draft-vasseur-mpls-loose-path-reopt-00.txt January 2003 Jean-Philippe Vasseur(Editor) Cisco Systems, Inc. Yuichi Ikejiri NTT Communications Corporation IETF Internet Draft Expires: July, 2003 January, 2003 draft-vasseur-mpls-loose-path-reopt-00.txt Reoptimization of an explicit loosely routed MPLS TE 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-mpls-loose-path-reopt-00.txt January 2003 Abstract The aim of this document is to propose a mechanism for the reoptimization of loosely routed explicit paths. 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). The path calculation (ERO expansion) to reach a loose hop is made on the previous hop defined in the TE LSP path. This draft proposes a mechanism that allows - the TE LSP Head-end LSR to trigger a reoptimization on every loose hops along the path, - an LSR to signal to the TE LSP head-end that a better path exists to reach a loose than the path in use. A better path is defined as a path with a lower cost, where the cost is defined by the metric used to compute the path. This primarily applies to inter-area TE when the path is defined as a list of loose hops (generally the loose hops are the ABRs) but the following mechanism is also applicable to any loosely routed explicit paths within a single routing domain. 1. Establishment of an explicit loosely routed TE LSP path 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 specified in the ERO object of the Path message with the L flag of the Ipv4 prefix sub-object set, as defined in RFC3209. In this case, each LSR along path can perform a partial route computation to reach the next loose hop and then performs an ERO expand, before forwarding the RSVP Path message downstream. Note that the examples in the rest of this draft will be 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, this mechanism could also be used in the context of loosely routed paths in the context of TE LSPs spanning several autonomous systems. Also examples will be provided with OSPF as the IGP but the mechanisms similarly apply to IS-IS. An example of an explicit loosely routed TE LSP signalling (see also [MULTI-AREA-TE scenario 1] <---area 1--><-area 0--><-area 2-> R1---R2----R3---R6 R8-----R10 | | | / |\ | | | | -- | --\ | | | |/ | \| |---R4----R5---R7----R9-----R11 Vasseur and Ikejiri 2 draft-vasseur-mpls-loose-path-reopt-00.txt January 2003 Assumptions - R3, R5, R8 and R9 are ABRs - A TE LSP1 from R1 (Head-End LSR) to R11 (Tail-end LSR) is defined with the following loosely routed path: R1-R3-R8-R11. R3, R8 and R11 are defined as loose hops. Step 1: the TE LSP 1 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 LSP constraint 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 expand 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. ... 2. Reoptimization of an explicit loosely routed TE LSP Once the TE LSP is set up, the TE LSP 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). Currently if the better path is not visible from the Head-end LSR, it cannot make use of this better path and perform a make before break when appropriate. This is for instance the case in the example above as the better path appears in an area, which is not the Head-end area. This draft 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 it can perform a make before break. Vasseur and Ikejiri 3 draft-vasseur-mpls-loose-path-reopt-00.txt January 2003 3. Signalling extensions 3.1. ERO expansion signaling request For EXPLICIT ROUTE object C-Type 1 and 7, we define a new flag in the SESSION_ATTRIBUTE object: ERO expansion request: 0x20 This flag indicates to every LSR having a loose hops specified as their next hop in the ERO that a new ERO expansion is requested. 3.2. New Path Error sub-code The format of a Path Error is the following: <PathErr message> ::= <Common Header> [ <INTEGRITY> ] <SESSION> <ERROR_SPEC> [ <POLICY_DATA> ...] [ <sender descriptor> ] <sender descriptor> ::= (see earlier definition) 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: 4 Better path exists 5 No better path 4. Mode of operation 4.1. TE LSP reroute Vasseur and Ikejiri 4 draft-vasseur-mpls-loose-path-reopt-00.txt January 2003 The TE LSP reroute (make before break) is always considered as being performed on the Head-end LSR. 4.2. Reoptimization triggers There are two possible reoptimization triggers: - timer-based: a reoptimization is triggered (look for a more optimal path) when a configurable timer expires, - event-driven: a reoptimization is triggered when a particular event happens (such as a ''Link-UP'' event). 4.3. Head-end reoptimization request versus mid-point reoptimization indication The need for reoptimization (a better path exists) of an explicit loosely routed TE LSP can be either: - requested by the Head-end LSR, - determined by any mid-point LSR whose next hop is a loose hop having detected that a better path (than the existing path) exists. 4.3.1. Head-end reoptimization request In this mode, when a timer-based reoptimization is triggered on the head-end LSR (reoptimization timer has fired) or the operator manually requests a reoptimization, the head-end LSR immediately sends a Path message with the ''ERO expansion request'' bit of the SESSION_ATTRIBUTE object set. Upon receiving a Path message with the ''ERO expansion request'' bit of the SESSION_ATTRIBUTE object set, every LSR, for which the next abstract node contained in the ERO is defined as a loose hop, must perform a new ERO expansion (path re evaluation): - if a better path can be found to reach the next loose hop (than the path currently in use), the LSR must immediately send a Path Error to the head-end (Error code 25 (Notify), sub- code=4 (better path exists)), - if no better path can be found, the LSR must send a Path Error to the head-end (Error code 25 (Notify), sub-code=5 (no better path)) By better path, we mean a path having a lower cost. By default, an LSR uses the IGP metric in their CSPF to detect the shortest path that obeys a set of constraints. Note that the head-end 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. Let call Ln the list of LSRs defined as loose hops in the ERO sent in the Path message by the Head-end LSR: Ln=<l1, l2, ..., ln>. Let's now Vasseur and Ikejiri 5 draft-vasseur-mpls-loose-path-reopt-00.txt January 2003 call Pn=<p1, p2, ..., pn> the list of LSRs pi such that li is a next (loose) hop of pi for i=1...n Example 2: <---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-R8-R11. R3, R8 and R11 are defined as loose hops. Ln=<R3,R8,R11> Pn=<R1,R3,R8> As soon as a positive response is received from an LSR pi (sub-code=4, ''Better path exists''), the Head-end LSR must: - clear the ''ERO expansion request'' bit, - perform a make before break If no positive response is received then the Head-end LSR must keep refreshing the current TE LSP. The Head-end LSR must not clear the ''ERO expansion request'' bit until: - all LSRs <p1, ..pn> have responded to the request, or - at least 4 consecutive Path messages have been sent with this bit set. Note that if RSVP Path messages are sent in reliable mode (see RFC2961), then this number of 4 can be reduced to 1. 4.3.2. Mid-point reoptimization indication In this mode, an 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)). If a better path is found compared to the existing one, the LSR sends a Path Error to the head-end (Error code 25 (Notify), sub-code=4 (better path exists)). The Head-end LSR must then immediately perform a make before break. 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. Vasseur and Ikejiri 6 draft-vasseur-mpls-loose-path-reopt-00.txt January 2003 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 retopimization timer on the mid-point has fired), the more optimal path should be cached on the mid-point for a limited amount of time to avoid having to recompute a route once the Head-LSR performs a make before break. 5. Interoperability An LSR non supporting the ''ERO expansion request'' bit of the SESSION- ATTRIBUTE object should just ignore it. Any Head-end LSR non supporting this draft receiving a Path Error Notify message with sub-code = 4 or 5 should just ignore the Path message. 6. Security Considerations The practice described in this draft does not raise specific security issues beyond those of existing TE. 7. Acknowledgment The authors would like to thank Carol Iturralde and Miya Khono for their useful and valuable comments. 8. Intellectual Property The contributor represents that he has disclosed the existence of any proprietary or intellectual property rights in the contribution that are reasonably and personally known to the contributor. The contributor does not represent that he personally knows of all potentially pertinent proprietary and intellectual property rights owned or claimed by the organization he represents (if any) or third parties. References [TE-REQ] Awduche et al, Requirements for Traffic Engineering over MPLS, RFC2702, September 1999. Vasseur and Ikejiri 7 draft-vasseur-mpls-loose-path-reopt-00.txt January 2003 [OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft- katz-yeung-ospf-traffic-05.txt, June 2001. [ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft- ietf-isis-traffic-03.txt, June 2001. [RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC3209, December 2001. [METRICS] Fedyk et al, ''Multiple Metrics for Traffic Engineering with IS-IS and OSPF'', draft-fedyk-isis-ospf-te-metrics-01.txt, November 2000. [DS-TE] Le Faucheur et al, ''Requirements for support of Diff-Serv-aware MPLS Traffic Engineering'', draft-ietf-tewg-diff-te-reqts-01.txt, June 2001. [MULTI-AREA-TE] Kompella at all, ''Multi-area MPLS Traffic Engineering'', draft-kompella-mpls-multiarea-te-03.txt, June 2002. [PATH-COMP] Vasseur et al, ''RSVP Path computation request and reply messages'', draft-vasseur-mpls-computation-rsvp-03.txt, November 2001. [SEC-METRIC] Le Faucheur et all,'' Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering Metric'', draft-ietf- tewg-te-metric-igp-02.txt, September, 2002. Authors' addresses: Jean Philippe Vasseur Cisco Systems, Inc. 300 Apollo Drive Chelmsford, MA 01824 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 Vasseur and Ikejiri 8