GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc5283

Network Working Group B. Decraene Request for Comments: 5283 JL. Le Roux Category: Standards Track France Telecom

                                                              I. Minei
                                                Juniper Networks, Inc.
                                                             July 2008
      LDP 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 Contents

 1. Introduction ....................................................2
 2. Conventions Used in This Document ...............................2
 3. Terminology .....................................................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. References ......................................................9
    9.1. Normative References .......................................9
    9.2. Informative References .....................................9
 10. Acknowledgments ...............................................11

Decraene, et al. Standards Track [Page 1] RFC 5283 LDP Extension for Inter-Area LSPs July 2008

1. 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 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.

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 2008

4. 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:
  1. prefix up when a new IP prefix appears in the RIB,
  1. prefix down when an existing IP prefix disappears,
  1. 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:
  1. 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.
  1. 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.
  1. 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 2008

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 | 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 LDP

Decraene, 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 2008

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 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:
  1. 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.
  1. 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 of

Decraene, 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:
  1. 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).
  1. 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. References

9.1. 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 2008

10. 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.net

Decraene, et al. Standards Track [Page 11] RFC 5283 LDP Extension for Inter-Area LSPs July 2008

Full 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.
 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 in BCP 78 and BCP 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 at
 http://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]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5283.txt · Last modified: 2008/07/18 19:38 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki