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

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.

   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.

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].

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

   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
   (non-hierarchical) IGP.  MPLS TE affinities can be used to explicitly
   keep continental traffic (traffic originating and terminating within
   a continent) from traversing transoceanic resources.

   Another example of using MPLS TE affinities to exclude certain
   traffic from a subset of circuits might be to keep inter-regional
   LSPs off of circuits that are reserved for intra-regional traffic.

   Still another example is the situation in a heterogeneous network
   consisting of links with different capacities, e.g., OC-12, OC-48,
   and OC-192.  In such networks, affinities can be used to force some
   types of traffic to only traverse links with a given capacity, e.g.
   OC-48.

3.4 Re-optimization After Restoration

   After the occurrence of a network failure, it may be desirable to
   calculate a new set paths for LSPs to optimizes performance over the
   residual topology.  This re-optimization is complementary to the
   fast-reroute operation used to reduce packet losses during routing
   transients under network restoration.  Traffic protection can also be
   accomplished by associating a primary LSP with a set of secondary
   LSPs, hot-standby LSPs, or a combination thereof [11].

4. Implementation Considerations

4.1 Architectural and Operational Considerations

   When deploying TE solutions in a service provider environment, the
   impact of administrative policies and the selection of nodes that
   will serve as endpoints for LSP-tunnels should be carefully
   considered.  As noted in [9], when devising a virtual topology for
   LSP-tunnels, special consideration should be given to the tradeoff
   between the operational complexity associated with a large number of
   LSP-tunnels and the control granularity that large numbers of LSP-
   tunnels allow.  In other words, a large number of LSP-tunnels allow
   greater control over the distribution of traffic across the network,
   but increases network operational complexity.  In large networks, it
   may be advisable to start with a simple LSP-tunnel virtual topology
   and then introduce additional complexity based on observed or
   anticipated traffic flow patterns [9].

   Administrative policies should guide the amount of bandwidth to be
   allocated to an LSP.  One may choose to set the bandwidth of a
   particular LSP to a statistic of the measured observed utilization
   over an interval of time, e.g., peak rate, or a particular percentile
   or quartile of the observed utilization.  Sufficient over-
   subscription (of LSPs) or under-reporting bandwidth on the physical
   links should be used to account for flows that exceed their normal
   limits on an event-driven basis.  Flows should be monitored for
   trends that indicate when the bandwidth parameter of an LSP should be
   resized.  Flows should be monitored constantly to detect unusual
   variance from expected levels.  If an unpoliced flow greatly exceeds
   its assigned bandwidth, action should be taken to determine the root
   cause and remedy the problem.  Traffic policing is an option that may
   be applied to deal with congestion problems, especially when some
   flows exceed their bandwidth parameters and interfere with other
   compliant flows.  However, it is usually more prudent to apply
   policing actions at the edge of the network rather than within the
   core, unless under exceptional circumstances.

   When creating LSPs, a hierarchical network approach may be used to
   alleviate scalability problems associated with flat full mesh virtual
   topologies.  In general, operational experience has shown that very
   large flows (between city pairs) are long-lived and have stable
   characteristics, while smaller flows (edge to edge) are more dynamic
   and have more fluctuating statistical characteristics.  A
   hierarchical architecture can be devised consisting of core and edge
   networks in which the core is traffic engineered and serves as an
   aggregation and transit infrastructure for edge traffic.

   However, over-aggregation of flows can result in a stream so large
   that it precludes the constraint-based routing algorithm from finding
   a feasible path through a network.  Splitting a flow by using two or
   more parallel LSPs and distributing the traffic across the LSPs can
   solve this problem, at the expense of introducing more state in the
   network.

   Failure scenarios should also be addressed when splitting a stream of
   traffic over several links.  It is of little value to establish a
   finely balanced set of flows over a set of links only to find that
   upon link failure the balance reacts poorly, or does not revert to
   the original situation upon restoration.

4.2 Network Management Aspects

   Networks planning to deploy MPLS for traffic engineering must
   consider network management aspects, particularly performance and
   fault management [12].  With the deployment of MPLS in any
   infrastructure, some additional operational tasks are required, such
   as constant monitoring to ensure that the performance of the network
   is not impacted in the end-to-end delivery of traffic.  In addition,
   traffic characteristics, such as latency across an LSP, may also need
   to be assessed on a regular basis to ensure that service-level
   guarantees are achieved.

   Obtaining information on LSP behavior is critical in determining the
   stability of an MPLS network.  When LSPs transition or path changes
   occur, packets may be dropped which impacts network performance.  It
   should be the goal of any network deploying MPLS to minimize the
   volatility of LSPs and reduce the root causes that induce this
   instability.  Unfortunately, there are very few, if any, NMS systems
   that are available at this time with the capability to provide the
   correct level of management support, particularly root cause
   analysis.  Consequently, most early adopters of MPLS develop their
   own management systems in-house for the MPLS domain.  The lack of
   availability of commercial network management systems that deal
   specifically with MPLS-related aspects is a significant impediment to
   the large-scale deployment of MPLS networks.

   The performance of an MPLS network is also dependent on the
   configured values of bandwidth for each LSP.  Since congestion is a
   common cause of performance degradation in operational networks, it
   is important to proactively avoid these situations.  While MPLS was
   designed to minimize congestion on links by utilizing bandwidth
   reservations, it is still heavily reliant on user configurable data.
   If the LSP bandwidth value does not properly represent the traffic
   demand of that LSP, over-utilization may occur and cause significant
   congestion within the network.  Therefore, it is important to
   develop, deploy, and maintain a good modeling tool for determining
   LSP bandwidth size.  Lack of this capability may result in sub-
   optimal network performance.

4.3 Capacity Engineering Aspects

   Traffic engineering has a goal of ensuring traffic performance
   objectives for different services.  This requires that the different
   network elements be dimensioned properly to handle the expected load.
   More specifically, in mapping given user demands onto network
   resources, network dimensioning involves the sizing of the network
   elements, such as links, processors, and buffers, so that performance
   objectives can be met at minimum cost.  Major inputs to the
   dimensioning process are cost models, characterization of user
   demands and specification of performance objectives.

   In using MPLS, dimensioning involves the assignment of resources such
   as bandwidth to a set of pre-selected LSPs for carrying traffic, and
   mapping the logical network of LSPs onto a physical network of links
   with capacity constraints.  The dimensioning process also determines
   the link capacity parameters or thresholds associated with the use of
   some bandwidth reservation scheme for service protection.  Service
   protection controls the QoS for certain service types by restricting
   access to bandwidth, or by giving priority access to one type of
   traffic over another.  Such methods are essential, e.g., to prevent
   starvation of low-priority flows, to guarantee a minimum amount of
   resources for flows with expected short duration, to improve the
   acceptance probability for flows with high bandwidth requirements, or
   to maintain network stability by preventing performance degradation
   in case of a local overload.

4.4 Network Measurement Aspects

   Network measurement entails robust statistics collection and systems
   development.  Knowing *what* to do with these measurements is often
   where the secret-sauce is.  Examples for different applications of
   measurements are described in [13].  For instance, to ensure that the
   QoS objectives have been met, performance measurements and
   performance monitoring are required so that real-time traffic control

   actions, or policy-based actions, can be taken.  Also, to
   characterize the traffic demands, traffic measurements are used to
   estimate the offered loads from different service classes and to
   provide forecasting of future demands for capacity planning purposes.
   Forecasting and planning may result in capacity augmentation or may
   lead to the introduction of new technology and architecture.

   To avoid QoS degradation at the packet level, measurement-based
   admission control can be employed by using online measurements of
   actual usage.  This is a form of preventive control to ensure that
   the QoS requirements of different service classes can be met
   simultaneously, while maintaining network efficiency at a high level.
   However, it requires proper network dimensioning to keep the
   probability for the refusal of connection/flow requests sufficiently
   low.

5. Limitations

   Significant resources can be expended to gain a proper understanding
   of how MPLS works.  Furthermore, significant engineering and testing
   resources may need to be invested to identify problems with vendor
   implementations of MPLS.  Initial deployment of MPLS software and the
   configurations management aspects to support TE can consume
   significant engineering, operations, and system development
   resources.  Developing automated systems to create router
   configurations for network elements can require significant software
   development and hardware resources.  Getting to a point where
   configurations for routers are updated in an automated fashion can be
   a time consuming process.  Tracking manual tweaks to router
   configurations, or problems associated with these can be an endless
   task.  What this means is that much more is required in the form of
   various types of tools to simplify and automate the MPLS TE function.

   Certain architectural choices can lead to operational, protocol, and
   router implementation scalability problems.  This is especially true
   as the number of LSP-tunnels or router configuration data in a
   network increases, which can be exacerbated by designs incorporating
   full meshes, which create O(N^2) number of LSPs, where N is the
   number of network-edge nodes.  In these cases, minimizing N through
   hierarchy, regionalization, or proper selection of tunnel termination
   points can affect the network's ability to scale.  Loss of scale in
   this sense can be via protocol instability, inability to change
   network configurations to accommodate growth, inability for router
   implementations to be updated, hold or properly process
   configurations, or loss of ability to adequately manage the network.

   Although widely deployed, MPLS TE is a new technology when compared
   to the classic IP routing protocols such as IS-IS, OSPF, and BGP.
   MPLS TE also has more configuration and protocol options.  As such,
   some implementations are not battle-hardened and automated testing of
   various configurations is difficult if not infeasible.  Multi-vendor
   environments are beginning to appear, although additional effort is
   usually required to ensure full interoperability.

   Common approaches to TE in service provider environments switch the
   forwarding paradigm from connectionless to connection oriented.
   Thus, operational analysis of the network may be complicated in some
   regards (and improved in others).  Inconsistencies in forwarding
   state result in dropped packets whereas with connectionless methods
   the packet will either loop and drop, or be misdirected onto another
   branch in the routing tree.

   Currently deployed MPLS TE approaches can be adversely affected by
   both internal and external router and link failures.  This can create
   a mismatch between the signaled capacity and the traffic an LSP-
   tunnel carries.

   Many routers in service provider environments are already under
   stress processing the software workload associated with running IGP,
   BGP, and IPC.  Enabling TE in an MPLS environment involves adding
   traffic engineering databases and processes, adding additional
   information to be carried by the routing processes, and adding
   signaling state and processing to these network elements.  Additional
   traffic measurements may also need to be supported.  In some
   environments, this additional load may not be feasible.

   MPLS in general and MPLS-TE in particular is not a panacea for lack
   of network capacity, or lack of proper capacity planning and
   provisioning in the network dimensioning process.  MPLS-TE may cause
   network traffic to traverse greater distances or to take paths with
   more network elements, thereby incurring greater latency.  Generally,
   this added inefficiency is done to prevent shortcomings in capacity
   planning or available resources path to avoid hot spots.  The ability
   of TE to accommodate more traffic on a given topology can also be
   characterized as a short-term gain during periods of persistent
   traffic growth.  These approaches cannot achieve impossible mappings
   of traffic onto topologies.  Failure to properly capacity plan and
   execute will lead to congestion, no matter what technology aids are
   employed.

6. Conclusion

   The applicability of traffic engineering in Internet service provider
   environments has been discussed in this document.  The focus has been
   on the use of MPLS-based approaches to achieve traffic engineering in
   this context.  The applicability of traffic engineering and
   associated management and deployment considerations have been
   described, and the limitations highlighted.

   MPLS combines the ability to monitor point-to-point traffic
   statistics between two routers and the capability to control the
   forwarding paths of subsets of traffic through a given network
   topology.  This makes traffic engineering with MPLS applicable and
   useful for improving network performance by effectively mapping
   traffic flows onto links within service provider networks.  Tools
   that simplify and automate the MPLS TE functions and activation help
   to realize the full potential.

7. Security Considerations

   This document does not introduce new security issues.  When deployed
   in service provider networks, it is mandatory to ensure that only
   authorized entities are permitted to initiate establishment of LSP-
   tunnels.

8. References

   1  Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label
      Switching Architecture," RFC 3031, January 2001.

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

   3  X. Xiao, A. Hannan, B. Bailey, and L. Ni, "Traffic Engineering
      with MPLS in the Internet," IEEE Network, March/April 2000.

   4  V. Springer, C. Pierantozzi, and J. Boyle, "Level3 MPLS Protocol
      Architecture," Work in Progress.

   5  T. Li, and H. Smit, "IS-IS Extensions for Traffic Engineering,"
      Work in Progress.

   6  D. Katz, D. Yeung, and K. Kompella, "Traffic Engineering
      Extensions to OSPF," Work in Progress.

   7  Awduche, D., Berger, L., Gan, D.H., Li, T., Srinivasan, V. and G.
      Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels," RFC 3209,
      December 2001.

   8  Jamoussi, B. (Editor), "Constraint-Based LSP Setup using LDP," RFC
      3212, January 2002.

9 Awduche, D., Hannan, A. and X. Xiao, "Applicability Statement for
Extensions to RSVP for LSP-Tunnels," RFC 3210, December 2001.

   10 Awduche, D., Chiu, A., Elwalid, A., Widjaja, I. and X. Xiao,
      "Overview and Principles of Internet Traffic Engineering", RFC
      3272, May 2002.

   11 W.S. Lai, D. McDysan, J. Boyle, M. Carlzon, R. Coltun, T.
      Griffin, E. Kern, and T. Reddington, "Network Hierarchy and
      Multilayer Survivability," Work in Progress.

12 D. Awduche, "MPLS and Traffic Engineering in IP Networks," IEEE
Communications Magazine, December 1999.

   13 W.S. Lai, B. Christian, R.W. Tibbs, and S. Van den Berghe, "A
      Framework for Internet Traffic Engineering Measurement," Work in
      Progress.

9. Acknowledgments

   The effectiveness of the MPLS protocols for traffic engineering in
   service provider networks is in large part due to the experience
   gained and foresight given by network engineers and developers
   familiar with traffic engineering with ATM in these environments.  In
   particular, the authors wish to acknowledge the authors of RFC 2702
   for the clear articulation of the requirements, as well as the
   developers and testers of code in deployment today for keeping their
   focus.

10. Authors' Addresses

   Jim Boyle
   Protocol Driven Networks
   Tel: +1 919-852-5160
   EMail: jboyle@pdnets.com

   Vijay Gill
   AOL Time Warner, Inc.
   12100 Sunrise Valley Drive
   Reston, VA 20191
   EMail: vijay@umbc.edu

   Alan Hannan
   RoutingLoop Intergalactic
   112 Falkirk Court
   Sunnyvale, CA 94087, USA
   Tel: +1 408-666-2326
   EMail: alan@routingloop.com

   Dave Cooper
   Global Crossing
   960 Hamlin Court
   Sunnyvale, CA 94089, USA
   Tel: +1 916-415-0437
   EMail: dcooper@gblx.net

   Daniel O. Awduche
   Movaz Networks
   7926 Jones Branch Drive, Suite 615
   McLean, VA 22102, USA
   Tel: +1 703-298-5291
   EMail: awduche@movaz.com

   Blaine Christian
   Worldcom
   22001 Loudoun County Parkway, Room D1-2-737
   Ashburn, VA 20147, USA
   Tel: +1 703-886-4425
   EMail: blaine@uu.net

   Wai Sum Lai
   AT&T
   200 Laurel Avenue
   Middletown, NJ 07748, USA
   Tel: +1 732-420-3712
   EMail: wlai@att.com

11. Full Copyright Statement

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Funding for the RFC Editor function is currently provided by the
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