Jean-Philippe Vasseur(Editor) Cisco Systems, Inc. Yuichi Ikejiri NTT Communications Corporation IETF Internet Draft Expires: August, 2004 February, 2004 draft-vasseur-ccamp-loose-path-reopt-00.txt Reoptimization of MPLS Traffic Engineering loosely routed explicit 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-00.txt February 2004 Table of content 3.1. ERO expansion signaling request 4 3.2. New Path Error sub-code 4 4.1. TE LSP reroute 5 4.2. Reoptimization triggers 5 4.3. Head-end request versus mid-point indication 6 4.3.1. Head-end request 6 4.3.2. Mid-point indication 7 4.3.3. ERO caching 8 Abstract The aim of this document is to propose a mechanism for the reoptimization of MPLS Traffic Engineering loosely routed explicit LSP paths. A loosely routed explicit 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 TE LSP head-end LSR that a better path exists to reach a loose hop (compared to the current path in use). A better path is defined as a path with a lower cost, where the cost is determined by the metric used to compute the path. This primarily applies to inter-area TE LSPs and inter-AS TE LSPs (see [INTER-AREA-AS]) when the path is defined as a list of loose hops (generally the loose hops are the ABRs/ASBRs) but the following mechanism is also applicable to any loosely routed explicit path within a single routing domain. 1. Establishment of a loosely routed explicit TE LSP A loosely routed explicit path is as 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). Loose hops are listed in the ERO object of the RSVP Path message with the L flag of the Ipv4 prefix sub-object set, as defined in [RSVP-TE]. In this case, each LSR along path can perform a partial route computation to reach the next loose hop and then performs an ERO expansion, before forwarding the RSVP Path message downstream. Note that the examples in the rest of this document are provided in the context of MPLS inter-area TE but the proposed mechanism also applies to loosely routed path within a single routing domain. Furthermore, Vasseur and Ikejiri 2 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 this mechanism could also be used for loosely routed TE LSP in the context of TE LSPs spanning several autonomous systems and as such abides by the requirements for inter-AS TE define in [INTER-AS-TE-REQS] 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 - A TE LSP1 path from R1 (head-End LSR) to R11 (tail-end LSR) is defined as the following loosely routed path: R1-R3(loose)-R8(loose)- R11(loose):R3, R8 and R11 are defined as loose hops. Step 1: LSP 1ªs Head-end (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) The R1-R2-R3 path obeys the TE LSP1ªs 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. The new ERO becomes: R6(S)-R7(S)- R8(S)-R11(L). Step 4: the same procedure applies at R8 to reach LSP1ªs destination: R11. 2. Reoptimization of a loosely routed explicit TE LSP path Once the 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 (suppose in the example above that a link between R6 and R8 is added 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, it cannot make use of this better path (and perform a make before break) when appropriate. Vasseur and Ikejiri 3 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 This is the case in the example above as the better path does not appear in the head-end area. This document proposes a mechanism that allows: - The TE LSP 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, - An 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 TE LSP 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 - - see [SEC-METRIC], [METRIC]). Then once the existence of a better path is notified to the head-end LSR, the head-end LSR can decide (depending on the TE LSP characteristics) whether to perform a TE LSP graceful reoptimization. This allows the Head-end LSR 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. 3. Signalling extensions 3.1. 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. 3.2. New Path Error sub-code The format of a Path Error is the following: ::= [ ] [ ...] [ ] ::= (see earlier definition) Vasseur and Ikejiri 4 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 IPv4 ERROR_SPEC object: Class = 6, C-Type = 1 +-------------+-------------+-------------+-------------+ | IPv4 Error Node Address (4 bytes) | +-------------+-------------+-------------+-------------+ | Flags | Error Code | Error Value | +-------------+-------------+-------------+-------------+ Various Error Codes and Error values have been defined in RFC2205 and RFC3209. The ERROR-CODE 25 corresponds to a Path Error - Notify Error. We propose to add two new sub-codes: 6 Better path exists 7 Local link maintenance required 8 Local node maintenance required See details about Local maintenance required modes in section 4.3.2 4. Mode of operation 4.1. TE LSP reroute The notification process of a better 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. In particular, when a better path is discovered, one could conceivably envisage reoptimizing the TE LSP on a mid-point LSR or on the head-end LSR of the TE LSP. In the former case, this may not be desirable in several circumstances: indeed, the reoptimization process inevitably generates some jitter and potentially packet reordering. Furthermore, the only LSR having the complete view of the end to end path and TE LSP set of attributes/constraints is the head-end LSR. For those reasons, this document applies to the context of a head-end LSR reoptimization. It is just worth mentioning that in some other contexts, mid-point reoptimization may also be desirable. 4.2. Reoptimization triggers There are two possible reoptimization triggers: - 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 ªªLink-UPªª event). Vasseur and Ikejiri 5 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 It is RECOMMENDED for an implementation supporting the extensions proposed in this document to support both modes. 4.3. Head-end request versus mid-point indication This document defines two modes: - ªªHead-end requesting modeªª: the request for a new path evaluation of an explicit loosely routed TE LSP is requested by the head-end LSR. - ªªMid-point indication ªª: 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 make before break. 4.3.1. Head-end request 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 ªªERO Expansion requestªª 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 ªªERO expansion request ªª bit set, every LSR for which the next abstract node contained in the ERO is defined as a loose hop, MUST perform the following set of actions: - 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 ªªBetter path exists ªª to the head-end LSR will notify the head-end LSR of the existence of a better path in a downstream area/AS. Hence, triggering additional ERO expansions on downstream nodes is unnecessary. The only motivation to forward subsequent RSVP Path messages with the ªª Expansion request bit ªª 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 Vasseur and Ikejiri 6 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 to reduce the signaling setup delay, should a reoptimization be performed by the head-end LSR. - No better path can be found: as previously stated, 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. Note: by better path, we mean a path having a lower cost. By default, an LSR uses the IGP metric in their CSPF 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 the TE metric to determine the best path. Example: Let call Ln the list of LSRs defined as loose hops in the ERO sent in the RSVP Path message by the head-end LSR: Ln=. Letªs now call Pn= the list of LSRs pi such that li is a next (loose) hop of pi for i=1+n <---area 1--><-area 0--><-area 2-> R1---R2----R3---R6 R8-----R10 | | | / |\ | | | | -- | --\ | | | |/ | \| |---R4----R5---R7----R9-----R11 A TE LSP1 from R1 (head-End LSR) to R11 (tail-end LSR) is defined with the following loosely routed path: R1-R3(loose)-R8(loose)-R11(loose). R3, R8 and R11 are defined as loose hops. Ln= Pn= As soon as a positive response is received from an LSR pi (sub-code=6, ªª Better path existsªª), the head-end LSR is informed of a more optimal path. Note that if the RSVP Path message with the ªªERO expansion request ªª 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]. 4.3.2. Mid-point indication Vasseur and Ikejiri 7 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 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 by 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 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. 4.3.3. 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 : 5. Interoperability Comment : An LSR not supporting the ªªERO expansion request ªª bit of the SESSION- ATTRIBUTE object SHOULD just ignore it. Any head-end LSR not supporting this draft receiving a Path Error Notify message with sub-code = 6, 7 or 8 MUST just silently ignore the Path message. Vasseur and Ikejiri 8 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 6. Security Considerations The practice described in this document does not raise specific security issues beyond those of existing TE. 7. Acknowledgment The authors would like to thank Carol Iturralde, Miya Kohno, Francois Le Faucheur, Philip Matthews and Jim Gibson for their useful and valuable comments. 8. 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 Vasseur and Ikejiri 9 draft-vasseur-ccamp-loose-path-reopt-00.txt February 2004 [TE-REQ] Awduche et al, Requirements for Traffic Engineering over MPLS, RFC2702, September 1999. [METRICS] Fedyk et al,ªªMultiple Metrics for Traffic Engineering with IS-IS and OSPFªª, draft-fedyk-isis-ospf-te-metrics-01.txt, November 2000. [DS-TE] Le Faucheur et al, ªªRequirements for support of Diff-Serv-aware MPLS Traffic Engineeringªª, draft-ietf-tewg-diff-te-reqts-01.txt, June 2001. [MULTI-AREA-TE] Kompella et al, ªªMulti-area MPLS Traffic Engineeringªª, draft-kompella-mpls-multiarea-te-03.txt, June 2002. [SEC-METRIC] Le Faucheur et al,ªª Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering Metricªª, draft-ietf- tewg-te-metric-igp-02.txt, September, 2002. [INTER-AS-TE-REQS] Zhang et al, ªªMPLS Inter-AS Traffic Engineering requirementsªª, draft-ietf-tewg-interas-mpls-te-req-06.txt, February 2004, Work in progress. [INTER-AREA-AS] Vasseur and Ayyangar, ªªInter-area and Inter-AS Traffic Engineeringªª, draft-vasseur-inter-area-AS-TE-00.txt, February 2004, work in progress. [REFRESH-REDUCTION] Berger et al, ªªRSVP Refresh Overhead Reduction Extensionsªª, April 2001 Authors' addresses: Jean-Philippe Vasseur Cisco Systems, Inc. 300 Beaver Brook Road Boxborough , MA - 01719 USA Email: jpv@cisco.com Yuichi Ikejiri NTT Communications Corporation 1-1-6, Uchisaiwai-cho, Chiyoda-ku Tokyo 100-8019 JAPAN Email: y.ikejiri@ntt.com Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. 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