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LDP Extension for Inter-Area Label Switched Paths (LSPs)

The information below is for an old version of the document that is already published as an RFC.
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This is an older version of an Internet-Draft that was ultimately published as RFC 5283.
Authors Ina Minei , Jean-Louis Le Roux , Bruno Decraene
Last updated 2018-12-20 (Latest revision 2008-06-18)
Replaces draft-decraene-mpls-ldp-interarea
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Network Working Group                                    B. Decraene 
  Internet Draft                                          J.L. Le Roux 
  Document: draft-ietf-mpls-ldp-interarea-04.txt        France Telecom 
  Intended status: Standards Track                                     
  Expiration Date: December 2008                              I. Minei 
                                                Juniper Networks, Inc. 
                                                             June 2008 
                    LDP extension for Inter-Area LSP 
Status of this Memo 
   By submitting this Internet-Draft, each author represents that any 
   applicable patent or other IPR claims of which he or she is aware 
   have been or will be disclosed, and any of which he or she becomes 
   aware will be disclosed, in accordance with Section 6 of BCP 79. 
   Internet-Drafts are working documents of the Internet Engineering 
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   To facilitate the establishment of Label Switched Paths (LSP) 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 table 
   (RIB). Matching is defined by an IP longest match search and does not 
   mandate an exact match. 

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Table of Contents 
   1.    Conventions used in this document...........................2 
   2.    Terminology.................................................2 
   3.    Introduction................................................2 
   4.    Problem statement...........................................3 
   5.    Longest Match Label Mapping Message Procedure...............4 
   6.    Application examples........................................6 
   6.1.  Inter-area LSPs.............................................6 
   6.2.  Use of static routes........................................7 
   7.    Caveats for deployment......................................8 
   7.1.  Deployment considerations...................................8 
   7.2.  Routing convergence time considerations.....................8 
   8.    Security Considerations.....................................9 
   9.    IANA Considerations.........................................9 
   10.   References..................................................9 
   10.1. Normative References........................................9 
   10.2. Informative References......................................9 
   11.   Acknowledgments............................................10 
   12.   Authors' Addresses.........................................11 
1. Conventions used in this document 
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 
   document are to be interpreted as described in [RFC 2119]. 
2. 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. 
3. 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. 

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   However, [LDP] recommends that the IP address of the FEC Element 
   should *exactly* match an entry in the IP RIB: according to [LDP] 
   section (Label Mapping Messages Procedures) "A Label 
   Switching Router  (LSR) 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 setup unless the specific (e.g. /32 for IPv4) 
   loopback addresses of all the LERs are redistributed across all 
   The problem statement is discussed in section 4. Then, in section 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. 
4.      Problem statement 
   Provider based MPLS (Multi Protocol Label Switching) networks are 
   expanding with the success of Layer 3 VPN (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 two aggregation networks may be replaced by IP/MPLS 
   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 have currently 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 
   IGP route leaking consists in redistributing all specific PE loopback 
   addresses across area boundaries. As a result, LDP finds in the 
   Routing Information Base (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 FIB maintained 
   by every LSR of the domain (whatever the area/level it belongs to) 
   cannot be minimized. 

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   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 setup 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 P 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 which started with LDP and IGP route leaking to quickly 
   set-up the first inter-area LSPs. 
   Service providers may also setup 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 which allows setting up contiguous inter-area LSPs while 
   avoiding leaking of specific PE loopback addresses across area 
   boundaries, and hence 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 [ASSIGNED_AF]). 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. 

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   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 such as the FEC element is a superset of a RIB 
   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. 
   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 has already 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 which 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. E.g. the FEC 
        elements and used the IP RIB entry and a new more specific IP RIB entry 
        appears. This may result in changing the LSR used as next hop 
        and hence the Next Hop Label Forwarding Entry (NHLFE) for this 
     - When a prefix disappears in the RIB, the LSR MUST check all FEC 
        elements which 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. 

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   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. 
6. Application examples 
6.1. 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 | 
      |          |                          |             | 
      |PE4      ABR2                       ABR1       PE2 | 
      |          |        P3                |             | 
      |          |                          |         PE3 | 
      +----------+                          +-------------+ 
                 |                          | 
     Figure 1: An IGP domain with two areas attached to the Backbone 
   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:                                     
   The area border router ABR1 advertises in the backbone area: 
     - the aggregated IP prefix in the IGP 
     - all the specific IP FEC elements (/32) in LDP 
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   In the "backbone" area "B", the records available are: 
   IGP RIB                          LDP FEC elements:            
   The area border router ABR2 advertises in the area "A": 
     - an aggregated IP prefix 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:            
   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 ASBRs. 
6.2. Use of static routes 
   Consider the following example where a LER is dual-connected to two 
                    |         | 
                   LER        | 
                    |         | 
                 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 BFD 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. 

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7. Caveats for deployment 
7.1. 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 FEC 
   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 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 LSR 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's loopback 
   addresses in the IGP and now considering performing IP aggregation on 
   ABR should be aware that this may result in suboptimal routing as 
   discussed in [RFC 2966]. 
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 [1] 
   and therefore the bottleneck that should be addressed in priority. 
   The solution documented in this draft 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 of 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 of this 
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   Compared to [LDP], for all failures except LER failure (i.e. links, 
   Ps and ABRs nodes), the failure notification and the convergence is 
   unchanged. For LER failure, given that the IGP aggregates IP routes 
   on ABRs and no longer advertise 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 dependant). 
   - Advertise LER reachability in the IGP for the purpose of the 
     control plane in a way that do 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 Consideration 
   section of [LDP] is concerned. 
9. IANA Considerations 
   This document has no actions for IANA. 
10.     References 
10.1.   Normative References 
     [LDP]     Andersson, L., Minei, I., Thomas, B., "LDP 
          Specification", RFC 5036, October 2007 
     [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 
          Requirement Levels", RFC 2119, March 1997 
10.2.   Informative References 
     [L3-VPN]  Rosen, E., Rekhter, Y. ," BGP/MPLS IP Virtual Private 
          Networks (VPNs) ", RFC 4374, February 2006 
     [MP-BGP]  Bates, T., Chandra, R., Katz, D., Rekhter, Y., 
          "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007 
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     [MPLS-BGP] Rekhter, Y., Rosen, E., "Carrying Label Information in 
          BGP-4", RFC 3107, May 2001 
     [OSPFv2]  Moy, J.,"OSPF Version 2", RFC 2328, April 1998 
     [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., Rekhter, Y., "Virtual Private LAN Service 
          (VPLS) Using BGP for Auto-discovery and Signaling", RFC 4761, 
          January 2007. 
     [VPLS-LDP] Lasserre, M., Kompella, V., "Virtual Private LAN Service 
          (VPLS) Using Label Distribution Protocol (LDP) Signaling", RFC 
          4762, January 2007. 
     [RFC 2966] Li, T., Przygienda, T., Smit, H., "Domain-wide Prefix 
          Distribution with Two-Level IS-IS", RFC 2966, October 2000. 
     [RSVP-TE] Farrel, Ayyangar, Vasseur, "Inter domain MPLS and GMPLS 
          Traffic Engineering - RSVP-TE extensions", RFC 5151, February 
     [LDP-MIB] Cucchiara, J., Sjostrand, H., Luciani, J., "Definitions 
          of Managed Objects for the Multiprotocol Label Switching 
          (MPLS), Label Distribution Protocol (LDP)", RFC 3815, June 
     [BFD] Katz, D., Ward, D., "Bidirectional Forwarding Detection", 
          draft-ietf-bfd-base-08.txt, March 2008. 
     [1] Francois, P., Filsfils, C., and Evans, J. 2005. "Achieving sub-
          second IGP convergence in large IP networks". ACM SIGCOMM 
          Computer Communications Review, July 2005 
11.     Acknowledgments 
   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 review and comments. 

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12.     Authors' Addresses 
   Bruno Decraene 
   France Telecom 
   38 rue du General Leclerc 
   92794 Issy Moulineaux cedex 9 
   Jean-Louis Le Roux 
   France Telecom 
   2, avenue Pierre-Marzin 
   22307 Lannion Cedex 
   Ina Minei 
   Juniper Networks 
   1194 N. Mathilda Ave. 
   Sunnyvale, CA 94089 
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Copyright Statement 
   Copyright (C) The IETF Trust (2008). 
   This document is subject to the rights, licenses and restrictions 
   contained in BCP 78, and except as set forth therein, the authors 
   retain all their rights. 
Disclaimer of Validity 
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   Funding for the RFC Editor function is currently provided by the 
   Internet Society. 

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