PCE Working Group J.L. Le Roux (Editor)
Internet Draft France Telecom
Category: Informational
Expires: January 2006
July 2005
Requirements for Path Computation Element (PCE) Discovery
draft-ietf-pce-discovery-reqs-00.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document presents a set of requirements for a Path Computation
Element (PCE) discovery mechanism that would allow a Path Computation
Client (PCC) to discover dynamically and automatically a set of PCEs
along with their capabilities. It is intended that solutions that
specify procedures and protocol extensions for such PCE discovery
satisfy these requirements.
Le Roux et al. [Page 1]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119.
Table of Contents
1. Contributors................................................2
2. Terminology.................................................3
3. Introduction................................................3
4. Problem Statement and Requirements overview.................4
4.1. Problem Statement...........................................4
4.2. Requirements overview.......................................5
5. Example of application scenario.............................6
6. Detailed Requirements.......................................7
6.1. PCE Information to be disclosed.............................7
6.1.1. Discovery of PCE Location...................................7
6.1.2. Discovery of PCE computation scopes and domain(s) under
control...................................................7
6.1.3. Discovery of PCE Capabilities...............................8
6.1.4. Discovery of Alternate PCEs.................................9
6.2. Scope of PCE Discovery......................................9
6.3. PCE Information Synchronization.............................9
6.4. Detecting PCE Liveliness...................................10
6.5. Discovery of PCE capacity and congestion...................10
6.6. Extensibility..............................................10
6.7. Scalability................................................10
6.8. PCE Selection..............................................10
7. Security Considerations....................................11
8. Acknowledgments............................................11
9. References.................................................11
10. Authors' Addresses:........................................12
11. Intellectual Property Statement............................13
1. Contributors
The following are the authors that contributed to the present
document:
Jean-Louis Le Roux (France Telecom)
Paul Mabey (Qwest Communications)
Eiji Oki (NTT)
Richard Rabbat (Fujitsu)
Ting Wo Chung (Bell Canada)
Raymond Zhang (BT Infonet)
Le Roux et al. [Page 2]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
2. Terminology
Terminology used in this document
LSR: Label Switch Router
TE-LSP: Traffic Engineered Label Switched Path
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.
PCC: Path Computation Client: any client application requesting a
path computation to be performed by a Path Computation Element.
IGP Area: OSPF Area or ISIS level/area
ABR: IGP Area Border Router (OSPF ABR or ISIS L1L2 router)
Intra-area TE LSP: A TE LSP whose path does not cross IGP area
boundaries.
Inter-area TE LSP: A TE LSP whose path transits through
two or more IGP areas.
Inter-AS MPLS TE LSP: A TE LSP whose path transits
through two or more ASes or sub-ASes (BGP confederations).
Domain: any collection of network elements within a common sphere
of address management or path computational responsibility.
Examples of domains include IGP areas and Autonomous Systems.
3. Introduction
The PCE Architecture [PCE-ARCH] defines a Path Computation Element
(PCE) as an entity capable of computing TE-LSPs paths satisfying a
set of constraints. The PCE function can be located on a router/LSR
(composite PCE) or on a network server (external PCE).
A PCE serves TE-LSP path computation requests sent by Path
Computation Clients (PCC).
A Path Computation Client (PCC) is a client application requesting a
path computation to be performed by a PCE. This can be, for instance,
an LSR requesting a path for a TE-LSP for which it is the head-end,
or a PCE requesting a path computation of another PCE (inter-PCE
communication). The communication between a PCC and a PCE requires a
client-server protocol whose requirements are listed in [PCE-COM-
REQ].
There are several motivations for the adoption of a PCE-based
architecture to perform a TE-LSP path computation. They are listed in
[PCE-ARCH]. This includes applications such as CPU intensive path
Le Roux et al. [Page 3]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
computation, inter-domain path computation and backup path
computation.
The PCE architecture requires, of course, that a PCC be aware of the
location and capabilities of one or more PCEs in its domain, and also
potentially of some relevant PCEs in other domains (in the context of
inter-domain path computation).
In that context it would be highly desirable to define a mechanism
for automatic and dynamic PCE discovery, which would allow PCCs to
automatically discover a set of PCEs along with their capabilities,
and to dynamically detect new PCEs or any modification of the PCE
capabilities. This includes the discovery by a PCC of a set of one or
more PCEs in its domain, and potentially in some other domains. The
latter is a desirable function in the case of inter-domain path
computation for example.
This document lists a set of functional requirements for such an
automatic and dynamic PCE discovery mechanism. Section 3 points out
the problem statement. Section 4 illustrates an application scenario.
Finally section 5 addresses detailed requirements.
It is intended that solutions that specify procedures and protocol
extensions for such PCE discovery satisfy these requirements. There
is no intent either to specify solution specific requirements or to
make any assumption on the protocol(s) that could be used for the
discovery.
Note that requirements listed in this document apply equally to MPLS-
TE and GMPLS-capable PCEs.
It is also important to note that the notion of a PCC encompasses a
PCE acting as PCC when requesting a TE-LSP path computation of
another PCE (inter-PCE communication). Thus this document does not
make the distinction between PCE discovery by PCCs and PCE discovery
by PCEs.
4. Problem Statement and Requirements overview
4.1. Problem Statement
A routing domain may in practice be comprised of multiple PCEs:
-The path computation load may be balanced among a set of PCEs
to improve scalability;
-For the purpose of redundancy, primary and backup PCEs may be
used;
-PCEs may have distinct path computation capabilities (multi-
constrained path computation, backup path computation...);
-In an inter-domain context there can be several PCEs with
distinct path computation scopes (intra-area, inter-area,
inter-AS, inter-layer), each PCE being responsible for path
computation in one or more domains within its scope.
Le Roux et al. [Page 4]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
As an example, in a multi-area network made of one backbone area and
N peripheral areas, and where inter-area MPLS-TE path computation
relies on multiple-PCE path computation with ABRs acting as PCEs, the
backbone area would comprise at least N PCEs. In existing multi-area
networks, N can be quite large (e.g. beyond fifty).
In order to allow for efficient PCE selection by PCCs and efficient
load balancing of requests, a PCC has to know the location of PCEs in
its domain, along with their capabilities, and also potentially of
some relevant PCEs in other domains (for inter-domain path
computation purpose).
Such PCE information could be learnt through manual configuration, on
each PCC, of the set of PCEs along with their capabilities and
scope(s). Such manual configuration approach may be sufficient, and
even desired in some particular situations, but it obviously faces
several limitations:
-This may imply a substantial configuration overhead (see the
above example with N PCEs);
-This would not allow a PCC to dynamically detect that a new
PCE is available, that an existing PCE is no longer available,
or that there is a change in the PCE's capabilities.
Furthermore, as with any manual configuration approach, this may lead
to undesirable configuration errors.
Hence, an automated PCE discovery mechanism allowing a PCC to
dynamically discover a set of PCEs and their capabilities is highly
desirable.
4.2. Requirements overview
A PCE discovery mechanism that satisfies the requirements set forth
in this document MUST allow a PCC to automatically discover the
location of one or more PCEs in its domain and also, potentially, of
PCEs in other domains, of interest for inter-domain path computation
purpose.
A PCE discovery mechanism MUST allow discovering the path computation
scope(s) of a PCE. It MUST also allow a PCC to discover the set of
one or more domains under the path computation responsibility of a
PCE.
A PCE discovery mechanism MUST allow PCCs to dynamically discover
that a new PCE has appeared or that there is a change in PCE
information.
It MUST also allow PCCs to dynamically discover that a PCE is no
longer available.
A PCE discovery mechanism SHOULD also allow PCCs to learn about a set
of PCE path computation capabilities.
Le Roux et al. [Page 5]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
5. Example of application scenario
<----------------AS1--------------------> <----AS2---
Area 1 Area 0 Area 2
R1---------R3-----R5-------R6-----------R9----------R11----R13
| | | | |
| | | | |
R2---------R4-----R7-------R8-----------R10---------R12----R14
S1
Figure 1.
Figure 1 above illustrates a network with several PCEs:
-The ABR R3 is a composite PCE that can take part in inter area path
computation. It can compute paths in area 1 and area 0.
-The ABR R6 is a composite PCE that can take part in inter-area path
computation. It can compute paths in area 0 and area2
-The ASBR R9 is a composite PCE that can take part in inter-AS path
computation, responsible for path computation in AS1 towards AS2.
-The ASBR R12 is a composite PCE that can take part in inter-AS path
computation, responsible for path computation in AS2 towards AS1.
-The server S1 is an external PCE that can be used to compute diverse
paths and backup paths in area 1.
The PCE discovery mechanism will allow:
-each LSR in area 1 and 0 to dynamically discover R3, as a PCE for
inter-area path computation as well as its path computation domains:
area1 and area0.
-each LSR in area 0 and 2 to dynamically discover R6, as a PCE for
inter-area path computation, as well as its path computation domains:
area2 and area0.
-each LSR in AS1 and some PCEs in AS2 to dynamically discover R9 as a
PCE for inter-AS path computation as well as its path computation
domain: AS1
-each LSR in area 1 to dynamically discover S1, as a PCE for diverse
path computation and backup path computation in area1.
Le Roux et al. [Page 6]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
6. Detailed Requirements
6.1. PCE Information to be disclosed
The PCE discovery mechanism MUST disclose some PCE information that
will allow PCCs to select appropriate PCEs.
This section details the kind of information that has to be
disclosed.
6.1.1. Discovery of PCE Location
The PCE discovery mechanism MUST allow discovering, for a given PCE,
the IPv4 and/or IPv6 address to be used to reach the PCE. This
address will typically be a loop-back address that is always
reachable, if there is any connectivity to the PCE.
This address will be used by PCCs to communicate with a PCE, thanks
to a PCC-PCE communication protocol.
6.1.2. Discovery of PCE computation scopes and domain(s) under control
Inter-domain path computation is a key application of the PCE
architecture. This can rely on a multiple-PCE path computation,
where PCEs in each domain compute a part of the end-to-end path and
collaborate with each other to find the end-to-end-path. This can
also rely on a single-PCE path computation where a PCE has visibility
inside multiple domains and can compute an inter-domain path.
Hence the PCE discovery mechanism MUST allow discovering the path
computation scope of a PCE, i.e. if a PCE can be used to compute or
to take part in the computation of intra-area, inter-area or inter-AS
TE-LSP. Note that these path computation scopes are not mutually
exclusive.
Also the PCE discovery mechanism MUST allow discovering the set of
one or more domains under the path computation responsibility of a
PCE, i.e. where a PCE has visibility and can compute TE-LSP paths.
These domains can be identified using a domain identifier: For
instance, an IGP area can be identified by the Area ID (OSPF or
ISIS), and an AS can be identified by the AS number.
Le Roux et al. [Page 7]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
6.1.3. Discovery of PCE Capabilities
In the case where there are several PCEs with distinct capabilities,
available, a PCC has to select one or more appropriate PCEs.
For that purpose the PCE discovery mechanism SHOULD allow the
discovery of some PCE capabilities.
For the sake of illustration this could include for instance some
path computation related capabilities:
-The capability to compute MPLS-TE and/or GMPLS paths;
-The type of link and path constraints supported: e.g.
bandwidth, affinities, delay;
-The objective functions supported: e.g. shortest constrained
path, shortest bounded delay path;
-The capability to compute multiple paths in a synchronized
manner: e.g. diverse path computation, load balancing
computation;
-Some GMPLS specific capabilities: e.g. the supported interface
switching capabilities, the capability to compute multi-layer
paths.
And this could also include some general capabilities:
-The capability to handle request prioritization;
-The capability to authenticate PCCs and to be authenticated.
Such information regarding PCE capabilities could then be used by a
PCC to select an appropriate PCE from a list of candidate PCEs.
Note that the description of general and path computation specific
PCE capabilities is out of the scope of this document. It is expected
that this will be described in a separate document.
It is paramount that dynamic capability discovery MUST NOT generate
an excessive amount of information and SHOULD be limited to a small
set of generic capabilities.
If required, the exhaustive discovery of detailed capabilities could
be ensured by means of the PCC-PCE communication protocol.
Actually a tradeoff should be found between capability discovery by
the PCE discovery mechanism and by the PCC-PCE communication
protocol. One of the objectives of the PCE discovery mechanism is to
help PCCs to select appropriate PCEs and limit the likelihood of PCC-
PCE communication rejections that may occur in case a PCE cannot
support a given capability.
Le Roux et al. [Page 8]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
6.1.4. Discovery of Alternate PCEs
In the case of a PCE failure, a PCC has to select another PCE, if one
is available. It could be useful in various situations, to indicate a
set of one or more alternate PCEs that can be selected in case a
given PCE fails.
Hence the PCE Discovery mechanism SHOULD allow the advertising, for a
given PCE of the location of one or more assigned alternate PCEs.
6.2. Scope of PCE Discovery
The PCE Discovery mechanism MUST allow the control of the scope of
the PCE information discovery (IGP Area, AS, set of AS) on a per PCE
basis. In other words it MUST allow to control to which PCC or group
of PCCs the information related to a PCE may be disclosed.
The choice for the discovery scope of a given PCE MUST include the
followings:
-All PCCs in a single IGP area
-All PCCs in a set of adjacent IGP areas
-All PCCs in a single AS
-All PCCs in a set of ASes
-A set of one or more PCCs in a set of one or more ASes
Particularly this also implies that the PCE Discovery mechanism MUST
allow for the discovery of PCE information across IGP areas and
across AS boundaries.
Note that it MUST be possible to deactivate PCE discovery on a per
PCE basis.
6.3. PCE Information Synchronization
The PCE discovery mechanism MUST allow a PCC to detect any change in
the information related to a PCE (e.g. capability modifications).
In addition it MUST be possible to dynamically detect new PCEs.
The PCE Discovery Mechanism SHOULD allow such detection under 60
seconds.
Note that PCE capabilities are expected to be fairly stable and not
to change frequently.
Le Roux et al. [Page 9]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
6.4. Detecting PCE Liveliness
The PCE discovery mechanism MUST allow a PCC to detect when a PCE is
no longer alive. This allows a PCC to rapidly switch to another PCE
(for instance a predefined alternate PCE), and thus minimizes path
computation service disruption.
The PCE discovery mechanism SHOULD allow such PCE liveliness
detection under 60 seconds.
6.5. Discovery of PCE capacity and congestion
PCE WG feedback is requested on the following items:
-Is there a need for the discovery of PCE capacity in terms of
computation power? This static parameter could be used to
ensure weighted load balancing of requests in case PCEs do not
have the same capacity.
-Would it be useful that a PCE report its status as "congested"
in case it is too busy? PCCs may then use this dynamic
information to prefer a different PCE.
6.6. Extensibility
The PCE discovery mechanism MUST be flexible and extensible so as to
easily allow for the addition of some additional PCE information that
could be defined in the future.
6.7. Scalability
The PCE discovery mechanism MUST be designed to scale well with an
increase of any of the following parameters:
-Number of PCCs;
-Number of PCEs;
-Number of IGP Areas in the discovery scope;
-Number of ASs in the discovery scope.
Particularly, in case routing protocols (IGP, BGP) are extended to
support PCE discovery, the extensions MUST NOT cause a degradation in
routing protocol performance. The same applies to a signaling
solution that could serve for this communication.
6.8. PCE Selection
A PCC may have to select among a set of candidate PCEs that have the
same capabilities. A specific PCE selection procedure SHOULD be
defined in order to ensure consistent behaviour when doing load
balancing and avoid that all PCCs send the requests to only one PCE.
The precise requirements and mechanisms for this function are out of
the scope of this document. It is expected that this requirement will
be covered in another document.
Le Roux et al. [Page 10]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
7. Security Considerations
PCE discovery and particularly inter-AS PCE discovery may raise new
security issues. PCE discovery procedures or protocol extensions MUST
deliver the operational security objectives where required. The
overall security objectives of confidentiality, integrity and
availability may take on varying level of importance. These
objectives MAY be met by other established means and protocols.
The PCE discovery mechanism MUST be able to restrict the scope of
discovery to a set of authorized PCCs. The identity of any PCE MUST
only be learnt by authorized PCCs. It MUST be possible to encrypt
discovery information.
Note that a threat analysis of the proposed procedures and/or
protocol extensions SHOULD address masquerade, eavesdropping,
unauthorized access, loss or corruption of information (includes
replay attacks), repudiation, forgery and denial of service attacks.
8. Acknowledgments
We would like to thank Benoit Fondeviole, Thomas Morin, Emile
Stephan, Jean-Philippe Vasseur, Dean Cheng and Mohamed Boucadair for
their useful comments and suggestions.
9. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3667] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC
3667, February 2004.
[RFC3668] Bradner, S., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3668, February 2004.
[PCE-ARCH] Farrel, A., Vasseur, J.P., Ash, J., "Path Computation
Element (PCE) Architecture", draft-ietf-pce-architecture-00.txt, work
in progress.
[PCE-COM-REQ] Ash, J., Le Roux, J.L., " PCE Communication Protocol
Generic Requirements", draft-ietf-pce-comm-protocol-gen-reqs-00.txt,
work in progress.
Le Roux et al. [Page 11]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
10. Authors' Addresses:
Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
FRANCE
Email: jeanlouis.leroux@francetelecom.com
Paul Mabey
Qwest Communications
950 17th Street,
Denver, CO 80202,
USA
Email: pmabey@qwest.com
Eiji Oki
NTT
Midori-cho 3-9-11
Musashino-shi, Tokyo 180-8585,
JAPAN
Email: oki.eiji@lab.ntt.co.jp
Richard Rabbat
Fujitsu Laboratories of America
1240 East Arques Ave, MS 345
Sunnyvale, CA 94085
USA
Email: richard@us.fujitsu.com
Ting Wo Chung
Bell Canada
181 Bay Street, Suite 350
Toronto, Ontario, M5J 2T3
CANADA,
Email: ting_wo.chung@bell.ca
Raymond Zhang
BT Infonet
2160 E. Grand Ave.
El Segundo, CA 90025
USA
Email: raymond_zhang@infonet.com
Le Roux et al. [Page 12]
Internet Draft draft-ietf-pce-discovery-reqs-00.txt June 2005
11. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Le Roux et al. [Page 13]
