Internet DRAFT - draft-yasukawa-pce-p2mp-app


Network Working Group                                        S. Yasukawa
Internet Draft                                                       NTT
Category: Informational                               A. Farrel (Editor)
Created: February 15, 2008                            Old Dog Consulting
Expires: August 15, 2008

         Applicability of the Path Computation Element (PCE) to
     Point-to-Multipoint (P2MP) Multiprotocol Label Switching (MPLS)
       and Generalized MPLS (GMPLS) Traffic Engineering (TE)


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   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.

Yasukawa and Farrel                                             [Page 1]
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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 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 colocated 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]. 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
   constrainted 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 is 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
   LSP in certain types of network, 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 mlticast rendezvous points (perhaps to ensure appropriate

Yasukawa and Farrel                                             [Page 2]
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   policing of receivers in the P2MP tree, or perhaps to make protection
   and resilliency 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
   management plane operation necessary to maintain existing LSPs. The
   PCE architecture offers a way to offload such path computations from

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 colocated with the LSR that makes the request, or
   may be present in a computation-capable PCE server within the

Yasukawa and Farrel                                             [Page 3]
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   network. The PCE server may be another LSR in the network, a
   dedicated server, or a functional component of an 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

   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.

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 there exist algorithms 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 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 should 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 which is to off-load P2MP path
   computations from LSRs to remotely located PCEs. Such PCE function

Yasukawa and Farrel                                             [Page 4]
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   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).

2.2.2. PCE Congestion

   Since P2MP path computations are resource-intensive, it may be that
   the introduction of P2MP LSPs into an established PCE network will
   cause congestion 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 [PCE-POLICY].

   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 congestion 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 to other less-burdened
   PCEs within the network using the PCE cooperation model described in

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 congestion 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 as it discovers their
   existence. Further and more detailed exchanges of PCE capabilities
   can be made directly between the PCEs and the LSRs.

Yasukawa and Farrel                                             [Page 5]
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3. Fragmenting the P2MP Tree

   A way to reduce the computational burden on a single PCE of computing
   a large P2MP tree 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 exgress 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 computaiton response to PCE A. The mechanisms for both of
   these approaches are described in the PCE architecture [RFC4655].

                         \        \
                          \        \
                           j        x
                               \  \
                                \  \
                                 s  z

       Figure 1 : A P2MP Tree with Intermediate Computation Points

   A futher 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 [BRPC]. Indeed,
   if the LSRs K, L, and M were ASBRs or Area Border Routers (ABRs) the

Yasukawa and Farrel                                             [Page 6]
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   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, nor 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

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, and
   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 sope 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, and 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

                           \     \
                            \     \
                          E  h  K  o

           Figure 2 : Using Centralised Replication Points

Yasukawa and Farrel                                             [Page 7]
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   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 replicaiton 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 on 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 suboptimal 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.

   Further, as resource availability changes in the network due to other
   LSPs being released or network resources being brough 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

Yasukawa and Farrel                                             [Page 8]
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   Network-wide reoptimization of multiple LSPs [GCO] can achieve far
   greater improvements in optimality within congested 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 diconnected 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

   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.

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 sharing of
   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

   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

Yasukawa and Farrel                                             [Page 9]
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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. 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.

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 nervous LSRs 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

Yasukawa and Farrel                                            [Page 10]
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   [PCE-P2MP-REQ]. The PCE Communicaiton Protocol (PCEP) is defined in
   [PCEP], but extensions will be necessary to support P2MP computation

   As described in the body of this document, LSRs need to be able to
   recognise 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 [PCEP],
   but extensions will be required to describe P2MP capablities.

   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].

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 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.

   It would be possible to consider applying different authorization
   policies for P2MP path computation requests compared to other

10. IANA Considerations

   This document makes no requests for IANA action.

11. Acknowledgments

   The authors would like to thank Jerry Ash and Jean-Louis Le Roux for
   their thoughtful comments.

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12. References

12.1. Normative Reference

   [RFC4655]      Farrel, A., Vasseur, J.P., and Ash, G., "A Path
                  Computation Element (PCE)-Based Architecture",
                  RFC 4655, August 2006.

12.2. Informative Reference

   [RFC4461]      S. Yasukawa, Editor, "Signaling Requirements for
                  Point-to-Multipoint Traffic Engineered MPLS LSPs",
                  RFC4461, April 2006.

   [RFC4657]      Ash, J., and Le Roux, J.L., "Path Computation Element
                  (PCE) Communication Protocol Generic Requirements",
                  RFC 4657, September 2006.

   [RFC4834]      Morin, T., "Requirements for Multicast in Layer 3
                  Provider-Provisioned Virtual Private Networks
                  (PPVPNs)", RFC 4834, April 2007.

   [RFC4875]      Aggarwal, R., Papadimitriou, D., and Yasukawa, S.,
                  "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.P, and Le Roux, J.L., Editors, "IGP Routing
                  Protocol Extensions for Discovery of Traffic
                  Engineering Node Capabilities", RFC 5073, December

   [RFC5088]      Le Roux, J.L., and Vasseur, J.P., Editors, "OSPF
                  Protocol Extensions for Path Computation Element (PCE)
                  Discovery", RFC 5088, January 2008.

   [RFC5089]      Le Roux, J.L., and Vasseur, J.P., Editors, "IS-IS
                  Protocol Extensions for Path Computation Element (PCE)
                  Discovery", RFC 5089, January 2008.

   [BRPC]         J.P. Vasseur, Editor, "A Backward Recursive PCE-based
                  Computation (BRPC) procedure to compute shortest
                  inter-domain Traffic Engineering Label Switched
                  Paths", draft-ietf-pce-brpc, work in progress.

   [GCO]          Lee, Y., Le Roux, JL., King, D., and Oki, E., "Path
                  Computation Element Communication Protocol (PCECP)
                  Requirements and Protocol Extensions In Support of
                  Global Concurrent Optimization", draft-ietf-pce-
                  global-concurrent-optimization, work in progress.

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   [PCE-P2MP-REQ] Yasukawa, S., and Farrel, A., "PCC-PCE Communication
                  Requirements for Point to Multipoint Multiprotocol
                  Label Switching Traffic Engineering (MPLS-TE)",
                  draft-yasukawa-pce-p2mp-req, work in progress.

   [PCE-POLICY]   Bryskin, I., Papadimitriou, D., and Berger, L.,
                  "Policy-Enabled Path Computation Framework",
                  draft-ietf-pce-policy-enabled-path-comp, work in

   [PCEP]         Vasseur, J.P, and Le Roux, J.L., Editors, "Path
                  Computation Element (PCE) communication Protocol
                  (PCEP) - Version 1", draft-ietf-pce-pcep, work in

13. Authors' Addresses

   Seisho Yasukawa
   NTT Corporation
   (R&D Strategy Department)
   3-1, Otemachi 2-Chome Chiyodaku, Tokyo 100-8116 Japan
   Phone: +81 3 5205 5341

   Adrian Farrel
   Old Dog Consulting

14. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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Yasukawa and Farrel                                            [Page 13]
draft-yasukawa-pce-p2mp-app-02.txt                         February 2008

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Yasukawa and Farrel                                            [Page 14]