Network Working Group Adrian Farrel
IETF Internet Draft Old Dog Consulting
Proposed Status: Informational Jean-Philippe Vasseur
Expires: January 2006 Cisco Systems, Inc.
Jerry Ash
AT&T
July 2005
draft-ietf-pce-architecture-01.txt
Path Computation Element (PCE) Architecture
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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-layer 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. This
document does not attempt to provide a detailed description of all
the architectural components, but rather it describes a set of
building blocks for the PCE architecture from which solutions may be
constructed.
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Table of Contents
1. Introduction ................................................... 3
2. Terminology .................................................... 3
3. Definitions .................................................... 4
4. Motivation for a PCE-based Architecture ........................ 6
4.1. CPU-intensive Path Computation/Global Optimization ....... 6
4.2. Partial Visibility ....................................... 6
4.3. Absence of the TED or Use of Non-TE-Enabled IGP .......... 7
4.4. Node Outside the Routing Domain .......................... 7
4.5. Network Element Lacks Control Plane or Routing Capability 8
4.6. Backup Path Computation for Bandwidth Protection ......... 8
4.7. Multi-Layer Networks ..................................... 8
5. Overview of the PCE-Based Architecture ......................... 9
5.1. Composite PCE Node ....................................... 9
5.2. External PCE ............................................ 10
5.3. Multiple PCE Path Computation ........................... 11
5.4. Multiple PCE Path Computation with Inter-PCE Communication
.............................................. 12
5.5. Areas for Standardization ............................... 13
6. PCE Architectural Considerations .............................. 13
6.1. Centralized Computation Model ........................... 14
6.2. Distributed Computation Model ........................... 14
6.3. Synchronization ......................................... 14
6.4. PCE Discovery and Load Balancing ........................ 15
6.5. Detecting PCE Liveness .................................. 16
6.6. PCC-PCE & PCE-PCE Communication ......................... 16
6.7. PCE TED Synchronization ................................. 18
6.8. Stateful Versus Stateless PCEs .......................... 19
6.9. Monitoring .............................................. 21
6.10. Policy and Confidentiality ............................ 21
6.11. Unsolicited Interactions ............................... 21
7. Evaluation Metrics ............................................ 22
8. Manageability Considerations .................................. 23
8.1 Information and Data Models .............................. 23
8.2 Liveness Detection and Monitoring ........................ 24
8.3 Verifying Correct Operation .............................. 24
8.4 Requirements on Other Protocols and Functional Components 25
8.5 Impact on Network Operation .............................. 25
9. Security Considerations ....................................... 26
10. IANA Considerations .......................................... 26
11. Acknowledgements ............................................. 27
12. Intellectual Property Considerations ......................... 27
13. Normative References ......................................... 27
14. Informational References ..................................... 27
15. Authors' Addresses ........................................... 28
16. Full Copyright Statement ..................................... 29
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1. Introduction
Constraint-based path computation is a fundamental building block for
traffic engineering in MPLS and GMPLS networks. Path computation in
large, multi-domain networks is highly complex and may require
special computational components and cooperation between the elements
in different domains. This document specifies the architecture for a
Path Computation Element (PCE)-based model to address this problem
space.
This document describes a set of building blocks for the PCE
architecture from which solutions may be constructed. For example, it
discusses PCE-based implementations including composite, external,
and multiple PCE path computation. Furthermore, it discusses
architectural considerations including centralized computation,
distributed computation, synchronization, PCE discovery and load
balancing, detection of PCE liveness, PCC-PCE and PCE-PCE
communication, TED synchronization, stateful and stateless PCEs,
monitoring, policy and confidentiality, and evaluation metrics.
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.
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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 of
applying computational constraints during the computation. 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 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 computation 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 path computation
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 intra-domain, inter-domain, and
inter-layer contexts.
a. Inter-domain path computation may involve the correlation of
topology and routing information between domains.
b. Inter-layer path computation refers to the use of PCE where
multiple 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) a. In "single PCE path computation," a single PCE is used to
compute a given path in a domain. There may be multiple PCEs in
a domain, but only one PCE per domain is involved in any single
path computation.
b. In "multiple PCE path computation," multiple PCEs are used to
compute a given path in a domain.
3) a. "Centralized computation model" refers to a model whereby all
paths in a domain are computed by a single, centralized PCE.
b. Conversely, "Distributed computation model" refers to the
computation of paths in a domain being shared among multiple
PCEs.
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Paths that span multiple domains may be computed using the
distributed model with one or more PCEs 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 PCEs.
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 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 ASs may be specified by other means (not
determined by a PCE). Furthermore, different path computation
models may be used for different TE LSPs.
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.
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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.
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 models but would apply to specific existing
or future 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 resolution of 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.
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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 that has access to the complete set of topology
information. 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 reoptimization, 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
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.
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4.5. Network Element Lacks Control Plane or Routing Capability
It is common in legacy optical networks for the network elements not
to have a control plane or routing capability. Such network elements
only have a data plane and a 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.
That is, the PCC would be an element of the management plane, perhaps
residing in the NMS or OSS.
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 complete bandwidth sharing between backup 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
the paths of backup tunnels to protect against the failure of
adjacent network links or nodes.
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 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.
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In this case, the PCEs are 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 gives an overview of the 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 Node
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 this conversion may require path computation
which is requested from a 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 a PCE that is external to the requesting network
element. A service request is received by the head-end node and
before it can initiate signaling to establish the service, it makes
a path computation request to the external PCE. The PCE uses the TED
as input to the computation 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 be an LSR or router performing forwarding in its own right (i.e.
it may be a composit PCE node), but those functions are purely
orthogonal to the operation of the function in the instance being
considered here.
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. This may be the case when
the path returned is a partial path that does not reach the intended
destination or when the computed path is loose. The downstream
network element consults another PCE to establish the next hop(s) in
the path.
Note that either or both PCEs in this case could be composite PCE
nodes as in Section 5.1.
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---------- ----------
| | | |
| 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
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 may be 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
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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. This model does not provide a distributed computaiton
algorithm, but allows distinct PCEs to be responsible for computation
of parts (segments) of the path.
PCE-PCE communication is discussed further in Section 6.6.
Note that a PCC might not 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
The following areas require standardization within the PCE
architecture.
- communication between PCCs and PCEs, and between cooperating PCEs
- requirements for extensions to existing routing and/or 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
This section provides a list of the PCE architectural components.
Specific realizations and implementation details (state 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.
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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 designated 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.
This model has a single point of failure: the PCE. In order to avoid
this issue, the centralized computation model may designate a backup
PCE that can take over the computation responsibility in a controlled
manner in the event of a failure of the primary PCE. Note that at any
moment in time there is only one active PCE in any domain.
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
for a discussion of PCE discovery which impacts on this choice. 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 area where the ingress LSR /
composite PCE node is responsible for computing the path or for
contacting an external PCE. Conversely, multiple PCE path computation
implies that more than one PCE is involved in the computation of a
single path. An example of this is where loose hop expansion is
performed by transit LSRs / composite PCE nodes on an MPLS TE LSP.
Another example is the use of multiple cooperating PCEs to compute
the path of a single LSP.
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 of non-synchronized path computation, the PCE will make
multiple individual path computations to generate the paths and the
PCC may send its individual requests to different PCEs.
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Alternatively, the PCC may send a single request to a PCE asking for
a set of paths to be computed but specifying that non-synchronized
path computation is acceptable. The PCE may compute each path in turn
exactly as it would have done had the PCC made multiple requests, and
the PCE may devolve some computations to other PCEs if it chooses.
Conversely, the PCC may issue a single request to the PCE asking for
all of the paths to be computed in a synchronized manner. The PCE
will then perform simultaneous computation of the set of requested
path. Such synchronized computation can often provide 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
that the PCE supplies a set of alternate paths for use by the PCC
should the establishment of the LSP using the principal path fail to
complete. While alternate paths may not always be successful if the
first path fails, including alternate paths in a PCE response could
have less overhead than having the PCC make separate requests for
subsequent path computations as the need arises. This technique is
used in some existing CSPF implementations.
6.4. PCE Discovery and Load Balancing
The PCE architecture requires that the PCC/PCE know 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.
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A PCE SHOULD advertise its capabilities, such as:
- set of constraints that it can account for (diversity, SRLGs,
Optical impairments, wavelength 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 that the PCE is unable to
perform the service, but specifying the things that the PCE can do.
Note that the parameters mentioned above are not meant to be
exhaustive and are listed for the sake of illustration.
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 inability of a PCE to service a request (perhaps due to excessive
load) may be reported to the PCC through a failure message, but the
failure of a PCE or the communications mechanism while processing a
request cannot be reported in this way. Further, in the case of
excessive load, the PCE may not have sufficient resources to send a
failure message. Thus the PCC should employ other mechanisms such as
protocol timers to determine the liveness of the PCE. This is
particularly important in the case of inter-domain path computation
where the PCE liveness may not be detected by means of the IGP that
runs in the PCC's domain.
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.
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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.
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(s),
and potentially with advice about which constraints might be relaxed
to be more likely to achieve a positive result in a future request.
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.
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.
An important feature of PCEs that are cooperating to compute a path
is that they apply compatible or identical computation algorithms.
This may require coordination through the communication between the
PCEs.
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Note that when multiple PCEs cooperate to compute a path it is
important that they have a coordinated view of the meaning of
constraints such as resource affinities and class of service. This
is particularly significant where the PCEs are responsible for
different domains. It is assumed that this is a matter of policy
between domains and between PCEs, and is achieved through
configuration not through protocol communications.
No assumption is made in this architecture 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
through the use of extensions to the IGP [RFC3630, RFC3748]. 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 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.
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3) Information in the TED can include information obtained from
sources other than the IGP. For example, information about link
usage policies can be configured by the operator. Path computation
can also act on a far wider set of information that includes data
about the LSPs provisioned within the network. This information
can include LSP routes, reserved bandwidth, and measured traffic
volume passing through the LSP.
Such LSP information can enhance LSP reoptimization to provide
"full network" reoptimization, and can allow traffic fluctuations
to be taken into account. Detailed LSP information may also
facilitate reconfiguration of the Virtual Network Topology (VNT)
[MRN], in which lower layer LSPs such as optical paths provide TE
links for use by the higher layer, since this reconfiguration is
also a "full network" problem.
Note that synchronization techniques may apply to both intra- and
inter-domain TEDs. Further, the techniques can be mixed for use in
different domains. The degree of synchronization between the PCE and
the network is subject to implementation and/or policy. However,
better synchronization generally 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 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).
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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 model could
require substantial control plane resources. If there are multiple
PCEs in the network, LSP computation and information is distributed
among PCEs and so the resources required to perform the computations
are 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
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 and 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).
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It should be observed that the stateless PCE does operate on
information about network state. The TED contains link state and
bandwidth availability information as distributed by the IGPs or
collected through some other means. This information could be
further enhanced to provide increased granularity and more detail to
cover, for example, the current bandwidth usage on certain links
according to resource affinities or forwarding equivalence classes.
Such information is, however, not PCE state information and so a
model that uses it is still described as stateless in the PCE
context.
A limited form of statefulness might be applied within an otherwise
stateful PCE. The PCE may retain some context from paths it has
recently computed so that it avoid suggesting the use of the same
resources for other LSPs.
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.
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.
6.11. Unsolicited Interactions
It may be that the PCC-PCE communications (see section 6.6) can be
usefully extended beyond a simple request/response interaction. For
example, the PCE and PCC could exchange capabilities using this
protocol. Additionally, the protocol could be used to collect and
report information in support of a stateful PCE.
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Further, it may be the case that a PCE is able to update a path that
it computed earlier (perhaps in reaction to a change in the network),
and in this case the PCE-PCC communication could support an
"unsolicited" path computation message to supply this new path to the
PCC. It should be noted, however, that this function would require
that the PCE retained a record of previous computations and had a
clear trigger for performing recomputations. The PCC would also need
to be able to identify the new path with the old path and determine
whether it should act on the new path. Note that the PCE-PCC
interaction is not a management interaction and the PCC is not
obliged to utilize any additional path supplied by the PCE.
These functions fit easily within the architecture described here
but are left for further discussion within separate requirements
documents.
7. Evaluation Metrics
Evaluation metrics that may be used to evaluate the efficiency and
applicability of any PCE-based solution are listed below. Note that
these metrics are not being used to determine paths, but are used to
evaluate potential solutions to the PCE architecture.
- Optimality: The ability to maximize network utilization and
minimize cost, considering QoS objectives, multiple regions and
network layers. Note that models that require the sequential
involvement of multiple PCEs (for example, the multiple PCE model
described in section 5.3) have an inherent risk of lower quality
paths and might create path loops unless careful policy is applied.
- Scalability: The implications of routing, LSP signaling and PCE
communication overhead such as the number of messages and the size
of messages (includes LSAs, crankbacks, queries, distribution
mechanisms, etc.).
- Load sharing: The ability to allow multiple PCEs to spread the path
computation load by allowing multiple PCEs to each take
responsibility for a subset of the total path computation requests.
- 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.
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- Path computation time: The time to compute individual paths,
multiple diverse paths, and to satisfy bulk path computation
requests. (Note that such a metric can only be applied to problems
that are not NP-complete.)
- 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. Manageability Considerations
The PCE architecture introduces several elements that are subject to
manageability. The PCE itself must be managed as must its
communications with PCCs and other PCEs. The mechanism by which PCEs
and PCCs discover each other are also subject to manageability.
Many of the issues of manageability are already covered in other
sections of this document.
8.1 Information and Data Models
It is expected that the operations of PCEs and PCCs will be modeled
and controlled through appropriate MIB modules. These will be
relatively simple constructs since the relationships between PCEs and
PCCs are quite simple. The tables in the new MIB modules will need to
reflect the relationships between entities and to control and report
on configurable options.
Statistics gathering will form an important part of the operation of
PCEs. The operator must be able to determine the historical
interactions of a PCC with its PCEs, the performance that it has
seen, and success rate of its requests. Similarly, it is important
for an operator to be able to inspect a PCE and determine its load
and whether an individual PCC is responsible for a disproportionate
amount of the load. It will also be important to be able to record
and inspect statistics about the communications between the PCC and
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PCE, including issues such as malformed messages, unauthorized
messages and messages discarded owing to congestion. In this respect
there is clearly an overlap between manageability and security.
Statistics for the PCE architecture can be made available through
appropriate tables in the new MIB modules.
The new MIB modules should also be used to provide notifications
(formerly known as traps) when key thresholds are crossed or when
important events occur. Great care must be exercised to ensure that
the network is not flooded with SNMP notifications. Thus it might be
inappropriate to issue a notification every time that a PCE receives
a request to compute a path. In any case, full control must be
provided through the MIB modules to allow notifications to be
disabled.
8.2 Liveness Detection and Monitoring
Section 6.5 discusses the importance of a PCC being able to detect
the liveness of a PCE. PCE-PCC communications techniques must enable
a PCC to determine the liveness of a PCE both before it sends a
request and in the period between sending a request and receiving a
response.
It is less important for a PCE to know about the liveness of PCCs,
and within the simple request/response model, this is only helpful:
- to gain a predictive view of the likely loading of a PCE in the
future
- to allow a PCE to abandon processing of a received request.
8.3 Verifying Correct Operation
Correct operation for the PCE architecture can be classified as
determining the correct point-to-point connectivity between PCCs and
PCEs, and assessing the validity of the computed paths. The former is
a security issue that may be enhanced by authentication and monitored
through event logging and records as described in Section 8.1.
Verifying computed paths is more complex. The information to perform
this function can, however, be made available to the operator through
MIB tables provided full records are kept of the constraints passed
on the request, the path computed and provided on the response, and
any additional information supplied by the PCE such as the constraint
relaxation policies applied.
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8.4 Requirements on Other Protocols and Functional Components
At the architectural stage it is impossible to make definitive
statements about the impact on other protocols and functional
components since the solutions work has not been completed. However,
it is possible to make some observations.
- Dependence on underlying transport protocols
PCE-PCC communications may choose to utilize underlying protocols
to provide transport mechanisms. In this case some of the
manageability considerations described in the previous sections may
be devolved to those protocols.
- Re-use of existing protocols for discovery
Without prejudicing the requirements and solutions work for PCE
discovery (see Section 6.4) it is possible that use will be made of
existing protocols to facilitate this function. In this case some
of the manageability considerations described in the previous
sections may be devolved to those protocols.
- Impact on LSRs and LSP signaling
The primary example of a PCC identified in this architecture is an
MPLS LSR. Consideration must therefore be given to the
manageability of the LSRs and the additional manageability
constraints applicable to the LSP signaling protocols.
As well as allowing the PCC management described in the previous
sections, an LSR must be configurable to determine whether it will
use a remote PCE at all - the options being to use hop-by-hop
routing or to supply the PCE function itself. It is likely to be
important to be able to distinguish within an LSR whether the path
used for an LSP was supplied in a signaling message, by an
operator, or by a PCE, and in the case where it was supplied in a
signaling message whether it was enhanced or expanded by a PCE.
8.5 Impact on Network Operation
This architecture may have two impacts on the operation of a network.
It increases LSP setup times while requests are sent to and processed
by a remote PCE, and it may cause congestion within the network if a
significant number of computation requests are issued in a small
period of time. These issues are most severe in busy networks and
after network failures although the effect may be mitigated if the
protection paths are precomputed.
Issues of potential congestion during recovery from failures may be
mitigated through the use of pre-established protection schemes such
as fast reroute.
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It is important that network congestion be managed proactively
because it may be impossible to manage it reactively once the network
is congested. It should be possible for an operator to rate limit the
requests that a PCC sends to a PCE, and a PCE should be able to
report impending congestion (according to a configured threshold)
both to the operator and to its PCCs.
9. 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
- 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.
10. IANA Considerations
This informational document makes no requests for IANA action.
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11. Acknowledgements
The authors would like to extend their warmest thanks to (in
alphabetical order) Arthi Ayyangar, Zafar Ali, Mohamed Boucadair,
Igor Bryskin, Dean Cheng, Vivek Dubey, Kireeti Kompella,
Jean-Louis Le Roux, Stephen Morris, Eiji Oki, Dimitri Papadimitriou,
Richard Rabbat, Takao Shimizu, and Raymond Zhang for their review and
suggestions.
12. 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.
13. Normative References
[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.
14. 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.
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[RFC3209] Awduche, D., et. all, "Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3630] Katz et al., "Traffic Engineering (TE) Extensions to
OSPF Version 2", RFC3630, September 2003.
[RFC3473] Berger, L., et. al., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling - Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions",
RFC 3473, January 2003.
[RFC3748] Smit, H. and Li, T., "Intermediate System to
Intermediate System (IS-IS) - Extensions for Traffic
Engineering (TE)", RFC3784, June 2004.
[RFC4105] Le Roux, J., Vasseur, JP, Boyle, J., "Requirements for
Support of Inter-Area and Inter-AS MPLS Traffic
Engineering", RFC 4105, June 2005.
[INTER-AS] Zhang, R., Vasseur, JP., et. al., "MPLS Inter-AS Traffic
Engineering requirements",
draft-ietf-tewg-interas-mpls-te-req, work in progress.
[MRN] Shiomoto, K., et. al., "Requirements for GMPLS-based
multi-region and multi-layer networks",
draft-shiomoto-ccamp-gmpls-mrn-reqs, work in progress.
15. Authors' Addresses
Adrian Farrel
Old Dog Consulting
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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16. 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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