Network Working Group J.-L. Le Roux (Editor)
Internet Draft France Telecom
Category: Informational
Expires: August 2006
February 2006
PCE Communication Protocol (PCECP) Specific Requirements for Inter-Area
(G)MPLS Traffic Engineering
draft-ietf-pce-pcecp-interarea-reqs-01.txt
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Abstract
For scalability purposes a network may comprise multiple IGP areas.
An inter-area TE-LSP is an LSP that transits through at least two IGP
areas. In a multi-area network, topology visibility remains local to
a given area, and a head-end LSR cannot compute alone an inter-area
shortest constrained path. One key application of the Path
Computation Element (PCE) based architecture is the computation of
inter-area TE-LSP paths. This document lists a detailed set of PCE
Communication Protocol (PCECP) specific requirements for support of
inter-area TE-LSP path computation. It complements generic
requirements for a PCE Communication Protocol.
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................................................3
2. Terminology.................................................3
3. Introduction................................................3
4. Motivations for PCE-based Inter-Area Path Computation.......4
5. Detailed Inter-Area Specific Requirements on PCECP..........5
5.1. Control of area crossing....................................5
5.2. Area Recording..............................................6
5.3. Strict Explicit Path and Loose Path.........................6
5.4. PCE-list Enforcement and Recording in Multiple PCE
Computation.................................................6
5.5. Inclusion of Area IDs in Request............................6
5.6. Inter-area Diverse Path computation.........................7
5.7. Inter-Area Policies.........................................7
6. Manageability Considerations................................7
7. Security Considerations.....................................8
8. Acknowledgments.............................................8
9. Informative References......................................8
10. Editor Address:.............................................9
11. Contributors' Addresses.....................................9
12. Intellectual Property Statement............................10
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1. Contributors
The following are the authors that contributed to the present
document:
Jerry Ash (AT&T)
Nabil Bitar (Verizon)
Dean Cheng (Cisco)
Kenji Kumaki (KDDI)
J.L. Le Roux (France Telecom)
Eiji Oki (NTT)
Raymond Zhang (BT Infonet)
Renhai Zhang (Huawei)
2. Terminology
LSR: Label Switching Router.
LSP: MPLS Label Switched Path.
TE-LSP: Traffic Engineering Label Switched Path.
IGP area: OSPF Area or IS-IS level.
ABR: IGP Area Border Router, a router that is attached to more
than one IGP areas (ABR in OSPF or L1/L2 router in IS-IS).
Inter-Area TE LSP: TE LSP that traverses more than one IGP area.
CSPF: Constrained Shortest Path First.
SRLG: Shared Risk Link Group.
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 application that request path
computation to be performed by a PCE.
PCECP: PCE Communication Protocol, a protocol for communication
between PCCs and PCEs, and between PCEs.
3. Introduction
[RFC4105] lists a set of motivations and requirements for setting up
TE-LSPs across IGP area boundaries. These LSPs are called inter-area
TE-LSPs. These requirements include the computation of inter-
area shortest constrained paths with key guideline being to respect
the IGP hierarchy concept, and particularly the containment of
topology information. The main challenge with inter-area MPLS-TE
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relies actually on path computation. Indeed the head-end LSR cannot
compute a constrained path across areas, as its topology visibility
is limited to its own area.
Inter-area path computation is one of the key applications of the PCE
based architecture [PCE-ARCH]. The computation of optimal inter-area
paths may be achieved using the services of one or more PCEs.
Such PCE-based inter-area path computation could rely for instance on
a single multi-area PCE that has the TE database of all the areas in
the IGP domain and can directly compute an end-to-end constrained
shortest path. Alternatively, this could rely on the cooperation
between PCEs whereby each PCE covers one or more IGP areas and the
full set of PCEs covers all areas.
The generic requirements for a PCE Communication Protocol (PCECP),
which allows a PCC to send path computation requests to a PCE and the
PCE to sent path computation responses to a PCC, are set forth in
[PCE-COM-REQ]. The use of a PCE-based approach for inter-area path
computation implies specific requirements on a PCE Communication
Protocol, in addition to the generic requirements already listed in
[PCE-COM-REQ]. This document complements these generic requirements
by listing a detailed set of PCECP requirements specific to inter-
area path computation.
It is expected that a solution for a PCECP satisfies these
requirements.
Note that PCE-based inter-area path computation may require a
mechanism for an automatic PCE discovery across areas, which is out
of the scope of this document. Detailed requirements for such a
mechanism are discussed in [PCE-DISCO-REQ].
4. Motivations for PCE-based Inter-Area Path Computation
IGP hierarchy allows improving IGP scalability, by dividing the IGP
domain into areas and limiting the flooding scope of topology
information to area boundaries. A router in an area has full topology
information for its own area but only reachability to destinations in
other areas._ Thus, a head-end LSR cannot compute an end-to-end
constrained path that traverses more than one IGP area.
A solution for computing inter-area TE-LSP path currently relies on a
per domain path computation ([PD-COMP]). It is based on loose hop
routing with an ERO expansion on each ABR. This can allow setting up
a constrained path, but faces two major limitations:
- This does not allow computing an optimal constrained path
- This may lead to several crankback signaling messages and
hence delay the LSP setup, and also invoke possible alternate
routing activities.
Note that, here, by optimal constrained path we mean the shortest
constrained path across multiple areas, taking into account either
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the IGP or TE metric [METRIC]. In other words, such a path is the
path that would have been computed by making use of some CSPF
algorithm in the absence of multiple IGP areas.
The PCE based architecture [PCE-ARCH] is well suited to inter-area
path computation, as it allows overcoming the path computation
limitations resulting from the limited topology visibility, by
introducing path computation entities with more topology visibility,
or by allowing cooperation between path computation entities in each
area.
There are two main approaches for the computation of an inter-area
optimal path:
- Single PCE computation: The path is computed by a single PCE that
has topology visibility in all areas and can alone compute an end-
to-end optimal constrained path.
- Multiple PCE computation with inter-PCE communication: the path
computation is distributed on multiple PCEs, which have partial
topology visibility. They compute path segments in their areas of
visibility and collaborate with each other so as to arrive at an
end-to-end optimal constrained path. Such collaboration is ensured
thanks to inter-PCE communication.
Note that the use of a PCE-based approach, to perform inter-area path
computation implies specific functional requirements in a PCECP, in
addition to the generic requirements listed in [PCE-COM-REQ]. These
specific requirements are discussed in next section.
5. Detailed Inter-Area Specific Requirements on PCECP
This section lists a set of additional requirements for the PCECP
that complement requirements listed in [PCE-COM-REQ] and are specific
to inter-area (G)MPLS TE path computation.
5.1. Control of area crossing
In addition to the path constraints specified in Section 6.1.16 of
[PCE-COM-REQ], the request message MUST allow indicating whether area
crossing is allowed or not.
Indeed, when the source and destination reside in the same IGP area,
there may be intra-area and inter-area feasible paths. As set forth
in [RFC4105], if the shortest path is an inter-area path, an operator
either may want to avoid, as far as possible, crossing areas and thus
may prefer selecting a sub-optimal intra-area path or, conversely,
may prefer to use a shortest path, even if it crosses areas.
Also, when the source and destinations reside in the same area it may
be useful to know whether the returned path is an inter-area path.
Hence the response message MUST allow indicating whether the computed
path is crossing areas.
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5.2. Area Recording
It may be useful for the PCC to know the set of areas crossed by an
inter-area path and the corresponding path segments. Hence the
response message MUST support the inclusion of the identifiers of the
crossed areas and MUST allow identifying the corresponding path
segments.
5.3. Strict Explicit Path and Loose Path
A Strict Explicit Path is defined as a set of strict hops, while a
Loose Path is defined as a set of at least one loose hop and zero,
one ore more strict hops. An inter-area path may be strictly explicit
or loose (e.g. a list of ABRs as loose hops). It may be useful to
indicate to the PCE if a Strict Explicit path is required or not.
Hence the PCECP request message MUST allow indicating whether a
Strict Explicit Path is required/desired.
5.4. PCE-list Enforcement and Recording in Multiple PCE Computation
In case of multiple-PCE inter-area path computation, a PCC may want
to indicate a preferred list of PCEs to be used. Hence the PCECP
request message MUST support the inclusion of a list of preferred
PCEs. Note that this requires that a PCC in one area have knowledge
of PCEs in other areas. This could rely on configuration or on a PCE
discovery mechanism, allowing discovery across area boundaries (see
[PCE-DISCO-REQ]).
Also it would be useful to know the list of PCEs which effectively
participated in the computation. Hence the request message MUST
support a request for PCE recording and the response message MUST
support the recording of the set of one or more PCEs that took part
in the computation.
It may also be useful to know the path segments computed by each PCE.
Hence the request message SHOULD allow a request for the
identification of path segments computed by a PCE, and the response
message SHOULD allow identifying the path segments computed by each
PCE.
5.5. Inclusion of Area IDs in Request
The knowledge of the areas in which the source and destination lie
would allow selection of appropriate cooperating PCEs. A PCE may not
be able to determine the location of the source and destination and
in such a case it would be useful that a PCC indicates the source and
destination area IDs.
For that purpose the request message MUST support the inclusion of
the source and destination area IDs. Note that this information could
be learned by the PCC through configuration.
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5.6. Inter-area Diverse Path computation
For various reasons, including protection and load balancing, the
computation of diverse inter-area paths may be required.
There are various levels of diversity in an inter-area context:
-Per-area diversity (intra-area path segments are link, node or
SRLG disjoint)
-Inter-area diversity (end-to-end inter-area paths are link,
node or SRLG disjoint)
Note that two paths may be disjoint in the backbone area but non-
disjoint in peripheral areas. Also two paths may be node disjoint
within areas but may share ABRs, in which case path segments within
an area are node disjoint but end-to-end paths are not node-disjoint.
The request message MUST allow requesting the computation of a set of
inter-area diverse paths between the same node pair or between
distinct node pairs. It MUST allow indicating the required level of
intra-area diversity (link, node, SRLG) on a per area basis, as well
as the level of inter-area diversity (shared ABRs or ABR
disjointness).
The response message MUST allow indicating the level of diversity of
a set of computed loose paths.
Note that specific objective functions may be requested for diverse
path computation, such as minimizing the cumulated cost of a set of
diverse paths as set forth in [PCE-COM-REQ].
5.7. Inter-Area Policies
As already defined in Section 5.1, a request message MUST allow
indicating whether area crossing is allowed or not.
A PCE may want to apply policies based on the initiating PCC.
In a multiple-PCE computation the address of the initiating PCC may
no longer be part of the request messages sent between PCEs.
Hence, the request message MUST support the inclusion of the address
of the originating PCC.
Note that in some cases it is important to contain an inter-area
path within a single AS. Hence the request message MUST allow
indicating that AS crossing is not authorized.
6. Manageability Considerations
The inter-area application does not imply new manageability
requirements beyond those already defined in [PCE-COM-REQ].
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7. Security Considerations
IGP areas are administrated by the same entity. Hence the inter-area
application does not imply a new trust model, or new security issues
beyond those already defined in [PCE-COM-REQ].
8. Acknowledgments
We would also like to thank Adrian Farrel, Jean-Philippe Vasseur,
Bruno Decraene, Yannick Le Louedec and Dimitri Papadimitriou for
their useful comments and suggestions.
9. Informative 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.
[BCP79] Bradner, S., "Intellectual Property Rights in IETF
Technology", RFC 3979, March 2005.
[RFC4105] Le Roux J.L., Vasseur J.P., Boyle, J., et al. "Requirements
for inter-area MPLS-TE" RFC 4105, June 2005.
[PCE-ARCH] A. Farrel, JP. Vasseur and J. Ash, Path Computation
Element (PCE) Based Architecture, work in progress.
[PCE-COM-REQ] J. Ash, J.L Le Roux et. al., PCE Communication
Protocol Generic Requirements, work in progress.
[PCE-DISC-REQ] J.L. Le Roux et. al., Requirements for Path
Computation Element (PCE) Discovery, work in progress.
[PD-COMP] Vasseur, J.P., Ayyangar, A., Zhang, R., "A Per-domain path
computation method for computing Inter-domain Traffic Engineering
(TE) Label Switched Path (LSP)", work in progress.
[METRIC] Le Faucheur, F., Uppili, R., Vedrenne, A., Merckx, P.,
and T. Telkamp, "Use of Interior Gateway Protocol(IGP) Metric as a
second MPLS Traffic Engineering (TE) Metric", BCP 87, RFC 3785, May
2004.
[ID-RSVP] Ayyangar, A., Vasseur, J.P., "Inter domain GMPLS Traffic
Engineering - RSVP-TE extensions", work in progress.
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10. Editor Address:
Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
FRANCE
Email: jeanlouis.leroux@francetelecom.com
11. Contributors' Addresses
Jerry Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748, USA
Phone: +1-(732)-420-4578
Email: gash@att.com
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145
Email: nabil.bitar@verizon.com
Dean Cheng
Cisco Systems Inc.
3700 Cisco Way
San Jose CA 95134 USA
Phone: +1 408 527 0677
Email: dcheng@cisco.com
Kenji Kumaki
KDDI Corporation
Garden Air Tower
Iidabashi, Chiyoda-ku,
Tokyo 102-8460, JAPAN
Phone: +81-3-6678-3103
Email: ke-kumaki@kddi.com
Eiji Oki
NTT
Midori-cho 3-9-11
Musashino-shi, Tokyo 180-8585, JAPAN
Email: oki.eiji@lab.ntt.co.jp
Raymond Zhang
BT INFONET Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90245 USA
Email: raymond_zhang@bt.infonet.com
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Renhai Zhang
Huawei Technologies
No. 3 Xinxi Road, Shangdi,
Haidian District,
Beijing City,
P. R. China
Email: zhangrenhai@huawei.com
12. Intellectual Property Statement
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this standard. Please address the information to the IETF at
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Copyright (C) The Internet Society (2006). This document is subject
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except as set forth therein, the authors retain all their rights.
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