Network Working Group J.-L. Le Roux (Editor) Internet Draft France Telecom Category: Informational Expires:
August 2006 February 2006January 2007 PCE Communication Protocol (PCECP) Specific Requirements for Inter-Area (G)MPLS Traffic Engineering draft-ietf-pce-pcecp-interarea-reqs-01.txtdraft-ietf-pce-pcecp-interarea-reqs-02.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. 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 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.................................................6Computation...............................................6 5.5. Inclusion of Area IDs in Request............................6 5.6. Inter-area Diverse Path computation.........................7 5.7. Inter-AreaInter-area Policies.........................................7 6. Manageability Considerations................................7Considerations................................8 7. Security Considerations.....................................8 8. Acknowledgments.............................................8 9. IANA Considerations.........................................8 10. References..................................................8 10.1. Normative References........................................8 10.2. Informative References......................................8 10.11. Editor Address:.............................................9 11.12. Contributors' Addresses.....................................9 12.13. Intellectual Property Statement............................10 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 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 pathpath; - 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 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. 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. 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-AreaInter-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 wantIn addition to apply policies based onthe initiating PCC. In a multiple-PCE computationpolicy requirements discussed in [PCE-COM-REQ], the addressapplication of the initiating PCC may no longerinter-area path computation policies requires some additional information to be part ofcarried in the PCECP request messages sent between PCEs. Hence, themessages: - The request message MUST supportallow for the inclusion of the address of the originating PCC. Note thatThis may be useful in some cases it is important to contain an inter-area path withina single AS. Hencemultiple PCE computation, so as to apply policies not only based on the PCECP peer but also based on the originating PCC; - The request message MUST allow indicating that ASwhether area/AS crossing is not authorized.allowed or not. Note that work on supported policy models and the corresponding requirements/implications is being undertaken as a separate work item in the PCE working group ([PCE-POL-FMWK]). 6. Manageability Considerations The inter-area application does not imply new manageability requirements beyond those already defined in [PCE-COM-REQ]. 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 andLouedec, Dimitri Papadimitriou and Lou Berger for their useful comments and suggestions. 9. InformativeIANA Considerations This document makes no requests for IANA action. 10. References 10.1. Normative 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. 10.2. Informative References [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. 10.[PCE-POL-FMWK] I. Bryskin, D. Papadimitriou, L. Berger " Policy- Enabled Path Computation Framework", draft-bryskin-pce-policy- enabled-path-comp, work in progress. 11. Editor Address: Jean-Louis Le Roux France Telecom 2, avenue Pierre-Marzin 22307 Lannion Cedex FRANCE Email: email@example.com 11.12. Contributors' Addresses Jerry Ash AT&T Room MT D5-2A01 200 Laurel Avenue Middletown, NJ 07748, USA Phone: +1-(732)-420-4578 Email: firstname.lastname@example.org Nabil Bitar Verizon 40 Sylvan Road Waltham, MA 02145 Email: email@example.com Dean Cheng Cisco Systems Inc. 3700 Cisco Way San Jose CA 95134 USA Phone: +1 408 527 0677 Email: firstname.lastname@example.org Kenji Kumaki KDDI Corporation Garden Air Tower Iidabashi, Chiyoda-ku, Tokyo 102-8460, JAPAN Phone: +81-3-6678-3103 Email: email@example.com Eiji Oki NTT Midori-cho 3-9-11 Musashino-shi, Tokyo 180-8585, JAPAN Email: firstname.lastname@example.org Raymond Zhang BT INFONET Services Corporation 2160 E. Grand Ave. El Segundo, CA 90245 USA Email: email@example.com Renhai Zhang Huawei Technologies No. 3 Xinxi Road, Shangdi, Haidian District, Beijing City, P. R. China Email: firstname.lastname@example.org 12.13. 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