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RFC 3213 - Applicability Statement for CR-LDP

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Network Working Group                                             J. Ash
Request for Comments: 3213                                          AT&T
Category: Informational                                        M. Girish
                                                           Atoga Systems
                                                                 E. Gray
                                                             B. Jamoussi
                                                               G. Wright
                                                   Nortel Networks Corp.
                                                            January 2002

                   Applicability Statement for CR-LDP

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 Internet Society (2002).  All Rights Reserved.


   This document discusses the applicability of Constraint-Based LSP
   Setup using LDP.  It discusses possible network applications,
   extensions to Label Distribution Protocol (LDP) required to implement
   constraint-based routing, guidelines for deployment and known
   limitations of the protocol.  This document is a prerequisite to
   advancing CR-LDP on the standards track.

1. Introduction

   As the Internet evolves, additional capabilities are required to
   ensure proper treatment of data [3], voice, video and other delay
   sensitive traffic [4].  MPLS enhances source routing and allows for
   certain techniques, used in circuit switching, in IP networks.  This
   permits a scalable approach to handling these diverse transmission
   requirements.  CR-LDP [1] is a simple, scalable, open, non-
   proprietary, traffic engineering signaling protocol for MPLS IP

   CR-LDP provides mechanisms for establishing explicitly routed Label
   Switched Paths (LSPs).  These mechanisms are defined as extensions to
   LDP [2].  Because LDP is a peer-to-peer protocol based on the

   establishment and maintenance of TCP sessions, the following natural
   benefits exist:

      CR-LDP messages are reliably delivered by the underlying TCP, and
      State information associated with explicitly routed LSPs does not
      require periodic refresh.

      CR-LDP messages are flow controlled (throttled) through TCP.

   CR-LDP is defined for the specific purpose of establishing and
   maintaining explicitly routed LSPs.  Additional optional capabilities
   included have minimal impact on system performance and requirements
   when not in use for a specific explicitly routed LSP.  Optional
   capabilities provide for negotiation of LSP services and traffic
   management parameters over and above best-effort packet delivery
   including bandwidth allocation, setup and holding priorities.  CR-LDP
   optionally allows these parameters to be dynamically modified without
   disruption of the operational (in-service) LSP [4].

   CR-LDP allows the specification of a set of parameters to be signaled
   along with the LSP setup request.  Moreover, the network can be
   provisioned with a set of edge traffic conditioning functions (which
   could include marking, metering, policing and shaping).  This set of
   parameters along with the specification of edge conditioning
   functions can be shown to be adequate and powerful enough to
   describe, characterize and parameterize a wide variety of QoS
   scenarios and services including IP differentiated services [5],
   integrated services [6], ATM service classes [7], and frame relay

   CR-LDP is designed to adequately support the various media types that
   MPLS was designed to support (ATM, FR, Ethernet, PPP, etc.).  Hence,
   it will work equally well for Multi-service switched networks, router
   networks, or hybrid networks.

   This applicability statement does not preclude the use of other
   signaling and label distribution protocols for the traffic
   engineering application in MPLS based networks.  Service providers
   are free to deploy whatever signaling protocol meets their needs.

   In particular CR-LDP and RSVP-TE [9] are two signaling protocols that
   perform similar functions in MPLS networks.  There is currently no
   consensus on which protocol is technically superior.  Therefore,
   network administrators should make a choice between the two based
   upon their needs and particular situation.  Applicability of RSVP-TE
   is described in [10].

2. Applicability of extensions to LDP

   To provide support of additional LSP services, CR-LDP extensions are
   defined in such a way as to be directly translatable to objects and
   messages used in other protocols defined to provide similar services
   [9].  Implementations can take advantage of this fact to:

      Setup LSPs for provision of an aggregate service associated with
      the services being provided via these other protocols.

      Directly translate protocol messages to provide services defined
      in a non-CR-LDP portion of the network.

      Describe, characterize and parameterize a wide variety of QoS
      scenarios and services including IP differentiated services,
      integrated services, ATM service classes, and frame relay.

   Steady state information required for proper maintenance of an LSP
   may be as little as 200 bytes or less.  It is not unreasonable to
   anticipate that CR-LDP implementations may support in excess of one
   hundred thousand or one million LSPs switched through a single Label
   Switching Router (LSR) under fairly stable conditions.

   Because CR-LDP provides for low overhead per LSP - both in terms of
   needed state information and control traffic - CR-LDP is applicable
   in those portions of the Internet where very large numbers of LSPs
   may need to be switched at each LSR.  An example of this would be
   large backbone networks using MPLS exclusively to transport very
   large numbers of traffic streams between a moderately large number of
   MPLS edge nodes.

   CR-LDP may also be applicable as a mediating service between networks
   providing similar service extensions using widely varying signaling

3. Implementation and deployment considerations in relation to LDP

   LDP specifies the following label distribution and management modes
   (which can be combined in various logical ways described in LDP):

      . Downstream On Demand label distribution
      . Downstream Unsolicited label distribution
      . Independent Label Distribution Control
      . Ordered Label Distribution Control
      . Conservative Label Retention Mode
      . Liberal Label Retention Mode

   The applicability of LDP is described in [11].

   In networks where only Traffic Engineered LSPs are required, the CR-
   LDP implementation and deployment does NOT require all the
   functionality defined in the LDP specification.  The basic Discovery,
   Session, and Notification messages are required.  However, CR-LDP
   requires one specific combination of the label distribution modes:

      . Downstream On Demand Ordered label distribution and
        conservative Label Retention Mode

   Although CR-LDP is defined as an extension to LDP, support for
   Downstream Unsolicited Label Advertisement and Independent Control
   modes is not required for support of Strict Explicit Routes.  In
   addition, implementations of CR-LDP MAY be able to support Loose
   Explicit Routes via the use of 'Abstract Nodes' and/or 'Hierarchical
   Explicit Routing', without using LDP for hop-by-hop LSP setup.

   CR-LDP also includes support for loose explicit routes.  Use of this
   capability allows the network operator to define an 'explicit path'
   through portions of their network with imperfect knowledge of the
   entire network topology.  Proper use of this capability may also
   allow CR-LDP implementations to inter-operate with 'vanilla' LDP
   implementations - particularly if it is desired to set up an
   explicitly routed LSP for best-effort packet delivery via a loosely
   defined path.

   Finally, in networks where both Routing Protocol-driven LSPs (a.k.a.
   hop-by-hop LSPs) and Traffic Engineered LSPs are required, a single
   protocol (LDP, with the extensions defined in CR-LDP) can be used for
   both TE and Hop-by-Hop LSPs.  New protocols do not have to be
   introduced in the network to provide TE-LSP signaling.

4. Limitations

   CR-LDP specification only supports point-to-point LSPs.  Multi-
   point-to-point and point-to-multi-point are for further study (FFS).

   CR-LDP specification only supports unidirectional LSP setup.  Bi-
   directional LSP setup is FFS.

   CR-LDP specification only supports a unique label allocation per LSP
   setup.  Multiple label allocations per LSP setup are FFS.

5. Security Considerations

   No additional security issues are introduced in this document.  As an
   extension to LDP, CR-LDP shares the security concerns associated with

6. Acknowledgements

   The authors would like to thank the following people for their
   careful review of the document and their comments: Loa Andersson,
   Peter Ashwood-Smith, Anoop Ghanwani, Juha Heinanen, Jon Weil and
   Adrian Farrel.

7. References

   [1]  Jamoussi, B., Andersson, L., Callon, R., Dantu, R., Wu, L.,
        Doolan, P., Worster, T., Feldman, N., Fredette, A., Girish, M.,
        Gray, E., Heinanen, J., Kilty, T. and A. Malis, "Constraint-
        based LSP Setup Using LDP", RFC 3212, January 2002.

   [2]  Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
        Thomas, "LDP Specification", RFC 3036, January 2001.

   [3]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
        McManus, "Requirements for Traffic Engineering Over MPLS", RFC
        2702, September 1999.

   [4]  Ash, B., Lee, Y., Ashwood-Smith, P., Jamoussi, B., Fedyk, D.,
        Skalecki, D. and L. Li, "LSP Modification using CR-LDP", RFC
        3214, January 2002.

   [5]  Blake S., Black, D., Carlson, M., Davies, E., Wang, Z. and W.
        Weiss, "An Architecture for Differentiated Services", RFC 2475,
        December 1998.

   [6]  Shenker, S. and  J. Wroclawski, "General Characterization
        Parameters for Integrated Service Network Elements", RFC 2215,
        September 1997.

   [7]  ATM Forum Traffic Management Specification Version 4.1 (AF-TM-
        0121.000), March 1999.

        SERVICE, ITU (CCITT) Recommendation I.370, 1991.

   [9]  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.

   [10] Awduche, D., Hannan, A. and X. Xiao, "Applicability Statement
        for Extensions to RSVP for LSP-Tunnels", RFC 3210, December

   [11] Thomas, B. and E. Gray, "LDP Applicability", RFC 3037, January

8. Author's Addresses

   Gerald R. Ash
   Room MT D5-2A01
   200 Laurel Avenue
   Middletown, NJ 07748
   Phone: 732-420-4578
   Fax:   732-368-8659
   EMail: gash@att.com

   Eric Gray
   600 Federal Drive
   Andover, MA  01810
   Phone: (978) 689-1610
   EMail: eric.gray@sandburst.com

   Gregory Wright
   Nortel Networks Corp.
   P O Box 3511 Station C
   Ottawa, ON K1Y 4H7
   Phone: +1 613 765-7912
   EMail: gwright@nortelnetworks.com

   M. K. Girish
   Atoga Systems
   49026 Milmont Drive
   Fremont, CA 94538
   EMail: muckai@atoga.com

   Bilel Jamoussi
   Nortel Networks Corp.
   600 Technology Park Drive
   Billerica, MA 01821
   phone: +1 978-288-4506
   EMail: Jamoussi@nortelnetworks.com

9. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an


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   Internet Society.


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