draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 IETF Internet Draft Jean-Philippe Vasseur (Editor) Proposed Status : Standard Cisco Systems, Inc Expires: October 2004 Yuichi Ikejiri NTT Communications Corporation April 2004 draft-vasseur-ccamp-loose-path-reopt-01.txt Reoptimization of MPLS Traffic Engineering loosely routed LSP paths 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. Vasseur and Ikejiri 1 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 Table of content 1. Introduction 2. Establishment of a loosely routed TE LSP 3. Reoptimization of a loosely routed TE LSP path 4. Signalling extensions 4.1 ERO expansion signaling request 4.2 New Path Error sub-codes 5. Mode of operation 5.1 Head-end reoptimization control 5.2 Reoptimization triggers 5.3 Head-end request versus mid-point explicit notification modes 5.3.1 Head-end request mode 5.3.2 Mid-point explicit notification mode 5.3.3 ERO cashing 6. Interoperability 7. Security considerations 8. Acknowledgments 9. Intellectual property Abstract The aim of this document is to propose a mechanism for the reoptimization of MPLS Traffic Engineering loosely routed LSP paths. A loosely routed LSP path is a path specified as a combination of strict and loose hop(s) that contains at least one loose hop and zero or more strict hop(s). The path calculation (which implies an ERO expansion) to reach a loose hop is performed by the previous hop defined in the TE LSP path. This document proposes a mechanism that allows: - The TE LSP head-end LSR to trigger a new ERO expansion on every hop having a next hop defined as a loose hop, - A mid-point LSR to signal to the head-end LSR that either a better path exists to reach a loose hop (compared to the current path in use) or that the TE LSP must be reoptimized because of some maintenance required on the TE LSP path. A better path is defined as a lower cost path, where the cost is determined by the metric used to compute the path. The proposed mechanism applies to intra-domain and inter-domain packet and non-packet TE LSPs when the path is defined as a list of loose hops. Examples of domains are IGP areas and Autonomous Systems. 1. Introduction The Traffic Engineering Work Group has specified a set of requirements for inter-area [INTER-AREA-TE-REQ] and inter-AS [INTER-AS-TE-REQ] MPLS Traffic Engineering. Both requirements documents specify the need for some mechanism providing an option for the head-end to control the Vasseur and Ikejiri 2 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 reoptimization process, should a more optimal path exist in a downstream domain (IGP area or Autonomous System). This document proposed a solution to meet this requirement, in addition to some mechanism to discover the existence of such a more optimal path or the need to reoptimize due to some maintenance required in a downstream domain. 2. Establishment of a loosely routed TE LSP A loosely routed explicit path is a path specified as a combination of strict and loose hop(s) that contains at least one loose hop and a set of zero or more strict hop(s). Loose hops are listed in the ERO object of the RSVP Path message with the L flag of the Ipv4 or the IPv6 prefix sub-object set, as defined in [RSVP-TE]. In this case, each LSR along path whose next hop is specified as a loose hop triggers a path computation (also referred to as an ERO expansion), before forwarding the RSVP Path message downstream. The path computation may be either be performed by means of CSPF or any Path Computation Element (PCE) and can be partial (up to the next loose hop) and complete (up to the TE LSP destination). Note that the examples in the rest of this document are provided in the context of MPLS inter-area TE but the proposed mechanism equally applies to loosely routed explicit paths within a single routing domain and across multiple Autonomous Systems. The examples below are provided with OSPF as the IGP but the described set of mechanisms similarly apply to IS-IS. An example of an explicit loosely routed TE LSP signaling. <---area 1--><-area 0--><-area 2-> R1---R2----R3---R6 R8-----R10 | | | / |\ | | | | -- | --\ | | | |/ | \| R4---------R5---R7----R9-----R11 Assumptions - R3, R5, R8 and R9 are ABRs - The path an inter-area TE LSP T1 from R1 (head-End LSR) to R11 (tail- end LSR) is defined on R1 as the following loosely routed path: R1- R3(loose)-R8(loose)-R11(loose). R3, R8 and R11 are defined as loose hops. Step 1: R1 builds the following ERO object: R1(S)-R2(S)-R3(S)-R8(L)- R11(L) where: S: Strict hop (L=0) L: Loose hop (L=1) Vasseur and Ikejiri 3 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 The R1-R2-R3 path obeys T1Æs set of constraints Step 2: the RSVP Path message is then forwarded by R1 following the ERO path and reaches R3 with the following content: R8(L)-R11(L) Step 3: R3 determines that the next hop (R8) is a loose hop (not directly connected to R3) and then performs an ERO expansion operation to reach the next loose hops R8 either by means of CSPF or any other PCE-based path computation method. The new ERO becomes: R6(S)-R7(S)- R8(S)-R11(L) Note: in this example, the assumption is made that the path is computed on a per loose hop basis, also referred to a partial route computation. Note that PCE-based mechanisms may also allow for full route computation (up to the final destination). Step 4: the same procedure applies at R8 to reach T1Æs destination: R11. 3. Reoptimization of a loosely routed TE LSP path Once a loosely routed explicit TE LSP is set up, it is maintained through normal RSVP procedures. Then a more optimal path might appear between an LSR and its next loose hop(for the sake of illustration, suppose in the example above that a link between R6 and R8 is added or restored that provides a shorter path between R3 and R8 (R3-R6-R8) than the existing R3-R6-R7-R8 path). Since the better path is not visible from the head-end LSR by means of the IGP because it does not belong to the head-end IGP area, the head-end cannot make use of this better path (and perform a make before break) when appropriate. Hence some mechanism is required to detect the existence of such a better path and notifies the head-end accordingly. This document proposes a mechanism that allows: - A head-end LSR to trigger on every LSR whose next hop is a loose hop the re-evaluation of the current path in order to detect a potential more optimal path, - A mid-point LSR whose next hop is a loose-hop to signal (using a new ERROR-SPEC sub-code carried in a Path Error Notify message) to the head-end that a better path exists (a path with a lower cost, where the cost is defined by the metric used to compute the path û see [SEC-METRIC], [METRIC]). Then once the existence of such a better path is notified to the head- end, the head-end LSR can decide (depending on the TE LSP characteristics) whether to perform a TE LSP graceful reoptimization. There is another scenario whereby notifying the head-end of the Vasseur and Ikejiri 4 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 existence of a better path is desirable: if the current path is about the fail due to some (link or node) required maintenance. This allows the head-end to reoptimize a TE LSP making use of the non disruptive make before break procedure if and only if a better path exists and if such a reoptimized is desired. 4. Signalling extensions 4.1. ERO expansion signaling request The following new flag of the SESSION_ATTRIBUTE object (C-Type 1 and 7) is defined: ERO Expansion request: 0x20 This flag indicates that a new ERO expansion is requested. Note: in case of link bundling for instance, although the resulting ERO might be identical, this might give the opportunity for a mid-point LSR to locally select another link within a bundle, although strictly speaking, the ERO has not changed. 4.2. New Path Error sub-code As defined in [RSVP-TE], the ERROR-CODE 25 an ERROR SPEC object corresponds to a Path Error - Notify Error. This document proposes to add three new sub-codes: 6 Better path exists 7 Local link maintenance required 8 Local node maintenance required The details about the local maintenance required modes are detailed in section 5.3.2 5. Mode of operation 5.1. Head-end reoptimization control The notification process of a better path (shorter path or new path due to some maintenance required on the current path) is by nature de- correlated from the reoptimization operation. In other words, the location where a potentially more optimal path is discovered does not have to be where the TE LSP is actually reoptimized. This document applies to the context of a head-end reoptimization. 5.2. Reoptimization triggers There are two possible reoptimization triggers: Vasseur and Ikejiri 5 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 - Timer-based: a reoptimization is triggered (process evaluating whether a more optimal path can be found) when a configurable timer expires, - Event-driven: a reoptimization is triggered when a particular network event occurs (such as a ôLink-UPö event), - Operator-driven: a reoptimization is manually triggered by the Operator. It is RECOMMENDED for an implementation supporting the extensions proposed in this document to support both modes. 5.3. Head-end request versus mid-point explicit notification modes This document defines two modes: - ôHead-end requesting modeö: the request for a new path evaluation of a loosely routed TE LSP is requested by the head- end LSR. - ôMid-point explicit notificationö: a mid-point LSR having determined that a better path (than the current path is use) exists or having the desire to perform a link/node local maintenance explicitly notifies the head-end LSR which will in turn decide whether to perform a reoptimization. 5.3.1. Head-end request mode In this mode, when a timer-based reoptimization is triggered on the head-end LSR or the operator manually requests a reoptimization, the head-end LSR immediately sends an RSVP Path message with the ôERO Expansion requestö bit of the SESSION-ATTRIBUTE object set. This bit is then cleared in subsequent RSVP path messages sent downstream. Upon receiving a Path message with the ôERO expansion requestö bit set, every LSR for which the next abstract node contained in the ERO is defined as a loose hop, performs the following set of actions: 1) A new ERO expansion is triggered and the newly computed path is compared to the existing path: - If a better path can be found, the LSR MUST immediately send a Path Error to the head-end LSR (Error code 25 (Notify), sub- code=6 (better path exists)). At this point, the LSR MAY decide to clear the ERO expansion request bit of the SESSION-ATTRIBUTE object in subsequent RSVP Path messages sent downstream: this mode is the RECOMMENDED mode. The sending of a Path Error Notify message ôBetter path existsö to the head-end LSR will notify the head-end LSR of the Vasseur and Ikejiri 6 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 existence of a better path (e.g in a downstream area/AS or in another location within a single domain). Hence, triggering additional ERO expansions on downstream nodes is unnecessary. The only motivation to forward subsequent RSVP Path messages with the ôExpansion request bitö of the SESSION-ATTRIBUTE object set would be to trigger path re-evaluation on downstream nodes that could in turn cache some potentially better paths downstream with the objective to reduce the signaling setup delay, should a reoptimization be performed by the head-end LSR. - If no better path can be found, the recommended mode is for an LSR to relay the request (by setting the ERO expansion bit of the SESSION-ATTRIBUTE object in RSVP path message sent downstream) only if no better path has been found on this mid- point LSR. By better path, we mean a path having a lower cost. By default, an LSR uses the TE metric to compute the shortest path that obeys a set of constraints. Note that the head-end LSR might use the METRIC-TYPE object (defined in [PATH-COMP]) in its path message to request the LSR having a next hop defined as a loose hop in the ERO to use another metric to determine the best path. If the RSVP Path message with the ôERO expansion requestö bit set is lost, then the next request will be sent when the reoptimization event will trigger on the head-end LSR. The solution to handle RSVP reliable messaging has been defined in [REFRESH-REDUCTION]. The network administrator may decide to establish some local policy specifying to ignore such request or to consider those requests not more frequently than a certain rate. The proposed mechanism does not make any assumption of the path computation method performed by the ERO expansion process: it can either be CSPF or PCE based. 5.3.2. Mid-point explicit notification mode In this mode, a mid-point LSR whose next abstract node is a loose hop can locally trigger an ERO expansion (when a configurable timer expires or on event-driven basis (link-up event for example) or the user explicitly requests it). If a better path is found compared to the existing one, the LSR sends a Path Error to the head-end LSR (Error code 25 (Notify), sub-code=6 (better path exists)). There are other circumstances in which a mid-point LSR MAY send an RSVP Path Error Notify message with the objective for the TE LSP to be rerouted by its head-end LSR: when a link or a node will go down for local maintenance reasons. In this case, the mid-point LSR where the local maintenance must be performed is responsible for sending an RSVP Vasseur and Ikejiri 7 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 Path Error Notify message with the sub-code=7 or 8 depending on the affected network element (link or node). Then the first upstream node having performed the ERO expansion MUST perform the following set of actions: - The link (sub-code=7) or the node (sub-code=8) MUST be locally registered for further reference (the TE database must be updated) - The RSVP Path Error message MUST be immediately forwarded unchanged upstream to the head-end LSR. Upon, receiving a Path Error Notify message with sub-code 7 or 8, the Head-end LSR MUST perform a TE LSP reoptimization. Note that those modes are not exclusive: both the timer and even-driven reoptimization triggers can be implemented on the head-end and/or any mid-point LSR with potentially different timer values for the timer driven reoptimization case. A head-end LSR MAY decide upon receiving an explicit mid-point notification to delay its next ERO expansion request. 5.3.3. ERO caching Once a mid-point LSR has determined that a better path exists (after a reoptimization request has been received by the head-end LSR or the reoptimization timer on the mid-point has fired), the more optimal path MAY be cached on the mid-point LSR for a limited amount of time to avoid having to recompute a route once the head-LSR performs a make before break. This mode is optional. Comment : 6. Interoperability Comment : An LSR not supporting the ôERO expansion requestö bit of the SESSION- ATTRIBUTE object SHOULD just ignore it. Any head-end LSR not supporting a Path Error Notify message with sub- code = 6, 7 or 8 MUST just silently ignore such Path Error messages. 7. Security Considerations The practice described in this document does not raise specific security issues beyond those of existing TE. 8. Acknowledgments The authors would like to thank Carol Iturralde, Miya Kohno, Francois Le Faucheur, Philip Matthews, Jim Gibson, Raymond Zhang, Jean-Louis Le Roux and Kenji Kumaki for their useful and valuable comments. Vasseur and Ikejiri 8 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 9. Intellectual Property 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 BCP-11. 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. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights. Normative References [RFC] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels," RFC 2119. [RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC3209, December 2001. Informative references [TE-REQ] Awduche et al, Requirements for Traffic Engineering over MPLS, RFC2702, September 1999. [METRICS] Fedyk et al, ôMultiple Metrics for Traffic Engineering with IS-IS and OSPFö, draft-fedyk-isis-ospf-te-metrics-01.txt, November 2000. [DS-TE] Le Faucheur et al, ôRequirements for support of Diff-Serv-aware MPLS Traffic Engineeringö, draft-ietf-tewg-diff-te-reqts-01.txt, June 2001. Vasseur and Ikejiri 9 draft-vasseur-ccamp-loose-path-reopt-01.txt April 2004 [MULTI-AREA-TE] Kompella et al, ôMulti-area MPLS Traffic Engineeringö, draft-kompella-mpls-multiarea-te-03.txt, June 2002. [SEC-METRIC] Le Faucheur et all,ô Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering Metricö, draft-ietf- tewg-te-metric-igp-02.txt, September, 2002. [INTER-AREA-TE-REQ], Le Roux, Vasseur, Boyle et al. ½ Requirements for Inter-area MPLS Traffic Engineering ©, draft-ietf-tewg-interarea-mpls- te-req-01, April 2004 (Work in progress). [INTER-AS-TE-REQ] Zhang et al, ôMPLS Inter-AS Traffic Engineering requirementsö, draft-ietf-tewg-interas-mpls-te-req-06.txt, February 2004, Work in progress. [INTER-AREA-AS] Vasseur and Ayyangar, ôInter-area and Inter-AS Traffic Engineeringö, draft-vasseur-inter-area-AS-TE-00.txt, February 2004, work in progress. [REFRESH-REDUCTION] Berger et al, ôRSVP Refresh Overhead Reduction Extensionsö, April 2001 Authors' addresses: Formatted: Jean-Philippe Vasseur Cisco Systems, Inc. 300 Beaver Brook Road Boxborough , MA - 01719 USA Email: jpv@cisco.com Formatted: Formatted: Yuichi Ikejiri Formatted: NTT Communications Corporation Field Code 1-1-6, Uchisaiwai-cho, Chiyoda-ku Tokyo 100-8019 JAPAN Email: y.ikejiri@ntt.com Formatted: Formatted: Full Copyright Statement Formatted: Field Code 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 in 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. 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