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Versions: 00 01 draft-ietf-pce-architecture

IETF Internet Draft PCE Working Group                     Adrian Farrel
Proposed Status: Informational                       Old Dog Consulting
                                                  Jean-Philippe Vasseur
                                                    Cisco Systems, Inc.
                                                              Jerry Ash
                                                                   AT&T
                                                          February 2005


                   draft-ash-pce-architecture-01.txt

             Path Computation Element (PCE) Architecture


Status of this Memo

By submitting this Internet-Draft, I certify that any applicable patent
or other IPR claims of which I am aware have been disclosed, and any of
which I become aware will be disclosed, in accordance with RFC 3668.

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Abstract

Constraint-based path computation is a fundamental building block for
traffic engineering systems such as Multiprotocol Label Switching (MPLS)
and Generalized Multiprotocol Label Switching (GMPLS) networks. Path
computation in large, multi-domain, multi-region or multi-layers
networks is highly complex and may require special computational
components and cooperation between the different network domains.

This document specifies the architecture for a Path Computation Element
(PCE)-based model to address this problem space.


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Table of Contents

1. Conventions used in this document . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Motivation for a PCE-based Architecture . . . . . . . . . . . . . 4
   4.1. CPU-intensive Path Computation/Global Optimization . . . . . 5
   4.2. Partial Visibility . . . . . . . . . . . . . . . . . . . . . 5
   4.3. Absence of the TED or Use of Non-TE-Enabled IGP  . . . . . . 5
   4.4. Node Outside the Routing Domain  . . . . . . . . . . . . . . 6
   4.5. Network Element Lacks Control Plan or Routing Capability . . 6
   4.6. Backup Path Computation for Bandwidth Protection . . . . . . 6
   4.7. Multi-Layer Networks . . . . . . . . . . . . . . . . . . . . 6
5. Overview of the PCE-Based Architecture  . . . . . . . . . . . . . 7
   5.1. Composite PCE  . . . . . . . . . . . . . . . . . . . . . . . 7
   5.2. External PCE . . . . . . . . . . . . . . . . . . . . . . . . 8
   5.3. Multiple PCE Path Computation  . . . . . . . . . . . . . . . 9
   5.4. Multiple PCE Path Computation with Inter-PCE Communication . 10
   5.5. Areas for Standardization  . . . . . . . . . . . . . . . . . 10
6. PCE Architectural Considerations  . . . . . . . . . . . . . . . . 10
   6.1. Centralized Computation Model  . . . . . . . . . . . . . . . 11
   6.2. Distributed Computation Model  . . . . . . . . . . . . . . . 11
   6.3. Synchronization  . . . . . . . . . . . . . . . . . . . . . . 11
   6.4. PCE Discovery and Load Balancing . . . . . . . . . . . . . . 12
   6.5. Detecting PCE Liveness . . . . . . . . . . . . . . . . . . . 13
   6.6. PCC-PCE & PCE-PCE Communication  . . . . . . . . . . . . . . 13
   6.7. PCE TED Synchronization  . . . . . . . . . . . . . . . . . . 14
   6.8. Stateful Versus Stateless PCEs . . . . . . . . . . . . . . . 15
   6.9. Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.10. Policy and Confidentiality  . . . . . . . . . . . . . . . . 16
7. PCE Evaluation Metrics  . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 18
11. Intellectual Property Considerations . . . . . . . . . . . . . . 18
12. Normative References . . . . . . . . . . . . . . . . . . . . . . 18
13. Informational References . . . . . . . . . . . . . . . . . . . . 19
14. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
15. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 20


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1. Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

2. Terminology

CSPF: Constraint-based Shortest Path First.

LER: Label Edge Router.

LSDB: Link State Database.

LSP: Label Switched Path.

LSR: Label Switching Router.

PCC: Path Computation Client : any client application requesting a path
computation to be performed by the Path Computation Element.
PCE: Path Computation Element: an entity (component, application or
network node) that is capable of computing a network path or route based
on a network graph and applying computational constraints (see further
description in section 3).

TED: Traffic Engineering Database which contains the topology and
resource information of the domain. The TED may be fed by IGP extensions
or potentially by other means.

TE LSP: Traffic Engineering MPLS Label Switched Path.

3. Definitions

A Path Computation Element (PCE) is an entity that is capable of
computing a network path or route based on a network graph, and applying
computational constraints. The PCE entity is an application that can be
located within a network node or component, on an out-of-network server,
etc. For example, a PCE would be able to compute the path of a TE LSP by
operating on the TED and considering the bandwidth and other constraints
applicable to the TE LSP service request.

A domain is any collection of network elements within a common sphere of
address management or path computational responsibility. Examples of
domains include IGP areas, Autonomous Systems (ASs), multiple ASs within
a service provider network, or multiple ASs across multiple service
provider networks.  However, domains of computational responsibility may
also exist as sub-domains of areas or ASs.

In order to fully characterize a PCE and clarify these definitions, the
following important considerations must also be examined:

1) Path computation is applicable in both intra-domain, inter-domain,
and inter-layer contexts. Inter-domain path computation may involve the
correlation of topology and routing information between domains.
Inter-layer path computation refers to the use of PCE where multiple

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layers are involved and when the objective is to perform path
computation at one or multiple layers while taking into account
topology and resource information at these layers.  Overlapping domains
are not within the scope of this document.  In the inter-domain case,
the domains may belong to a single or multiple Service Providers.

2) In "single PCE path computation," a single PCE is used to compute a
given path in a domain. In "multiple PCE path computation," multiple
PCEs are used to compute a given path in a domain.

3) "Centralized computation model" refers to a model whereby all paths
in a domain are computed by a single, centralized PCE. Conversely,
"Distributed computation model" refers to the computation of paths in a
domain being shared among multiple PCEs. Paths that span multiple
domains may be computed using the distributed model with a PCE
responsible for each domain, or the centralized model by defining a
domain that encompasses all of the other domains. From these
definitions, a centralized computation model inherently uses single PCE
path computation. However, a distributed computation model could use
either single PCE path computation or multiple PCE path computations.
There would be no such thing as a centralized model which uses multiple
PCE path computations.

4) The PCE may or may not be located at the head-end of the path. For
example, a conventional intra-domain solution is to have path
computation performed by the head-end LSR of an MPLS TE LSP; in this
case, the head-end LSR contains a PCE. But solutions also exist where
other nodes on the path must contribute to the path computation (for
example, loose hops) making them PCEs in their own right. At the same
time, the path computation may be made by some other PCE physically
distinct from the computed path.

5) The path computed by the PCE may be an 'explicit PCE path' (that is,
the full explicit path from start to destination, made of a list of
strict hops) or a 'strict/loose PCE path' (that is, a mix of strict and
loose hops comprising of at least one loose hop representing the
destination), where a hop may be an abstract node such as an AS.

6) A PCE-based path computation model does not mean to be exclusive and
can be used in conjunction with other path computation models. For
instance, the path of an inter-AS TE LSP may be computed using a
PCE-based path computation model in some IGP areas, whereas the set of
traversed ASes may be specified by other means (not determined by any
PCE).

7) This document does not make any assumptions about the nature or
implementation of a PCE. A PCE could be implemented on a router, an LSR,
a dedicated network server, etc. Moreover, the PCE function is
orthogonal to the forwarding capability of the node on which it is
implemented.

4. Motivation for a PCE-based Architecture

Several motivations for a PCE-based architecture (described in section
5) are listed below. This list is not meant to be exhaustive and is
provided for the sake of illustration.

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It should be highlighted that the aim of this section is to provide some
application examples for which a PCE-based path may be suitable: this
also clearly states that such a model does not aim to replace existing
path computation model but would apply to specific existing situations.

4.1. CPU-intensive Path Computation/Global Optimization

There are many situations where the computation of a path may be highly
CPU-intensive: examples of CPU-intensive path computations include the
resolutions of NP-complete problems such as:

- Global optimization in placing a set of TE LSPs within a domain so as
to optimize an objective function (for example, minimization of the
maximum link utilization)

- Multi-criteria path computation (for example, delay and link
utilization, inclusion of switching capabilities, adaptation features,
encoding types and optical constraints within a GMPLS optical network)

- Computation of minimal cost Point to Multipoint trees (Steiner
trees).

In these situations, it may not be possible or desirable for a router to
perform path computation because of the constraints on its CPU, in which
case the path computation may be off-loaded to some other PCE(s).

4.2. Partial Visibility

There are several scenarios where the node responsible for path
computation has limited visibility of the network topology to the
destination. This limitation may occur, for instance, when an ingress
router attempts to establish an LSP to a destination that lies in a
separate domain, since TE information is not exchanged across the domain
boundaries. In such cases, it is possible to use loose routes to
establish the LSP, relying on routers at the domain borders to establish
the next piece of the path, however, it is not possible to guarantee
that the optimal (shortest) path will be used, nor even that a viable
path will be discovered except, possibly, through repeated trial and
error using crankback or other signaling extensions.

This problem of inter-domain path computation may most probably be
addressed through distributed computation with cooperation among PCEs
within each of the domains, or perhaps by using a central "all-seeing"
PCE. In this latter case there are challenges of scalability (both the
size of the TED and the responsiveness of a single PCE handling requests
for many domains) and of preservation of confidentiality when the
domains belong to different Service Providers.

Note that the issues described here can be further highlighted in the
context of LSP re-optimization, or the establishment of multiple diverse
LSPs for protection or load sharing.

4.3. Absence of the TED or use of Non-TE-Enabled IGP

The traffic engineering database (TED) may be a large drain on the

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resources of a network node (such as an edge router or LER) both from a
memory perspective and because it may require non-negligible CPU
activity to maintain. The use of a distinct PCE may be appropriate in
such circumstances, and a separate node can be used to establish and
maintain the TED, and to make it available for path computation.

The IGPs run within some networks are not sufficient to build a full
TED. For example, a network may run OSPF/IS-IS without the
OSPF-TE/ISIS-TE extensions, or some routers in the network may not
support the TE extensions. In these cases, in order to successfully
compute paths through the network, the TED must be constructed or
supplemented through configuration action, and updated as network
resources are reserved or released. Such a TED could be distributed to
each router so that each router can perform path computation, or held
centrally (on a distinct node that supports PCE) for centralized path
computation.

4.4. Node Outside the Routing Domain

An LER might not be part of the routing domain for administrative
reasons (for example, a customer-edge (CE) router connected to the
provider-edge (PE) router in the context of MPLS VPN [RFC2547] and for
which it is desired to provide a CE to CE TE LSP path).

This scenario suggests a solution that does not involve doing
computation on the ingress router, and that does not rely on static
loose hops configuration in which case optimal shortest paths could not
be achieved.  A distinct PCE-based solution can help here. Note that the
PCE in this case may, itself, provide a path that includes loose hops.

4.5. Network Element Lacks Control Plan or Routing Capability

It is common in legacy optical networks for the network elements not to
have a control plane or routing capability. On such network elements
there only exists the data plane and management plane, and all
cross-connections are made from the management plane. It is desirable in
this case to run the path computation on the PCE, and send the
cross-connection commands to each node on the computed path. This
scenario is important for ASON-capable networks, and may also be used
for interworking between GMPLS-capable and GMPLS-incapable networks.

4.6. Backup Path Computation for Bandwidth Protection

A PCE can be used to compute backup paths in the context of fast reroute
protection of TE-LSPs. In this model all backup TE-LSPs protecting a
given facility are computed in a coordinated manner by a PCE. This
allows ensuring complete bandwidth sharing between bypass tunnels
protection independent elements, while avoiding any extensions to LSP
signaling. Both centralized and distributed computation models are
applicable. In the distributed case each LSR can be a PCE to compute its
own protection.

4.7. Multi-Layer Networks

A server-layer network of one switching capability may support multiple
networks of another (more granular) switching capability. For example, a

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TDM network may provide connectivity for client-layer networks such as
IP, MPLS or Layer 2 [MRN].

The server-layer network is unlikely to provide the same connectivity
paradigm as the client networks so that bandwidth granularity in the
server-layer network may be much coarser than in the client-layer
network. Similarly, there is likely to be a management separation
between the two networks providing independent address spaces.
Further, where multiple client-layer networks make use of the same
server-layer network, those client-layer networks may have
independent policies, control parameters, address spaces and routing
preferences.

The different client and server layer networks may be considered as
distinct path computation regions within a PCE domain, and so the PCE
architecture is
useful to allow path computation from one client-layer network region,
across the server-layer network to another client-layer network region.

In this case, the PCE is responsible for resolving address space issues,
handling differences in policy and control parameters, and coordinating
resources between the networks. Note that, because of the differences
in bandwidth granularity, connectivity across the server-layer network
may be provided through virtual TE links or Forwarding Adjacencies:
the PCE may offer a point of control responsible for the decision to
provision new TE links or Forwarding Adjacencies across the
server-layer network.

5. Overview of the PCE-Based Architecture

This section is intended to give an overview of the network architecture
of the PCE model. It needs to be read in conjunction with the details
provided in the next section to provide a full view of the flexibility
of the model.

5.1. Composite PCE

Figure 1 below shows the components of a typical composite PCE node
(that is, a router that also implements the PCE functionality) that
utilizes path computation. The routing protocol is used to exchange TE
information from which the TED is constructed. Service requests to
provision TE LSPs are received by the node and converted into signaling
requests, but these may first require path computation which is
requested from a Path Computation Element, the PCE. The PCE operates on
the TED in order to respond with the requested path.


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          ---------------
         |   ---------   | Routing   ----------
         |  |         |  | Protocol |          |
         |  |   TED   |<-+----------+->        |
         |  |         |  |          |          |
         |   ---------   |          |          |
         |      |        |          |          |
         |      | Input  |          |          |
         |      v        |          |          |
         |   ---------   |          |          |
         |  |         |  |          | Adjacent |
         |  |   PCE   |  |          |   Node   |
         |  |         |  |          |          |
         |   ---------   |          |          |
         |      ^        |          |          |
         |      |Request |          |          |
         |      |Response|          |          |
         |      v        |          |          |
         |   ---------   |          |          |
Service  |  |         |  | Signaling|          |
 Request |  |Signaling|  | Protocol |          |
   ------+->| Engine  |<-+----------+->        |
         |  |         |  |          |          |
         |   ---------   |           ----------
          ---------------

             Figure 1. Composite PCE Node

Note that the routing adjacency between the composite PCE node and any
other router may be performed by means of direct connectivity or any
tunneling mechanism.

5.2. External PCE

Figure 2 shows PCE support that is external from the requesting network
element. A service request is received by the head-end node and before
it can signal to establish the service it makes a request to the
external PCE for a path to be computed. The PCE makes the computation
using the TED and returns a response.


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        ----------
       |  -----   |
       | | TED |<-+------------>
       |  -----   |  TED synchronization
       |    |     |  mechanism (for example, routing protocol)
       |    |     |
       |    v     |
       |  -----   |
       | | PCE | |
       |  -----   |
        ----------
            ^
            | Request/
            | Response
            v
Service ----------  Signaling   ----------
Request| Head-End | Protocol   | Adjacent |
  ---->|  Node    |<---------->|   Node   |
        ----------              ----------

             Figure 2. External PCE Node

Note that in this case, the node that supports the PCE function may also
perform forwarding, but those functions are purely orthogonal.

5.3. Multiple PCE Path Computation

Figure 3 illustrates how multiple PCE path computations may be performed
along the path of a signaled service. As in the previous example, the
head-end PCC makes a request to an external PCE, but the path that is
returned is such that the next network element finds it necessary to
perform further computation. It consults another PCE to establish the
next hop in the path.

Note that either or both PCEs in this case could be co-resident with the
network node as in Section 5.1.

        ----------              ----------
       |          |            |          |
       |   PCE    |            |   PCE    |
       |          |            |          |
       |   -----  |            |   -----  |
       |  | TED | |            |  | TED | |
       |   -----  |            |   -----  |
        ----------              ----------
            ^                        ^
            | Request/               | Request/
            | Response               | Response
            v                        v
Service ----------  Signaling   -------------  Signaling  ------------
Request| Head-End | Protocol   |Intermediate | Protocol  |Intermediate|
  ---->| Node     |<---------->|    Node     |<--------->|    Node    |
        ----------              -------------             ------------

             Figure 3. Multiple PCE Path Computation


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5.4. Multiple PCE Path Computation with Inter-PCE Communication

The PCE in Section 5.3 was not able to supply a full path for the
requested service and this resulted in the adjacent node needing to make
its own computation request. As illustrated in Figure 4, the same
problem is solved by introducing inter-PCE communication and cooperation
between PCEs so that the PCE consulted by the head-end network node
makes a request of another PCE to help with the computation.

        ----------                                     ----------
       |          |   Inter-PCE Request/Response      |          |
       |   PCE    |<--------------------------------->|   PCE    |
       |          |                                   |          |
       |   -----  |                                   |   -----  |
       |  | TED | |                                   |  | TED | |
       |   -----  |                                   |   -----  |
        ----------                                     ----------
            ^
            | Request/
            | Response
            v
Service ----------  Signaling   ----------  Signaling   ----------
Request| Head-End | Protocol   | Adjacent | Protocol   | Adjacent |
  ---->|  Node    |<---------->|   Node   |<---------->|   Node   |
        ----------              ----------              ----------

  Figure 4. Multiple PCE Path Computation with Inter-PCE Communication

Multiple PCE path computation with inter-PCE communication involves
coordination between distributed PCEs such that the result of the
computation performed by one PCE depends on information supplied by
other PCEs. PCE-PCE communication is discussed further in section 6.6.

Note that a PCC cannot see the difference between centralized
computation, and multiple PCE path computation with inter-PCE
communication. That is, the PCC network node or component that requests
the computation makes a single request and receives a full or partial
path in response, but the response is actually achieved through the
coordinated, cooperative efforts of more than one PCE.

5.5 Areas for Standardization

According to the PCE charter, the following are the standardization
areas that the PCE working group will address:

- communication between PCCs and PCEs, and between cooperating PCEs
- requirements for extensions to existing routing and signaling
protocols in support of PCE discovery and signaling of inter-domain
paths
- definition of metrics to evaluate path quality, scalability,
  responsiveness and robustness of path computation models.

6. PCE Architectural Considerations

The aim of this section is to provide a list of the PCE architectural
components. Specific realizations and implementation details (state

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machines or algorithms, etc.) of PCE-based solutions are out of the
scope of this document.

Note also that PCE-based path computation does not affect in any way the
use of the computed paths. For example, the use of PCE does not change
the way in which Traffic Engineering LSPs are signaled, maintained and
torn down, but strictly relates to the path computation aspects of such
TE LSPs.

6.1. Centralized Computation Model

A "centralized computation model" considers that all path computations
for a given domain will be performed by a single, centralized PCE. This
may be a dedicated server (for example, an external PCE node), or a
nominated router (for example, a composite PCE node) in the network. In
this model, all PCCs in the domain would send their path computation
requests to the central PCE. While a domain in this context might be an
IGP area or AS, it might also be a sub-group of network nodes that is
defined by its dependence on the PCE.

6.2. Distributed Computation Model

A "distributed computation model" refers to a domain or network that may
include multiple PCEs, and where computation of paths is shared among
the PCEs. A given path may in turn be computed by a single PCE ("single
PCE path computation") or multiple PCEs ("multiple PCE path
computation"). A PCC may be linked to a particular PCE, or may be able
to choose freely among several PCEs - the method of choice between PCEs
is out of scope of this document, but see section 6.4. It will often be
the case that the computation of an individual path is performed
entirely by a single PCE. For example, this is usually the case in MPLS
TE within a single IGP where the ingress LSR/composite node is
responsible for computing the path or for contacting an external PCE.
Conversely, multiple PCE path computation implies the involvement of
more than one PCE in the computation of a single path. An example of
this is where loose hop expansion is performed by transit LSRs/composite
nodes on an MPLS TE LSP. Another example is the use of multiple
cooperative PCE involved in the computation of a single LSP path.

6.3. Synchronization

It is often the case that multiple paths need to be computed to support
a single service (for example, for protection or load sharing).
A PCC that determines that it requires more than one path to be computed
may send a series of individual requests to the PCE. In this case, the
PCE may make multiple individual path computations to generate the set
of paths - the resultant paths are non-synchronized and are exactly
those that would have been generated had the PCC made multiple requests.
In this case of non-synchronized path computation, the path computation
of a set of TE LSPs can be shared among a set of PCEs (that is, one path
computed by each PCE). Furthermore, each PCE can be backed up by one or
more PCEs, should it fail.

Conversely, the PCC may issue a single request to the PCE asking for all
of the paths. The PCE will then in turn perform the simultaneous
computation of the set of requested path. Such synchronized computation

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usually provides more optimal results.

The involvement of more than one PCE in the computation of a series of
paths is by its nature non-synchronized. However, a set of cooperating
PCEs may be synchronized under the control of a single PCE. For example,
a PCC may send a request to a PCE which invokes domain specific
computations by other PCEs before supplying a result to the PCC.

It is desirable to add a parameter to the PCC-PCE protocol to request
alternate paths should the primary path fail to complete. While
alternate paths may not always be successful if the primary fails,
including alternate paths in a PCE response could perhaps have less
overhead than having the PCC make separate requests for a second path,
third path, etc. This technique is used in some existing CSPF
implementations.

6.4. PCE Discovery and Load Balancing

The PCE architecture requires that the PCC knows the location of one or
more PCEs that it can use for the computation of a path. Such knowledge
may come through a discovery mechanism that simply relies on local
configuration, or can imply dynamic PCE discovery along with various
static (for example, Boolean capability) or dynamically computed
variables (for example, computing resources). Proxy PCE advertisement
whereby the existence of a PCE is advertised via a proxy PCE is a viable
alternative, should the PCE be incapable of such advertisement itself.
In this later case, it is a requirement for the proxy to adequately
advertise the PCE status and capability in a timely and synchronized
fashion.

In the event that multiple PCEs are available to serve a particular path
computation request, the PCC must select a PCE to satisfy the request.
The details of such a selection, in order for instance to efficiently
share the computation load across multiple PCEs, is local to the PCC and
out of the scope of this document.

A PCE SHOULD advertise its capabilities, such as:

- set of constraints that it can account for (diversity, SRLGs,
  Optical impairments, wavelengh continuity, etc.)
- number of switching capability layers (and which ones)
- number of path selection criteria (and which ones)
- whether it is a stateless PCE or it can send updates about
  better paths that might be available in the future
- whether it can compute P2MP trees (and which types)
- whether it can ensure resource sharing between backup tunnels

This information would help a PCC that dynamically learns about
PCEs  available on the network to decide which of them to use.
Alternatively, a PCC might ask a PCE to perform a particular type
of service and receive a response that says it is unable to
perform the service, specify the things it can do. Note that the
parameters mentioned above are not meant to be exhaustive and are
listed for the sake of illustration.


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6.5. Detecting PCE Liveness

The ability to detect a PCE's liveness is a mandatory piece of the
overall architecture and could be achieved by several means. If some
form of regular advertisement (such as through IGP extensions) is used
for PCE discovery, it is expected that the PCE liveness will be
determined by means of status advertisement (for example, IGP LSA/LSPs).

The failure of a PCE while processing a request, or the inability of a
PCE to service a request (perhaps due to excessive load) may be
determined by the PCC through the use of timers. This is particularly
true in the case of inter-domain path computation where the PCE liveness
may not be detected by means of the IGP. The detection of a PCE failure
can be achieved by using the PCC-PCE protocol, much like the mechanisms
involving timers used in RSVP and LDP.

6.6. PCC-PCE & PCE-PCE Communication

Once the PCC has selected a PCE, and provided that the PCE is not local
to the PCC, a request/response protocol is required for the PCC to
communicate the path computation requests to the PCE and for the PCE to
return the path computation response.

The path computation request may include a significant set of
requirements including

- the source and destination of the path
- the bandwidth and other QoS parameters desired
- resources, resource affinities and shared risk link groups (SRLGs) to
  use/avoid
- the number of disjoint paths required and if near-disjoint paths are
  acceptable
- the level of robustness of the path resources
- and so on.

The level of robustness of the path resources covers a qualitative
assessment of the vulnerability of the resources that may be used.  For
example, one might grade resources based on empirical evidence (mean
time between failures), on known risks (there is major building work
going on near this conduit), or on prejudice (vendor X's software is
always crashing).  A PCC could request that only robust resources be
used, or allow any resource.  Of course, this information does not
comprise part of the TE information advertise by IGPs. It must come from
somewhere else.

In case of a positive response from the PCE, one or more paths would be
returned to the requesting node. In the event of a failure to compute
the desired path(s), an error is returned together with as much
information as possible about the reasons for the failure, and
potentially advice about which constraints might be relaxed to be more
likely to achieve a positive result.

Note that the resultant path(s) may be made up of a set of strict or
loose hops, or any combination of strict and loose hops. Moreover, a hop
may have the form of a non-explicit abstract node.


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A request/response protocol is also required for a PCE to communicate
path computation requests to another PCE and for the PCE to return the
path computation response. The path computation request may include a
significant set of requirements including those defined above. In case
of a positive response from the PCE, one or more paths would be returned
to the requesting PCE. In the event of a failure to compute the desired
path(s), an error is returned together with as much information as
possible about the reasons for the failure, and potentially advice about
which constraints might be relaxed to be more likely to achieve a
positive result.  Note that the resultant path(s) may be made up of a
set of strict or loose hops, or any combination of strict and loose
hops. Moreover, a hop may have the form of a non-explicit abstract node.

No assumption is made at this stage about whether the PCC-PCE and
PCE-PCE communication protocols are identical.

6.7. PCE TED Synchronization

As previously described, the PCE operates on a TED. Information on
network status to build the TED may be provided in the domain by various
means:

1) Participation in IGP distribution of TE information. The
standard method of distribution of TE information within an IGP area is
using extensions to the IGP. This mechanism allows participating nodes
to build a TED, and this is the standard technique, for example, within
a single area MPLS network. A node that hosts the PCE function may
collect TE information in this way by maintaining at least one routing
adjacency with a router in the domain. The PCE node may be adjacent or
non-adjacent (via some tunneling techniques) to the router. Such a
technique provides a mechanism for ensuring that the TED is efficiently
synchronized with the network state and is the normal case, for example,
when the PCE is co-resident with the LSRs in an MPLS network.

2) Out-of-band TED synchronization. It may not be convenient or
possible for a PCE node to participate in the IGPs of one or more
domains (for example, when there are very many domains, when IGP
participation is not desired, or when some domains are not running
TE-aware IGPs). In this case some mechanism may need to be defined to
allow the PCE node to retrieve the TED from each domain. Such a
mechanism could be incremental (like the IGP in the previous case), or
could involve a bulk transfer of the complete TED. The latter might
significantly limit the capability to ensure TED synchronization which
might result in an increase in the failure rate of computed paths.
Consideration should also be given to the impact of the TED distribution
on the network and on the network node within the domain that is asked
to distribute the database. This is particularly relevant in the case of
frequent network state changes.

3) Information in the TED can include LSP information obtained
from sources other than the IGP.  For example, this information
can include LSP routes, reserved bandwidth, and measured traffic
volume passing through the LSP.  Such LSP information is required
to perform LSP re-optimization, as described in Sections 4.4 and
7, which can be take into account the traffic fluctuations.  Also,
such LSP information is needed to reconfigure virtual network

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topology (VNT), in which lower layer LSPs such as optical paths
form the VNT.  The VNT is used for routing of higher-region
traffic such as IP traffic.

Note that synchronization techniques apply to both intra- and
inter-domain TEDs. Further, the techniques can be mixed for use with
different domains. The degree of synchronization between the PCE and the
network is subject to implementation and/or policy.  However, better
synchronization leads to paths that are more likely to succeed.

It must also be highlighted that the PCE may have access to only a
partial TED: for instance in the case of inter-domain path computation
where each such domain may be managed by different entities. In such
cases, each PCE may have a access to a partial TED and cooperative
techniques between PCEs may be used to achieve end-to-end path
computation without any requirement for any PCE to handle the complete
TED related to the set of traversed domains by the LSP path in question.

6.8. Stateful Versus Stateless PCEs

A PCE can be either stateful or stateless. In the former case, there is
a strict synchronization between the PCE and not only the network states
(in term of topology and resource information), but also the set of
computed paths and reserved resources in use in the network. In other
words, the PCE utilizes information from the TED as well as information
about existing paths (for example, TE LSPs) in the network when
processing new requests. Note that although this allows for optimal path
computation and increased path computation success, stateful PCEs
require reliable state synchronization mechanisms, with potentially
significant control plane overhead and the maintenance of a large amount
of data/states (for example, full mesh of TE LSPs).

For example, if there is only one PCE in the domain, all LSP computation
is done by this PCE, which can then track all the existing LSPs and stay
synchronized. However, this could require substantial control plane
resources to accomplish. If there are multiple PCEs in the network, LSP
computation and information is distributed among PCEs and the resources
required is also distributed. However, synchronization issues discussed
in Section 6.7 also come into play.

The maintenance of a stateful database can be non-trivial.  However, in
a single centralized PCE environment, a stateful PCE is almost a simple
matter of remembering all of the LSPs the PCE has computed, if it can
also be known that the LSPs were actually set up, and when they were
torn down.  Out-of-band TED synchronization can also be complex with
multiple PCE setup in a distributed PCE computation model, and could be
prone to race conditions, scalability concerns, etc.  Even if the PCE
has detailed information on all paths, priorities, and layers, taking
such information into account for path computation could be highly
complex.  PCEs might synchronize state by communicating with each other,
but when LSPs are set up using distributed computation performed among
several PCEs, the problem of synchronization becomes larger and more
complex.

There is benefit in knowing which LSPs exist, and their routing, to
support such applications as placing a high priority LSP in a crowded

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network such that it preempts as few other LSPs as possible.  Note that
preempting based on the minimum number of links might not result in the
smallest number of LSPs being disrupted.  Another application concerns
the construction and maintenance of a Virtual Network Topology [MRN].
It is also helpful to understand which other LSPs exist in the network
in order to decide how to manage the forward adjacencies that exist or
need to be set up. The cost-benefit of stateful PCE computation would be
helpful to determine if the benefit in path computation is sufficient to
offset the additional drain on the network computational resources.

Conversely, stateless PCEs do not have to remember any computed path and
each set of request(s) is processed independently of each other. For
example, stateless PCEs may compute paths based on current TED
information, which could be out of sync with actual network state given
other recent PCE-computed paths changes. Note that a PCC may include a
set of previously computed paths in its request, in order to take them
into account, for instance to avoid double bandwidth accounting, or to
try to minimize changes (minimum perturbation problem).

6.9. Monitoring

PCE Monitoring is undoubtedly of the utmost importance in any PCE
architecture. This must include the collection of variables related to
the PCE status and operation. For example, it will be necessary to
understand the way in which the TED is being kept synchronized, the rate
of arrival of new requests and the computation times, the range of PCCs
that are using the PCE, and the operation of any PCC-PCE protocol.

6.10. Policy and Confidentiality

As stated in [INTER-AS], the case of inter-provider TE LSP path
computation requires the ability to compute a path while preserving
confidentiality across multiple Service Providers cores. Thus any PCE
architecture solution must support the ability to return partial paths
by means of loose hops (for example, where each loose hops would for
instance identify a boundary LSR). Confidentiality and security of
PCC-PCE and PCE-PCE messages must also be ensured.

As mentioned in section 6.9, the ability to compute a path at the
request of the head end PCC, but to supply the path in segments to the
domain boundary PCCs may also be desirable.

7. PCE Evaluation Metrics

PCE evaluation metrics that may be used to evaluate the efficiency and
applicability of any PCE-based solution are listed below.

- Optimality: The ability to maximize network utilization and minimize
cost, considering QoS objectives, multiple regions and network layers.

- Scalability: The implications of routing and signaling overhead
(includes LSAs, crankbacks, queries, distribution mechanisms, etc.).

- Load sharing: The ability to allow multiple PCEs to spread the path
computation load.


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- Multi-path computation: The ability to compute multiple and
potentially diverse paths to satisfy load-sharing of traffic and
protection/restoration needs including end-to-end diversity and
protection within individual domains.

- Reoptimization: The ability to perform TE LSP path reoptimization.
This also includes the ability to perform inter-layer correlation when
considering the reoptimization at any specific layer.

- Path computation time. The time to compute individual paths, multiple
diverse paths, and to satisfy bulk path computation requests.

- Network stability: The ability to minimize any perturbation on
existing TE state resulting from the computation and establishment of
new TE paths.

- Ability to maintain accurate synchronization between TED and network
topology and resource states.

- Speed with which TED synchronization is achieved.

- Impact of the synchronization process on the data flows in the
network.

Note that other metrics may also be considered. Such metrics should be
used when evaluating a particular PCE-based architecture. It must also
be highlighted that the potential tradeoffs of the optimization of such
metrics should be evaluated (for instance, increasing the path
optimality is likely to have consequences on the computation time).

8. Security Considerations

The impact of the use of a PCE-based architecture MUST be considered in
the light of the impact that it has on the security of the existing
routing and signaling protocols and techniques in use within the
network. There is unlikely to be any impact on intra-domain security,
but an increase in inter-domain information flows and the facilitation
of inter-domain path establishment may increase the vulnerability to
security attacks.

Of particular relevance are the implications for confidentiality
inherent in a PCE-based architecture for multi-domain networks. It is
not necessarily the case that a multi-domain PCE solution will
compromise security, but solutions MUST examine their effects in this
area.

Applicability statements for particular combinations of signaling,
routing and path computation techniques are expected to contain detailed
security sections.

It should be observed that the use of a non-local PCE (that is, not
co-resident with the PCC) does introduce additional security issues.
Most notable amongst these are:

- Interception of PCE requests or responses


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- Impersonation of PCE

- Falsification of TE information

- Denial of service attacks on PCE or PCE communication mechanisms.

It is expected that PCE solutions will address these issues in detail
using authentication and security techniques.

9. IANA Considerations

This document makes no requests for IANA action.

10. Acknowledgements

The authors would like to extend their warmest thanks to (in
alphabetical order) Zafar Ali, Dean Cheng, Kireeti Kompella, Jean-Louis
Le Roux, Eiji Oki, Dimitri Papadimitriou, and Raymond Zhang for their
Review and suggestions.

11. Intellectual Property Considerations

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; nor does it represent that it has made any
independent effort to identify any such rights. Information on the
procedures with respect to rights in RFC documents can be found in BCP
78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can be
obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
that may cover technology that may be required to implement this
standard. Please address the information to the IETF at
ietf-ipr@ietf.org.

12. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3667] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3667,
February 2004.

[RFC3668] Bradner, S., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3668, February 2004.


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13. Informational References

[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell and J. McManus,
"Requirements for Traffic Engineering over MPLS", RFC 2702, September
1999.

[RFC2547] Rosen, E. and  Rekhter, Y. "BGP/MPLS VPNs", RFC2547, March
1999.

[RFC3209] Awduche, D., et. all, "Extensions to RSVP for LSP Tunnels",
RFC 3209, December 2001.

[RFC3473] Berger, L., et. al., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling - Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

[INTER-AREA] Le Roux, J., Vasseur, JP, Boyle, J., "Requirements for
Support of Inter-Area and Inter-AS MPLS Traffic Engineering",
draft-ietf-tewg- interarea-mpls-te-req-03.txt, November 2004 (work in
progress).

[INTER-AS] Zhang, R., Vasseur, JP., et. al., "MPLS Inter-AS Traffic
Engineering requirements", draft-ietf-tewg-interas-mpls-te-req-09.txt,
September 2004 (work in progress).

[MRN] Papadimitriou, D., et. al., "Generalized MPLS Architecture for
Multi-Region Networks,"draft-vigoureux-shiomoto-ccamp-gmpls-mrn-04.txt,
February 2004 (work in progress).

14. Authors' Addresses

Adrian Farrel
Old Dog Consulting
Phone: +44 (0) 1978 860944
Fax:   +44 (0) 870-130-5411
EMail: adrian@olddog.co.uk

Jean-Philippe Vasseur
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough , MA - 01719
USA
Email: jpv@cisco.com

Jerry Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748, USA
Phone: +1-(732)-420-4578
Fax:   +1-(732)-368-8659
Email: gash@att.com


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15. Full Copyright Statement

Copyright (C) The Internet Society (2005).  This document is subject to
the rights, licenses and restrictions contained in BCP 78, and except as
set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR
IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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