Network Working Group A. Farrel
Request for Comments: 4655 Old Dog Consulting
Category: Informational J.-P. Vasseur
Cisco Systems, Inc.
J. Ash
AT&T
August 2006
A Path Computation Element (PCE)-Based Architecture
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
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 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.
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 .............................6
4.2. Partial Visibility .........................................7
4.3. Absence of the TED or Use of Non-TE-Enabled IGP ............7
4.4. Node Outside the Routing Domain ............................8
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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 .......................................9
4.8. Path Selection Policy ......................................9
4.9. Non-Motivations ...........................................10
4.9.1. The Whole Internet .................................10
4.9.2. Guaranteed TE LSP Establishment ....................10
5. Overview of the PCE-Based Architecture .........................11
5.1. Composite PCE Node ........................................11
5.2. External PCE ..............................................12
5.3. Multiple PCE Path Computation .............................13
5.4. Multiple PCE Path Computation with Inter-PCE
Communication .............................................14
5.5. Management-Based PCE Usage ................................15
5.6. Areas for Standardization .................................16
6. PCE Architectural Considerations ...............................16
6.1. Centralized Computation Model .............................16
6.2. Distributed Computation Model .............................17
6.3. Synchronization ...........................................17
6.4. PCE Discovery and Load Balancing ..........................18
6.5. Detecting PCE Liveness ....................................20
6.6. PCC-PCE and PCE-PCE Communication .........................20
6.7. PCE TED Synchronization ...................................22
6.8. Stateful versus Stateless PCEs ............................23
6.9. Monitoring ................................................25
6.10. Confidentiality ..........................................25
6.11. Policy ...................................................26
6.11.1. PCE Policy Architecture ...........................26
6.11.2. Policy Realization ................................28
6.11.3. Type of Policies ..................................28
6.11.4. Relationship to Signaling .........................29
6.12. Unsolicited Interactions .................................30
6.13. Relationship with Crankback ..............................30
7. The View from the Path Computation Client ......................31
8. Evaluation Metrics .............................................32
9. Manageability Considerations ...................................33
9.1. Control of Function and Policy ............................33
9.2. Information and Data Models ...............................34
9.3. Liveness Detection and Monitoring .........................34
9.4. Verifying Correct Operation ...............................35
9.5. Requirements on Other Protocols and Functional
Components ................................................35
9.6. Impact on Network Operation ...............................36
9.7. Other Considerations ......................................36
10. Security Considerations .......................................37
11. Acknowledgements ..............................................37
12. Informative References ........................................38
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1. Introduction
Constraint-based path computation is a fundamental building block for
traffic engineering in MPLS [RFC3209] and GMPLS [RFC3473] networks.
[RFC2702] describes requirements for traffic engineering in MPLS
networks, while [RFC4105] and [RFC4216] describe traffic engineering
requirements in inter-area and inter-AS environments, respectively.
Path computation in large, multi-domain networks is 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, communication between Path
Computation Clients (PCCs) and the PCE (PCC-PCE communication) and
PCE-PCE communication, Traffic Engineering Database (TED)
synchronization, stateful and stateless PCEs, monitoring, policy and
confidentiality, and evaluation metrics.
The model of the Internet is to distribute network functionality
(e.g., routing) within the network. PCE functionality is not
intended to contradict this model and can be used to match the model
exactly, for example, when the PCE functionality coexists with each
Label Switching Router (LSR) in the network. PCE is also able to
augment functionality in the network where the Internet model cannot
supply adequate solutions, for example, where traffic engineering
information is not exchanged between network domains.
2. Terminology
CSPF: Constraint-based Shortest Path First.
LER: Label Edge Router.
LSDB: Link State Database.
LSP: Label Switched Path.
LSR: Label Switching Router.
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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 Interior
Gateway Protocol (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 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 (ASes), and multiple
ASes within a Service Provider network. Domains of path computation
responsibility may also exist as sub-domains of areas or ASes.
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 association of
topology, routing, and policy information from multiple domains
from which relationships may be deduced in order to help in
performing path computation.
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.
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Overlapping domains are not within the scope of this document. In
the inter-domain case, the domains may belong to a single or to
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.
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 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 that 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 path" (that is,
the full explicit path from start to destination, made of a list
of strict hops) or a "strict/loose 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
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a PCE-based path computation model in some ASes, whereas the set
of traversed ASes 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.
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.
As can be seen from these examples, PCE does not replace the existing
Internet model where intelligence is distributed within the network.
Instead, it builds on this model and makes use of distributed centers
of information or computational ability. PCE should not, therefore,
necessarily be seen as a centralized, "all-seeing oracle in the sky",
but as the cooperative operation of distributed functionality used to
address specific challenges such as the computation of a shortest
inter-domain constrained path.
4.1. CPU-Intensive Path Computation
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:
- 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)
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- Computation of minimal cost Point to Multipoint trees (Steiner
trees)
In these situations, it may not be possible or desirable for some
routers to perform path computation because of the constraints on
their CPUs, in which case the path computations may be off-loaded to
some other PCE(s) that may, themselves, be routers or may be
dedicated PCE servers.
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 a TE 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 TE 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, or 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, and potentially using crankback between
the domains to dynamically resolve provisioning issues.
Alternatively, a central "all-seeing" PCE that has access to the
complete set of topology information may be used, but in this 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 TE LSP reoptimization, or the establishment of multiple
diverse TE 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).
Maintaining the TED may require a lot of memory and may require non-
negligible CPU activity. 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.
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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 the routers that need to perform path computation or held
centrally (on a distinct node that supports PCE) for centralized
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 [RFC4364] 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 (TE LSP head-end, CE) router, and that
does not rely on the configuration of static loose hops. In this
case, optimal shortest paths cannot be guaranteed. A solution that a
distinct PCE 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 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 to 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 Network Management System (NMS) or Operations Support
System (OSS).
This scenario is important for Automatically Switched Optical Network
(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
protecting independent elements, while avoiding any extensions to TE
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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 Time-Division Multiplexing (TDM) network may provide
connectivity for client-layer networks such as IP, MPLS, or Layer 2
[MLN].
The server-layer network is unlikely to provide the same connectivity
paradigm as the client networks, so 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.
Furthermore, 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
distinct path computation regions within a PCE domain, 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 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.
4.8. Path Selection Policy
A PCE may have a local policy that impacts path computation and
selection in response to a path computation request. Such policy may
act on information provided by the requesting PCC. The result of
applying such policy includes, for example, rejection of the path
computation request, or provision of a path that does not meet all of
the requested constraints. Further, the policy may support
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administratively configured paths, or selection among transit
providers. Inclusion of policy within PCE may simplify the
application of policy within the path computation/selection process.
Similarly, a PCC may apply local policy to the selection of a PCE to
compute a specific path, and to the constraints that are requested.
In a PCE context, the policy may be sensitive to the type of path
that is being computed. For example, a different set of policies may
be applied for an intra-area or single-layer path than would be
provided for an inter-area or multi-layer path.
Note that synchronization of policy between PCEs or between PCCs and
PCEs may be necessary. Such issues are outside the scope of the PCE
architecture, but within scope for the PCE policy framework and
application which is described in a separate document.
4.9. Non-Motivations
4.9.1. The Whole Internet
PCE is not considered to be a solution that is applicable to the
entire Internet. That is, the applicability of PCE is limited to a
set of domains with known relationships. The scale of this
limitation is similar to the peering relationships between Service
Providers.
4.9.2. Guaranteed TE LSP Establishment
When two or more paths for TE LSPs are computed on the same set of TE
link state information, it is possible that the resultant paths will
compete for limited resources within the network. This may result in
success for only the first TE LSP to be signaled, or it might even
mean that no TE LSP can be established.
Batch processing of computation requests, back-off times, computation
of alternate paths, and crankback can help to mitigate this sort of
problem, and PCE may also improve the chances of successful TE LSP
setup. However, a single, centralized PCE is not viewed as a
solution that can guarantee TE LSP establishment since the potential
for network failures or contention for resources still exists where
the centralized TED cannot fully reflect current (i.e., real-time)
network state.
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5. Overview of the PCE-Based Architecture
This section 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
that is requested from a PCE. The PCE operates on the TED subject to
local policy in order to respond with the requested path.
---------------
| --------- | 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.
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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
subject to local policy as input to the computation and returns a
response.
----------
| ----- |
| | 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 composite PCE node), but those functions are
purely orthogonal to the operation of the function in the instance
being considered here.
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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. In this case, all policy decisions are made independently
at each PCE based on information passed from the PCC.
Note that either or both PCEs in this case could be composite PCE
nodes, 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 as a result the adjacent node needs 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
Multiple PCE path computation with inter-PCE communication involves
coordination between distinct PCEs such that the result of the
computation performed by one PCE depends on path fragment information
supplied by other PCEs. This model does not provide a distributed
computation algorithm, but it 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.
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In this model, all policy decisions may be made independently at each
PCE based on computation information passed from the previous PCE.
Alternatively, there may be explicit communication of policy
information between PCEs.
5.5. Management-Based PCE Usage
It must be observed that the PCC is not necessarily an LSR. For
example, in Figure 5 the NMS supplies the head-end LSR with a fully
computed explicit path for the TE LSP that it is to establish through
signaling. The NMS uses a management plane mechanism to send this
request and encodes the data using a representation such as the TE
MIB module [RFC3812].
The NMS constructs the explicit path that it supplies to the head-end
LSR using information provided by the operator. It consults the PCE,
which returns a path for the NMS to use.
Although Figure 5 shows the PCE as remote from the NMS, it could, of
course, be collocated with the NMS.
-----------
| ----- |
Service | | TED |<-+----------->
Request | ----- | TED synchronization
| | | | mechanism (for example,
v | | | routing protocol)
------------- Request/ | v |
| | Response| ----- |
| NMS |<--------+> | PCE | |
| | | ----- |
------------- -----------
Service |
Request |
v
---------- Signaling ----------
| Head-End | Protocol | Adjacent |
| Node |<---------->| Node |
---------- ----------
Figure 5. Management-Based PCE Usage
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5.6. Areas for Standardization
The following areas require standardization within the PCE
architecture.
- communication between PCCs and PCEs, and between cooperating PCEs,
including the communication of policy-related information
- requirements for extending 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, robustness, and policy support of path computation
models.
- MIB modules related to communication protocols, routing and
signaling extensions, metrics, and PCE monitoring information
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 it strictly relates to the path
computation aspects of such TE LSPs.
This section presents an architectural view of PCE. That is, it
describes the components that exist and how they interact. Note that
the architectural model, and in particular the functional model, may
be perceived differently by different components of the PCE system.
For example, the PCC will not be aware of whether a PCE consults
other PCEs. The PCC view of the PCE architecture is discussed in
Section 7.
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
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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. Any policies
present on the primary PCE should also be present on the backup,
although the primary policies may themselves be subject to policy
governing how they are implemented on the backup. 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 that affects this choice.
Implementation of policy should be consistent across the set of
available PCEs.
Often, 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 TE
LSP across multiple domains.
6.3. Synchronization
Often, 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 requests, the PCE may 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. On the other hand, the PCE is not prohibited from
performing all computations together in a synchronized manner as
described below.
The PCC may also issue a single request to the PCE asking for all the
paths to be computed in a synchronized manner. The PCE will then
perform simultaneous computation of the set of requested paths. Such
synchronized computation can often provide better 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 that 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 supply a set of alternate paths for use by the PCC,
should the establishment of the TE 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
In order that a PCC can communicate efficiently with a PCE, it must
know the location of the PCE. That is, it is an architectural
decision made here that PCC requests be targeted to a specific PCE,
and not broadcast to the network for any PCE to respond. This
decision means that only the selected PCE will operate on any single
request, and it saves network resources during request propagation
and processing resources at the PCEs that are not required to
respond.
The knowledge of the location of a PCE may be achieved through local
configuration at the PCC or may rely on a protocol-based discovery
mechanism that may be governed by policy.
Where more than one PCE is known to a PCC, the PCC must have
sufficient information to select an appropriate PCE for its purposes,
under the control of policy. Such a selection procedure allows for
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load sharing between PCEs and supports PCEs with different
computation capabilities including different visibility scopes.
Thus, the information available to the PCC must include details of
the PCE capabilities, which may be fixed or may vary dynamically in
time.
The PCC may learn PCE capabilities through static configuration, or
it may discover the information dynamically. Note that even when the
location of the PCE is configured at the PCC, the PCC may still
discover the PCE capabilities dynamically. Dynamic PCE capabilities
cannot be configured and can only be discovered.
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 case, it is a requirement that
the proxy 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 (for instance, to
efficiently share the computation load across multiple PCEs or to
request secondary computations after partial or failed computations)
are local to the PCC, may be based on policy, and are out of the
scope of this document.
PCE capabilities that may be advertised or configured could include
(and are not be limited to):
- a set of constraints that it can account for (diversity, shared
risk link groups (SRLGs), optical impairments, wavelength
continuity, etc.)
- computational capacity (for example, the number of computations it
can perform per second)
- the number of switching capability layers (and which ones)
- the 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 to decide which PCE to use.
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Requirements for PCE advertisement will be documented separately.
Note that there is no restriction within the architecture about how
location and capabilities are advertised, and the two elements should
be considered functionally distinct.
A PCC might also ask a PCE to perform a particular type of service
without knowledge of the PCE's capabilities and receive a response
that says that the PCE is unable to perform the service. The
response could specify the capabilities of the PCE and might also
suggest another PCE that has the requested capabilities.
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. Furthermore, 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 and 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. Discussion of the
security requirements and implications for this protocol is provided
in Section 10 of this document.
The path computation request may include a significant set of
requirements, including the following:
- the source and destination of the path
- the bandwidth and other Quality of Service (QoS) parameters desired
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- resources, resource affinities, and shared risk link groups (SRLGs)
to use/avoid
- the number of disjoint paths required and whether near-disjoint
paths are acceptable
- the levels of resiliency, reliability, and robustness of the path
resources
- policy-related information
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 it could 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
so that a positive result is more likely 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 so that a positive result is more likely. 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 and
coordinated policies. 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 costs, 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.
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 or
GMPLS 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 or GMPLS 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 it 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, or the computation of sub-
optimal 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 TE LSPs provisioned within the network.
This information can include TE LSP routes, reserved bandwidth,
and measured traffic volume passing through the TE LSP.
Such TE LSP information can enhance TE LSP (re)optimization to
provide "full network" (re)optimization and can allow traffic
fluctuations to be taken into account. Detailed TE LSP
information may also facilitate reconfiguration of the Virtual
Network Topology (VNT) [MLN], in which lower-layer TE 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. Furthermore, 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.
Note also 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 that any PCE handle the complete TED related
to the set of traversed domains by the TE LSP 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 TE LSP
computation is done by this PCE, which can then track all the
existing TE LSPs and stay synchronized (each TE LSP state change must
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be tracked by the PCE). However, this model could require
substantial control plane resources. If there are multiple PCEs in
the network, TE LSP computation and information are 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 the TE LSPs the PCE has computed,
that the TE LSPs were actually set up (if this can be known), 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 TE LSPs are set up using
distributed computation performed among several PCEs, the problems of
synchronization and race condition avoidance become larger and more
complex.
There is benefit in knowing which TE LSPs exist, and their routing,
to support such applications as placing a high-priority TE LSP in a
crowded network such that it preempts as few other TE LSPs as
possible (also known as the "minimal perturbation" problem). Note
that preempting based on the minimum number of links might not result
in the smallest number of TE LSPs being disrupted. Another
application concerns the construction and maintenance of a Virtual
Network Topology [MLN]. It is also helpful to understand which other
TE 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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Note 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
stateless PCE. The PCE may retain some context from paths it has
recently computed so that it avoids suggesting the use of the same
resources for other TE 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. Confidentiality
As stated in [RFC4216], the case of inter-provider TE LSP computation
requires the ability to compute a path while preserving
confidentiality across multiple Service Providers cores. That is,
one Service Provider must not be required to divulge any information
about its resources or topology in order to support inter-provider TE
LSP path computation. Thus, any PCE architecture solution must
support the ability to return partial paths by means of loose hops
(for example, where each loose hop would, for instance, identify a
boundary LSR).
This requirement is not a security issue, but relates to Service
Provider policy. Confidentiality, integrity, and authentication of
PCC-PCE and PCE-PCE messages must also be ensured and are described
in Section 10.
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.
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6.11. Policy
Policy impacts multiple aspects of the PCE architecture. There are
two applications of policy for consideration:
- application of policy within an architectural entity (PCC or PCE)
- application of policy to PCE-related communications
As directly applicable to TE LSPs, policy forms part of the signaling
mechanism for the establishment of the TE LSPs and is not described
here.
It is envisioned that policy will be largely applied as a local
matter within each PCC and PCE. However, this document needs to
define policy models that can be supported within the PCE
architecture and by PCE-related communication.
Some example policies include:
- selection of a PCE by a PCC
- rejection of a request by the PCE based on the identity of the
requesting PCC
- selection by the PCE of a path or application of additional
constraints to a computation based on the PCC, the computation
target, the time of day, etc.
6.11.1. PCE Policy Architecture
Two examples of the use of policy components within the PCE
architecture are illustrated in Figures 6 and 7. Policy components
could equally be applied to the other PCE configurations shown in
Section 5. In each configuration, policy may be consulted before a
response is provided by a PCE and may also be consulted by the
PCC/PCE that receives the response.
A PCE may have a local policy that impacts the paths selected to
satisfy a particular PCE request. A policy may be applied based on
any information provided from a PCC.
In Figure 6, the policy component is shown providing input to the PCE
component. This policy component may consult an external policy
database, but this is outside the scope of this document.
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------------------------------
| --------- | Routing ----------
| | | | Protocol | |
| | TED |<-+----------+-> |
| | | | | |
| --------- | | |
| | | | |
| | Input | | |
| v | | |
| --------- --------- | | |
| | Policy | | | | | Adjacent |
| |Component|--->| PCE | | | Node |
| | | | | | | |
| --------- --------- | | |
| ^ | | |
| |Request | | |
| |Response| | |
| v | | |
| --------- | | |
Service | | | | Signaling| |
Request | |Signaling| | Protocol | |
------+---------------->| Engine |<-+----------+-> |
| | | | | |
| --------- | ----------
------------------------------
Figure 6. Policy Component in the Composite PCE Node
Note that policy information may be conveyed on the internal
interfaces, and on the external protocol interfaces.
Figure 7 displays the case of a distinct PCE function through the
example of the multiple PCE with inter-PCE communication example
(compare with Figure 4). Each PCE takes input from local policy as
part of the router computation/determination process. The local
policy components may consult external policy components or
databases, but that is out of the scope of this document.
Note that policy information may be conveyed on the external protocol
interfaces, including the inter-PCE interface.
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------------------ ------------------
| | Inter-PCE Request/Response| |
| PCE |<------------------------->| PCE |
| | | |
| ------ ----- | | ------ ----- |
| |Policy| | TED | | | |Policy| | TED | |
| ------ ----- | | ------ ----- |
------------------ ------------------
^
| Request/
| Response
v
Service ---------- Signaling ---------- Signaling ----------
Request| Head-End | Protocol | Adjacent | Protocol | Adjacent |
---->| Node |<---------->| Node |<---------->| Node |
---------- ---------- ----------
Figure 7. Policy Components in Multiple PCEs
6.11.2. Policy Realization
There are multiple options for how policy information is coordinated.
- Policy decisions may be made by PCCs before consulting PCEs. This
type of decision includes selection of PCE, application of
constraints, and interpretation of service requests.
- Policy decisions may be made independently at a PCE, or at each
cooperating PCE. That is, the PCE(s) may make policy decisions
independent of other policy decisions made at PCCs or other PCEs.
- There may also be explicit communication of policy information
between PCC and PCE, or between PCEs to achieve some level of
coordination of policy between entities. The type of information
conveyed to support policy has important implications on what
policies may be applied at each PCE, and the requirements for the
exchange of policy information inform the choice or implementation
of communication protocols including PCC-PCE, PCE-PCE, and
discovery protocols.
6.11.3. Type of Policies
Within the context of PCE, we identify several types of policies:
o User-specific policies operate on information that is specific to
the user of a service or the service itself, that is, the service
for which the path is being computed, not the computation service.
Examples of such information includes the contents of objects of a
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signaling or provisioning message, the port ID over which the
message was received, a VPN ID, a reference point type, or the
identity of the user initiating the request. User-specific
policies could be applied by a PCC while building a path
computation request, or by a PCE while processing the request
provided that sufficient information is supplied by the PCC to the
PCE.
o Request-specific policies operate on information that is specific
to a path computation request and is carried in the request.
Examples of such information include constraints, diversities,
constraint and diversity relaxation strategies, and optimization
functions. Request-specific policies directly affect the path
selection process because they specify which links, nodes, path
segments, and/or paths are not acceptable or, on the contrary, may
be desirable in the resulting paths.
o Domain-specific policies operate on the identify of the domain in
which the requesting PCC exists, and upon the identities of the
domains through which the resulting paths are routed. These
policies have the same effect as user-specific policies, with the
difference that they can be applied to a group of users rather than
an individual user. One example of domain-specific policy is a
restriction on what information a PCE publishes within a given
domain. In such a case, PCEs in some domains may advertise just
their presence, while others may advertise details regarding their
capabilities, client authentication process, and computation
resource availability.
6.11.4. Relationship to Signaling
When a path for an inter-domain TE LSP is being computed, it is not
required to consider signaling plane policy. However, failure to do
so may result in the TE LSP failing to be established, or being
assigned fewer resources than intended resulting in a substandard
service. Thus, where a PCE invoked by a head-end LSR has visibility
into other domains, it should be capable of applying policy
considerations to the computation and should be aware of the inter-
domain policy agreements. Where path computation is the result of
cooperation between PCEs, each of which is responsible for a
particular domain, the policy issues should, where possible, be
resolved at the time of computation so that the TE LSP is more likely
to be signaled successfully. In this context, policy violation
during inter-domain TE LSP computation may lead to path computation
interruption, about which the requester should be notified along with
the cause.
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6.12. 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.
Furthermore, 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 or a change in policy), and in this case the PCE-PCC
communication could support an "unsolicited" path computation message
to supply this new path to the PCC. Note, 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.
Further, the PCC should be able to report the outcome of such path
changes to the requesting PCE. 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.
6.13. Relationship with Crankback
Crankback routing is a mechanism whereby a failure to establish a
path or a failure of an existing path may be corrected by a new path
computation and fresh signaling. Crankback routing relies on the
distribution of crankback information along with the failure
notification so that the new computation can be performed avoiding
the failure or blockage point.
In the context of PCE, crankback information may be passed back to
the head-end where the process of computation and signaling can be
repeated using the failed resource as an exclusion in the computation
process. But crankback may be used to attempt to correct the problem
at intermediate points along the path. Such crankback recomputation
nodes are most likely to be domain boundaries where the PCC had
already invoked a PCE. Thus, a failure within a domain is reported
to the ingress domain boundary, which will attempt to compute an
alternate path across the domain. Failing this, the problem may be
reported to the previous domain and communicated to the ingress
boundary for that domain, which may attempt to select a more
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successful path either by choosing a different entry point into the
next domain, or by selecting a route through a different set of
domains.
7. The View from the Path Computation Client
The view of the PCE architecture, and particularly the functional
model, is subtly different from the PCC's perspective. This is
partly because the PCC has limited knowledge of the way in which the
PCEs cooperate to answer its requests, but depends more on the fact
that the PCC is concerned with different questions.
The PCC is interested in the following:
- Selecting a PCE that is able to promptly provide a computed path
that meets the supplied constraints.
- How many computation requests will the PCC have to send? Will the
desired path be computed by the first PCE contacted (possibly in
cooperation with other PCEs), or will the PCC have to consult other
PCEs to fill in gaps in the path?
- How many other path computations will need to be issued from within
the network in order to establish the TE LSP?
This last question might be considered out of scope for the head-end
LSR, but an important constraint that the PCC may wish to apply is
that the path should be computed in its entirety and supplied without
loose hops or non-simple abstract nodes.
Thus, with its limited perspective, the PCC will see Multiple PCE
Path Computation (Section 5.3) as important and will distinguish two
subcases. The first is as shown in Figure 3 with subsequent
computation requests made by other PCCs along the path of the TE LSP.
In the second, multiple computation requests are issued by the head-
end LSR. On the other hand, the PCC will not be aware of Multiple
PCE Path Computation with Inter-PCE Communication (Section 5.4),
which it will perceive as no different from the simple External PCE
Node case (Section 5.2).
The PCC, therefore, will be acutely aware that a Centralized PCE
Model (Section 6.1) might still require Multiple PCE Path
Computations with the head-end or subsequent PCCs required to issue
further requests to the central PCE. Conversely, the PCC may be
protected from the Distributed PCE Model (Section 6.2) because the
first PCE it consults uses inter-PCE communication to achieve a
complete computation result so that no further computation requests
are required.
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These distinctions can be completely classified by determining
whether the computation response includes all necessary paths, and
whether those paths are fully explicit (that is, containing only
strict hops between simple abstract nodes).
8. 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) might create path loops unless careful
policy is applied.
- Scalability: The implications of routing, TE LSP signaling, and PCE
communication overhead, such as the number of messages and the size
of messages (including LSAs, crankback information, queries,
distribution mechanisms, etc.).
- Load sharing: The ability to allow multiple PCEs to spread the path
computation load by allowing multiple PCEs each to 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.
- Path computation time: The time to compute individual paths and
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.
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- Speed with which TED synchronization is achieved.
- Impact of the synchronization process on the data flows in the
network.
- Ability to deal with situations where paths satisfying a required
set of constraints cannot be found by the PCE.
- Policy: Application of policy to the PCC-PCE and PCE-PCE
communications as well as to the computation of paths that respect
inter-domain TE LSP establishment policies.
Note that other metrics may also be considered. Such metrics should
be used when evaluating a particular PCE-based architecture. 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).
9. 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.
9.1. Control of Function and Policy
It must be possible to enable and disable the PCE function at a PCE,
and this will lead to the PCE accepting, rejecting, or simply not
receiving requests from PCCs. Graceful shutdown of the PCE function
should also be considered so that in controlled circumstances (such
as software upgrade) a PCE does not just 'disappear' but warns its
PCCs and gracefully handles any queued computation requests (perhaps
by completing them, forwarding them to another PCE, or rejecting
them).
Similarly it must be possible to control the application of policy at
the PCE through configuration. This control may include the
restriction of certain functions or algorithms, the configuration of
access rights and priorities for PCCs, and the relationships with
other PCEs both inside and outside the domain.
The policy configuration interface is yet to be determined. The
interface may be purely a local matter, or it may be supported via a
standardized interface (such as a MIB module).
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9.2. Information and Data Models
It is expected that the operations of PCEs and PCCs will be modeled
and controlled through appropriate MIB modules. 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 the 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 PCE, including issues such as malformed messages,
unauthorized messages, and messages discarded because of 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 when
key thresholds are crossed or when important events occur. Great
care must be exercised to ensure that the network is not flooded with
Simple Network Management Protocol (SNMP) notifications. Thus, it
might be inappropriate to issue a notification every time a PCE
receives a request to compute a path. In any case, full control must
be provided to allow notifications to be disabled using, for example,
the mechanisms defined in the SNMP-NOTIFICATION-MIB module in
[RFC3413].
9.3. 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, or
- to allow a PCE to abandon processing of a received request.
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9.4. 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 as 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
9.1. It may also be a routing issue to ensure that PCC-PCE
connectivity is possible.
Verifying computed paths is more complex. The information to perform
this function can, however, be made available to the operator through
MIB tables, provided that 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.
9.5. 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 solution's 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 TE LSP signaling
The primary example of a PCC identified in this architecture is an
MPLS or a GMPLS LSR. Consideration must therefore be given to the
manageability of the LSRs and the additional manageability
constraints applicable to the TE LSP signaling protocols.
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In addition to 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
route used for a TE LSP was supplied in a signaling message from
another LSR, 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.
- Reuse of existing policy models and mechanisms
As policy support mechanisms can be quite extensive, it is
worthwhile to explore to what extent this prior work can be
leveraged and applied to PCE. This desire to leverage prior work
should not be interpreted as a requirement to use any particular
solution or protocol.
9.6. Impact on Network Operation
This architecture may have two impacts on the operation of a network.
It increases TE 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 or if the path
computation load is distributed among a set of PCEs.
Issues of potential congestion during recovery from failures may be
mitigated through the use of pre-established protection schemes such
as fast reroute.
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.
9.7. Other Considerations
No other management considerations have been identified.
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10. 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. The impact may be less likely to be an issue in the
case of intra-domain use of PCE, 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.
Note that the use of a non-local PCE (that is, one not co-resident
with the PCC) does introduce additional security issues. Most
notable among these are:
- interception of PCE requests or responses;
- impersonation of PCE or PCC;
- falsification of TE information, policy information, or PCE
capabilities; and
- 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.
11. Acknowledgements
The authors would like to extend their warmest thanks to (in
alphabetical order) Arthi Ayyangar, Zafar Ali, Lou Berger, Mohamed
Boucadair, Igor Bryskin, Dean Cheng, Vivek Dubey, Kireeti Kompella,
Jean-Louis Le Roux, Stephen Morris, Eiji Oki, Dimitri Papadimitriou,
Richard Rabbat, Payam Torab, Takao Shimizu, and Raymond Zhang for
their review and suggestions. Lou Berger provided valuable and
detailed contributions to the discussion of policy in this document.
Thanks also to Pekka Savola, Russ Housley and Dave Kessens for review
and constructive discussions during the final stages of publication.
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12. Informative References
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, September 1999.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62, RFC
3413, December 2002.
[RFC3473] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.
[RFC3748] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE)",
RFC 3784, June 2004.
[RFC3812] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Traffic Engineering
(TE) Management Information Base (MIB)", RFC 3812, June
2004.
[RFC4105] Le Roux, J.-L., Vasseur, J.-P., and J. Boyle,
"Requirements for Inter-Area MPLS Traffic Engineering",
RFC 4105, June 2005.
[RFC4216] Zhang, R. and J.-P. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.
[MLN] Shiomoto, K., Papdimitriou, D., Le Roux, J.-L., Vigoureux,
M., and D. Brungard, "Requirements for GMPLS-based multi-
region and multi-layer networks (MRN/MLN)", Work in
Progress, June 2006.
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Authors' Addresses
Adrian Farrel
Old Dog Consulting
EMail: adrian@olddog.co.uk
Jean-Philippe Vasseur
1414 Massachussetts Avenue
Boxborough, MA 01719
USA
EMail: jpv@cisco.com
Jerry Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748,
USA
Phone: (732)-420-4578
Fax: (732)-368-8659
EMail: gash@att.com
Farrel, et al. Informational [Page 39]
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Farrel, et al. Informational [Page 40]
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