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Network Working Group                                   JP. Vasseur, Ed.Request for Comments: 4736                           Cisco Systems, Inc.Category: Informational                                       Y. Ikejiri                                          NTT Communications Corporation                                                                R. Zhang                                                              BT Infonet                                                           November 2006Reoptimization of Multiprotocol Label Switching (MPLS) TrafficEngineering (TE) Loosely Routed Label Switched Path (LSP)Status of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2006).Abstract   This document defines a mechanism for the reoptimization of loosely   routed MPLS and GMPLS (Generalized Multiprotocol Label Switching)   Traffic Engineering (TE) Label Switched Paths (LSPs) signaled with   Resource Reservation Protocol Traffic Engineering (RSVP-TE).  This   document proposes a mechanism that allows a TE LSP head-end Label   Switching Router (LSR) to trigger a new path re-evaluation on every   hop that has a next hop defined as a loose or abstract hop and a   mid-point LSR to signal to the head-end LSR that a better path exists   (compared to the current path) or that the TE LSP must be reoptimized   (because of maintenance required on the TE LSP path).  The proposed   mechanism applies to the cases of intra- and inter-domain (Interior   Gateway Protocol area (IGP area) or Autonomous System) packet and   non-packet TE LSPs following a loosely routed path.Vasseur, et al.              Informational                      [Page 1]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006Table of Contents1. Introduction ....................................................32. Terminology .....................................................32.1. Requirements Language ......................................43. Establishment of a Loosely Routed TE LSP ........................44. Reoptimization of a Loosely Routed TE LSP Path ..................65. Signaling Extensions ............................................75.1. Path Re-Evaluation Request .................................75.2. New Error Value Sub-Codes ..................................76. Mode of Operation ...............................................76.1. Head-End Reoptimization Control ............................76.2. Reoptimization Triggers ....................................8      6.3. Head-End Request versus Mid-Point Explicit           Notification Functions .....................................86.3.1. Head-End Request Function ...........................86.3.2. Mid-Point Explicit Notification ....................106.3.3. ERO Caching ........................................107. Applicability and Interoperability .............................118. IANA Considerations ............................................119. Security Considerations ........................................1110. Acknowledgements ..............................................1211. References ....................................................1211.1. Normative References .....................................1211.2. Informative References ...................................12Vasseur, et al.              Informational                      [Page 2]

RFC 4736               MPLS-TE Loosely Routed LSP          November 20061.  Introduction   This document defines a mechanism for the reoptimization of loosely   routed MPLS and GMPLS (Generalized Multiprotocol Label Switching)   Traffic Engineering LSPs signaled with RSVP-TE (see [RFC3209] and   [RFC3473]).  A loosely routed LSP is defined as one that does not   contain a full, explicit route identifying each LSR along the path of   the LSP at the time it is signaled by the ingress LSR.  Such an LSP   is signaled with no Explicit Route Object (ERO), with an ERO that   contains at least one loose hop, or with an ERO that contains an   abstract node that is not a simple abstract node (that is, an   abstract node that identifies more than one LSR).   The Traffic Engineering Working Group (TE WG) has specified a set of   requirements for inter-area and inter-AS MPLS Traffic Engineering   (see [RFC4105] and [RFC4216]).  Both requirements documents specify   the need for some mechanism providing an option for the head-end LSR   to control the reoptimization process should a more optimal path   exist in a downstream domain (IGP area or Autonomous System).  This   document defines a solution to meet this requirement and proposes two   mechanisms:   (1) The first mechanism allows a head-end LSR to trigger a new path       re-evaluation on every hop that has a next hop defined as a loose       hop or abstract node and get a notification from the mid-point as       to whether a better path exists.   (2) The second mechanism allows a mid-point LSR to explicitly signal       to the head-end LSR either that a better path exists to reach a       loose/abstract hop (compared to the current path) or that the TE       LSP must be reoptimized because of some maintenance required       along the TE LSP path.  In this case, the notification is sent by       the mid-point LSR without being polled by the head-end LSR.   A better path is defined as a lower cost path, where the cost is   determined by the metric used to compute the path.2.  Terminology   ABR: Area Border Router.   ERO: Explicit Route Object.   LSR: Label Switching Router.   TE LSP: Traffic Engineering Label Switched Path.   TE LSP head-end: head/source of the TE LSP.Vasseur, et al.              Informational                      [Page 3]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006   TE LSP tail-end: tail/destination of the TE LSP.   Interior Gateway Protocol Area (IGP Area): OSPF Area or IS-IS level.   Intra-area TE LSP: A TE LSP whose path does not transit across areas.   Inter-area TE LSP: A TE LSP whose path transits across at least two   different IGP areas.   Inter-AS MPLS TE LSP: A TE LSP whose path transits across at least   two different Autonomous Systems (ASes) or sub-ASes (BGP   confederations).2.1.  Requirements Language   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 inRFC 2119 [RFC2119].3.  Establishment of a Loosely Routed TE LSP   The aim of this section is purely to summarize the mechanisms   involved in the establishment of a loosely routed TE LSP, as   specified in [RFC3209].  The reader should seeRFC 3209 for a more   detailed description of these mechanisms.   In the context of this document, a loosely routed LSP is defined as   one that does not contain a full, explicit route identifying each LSR   along the path of the LSP at the time it is signaled by the ingress   LSR.  Such an LSP is signaled with no ERO, with an ERO that contains   at least one loose hop, or with an ERO that contains an abstract node   that is not a simple abstract node (that is, an abstract node that   identifies more than one LSR).  As specified in [RFC3209], 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.   Each LSR along the path whose next hop is specified as a loose hop or   a non-specific abstract node triggers a path computation (also   referred to as an ERO expansion), before forwarding the RSVP Path   message downstream.  The computed path may be either partial (up to   the next loose hop) or complete (set of strict hops up to the TE LSP   destination).   Note that although the examples in the rest of this document are   provided in the context of MPLS inter-area TE, the proposed mechanism   applies equally to loosely routed paths within a single routingVasseur, et al.              Informational                      [Page 4]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006   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 follows.   <---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 of 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 determines that the next hop (R3) is a loose hop (not     directly connected to R1) and then performs an ERO expansion     operation to reach the next loose hops R3.  The new ERO becomes:     R2(S)-R3(S)-R8(L)-R11(L), where S is a strict hop (L=0) and L is a     loose hop (L=1).     The R1-R2-R3 path satisfies T1's set of constraints.     Step 2: The RSVP Path message is then forwarded by R1 following the     path specified in the ERO object 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).     Note: In this example, the assumption is made that the path is     computed on a per-loose-hop basis, also referred to as a partial     route computation.  Note that other path computation techniques may     result in complete paths (set of strict hops up to the final     destination).     Step 4: The same procedure is repeated by R8 to reach T1's     destination (R11).Vasseur, et al.              Informational                      [Page 5]

RFC 4736               MPLS-TE Loosely Routed LSP          November 20064.  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.  During the TE LSP lifetime, a more   optimal path might appear between an LSR and its next loose hop (for   the sake of illustration, suppose that in the example above a link   between R6 and R8 is added or restored that provides a preferable   path between R3 and R8 (R3-R6-R8) than the existing R3-R6-R7-R8   path).  Since a preferable (e.g., shorter) path might not be visible   from the head-end LSR by means of the IGP if the head-end LSR does   not belong to the same IGP area where the associated topology change   occurred, the head-end cannot make use of this shorter path (and   reroute the LSP using a make-before-break technique as described in   [RFC3209]) when appropriate.  Thus, a new mechanism specified in this   document is required to detect the existence of such a preferable   path and to notify the head-end LSR accordingly.   This document defines a mechanism that allows   - a head-end LSR to trigger on every LSR whose next hop is a loose     hop or an abstract node the re-evaluation of the current path in     order to detect a potentially more optimal path; and   - a mid-point LSR whose next hop is a loose-hop or an abstract node     to signal (using a new Error Value sub-code carried in a RSVP     PathErr message) to the head-end LSR that a preferable path exists     (a path with a lower cost, where the cost definition is determined     by some metric).   Once the head-end LSR has been notified of the existence of such a   preferable path, it can decide (depending on the TE LSP   characteristics) whether to perform a TE LSP graceful reoptimization   such as the "make-before-break" procedure.   There is another scenario whereby notifying the head-end LSR of the   existence of a better path is desirable: if the current path is about   to fail due to some (link or node) required maintenance.   This mechanism allows the head-end LSR to reoptimize a TE LSP by   making use of the non-disruptive make-before-break procedure if and   only if a preferable path exists and if such a reoptimization is   desired.Vasseur, et al.              Informational                      [Page 6]

RFC 4736               MPLS-TE Loosely Routed LSP          November 20065.  Signaling Extensions   A new flag in the SESSION ATTRIBUTE object and new Error Value sub-   codes in the ERROR SPEC object are proposed in this document.5.1.  Path Re-Evaluation Request   The following new flag of the SESSION_ATTRIBUTE object (C-Type 1 and   7) is defined:   Path re-evaluation request: 0x20   This flag indicates that a path re-evaluation (of the current path in   use) is requested.  Note that this does not trigger any LSP Reroute   but instead just signals a request to evaluate whether a preferable   path exists.   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.  However,   strictly speaking, the ERO has not changed.5.2.  New Error Value Sub-Codes   As defined in [RFC3209], the Error Code 25 in the ERROR SPEC object   corresponds to a Notify Error.   This document adds three new Error Value sub-codes:   6 Preferable path exists   7 Local link maintenance required   8 Local node maintenance required   The details about the local maintenance required modes are inSection6.3.2.6.  Mode of Operation6.1.  Head-End Reoptimization Control   The notification process of a preferable 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 preferable 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 LSR reoptimization.Vasseur, et al.              Informational                      [Page 7]

RFC 4736               MPLS-TE Loosely Routed LSP          November 20066.2.  Reoptimization Triggers   There are several 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 "Link-UP" event).   - Operator-driven: A reoptimization is manually triggered by the     Operator.   It is RECOMMENDED that an implementation supporting the extensions   proposed in this document support the aforementioned modes as path   re-evaluation triggers.6.3.  Head-End Request versus Mid-Point Explicit Notification Functions   This document defines two functions:   1) "Head-end requesting function": The request for a new path      evaluation of a loosely routed TE LSP is requested by the head-end      LSR.   2) "Mid-point explicit notification function": Having determined that      a preferable path (other than the current path) exists or having      the need to perform a link/node local maintenance, a mid-point LSR      explicitly notifies the head-end LSR, which will in turn decide      whether to perform a reoptimization.6.3.1.  Head-End Request Function   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 "Path re-evaluation   request" bit of the SESSION-ATTRIBUTE object set.  This bit is then   cleared in subsequent RSVP path messages sent downstream.  In order   to handle the case of a lost Path message, the solution consists of   relying on the reliable messaging mechanism described in [RFC2961].   Upon receiving a Path message with the "Path re-evaluation request"   bit set, every LSR for which the next abstract node contained in the   ERO is defined as a loose hop/abstract node performs the following   set of actions:Vasseur, et al.              Informational                      [Page 8]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006   A path re-evaluation is triggered, and the newly computed path is   compared to the existing path:   - If a preferable path can be found, the LSR performing the path re-     evaluation MUST immediately send an RSVP PathErr to the head-end     LSR (Error code 25 (Notify), Error sub-code=6 (better path     exists)).  At this point, the LSR MAY decide not to propagate this     bit in subsequent RSVP Path messages sent downstream for the re-     evaluated TE LSP; this mode is the RECOMMENDED mode for the reasons     described below.     The sending of an RSVP PathErr Notify message "Preferable path     exists" to the head-end LSR will notify the head-end LSR of the     existence of a preferable path (e.g., in a downstream area/AS or in     another location within a single domain).  Therefore, triggering     additional path re-evaluations on downstream nodes is unnecessary.     The only motivation to forward subsequent RSVP Path messages with     the "Path re-evaluation request" bit 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 preferable path can be found, the recommended mode is for an     LSR to relay the request (by setting the "Path re-evaluation" bit     of the SESSION-ATTRIBUTE object in RSVP path message sent     downstream).     Note that, by preferable path, we mean a path with a lower cost.     If the RSVP Path message with the "Path re-evaluation request" bit     set is lost, then the next request will be sent when the next     reoptimization trigger will occur on the head-end LSR.  The     solution to handle RSVP reliable messaging has been defined in     [RFC2961].     The network administrator may decide to establish some local policy     specifying to ignore such request or not to consider those requests     more frequently than at a certain rate.     The proposed mechanism does not make any assumption of the path     computation method performed by the ERO expansion process.Vasseur, et al.              Informational                      [Page 9]

RFC 4736               MPLS-TE Loosely Routed LSP          November 20066.3.2.  Mid-Point Explicit Notification   By contrast with the head-end request function, in this case, a mid-   point LSR whose next hop is a loose hop or an abstract node can   locally trigger a path re-evaluation when a configurable timer   expires, some specific events occur (e.g., link-up event), or the   user explicitly requests it.  If a preferable path is found, the LSR   sends an RSVP PathErr to the head-end LSR (Error code 25 (Notify),   Error sub-code=6 ("preferable path exists").   There is another circumstance whereby any mid-point LSR MAY send an   RSVP PathErr 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 LSR where a local maintenance   must be performed is responsible for sending an RSVP PathErr message   with Error code 25 and Error sub-code=7 or 8, depending on the   affected network element (link or node).  Then the first upstream   node that has 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 PathErr message MUST be immediately forwarded upstream to     the head-end LSR.  Note that in the case of TE LSP spanning     multiple administrative domains, it may be desirable for the     boundary LSR to modify the RSVP PathErr message and insert its own     address for confidentiality.   Upon receiving an RSVP PathErr message with Error code 25 and Error   sub-code 7 or 8, the head-end LSR SHOULD perform a TE LSP   reoptimization.   Note that the two functions (head-end and mid-point driven) are not   exclusive of each other: both the timer and event-driven   reoptimization triggers can be implemented on the head-end or on any   mid-point LSR with a potentially different timer value for the   timer-driven reoptimization case.   A head-end LSR MAY decide upon receiving an explicit mid-point   notification to delay its next path re-evaluation request.6.3.3.  ERO Caching   Once a mid-point LSR has determined that a preferable path exists   (after a reoptimization request has been received by the head-end LSR   or the reoptimization timer on the mid-point has expired), the more   optimal path MAY be cached on the mid-point LSR for a limited amountVasseur, et al.              Informational                     [Page 10]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006   of time to avoid having to recompute a path once the head-LSR   performs a make-before-break.  This mode is optional.  A default   value of 5 seconds for the caching timer is suggested.7.  Applicability and Interoperability   The procedures described in this document are entirely optional   within an MPLS or GMPLS network.  Implementations that do not support   the procedures described in this document will interoperate   seamlessly with those that do.  Further, an implementation that does   not support the procedures described in this document will not be   impacted or implicated by a neighboring implementation that does   implement the procedures.   An ingress implementation that chooses not to support the procedures   described in this document may still achieve re-optimization by   periodically issuing a speculative make-before-break replacement of   an LSP without trying to discovery whether a more optimal path is   available in a downstream domain.  Such a procedure would not be in   conflict with any mechanisms already documented in [RFC3209] and   [RFC3473].   An LSR not supporting the "Path re-evaluation request" bit of the   SESSION-ATTRIBUTE object SHALL forward it unmodified.   A head-end LSR not supporting an RSVP PathErr with Error code 25   message and Error sub-code = 6, 7, or 8 MUST just silently ignore   such an RSVP PathErr message.8.  IANA Considerations   IANA assigned three new error sub-code values for the RSVP PathErr   Notify message (Error code=25):   6 Preferable path exists   7 Local link maintenance required   8 Local node maintenance required9.  Security Considerations   This document defines a mechanism for a mid-point LSR to notify the   head-end LSR of the existence of a preferable path or the need to   reroute the TE LSP for maintenance purposes.  Hence, in the case of a   TE LSP spanning multiple administrative domains, it may be desirable   for a boundary LSR to modify the RSVP PathErr message (Code 25, Error   sub-code = 6, 7, or 8) so as to preserve confidentiality acrossVasseur, et al.              Informational                     [Page 11]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006   domains.  Furthermore, a head-end LSR may decide to ignore explicit   notification coming from a mid-point residing in another domain.   Similarly, an LSR may decide to ignore (or to accept up to a pre-   defined rate) path re-evaluation requests originated by a head-end   LSR of another domain.10.  Acknowledgements   The authors would like to thank Carol Iturralde, Miya Kohno, Francois   Le Faucheur, Philip Matthews, Jim Gibson, Jean-Louis Le Roux, Kenji   Kumaki, Anca Zafir, and Dimitri Papadimitriou for their useful   comments.  A special thanks to Adrian Farrel for his very valuable   inputs.11.  References11.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2961]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,              and S. Molendini, "RSVP Refresh Overhead Reduction              Extensions",RFC 2961, April 2001.   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP              Tunnels",RFC 3209, December 2001.   [RFC3473]  Berger, L., "Generalized Multi-Protocol Label Switching              (GMPLS) Signaling Resource ReserVation Protocol-Traffic              Engineering (RSVP-TE) Extensions",RFC 3473, January 2003.11.2.  Informative References   [RFC4105]  Le Roux, J.-L., Vasseur, J.-P., and J. Boyle,              "Requirements for Inter-Area MPLS Traffic Engineering",RFC 4105, June 2005.   [RFC4216]  Zhang, R. and J.-P. Vasseur, "MPLS Inter-Autonomous System              (AS) Traffic Engineering (TE) Requirements",RFC 4216,              November 2005.Vasseur, et al.              Informational                     [Page 12]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006Authors' Addresses   JP Vasseur (Editor)   Cisco Systems, Inc   1414 Massachusetts Avenue   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   Raymond Zhang   BT Infonet   2160 E. Grand Ave.   El Segundo, CA  90025   USA   EMail: raymond_zhang@bt.infonet.comVasseur, et al.              Informational                     [Page 13]

RFC 4736               MPLS-TE Loosely Routed LSP          November 2006Full Copyright Statement   Copyright (C) The IETF Trust (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,   AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights 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; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Vasseur, et al.              Informational                     [Page 14]