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Network Working Group                                        B. DecraeneRequest for Comments: 5283                                   JL. Le RouxCategory: Standards Track                                 France Telecom                                                                I. Minei                                                  Juniper Networks, Inc.                                                               July 2008LDP Extension for Inter-Area Label Switched Paths (LSPs)Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   To facilitate the establishment of Label Switched Paths (LSPs) that   would span multiple IGP areas in a given Autonomous System (AS), this   document describes a new optional Longest-Match Label Mapping   Procedure for the Label Distribution Protocol (LDP).   This procedure allows the use of a label if the Forwarding   Equivalence Class (FEC) Element matches an entry in the Routing   Information Base (RIB).  Matching is defined by an IP longest-match   search and does not mandate an exact match.Table of Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................23. Terminology .....................................................24. Problem Statement ...............................................35. Longest-Match Label Mapping Message Procedure ...................46. Application Examples ............................................66.1. Inter-Area LSPs ............................................66.2. Use of Static Routes .......................................77. Caveats for Deployment ..........................................87.1. Deployment Considerations ..................................87.2. Routing Convergence Time Considerations ....................88. Security Considerations .........................................99. References ......................................................99.1. Normative References .......................................99.2. Informative References .....................................910. Acknowledgments ...............................................11Decraene, et al.            Standards Track                     [Page 1]

RFC 5283           LDP Extension for Inter-Area LSPs           July 20081.  Introduction   Link state Interior Gateway Protocols (IGPs) such as OSPF [OSPFv2]   and IS-IS [IS-IS] allow the partition of an autonomous system into   areas or levels so as to increase routing scalability within a   routing domain.   However, [LDP] recommends that the IP address of the FEC Element   should *exactly* match an entry in the IP Routing Information Base   (RIB).  According to [LDP], section 3.5.7.1 ("Label Mapping Messages   Procedures"):      An LSR [Label Switching Router] receiving a Label Mapping message      from a downstream LSR for a Prefix SHOULD NOT use the label for      forwarding unless its routing table contains an entry that exactly      matches the FEC Element.   Therefore, MPLS LSPs between Label Edge Routers (LERs) in different   areas/levels are not set up unless the specific (e.g., /32 for IPv4)   loopback addresses of all the LERs are redistributed across all   areas.   The problem statement is discussed insection 4.  Then, insection 5   we extend the Label Mapping Procedure defined in [LDP] so as to   support the setup of contiguous inter-area LSPs while maintaining IP   prefix aggregation on the ABRs.  This consists of allowing for   longest-match-based Label Mapping.2.  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 [RFC2119].3.  Terminology   IGP Area: OSPF Area or IS-IS level   ABR: OSPF Area Border Router or IS-IS L1/L2 router   LSP: Label Switched Path   Intra-area LSP: LSP that does not traverse any IGP area boundary.   Inter-area LSP: LSP that traverses at least one IGP area boundary.Decraene, et al.            Standards Track                     [Page 2]

RFC 5283           LDP Extension for Inter-Area LSPs           July 20084.  Problem Statement   Provider-based MPLS (Multiprotocol Label Switching) networks are   expanding with the success of Layer 3 Virtual Private Networks   [L3-VPN] and the new deployments of Layer 2 VPNs ([VPLS-BGP],   [VPLS-LDP]).  Service providers' MPLS backbones are significantly   growing both in terms of density with the addition of Provider Edge   (PE) routers to connect new customers and in terms of footprint as   traditional Layer 2 aggregation networks may be replaced by IP/MPLS   networks.  As a consequence many providers need to introduce IGP   areas.  Inter-area LSPs (that is, LSPs that traverse at least two IGP   areas) are required to ensure MPLS connectivity between PEs located   in distinct IGP areas.   To set up the required MPLS LSPs between PEs in different IGP areas,   service providers currently have three solutions: 1) LDP with IGP   route leaking, 2) BGP [MPLS-BGP] over LDP with MPLS hierarchy, and 3)   inter-area RSVP-TE (Resource Reservation Protocol-Traffic Engineering   [RSVP-TE]).   IGP route leaking consists of redistributing all specific PE loopback   addresses across area boundaries.  As a result, LDP finds in the RIB   an exact match for its FEC and sets up the LSP.  As a consequence,   the potential benefits that a multi-area domain may yield are   significantly diminished since a lot of addresses have to be   redistributed by ABRs, and the number of IP entries in the IGP Link   State Database (LSDB), RIB, and Forwarding Information Base (FIB)   maintained by every LSR of the domain (whatever the area/level it   belongs to) cannot be minimized.   Service providers may also set up these inter-area LSPs by using MPLS   hierarchy with BGP [MPLS-BGP] as a label distribution protocol   between areas.  The BGP next hop would typically be the ABRs, and the   BGP-created LSPs would be nested within intra-area LSPs set up by LDP   between PEs and ABRs and between ABRs.   This solution is not adequate for service providers which don't want   to run BGP on their provider routers as it requires BGP on all ABRs.   In addition, MPLS hierarchy does not allow locally protecting the LSP   against ABR failures (IP/LDP Fast Reroute), and hence ensuring sub-   50ms recovery upon ABR failure.  The resulting convergence time may   not be acceptable for stringent Service Level Agreements (SLAs)   required for voice or mission-critical applications.  Finally, this   solution requires a significant migration effort for service   providers that started with LDP and IGP route leaking to quickly set   up the first inter-area LSPs.Decraene, et al.            Standards Track                     [Page 3]

RFC 5283           LDP Extension for Inter-Area LSPs           July 2008   Service providers may also set up these inter-area LSPs by using   inter-area RSVP-TE [RSVP-TE].  This is a relevant solution when RSVP-   TE is already used for setting up intra-area LSPs, and inter-area   traffic engineering features are required.  In return, this is not a   desired solution when LDP is already used for setting up intra-area   LSPs, and inter-area traffic engineering features are not required.   To avoid the above drawbacks, there is a need for an LDP-based   solution that allows setting up contiguous inter-area LSPs while   avoiding leaking of specific PE loopback addresses across area   boundaries, thereby keeping all the benefits of IGP hierarchy.   In that context, this document defines a new LDP Label Mapping   Procedure so as to support the setup of contiguous inter-area LSPs   while maintaining IP prefix aggregation on the ABRs.  This procedure   is similar to the one defined in [LDP] but performs an IP longest   match when searching the FEC element in the RIB.5.  Longest-Match Label Mapping Message Procedure   This document defines a new Label Mapping Procedure for [LDP].  It is   applicable to IPv4 and IPv6 prefix FEC elements (address families 1   and 2 as per the "Address Family Numbers" registry on the IANA site).   It SHOULD be possible to activate/deactivate this procedure by   configuration, and it SHOULD be deactivated by default.  It MAY be   possible to activate it on a per-prefix basis.   With this new Longest-Match Label Mapping Procedure, an LSR receiving   a Label Mapping message from a neighbor LSR for a Prefix Address FEC   Element FEC1 SHOULD use the label for MPLS forwarding if its routing   table contains an entry that matches the FEC Element FEC1 and the   advertising LSR is a next hop to reach FEC1.  If so, it SHOULD   advertise the received FEC Element FEC1 and a label to its LDP peers.   By "matching FEC Element", one should understand an IP longest match.   That is, either the LDP FEC element exactly matches an entry in the   IP RIB or the FEC element is a subset of an IP RIB entry.  There is   no match for other cases (i.e., if the FEC element is a superset of a   RIB entry, it is not considered a match).   Note that LDP re-advertises to its peers the specific FEC element   FEC1, and not the aggregated prefix found in the IP RIB during the   longest-match search.   Note that with this Longest-Match Label Mapping Procedure, each LSP   established by LDP still strictly follows the shortest path(s)   defined by the IGP.Decraene, et al.            Standards Track                     [Page 4]

RFC 5283           LDP Extension for Inter-Area LSPs           July 2008   FECs selected by this Longest-Match Label Mapping Procedure are   distributed in an ordered way.  In case of LER failure, the removal   of reachability to the FEC occurs using LDP ordered label   distribution mode procedures.  As defined in [LDP] in section A.1.5,   the FEC will be removed in an ordered way through the propagation of   Label Withdraw messages.  The use of this (un)reachability   information by application layers using this MPLS LSP (e.g.,   [MP-BGP]) is outside the scope of this document.   As per [LDP], LDP already has some interactions with the RIB.  In   particular, it needs to be aware of the following events:      - prefix up when a new IP prefix appears in the RIB,      - prefix down when an existing IP prefix disappears,      - next-hop change when an existing IP prefix has a new next hop        following a routing change.   With this Longest-Match Label Mapping Message Procedure, multiple   FECs may be concerned by a single RIB prefix change.  The LSR MUST   check all the FECs that are a subset of this RIB prefix.  So, some   LDP reactions following a RIB event are changed:      - When a new prefix appears in the RIB, the LSR MUST check if this        prefix is a better match for some existing FECs.  For example,        the FEC elements 192.0.2.1/32 and 192.0.2.2/32 used the IP RIB        entry 192.0.2.0/24, and a new more specific IP RIB entry        192.0.2.0/26 appears.  This may result in changing the LSR used        as next hop and hence the Next Hop Label Forwarding Entry        (NHLFE) for this FEC.      - When a prefix disappears in the RIB, the LSR MUST check all FEC        elements that are using this RIB prefix as best match.  For each        FEC, if another RIB prefix is found as best match, LDP MUST use        it.  This may result in changing the LSR used as next hop and        hence the NHLFE for this FEC.  Otherwise, the LSR MUST remove        the FEC binding and send a Label Withdraw message.      - When the next hop of a RIB prefix changes, the LSR MUST change        the NHLFE of all the FEC elements using this prefix.   Future work may define new management objects to the MPLS LDP MIB   modules [LDP-MIB] to activate/deactivate this Longest-Match Label   Mapping Message Procedure, possibly on a per-prefix basis.Decraene, et al.            Standards Track                     [Page 5]

RFC 5283           LDP Extension for Inter-Area LSPs           July 20086.  Application Examples6.1.  Inter-Area LSPs   Consider the following example of an autonomous system with one   backbone area and two edge areas:                        Area "B"                 Level 2 / Backbone area              +--------------------------+     Area "A" |                          |  Area "C"              |                          |     Level 1  |                          |  Level 1 / area              |        P1                |   +----------+                          +-------------+   |          |                 P2       |         PE1 | 192.0.2.1/32   |          |                          |             |   |PE4      ABR2                       ABR1       PE2 | 192.0.2.2/32   |          |        P3                |             |   |          |                          |         PE3 | 192.0.2.3/32   +----------+                          +-------------+              |                          |              +--------------------------+              Figure 1: An IGP domain with two areas                  attached to the Backbone Area.   Note that this applies equally to IS-IS and OSPF.  An ABR refers here   either to an OSPF ABR or to an IS-IS L1/L2 node.   All routers are MPLS enabled, and MPLS connectivity (i.e., an LSP) is   required between all PE routers.   In the "egress" area "C", the records available are:   IGP RIB                          LDP FEC elements:     192.0.2.1/32                      192.0.2.1/32     192.0.2.2/32                      192.0.2.2/32     192.0.2.3/32                      192.0.2.3/32   The area border router ABR1 advertises in the backbone area:      - the aggregated IP prefix 192.0.2.0/26 in the IGP      - all the specific IP FEC elements (/32) in LDPDecraene, et al.            Standards Track                     [Page 6]

RFC 5283           LDP Extension for Inter-Area LSPs           July 2008   In the "backbone" area "B", the records available are:   IGP RIB                          LDP FEC elements:     192.0.2.0/26                     192.0.2.1/32                                      192.0.2.2/32                                      192.0.2.3/32   The area border router ABR2 advertises in the area "A":      - an aggregated IP prefix 192.0.2.0/24 in the IGP      - all the individual IP FEC elements (/32) in LDP   In the "ingress" area "A", the records available are:   IGP RIB                          LDP FEC elements:     192.0.2.0/24                     192.0.2.1/32                                      192.0.2.2/32                                      192.0.2.3/32   In this situation, one LSP is established between the ingress PE4 and   every egress PE of area C while maintaining IP prefix aggregation on   the ABRs.6.2.  Use of Static Routes   Consider the following example where a LER is dual-connected to two   LSRs:                              +--------LSR1----                              |         |                             LER        |                              |         |                              +--------LSR2----                 Figure 2: LER dual-connected to two LSRs.   In some situations, especially on the edge of the network, it is   valid to use static IP routes between the LER and the two LSRs.  If   necessary, the Bidirectional Forwarding Detection protocol [BFD] can   be used to quickly detect loss of connectivity.   The LDP specification defined in [LDP] would require on the ingress   LER the configuration and the maintenance of one IP route per egress   LER and per outgoing interface.   The Longest-Match Label Mapping Procedure described in this document   only requires one IP route per outgoing interface.Decraene, et al.            Standards Track                     [Page 7]

RFC 5283           LDP Extension for Inter-Area LSPs           July 20087.  Caveats for Deployment7.1.  Deployment Considerations   LSRs compliant with this document are backward compatible with LSRs   that comply with [LDP].   For the successful establishment of end-to-end MPLS LSPs whose FECs   are aggregated in the RIB, this specification must be implemented on   all LSRs in all areas where IP aggregation is used.  If an LSR on the   path does not support this procedure, then the LSP initiated on the   egress LSR stops at this non-compliant LSR.  There are no other   adverse effects.   This extension can be deployed incrementally:      - It can be deployed on a per-area or per-routing-domain basis and        does not necessarily require an AS-wide deployment.  For        example, if all specific IP prefixes are leaked in the IGP        backbone area and only stub areas use IP aggregation, LSRs in        the backbone area don't need to be compliant with this document.      - Within each routing area, LSRs can be upgraded independently, at        any time, in any order, and without service disruption.  During        deployment, if those LSPs are already used, one should only make        sure that ABRs keep advertising the specific IP prefixes in the        IGP until all LSRs of this area are successfully upgraded.        Then, the ABRs can advertise the aggregated prefix only and stop        advertising the specific ones.   A service provider currently leaking specific LER loopback addresses   in the IGP and considering performing IP aggregation on ABR should be   aware that this may result in suboptimal routing as discussed in   [RFC2966].7.2.  Routing Convergence Time Considerations   IP and MPLS traffic restoration time is based on two factors: the   Shortest Path First (SPF) calculation in the control plane and   Forwarding Information Base (FIB) / Label FIB (LFIB) update time in   the forwarding plane.  The SPF calculation scales O(N*Log(N)) where N   is the number of Nodes.  The FIB/LFIB update scales O(P) where P is   the number of modified prefixes.  Currently, with most routers   implementations, the FIB/LFIB update is the dominant component   [IGP-CONV], and therefore the bottleneck that should be addressed in   priority.  The solution documented in this document reduces the link   state database size in the control plane and the number of FIB   entries in the forwarding plane.  As such, it solves the scaling ofDecraene, et al.            Standards Track                     [Page 8]

RFC 5283           LDP Extension for Inter-Area LSPs           July 2008   pure IP routers sharing the IGP with MPLS routers.  However, it does   not decrease the number of LFIB entries so is not sufficient to solve   the scaling of MPLS routers.  For this, an additional mechanism is   required (e.g., introducing some MPLS hierarchy in LDP).  This is out   of scope for this document.   Compared to [LDP], for all failures except LER failure (i.e., links,   provider routers, and ABRs), the failure notification and the   convergence is unchanged.  For LER failure, given that the IGP   aggregates IP routes on ABRs and no longer advertises specific   prefixes, the control plane and more specifically the routing   convergence behavior of protocols (e.g., [MP-BGP]) or applications   (e.g., [L3-VPN]) may be changed in case of failure of the egress LER   node.  For protocols and applications which need to track egress LER   availability, several solutions can be used, for example:   - Rely on the LDP ordered label distribution control mode -- as     defined in [LDP] -- to know the availability of the LSP toward the     egress LER.  The egress to ingress propagation time of that     unreachability information is expected to be comparable to the IGP     (but this may be implementation dependent).   - Advertise LER reachability in the IGP for the purpose of the     control plane in a way that does not create IP FIB entries in the     forwarding plane.8.  Security Considerations     The Longest-Match Label Mapping procedure described in this     document does not introduce any change as far as the Security     Considerations section of [LDP] is concerned.9.  References9.1.  Normative References   [LDP]         Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,                 "LDP Specification",RFC 5036, October 2007.   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate                 Requirement Levels",BCP 14,RFC 2119, March 1997.9.2.  Informative References   [L3-VPN]      Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private                 Networks (VPNs)",RFC 4364, February 2006.Decraene, et al.            Standards Track                     [Page 9]

RFC 5283           LDP Extension for Inter-Area LSPs           July 2008   [MP-BGP]      Bates, T., Chandra, R., Katz, D., and Y. Rekhter,                 "Multiprotocol Extensions for BGP-4",RFC 4760, January                 2007.   [MPLS-BGP]    Rekhter, Y. and E. Rosen, "Carrying Label Information                 in BGP-4",RFC 3107, May 2001.   [IS-IS]       Callon, R., "Use of OSI IS-IS for routing in TCP/IP and                 dual environments",RFC 1195, December 1990.   [VPLS-BGP]    Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual                 Private LAN Service (VPLS) Using BGP for Auto-Discovery                 and Signaling",RFC 4761, January 2007.   [VPLS-LDP]    Lasserre, M., Ed., and V. Kompella, Ed., "Virtual                 Private LAN Service (VPLS) Using Label Distribution                 Protocol (LDP) Signaling",RFC 4762, January 2007.   [RFC2966]     Li, T., Przygienda, T., and H. Smit, "Domain-wide                 Prefix Distribution with Two-Level IS-IS",RFC 2966,                 October 2000.   [RSVP-TE]     Farrel, A., Ed., Ayyangar, A., and JP. Vasseur,                 "Inter-Domain MPLS and GMPLS Traffic Engineering --                 Resource Reservation Protocol-Traffic Engineering                 (RSVP-TE) Extensions",RFC 5151, February 2008.   [LDP-MIB]     Cucchiara, J., Sjostrand, H., and J. Luciani,                 "Definitions of Managed Objects for the Multiprotocol                 Label Switching (MPLS), Label Distribution Protocol                 (LDP)",RFC 3815, June 2004.   [BFD]         Katz, D. and D. Ward, "Bidirectional Forwarding                 Detection", Work in Progress, March 2008.   [IGP-CONV]    Francois, P., Filsfils, C., and Evans, J., "Achieving                 sub-second IGP convergence in large IP networks".  ACM                 SIGCOMM Computer Communications Review, July 2005.   [OSPFv2]      Moy, J., "OSPF Version 2", STD 54,RFC 2328, April                 1998.Decraene, et al.            Standards Track                    [Page 10]

RFC 5283           LDP Extension for Inter-Area LSPs           July 200810.  Acknowledgments   The authors would like to thank Yakov Rekhter, Stefano Previdi, Vach   Kompella, Bob Thomas, Clarence Filsfils, Kireeti Kompella, Luca   Martini, Sina Mirtorabi, Dave McDysan, Benoit Fondeviole, Gilles   Bourdon, and Christian Jacquenet for the useful discussions on this   subject, their reviews, and comments.Authors' Addresses   Bruno Decraene   France Telecom   38 rue du General Leclerc   92794 Issy Moulineaux cedex 9   France   EMail: bruno.decraene@orange-ftgroup.com   Jean-Louis Le Roux   France Telecom   2, avenue Pierre-Marzin   22307 Lannion Cedex   France   EMail: jeanlouis.leroux@orange-ftgroup.com   Ina Minei   Juniper Networks   1194 N. Mathilda Ave.   Sunnyvale, CA 94089   EMail: ina@juniper.netDecraene, et al.            Standards Track                    [Page 11]

RFC 5283           LDP Extension for Inter-Area LSPs           July 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   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.Decraene, et al.            Standards Track                    [Page 12]