Network Working Group S. Yasukawa
Internet Draft NTT
Category: Informational A. Farrel (Editor)
Expires: August 2007 Old Dog Consulting
February 2007
Applicability of the Path Computation Element (PCE) to
Point-to-Multipoint (P2MP) Multiprotocol Label Switching (MPLS)
and Generalized MPLS (GMPLS) Traffic Engineering (TE)
draft-yasukawa-pce-p2mp-app-00.txt
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Abstract
The Path Computation Element (PCE) provides path computation
functions in support of traffic engineering in Multi-Protocol 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
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.
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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 [P2MP-RSVP]. P2MP MPLS TE
networks are considered in support of various features including
layer 3 multicast VPNs, 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 [TE-NODE-CAP].
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 (for example, 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, administration policies may dictate that replication
should be concentrated on specific key replication nodes behaving
like IP mlticast rendezvous points (perhaps to ensure appropriate
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policing of receivers in the P2MP tree, or perhaps to make proteciton
and resilliency easier to implement).
Path computation for P2MP TE LSPs presents a significant challenge
because of the complexity of the computation. 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 may 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 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) where more planning is
normally a feature of LSP provisioning and where longer setup times
are tollerated.
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
network. The PCE server may be another LSR in the network, a
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dedicated server, or a function of an NMS. Further, 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.
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, this load is an order of magnitude greater than the
combined load of computing a single optimal path to each leaf of the
P2MP 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.
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 withint the network. It is obviously critically important that
existing traffic should not be disrupted by the computation of a path
for a new LSP.
Nor is it 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 ingress 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
can be provided on dedicated or high-capacity network elements (such
as dedicated servers, or high-end routers such as might be located as
Autonomous System Border Routers - ASBRs).
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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.
Further, 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
[RFC4655].
2.2.3. PCE Capabilities
As indicated above, an LSR may choose between PCEs that are available
to select the PCE 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.
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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 fo 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 [RC4655].
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 Cmputation 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
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BRPC technique would be particularly applicable.
4. Central Replication Points
A deployment model for P2MP LSPs is to use centralised, 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
o.
f--i--m
/
/
a--b--c--D--g--J--n
\ \
\ \
E h K o
\
\
l--p
Figure 2 : Using Centralised 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 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
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better security to protect against rogue leaf attachment) or network
loading concerns (for example, LSR E may already be handling a vary
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
reoptimization.
Network wide, reoptimization of multiple LSPs can achieve for 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.
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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 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
'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 have 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 ssues are more readily avoided if the paths are computed in
parallel.
The computation of link disjoint P2P paths is 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.
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3. 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 of the size of P2MP
computation requests and the additional loading exerted on the PCEs.
3.1. Control of Function and Policy
As already described, individual PCEs may choose 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.
3.2. 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.
3.3. 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.
3.4. 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 nervouse LSRs may make identical requests
to separate PCEs and compare the responses.
3.5. Requirements on Other Protocols and Functional Components
As is clear from the PCE architecture, a communications protocol is
necessary to allow LSRs to send computaiton 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
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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
requests.
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
[PCE-OSPF] and [PCE-ISIS], 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 [TE-NODE-CAP].
3.6. 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.
Further, 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.
4. 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
requests.
5. IANA Considerations
This document makes no requests for IANA action.
6. Acknowledgments
TBD
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7. References
7.1. Normative Reference
[RFC4655] Farrel, A., Vasseur, J.P., and Ash, G., "A Path
Computation Element (PCE)-Based Architecture",
RFC 4655, August 2006.
7.2. 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.
[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.
[P2MP-RSVP] Aggarwal, R., Papadimitriou, D., and Yasukawa, S.,
"Extensions to RSVP-TE for Point to Multipoint TE
LSPs", draft-ietf-mpls-rsvp-te-p2mp, work in progress.
[PCE-ISIS] Le Roux, J.L., and Vasseur, J.P., Editors, "IS-IS
protocol extensions for Path Computation Element (PCE)
Discovery", draft-ietf-pce-disco-proto-isis, work in
progress.
[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-OSPF] Le Roux, J.L., and Vasseur, J.P., Editors, "OSPF
protocol extensions for Path Computation Element (PCE)
Discovery", draft-ietf-pce-disco-proto-ospf, 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
progress.
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[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
progress.
[TE-NODE-CAP] Vasseur, J.P, and Le Roux, J.L., Editors, "IGP Routing
Protocol Extensions for Discovery of Traffic
Engineering Node Capabilities", draft-ietf-ccamp-te-
node-cap, work in progress.
8. 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
Email: s.yasukawa@hco.ntt.co.jp
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
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Yasukawa and Farrel Expires August 2007 [Page 14]