rfc3346.txt

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Network Working Group                                           J. Boyle
Request for Comments: 3346                                       PD Nets
Category: Informational                                          V. Gill
                                                   AOL Time Warner, Inc.
                                                               A. Hannan
                                                             RoutingLoop
                                                               D. Cooper
                                                         Global Crossing
                                                              D. Awduche
                                                          Movaz Networks
                                                            B. Christian
                                                                Worldcom
                                                                W.S. Lai
                                                                    AT&T
                                                             August 2002


       Applicability Statement for Traffic Engineering with MPLS

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.

Abstract

   This document describes the applicability of Multiprotocol Label
   Switching (MPLS) to traffic engineering in IP networks.  Special
   considerations for deployment of MPLS for traffic engineering in
   operational contexts are discussed and the limitations of the MPLS
   approach to traffic engineering are highlighted.















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RFC 3346    Applicability Statement for Traffic Engineering  August 2002


Table of Contents

   1.  Introduction....................................................2
   2.  Technical Overview of ISP Traffic Engineering...................3
   3.  Applicability of Internet Traffic Engineering...................4
   3.1 Avoidance of Congested Resources................................4
   3.2 Resource Utilization in Network Topologies with Parallel Links..5
   3.3 Implementing Routing Policies using Affinities..................5
   3.4 Re-optimization After Restoration...............................6
   4.  Implementation Considerations...................................6
   4.1 Architectural and Operational Considerations....................6
   4.2 Network Management Aspects......................................7
   4.3 Capacity Engineering Aspects....................................8
   4.4 Network Measurement Aspects.....................................8
   5.  Limitations.....................................................9
   6.  Conclusion.....................................................11
   7.  Security Considerations........................................11
   8.  References.....................................................11
   9.  Acknowledgments................................................12
   10. Authors' Addresses.............................................13
   11. Full Copyright Statement.......................................14

1. Introduction

   It is generally acknowledged that one of the most significant initial
   applications of Multiprotocol Label Switching (MPLS) is traffic
   engineering (TE) [1][2] in IP networks.  A significant community of
   IP service providers have found that traffic engineering of their
   networks can have tactical and strategic value [2, 3, 4].  To support
   the traffic engineering application, extensions have been specified
   for Interior Gateway Protocols (IGP) IS-IS [5] and OSPF [6], and to
   signaling protocols RSVP [7] and LDP [8].  The extensions for IS-IS,
   OSPF, and RSVP have all been developed and deployed in large scale in
   many networks consisting of multi-vendor equipment.

   This document discusses the applicability of TE to Internet service
   provider networks, focusing on the MPLS-based approach.  It augments
   the existing protocol applicability statements and, in particular,
   relates to the operational applicability of RSVP-TE [9].  Special
   considerations for deployment of MPLS in operational contexts are
   discussed and the limitations of this approach to traffic engineering
   are highlighted.









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RFC 3346    Applicability Statement for Traffic Engineering  August 2002


2. Technical Overview of ISP Traffic Engineering

   Traffic engineering (TE) is generally concerned with the performance
   optimization of operational networks [2].  In contemporary practice,
   TE means mapping IP traffic flows onto the existing physical network
   topology in the most effective way to accomplish desired operational
   objectives.  Techniques currently used to accomplish this include,
   but are not limited to:

          1.  Manipulation of IGP cost (metrics)
          2.  Explicit routing using constrained virtual-circuit
              switching techniques such as ATM or Frame Relay SPVCs
          3.  Explicit routing using constrained path setup techniques
              such as MPLS

   This document is concerned primarily with MPLS techniques.
   Specifically, it deals with the ability to use paths other than the
   shortest paths selected by the IGP to achieve a more balanced network
   utilization, e.g., by moving traffic away from IGP-selected shortest
   paths onto alternate paths to avoid congestion in the network.  This
   can be achieved by using explicitly signaled LSP-tunnels.  The
   explicit routes to be used may be computed offline and subsequently
   downloaded and configured on the routers using an appropriate
   mechanism.  Alternatively, the desired characteristics of an LSP
   (such as endpoints, bandwidth, affinities) may be configured on a
   router, which will then use an appropriate algorithm to compute a
   path through the network satisfying the desired characteristics,
   subject to various types of constraints.  Generally, the
   characteristics associated with LSPs may include:

          o  Ingress and egress nodes
          o  Bandwidth required
          o  Priority
          o  Nodes to include or exclude in the path
          o  Affinities to include or exclude in the path
          o  Resilience requirements

   Affinities are arbitrary, provider-assigned, attributes applied to
   links and carried in the TE extensions for the IGPs.  Affinities
   impose a class structure on links, which allow different links to be
   logically grouped together.  They can be used to implement various
   types of policies, or route preferences that allow the inclusion or
   exclusion of groups of links from the path of LSPs.  Affinities are
   unique to MPLS and the original requirement for them was documented
   in [2].






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3. Applicability of Internet Traffic Engineering

   As mentioned in [2] and [7], traffic engineering with MPLS is
   appropriate to establish and maintain explicitly routed paths in an
   IP network for effective traffic placement.  LSP-tunnels can be used
   to forward subsets of traffic through paths that are independent of
   routes computed by conventional IGP Shortest Path First (SPF)
   algorithms.  This gives network operators significant flexibility in
   controlling the paths of traffic flows across their networks and
   allows policies to be implemented that can result in the performance
   optimization of networks.  Examples of scenarios where MPLS-based TE
   capabilities are applicable in service provider environments are
   given below.  The applicability of MPLS is certainly not restricted
   to these scenarios.

3.1 Avoidance of Congested Resources

   In order to lower the utilization of congested link(s), an operator
   may utilize TE methods to route a subset of traffic away from those
   links onto less congested topological elements.  These types of
   techniques are viable when segments of the network are congested
   while other parts are underutilized.

   Operators who do not make extensive use of LSP-tunnels may adopt a
   tactical approach to MPLS TE in which they create LSP-tunnels only
   when necessary to address specific congestion problems.  For example,
   an LSP can be created between two nodes (source and destination) that
   are known to contribute traffic to a congested network element, and
   explicitly route the LSP through a separate path to divert some
   traffic away from the congestion.  On the other hand, operators who
   make extensive use of LSP-tunnels, either for measurement or
   automated traffic control, may decide to explicitly route a subset of
   the LSPs that traverse the point of congestion onto alternate paths.
   This can be employed to respond quickly when the bandwidth parameter
   associated with the LSPs does not accurately represent the actual
   traffic carried by the LSPs, and the operator determines that
   changing the bandwidth parameter values might not be effective in
   addressing the issue or may not have lasting impact.

   There are other approaches that measure traffic workloads on LSPs and
   utilize these empirical statistics to configure various
   characteristics of LSPs.  These approaches, for example, can utilize
   the derived statistics to configure explicit routes for LSPs (also
   known as offline TE [10]).  They can also utilize the statistics to
   set the values of various LSP attributes such as bandwidths,
   priority, and affinities (online TE).  All of these approaches can be
   used both tactically and strategically to react to periods of
   congestion in a network.  Congestion may occur as a result of many



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   factors: equipment or facility failure, longer than expected
   provisioning cycles for new circuits, and unexpected surges in
   traffic demand.

3.2 Resource Utilization in Network Topologies with Parallel Links

   In practice, many service provider networks contain multiple parallel
   links between nodes.  An example is transoceanic connectivity which
   is often provisioned as numerous low-capacity circuits, such as
   NxDS-3 (N parallel DS-3 circuits) and  NxSTM-1 (N parallel STM-1
   circuits).  Parallel circuits also occur quite often in bandwidth-
   constrained cities.  MPLS TE methods can be applied to effectively
   distribute the aggregate traffic workload across these parallel
   circuits.

   MPLS-based approaches commonly used in practice to deal with parallel
   links include using LSP bandwidth parameters to control the
   proportion of demand traversing each link, explicitly configuring
   routes for LSP-tunnels to distribute them across the parallel links,
   and using affinities to map different LSPs onto different links.
   These types of solutions are also applicable in networks with
   parallel and replicated topologies, such as an NxOC-3/12/48 topology.

3.3 Implementing Routing Policies using Affinities

   It is sometimes desirable to restrict certain types of traffic to
   certain types of links, or to explicitly exclude certain types of
   links in the paths for some types of traffic.  This might be needed
   to accomplish some business policy or network engineering objectives.
   MPLS resource affinities provide a powerful mechanism to implement
   these types of objectives.

   As a concrete example, suppose a global service provider has a flat