
quickly find suitable new branches that can be grafted onto the
existing tree to reconnect the disconnected leaves.
In fact, immediately after a network failure there may be a very
large number of path computations required in order to restore
multiple P2P and P2MP LSPs. The PCEs will be heavily loaded, and it
is important that computation requests are restricted to only the
'essential'.
In this light, it is useful to note that the simple repair
computations described in the first paragraph of this section may be
applied to achieve a first repair of the LSPs, while more
sophisticated reoptimization computations can be deferred until the
network is stable and the load on the PCEs has been reduced. Those
reoptimizations can be computed as described in Section 5.
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7. Disjoint Paths
Disjoint paths are required for end-to-end protection services and
sometimes for load balancing. They may require to be fully disjoint
(except at the ingress and egress!), link disjoint (allowing common
nodes on the paths), or best-effort disjoint (allowing shared links
or nodes when no other path can be found).
It is possible to compute disjoint paths sequentially, but this can
lead to blocking problems where the second path cannot be placed.
Such issues are more readily avoided if the paths are computed in
parallel.
The computation of link disjoint P2P paths may be non-trivial and may
be the sort of task that an LSR offloads to a PCE because of its
complexity. The computation of disjoint P2MP paths is considerably
more difficult and is therefore a good candidate to be offloaded to a
PCE that has dedicated resources. In fact, it may well be the case
that not all P2MP-capable PCEs can handle disjoint path requests and
it may be necessary to select between PCEs based on their
capabilities.
8. Manageability Considerations
The use of PCE to compute P2MP paths has many of the same
manageability considerations as when it is used for P2P LSPs
[RFC5440]. There may be additional manageability implications for
the size of P2MP computation requests and the additional loading
exerted on the PCEs.
8.1. Control of Function and Policy
As already described, individual PCEs may choose to not be capable of
P2MP computation, and where this function is available, it may be
disabled by an operator, or may be automatically withdrawn when the
PCE becomes loaded or based on other policy considerations.
Further, a PCE may refuse any P2MP computation request or pass it on
to another PCE based on policy.
8.2. Information and Data Models
P2MP computation requests necessitate considerably more information
exchange between the LSR and the PCE than is required for P2P
computations. This will result in much larger data sets to be
controlled and modeled, and will impact the utility of any management
data models, such as MIB modules. Care needs to be taken in the
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design of such data models, and the use of other management protocols
and data modeling structures, such as NETCONF [RFC4741] and the
NETCONF Data Modeling Language (NETMOD), could be considered.
8.3. Liveness Detection and Monitoring
PCE liveness detection and monitoring is unchanged from P2P
operation, but it should be noted that P2MP requests will take longer
to process than P2P requests, meaning that the time between request
and response will be, on average, longer. This increases the chance
of a communications failure between request and response and means
that liveness detection is more important.
8.4. Verifying Correct Operation
Correct operation of any communication between LSRs and PCEs is
exactly as important as it is for P2P computations.
The correct operation of path computation algorithms implemented at
PCEs is out of scope, but LSRs that are concerned that PCE algorithms
might not be operating correctly may make identical requests to
separate PCEs and compare the responses.
8.5. Requirements on Other Protocols and Functional Components
As is clear from the PCE architecture, a communications protocol is
necessary to allow LSRs to send computation requests to PCEs and for
PCEs to cooperate. Requirements for such a protocol to handle P2P
path computations are described in [RFC4657], and additional
requirements in support of P2MP computations are described in
[PCE-P2MP]. The PCE Communication Protocol (PCEP) is defined in
[RFC5440], but extensions will be necessary to support P2MP
computation requests.
As described in the body of this document, LSRs need to be able to
recognize which PCEs can perform P2MP computations. Capability
advertisement is already present in the PCE Discovery protocols
([RFC5088] and [RFC5089]) and can also be exchanged in PCEP
([RFC5440]), but extensions will be required to describe P2MP
capabilities.
As also described in this document, the PCE needs to know the branch
capabilities of the LSRs and store this information in the TED. This
information can be distributed using the routing protocols as
described in [RFC5073].
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8.6. Impact on Network Operation
The use of a PCE to perform P2MP computations may have a beneficial
impact on network operation if it can offload processing from the
LSRs, freeing them up to handle protocol operations.
Furthermore, the use of a PCE may enable more dynamic behavior in
P2MP LSPs (such as the addition of new egresses, reoptimization, and
failure recovery) than is possible using more traditional
management-based planning techniques.
9. Security Considerations
The use of PCE to compute P2MP paths does not raise any additional
security issues beyond those that generally apply to the PCE
architecture. See [RFC4655] for a full discussion.
Note, however, that P2MP computation requests are more CPU-intensive
and also use more link bandwidth. Therefore, if the PCE was attacked
by the injection of spurious path computation requests, it would be
more vulnerable through a smaller number of such requests.
Thus, the use of message integrity and authentication mechanisms
within the PCE protocol should be used to mitigate attacks from
devices that are not authorized to send requests to the PCE. It
would be possible to consider applying different authorization
policies for P2MP path computation requests compared to other
requests.
10. Acknowledgments
The authors would like to thank Jerry Ash and Jean-Louis Le Roux for
their thoughtful comments. Lars Eggert, Dan Romascanu, and Tim Polk
provided useful comments during IESG review.
11. References
11.1. Normative References
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
11.2. Informative References
[RFC4461] Yasukawa, S., Ed., "Signaling Requirements for Point-to-
Multipoint Traffic-Engineered MPLS Label Switched Paths
(LSPs)", RFC 4461, April 2006.
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[RFC4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006.
[RFC4741] Enns, R., Ed., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[RFC4834] Morin, T., Ed., "Requirements for Multicast in Layer 3
Provider-Provisioned Virtual Private Networks (PPVPNs)",
RFC 4834, April 2007.
[RFC4875] Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
Yasukawa, Ed., "Extensions to Resource Reservation
Protocol - Traffic Engineering (RSVP-TE) for Point-to-
Multipoint TE Label Switched Paths (LSPs)", RFC 4875, May
2007.
[RFC5073] Vasseur, J., Ed., and J. Le Roux, Ed., "IGP Routing
Protocol Extensions for Discovery of Traffic Engineering
Node Capabilities", RFC 5073, December 2007.
[RFC5088] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
Zhang, "OSPF Protocol Extensions for Path Computation
Element (PCE) Discovery", RFC 5088, January 2008.
[RFC5089] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
Zhang, "IS-IS Protocol Extensions for Path Computation
Element (PCE) Discovery", RFC 5089, January 2008.
[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
December 2008.
[RFC5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
March 2009.
[RFC5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
"A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441, April
2009.
[RFC5557] Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
Computation Element Communication Protocol (PCEP)
Requirements and Protocol Extensions in Support of Global
Concurrent Optimization", RFC 5557, July 2009.
Yasukawa & Farrel Informational [Page 14]RFC 5671 PCE for P2MP MPLS and GMPLS TE October 2009
[PCE-P2MP] Yasukawa, S., and Farrel, A., "PCC-PCE Communication
Requirements for Point to Multipoint Multiprotocol Label
Switching Traffic Engineering (MPLS-TE)", Work in
Progress, May 2008.
Authors' Addresses
Seisho Yasukawa
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-Shi, Tokyo 180-8585,
Japan
EMail: yasukawa.seisho@lab.ntt.co.jp
Adrian Farrel
Old Dog Consulting
EMail: adrian@olddog.co.uk
Yasukawa & Farrel Informational [Page 15]
