TEWG Working Group JL Le Roux,(Ed.) Internet Draft France Telecom JP Vasseur, (Ed.) Cisco System Inc. Jim Boyle, (Ed.) PDNETs Category: Informational Expires: November 2004 June 2004 Requirements for Inter-area MPLS Traffic Engineering draft-ietf-tewg-interarea-mpls-te-req-02.txt Status of this Memo 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 lists a detailed set of functional requirements for the support of inter-area MPLS Traffic Engineering (inter-area MPLS TE). It is intended that solutions that specify procedures and protocol extensions for inter-area MPLS-TE satisfy these requirements. Le Roux, et. al. Informational - Expires November 2004 [Page 1] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 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. Introduction................................................3 2. Contributing Authors........................................4 3. Terminology.................................................5 4. Current intra-area uses of MPLS Traffic Engineering.........5 4.1. Intra-area MPLS Traffic Engineering Architecture............5 4.2. Intra-area MPLS Traffic Engineering Applications............6 4.2.1. Intra-area resource optimization............................6 4.2.2. Intra-area QoS guarantees...................................6 4.2.3. Fast recovery within an area................................7 4.3. Intra-area MPLS-TE and routing..............................7 5. Problem Statement, Requirements and Objectives of inter area MPLS-TE..............................................8 5.1. Inter-Area Traffic Engineering Problem Statement............8 5.2. Motivations for inter-area MPLS-TE..........................9 5.3. Key Objectives for an inter-area MPLS-TE solution...........9 5.3.1. Preserve the IGP hierarchy concept..........................9 5.3.2. Preserve Scalability.......................................10 6. Application Scenario.......................................11 7. Detailed requirements for inter-area MPLS-TE...............12 7.1. Inter-area MPLS TE operations and interoperability.........12 7.2. Inter-Area TE-LSP signalling...............................12 7.3. Path optimality............................................12 7.4. Inter-Area MPLS-TE Routing.................................13 7.5. Inter-Area MPLS-TE Path computation........................13 7.6. Inter-area Crankback Routing...............................13 7.7. Support of diversely routed inter-area TE LSPs.............14 7.8. Intra/Inter-area Path selection policy.....................15 7.9. Reoptimization of inter-area TE LSP........................15 7.10. Inter-area LSP Recovery....................................16 7.10.1. Rerouting of inter-area TE LSPs................... ........16 7.10.2. Fast recovery of inter-area TE LSP................ ........16 7.11. DS-TE support..............................................17 7.12. Hierarchical LSP support...................................17 7.13. Hard/Soft pre-emption......................................17 7.14. Auto-discovery of TE meshes................................17 7.15. Inter-area MPLS TE fault management requirements...........17 7.16. Inter-area MPLS-TE and routing.............................18 8. Evaluation criteria........................................18 8.1. Performances...............................................18 8.2. Complexity and risks.......................................18 8.3. Backward Compatibility.....................................19 9. Security Considerations....................................19 10. Acknowledgements...........................................19 Le Roux, et. al. Informational - Expires November 2004 [Page 2] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 11. Normative References.......................................19 12. Informative References.....................................19 13. Intellectual Property Statement............................21 14. Editors' Address:..........................................21 1. Introduction The set of MPLS Traffic Engineering components, defined in [RSVP-TE], [OSPF-TE] and [ISIS-TE], which supports the requirements defined in [TE-REQ], is used today by many network operators to achieve major Traffic Engineering objectives defined in [TE-OVW] and summarized below: -Aggregated Traffic measurement -Optimization of network resources utilization -Support for services requiring end-to-end QoS guarantees -Fast recovery against link/node/SRLG failures However, the current set of MPLS Traffic Engineering components have to date been limited to use within a single IGP area. This document discusses the requirements for an inter-area MPLS Traffic Engineering mechanism that may be used to achieve the same set of objectives across multiple IGP areas. Basically, it would be useful to extend MPLS TE capabilities across IGP areas to support inter-area resources optimization, to provide strict QoS guarantees between two edge routers located within distinct areas, and to protect inter-area traffic against ABR failures. This document firstly addresses current uses of MPLS Traffic Engineering within a single IGP area. This helps, then, in discussing a set of functional requirements a solution must or should satisfy in order to support inter-area MPLS Traffic Engineering. Since the scope of requirements will vary between operators, some requirements will be mandatory (MUST) whereas others will be optional (SHOULD). Finally, a set of evaluation criteria for any solution meeting these requirements is given. Le Roux, et. al. Informational - Expires November 2004 [Page 3] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 2. Contributing Authors This document was the collective work of several. The text and content of this document was contributed by the editors and the co-authors listed below (The contact information for the editors appears in section 14, and is not repeated below): Ting-Wo Chung Yuichi Ikejiri Bell Canada NTT Communications Corporation 181 Bay Street, Suite 350, 1-1-6, Uchisaiwai-cho, Toronto, Chiyoda-ku, Tokyo 100-8019 Ontario, Canada, M5J 2T3 JAPAN Email: ting_wo.chung@bell.ca Email: y.ikejiri@ntt.com Raymond Zhang Parantap Lahiri Infonet Services Corporation MCI 2160 E. Grand Ave. 22001 loudoun Cty Pky El Segundo, CA 90025 Ashburn, VA 20147 USA USA Email: raymond_zhang@infonet.com E-mail: parantap.lahiri@mci.com Kenji Kumaki KDDI Corporation Garden Air Tower Iidabashi, Chiyoda-ku, Tokyo 102-8460, JAPAN E-mail : ke-kumaki@kddi.com Le Roux, et. al. Informational - Expires November 2004 [Page 4] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 3. Terminology LSR: Label Switching Router TE-LSP: MPLS Traffic Engineering Label Switched Path Inter-area TE-LSP : TE-LSP whose head-end LSR and tail-end LSR do not reside within the same IGP area or both head-end LSR and tail-end LSR are in the same IGP area but the TE LSP transiting path may be across different IGP areas. IGP area: OSPF area or IS-IS level. ABR: Area Border Router, router used to connect two IGP areas (ABR in OSPF or L1/L2 router in IS-IS). CSPF: Constraint-based Shortest Path First. 4. Current intra-area uses of MPLS Traffic Engineering This section addresses architecture, capabilities and uses of MPLS-TE within a single IGP area. It first summarizes the current MPLS-TE architecture, then addresses various MPLS-TE capabilities, and finally lists various approaches to integrate MPLS-TE into routing. This section is intended to help defining the requirements for MPLS- TE extensions across multiple IGP areas. 4.1. Intra-area MPLS Traffic Engineering Architecture The MPLS-TE control plane allows establishing explicitly routed MPLS LSPs whose path obeys a set of TE constraints. It is used to achieve major TE objectives such as resource usage optimization, QoS guarantee and fast failure recovery. It basically consists of three main components: -The Routing component, responsible for the discovery of the TE topology. This is ensured thanks to extensions of link state IGP:[ISIS-TE], [OSPF-TE]. -The Path Computation component, responsible for the placement of the LSP. It is performed on the Head-End LSR thanks to a CSPF algorithm, which takes TE topology and LSP Constraints as input. -The Signalling component, responsible for the establishment of the LSP (explicit routing, label distribution an resources reservation) along the computed path. This is ensured thanks to [RSVP-TE] Le Roux, et. al. Informational - Expires November 2004 [Page 5] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 4.2. Intra-area MPLS Traffic Engineering Applications 4.2.1. Intra-area resource optimization MPLS-TE can be used within an area to redirect paths of aggregated flows away from over-utilized resources within a network. In a small scale, this may be done by explicitly configuring a path to be used between two routers. In a grander scale, a mesh of LSPs can be established between central points in a network. LSPs paths can be defined statically in configuration or arrived at by an algorithm that determines the shortest path given constraints such as bandwidth or other administrative constraints. In this way, MPLS-TE allows for greater control over how traffic demands are routed over a network topology and utilize a network's ressources. As mentioned in Section 1, uses to date have been limited to a single IGP area. Note also that TE-LSPs allow to measure traffic matrix in a simple and scalable manner. Basically, aggregated traffic rate between two LSRs is easily measured by accounting of traffic sent onto a TE LSP provisioned between the two LSRs in question. 4.2.2. Intra-area QoS guarantees The DiffServ IETF working group has defined a set of mechanisms described in [DIFF-ARCH], [DIFF-AF] and [DIFF-EF] or [MPLS-DIFF] that can be activated at the edge or over a DiffServ domain to contribute to the enforcement of a (set of) QoS policy(ies), which can be expressed in terms of maximum one-way transit delay, inter-packet delay variation, loss rate, etc. Many Operators have some or full deployment of DiffServ implementations in their networks today, either across the entire network or at least at the edge of the network. In situations where strict QoS bounds are required, admission control inside the backbone of a network is in some cases required in addition to current DiffServ mechanisms. When the propagation delay can be bounded, the performance targets, such as maximum one-way transit delay may be guaranteed by providing bandwidth guarantees along the DiffServ-enabled path. MPLS-TE can be simply used with DiffServ: in that case, it only ensures aggregate QoS guarantees for the whole traffic. It can also be more intimately combined with DiffServ to perform per-class of service admission control and resource reservation. This requires extensions to MPLS-TE called DiffServ Aware TE and defined in [DS-TE- PROTO]. DS-TE allows ensuring strict end-to-end QoS guarantees. For instance, an EF DS-TE LSP may be provisioned between voice gateways within the same area to ensure strict QoS to VoIP traffic. Le Roux, et. al. Informational - Expires November 2004 [Page 6] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 MPLS-TE allows computing intra-area shortest paths satisfying various constraints including bandwidth. For the sake of illustration, if the IGP metrics reflects the propagation delay, it allows finding a minimum propagation delay path satisfying various constraints like bandwidth. 4.2.3. Fast recovery within an area As quality sensitive applications are deployed, one of the key requirements is to provide fast recovery mechanisms, allowing to guarantee traffic recovery on the order of tens of msecs, in case of network element failure. Note that this cannot be achieved by relying only on IGP rerouting. Various recovery mechanisms can be used to protect traffic carried onto TE LSPs. They are defined in [MPLS-RECOV]. Protection mechanisms are based on the provisioning of backup LSPs that are used to recover traffic in case of failure of protected LSPs. Among those protection mechanisms, local protection, also called Fast Reroute is intended to achieve sub-50ms recovery in case of link/node/SRLG failure along the LSP path [FAST-REROUTE]. Fast Reroute is currently used by many operators to protect sensitive traffic inside an IGP area. [FAST-REROUTE] defines two modes for backup LSPs. The first one, called one-to-one backup, consists in setting up a detour LSP per protected LSP and per element to protect. The second one called facility-backup consists in setting up one or several bypass LSPs to protect a given facility (link or node). In case of failure, all protected LSPs are nested into the bypass LSPs (benefiting from the MPLS label stacking property). 4.3. Intra-area MPLS-TE and routing There are several possibilities to direct traffic into intra-area TE LSPs: 1) Static routes to the LSP destination address or any other addresses. 2) IGP routes beyond the LSP destination, from an IGP SPF perspective (IGP shortcuts). 3) BGP routes announced by a (MP-)BGP peer that is reachable through the TE-LSP by means of a single static route to the corresponding BGP next-hop address (option 1) or by means of IGP shortcuts (option 2). This is often called BGP recursive routing. 4) The LSP can be advertised as a link into the IGP to become part of IGP database for all nodes, and thus taken into account during SPF for all nodes. Note that, even if similar in concept, this is different from the notion of Forwarding- Adjacency, as defined in [LSP-HIER], where the LSP is Le Roux, et. al. Informational - Expires November 2004 [Page 7] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 advertised as a TE-link into the IGP-TE, to become part of the TE database and taken into account in CSPF. 5. Problem Statement, Requirements and Objectives of inter-area MPLS-TE 5.1. Inter-Area Traffic Engineering Problem Statement As described in section 4, MPLS-TE is deployed today by many operators to optimize network bandwidth usage, to provide strict QoS guarantees and to ensure sub-50ms recovery in case of link/node/SRLG failure. However, MPLS-TE mechanisms are currently limited to a single IGP area. The limitation comes essentially from the Routing and Path Computation components and less from the Signaling component. This is basically due to the fact that hierarchy limits topology visibility of head-end LSRs to their IGP area, and consequently head- end LSRs can no longer run a CSPF algorithm to compute the shortest constrained path to the tail-end, as CSPF requires the whole topology to compute an end-to-end shortest constrained path. Several operators have multi-area networks and many operators that are still using a single IGP area may have to migrate to a multi-area environment, as their network grows and single area scalability limits are approached. Hence, those operators may require inter-area traffic engineering to: - Perform inter-area resource optimization. - Provide inter-area QoS guarantees for traffic between edge nodes located in different areas. - Provide fast recovery across areas, to protect inter-area traffic in case of link or node failure, including ABR node failures. For instance an operator running a multi-area IGP may have Voice gateways located in different areas. Such VoIP transport requires inter-area QoS guarantees and inter-area fast protection. One possible approach for inter-area traffic engineering could consist of deploying MPLS-TE on a per-area basis, but such an approach has several limitations: - Traffic aggregation at the ABR levels implies some constraints that do no lead to efficient traffic engineering. Actually such per-area TE approach might lead to sub-optimal resource utilization, by optimizing resources independently in each area. And what many operators want is to optimize their resources as a whole, in other words as if there was only one area (flat network). - This does not allow computing an inter-area constrained shortest path and thus does not ensure end-to-end QoS guarantees across areas. Le Roux, et. al. Informational - Expires November 2004 [Page 8] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 - Inter-area traffic cannot be protected with local protection mechanisms such as [FAST-REROUTE] in case of ABR failure. 5.2. Motivations for inter-area MPLS-TE For the reasons mentioned above, it is highly desired to extend the current set of MPLS-TE mechanisms across multiple IGP areas in order to support the intra area applications described in section 4 across areas. Inter-area MPLS-TE extensions are highly desired to provide: - Inter-area resources optimization. - Strict inter-area QoS guarantees. - Fast recovery across areas, particularly in order to protect inter-area traffic against ABR failures. Basically, the solution MUST allow setting up inter-area TE LSPs, ie LSPs whose path crosses at least two IGP areas. It may be desired to compute inter-area shortest path that satisfy some bandwidth constraints or any other constraints, as currently possible within a single IGP area. For the sake of illustration, if the IGP metrics reflects the propagation delay, it may be needed to be able to find the optimal (shortest) path satisfying some constraints (i.e bandwidth) across multiple IGP areas: such a path would be the inter-area path offering the minimal propagation delay. Thus the solution SHOULD provide the ability to compute an inter-area shortest path satisfying a set of constraints (i.e. bandwidth). 5.3. Key Objectives for an inter-area MPLS-TE solution Any solution for inter-area MPLS-TE should be designed having as key objectives to preserve IGP hierarchy concept, and to preserve routing and signaling scalability. 5.3.1. Preserve the IGP hierarchy concept The absence of a full link state topology database makes the computation of an end-to-end optimal path by the head-end LSR not possible without further signaling and routing extensions. There are several reasons that network operators choose to break up their network into different areas. These often include scalability and containment of routing information. The latter can help isolate most of a network from receiving and processing updates that are of no consequence to its routing decisions. Containment of routing information MUST not be compromised to allow inter-area traffic engineering. Information propagation for path-selection MUST continue to be localized. In other words, the solution MUST entirely preserve the concept of IGP hierarchy. Le Roux, et. al. Informational - Expires November 2004 [Page 9] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 5.3.2. Preserve Scalability Being able to achieve the requirements listed in this document MUST be performed while preserving the IGP scalability, which is of the utmost importance. The hierarchy preservation objective addressed in the above section is actually an element to preserve IGP scalability. The solution MUST also not increase IGP load which could compromise IGP scalability. In particular, a solution satisfying those requirements MUST not require for the IGP to carry some unreasonable amount of extra information and MUST not unreasonably increase the IGP flooding frequency. Likewise, the solution MUST also preserve scalability of RSVP-TE ([RSVP-TE]). Additionally, the base specification of MPLS TE is architecturally structured and relatively devoid of excessive state propagation in terms of routing or signaling. Its strength in extensibility can also be seen as an Achilles heel, as there is really no limit to what is possible with extensions. It is paramount to maintain architectural vision and discretion when adapting it for use for inter-area MPLS-TE. Additional information carried within an area, or propagated outside of an area (via routing or signaling) should neither be excessive, patchwork, nor non-relevant. Particularly, as mentioned in 5.2 it may be desired, for some inter- area TE LSP carrying highly sensitive traffic, to compute a shortest inter-area path satisfying a set of constraints like bandwidth. This may require an additional routing mechanism, as base CSPF at head-end can not longer be used due to the lack of topology and resources information. Such routing mechanism MUST not compromise the scalability of the overall system. Le Roux, et. al. Informational - Expires November 2004 [Page 10] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 6. Application Scenario ---area1--------area0------area2-- ------R1-ABR1-R2-------ABR3------- | \ | / | | | R0 \ | / | R4 | | R5 \ |/ | | ---------ABR2----------ABR4------- - ABR1, ABR2: Area0-Area1 ABRs - ABR3, ABR4: Area0-Area2 ABRs - R0, R1, R5: LSRs in area 1 - R2: an LSR in area 0 - R4: an LSR in area 2 Typically, an inter-area TE LSP will be set up between R0 and R4 where both LSRs belong to different IGP areas. Note that the solution MUST support the capability to protect such an inter-area TE LSP from the failure on any link/SRLG/Node within any area and the failure of any traversed ABR. For instance, if the TE-LSP R0->R4 goes through R1->ABR1->R2, then it can be protected against ABR1 failure, thanks to a backup LSP (detour or bypass) that may follow the alternate path R1->ABR2->R2. For instance R0 and R4 may be two voice gateways located in distinct areas. An inter-area DS-TE LSP with class-type EF, is setup from R1 to R4 to route VoIP traffic classified as EF. Per-class inter-area constraint based routing allows to route the DS-TE LSP over a path that will ensure strict QoS guarantees for VoIP traffic. In another application R0 and R4 may be two pseudo wire gateways residing in different areas. An inter-area LSP may be setup to carry pseudo wires. In some cases, it might also be possible to have an inter-area TE LSP from R0 to R5 transiting via the backbone area (or any other levels with IS-IS). Basically, there may be cases where there is no longer enough resources on any intra area path R0-to-R5, while there is a feasible inter-area path through the backbone area. Le Roux, et. al. Informational - Expires November 2004 [Page 11] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 7. Detailed requirements for inter-area MPLS-TE 7.1. Inter-area MPLS TE operations and interoperability The inter-area MPLS TE solution MUST be consistent with requirements discussed in [TE-REQ] and the derived solution MUST be such that it will interoperate seamlessly with current intra-area MPLS TE mechanisms and inherit its capability sets from [RSVP-TE]. The proposed solution MUST allow provisioning at the head-end with end-to-end RSVP signalling (potentially with loose paths) traversing across the interconnected ABRs, without further provisioning required along the transit path. 7.2. Inter-Area TE-LSP signalling The solution MUST allow for the signalling of inter-area TE-LSPs, using RSVP-TE. The proposed solution MUST allow the head-end LSR to explicitly specify a set of LSRs, including ABRs, by means of strict or loose hops for the inter-area TE LSP. In addition, the proposed solution SHOULD also provide the ability to specify and signal certain resources to be explicitly excluded in the inter-area TE LSP path establishment. 7.3. Path optimality In the context of this requirement document, an optimal path is defined as the shortest 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 making use of some CSPF algorithm in the absence of multiple IGP areas. As already mentioned in 5.2, the solution SHOULD provide the capability to dynamically compute an optimal path satisfying a set of specified constraints defined in [TE-REQ] across multiple IGP areas. Note that this requirement document does not mandate that all inter- area TE LSPs require the computation of an optimal (shortest) inter- area path: some inter-area TE LSP paths may be computed via some mechanisms not guaranteeing an optimal end to end path whereas some other inter-area TE LSP paths carrying sensitive traffic could be computed making use of some mechanisms allowing to dynamically compute an optimal end-to-end path. Note that regular constraints like bandwidth, affinities, IGP/TE metric optimization, path Le Roux, et. al. Informational - Expires November 2004 [Page 12] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 diversity, etc, MUST be taken into account in the computation of an optimal end-to-end path. 7.4. Inter-Area MPLS-TE Routing As already mentioned in 5.1, IGP hierarchy does not allow the Head- End LSR computing an end-to-end optimal path. Additional mechanisms are required to compute an optimal path. These additional mechanisms MUST not alter the IGP hierarchy principles. Particularly, in order to maintain containment of routing information and preserve the overall IGP scalability, the solution MUST preclude the leaking across area of any TE Topology related information even in a summarized form. Conversely, this does not preclude the leaking of non topology related information, that are not taken into account during path selection, such as static TE Node information like TE router ids. 7.5. Inter-Area MPLS-TE Path computation Several methods may be used for path computation, as follows: -Per-area path computation based on ERO expansion on the Head- End LSR and on ABRs, with two options for ABR selection: -Static configuration of ABRs as loose hops at the head-end LSR. -Dynamic ABR selection. -Inter-area end-to-end path computation, that may be based for instance on a recursive constraint based searching thanks to collaboration between ABRs. Note that any path computation method may be used provided that it respect key objectives pointed out in 5.3. In case a solution supports more than one method, it SHOULD allow the operator to select by configuration, and on a per-LSP basis, the desired option. 7.6. Inter-area Crankback Routing Crankback routing, as defined in [CRANKBACK] may be used for inter- area TE-LSP. Basically for paths computed thanks to ERO expansions with a dynamic selection of downstream ABRs, crankback routing can be used when there is no feasible path from a selected downstream ABR to the destination: The upstream ABR or Head-End LSR, selects another downstream ABR, and perform ERO expansion. Note that such method does not allow computing and optimal path but just a feasible path. Le Roux, et. al. Informational - Expires November 2004 [Page 13] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 Note also that there can be 0(N^2) LSP setup failures before finding a feasible path where N is the average number of ABR between two areas. This may have a non negligible impact on the LSP setup delay. Crankback may also be used for inter-area LSP recovery: Basically in case a link/node/SRLG failure occurs in the backbone or tail-end area, the ABR upstream to the failure computes an alternate path an reroutes locally the LSP. An inter-area MPLS-TE solution MAY support [CRANKBACK]. A solution that supports [CRANKBACK], MUST allow to activate/deactivate it via signaling, on a per-LSP basis. 7.7. Support of diversely routed inter-area TE LSPs There are several cases where the ability to compute diversely routed TE LSP paths may be desirable. For instance, in case of LSP protection, primary and backup LSPs should be diversely routed. Another example is the requirement to set up multiple diversely routed TE LSPs between a pair of LSRs residing in different IGP areas. For instance when a single TE-LSP satisfying the bandwidth constraint could not be found between two end-points, a solution would consist of setting up multiple TE-LSPs such that the sum of their bandwidth satisfy the bandwidth requirement. In this case, it may be desirable to have these TE-LSPs diversely routed in order to minimize the impact of a failure, on the traffic between the two end- points. Hence, the solution MUST be able to establish diversely routed inter- area TE LSPs, when diverse patsh exist. It MUST support all kinds of path diversity (link, node, SRLG). The solution SHOULD allow computing an optimal placement of diversely routed LSP. There may be various criteria to determine such an optimal placement. For instance the placement of two diversely routed LSPs, for load-balancing purpose may consists of minimizing their cumulative cost. The placement of two diversely routed LSPs for protection purpose may consists in minimizing the cost of the primary LSP while bounding the cost, or hop count of the backup LSP. Le Roux, et. al. Informational - Expires November 2004 [Page 14] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 7.8. Intra/Inter-area Path selection policy For inter-area TE LSPs whose head-end and tail-end LSRs reside in the same IGP area, there may be intra-area and inter-area feasible paths. In case the shortest path is an inter-area path, an operator may either want to avoid, as far as possible, crossing area and thus prefer selecting a sub-optimal intra-area path, or conversely may prefer to use a shortest path, even if it crosses areas. Thus, the solution MUST allow to enable/disable IGP area crossing, on a per-LSP basis, for TE LSPs whose head-end and tail-end reside in the same IGP area. 7.9. Reoptimization of inter-area TE LSP The solution MUST provide the ability to reoptimize in a minimally disruptive manner (make before break) an inter-area TE LSP, should a more optimal path appear in any traversed IGP area. The operator should be able to parameter such a reoptimization on a timer or event-driven basis. It should also be possible to trigger such a reoptimization manually. The solution SHOULD provide the ability to locally reoptimize an inter-area TE-LSP within an area, i.e. retaining the same set of transit ABRs. The reoptimization process in that case, MAY be controlled by the head-end LSR of the inter-area LSP, or by an ABR. The ABR should check for local optimality of the inter-area TE LSPs established through it, based on a timer or triggered by an event. Option of providing manual trigger to check for optimality should also be provided. In some cases it is important to restrict the control of reoptimization to the Head-End LSR only. Thus, the solution MUST allow to activate/deactivate ABR control of reoptimization, via signalling on a per LSP-basis. The solution SHOULD also provide the ability to perform an end-to-end reoptimization, resulting potentially in a change on the set of transit ABRs. Such reoptimization can be controlled only by the HE LSR. In case of head-end control of reoptimization, the solution SHOULD provide the ability for the inter-area head-end LSR to be informed of the existence of a more optimal path in a downstream area and keep a strict control on the reoptimization process. Hence, the inter-area head-end LSR, once informed of a more optimal path in some downstream IGP areas, could decide (or not) to gracefully perform a make-before- break reoptimization, according to the inter-area TE LSP characteristics. Le Roux, et. al. Informational - Expires November 2004 [Page 15] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 7.10. Inter-area LSP Recovery. 7.10.1. Rerouting of inter-area TE LSPs The solution MUST support rerouting of an inter-area TE LSP in case of SRLG/link/node failure or pre-emption. Such rerouting may be controlled by the Head-End LSR or by an ABR (see section 7.6 on crankback). 7.10.2. Fast recovery of inter-area TE LSP The solution MUST provide the ability to benefit from fast recovery making use of the local protection techniques specified in [FAST- REROUTE] in both the case of an intra-area network element failure (link/SRLG/Node) and an ABR node failure. Note that different protection techniques SHOULD be usable in different parts of the network to protect an inter-area TE LSP. This is of the utmost importance in particular in the case of an ABR node failure that typically carries a great deal of inter-area traffic. Moreover, the solution SHOULD allow computing and setting up a backup tunnel following an optimal path that offers bandwidth guarantees during failure along with other potential constraints (like bounded propagation delay increase along the backup path). The solution SHOULD allow to protect ABRs while providing the same level of performances (recovery delay, bandwidth consumption) as provided today within an area. Note that some signalling approaches may have an impact on FRR performances (recovery delay, bandwidth consumption). Typically, when some intra-area LSPs are used to set up the inter-area TE LSP, then the protection of ABR using [FAST-REROUTE] may lead to higher recovery delays and also higher resources consumption. The use of [FAST-REROUTE] to protect ABRs, while ensuring the same level of performances, currently requires to use a single end-to-end RSVP session, with no use of any intra-area LSP. Thus, the solution MUST provide the ability, via signalling on a per-LSP basis, to allow/preclude the use of intra-area LSPs to set up the inter-area LSPs. Le Roux, et. al. Informational - Expires November 2004 [Page 16] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 7.11. DS-TE support The proposed inter-area MPLS TE solution SHOULD also satisfy core requirements documented in [DSTE-REQ] and interoperate seamlessly with current intra-area MPLS DS-TE mechanism [DSTE-PROTO]. 7.12. Hierarchical LSP support In case of large inter-area MPLS deployment potentially involving a large number of LSRs, it can be desirable/necessary to introduce some level of hierarchy in order to reduce the number of states on LSRs (it is worth mentioning that such a solution implies other challenges). Hence, the proposed solution SHOULD allow inter- area TE LSP aggregation (also referred to as LSP nesting) such that individual TE LSPs can be carried onto one or more aggregating LSP(s). One such mechanism, for example is described in [LSP-HIER]. 7.13. Hard/Soft pre-emption As defined in [MPLS-PREEMPT], there are two pre-emption models applicable to MPLS: Soft and Hard Pre-emption. An inter-area MPLS-TE solution SHOULD support the two models. In case of hard pre-emption, the pre-empted inter-area TE-LSP should be rerouted, following requirements defined in section 7.10.1. In case of soft pre-emption, the pre-empted inter-area TE-LSP should be re-optimized, following requirements defined in section 7.9. 7.14. Auto-discovery of TE meshes Because the number of LSRs participating in some TE mesh might be quite large, it might be desirable to provide some discovery mechanisms allowing an LSR to automatically discover the LSRs members of the TE mesh(es) that it belongs to. The discovery mechanism SHOULD be applicable across multiple IGP areas, and SHOULD not impact the IGP scalability, provided that IGP extensions are used for such a discovery mechanism. 7.15. nter-area MPLS TE fault management requirements The proposed solution SHOULD be able to interoperate with fault detection mechanisms of intra-area MPLS TE. The solution SHOULD support[LSP-PING] and [MPLS-TTL]. Le Roux, et. al. Informational - Expires November 2004 [Page 17] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 The solution SHOULD also support for fault detection on backup LSPs, in case [FAST-REROUTE] is deployed. 7.16. Inter-area MPLS-TE and routing In the case of intra-area MPLS TE, there are currently several possibilities to route traffic into an intra-area TE LSP. They are listed in section 4.2. In case of inter-area MPLS-TE, the solution MUST support static routing into the LSP, and also BGP recursive routing with a static route to the BGP next-hop address. ABRs propagate IP reacheability information (summary LSA in OSPF and IP reacheability TLV in ISIS), that MAY be used by the head-end LSR to route traffic to a destination beyond the TE LSP tail-head LSR (e.g. to an ASBR). The use of IGP shortcuts, MUST be precluded when TE LSP head-end and tail-end LSRs do not reside in the same IGP area. It MAY be used when they reside in the same area. The advertisement of an inter-area TE LSP as a link, into the IGP, to attract traffic to an LSP source MUST be precluded when TE LSP head- end and tail-end LSRs do not reside in the same IGP area. It MAY be used when they reside in the same area. 8. Evaluation criteria 8.1. Performances The solution SHOULD clearly be evaluated with respects to the following criteria: (1) Optimality of the computed inter-area TE LSP paths. (2) Optimality of the computed backup paths protecting against the failure of an ABR, capability to share bandwidth among backup tunnels protecting independent facilities. (3) Inter-area TE LSP set up time. (4) RSVP-TE and IGP scalability (state impact, number of messages, message size) 8.2. Complexity and risks The proposed solution(s) SHOULD not introduce complexity to the current operating network to such a degree that it would affect the stability and diminish the benefits of deploying such solution over SP networks. Le Roux, et. al. Informational - Expires November 2004 [Page 18] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 8.3. Backward Compatibility The deployment of inter-area MPLS TE SHOULD not have impact on existing MPLS TE mechanisms to allow for a smooth migration or co- existence. In particular the solution SHOULD allow the setup of an inter-area TE-LSP among transit LSRs that do not support inter-area extensions, provided that these LSRs do not participate in the inter- area TE procedure. For illustration purpose the solution MAY require inter-area extensions only on end-point LSRs, on ABRs, and potentially on PLRs protecting an ABR. 9. Security Considerations Inter-area MPLS-TE does not raise any new security issue, beyond those of intra-area MPLS-TE. 10. Acknowledgements We would like to thank Dimitri Papadimitriou, Adrian Farrel, Vishal Sharma and Arthi Ayyangar for their useful comments and suggestions. 11. Normative References [TE-REQ] Awduche et. al., "Requirements for Traffic Engineering over MPLS", RFC2702. [DSTE-REQ] Le faucheur, F., et al, ô Requirements for Support of Differentiated Services-aware MPLS Traffic Engineeringö, RFC3564. 12. Informative References [MPLS-ARCH] Rosen, et. al., "Multiprotocol Label Switching Architecture", RFC 3031. [TE-OVW] Awduche, et. al., "Overview and Principles of Internet Traffic Engineering", RFC 3272. [RSVP-TE] Awduche, et al, "Extensions to RSVP for LSP Tunnels", RFC 3209. [OSPF-TE] Katz, D., Yeung, D., Kompella, K., "Traffic Engineering Extensions to OSPF Version 2", RFC3630. [ISIS-TE] Li, T., Smit, H., "IS-IS extensions for Traffic Engineering", draft-ietf-isis-traffic-04.txt, work in progress. Le Roux, et. al. Informational - Expires November 2004 [Page 19] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 [TE-APP] Boyle, et. al., "Applicability Statement of Traffic Engineering", RFC 3346. [FAST-REROUTE] Ping Pan, et al, "Fast Reroute Extensions to RSVP-TE for LSP Tunnels", draft-ietf-mpls-rsvp-lsp-fastreroute-04.txt, work in progress. [LSPPING] Kompella, K., Pan, P., Sheth, N., Cooper, D.,Swallow, G., Wadhwa, S., Bonica, R., " Detecting Data Plane Liveliness in MPLS", Internet Draft "draft-ietf-mpls-lsp-ping-05.txt", work in progress. [MPLS-TTL] Agarwal, R., et al, "Time to Live (TTL) Processing in MPLS Networks", RFC 3443. [LSP-HIER] Kompella K., Rekhter Y., "LSP Hierarchy with Generalized MPLS TE", draft-ietf-mpls-lsp-hierarchy-08.txt, work in progress. [MPLS-RECOV] V. Sharma, F. Hellstrand, "Framework for Multi-Protocol Label Switching (MPLS)-based Recovery", RFC 3469. [CRANKBACK] Farrel, A., Ed., "Crankback Signaling Extensions for MPLS Signalingö, draft-ietf-ccamp-crankback-01.txt, work in progress. [MPLS-DIFF] Le Faucheur, et. al., "MPLS Support of Differentiated Services", RFC 3270. [DSTE-PROTO] Le faucheur, F., et al, ôProtocol extensions for support of Differentiated-Service-aware MPLS Traffic Engineeringö, draft- ietf-tewg-diff-te-proto-07.txt, work in progress. [DIFF-ARCH] Blake, et. al., "An Architecture for Differentiated Services", RFC 2475. [DIFF-AF] Heinanen, et. al., "Assured Forwarding PHB Group", RFC 2597. [DIFF-EF] Davie, et. al., "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246. [MPLS-PREEMPT] Farrel, A., "Interim Report on MPLS Pre-emption", draft-farrel-mpls-preemption-interim-00.txt, work in progress. [METRIC] Le Faucheur, et. Al., "Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering Metric", draft- ietf-tewg-te-metric-igp-02.txt, work in progress. Le Roux, et. al. Informational - Expires November 2004 [Page 20] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 13. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in RFC 2028. Copies of claims of rights made available for publication 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 implementors or users of this specification can be obtained from the IETF Secretariat. 14. Editors' Address: Jean-Louis Le Roux France Telecom 2, avenue Pierre-Marzin 22307 Lannion Cedex France Jean-Philippe Vasseur Cisco Systems, Inc. 300 Beaver Brook Road Boxborough , MA - 01719 USA Email: jpv@cisco.com Jim Boyle Email: jboyle@pdnets.com Le Roux, et. al. Informational - Expires November 2004 [Page 21] Internet Draft draft-ietf-tewg-interarea-mpls-te-req-02 June 2004 Full Copyright Statement "Copyright (C) The Internet Society (2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Le Roux, et. al. Informational - Expires November 2004 [Page 22]