Policy-Enabled Path Computation Framework
RFC 5394
| Document | Type | RFC - Informational (December 2008; No errata) | |
|---|---|---|---|
| Authors | Dimitri Papadimitriou , Gerald Ash , Igor Bryskin , Lou Berger | ||
| Last updated | 2015-10-14 | ||
| Replaces | draft-bryskin-pce-policy-enabled-path-comp | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 5394 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Ross Callon | ||
| Send notices to | (None) | ||
Network Working Group I. Bryskin
Request for Comments: 5394 Adva Optical
Category: Informational D. Papadimitriou
Alcatel
L. Berger
LabN Consulting
J. Ash
AT&T
December 2008
Policy-Enabled Path Computation Framework
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) 2008 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
The Path Computation Element (PCE) architecture introduces the
concept of policy in the context of path computation. This document
provides additional details on policy within the PCE architecture and
also provides context for the support of PCE Policy. This document
introduces the use of the Policy Core Information Model (PCIM) as a
framework for supporting path computation policy. This document also
provides representative scenarios for the support of PCE Policy.
Bryskin, et al. Informational [Page 1]
RFC 5394 Policy-Enabled Path Computation December 2008
Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Background ......................................................4
2.1. Motivation .................................................4
2.2. Policy Attributes ..........................................6
2.3. Representative Policy Scenarios ............................7
2.3.1. Scenario: Policy Configured Paths ...................7
2.3.2. Scenario: Provider Selection Policy ................10
2.3.3. Scenario: Policy Based Constraints .................12
2.3.4. Scenario: Advanced Load Balancing (ALB) Example ....14
3. Requirements ...................................................16
4. Path Computation Policy Information Model (PCPIM) ..............18
5. Policy-Enabled Path Computation Framework Components ...........20
6. Policy Component Configurations ................................21
6.1. PCC-PCE Configurations ....................................21
6.2. Policy Repositories .......................................24
6.3. Cooperating PCE Configurations ............................25
6.4. Policy Configuration Management ...........................27
7. Inter-Component Communication ..................................27
7.1. Policy Communication ......................................27
7.2. PCE Discovery Policy Considerations .......................29
8. Path Computation Sequence of Events ............................29
8.1. Policy-Enabled PCC, Policy-Enabled PCE ....................29
8.2. Policy-Ignorant PCC, Policy-Enabled PCE ...................31
9. Introduction of New Constraints ................................32
10. Security Considerations .......................................33
11. Acknowledgments ...............................................33
12. References ....................................................34
12.1. Normative References .....................................34
12.2. Informative References ...................................34
1. Introduction
The Path Computation Element (PCE) Architecture is introduced in
[RFC4655]. This document describes the impact of policy-based
decision making when incorporated into the PCE architecture and
provides additional details on, and context for, applying policy
within the PCE architecture.
Policy-based Management (PBM), see [RFC3198], is a network management
approach that enables a network to automatically perform actions in
response to network events or conditions based on pre-established
rules, also denoted as policies, from a network administrator. PBM
enables network administrators to operate in a high-level manner
through rule-based strategy (policies can be defined as a set of
rules and actions); the latter are translated automatically (i.e.,
Bryskin, et al. Informational [Page 2]
RFC 5394 Policy-Enabled Path Computation December 2008
dynamically, without human interference) into individual device
configuration directives, aimed at controlling a network as a whole.
Two IETF Working Groups have considered policy networking in the
past: The Resource Allocation Protocol (RAP) working group and the
Policy Framework working group.
A framework for policy-based admission control [RFC2753] was defined
and a protocol for use between Policy Enforcement Points (PEP) and
Policy Decision Points (PDP) was specified: Common Open Policy
Service (COPS) [RFC2748]. This document uses the terms PEP and PDP
to refer to the functions defined in the COPS context. This document
makes no assumptions nor does it require that the actual COPS
protocol be used. Any suitable policy exchange protocol (for
example, Simple Object Access Protocol (SOAP) [W3CSOAP]) may be
substituted.
The IETF has also produced a general framework for representing,
managing, sharing, and reusing policies in a vendor-independent,
interoperable, and scalable manner. It has also defined an
extensible information model for representing policies, called the
Policy Core Information Model (PCIM) [RFC3060], and an extension to
this model to address the need for QoS management, called the Quality
of Service (QoS) Policy Information Model (QPIM) [RFC3644]. However,
additional mechanisms are needed in order to specify policies related
to the path computation logic as well as its control.
In Section 2, this document presents policy-related background and
scenarios to provide a context for this work. Section 3 provides
requirements that must be addressed by mechanisms and protocols that
enable policy-based control over path computation requests and
decisions. Section 4 introduces PCIM as a core component in a
framework for providing policy-enabled path computation. Section 5
introduces a set of components that may be used to support policy-
enabled path computation. Sections 6, 7, and 8 provide details on
possible component configurations, communication, and events.
Section 10 discusses the ability to introduce new constraints with
minimal impact. It should be noted that this document, in Section 4,
only introduces PCIM; specific PCIM definitions to support path
computation will be discussed in a separate document.
1.1. Terminology
The reader is assumed to be familiar with the following terms:
BEEP: Blocks Extensible Exchange Protocol, see [RFC3080].
CIM: Common Information Model, see [DMTF].
COPS: Common Open Policy Service, see [RFC2748].
CSPF: Constraint-based Shortest Path First, see [RFC3630].
Bryskin, et al. Informational [Page 3]
RFC 5394 Policy-Enabled Path Computation December 2008
LSP: Label Switched Path, see [RFC3031].
LSR: Label Switching Router, see [RFC3031].
PBM: Policy-Based Management, see [RFC3198].
PC: Path Computation.
PCC: Path Computation Client, see [RFC4655].
PCCIM: Path Computation Core Information Model.
PCE: Path Computation Element, see [RFC4655].
PCEP: Path Computation Element Communication Protocol,
see [PCEP].
PCIM: Policy Core Information Model, see [RFC3060].
PDP: Policy Decision Point, see [RFC2753].
PEP: Policy Enforcement Point, see [RFC2753].
QPIM: QoS Policy Information Model, see [RFC3644].
SLA: Service Level Agreement.
SOAP: Simple Object Access Protocol, see [W3CSOAP].
TE: Traffic Engineering, see [RFC3209] and [RFC3473].
TED: Traffic Engineering Database, see [RFC3209] and [RFC3473].
TE LSP: Traffic Engineering MPLS Label Switched Path, see
[RFC3209] and [RFC3473].
WDM: Wavelength Division Multiplexing
2. Background
This section provides some general background on the use of policies
within the PCE architecture. It presents the rationale behind the
use of policies in the TE path computation process, as well as
representative policies usage scenarios. This information is
intended to provide context for the presented PCE policy framework.
This section does not attempt to present an exhaustive list of
rationales or scenarios.
2.1. Motivation
The PCE architecture as introduced in [RFC4655] includes policy as an
integral part of the PCE architecture. This section presents some of
the rationale for this inclusion.
Network operators require a certain level of flexibility to shape the
TE path computation process, so that the process can be aligned with
their business and operational needs. Many aspects of the path
computation may be governed by policies. For example, a PCC may use
policies configured by the operator to decide which optimization
criteria, constraints, diversities and their relaxation strategies to
request while computing path(s) for a particular service. Depending
on SLAs, TE and cost/performance ratio goals, path computation
requests may be issued differently for different services. A given
Service A, for instance, may require two Shared Risk Link Group
(SRLG)-disjoint paths for building end-to-end recovery scheme, while
Bryskin, et al. Informational [Page 4]
RFC 5394 Policy-Enabled Path Computation December 2008
for a Service B link-disjoint paths may be sufficient. Service A may
need paths with minimal end-to-end delay, while Service B may be
looking for shortest (minimal-cost) paths. Different constraint
relaxation strategies may be applied while computing paths for
Service A and for Service B, and so forth. So, based on distinct
service requirements, distinct or similar policies may be adopted
when issuing/handling path computation requests.
Likewise, a PCE may apply policies to decide which algorithm(s) to
use while performing path computations requested from a particular
PCC or for a particular domain, see [RFC4927]; whether to seek the
cooperation of other PCEs to satisfy a particular request or to
handle a request on its own (possibly responding with non-explicit
paths), or how the request should be modified before being sent to
other member(s) of a group of cooperating PCEs, etc.
Additional motivation for supporting policies within the PCE
architecture can be described as follows. Historically, a path
computation entity was an intrinsic part of an LSR's control plane
and always co-located with the LSR's signaling and routing
subsystems. This approach allowed for unlimited flexibility in
providing various path computation enhancements, such as: adding new
types of constraints, diversities and their relaxation strategies,
adopting new objective functions and optimization criteria, etc. All
that had to be done to support an enhancement was to upgrade the
control plane software of a particular LSR (and no other LSRs or any
other network elements).
With the introduction of the PCE architecture, the introduction of
new PCE capabilities becomes more complicated: it isn't enough for a
PCE to upgrade its own software. In order to take advantage of a
PCE's new capabilities, new advertising and signaling objects may
need to be standardized, all PCCs may need to be upgraded with new
software, and new interoperability problems may need to be resolved,
etc.
Within the context of the PCE architecture, it is therefore highly
desirable to find a way to introduce new path computation
capabilities without requiring modifying either the
discovery/communication protocols or the PCC software. One way to
achieve this objective is to consider path selection constraints,
their relaxations, and objective functions, as path computation
request-specific policies. Furthermore, such policies may be
configured and managed by a network operator as any other policies
and may be interpreted in real time by PCCs and PCEs.
Bryskin, et al. Informational [Page 5]
RFC 5394 Policy-Enabled Path Computation December 2008
There are a number of advantages and useful by-products of such an
approach:
- New path computation capabilities may be introduced without
changing PCE-PCC communication and discovery protocols or PCC
software. Only the PCE module providing the path computation
capabilities (referred to in this document as a path computation
engine) needs to be updated.
- Existing constraints, objective functions and their relaxations may
be aggregated and otherwise associated, thus producing new, more
complex objective functions that do not require a change of code
even on the PCEs supporting the functions.
- Different elements such as conditions, actions, variables, etc.,
may be reused by multiple constraints, diversities, and
optimizations.
- PCCs and PCEs need to handle other (that is, not request-specific)
policies. Path computation-related policies of all types can be
placed within the same policy repositories, managed by the same
policy management tools, and interpreted using the same mechanisms.
Also, policies need to be supported by PCCs and PCEs independent of
the peculiarities of a specific PCC-PCE communication protocol, see
[PCEP]. Thus, introducing a new (request-specific) type of policy
describing constraints and other elements of a path computation
request will be a natural and relatively inexpensive addition to
the policy-enabled path computation architecture.
2.2. Policy Attributes
This section provides a summary listing of the policy attributes that
may be included in the policy exchanges described in the scenarios
that follow. This list is provided for guidance and is not intended
to be exclusive. Implementation of this framework might include
additional policy attributes not listed here.
Identities
- LSP head-end
- LSP destination
- PCC
- PCE
Bryskin, et al. Informational [Page 6]
RFC 5394 Policy-Enabled Path Computation December 2008
LSP identifiers
- LSP head-end
- LSP destination
- Tunnel identifier
- Extended tunnel identifier
- LSP ID
- Tunnel name
Requested LSP qualities
- bandwidth
- traffic parameters
- LSP attributes
- explicit path inclusions
- explicit path exclusions
- link protection level
- setup priority
- holding priority
- preexisting LSP route
Requested path computation behavior
- objective function
- other LSPs to be considered
Additional policy information
- Transparent policy information as received in Resource
Reservation Protocol (RSVP)-TE
2.3. Representative Policy Scenarios
This section provides example scenarios of how policies may be
applied using the PCE policy framework within the PCE architecture
context. Actual networks may deploy one of the scenarios discussed,
some combination of the presented scenarios, or other scenarios (not
discussed). This section should not be viewed as limiting other
applications of policies within the PCE architecture.
2.3.1. Scenario: Policy Configured Paths
A very simple usage scenario for PCE policy would be to use PCE to
centrally administer configured paths. Configured paths are composed
of strict and loose hops in the form of Explicit Route Objects
(EROs), see [RFC3209], and are used by one or more LSPs. Typically,
such paths are configured at the LSP ingress. In the context of
policy-enabled path computation, an alternate approach is possible.
Bryskin, et al. Informational [Page 7]
RFC 5394 Policy-Enabled Path Computation December 2008
In particular, service-specific policies can be installed that will
provide configured path(s) for a specific service request. The
request may be identified based on service parameters such as
endpoints, requested QoS, or even a token that identifies the
initiator of a service request. The configured path(s) would then be
used as input to the path computation process, which would return
explicit routes by expanding of all specified loose hops.
Example of policy:
if(service_destination matches 10.132.12.0/24)
Use path: 10.125.13.1 => 10.125.15.1 => 10.132.12.1.
else
Compute path dynamically.
----------------------
| ----- |
| | TED |<-+------------>
| ----- | TED synchronization
| | | mechanism (e.g., routing protocol)
| | |
| v |
| ------ ----- | Inter-PCE Request/Response
| |Policy|<-->| PCE |<.+...........> (when present)
| ------ ----- |
----------------------
^
| Request/
| Response
v
Service ------------- Signaling
Request |[PCC][Policy]| Protocol
<------>| Node |<------->
or Signaling -------------
Protocol
Figure 1: Policy Enabled PCC and PCE
Path computation policies may be applied at either a PCC or a PCE,
see Figure 1. In the PCC case, the configured path would be
processed at the PCC and then passed to the PCE along with the PCE
request, probably in the form of (inclusion) constraints. When
applied at the PCE, the configured path would be used locally. Both
cases require some method to configure and manage policies. In the
PCC case, the real benefit would come when there is an automated
policy distribution mechanism.
Bryskin, et al. Informational [Page 8]
RFC 5394 Policy-Enabled Path Computation December 2008
------------------ -------------------
| | | |
| PCE | | PCE |
| | | |
| ------ ----- | | ----- ------ |
| |Policy| | TED | | | | TED | |Policy| |
| ------ ----- | | ----- ------ |
------------------ -------------------
^ ^
| Request/ | Request/
| Response | Response
v v
Service -------- Signaling ------------ Signaling ------------
Request|Head-End| Protocol |Intermediate| Protocol |Intermediate|
---->| Node |<--------->| Node |<--------->| Node |
-------- ------------ ------------
Figure 2: Multiple PCE Path Computation
------------------ ------------------
| | 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 3: Multiple PCE Path Computation with Inter-PCE Communication
Policy-configured paths may also be used in environments with
multiple (more than one) cooperating PCEs (see Figures 2 and 3). For
example, consider the case when there is limited TE visibility and
independent PCEs are used to determine path(s) within each area of
the TE visibility. In such a case, it may not be possible (or
desirable) to configure entire explicit path(s) on a single PCE.
However, it is possible to configure explicit path(s) for each area
of the TE visibility and each responsible PCE. One by one, the PCEs
would then map an incoming signaling request to appropriate
configured path(s). Note that to make such a scenario work, it would
Bryskin, et al. Informational [Page 9]
RFC 5394 Policy-Enabled Path Computation December 2008
likely be necessary to start and finish the configured paths on TE
domain boundary nodes. Clearly, consistent PCE Policy Repositories
are also critical in this example.
2.3.2. Scenario: Provider Selection Policy
A potentially more interesting scenario is applying PC policies in
multi-provider topologies. There are numerous interesting policy
applications in such topologies. A rudimentary example is simple
access control, that is, deciding which PCCs are permitted to request
inter-domain path computation.
A more complicated example is applying policy to determine which
domain or network provider will be used to support a particular PCE
request. Consider the topology presented in Figure 4. In this
example, there are multiple transit domains available to provide a
path from a source domain to a destination domain. Furthermore, each
transit domain may have one or more options for reaching a particular
domain. Each domain will need to select which of the multiple
available paths will be used to satisfy a particular PCE request.
In today's typical path computation process, TE reachability,
availability, and metric are the basic criteria for path selection.
However, policies can provide an important added consideration in the
decision process. For example, transit domain A may be more
expensive and provide lower delay or loss than transit domain B.
Likewise, a transit domain may wish to treat PCE requests from its
own customers differently than requests from other providers. In
both cases, computation based on traffic engineering databases will
result in multiple transit domains that provide reachability, and
policies can be used to govern which PCE requests get better service.
Bryskin, et al. Informational [Page 10]
RFC 5394 Policy-Enabled Path Computation December 2008
+-------+
+----------+Transit+----------+
+---+---+ | Domain| +---+---+
|Transit| | C | |Transit|
+--------+ Domain| +---+---+ | Domain+--------+
| | A +--+ | +--+ F | |
+--+---+ +---+---+ | | | +---+---+ +--+---+
|Source| | | +---+---+ | | |Target|
|Domain| | +---+Transit+---+ | |Domain|
+--+---+ | +---+ Domain|---+ | +--+---+
| +---+---+ | | D | | +---+---+ |
| |Transit| | +---+---+ | |Transit| |
+--------+ Domain+--+ | +--+ Domain+--------+
| B | | | G |
+---+---+ +---+---+ +---+---+
| |Transit| |
+----------+ Domain+----------+
| E |
+-------+
Figure 4: Multi-Domain Network with Multiple Transit Options
There are multiple options for differentiating which PCE requests use
a particular transit domain and get a particular (better or worse)
level of service. For example, a PCE in the source domain may use
user- and request-specific policies to determine the level of service
to provide. A PCE in the source domain may also use domain-specific
policies to choose which transit domains are acceptable. A PCE in a
transit domain may use request-specific policies to determine if a
request is from a direct customer or another provider, and then use
domain-specific policies to identify how the request should be
processed.
Example of policy:
if(path computation request issued by a PCC within Source Domain)
Route the path through Transit Domain A.
else
Route the path through Transit Domain B.
Bryskin, et al. Informational [Page 11]
RFC 5394 Policy-Enabled Path Computation December 2008
2.3.3. Scenario: Policy Based Constraints
Another usage scenario is the use of policy to provide constraints in
a PCE request. Consider an LSR with a policy enabled PCC, as shown
in Figure 1, which receives a service request via signaling,
including over a Network-Network Interface (NNI) or User Network
Interface (UNI) reference point, or receives a configuration request
over a management interface to establish a service. In either case,
the path(s) needed to support the service are not explicitly
specified in the message/request, and hence path computation is
needed.
In this case, the PCC may apply user- or service-specific policies to
decide how the path selection process should be constrained, that is,
which constraints, diversities, optimization criterion, and
constraint relaxation strategies should be applied in order for the
service LSP(s) to have a likelihood to be successfully established
and provide necessary QoS and resilience against network failures.
When deciding on the set of constraints, the PCC uses as an input all
information it knows about the user and service, such as the contents
of the received message, port ID over which message was received,
associated VPN ID, signaling/reference point type, request time, etc.
Once the constraints and other parameters of the required path
computation are determined, the PCC generates a path computation
request that includes the request-specific policies that describe the
determined set of constraints, optimizations, and other parameters
that indicate how the request is to be considered in the path
computation process.
Example of policy:
if(LSP belongs to a WDM layer network)
Compute the path with wavelength continuity constraint with the
maximum Optical Signal Noise Ratio (OSNR) at the path end
optimization.
else if(LSP belongs to a connection oriented Ethernet layer network)
Compute the path with minimum end-to-end delay.
else
Compute the shortest path.
The PCC may also apply server-specific policies in order to select
which PCE to use from the set of known (i.e., discovered or
configured) PCEs. The PCC may also use server-specific policies to
form the request to match the PCE's capabilities so that the request
will not be rejected and has a higher likelihood of being satisfied
in an efficient way. An example of a request modification as the
result of a server-specific policy is removing a constraint not
supported by the PCE. Once the policy processing is completed at the
Bryskin, et al. Informational [Page 12]
RFC 5394 Policy-Enabled Path Computation December 2008
PCC, and the path computation request resulting from the original
service request is updated by the policy processing, the request is
sent to the PCE.
Example of policy:
if(LSP belongs to a WDM layer network)
Identify a PCE supporting wavelength continuity and optical
impairment constraints;
Send a request to such PCE, requesting path computation with the
following constraints:
a) wavelength continuity;
b) maximum Polarization Mode Dispersion (PMD) at the path end.
if(the path computation fails) remove the maximum PMD constraint
and try the computation again.
The PCE that receives the request validates and otherwise processes
the request, applying the policies found in the request as well as
any policies that are available at the PCE, e.g., client- and domain-
specific policies. As a result of the policy processing, the PCE may
decide to reject the request.
Example of policy:
Authenticate the PCC requesting the path computation using the
PCC ID found in the path computation request;
Reject the request if the authentication fails.
The PCE also may decide to respond with one or several pre-computed
paths if user- or client-specific policies instruct the PCE to do so.
If the PCE decides to satisfy the request by performing a path
computation, it determines if it needs the cooperation of other PCEs
and defines parameters for path computations to be performed locally
and remotely. After that, the PCE instructs a co-located path
computation engine to perform the local path computation(s) and, if
necessary, sends path computation requests to one or more other PCEs.
It then waits for the responses from the local path computation
engine and, when used, the remote PCE. It then combines the
resulting paths and sends the result back to the requesting PCC. The
response may indicate policies describing the resulting paths, their
characteristics (summary cost, expected end-to-end delay, etc.), as
well as additional information related to the request, e.g., which
constraints were honored, which were dismissed, and which were
relaxed and in what way.
Bryskin, et al. Informational [Page 13]
RFC 5394 Policy-Enabled Path Computation December 2008
Example of policy:
if(the path destination belongs to domain A)
Instruct local path computation engine to perform the path
computation;
else
Identify the PCE supporting the destination domain;
Send path computation request to such PCE;
Wait for and process the response.
Send the path computation response to the requesting PCC.
The PCC processes the response and instructs the LSR to encode the
received path(s) into the outgoing signaling message(s).
2.3.4. Scenario: Advanced Load Balancing (ALB) Example
Figure 5 illustrates a problem that stems from the coupling between
BGP and IGP in the BGP decision process. If a significant portion of
the traffic destined for the data center (or customer network) enters
a PCE-enabled network from AS 1 and all IGP links' weights are the
same, then both PE3 and PE4 will prefer to reach the data center
using the routes advertised by PE2. PE5 will use the router-IDs of
PE1 and PE2 to break the tie and might therefore also select to use
the path through PE2 (if the router ID of PE2 is smaller than that of
PE1). Either way, the net result is that the link between PE2 and CE
will carry most of the traffic while the link between PE1 and the
Customer Edge (CE) will be mostly idle.
Bryskin, et al. Informational [Page 14]
RFC 5394 Policy-Enabled Path Computation December 2008
..............................
. AS 1 .
. .
. +---+ +---+ +----+ .
....|PE8|...|PE9|...|PE10|....
+---+ +---+ +----+
| | |
+---+ +---+ +---+
......|PE3|...|PE4|...|PE5|......
. +---+ +---+ +---+ .
.............. +---+ \ / ___/ +---+
. . _|PE2|_____+--+__/ / _|PE6|
. +--+ / +---+ |P1|_____+--+_______/ +---+
. Customer |CE|= . +--+ |P2| .
. Network +--+ \_+---+ \ +--+ .
. . |PE1|________+--+___/| x===x . PCE used
.............. +---+ |P3| | |PCE| . by all
. +--+ | x===x . AS0 nodes
. AS 0 +---+ .
..................|PE7|..........
+---+
Figure 5: Advanced Load Balancing
This is a common problem for providers and customers alike. Analysis
of Netflow records, see [IRSCP], for a large ISP network on a typical
day has shown that for 71.8% of multi-homed customers, there is a
complete imbalance, where the most loaded link carries all the
traffic and the least loaded link carries none.
PCE policies can address this problem by basing the routing decision
at the ingress routers on the offered load towards the multi-homed
customer. For example, in Figure 5, PCE policies could be configured
such that traffic load is monitored (e.g., based on Netflow data) at
ingress routers PE3 to PE7 towards the data center prefixes served by
egress routers PE1 and PE2. Using this offered load information, the
path computations returned by PCE, based on the enabled PCE policies,
can direct traffic to the appropriate egress router, on a per-ingress
router basis. For example, the PCE path computation might direct
traffic from both PE4 and PE5 to egress PE1, thus overriding the
default IGP based selection. Alternatively, traffic from each
ingress router to each egress link could be split 50-50.
This scenario is a good example of how a policy-governed PCE can
account for some information that was not or cannot be advertised as
TE link/node attributes, and, therefore, cannot be subject for
explicit path computation constraints. More generally, such
information can be pretty much anything. For example, traffic demand
Bryskin, et al. Informational [Page 15]
RFC 5394 Policy-Enabled Path Computation December 2008
forecasts, flow monitoring feedback, any administrative policies,
etc. Further examples are described in [IRSCP] of how PCE policies
might address certain network routing problems, such as selective
distributed denial-of-service (DDoS) blackholing, planned
maintenance, and VPN gateway selection.
Example of policy:
for(all traffic flows destined to Customer Network)
if(flow ingresses on PE3, PE4, or PE5)
Route the flow over PE1.
else
Route the flow over PE2.
3. Requirements
The following requirements must be addressed by mechanisms and
protocols that enable policy-based control over path computation
requests and decisions:
- (G)MPLS path computation-specific
The mechanisms must meet the policy-based control requirements
specific to the problem of path computation using RSVP-TE as the
signaling protocol on MPLS and GMPLS LSRs.
- Support for non-(G)MPLS PCCs
The mechanisms must be sufficiently generic to support non-(G)MPLS
(LSR) clients such as a Network Management System (NMS), or network
planner, etc.
- Support for many policies
The mechanisms must include support for many policies and policy
configurations. In general, the determination and configuration of
viable policies are the responsibility of the service provider.
- Provision for monitoring and accounting information
The mechanisms must include support for monitoring policy state and
provide access information. In particular, mechanisms must provide
usage and access information that may be used for accounting
purposes.
- Fault tolerance and recovery
The mechanisms must include provisions for fault tolerance and
recovery from failure cases such as failure of PCC/PCE PDPs,
disruption in communication that separate a PCC/PCE PDP from its
associated PCC/PCE PEPs.
Bryskin, et al. Informational [Page 16]
RFC 5394 Policy-Enabled Path Computation December 2008
- Support for policy-ignorant nodes
The mechanisms should not be mandatory for every node in a network.
Policy-based path computation control may be enforced at a subset
of nodes, for example, on boundary nodes within an administrative
domain. These policy-capable nodes will function as trusted nodes
from the point of view of the policy-ignorant nodes in that
administrative domain. Alternatively, policy may be applied solely
on PCEs with all PCCs being policy-ignorant nodes.
- Scalability
One of the important requirements for the mechanisms is
scalability. The mechanisms must scale at least to the same extent
that RSVP-TE signaling scales in terms of accommodating multiple
LSPs and network nodes in the path of an LSP. There are several
sensitive areas in terms of scalability of policy-based path
computation control. First, not every policy-aware node in an
infrastructure should be expected to contact a remote PDP. This
would cause potentially long delays in verifying requests.
Additionally, the policy control architecture must scale at least
as well as RSVP-TE protocol based on factors such as the size of
RSVP-TE messages, the time required for the network to service an
RSVP-TE request, local processing time required per node, and local
memory consumed per node. These scaling considerations are of
particular importance during re-routing of a set of LSPs.
- Security and denial-of-service considerations
The policy control architecture, protocols, and mechanisms must be
secure as far as the following aspects are concerned:
o First, the mechanisms proposed must minimize theft and denial-
of-service threats.
o Second, it must be ensured that the entities (such as PEPs and
PDPs) involved in policy control can verify each other's
identity and establish necessary trust before communicating.
- Inter-AS and inter-area requirements
There are several inter-AS policy-related requirements discussed in
[RFC4216] and [RFC5376], and inter-area policy-related requirements
discussed in [RFC4927]. These requirements must be addressed by
policy-enabled PCE mechanisms and protocols.
It should be noted that this document only outlines the communication
elements and mechanisms needed to allow a wide variety of possible
policies to be applied for path computation control. It does not
include any discussion of any specific policy behavior, nor does it
define or require use of specific policies.
Bryskin, et al. Informational [Page 17]
RFC 5394 Policy-Enabled Path Computation December 2008
4. Path Computation Policy Information Model (PCPIM)
The Policy Core Information Model (PCIM) introduced in [RFC3060] and
expanded in [RFC3460] presents the object-oriented information model
for representing general policy information.
This model defines two hierarchies of object classes:
- Structural classes representing policy information and control of
policies.
- Association classes that indicate how instances of the structural
classes are related to each other.
These classes can be mapped to various concrete implementations, for
example, to a directory that uses Lightweight Directory Access
Protocol version 3 (LDAPv3) as its access protocol.
Figure 6 shows an abstract from the class inheritance hierarchy for
PCIM.
Bryskin, et al. Informational [Page 18]
RFC 5394 Policy-Enabled Path Computation December 2008
ManagedElement (abstract)
|
+--Policy (abstract)
| |
| +---PolicySet (abstract)
| | |
| | +---PolicyGroup
| | |
| | +---PolicyRule
| |
| +---PolicyCondition (abstract)
| | |
| | +---PolicyTimePeriodCondition
| | |
| | +---VendorPolicyCondition
| | |
| | +---SimplePolicyCondition
| | |
| | +---CompoundPolicyCondition
| | |
| | +---CompoundFilterCondition
| |
| +---PolicyAction (abstract)
| | |
| | +---VendorPolicyAction
| | |
| | +---SimplePolicyAction
| | |
| | +---CompoundPolicyAction
| |
| +---PolicyVariable (abstract)
| | |
| | +---PolicyExplicitVariable
| | |
| | +---PolicyImplicitVariable
| | |
| | +---(subtree of more specific classes)
| |
| +---PolicyValue (abstract)
| |
| +---(subtree of more specific classes)
Figure 6: PCIM Class Inheritance
The policy classes and associations defined in PCIM are sufficiently
generic to allow them to represent policies related to anything.
Bryskin, et al. Informational [Page 19]
RFC 5394 Policy-Enabled Path Computation December 2008
Policy models for application-specific areas such as the Path
Computation Service may extend the PCIM in several ways. The
preferred way is to use the PolicyGroup, PolicyRule, and
PolicyTimePeriodCondition classes directly as a foundation for
representing and communicating policy information. Then, specific
subclasses derived from PolicyCondition and PolicyAction can capture
application-specific definitions of conditions and actions of
policies.
The Policy Quality of Service Information Model [RFC3644] further
extends the PCIM to represent QoS policy information for large-scale
policy domains. New classes introduced in this document describing
QoS- and RSVP-related variables, conditions, and actions can be used
as a foundation for the PCPIM.
Detailed description of the PCPIM will be provided in a separate
document.
5. Policy-Enabled Path Computation Framework Components
The following components are defined as part of the framework to
support policy-enabled path computation:
- PCE Policy Repository
A database from which PCE policies are available in the form of
instances of PCPIM classes. PCE Policies are configured and
managed by PCE Policy Management Tools;
- PCE Policy Decision Point (PCE-PDP)
A logical entity capable of retrieving relevant path computation
policies from one or more Policy Repositories and delivering the
information to associated PCE-PEP(s);
- PCE Policy Enforcement Point (PCE-PEP)
A logical entity capable of issuing device-specific Path
Computation Engine configuration requests for the purpose of
enforcing the policies;
- PCC Policy Decision Point (PCC-PDP)
A logical entity capable of retrieving relevant path computation
policies from one or more Policy Repositories and delivering the
information to associated PCC-PEP(s);
- PCC Policy Enforcement Point (PCC-PEP)
A logical entity capable of issuing device-specific Path
Computation Service User configuration requests for the purpose of
enforcing the policies.
Bryskin, et al. Informational [Page 20]
RFC 5394 Policy-Enabled Path Computation December 2008
From the policy perspective a PCC is logically decomposed into two
parts: PCC-PDP and PCC-PEP. When present, a PCC-PEP is co-located
with a Path Computation Service User entity that requires remote path
computation (for example, the GMPLS control plane of an LSR). The
PCC-PEP and PCC-PDP may be physically co-located (as per [RFC2748])
or separated. In the latter case, they talk to each other via such
protocols as SOAP [W3CSOAP] or BEEP [RFC3080].
Likewise, a PCE is logically decomposed into two parts: PCE-PEP and
PCE-PDP. When present, PCE-PEP is co-located with a Path Computation
Engine entity that actually provides the Path Computation Service
(that is, runs path computation algorithms). PCE-PEP and PCE-PDP may
be physically co-located or separated. In the later case, they
communicate using such protocols as SOAP and/or BEEP.
PCC-PDP/PCE-PDP may be co-located with, or separated from, an
associated PCE Policy Repository. In the latter case, the PDPs use
some access protocol (for example, LDAPv3 or SNMP). The task of PDPs
is to retrieve policies from the repository (or repositories) and
convey them to respective PEPs either in an unsolicited way or upon
the PEP's requests.
A PCC-PEP may receive policy information not only from PCC-PDP(s) but
also from PCE-PEP(s) via PCC-PCE communication and/or PCE discovery
protocols. Likewise, a PCE-PEP may receive policy information not
only from PCE-PDP(s) but also from PCC-PEP(s), via the PCC-PCE
communication protocol [PCEP].
Any given policy can be interpreted (that is, translated into a
sequence of concrete device specific configuration requests) either
on a PDP or on the associated PEP or partly on the PDP and partly on
the PEP.
Generally speaking, the task of the PCC-PEP is to select the PCE and
build path computation requests applying service-specific policies
provided by the PCC-PDP. The task of the PCE-PEP is to control path
computations by applying request-specific policies found in the
requests as well as client-specific and domain-specific policies
supplied by the PCE-PDP.
6. Policy Component Configurations
6.1. PCC-PCE Configurations
The PCE policy architecture supports policy being applied at a PCC
and at a PCE. While the architecture supports policy being applied
at both, there is no requirement for policy to always be applied at
both, or even at either. The use of policy in a network, on PCCs,
Bryskin, et al. Informational [Page 21]
RFC 5394 Policy-Enabled Path Computation December 2008
and on PCEs, is a specific network design choice. Some networks may
choose to apply policy only at PCCs (Figure 7), some at PCEs (Figure
8), and others at both PCCs and PCEs (Figure 9). Regardless of where
policy is applied, it must be applied in a consistent fashion in
order to achieve the intended results.
.........................
. .
. PCE Policy Management .
. .
.........................
.
.
--------- Policy -----------------------
| PCC-PDP |<--------- | PCE Policy Repository |
--------- -----------------------
^
| e.g., SOAP
v
--------- PCEP ---------
| PCC-PEP |<------------------------------------------->| PCE |
--------- PCC-PCE Communication Protocol ---------
Figure 7: Policies Applied on PCC Only
Along with supporting flexibility in where policy may be applied, the
PCE architecture is also flexible in terms of where specific types of
policies may be applied. Also, the PCE architecture allows for the
application of only a subset of policy types. [RFC4655] defines
several PC policy types. Each of these may be applied at either a
PCC or a PCE or both. Clearly, when policy is only applied at PCCs
or at PCEs, all PCE policy types used in the network must be applied
at those locations.
Bryskin, et al. Informational [Page 22]
RFC 5394 Policy-Enabled Path Computation December 2008
.........................
. .
. PCE Policy Management .
. .
.........................
.
.
----------------------- Policy ---------
| PCE Policy Repository | -------->| PCE-PDP |
----------------------- ---------
^
e.g., SOAP |
v
--------- PCEP ---------
| PCC |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 8: Policies Applied on Only
In the case where policy is only applied at a PCE, it is expected
that the PCC will pass to the PCE all information about the service
that it can gather in the path computation request (most likely in
the form of PCPIM policy variables). The PCE is expected to
understand this information and apply appropriate policies while
defining the actual parameters of the path computation to be
performed. Note that in this scenario, the PCC cannot apply server-
specific or any other policies, and PCE selection is static.
When applying policy at both the PCC and PCE, it is necessary to
select which types of policies are applied at each. In such
configurations, it is likely that the application of policy types
will be distributed across the PCC and PCE rather than applying all
of them at both. For example, user-specific and server-specific
policies may be applied at a PCC, request- and client-specific
policies may be applied at a PCE, while domain-specific policies may
be applied at both the PCC and PCE.
In the case when policy is only applied at a PCC, the PCC must apply
all the types of required policies, for example user-, service-,
server-, and domain-specific policies. The PCC uses the policies to
construct a path computation request that appropriately represents
the applied policies. The request will necessarily be limited to the
set of "basic" (that is, non-policy capable) constraints explicitly
defined by the PCC-PCE communication protocol.
Bryskin, et al. Informational [Page 23]
RFC 5394 Policy-Enabled Path Computation December 2008
6.2. Policy Repositories
Within the policy-enabled path computation framework policy
repositories may be used in a single or multiple PCE policy
repository configuration:
o) Single PCE Policy Repository
In this configuration, there is a single PCE Policy Repository shared
between PCCs and PCEs.
.........................
. .
. PCE Policy Management .
. .
.........................
.
.
--------- Policy a ----------------------- Policy b ---------
| PCC-PDP |<--------- | PCE Policy Repository | -------->| PCE-PDP |
--------- ----------------------- ---------
^ ^
| e.g., SOAP e.g., SOAP |
v v
--------- PCEP ---------
| PCC-PEP |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 9: Single PCC/PCE Policy Repository
o) Multiple PCE Policy Repositories
The repositories in this case may be fully or partially synchronized
by some discovery/synchronization management protocol or may be
completely independent. Note that the situation when PCE Policy
Repository A exactly matches PC Policy Repository B, results in the
single PCE Policy Repository configuration case.
Bryskin, et al. Informational [Page 24]
RFC 5394 Policy-Enabled Path Computation December 2008
-------------- --------------
| PCE Policy | | PCE Policy |
---| Repository A | | Repository B |---
| -------------- -------------- |
| |
| Policy a Policy b |
| |
v v
--------- ---------
| PCC-PDP | | PCE-PDP |
--------- ---------
^ ^
| e.g., SOAP e.g., SOAP |
v v
--------- PCEP ---------
| PCC-PEP |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 10: Multiple PCE/PCC Policy Repositories
6.3. Cooperating PCE Configurations
The previous section shows the relationship between PCCs and PCEs. A
parallel relationship exists between cooperating PCEs, and, in fact,
this relationship can be viewed as the same as the relationship
between PCCs and PCEs. The one notable difference is that there will
be cases where having a shared PCE Policy Repository will not be
desirable, for example, when the PCEs are managed by different
entities. Note that in this case, it still remains necessary for the
policies to be consistent across the domains in order to identify
usable paths. The other notable difference is that a PCE, while
processing a path computation request, may need to apply requester-
specific (that is, client-specific) policies in order to modify the
request before sending it to other cooperating PCE(s). This
relationship is particularly important as the PCE architecture allows
for configuration where all PCCs are not policy-enabled.
The following are example configurations. These examples do not
represent an exhaustive list and other configurations are expected.
o) Single Policy Repository
In this configuration, there is a single PCE Policy Repository shared
between PCEs. This configuration is likely to be useful within a
single administrative domain where multiple PCEs are provided for
redundancy or load distribution purposes.
Bryskin, et al. Informational [Page 25]
RFC 5394 Policy-Enabled Path Computation December 2008
.........................
. .
. PCE Policy Management .
. .
.........................
.
.
--------- Policy a ----------------------- Policy b ---------
| PCE-PDP |<--------- | PCE Policy Repository | -------->| PCE-PDP |
--------- ----------------------- ---------
^ ^
| e.g., SOAP e.g., SOAP |
v v
--------- ---------
| PCE-PEP |<------------------------------------------->| PCE-PEP |
--------- PCE-PCE Communication Protocol ---------
Figure 11: Single PCC Policy Repository
o) Multiple Policy Repositories
The repositories in this case may be fully or partially synchronized
by some discovery/synchronization management protocol(s) or may be
completely independent. In the multi-domain case, it is expected
that the repositories will be distinct, providing, however,
consistent policies.
-------------- --------------
| PCE Policy | | PCE Policy |
---| Repository A | | Repository B |---
| -------------- -------------- |
| |
| Policy a Policy b |
| |
v v
--------- ---------
| PCE-PDP | | PCE-PDP |
--------- ---------
^ ^
| e.g., SOAP e.g., SOAP |
v v
--------- PCEP ---------
| PCE-PEP |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 12: Multiple PCC Policy Repositories
Bryskin, et al. Informational [Page 26]
RFC 5394 Policy-Enabled Path Computation December 2008
6.4. Policy Configuration Management
The management of path computation policy information used by PCCs
and PCEs is largely out of scope of the described framework. The
framework assumes that such information is installed, removed, and
otherwise managed using typical policy management techniques. Policy
Repositories may be populated and managed via static configuration,
standard and proprietary policy management tools, or even dynamically
via policy management/discovery protocols and applications.
7. Inter-Component Communication
7.1. Policy Communication
Flexibility in the application of policy types is imperative from the
architecture perspective. However, this commodity implies added
complexity on the part of the PCE-related communication protocols.
One added complexity is that PCE communication protocols must carry
certain information to support various policy types that may be
applied. For example, in the case where policy is only applied at a
PCE, a PCC-PCE request must carry sufficient information for the PCE
to apply service- or user-specific policies. This does imply that a
PCC must have sufficient understanding of what policies can be
applied at the PCE. Such information may be obtained via local
configuration, static coding, or even via a PCE discovery mechanism.
The PCC must also have sufficient understanding to properly encode
the required information for each policy type.
Another added complexity is that PCE communication protocols must
also be able to carry information that may result from a policy
decision. For example, user- or service-specific policy applied at a
PCC may result in policy-related information that must be carried
along with the request for use by a PCE. This complexity is
particularly important as it may be used to introduce new path
computation parameters (e.g., constraints, objection functions, etc.)
without modification of the core PCC and PCE. This communication
will likely simply require the PCE communication protocols to support
opaque policy-related information elements.
A final added complexity is that PCE communication protocols must
also be able to support updated or unsolicited responses from a PCE.
For example, changes in PCE policy may force a change to a previously
provided path. Such updated or unsolicited responses may contain
information that the PCC must act on, and may contain policy
information that must be provided to a PCC.
Bryskin, et al. Informational [Page 27]
RFC 5394 Policy-Enabled Path Computation December 2008
PCC-PEP and PCE-PEP or a pair of PCE-PEPs communicate via a request-
response type PCC-PCE Communication Protocol, i.e., [PCEP]. This
document makes no assumptions as to what exact protocol is used to
support this communication. This document does assume that the
semantics of a path computation request are sufficiently abstract and
general, and support both PCE-PCC and PCE-PCE communication.
From a policy perspective, a path computation request should include
at a minimum:
o One or more source addresses;
o One or more destination addresses;
o Computation type (P2P (point to point), P2MP (point to multipoint),
MP2P (multipoint to point), etc.);
o Number of required paths;
o Zero or more policy descriptors in the following format:
<policy name>,
<policy variable1 name>, <param11>, <param12>,...,<param1N>
<policy variable2 name>, <param21>, <param12>,...,<param2N>
...
<policy variableM name>, <paramM1>, <paramM2>,...,<paramMN>
A successful path computation response, at minimum, should include
the list of computed paths and may include policies (in the form of
policy descriptors as in path computation request, see above) for use
in evaluating and otherwise applying the computed paths.
PCC-PCE Communication Protocol provides transport for policy
information and should not understand nor make any assumptions about
the semantics of policies specified in path computation requests and
responses.
Note: This document explicitly allows for (but does not require) the
PCC to decide that all necessary constraints, objective functions,
etc. pertinent to the computation of paths for the service in
question are to be determined by the PCE performing the computation.
In this case, the PCC will use a set of policies (more precisely,
PCPIM policy variables) describing the service-specific information.
These policies may be placed within the path computation request and
delivered to the PCE via a PCC-PCE communication protocol such as
[PCEP]. The PCE (more precisely, PCE-PEP) is expected to understand
this information and use it to determine the constraints and
optimization functions applying local policies (that is, policies
locally configured or provided by the associated PCE-PDP(s)).
Bryskin, et al. Informational [Page 28]
RFC 5394 Policy-Enabled Path Computation December 2008
7.2. PCE Discovery Policy Considerations
Dynamic PCE discovery allows for PCCs and PCEs to automatically
discover a set of PCEs (including information required for the PCE
selection). It also allows for PCCs and PCEs to dynamically detect
new PCEs or any modification of PCEs status. Policy can be applied
in two ways in this context:
1. Restricting the scope of information distribution for the
mandatory set of information (in particular the PCE presence and
location).
2. Restricting the type and nature of the optional information
distributed by the discovery protocol. The latter is also subject
to policy since the PCE architecture allows for distributing this
information using either PCE discovery protocol(s) or PCC-PCE
communication protocol(s). One important policy decision in this
context is the nature of the information to be distributed,
especially, when this information is not strictly speaking
"discovery" information, rather, the PCE state changes. Client-
specific and domain-specific policies may be applied when deciding
whether this information should be distributed and to which
clients of the path computation service (that is, which PCCs
and/or PCEs).
Another place where policy applies is at the administrative
boundaries. In multi-domain networks, multiple PCEs will communicate
with each other and across administrative boundaries. In such cases,
domain-specific policies would be applied to 1) filter the
information exchanged between peering PCEs during the discovery
process (to the bare minimum in most cases if at all allowed by the
security policy) and 2) limit the content of information being passed
in path computation request and responses.
8. Path Computation Sequence of Events
This section presents a non-exhaustive list of representative
scenarios.
8.1. Policy-Enabled PCC, Policy-Enabled PCE
When a GMPLS LSR receives a Setup (RSVP Path) message from an
upstream LSR, the LSR may decide to use a remote Path Computation
Entity. The following sequence of events occurs in this case:
- A PCC-PEP co-located with the LSR applies the service-specific
policies to select a PCE for the service path computation as well
as to build the path computation request (that is, to select a list
Bryskin, et al. Informational [Page 29]
RFC 5394 Policy-Enabled Path Computation December 2008
of policies, their variables, conditions and actions expressing
constraints, diversities, objective functions and relaxation
strategies appropriate for the service path computation). The
policies may be:
a) Statically configured on the PCC-PEP;
b) Communicated to the PCC-PEP by a remote or local PCC-PDP via
protocol such as SOAP either proactively (most of the cases) or
upon an explicit request by the PCC-PEP in cases when some
specifics of the new service have not been covered yet by the
policies so far known to the PCC-PEP).
The input for the decision process on the PCC-PEP is the
information found in the signaling message as well as any other
service-specific information such as port ID over which the message
was received, associated VPN ID, the reference point type (UNI,
E-NNI, etc.) and so forth. After the path computation request is
built, it is sent directly to the PCE-PEP using the PCC-PCE
Communication Protocol, e.g., [PCEP].
- PCE-PEP validates and otherwise processes the request applying the
policies found in the request- as well as client- and domain-
specific policies. The latter, again, may be either statically
configured on the PCE-PEP or provided by the associated local or
remote PCE-PDP via a protocol such as SOAP. The outcome of the
decision process is the following information:
a) Whether the request should be satisfied, rejected, or dismissed.
b) The sets of sources and destinations for which paths should be
locally computed.
c) The set of constraints, diversities, optimization functions, and
relaxations to be considered in each of locally performed path
computation.
d) The address of the next-in-chain PCE.
e) The path computation request to be sent to the next-in-chain
PCE.
The PCE-PEP instructs a co-located path computation engine to
perform the local path computation(s) and, if necessary, sends the
path computation request to the next-in-chain PCE using a PCC-PCE
Communication Protocol. Then, it waits for the responses from the
local path computation engine and the remote PCE, combines the
resulting paths, and sends them back to the PCC-PEP using the PCC-
Bryskin, et al. Informational [Page 30]
RFC 5394 Policy-Enabled Path Computation December 2008
PCE Communication Protocol. The response contains the resulting
paths as well as policies describing some additional information
(for example, which of constraints were honored, which were
dismissed, and which were relaxed and in what way).
- PCC-PEP instructs the signaling subsystem of the GMPLS LSR to
encode the received path(s) into the outgoing Setup message(s).
8.2. Policy-Ignorant PCC, Policy-Enabled PCE
This case parallels the previous example, but the user- and service-
specific policies should be applied at the PCE as the PCC is policy
ignorant. Again, when a GMPLS LSR has received a Setup (RSVP Path)
message from an upstream LSR, the LSR may decide to use a non-co-
located Path Computation Entity. The following sequence of events
occurs in this case:
- The PCC constructs a PCE request using information found in the
signaling/provisioning message as well as any other service-
specific information such as port ID over which the message was
received, associated VPN ID, the reference point type (UNI, E-NNI,
etc.) and so forth. This information is encoded in the request in
the form of policy variables. After the request is built, it is
sent directly to the PCE-PEP using a PCC-PCE Communication
Protocol.
- PCE-PEP validates and otherwise processes the request interpreting
the policy variables found in the request and applying user-,
service-, client-, and domain-specific policies to build the actual
path computation request. The policies, again, may be either
statically configured on the PCE-PEP or provided by the associated
local or remote PCE-PDP via a protocol such as SOAP. The outcome
of the decision process is the following information:
a) Whether the request should be satisfied, rejected, or dismissed.
b) The sets of sources and destinations for which paths should be
locally computed.
c) The set of constraints, diversities, optimization functions, and
relaxations to be considered in each of locally performed path
computation.
d) The address of the next-in-chain PCE.
e) The path computation request to be sent to the next-in-chain
PCE.
Bryskin, et al. Informational [Page 31]
RFC 5394 Policy-Enabled Path Computation December 2008
The PCE-PEP instructs a co-located path computation engine to
perform the local path computation(s) and, if necessary, sends the
path computation request to the next-in-chain PCE using the PCC-PCE
Communication Protocol. Then, it waits for the responses from the
local path computation engine and the remote PCE, combines the
resulting paths, and sends them back to the PCC-PEP using the PCC-
PCE Communication Protocol. The response contains the resulting
paths as well as policies describing some additional information
(for example, which of constraints were honored, which were
dismissed, and which were relaxed and in what way)
- PCC-PEP instructs the signaling sub-system of the GMPLS LSR to
encode the received path(s) into the outgoing Setup message(s).
9. Introduction of New Constraints
An important aspect of the policy-enabled path computation framework
discussed above is the ability to introduce new constraints with
minimal impact. In particular, only those components and mechanisms
that will use a new constraint need to be updated in order to support
the new constraint. Importantly, those components and mechanisms
that will not use the new constraint must not require any change in
order for the new constraint to be utilized. For example, the PCE
communication protocols must not require any changes to support new
constraints. Likewise, PCC and PCEs that will not process new
constraints must not require any modification.
Consider the case where a PCE has been upgraded with software
supporting optical physical impairment constraint, such as
Polarization Mode Dispersion (PMD), that previously was not supported
in the domain. In this case, one or more new policies will be
installed in the PCE Policy Repository (associated with the PCE)
defining the constraint (rules that determine application criteria,
set of policy variables, conditions, actions, etc.) and its
relaxation strategy (or strategies). The new policies will be also
propagated into other PCE Policy Repositories within the domain via
discovery and synchronization protocols or via local configuration.
PCE-PDPs and PCC-PDPs will then retrieve the corresponding policies
from the repository (or repositories). From then on, PCC-PDPs will
instruct associated PCC-PEPs to add the new policy information into
path computation requests for services with certain parameters (for
example, for services provisioned in the optical channel (OCh)
layer).
It is important to note that policy-enabled path computation model
naturally solves the PCE capability discovery issues. Suppose a PCE
working in a single PCE Policy Repository configuration starts to
support a new constraint. Once a corresponding policy installed in
Bryskin, et al. Informational [Page 32]
RFC 5394 Policy-Enabled Path Computation December 2008
the repository, it automatically becomes available for all repository
users, that is, PCCs. In the multi-repository case some policy
synchronization must be provided; however, this problem is one of the
management plane which is solved already.
10. Security Considerations
This document adds to the policy security considerations mentioned in
[RFC4655]. In particular, it is now necessary to consider the
security issues related to policy information maintained in PCE
Policy Repositories and policy-related transactions. The most
notable issues, some of which are also listed in [RFC4655], are:
- Unauthorized access to the PCE Policy Repositories;
- Interception of policy information when it is retrieved from the
repositories and/or transported from PDPs to PEPs;
- Interception of policy-related information in path computation
requests and responses;
o Impersonation of user and client identities;
o Falsification of policy information and/or PCE capabilities;
o Denial-of-service attacks on policy-related communication
mechanisms.
As with [RFC4655], it is expected that PCE solutions will address the
PCE aspects of these issues in detail.
11. Acknowledgments
Adrian Farrel contributed significantly to this document. We would
like to thank Bela Berde for fruitful discussions on PBM and policy-
driven path computation. We would also like to thank Kobus Van der
Merwe for providing insights and examples regarding PCE policy
applications.
Bryskin, et al. Informational [Page 33]
RFC 5394 Policy-Enabled Path Computation December 2008
12. References
12.1. Normative References
[RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
for Policy-based Admission Control", RFC 2753, January
2000.
[RFC3060] Moore, B., Ellesson, E., Strassner, J., and A. Westerinen,
"Policy Core Information Model -- Version 1
Specification", RFC 3060, February 2001.
[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.
[RFC3460] Moore, B., Ed., "Policy Core Information Model (PCIM)
Extensions", RFC 3460, January 2003.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
3473, January 2003.
[RFC3644] Snir, Y., Ramberg, Y., Strassner, J., Cohen, R., and B.
Moore, "Policy Quality of Service (QoS) Information
Model", RFC 3644, November 2003.
[RFC4216] Zhang, R., Ed., and J.-P. Vasseur, Ed., "MPLS Inter-
Autonomous System (AS) Traffic Engineering (TE)
Requirements", RFC 4216, November 2005.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
[RFC4927] Le Roux, J.-L., Ed., "Path Computation Element
Communication Protocol (PCECP) Specific Requirements for
Inter-Area MPLS and GMPLS Traffic Engineering", RFC 4927,
June 2007.
12.2. Informative References
[DMTF] Common Information Model (CIM) Schema, version 2.x.
Distributed Management Task Force, Inc. The components of
the CIM v2.x schema are available via links on the
following DMTF web page:
http://www.dmtf.org/standards/standard_cim.php.
Bryskin, et al. Informational [Page 34]
RFC 5394 Policy-Enabled Path Computation December 2008
[IRSCP] Van der Merwe, J., et al., "Dynamic Connectivity
Management with an Intelligent Route Service Control
Point," ACM SIGCOMM Workshop on Internet Network
Management (INM), Pisa, Italy, September 11, 2006.
[PCEP] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", Work in
Progress, November 2008.
[RFC2748] Durham, D., Ed., Boyle, J., Cohen, R., Herzog, S., Rajan,
R., and A. Sastry, "The COPS (Common Open Policy Service)
Protocol", RFC 2748, January 2000.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3080] Rose, M., "The Blocks Extensible Exchange Protocol Core",
RFC 3080, March 2001.
[RFC3198] Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
J., and S. Waldbusser, "Terminology for Policy-Based
Management", RFC 3198, November 2001.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
[RFC5376] Bitar, N., Zhang, R., and K. Kumaki, "Inter-AS
Requirements for the Path Computation Element
Communication Protocol (PCECP)", RFC 5376, November 2008.
[W3CSOAP] Hadley, M., Mendelsohn, N., Moreau, J., Nielsen, H., and
Gudgin, M., "SOAP Version 1.2 Part 1: Messaging
Framework", W3C REC REC-soap12-part1-20030624, June 2003.
Bryskin, et al. Informational [Page 35]
RFC 5394 Policy-Enabled Path Computation December 2008
Authors' Addresses
Igor Bryskin
ADVA Optical
7926 Jones Branch Drive
Suite 615
McLean, VA 22102
EMail: ibryskin@advaoptical.com
Dimitri Papadimitriou
Alcatel
Fr. Wellesplein 1,
B-2018 Antwerpen, Belgium
Phone: +32 3 240-8491
EMail: dimitri.papadimitriou@alcatel.be
Lou Berger
LabN Consulting, LLC
Phone: +1 301 468 9228
EMail: lberger@labn.net
Jerry Ash
AT&T
EMail: gash5107@yahoo.com
Bryskin, et al. Informational [Page 36]