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rfc:rfc3272

Network Working Group D. Awduche Request for Comments: 3272 Movaz Networks Category: Informational A. Chiu

                                                       Celion Networks
                                                            A. Elwalid
                                                            I. Widjaja
                                                   Lucent Technologies
                                                               X. Xiao
                                                      Redback Networks
                                                              May 2002
      Overview and Principles of Internet Traffic Engineering

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 memo describes the principles of Traffic Engineering (TE) in the
 Internet.  The document is intended to promote better understanding
 of the issues surrounding traffic engineering in IP networks, and to
 provide a common basis for the development of traffic engineering
 capabilities for the Internet.  The principles, architectures, and
 methodologies for performance evaluation and performance optimization
 of operational IP networks are discussed throughout this document.

Table of Contents

 1.0 Introduction...................................................3
    1.1 What is Internet Traffic Engineering?.......................4
    1.2 Scope.......................................................7
    1.3 Terminology.................................................8
 2.0 Background....................................................11
    2.1 Context of Internet Traffic Engineering....................12
    2.2 Network Context............................................13
    2.3 Problem Context............................................14
       2.3.1 Congestion and its Ramifications......................16
    2.4 Solution Context...........................................16
       2.4.1 Combating the Congestion Problem......................18
    2.5 Implementation and Operational Context.....................21

Awduche, et. al. Informational [Page 1] RFC 3272 Overview and Principles of Internet TE May 2002

 3.0 Traffic Engineering Process Model.............................21
    3.1 Components of the Traffic Engineering Process Model........23
    3.2 Measurement................................................23
    3.3 Modeling, Analysis, and Simulation.........................24
    3.4 Optimization...............................................25
 4.0 Historical Review and Recent Developments.....................26
    4.1 Traffic Engineering in Classical Telephone Networks........26
    4.2 Evolution of Traffic Engineering in the Internet...........28
       4.2.1 Adaptive Routing in ARPANET...........................28
       4.2.2 Dynamic Routing in the Internet.......................29
       4.2.3 ToS Routing...........................................30
       4.2.4 Equal Cost Multi-Path.................................30
       4.2.5 Nimrod................................................31
    4.3 Overlay Model..............................................31
    4.4 Constraint-Based Routing...................................32
    4.5 Overview of Other IETF Projects Related to Traffic
        Engineering................................................32
       4.5.1 Integrated Services...................................32
       4.5.2 RSVP..................................................33
       4.5.3 Differentiated Services...............................34
       4.5.4 MPLS..................................................35
       4.5.5 IP Performance Metrics................................36
       4.5.6 Flow Measurement......................................37
       4.5.7 Endpoint Congestion Management........................37
    4.6 Overview of ITU Activities Related to Traffic
        Engineering................................................38
    4.7 Content Distribution.......................................39
 5.0 Taxonomy of Traffic Engineering Systems.......................40
    5.1 Time-Dependent Versus State-Dependent......................40
    5.2 Offline Versus Online......................................41
    5.3 Centralized Versus Distributed.............................42
    5.4 Local Versus Global........................................42
    5.5 Prescriptive Versus Descriptive............................42
    5.6 Open-Loop Versus Closed-Loop...............................43
    5.7 Tactical vs Strategic......................................43
 6.0 Recommendations for Internet Traffic Engineering..............43
    6.1 Generic Non-functional Recommendations.....................44
    6.2 Routing Recommendations....................................46
    6.3 Traffic Mapping Recommendations............................48
    6.4 Measurement Recommendations................................49
    6.5 Network Survivability......................................50
       6.5.1 Survivability in MPLS Based Networks..................52
       6.5.2 Protection Option.....................................53
    6.6 Traffic Engineering in Diffserv Environments...............54
    6.7 Network Controllability....................................56
 7.0 Inter-Domain Considerations...................................57
 8.0 Overview of Contemporary TE Practices in Operational
     IP Networks...................................................59

Awduche, et. al. Informational [Page 2] RFC 3272 Overview and Principles of Internet TE May 2002

 9.0 Conclusion....................................................63
 10.0 Security Considerations......................................63
 11.0 Acknowledgments..............................................63
 12.0 References...................................................64
 13.0 Authors' Addresses...........................................70
 14.0 Full Copyright Statement.....................................71

1.0 Introduction

 This memo describes the principles of Internet traffic engineering.
 The objective of the document is to articulate the general issues and
 principles for Internet traffic engineering; and where appropriate to
 provide recommendations, guidelines, and options for the development
 of online and offline Internet traffic engineering capabilities and
 support systems.
 This document can aid service providers in devising and implementing
 traffic engineering solutions for their networks.  Networking
 hardware and software vendors will also find this document helpful in
 the development of mechanisms and support systems for the Internet
 environment that support the traffic engineering function.
 This document provides a terminology for describing and understanding
 common Internet traffic engineering concepts.  This document also
 provides a taxonomy of known traffic engineering styles.  In this
 context, a traffic engineering style abstracts important aspects from
 a traffic engineering methodology.  Traffic engineering styles can be
 viewed in different ways depending upon the specific context in which
 they are used and the specific purpose which they serve.  The
 combination of styles and views results in a natural taxonomy of
 traffic engineering systems.
 Even though Internet traffic engineering is most effective when
 applied end-to-end, the initial focus of this document document is
 intra-domain traffic engineering (that is, traffic engineering within
 a given autonomous system).  However, because a preponderance of
 Internet traffic tends to be inter-domain (originating in one
 autonomous system and terminating in another), this document provides
 an overview of aspects pertaining to inter-domain traffic
 engineering.
 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 RFC 2119.

Awduche, et. al. Informational [Page 3] RFC 3272 Overview and Principles of Internet TE May 2002

1.1. What is Internet Traffic Engineering?

 Internet traffic engineering is defined as that aspect of Internet
 network engineering dealing with the issue of performance evaluation
 and performance optimization of operational IP networks.  Traffic
 Engineering encompasses the application of technology and scientific
 principles to the measurement, characterization, modeling, and
 control of Internet traffic [RFC-2702, AWD2].
 Enhancing the performance of an operational network, at both the
 traffic and resource levels, are major objectives of Internet traffic
 engineering.  This is accomplished by addressing traffic oriented
 performance requirements, while utilizing network resources
 economically and reliably.  Traffic oriented performance measures
 include delay, delay variation, packet loss, and throughput.
 An important objective of Internet traffic engineering is to
 facilitate reliable network operations [RFC-2702].  Reliable network
 operations can be facilitated by providing mechanisms that enhance
 network integrity and by embracing policies emphasizing network
 survivability.  This results in a minimization of the vulnerability
 of the network to service outages arising from errors, faults, and
 failures occurring within the infrastructure.
 The Internet exists in order to transfer information from source
 nodes to destination nodes.  Accordingly, one of the most significant
 functions performed by the Internet is the routing of traffic from
 ingress nodes to egress nodes.  Therefore, one of the most
 distinctive functions performed by Internet traffic engineering is
 the control and optimization of the routing function, to steer
 traffic through the network in the most effective way.
 Ultimately, it is the performance of the network as seen by end users
 of network services that is truly paramount.  This crucial point
 should be considered throughout the development of traffic
 engineering mechanisms and policies.  The characteristics visible to
 end users are the emergent properties of the network, which are the
 characteristics of the network when viewed as a whole.  A central
 goal of the service provider, therefore, is to enhance the emergent
 properties of the network while taking economic considerations into
 account.
 The importance of the above observation regarding the emergent
 properties of networks is that special care must be taken when
 choosing network performance measures to optimize.  Optimizing the
 wrong measures may achieve certain local objectives, but may have

Awduche, et. al. Informational [Page 4] RFC 3272 Overview and Principles of Internet TE May 2002

 disastrous consequences on the emergent properties of the network and
 thereby on the quality of service perceived by end-users of network
 services.
 A subtle, but practical advantage of the systematic application of
 traffic engineering concepts to operational networks is that it helps
 to identify and structure goals and priorities in terms of enhancing
 the quality of service delivered to end-users of network services.
 The application of traffic engineering concepts also aids in the
 measurement and analysis of the achievement of these goals.
 The optimization aspects of traffic engineering can be achieved
 through capacity management and traffic management.  As used in this
 document, capacity management includes capacity planning, routing
 control, and resource management.  Network resources of particular
 interest include link bandwidth, buffer space, and computational
 resources.  Likewise, as used in this document, traffic management
 includes (1) nodal traffic control functions such as traffic
 conditioning, queue management, scheduling, and (2) other functions
 that regulate traffic flow through the network or that arbitrate
 access to network resources between different packets or between
 different traffic streams.
 The optimization objectives of Internet traffic engineering should be
 viewed as a continual and iterative process of network performance
 improvement and not simply as a one time goal.  Traffic engineering
 also demands continual development of new technologies and new
 methodologies for network performance enhancement.
 The optimization objectives of Internet traffic engineering may
 change over time as new requirements are imposed, as new technologies
 emerge, or as new insights are brought to bear on the underlying
 problems.  Moreover, different networks may have different
 optimization objectives, depending upon their business models,
 capabilities, and operating constraints.  The optimization aspects of
 traffic engineering are ultimately concerned with network control
 regardless of the specific optimization goals in any particular
 environment.
 Thus, the optimization aspects of traffic engineering can be viewed
 from a control perspective.  The aspect of control within the
 Internet traffic engineering arena can be pro-active and/or reactive.
 In the pro-active case, the traffic engineering control system takes
 preventive action to obviate predicted unfavorable future network
 states.  It may also take perfective action to induce a more
 desirable state in the future.  In the reactive case, the control
 system responds correctively and perhaps adaptively to events that
 have already transpired in the network.

Awduche, et. al. Informational [Page 5] RFC 3272 Overview and Principles of Internet TE May 2002

 The control dimension of Internet traffic engineering responds at
 multiple levels of temporal resolution to network events.  Certain
 aspects of capacity management, such as capacity planning, respond at
 very coarse temporal levels, ranging from days to possibly years.
 The introduction of automatically switched optical transport networks
 (e.g., based on the Multi-protocol Lambda Switching concepts) could
 significantly reduce the lifecycle for capacity planning by
 expediting provisioning of optical bandwidth.  Routing control
 functions operate at intermediate levels of temporal resolution,
 ranging from milliseconds to days.  Finally, the packet level
 processing functions (e.g., rate shaping, queue management, and
 scheduling) operate at very fine levels of temporal resolution,
 ranging from picoseconds to milliseconds while responding to the
 real-time statistical behavior of traffic.  The subsystems of
 Internet traffic engineering control include: capacity augmentation,
 routing control, traffic control, and resource control (including
 control of service policies at network elements).  When capacity is
 to be augmented for tactical purposes, it may be desirable to devise
 a deployment plan that expedites bandwidth provisioning while
 minimizing installation costs.
 Inputs into the traffic engineering control system include network
 state variables, policy variables, and decision variables.
 One major challenge of Internet traffic engineering is the
 realization of automated control capabilities that adapt quickly and
 cost effectively to significant changes in a network's state, while
 still maintaining stability.
 Another critical dimension of Internet traffic engineering is network
 performance evaluation, which is important for assessing the
 effectiveness of traffic engineering methods, and for monitoring and
 verifying compliance with network performance goals.  Results from
 performance evaluation can be used to identify existing problems,
 guide network re-optimization, and aid in the prediction of potential
 future problems.
 Performance evaluation can be achieved in many different ways.  The
 most notable techniques include analytical methods, simulation, and
 empirical methods based on measurements.  When analytical methods or
 simulation are used, network nodes and links can be modeled to
 capture relevant operational features such as topology, bandwidth,
 buffer space, and nodal service policies (link scheduling, packet
 prioritization, buffer management, etc.).  Analytical traffic models
 can be used to depict dynamic and behavioral traffic characteristics,
 such as burstiness, statistical distributions, and dependence.

Awduche, et. al. Informational [Page 6] RFC 3272 Overview and Principles of Internet TE May 2002

 Performance evaluation can be quite complicated in practical network
 contexts.  A number of techniques can be used to simplify the
 analysis, such as abstraction, decomposition, and approximation.  For
 example, simplifying concepts such as effective bandwidth and
 effective buffer [Elwalid] may be used to approximate nodal behaviors
 at the packet level and simplify the analysis at the connection
 level.  Network analysis techniques using, for example, queuing
 models and approximation schemes based on asymptotic and
 decomposition techniques can render the analysis even more tractable.
 In particular, an emerging set of concepts known as network calculus
 [CRUZ] based on deterministic bounds may simplify network analysis
 relative to classical stochastic techniques.  When using analytical
 techniques, care should be taken to ensure that the models faithfully
 reflect the relevant operational characteristics of the modeled
 network entities.
 Simulation can be used to evaluate network performance or to verify
 and validate analytical approximations.  Simulation can, however, be
 computationally costly and may not always provide sufficient
 insights.  An appropriate approach to a given network performance
 evaluation problem may involve a hybrid combination of analytical
 techniques, simulation, and empirical methods.
 As a general rule, traffic engineering concepts and mechanisms must
 be sufficiently specific and well defined to address known
 requirements, but simultaneously flexible and extensible to
 accommodate unforeseen future demands.

1.2. Scope

 The scope of this document is intra-domain traffic engineering; that
 is, traffic engineering within a given autonomous system in the
 Internet.  This document will discuss concepts pertaining to intra-
 domain traffic control, including such issues as routing control,
 micro and macro resource allocation, and the control coordination
 problems that arise consequently.
 This document will describe and characterize techniques already in
 use or in advanced development for Internet traffic engineering.  The
 way these techniques fit together will be discussed and scenarios in
 which they are useful will be identified.
 While this document considers various intra-domain traffic
 engineering approaches, it focuses more on traffic engineering with
 MPLS.  Traffic engineering based upon manipulation of IGP metrics is
 not addressed in detail.  This topic may be addressed by other
 working group document(s).

Awduche, et. al. Informational [Page 7] RFC 3272 Overview and Principles of Internet TE May 2002

 Although the emphasis is on intra-domain traffic engineering, in
 Section 7.0, an overview of the high level considerations pertaining
 to inter-domain traffic engineering will be provided.  Inter-domain
 Internet traffic engineering is crucial to the performance
 enhancement of the global Internet infrastructure.
 Whenever possible, relevant requirements from existing IETF documents
 and other sources will be incorporated by reference.

1.3 Terminology

 This subsection provides terminology which is useful for Internet
 traffic engineering.  The definitions presented apply to this
 document.  These terms may have other meanings elsewhere.
  1. Baseline analysis:

A study conducted to serve as a baseline for comparison to

          the actual behavior of the network.
  1. Busy hour:

A one hour period within a specified interval of time

          (typically 24 hours) in which the traffic load in a network
          or sub-network is greatest.
  1. Bottleneck:

A network element whose input traffic rate tends to be

          greater than its output rate.
  1. Congestion:

A state of a network resource in which the traffic incident

          on the resource exceeds its output capacity over an interval
          of time.
  1. Congestion avoidance:

An approach to congestion management that attempts to

          obviate the occurrence of congestion.
  1. Congestion control:

An approach to congestion management that attempts to remedy

          congestion problems that have already occurred.
  1. Constraint-based routing:

A class of routing protocols that take specified traffic

          attributes, network constraints, and policy constraints into
          account when making routing decisions.  Constraint-based
          routing is applicable to traffic aggregates as well as
          flows.  It is a generalization of QoS routing.

Awduche, et. al. Informational [Page 8] RFC 3272 Overview and Principles of Internet TE May 2002

  1. Demand side congestion management:

A congestion management scheme that addresses congestion

          problems by regulating or conditioning offered load.
  1. Effective bandwidth:

The minimum amount of bandwidth that can be assigned to a

          flow or traffic aggregate in order to deliver 'acceptable
          service quality' to the flow or traffic aggregate.
  1. Egress traffic:

Traffic exiting a network or network element.

  1. Hot-spot:

A network element or subsystem which is in a state of

          congestion.
  1. Ingress traffic:

Traffic entering a network or network element.

  1. Inter-domain traffic:

Traffic that originates in one Autonomous system and

          terminates in another.
  1. Loss network:

A network that does not provide adequate buffering for

          traffic, so that traffic entering a busy resource within the
          network will be dropped rather than queued.
  1. Metric:

A parameter defined in terms of standard units of

          measurement.
  1. Measurement Methodology:

A repeatable measurement technique used to derive one or

          more metrics of interest.
  1. Network Survivability:

The capability to provide a prescribed level of QoS for

          existing services after a given number of failures occur
          within the network.
  1. Offline traffic engineering:

A traffic engineering system that exists outside of the

          network.

Awduche, et. al. Informational [Page 9] RFC 3272 Overview and Principles of Internet TE May 2002

  1. Online traffic engineering:

A traffic engineering system that exists within the network,

          typically implemented on or as adjuncts to operational
          network elements.
  1. Performance measures:

Metrics that provide quantitative or qualitative measures of

          the performance of systems or subsystems of interest.
  1. Performance management:

A systematic approach to improving effectiveness in the

          accomplishment of specific networking goals related to
          performance improvement.
  1. Performance Metric:

A performance parameter defined in terms of standard units

          of measurement.
  1. Provisioning:

The process of assigning or configuring network resources to

          meet certain requests.
  1. QoS routing:

Class of routing systems that selects paths to be used by a

          flow based on the QoS requirements of the flow.
  1. Service Level Agreement:

A contract between a provider and a customer that guarantees

          specific levels of performance and reliability at a certain
          cost.
  1. Stability:

An operational state in which a network does not oscillate

          in a disruptive manner from one mode to another mode.
  1. Supply side congestion management:

A congestion management scheme that provisions additional

          network resources to address existing and/or anticipated
          congestion problems.
  1. Transit traffic:

Traffic whose origin and destination are both outside of the

          network under consideration.
  1. Traffic characteristic:

A description of the temporal behavior or a description of

          the attributes of a given traffic flow or traffic aggregate.

Awduche, et. al. Informational [Page 10] RFC 3272 Overview and Principles of Internet TE May 2002

  1. Traffic engineering system:

A collection of objects, mechanisms, and protocols that are

          used conjunctively to accomplish traffic engineering
          objectives.
  1. Traffic flow:

A stream of packets between two end-points that can be

          characterized in a certain way.  A micro-flow has a more
          specific definition: A micro-flow is a stream of packets
          with the same source and destination addresses, source and
          destination ports, and protocol ID.
  1. Traffic intensity:

A measure of traffic loading with respect to a resource

          capacity over a specified period of time.  In classical
          telephony systems, traffic intensity is measured in units of
          Erlang.
  1. Traffic matrix:

A representation of the traffic demand between a set of

          origin and destination abstract nodes.  An abstract node can
          consist of one or more network elements.
  1. Traffic monitoring:

The process of observing traffic characteristics at a given

          point in a network and collecting the traffic information
          for analysis and further action.
  1. Traffic trunk:

An aggregation of traffic flows belonging to the same class

          which are forwarded through a common path.  A traffic trunk
          may be characterized by an ingress and egress node, and a
          set of attributes which determine its behavioral
          characteristics and requirements from the network.

2.0 Background

 The Internet has quickly evolved into a very critical communications
 infrastructure, supporting significant economic, educational, and
 social activities.  Simultaneously, the delivery of Internet
 communications services has become very competitive and end-users are
 demanding very high quality service from their service providers.
 Consequently, performance optimization of large scale IP networks,
 especially public Internet backbones, have become an important
 problem.  Network performance requirements are multi-dimensional,
 complex, and sometimes contradictory; making the traffic engineering
 problem very challenging.

Awduche, et. al. Informational [Page 11] RFC 3272 Overview and Principles of Internet TE May 2002

 The network must convey IP packets from ingress nodes to egress nodes
 efficiently, expeditiously, and economically.  Furthermore, in a
 multiclass service environment (e.g., Diffserv capable networks), the
 resource sharing parameters of the network must be appropriately
 determined and configured according to prevailing policies and
 service models to resolve resource contention issues arising from
 mutual interference between packets traversing through the network.
 Thus, consideration must be given to resolving competition for
 network resources between traffic streams belonging to the same
 service class (intra-class contention resolution) and traffic streams
 belonging to different classes (inter-class contention resolution).

2.1 Context of Internet Traffic Engineering

 The context of Internet traffic engineering pertains to the scenarios
 where traffic engineering is used.  A traffic engineering methodology
 establishes appropriate rules to resolve traffic performance issues
 occurring in a specific context.  The context of Internet traffic
 engineering includes:
    (1)   A network context defining the universe of discourse, and in
          particular the situations in which the traffic engineering
          problems occur.  The network context includes network
          structure, network policies, network characteristics,
          network constraints, network quality attributes, and network
          optimization criteria.
    (2)   A problem context defining the general and concrete issues
          that traffic engineering addresses.  The problem context
          includes identification, abstraction of relevant features,
          representation, formulation, specification of the
          requirements on the solution space, and specification of the
          desirable features of acceptable solutions.
    (3)   A solution context suggesting how to address the issues
          identified by the problem context.  The solution context
          includes analysis, evaluation of alternatives, prescription,
          and resolution.
    (4)   An implementation and operational context in which the
          solutions are methodologically instantiated.  The
          implementation and operational context includes planning,
          organization, and execution.
 The context of Internet traffic engineering and the different problem
 scenarios are discussed in the following subsections.

Awduche, et. al. Informational [Page 12] RFC 3272 Overview and Principles of Internet TE May 2002

2.2 Network Context

 IP networks range in size from small clusters of routers situated
 within a given location, to thousands of interconnected routers,
 switches, and other components distributed all over the world.
 Conceptually, at the most basic level of abstraction, an IP network
 can be represented as a distributed dynamical system consisting of:
 (1) a set of interconnected resources which provide transport
 services for IP traffic subject to certain constraints, (2) a demand
 system representing the offered load to be transported through the
 network, and (3) a response system consisting of network processes,
 protocols, and related mechanisms which facilitate the movement of
 traffic through the network [see also AWD2].
 The network elements and resources may have specific characteristics
 restricting the manner in which the demand is handled.  Additionally,
 network resources may be equipped with traffic control mechanisms
 superintending the way in which the demand is serviced.  Traffic
 control mechanisms may, for example, be used to control various
 packet processing activities within a given resource, arbitrate
 contention for access to the resource by different packets, and
 regulate traffic behavior through the resource.  A configuration
 management and provisioning system may allow the settings of the
 traffic control mechanisms to be manipulated by external or internal
 entities in order to exercise control over the way in which the
 network elements respond to internal and external stimuli.
 The details of how the network provides transport services for
 packets are specified in the policies of the network administrators
 and are installed through network configuration management and policy
 based provisioning systems.  Generally, the types of services
 provided by the network also depends upon the technology and
 characteristics of the network elements and protocols, the prevailing
 service and utility models, and the ability of the network
 administrators to translate policies into network configurations.
 Contemporary Internet networks have three significant
 characteristics:  (1) they provide real-time services, (2) they have
 become mission critical, and (3) their operating environments are
 very dynamic.  The dynamic characteristics of IP networks can be
 attributed in part to fluctuations in demand, to the interaction
 between various network protocols and processes, to the rapid
 evolution of the infrastructure which demands the constant inclusion
 of new technologies and new network elements, and to transient and
 persistent impairments which occur within the system.

Awduche, et. al. Informational [Page 13] RFC 3272 Overview and Principles of Internet TE May 2002

 Packets contend for the use of network resources as they are conveyed
 through the network.  A network resource is considered to be
 congested if the arrival rate of packets exceed the output capacity
 of the resource over an interval of time.  Congestion may result in
 some of the arrival packets being delayed or even dropped.
 Congestion increases transit delays, delay variation, packet loss,
 and reduces the predictability of network services.  Clearly,
 congestion is a highly undesirable phenomenon.
 Combating congestion at a reasonable cost is a major objective of
 Internet traffic engineering.
 Efficient sharing of network resources by multiple traffic streams is
 a basic economic premise for packet switched networks in general and
 for the Internet in particular.  A fundamental challenge in network
 operation, especially in a large scale public IP network, is to
 increase the efficiency of resource utilization while minimizing the
 possibility of congestion.
 Increasingly, the Internet will have to function in the presence of
 different classes of traffic with different service requirements.
 The advent of Differentiated Services [RFC-2475] makes this
 requirement particularly acute.  Thus, packets may be grouped into
 behavior aggregates such that each behavior aggregate may have a
 common set of behavioral characteristics or a common set of delivery
 requirements.  In practice, the delivery requirements of a specific
 set of packets may be specified explicitly or implicitly.  Two of the
 most important traffic delivery requirements are capacity constraints
 and QoS constraints.
 Capacity constraints can be expressed statistically as peak rates,
 mean rates, burst sizes, or as some deterministic notion of effective
 bandwidth.  QoS requirements can be expressed in terms of (1)
 integrity constraints such as packet loss and (2) in terms of
 temporal constraints such as timing restrictions for the delivery of
 each packet (delay) and timing restrictions for the delivery of
 consecutive packets belonging to the same traffic stream (delay
 variation).

2.3 Problem Context

 Fundamental problems exist in association with the operation of a
 network described by the simple model of the previous subsection.
 This subsection reviews the problem context in relation to the
 traffic engineering function.

Awduche, et. al. Informational [Page 14] RFC 3272 Overview and Principles of Internet TE May 2002

 The identification, abstraction, representation, and measurement of
 network features relevant to traffic engineering is a significant
 issue.
 One particularly important class of problems concerns how to
 explicitly formulate the problems that traffic engineering attempts
 to solve, how to identify the requirements on the solution space, how
 to specify the desirable features of good solutions, how to actually
 solve the problems, and how to measure and characterize the
 effectiveness of the solutions.
 Another class of problems concerns how to measure and estimate
 relevant network state parameters.  Effective traffic engineering
 relies on a good estimate of the offered traffic load as well as a
 view of the underlying topology and associated resource constraints.
 A network-wide view of the topology is also a must for offline
 planning.
 Still another class of problems concerns how to characterize the
 state of the network and how to evaluate its performance under a
 variety of scenarios.  The performance evaluation problem is two-
 fold.  One aspect of this problem relates to the evaluation of the
 system level performance of the network.  The other aspect relates to
 the evaluation of the resource level performance, which restricts
 attention to the performance analysis of individual network
 resources.  In this memo, we refer to the system level
 characteristics of the network as the "macro-states" and the resource
 level characteristics as the "micro-states." The system level
 characteristics are also known as the emergent properties of the
 network as noted earlier.  Correspondingly, we shall refer to the
 traffic engineering schemes dealing with network performance
 optimization at the systems level as "macro-TE" and the schemes that
 optimize at the individual resource level as "micro-TE."  Under
 certain circumstances, the system level performance can be derived
 from the resource level performance using appropriate rules of
 composition, depending upon the particular performance measures of
 interest.
 Another fundamental class of problems concerns how to effectively
 optimize network performance.  Performance optimization may entail
 translating solutions to specific traffic engineering problems into
 network configurations.  Optimization may also entail some degree of
 resource management control, routing control, and/or capacity
 augmentation.

Awduche, et. al. Informational [Page 15] RFC 3272 Overview and Principles of Internet TE May 2002

 As noted previously, congestion is an undesirable phenomena in
 operational networks.  Therefore, the next subsection addresses the
 issue of congestion and its ramifications within the problem context
 of Internet traffic engineering.

2.3.1 Congestion and its Ramifications

 Congestion is one of the most significant problems in an operational
 IP context.  A network element is said to be congested if it
 experiences sustained overload over an interval of time.  Congestion
 almost always results in degradation of service quality to end users.
 Congestion control schemes can include demand side policies and
 supply side policies.  Demand side policies may restrict access to
 congested resources and/or dynamically regulate the demand to
 alleviate the overload situation.  Supply side policies may expand or
 augment network capacity to better accommodate offered traffic.
 Supply side policies may also re-allocate network resources by
 redistributing traffic over the infrastructure.  Traffic
 redistribution and resource re-allocation serve to increase the
 'effective capacity' seen by the demand.
 The emphasis of this memo is primarily on congestion management
 schemes falling within the scope of the network, rather than on
 congestion management systems dependent upon sensitivity and
 adaptivity from end-systems.  That is, the aspects that are
 considered in this memo with respect to congestion management are
 those solutions that can be provided by control entities operating on
 the network and by the actions of network administrators and network
 operations systems.

2.4 Solution Context

 The solution context for Internet traffic engineering involves
 analysis, evaluation of alternatives, and choice between alternative
 courses of action.  Generally the solution context is predicated on
 making reasonable inferences about the current or future state of the
 network, and subsequently making appropriate decisions that may
 involve a preference between alternative sets of action.  More
 specifically, the solution context demands reasonable estimates of
 traffic workload, characterization of network state, deriving
 solutions to traffic engineering problems which may be implicitly or
 explicitly formulated, and possibly instantiating a set of control
 actions.  Control actions may involve the manipulation of parameters
 associated with routing, control over tactical capacity acquisition,
 and control over the traffic management functions.
 The following list of instruments may be applicable to the solution
 context of Internet traffic engineering.

Awduche, et. al. Informational [Page 16] RFC 3272 Overview and Principles of Internet TE May 2002

    (1)   A set of policies, objectives, and requirements (which may
          be context dependent) for network performance evaluation and
          performance  optimization.
    (2)   A collection of online and possibly offline tools and
          mechanisms for measurement, characterization, modeling, and
          control of Internet traffic and control over the placement
          and allocation of network resources, as well as control over
          the mapping or distribution of traffic onto the
          infrastructure.
    (3)   A set of constraints on the operating environment, the
          network protocols, and the traffic engineering system
          itself.
    (4)   A set of quantitative and qualitative techniques and
          methodologies for abstracting, formulating, and solving
          traffic engineering problems.
    (5)   A set of administrative control parameters which may be
          manipulated through a Configuration Management (CM) system.
          The CM system itself may include a configuration control
          subsystem, a configuration repository, a configuration
          accounting subsystem, and a configuration auditing
          subsystem.
    (6)   A set of guidelines for network performance evaluation,
          performance optimization, and performance improvement.
 Derivation of traffic characteristics through measurement and/or
 estimation is very useful within the realm of the solution space for
 traffic engineering.  Traffic estimates can be derived from customer
 subscription information, traffic projections, traffic models, and
 from actual empirical measurements.  The empirical measurements may
 be performed at the traffic aggregate level or at the flow level in
 order to derive traffic statistics at various levels of detail.
 Measurements at the flow level or on small traffic aggregates may be
 performed at edge nodes, where traffic enters and leaves the network.
 Measurements at large traffic aggregate levels may be performed
 within the core of the network where potentially numerous traffic
 flows may be in transit concurrently.
 To conduct performance studies and to support planning of existing
 and future networks, a routing analysis may be performed to determine
 the path(s) the routing protocols will choose for various traffic
 demands, and to ascertain the utilization of network resources as
 traffic is routed through the network.  The routing analysis should
 capture the selection of paths through the network, the assignment of

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 traffic across multiple feasible routes, and the multiplexing of IP
 traffic over traffic trunks (if such constructs exists) and over the
 underlying network infrastructure.  A network topology model is a
 necessity for routing analysis.  A network topology model may be
 extracted from network architecture documents, from network designs,
 from information contained in router configuration files, from
 routing databases, from routing tables, or from automated tools that
 discover and depict network topology information.  Topology
 information may also be derived from servers that monitor network
 state, and from servers that perform provisioning functions.
 Routing in operational IP networks can be administratively controlled
 at various levels of abstraction including the manipulation of BGP
 attributes and manipulation of IGP metrics.  For path oriented
 technologies such as MPLS, routing can be further controlled by the
 manipulation of relevant traffic engineering parameters, resource
 parameters, and administrative policy constraints.  Within the
 context of MPLS, the path of an explicit label switched path (LSP)
 can be computed and established in various ways including: (1)
 manually, (2) automatically online using constraint-based routing
 processes implemented on label switching routers, and (3)
 automatically offline using constraint-based routing entities
 implemented on external traffic engineering support systems.

2.4.1 Combating the Congestion Problem

 Minimizing congestion is a significant aspect of Internet traffic
 engineering.  This subsection gives an overview of the general
 approaches that have been used or proposed to combat congestion
 problems.
 Congestion management policies can be categorized based upon the
 following criteria (see e.g., [YARE95] for a more detailed taxonomy
 of congestion control schemes): (1) Response time scale which can be
 characterized as long, medium, or short; (2) reactive versus
 preventive which relates to congestion control and congestion
 avoidance; and (3) supply side versus demand side congestion
 management schemes.  These aspects are discussed in the following
 paragraphs.
 (1) Congestion Management based on Response Time Scales
  1. Long (weeks to months): Capacity planning works over a relatively

long time scale to expand network capacity based on estimates or

 forecasts of future traffic demand and traffic distribution.  Since
 router and link provisioning take time and are generally expensive,
 these upgrades are typically carried out in the weeks-to-months or
 even years time scale.

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  1. Medium (minutes to days): Several control policies fall within the

medium time scale category. Examples include: (1) Adjusting IGP

 and/or BGP parameters to route traffic away or towards certain
 segments of the network; (2) Setting up and/or adjusting some
 explicitly routed label switched paths (ER-LSPs) in MPLS networks to
 route some traffic trunks away from possibly congested resources or
 towards possibly more favorable routes; (3) re-configuring the
 logical topology of the network to make it correlate more closely
 with the spatial traffic distribution using for example some
 underlying path-oriented technology such as MPLS LSPs, ATM PVCs, or
 optical channel trails.  Many of these adaptive medium time scale
 response schemes rely on a measurement system that monitors changes
 in traffic distribution, traffic shifts, and network resource
 utilization and subsequently provides feedback to the online and/or
 offline traffic engineering mechanisms and tools which employ this
 feedback information to trigger certain control actions to occur
 within the network.  The traffic engineering mechanisms and tools can
 be implemented in a distributed fashion or in a centralized fashion,
 and may have a hierarchical structure or a flat structure.  The
 comparative merits of distributed and centralized control structures
 for networks are well known.  A centralized scheme may have global
 visibility into the network state and may produce potentially more
 optimal solutions.  However, centralized schemes are prone to single
 points of failure and may not scale as well as distributed schemes.
 Moreover, the information utilized by a centralized scheme may be
 stale and may not reflect the actual state of the network.  It is not
 an objective of this memo to make a recommendation between
 distributed and centralized schemes.  This is a choice that network
 administrators must make based on their specific needs.
  1. Short (picoseconds to minutes): This category includes packet level

processing functions and events on the order of several round trip

 times.  It includes router mechanisms such as passive and active
 buffer management.  These mechanisms are used to control congestion
 and/or signal congestion to end systems so that they can adaptively
 regulate the rate at which traffic is injected into the network.  One
 of the most popular active queue management schemes, especially for
 TCP traffic, is Random Early Detection (RED) [FLJA93], which supports
 congestion avoidance by controlling the average queue size.  During
 congestion (but before the queue is filled), the RED scheme chooses
 arriving packets to "mark" according to a probabilistic algorithm
 which takes into account the average queue size.  For a router that
 does not utilize explicit congestion notification (ECN) see e.g.,
 [FLOY94], the marked packets can simply be dropped to signal the
 inception of congestion to end systems.  On the other hand, if the
 router supports ECN, then it can set the ECN field in the packet
 header.  Several variations of RED have been proposed to support
 different drop precedence levels in multi-class environments [RFC-

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 2597], e.g., RED with In and Out (RIO) and Weighted RED.  There is
 general consensus that RED provides congestion avoidance performance
 which is not worse than traditional Tail-Drop (TD) queue management
 (drop arriving packets only when the queue is full).  Importantly,
 however, RED reduces the possibility of global synchronization and
 improves fairness among different TCP sessions.  However, RED by
 itself can not prevent congestion and unfairness caused by sources
 unresponsive to RED, e.g., UDP traffic and some misbehaved greedy
 connections.  Other schemes have been proposed to improve the
 performance and fairness in the presence of unresponsive traffic.
 Some of these schemes were proposed as theoretical frameworks and are
 typically not available in existing commercial products.  Two such
 schemes are Longest Queue Drop (LQD) and Dynamic Soft Partitioning
 with Random Drop (RND) [SLDC98].
 (2) Congestion Management: Reactive versus Preventive Schemes
  1. Reactive: reactive (recovery) congestion management policies react

to existing congestion problems to improve it. All the policies

 described in the long and medium time scales above can be categorized
 as being reactive especially if the policies are based on monitoring
 and identifying existing congestion problems, and on the initiation
 of relevant actions to ease a situation.
  1. Preventive: preventive (predictive/avoidance) policies take

proactive action to prevent congestion based on estimates and

 predictions of future potential congestion problems.  Some of the
 policies described in the long and medium time scales fall into this
 category.  They do not necessarily respond immediately to existing
 congestion problems.  Instead forecasts of traffic demand and
 workload distribution are considered and action may be taken to
 prevent potential congestion problems in the future.  The schemes
 described in the short time scale (e.g., RED and its variations, ECN,
 LQD, and RND) are also used for congestion avoidance since dropping
 or marking packets before queues actually overflow would trigger
 corresponding TCP sources to slow down.
 (3) Congestion Management: Supply Side versus Demand Side Schemes
  1. Supply side: supply side congestion management policies increase

the effective capacity available to traffic in order to control or

 obviate congestion.  This can be accomplished by augmenting capacity.
 Another way to accomplish this is to minimize congestion by having a
 relatively balanced distribution of traffic over the network.  For
 example, capacity planning should aim to provide a physical topology
 and associated link bandwidths that match estimated traffic workload
 and traffic distribution based on forecasting (subject to budgetary
 and other constraints).  However, if actual traffic distribution does

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 not match the topology derived from capacity panning (due to
 forecasting errors or facility constraints for example), then the
 traffic can be mapped onto the existing topology using routing
 control mechanisms, using path oriented technologies (e.g., MPLS LSPs
 and optical channel trails) to modify the logical topology, or by
 using some other load redistribution mechanisms.
  1. Demand side: demand side congestion management policies control or

regulate the offered traffic to alleviate congestion problems. For

 example, some of the short time scale mechanisms described earlier
 (such as RED and its variations, ECN, LQD, and RND) as well as
 policing and rate shaping mechanisms attempt to regulate the offered
 load in various ways.  Tariffs may also be applied as a demand side
 instrument.  To date, however, tariffs have not been used as a means
 of demand side congestion management within the Internet.
 In summary, a variety of mechanisms can be used to address congestion
 problems in IP networks.  These mechanisms may operate at multiple
 time-scales.

2.5 Implementation and Operational Context

 The operational context of Internet traffic engineering is
 characterized by constant change which occur at multiple levels of
 abstraction.  The implementation context demands effective planning,
 organization, and execution.  The planning aspects may involve
 determining prior sets of actions to achieve desired objectives.
 Organizing involves arranging and assigning responsibility to the
 various components of the traffic engineering system and coordinating
 the activities to accomplish the desired TE objectives.  Execution
 involves measuring and applying corrective or perfective actions to
 attain and maintain desired TE goals.

3.0 Traffic Engineering Process Model(s)

 This section describes a generic process model that captures the high
 level practical aspects of Internet traffic engineering in an
 operational context.  The process model is described as a sequence of
 actions that a traffic engineer, or more generally a traffic
 engineering system, must perform to optimize the performance of an
 operational network (see also [RFC-2702, AWD2]).  The process model
 described here represents the broad activities common to most traffic
 engineering methodologies although the details regarding how traffic
 engineering is executed may differ from network to network.  This
 process model may be enacted explicitly or implicitly, by an
 automaton and/or by a human.

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 The traffic engineering process model is iterative [AWD2].  The four
 phases of the process model described below are repeated continually.
 The first phase of the TE process model is to define the relevant
 control policies that govern the operation of the network.  These
 policies may depend upon many factors including the prevailing
 business model, the network cost structure, the operating
 constraints, the utility model, and optimization criteria.
 The second phase of the process model is a feedback mechanism
 involving the acquisition of measurement data from the operational
 network.  If empirical data is not readily available from the
 network, then synthetic workloads may be used instead which reflect
 either the prevailing or the expected workload of the network.
 Synthetic workloads may be derived by estimation or extrapolation
 using prior empirical data.  Their derivation may also be obtained
 using mathematical models of traffic characteristics or other means.
 The third phase of the process model is to analyze the network state
 and to characterize traffic workload.  Performance analysis may be
 proactive and/or reactive.  Proactive performance analysis identifies
 potential problems that do not exist, but could manifest in the
 future.  Reactive performance analysis identifies existing problems,
 determines their cause through diagnosis, and evaluates alternative
 approaches to remedy the problem, if necessary.  A number of
 quantitative and qualitative techniques may be used in the analysis
 process, including modeling based analysis and simulation.  The
 analysis phase of the process model may involve investigating the
 concentration and distribution of traffic across the network or
 relevant subsets of the network, identifying the characteristics of
 the offered traffic workload, identifying existing or potential
 bottlenecks, and identifying network pathologies such as ineffective
 link placement, single points of failures, etc.  Network pathologies
 may result from many factors including inferior network architecture,
 inferior network design, and configuration problems.  A traffic
 matrix may be constructed as part of the analysis process.  Network
 analysis may also be descriptive or prescriptive.
 The fourth phase of the TE process model is the performance
 optimization of the network.  The performance optimization phase
 involves a decision process which selects and implements a set of
 actions from a set of alternatives.  Optimization actions may include
 the use of appropriate techniques to either control the offered
 traffic or to control the distribution of traffic across the network.
 Optimization actions may also involve adding additional links or
 increasing link capacity, deploying additional hardware such as
 routers and switches, systematically adjusting parameters associated
 with routing such as IGP metrics and BGP attributes, and adjusting

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 traffic management parameters.  Network performance optimization may
 also involve starting a network planning process to improve the
 network architecture, network design, network capacity, network
 technology, and the configuration of network elements to accommodate
 current and future growth.

3.1 Components of the Traffic Engineering Process Model

 The key components of the traffic engineering process model include a
 measurement subsystem, a modeling and analysis subsystem, and an
 optimization subsystem.  The following subsections examine these
 components as they apply to the traffic engineering process model.

3.2 Measurement

 Measurement is crucial to the traffic engineering function.  The
 operational state of a network can be conclusively determined only
 through measurement.  Measurement is also critical to the
 optimization function because it provides feedback data which is used
 by traffic engineering control subsystems.  This data is used to
 adaptively optimize network performance in response to events and
 stimuli originating within and outside the network.  Measurement is
 also needed to determine the quality of network services and to
 evaluate the effectiveness of traffic engineering policies.
 Experience suggests that measurement is most effective when acquired
 and applied systematically.
 When developing a measurement system to support the traffic
 engineering function in IP networks, the following questions should
 be carefully considered: Why is measurement needed in this particular
 context? What parameters are to be measured?  How should the
 measurement be accomplished?  Where should the measurement be
 performed? When should the measurement be performed?  How frequently
 should the monitored variables be measured?  What level of
 measurement accuracy and reliability is desirable? What level of
 measurement accuracy and reliability is realistically attainable? To
 what extent can the measurement system permissibly interfere with the
 monitored network components and variables? What is the acceptable
 cost of measurement? The answers to these questions will determine
 the measurement tools and methodologies appropriate in any given
 traffic engineering context.
 It should also be noted that there is a distinction between
 measurement and evaluation.  Measurement provides raw data concerning
 state parameters and variables of monitored network elements.
 Evaluation utilizes the raw data to make inferences regarding the
 monitored system.

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 Measurement in support of the TE function can occur at different
 levels of abstraction.  For example, measurement can be used to
 derive packet level characteristics, flow level characteristics, user
 or customer level characteristics, traffic aggregate characteristics,
 component level characteristics, and network wide characteristics.

3.3 Modeling, Analysis, and Simulation

 Modeling and analysis are important aspects of Internet traffic
 engineering.  Modeling involves constructing an abstract or physical
 representation which depicts relevant traffic characteristics and
 network attributes.
 A network model is an abstract representation of the network which
 captures relevant network features, attributes, and characteristics,
 such as link and nodal attributes and constraints.  A network model
 may facilitate analysis and/or simulation which can be used to
 predict network performance under various conditions as well as to
 guide network expansion plans.
 In general, Internet traffic engineering models can be classified as
 either structural or behavioral.  Structural models focus on the
 organization of the network and its components.  Behavioral models
 focus on the dynamics of the network and the traffic workload.
 Modeling for Internet traffic engineering may also be formal or
 informal.
 Accurate behavioral models for traffic sources are particularly
 useful for analysis.  Development of behavioral traffic source models
 that are consistent with empirical data obtained from operational
 networks is a major research topic in Internet traffic engineering.
 These source models should also be tractable and amenable to
 analysis.  The topic of source models for IP traffic is a research
 topic and is therefore outside the scope of this document.  Its
 importance, however, must be emphasized.
 Network simulation tools are extremely useful for traffic
 engineering.  Because of the complexity of realistic quantitative
 analysis of network behavior, certain aspects of network performance
 studies can only be conducted effectively using simulation.  A good
 network simulator can be used to mimic and visualize network
 characteristics under various conditions in a safe and non-disruptive
 manner.  For example, a network simulator may be used to depict
 congested resources and hot spots, and to provide hints regarding
 possible solutions to network performance problems.  A good simulator
 may also be used to validate the effectiveness of planned solutions
 to network issues without the need to tamper with the operational
 network, or to commence an expensive network upgrade which may not

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 achieve the desired objectives.  Furthermore, during the process of
 network planning, a network simulator may reveal pathologies such as
 single points of failure which may require additional redundancy, and
 potential bottlenecks and hot spots which may require additional
 capacity.
 Routing simulators are especially useful in large networks.  A
 routing simulator may identify planned links which may not actually
 be used to route traffic by the existing routing protocols.
 Simulators can also be used to conduct scenario based and
 perturbation based analysis, as well as sensitivity studies.
 Simulation results can be used to initiate appropriate actions in
 various ways.  For example, an important application of network
 simulation tools is to investigate and identify how best to make the
 network evolve and grow, in order to accommodate projected future
 demands.

3.4 Optimization

 Network performance optimization involves resolving network issues by
 transforming such issues into concepts that enable a solution,
 identification of a solution, and implementation of the solution.
 Network performance optimization can be corrective or perfective.  In
 corrective optimization, the goal is to remedy a problem that has
 occurred or that is incipient.  In perfective optimization, the goal
 is to improve network performance even when explicit problems do not
 exist and are not anticipated.
 Network performance optimization is a continual process, as noted
 previously.  Performance optimization iterations may consist of
 real-time optimization sub-processes and non-real-time network
 planning sub-processes.  The difference between real-time
 optimization and network planning is primarily in the relative time-
 scale in which they operate and in the granularity of actions.  One
 of the objectives of a real-time optimization sub-process is to
 control the mapping and distribution of traffic over the existing
 network infrastructure to avoid and/or relieve congestion, to assure
 satisfactory service delivery, and to optimize resource utilization.
 Real-time optimization is needed because random incidents such as
 fiber cuts or shifts in traffic demand will occur irrespective of how
 well a network is designed.  These incidents can cause congestion and
 other problems to manifest in an operational network.  Real-time
 optimization must solve such problems in small to medium time-scales
 ranging from micro-seconds to minutes or hours.  Examples of real-
 time optimization include queue management, IGP/BGP metric tuning,
 and using technologies such as MPLS explicit LSPs to change the paths
 of some traffic trunks [XIAO].

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 One of the functions of the network planning sub-process is to
 initiate actions to systematically evolve the architecture,
 technology, topology, and capacity of a network.  When a problem
 exists in the network, real-time optimization should provide an
 immediate remedy.  Because a prompt response is necessary, the real-
 time solution may not be the best possible solution.  Network
 planning may subsequently be needed to refine the solution and
 improve the situation.  Network planning is also required to expand
 the network to support traffic growth and changes in traffic
 distribution over time.  As previously noted, a change in the
 topology and/or capacity of the network may be the outcome of network
 planning.
 Clearly, network planning and real-time performance optimization are
 mutually complementary activities.  A well-planned and designed
 network makes real-time optimization easier, while a systematic
 approach to real-time network performance optimization allows network
 planning to focus on long term issues rather than tactical
 considerations.  Systematic real-time network performance
 optimization also provides valuable inputs and insights toward
 network planning.
 Stability is an important consideration in real-time network
 performance optimization.  This aspect will be repeatedly addressed
 throughout this memo.

4.0 Historical Review and Recent Developments

 This section briefly reviews different traffic engineering approaches
 proposed and implemented in telecommunications and computer networks.
 The discussion is not intended to be comprehensive.  It is primarily
 intended to illuminate pre-existing perspectives and prior art
 concerning traffic engineering in the Internet and in legacy
 telecommunications networks.

4.1 Traffic Engineering in Classical Telephone Networks

 This subsection presents a brief overview of traffic engineering in
 telephone networks which often relates to the way user traffic is
 steered from an originating node to the terminating node.  This
 subsection presents a brief overview of this topic.  A detailed
 description of the various routing strategies applied in telephone
 networks is included in the book by G. Ash [ASH2].
 The early telephone network relied on static hierarchical routing,
 whereby routing patterns remained fixed independent of the state of
 the network or time of day.  The hierarchy was intended to
 accommodate overflow traffic, improve network reliability via

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 alternate routes, and prevent call looping by employing strict
 hierarchical rules.  The network was typically over-provisioned since
 a given fixed route had to be dimensioned so that it could carry user
 traffic during a busy hour of any busy day.  Hierarchical routing in
 the telephony network was found to be too rigid upon the advent of
 digital switches and stored program control which were able to manage
 more complicated traffic engineering rules.
 Dynamic routing was introduced to alleviate the routing inflexibility
 in the static hierarchical routing so that the network would operate
 more efficiently.  This resulted in significant economic gains
 [HUSS87].  Dynamic routing typically reduces the overall loss
 probability by 10 to 20 percent (compared to static hierarchical
 routing).  Dynamic routing can also improve network resilience by
 recalculating routes on a per-call basis and periodically updating
 routes.
 There are three main types of dynamic routing in the telephone
 network.  They are time-dependent routing, state-dependent routing
 (SDR), and event dependent routing (EDR).
 In time-dependent routing, regular variations in traffic loads (such
 as time of day or day of week) are exploited in pre-planned routing
 tables.  In state-dependent routing, routing tables are updated
 online according to the current state of the network (e.g., traffic
 demand, utilization, etc.).  In event dependent routing, routing
 changes are incepted by events (such as call setups encountering
 congested or blocked links) whereupon new paths are searched out
 using learning models.  EDR methods are real-time adaptive, but they
 do not require global state information as does SDR.  Examples of EDR
 schemes include the dynamic alternate routing (DAR) from BT, the
 state-and-time dependent routing (STR) from NTT, and the success-to-
 the-top (STT) routing from AT&T.
 Dynamic non-hierarchical routing (DNHR) is an example of dynamic
 routing that was introduced in the AT&T toll network in the 1980's to
 respond to time-dependent information such as regular load variations
 as a function of time.  Time-dependent information in terms of load
 may be divided into three time scales: hourly, weekly, and yearly.
 Correspondingly, three algorithms are defined to pre-plan the routing
 tables.  The network design algorithm operates over a year-long
 interval while the demand servicing algorithm operates on a weekly
 basis to fine tune link sizes and routing tables to correct forecast
 errors on the yearly basis.  At the smallest time scale, the routing
 algorithm is used to make limited adjustments based on daily traffic
 variations.  Network design and demand servicing are computed using
 offline calculations.  Typically, the calculations require extensive
 searches on possible routes.  On the other hand, routing may need

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 online calculations to handle crankback.  DNHR adopts a "two-link"
 approach whereby a path can consist of two links at most.  The
 routing algorithm presents an ordered list of route choices between
 an originating switch and a terminating switch.  If a call overflows,
 a via switch (a tandem exchange between the originating switch and
 the terminating switch) would send a crankback signal to the
 originating switch.  This switch would then select the next route,
 and so on, until there are no alternative routes available in which
 the call is blocked.

4.2 Evolution of Traffic Engineering in Packet Networks

 This subsection reviews related prior work that was intended to
 improve the performance of data networks.  Indeed, optimization of
 the performance of data networks started in the early days of the
 ARPANET.  Other early commercial networks such as SNA also recognized
 the importance of performance optimization and service
 differentiation.
 In terms of traffic management, the Internet has been a best effort
 service environment until recently.  In particular, very limited
 traffic management capabilities existed in IP networks to provide
 differentiated queue management and scheduling services to packets
 belonging to different classes.
 In terms of routing control, the Internet has employed distributed
 protocols for intra-domain routing.  These protocols are highly
 scalable and resilient.  However, they are based on simple algorithms
 for path selection which have very limited functionality to allow
 flexible control of the path selection process.
 In the following subsections, the evolution of practical traffic
 engineering mechanisms in IP networks and its predecessors are
 reviewed.

4.2.1 Adaptive Routing in the ARPANET

 The early ARPANET recognized the importance of adaptive routing where
 routing decisions were based on the current state of the network
 [MCQ80].  Early minimum delay routing approaches forwarded each
 packet to its destination along a path for which the total estimated
 transit time was the smallest.  Each node maintained a table of
 network delays, representing the estimated delay that a packet would
 experience along a given path toward its destination.  The minimum
 delay table was periodically transmitted by a node to its neighbors.
 The shortest path, in terms of hop count, was also propagated to give
 the connectivity information.

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 One drawback to this approach is that dynamic link metrics tend to
 create "traffic magnets" causing congestion to be shifted from one
 location of a network to another location, resulting in oscillation
 and network instability.

4.2.2 Dynamic Routing in the Internet

 The Internet evolved from the APARNET and adopted dynamic routing
 algorithms with distributed control to determine the paths that
 packets should take en-route to their destinations.  The routing
 algorithms are adaptations of shortest path algorithms where costs
 are based on link metrics.  The link metric can be based on static or
 dynamic quantities.  The link metric based on static quantities may
 be assigned administratively according to local criteria.  The link
 metric based on dynamic quantities may be a function of a network
 congestion measure such as delay or packet loss.
 It was apparent early that static link metric assignment was
 inadequate because it can easily lead to unfavorable scenarios in
 which some links become congested while others remain lightly loaded.
 One of the many reasons for the inadequacy of static link metrics is
 that link metric assignment was often done without considering the
 traffic matrix in the network.  Also, the routing protocols did not
 take traffic attributes and capacity constraints into account when
 making routing decisions.  This results in traffic concentration
 being localized in subsets of the network infrastructure and
 potentially causing congestion.  Even if link metrics are assigned in
 accordance with the traffic matrix, unbalanced loads in the network
 can still occur due to a number factors including:
  1. Resources may not be deployed in the most optimal locations

from a routing perspective.

  1. Forecasting errors in traffic volume and/or traffic

distribution.

  1. Dynamics in traffic matrix due to the temporal nature of

traffic patterns, BGP policy change from peers, etc.

 The inadequacy of the legacy Internet interior gateway routing system
 is one of the factors motivating the interest in path oriented
 technology with explicit routing and constraint-based routing
 capability such as MPLS.

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4.2.3 ToS Routing

 Type-of-Service (ToS) routing involves different routes going to the
 same destination with selection dependent upon the ToS field of an IP
 packet [RFC-2474].  The ToS classes may be classified as low delay
 and high throughput.  Each link is associated with multiple link
 costs and each link cost is used to compute routes for a particular
 ToS.  A separate shortest path tree is computed for each ToS.  The
 shortest path algorithm must be run for each ToS resulting in very
 expensive computation.  Classical ToS-based routing is now outdated
 as the IP header field has been replaced by a Diffserv field.
 Effective traffic engineering is difficult to perform in classical
 ToS-based routing because each class still relies exclusively on
 shortest path routing which results in localization of traffic
 concentration within the network.

4.2.4 Equal Cost Multi-Path

 Equal Cost Multi-Path (ECMP) is another technique that attempts to
 address the deficiency in the Shortest Path First (SPF) interior
 gateway routing systems [RFC-2328].  In the classical SPF algorithm,
 if two or more shortest paths exist to a given destination, the
 algorithm will choose one of them.  The algorithm is modified
 slightly in ECMP so that if two or more equal cost shortest paths
 exist between two nodes, the traffic between the nodes is distributed
 among the multiple equal-cost paths.  Traffic distribution across the
 equal-cost paths is usually performed in one of two ways: (1)
 packet-based in a round-robin fashion, or (2) flow-based using
 hashing on source and destination IP addresses and possibly other
 fields of the IP header.  The first approach can easily cause out-
 of-order packets while the second approach is dependent upon the
 number and distribution of flows.  Flow-based load sharing may be
 unpredictable in an enterprise network where the number of flows is
 relatively small and less heterogeneous (for example, hashing may not
 be uniform), but it is generally effective in core public networks
 where the number of flows is large and heterogeneous.
 In ECMP, link costs are static and bandwidth constraints are not
 considered, so ECMP attempts to distribute the traffic as equally as
 possible among the equal-cost paths independent of the congestion
 status of each path.  As a result, given two equal-cost paths, it is
 possible that one of the paths will be more congested than the other.
 Another drawback of ECMP is that load sharing cannot be achieved on
 multiple paths which have non-identical costs.

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4.2.5 Nimrod

 Nimrod is a routing system developed to provide heterogeneous service
 specific routing in the Internet, while taking multiple constraints
 into account [RFC-1992].  Essentially, Nimrod is a link state routing
 protocol which supports path oriented packet forwarding.  It uses the
 concept of maps to represent network connectivity and services at
 multiple levels of abstraction.  Mechanisms are provided to allow
 restriction of the distribution of routing information.
 Even though Nimrod did not enjoy deployment in the public Internet, a
 number of key concepts incorporated into the Nimrod architecture,
 such as explicit routing which allows selection of paths at
 originating nodes, are beginning to find applications in some recent
 constraint-based routing initiatives.

4.3 Overlay Model

 In the overlay model, a virtual-circuit network, such as ATM, frame
 relay, or WDM, provides virtual-circuit connectivity between routers
 that are located at the edges of a virtual-circuit cloud.  In this
 mode, two routers that are connected through a virtual circuit see a
 direct adjacency between themselves independent of the physical route
 taken by the virtual circuit through the ATM, frame relay, or WDM
 network.  Thus, the overlay model essentially decouples the logical
 topology that routers see from the physical topology that the ATM,
 frame relay, or WDM network manages.  The overlay model based on ATM
 or frame relay enables a network administrator or an automaton to
 employ traffic engineering concepts to perform path optimization by
 re-configuring or rearranging the virtual circuits so that a virtual
 circuit on a congested or sub-optimal physical link can be re-routed
 to a less congested or more optimal one.  In the overlay model,
 traffic engineering is also employed to establish relationships
 between the traffic management parameters (e.g., PCR, SCR, and MBS
 for ATM) of the virtual-circuit technology and the actual traffic
 that traverses each circuit.  These relationships can be established
 based upon known or projected traffic profiles, and some other
 factors.
 The overlay model using IP over ATM requires the management of two
 separate networks with different technologies (IP and ATM) resulting
 in increased operational complexity and cost.  In the fully-meshed
 overlay model, each router would peer to every other router in the
 network, so that the total number of adjacencies is a quadratic
 function of the number of routers.  Some of the issues with the
 overlay model are discussed in [AWD2].

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4.4 Constrained-Based Routing

 Constraint-based routing refers to a class of routing systems that
 compute routes through a network subject to the satisfaction of a set
 of constraints and requirements.  In the most general setting,
 constraint-based routing may also seek to optimize overall network
 performance while minimizing costs.
 The constraints and requirements may be imposed by the network itself
 or by administrative policies.  Constraints may include bandwidth,
 hop count, delay, and policy instruments such as resource class
 attributes.  Constraints may also include domain specific attributes
 of certain network technologies and contexts which impose
 restrictions on the solution space of the routing function.  Path
 oriented technologies such as MPLS have made constraint-based routing
 feasible and attractive in public IP networks.
 The concept of constraint-based routing within the context of MPLS
 traffic engineering requirements in IP networks was first defined in
 [RFC-2702].
 Unlike QoS routing (for example, see [RFC-2386] and [MA]) which
 generally addresses the issue of routing individual traffic flows to
 satisfy prescribed flow based QoS requirements subject to network
 resource availability, constraint-based routing is applicable to
 traffic aggregates as well as flows and may be subject to a wide
 variety of constraints which may include policy restrictions.

4.5 Overview of Other IETF Projects Related to Traffic Engineering

 This subsection reviews a number of IETF activities pertinent to
 Internet traffic engineering.  These activities are primarily
 intended to evolve the IP architecture to support new service
 definitions which allow preferential or differentiated treatment to
 be accorded to certain types of traffic.

4.5.1 Integrated Services

 The IETF Integrated Services working group developed the integrated
 services (Intserv) model.  This model requires resources, such as
 bandwidth and buffers, to be reserved a priori for a given traffic
 flow to ensure that the quality of service requested by the traffic
 flow is satisfied.  The integrated services model includes additional
 components beyond those used in the best-effort model such as packet
 classifiers, packet schedulers, and admission control.  A packet
 classifier is used to identify flows that are to receive a certain
 level of service.  A packet scheduler handles the scheduling of

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 service to different packet flows to ensure that QoS commitments are
 met.  Admission control is used to determine whether a router has the
 necessary resources to accept a new flow.
 Two services have been defined under the Integrated Services model:
 guaranteed service [RFC-2212] and controlled-load service [RFC-2211].
 The guaranteed service can be used for applications requiring bounded
 packet delivery time.  For this type of application, data that is
 delivered to the application after a pre-defined amount of time has
 elapsed is usually considered worthless.  Therefore, guaranteed
 service was intended to provide a firm quantitative bound on the
 end-to-end packet delay for a flow.  This is accomplished by
 controlling the queuing delay on network elements along the data flow
 path.  The guaranteed service model does not, however, provide
 bounds on jitter (inter-arrival times between consecutive packets).
 The controlled-load service can be used for adaptive applications
 that can tolerate some delay but are sensitive to traffic overload
 conditions.  This type of application typically functions
 satisfactorily when the network is lightly loaded but its performance
 degrades significantly when the network is heavily loaded.
 Controlled-load service, therefore, has been designed to provide
 approximately the same service as best-effort service in a lightly
 loaded network regardless of actual network conditions.  Controlled-
 load service is described qualitatively in that no target values of
 delay or loss are specified.
 The main issue with the Integrated Services model has been
 scalability [RFC-2998], especially in large public IP networks which
 may potentially have millions of active micro-flows in transit
 concurrently.
 A notable feature of the Integrated Services model is that it
 requires explicit signaling of QoS requirements from end systems to
 routers [RFC-2753].  The Resource Reservation Protocol (RSVP)
 performs this signaling function and is a critical component of the
 Integrated Services model.  The RSVP protocol is described next.

4.5.2 RSVP

 RSVP is a soft state signaling protocol [RFC-2205].  It supports
 receiver initiated establishment of resource reservations for both
 multicast and unicast flows.  RSVP was originally developed as a
 signaling protocol within the integrated services framework for
 applications to communicate QoS requirements to the network and for
 the network to reserve relevant resources to satisfy the QoS
 requirements [RFC-2205].

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 Under RSVP, the sender or source node sends a PATH message to the
 receiver with the same source and destination addresses as the
 traffic which the sender will generate.  The PATH message contains:
 (1) a sender Tspec specifying the characteristics of the traffic, (2)
 a sender Template specifying the format of the traffic, and (3) an
 optional Adspec which is used to support the concept of one pass with
 advertising" (OPWA) [RFC-2205].  Every intermediate router along the
 path forwards the PATH Message to the next hop determined by the
 routing protocol.  Upon receiving a PATH Message, the receiver
 responds with a RESV message which includes a flow descriptor used to
 request resource reservations.  The RESV message travels to the
 sender or source node in the opposite direction along the path that
 the PATH message traversed.  Every intermediate router along the path
 can reject or accept the reservation request of the RESV message.  If
 the request is rejected, the rejecting router will send an error
 message to the receiver and the signaling process will terminate.  If
 the request is accepted, link bandwidth and buffer space are
 allocated for the flow and the related flow state information is
 installed in the router.
 One of the issues with the original RSVP specification was
 Scalability.  This is because reservations were required for micro-
 flows, so that the amount of state maintained by network elements
 tends to increase linearly with the number of micro-flows.  These
 issues are described in [RFC-2961].
 Recently, RSVP has been modified and extended in several ways to
 mitigate the scaling problems.  As a result, it is becoming a
 versatile signaling protocol for the Internet.  For example, RSVP has
 been extended to reserve resources for aggregation of flows, to set
 up MPLS explicit label switched paths, and to perform other signaling
 functions within the Internet.  There are also a number of proposals
 to reduce the amount of refresh messages required to maintain
 established RSVP sessions [RFC-2961].
 A number of IETF working groups have been engaged in activities
 related to the RSVP protocol.  These include the original RSVP
 working group, the MPLS working group, the Resource Allocation
 Protocol working group, and the Policy Framework working group.

4.5.3 Differentiated Services

 The goal of the Differentiated Services (Diffserv) effort within the
 IETF is to devise scalable mechanisms for categorization of traffic
 into behavior aggregates, which ultimately allows each behavior
 aggregate to be treated differently, especially when there is a
 shortage of resources such as link bandwidth and buffer space [RFC-
 2475].  One of the primary motivations for the Diffserv effort was to

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 devise alternative mechanisms for service differentiation in the
 Internet that mitigate the scalability issues encountered with the
 Intserv model.
 The IETF Diffserv working group has defined a Differentiated Services
 field in the IP header (DS field).  The DS field consists of six bits
 of the part of the IP header formerly known as TOS octet.  The DS
 field is used to indicate the forwarding treatment that a packet
 should receive at a node [RFC-2474].  The Diffserv working group has
 also standardized a number of Per-Hop Behavior (PHB) groups.  Using
 the PHBs, several classes of services can be defined using different
 classification, policing, shaping, and scheduling rules.
 For an end-user of network services to receive Differentiated
 Services from its Internet Service Provider (ISP), it may be
 necessary for the user to have a Service Level Agreement (SLA) with
 the ISP.  An SLA may explicitly or implicitly specify a Traffic
 Conditioning Agreement (TCA) which defines classifier rules as well
 as metering, marking, discarding, and shaping rules.
 Packets are classified, and possibly policed and shaped at the
 ingress to a Diffserv network.  When a packet traverses the boundary
 between different Diffserv domains, the DS field of the packet may be
 re-marked according to existing agreements between the domains.
 Differentiated Services allows only a finite number of service
 classes to be indicated by the DS field.  The main advantage of the
 Diffserv approach relative to the Intserv model is scalability.
 Resources are allocated on a per-class basis and the amount of state
 information is proportional to the number of classes rather than to
 the number of application flows.
 It should be obvious from the previous discussion that the Diffserv
 model essentially deals with traffic management issues on a per hop
 basis.  The Diffserv control model consists of a collection of
 micro-TE control mechanisms.  Other traffic engineering capabilities,
 such as capacity management (including routing control), are also
 required in order to deliver acceptable service quality in Diffserv
 networks.  The concept of Per Domain Behaviors has been introduced to
 better capture the notion of differentiated services across a
 complete domain [RFC-3086].

4.5.4 MPLS

 MPLS is an advanced forwarding scheme which also includes extensions
 to conventional IP control plane protocols.  MPLS extends the
 Internet routing model and enhances packet forwarding and path
 control [RFC-3031].

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 At the ingress to an MPLS domain, label switching routers (LSRs)
 classify IP packets into forwarding equivalence classes (FECs) based
 on a variety of factors, including, e.g., a combination of the
 information carried in the IP header of the packets and the local
 routing information maintained by the LSRs.  An MPLS label is then
 prepended to each packet according to their forwarding equivalence
 classes.  In a non-ATM/FR environment, the label is 32 bits long and
 contains a 20-bit label field, a 3-bit experimental field (formerly
 known as Class-of-Service or CoS field), a 1-bit label stack
 indicator and an 8-bit TTL field.  In an ATM (FR) environment, the
 label consists of information encoded in the VCI/VPI (DLCI) field.
 An MPLS capable router (an LSR) examines the label and possibly the
 experimental field and uses this information to make packet
 forwarding decisions.
 An LSR makes forwarding decisions by using the label prepended to
 packets as the index into a local next hop label forwarding entry
 (NHLFE).  The packet is then processed as specified in the NHLFE.
 The incoming label may be replaced by an outgoing label, and the
 packet may be switched to the next LSR.  This label-switching process
 is very similar to the label (VCI/VPI) swapping process in ATM
 networks.  Before a packet leaves an MPLS domain, its MPLS label may
 be removed.  A Label Switched Path (LSP) is the path between an
 ingress LSRs and an egress LSRs through which a labeled packet
 traverses.  The path of an explicit LSP is defined at the originating
 (ingress) node of the LSP.  MPLS can use a signaling protocol such as
 RSVP or LDP to set up LSPs.
 MPLS is a very powerful technology for Internet traffic engineering
 because it supports explicit LSPs which allow constraint-based
 routing to be implemented efficiently in IP networks [AWD2].  The
 requirements for traffic engineering over MPLS are described in
 [RFC-2702].  Extensions to RSVP to support instantiation of explicit
 LSP are discussed in [RFC-3209].  Extensions to LDP, known as CR-LDP,
 to support explicit LSPs are presented in [JAM].

4.5.5 IP Performance Metrics

 The IETF IP Performance Metrics (IPPM) working group has been
 developing a set of standard metrics that can be used to monitor the
 quality, performance, and reliability of Internet services.  These
 metrics can be applied by network operators, end-users, and
 independent testing groups to provide users and service providers
 with a common understanding of the performance and reliability of the
 Internet component 'clouds' they use/provide [RFC-2330].  The
 criteria for performance metrics developed by the IPPM WG are
 described in [RFC-2330].  Examples of performance metrics include
 one-way packet

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 loss [RFC-2680], one-way delay [RFC-2679], and connectivity measures
 between two nodes [RFC-2678].  Other metrics include second-order
 measures of packet loss and delay.
 Some of the performance metrics specified by the IPPM WG are useful
 for specifying Service Level Agreements (SLAs).  SLAs are sets of
 service level objectives negotiated between users and service
 providers, wherein each objective is a combination of one or more
 performance metrics, possibly subject to certain constraints.

4.5.6 Flow Measurement

 The IETF Real Time Flow Measurement (RTFM) working group has produced
 an architecture document defining a method to specify traffic flows
 as well as a number of components for flow measurement (meters, meter
 readers, manager) [RFC-2722].  A flow measurement system enables
 network traffic flows to be measured and analyzed at the flow level
 for a variety of purposes.  As noted in RFC 2722, a flow measurement
 system can be very useful in the following contexts: (1)
 understanding the behavior of existing networks, (2) planning for
 network development and expansion, (3) quantification of network
 performance, (4) verifying the quality of network service, and (5)
 attribution of network usage to users.
 A flow measurement system consists of meters, meter readers, and
 managers.  A meter observes packets passing through a measurement
 point, classifies them into certain groups, accumulates certain usage
 data (such as the number of packets and bytes for each group), and
 stores the usage data in a flow table.  A group may represent a user
 application, a host, a network, a group of networks, etc.  A meter
 reader gathers usage data from various meters so it can be made
 available for analysis.  A manager is responsible for configuring and
 controlling meters and meter readers.  The instructions received by a
 meter from a manager include flow specification, meter control
 parameters, and sampling techniques.  The instructions received by a
 meter reader from a manager include the address of the meter whose
 date is to be collected, the frequency of data collection, and the
 types of flows to be collected.

4.5.7 Endpoint Congestion Management

 [RFC-3124] is intended to provide a set of congestion control
 mechanisms that transport protocols can use.  It is also intended to
 develop mechanisms for unifying congestion control across a subset of
 an endpoint's active unicast connections (called a congestion group).
 A congestion manager continuously monitors the state of the path for

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 each congestion group under its control.  The manager uses that
 information to instruct a scheduler on how to partition bandwidth
 among the connections of that congestion group.

4.6 Overview of ITU Activities Related to Traffic Engineering

 This section provides an overview of prior work within the ITU-T
 pertaining to traffic engineering in traditional telecommunications
 networks.
 ITU-T Recommendations E.600 [ITU-E600], E.701 [ITU-E701], and E.801
 [ITU-E801] address traffic engineering issues in traditional
 telecommunications networks.  Recommendation E.600 provides a
 vocabulary for describing traffic engineering concepts, while E.701
 defines reference connections, Grade of Service (GOS), and traffic
 parameters for ISDN.  Recommendation E.701 uses the concept of a
 reference connection to identify representative cases of different
 types of connections without describing the specifics of their actual
 realizations by different physical means.  As defined in
 Recommendation E.600, "a connection is an association of resources
 providing means for communication between two or more devices in, or
 attached to, a telecommunication network."  Also, E.600 defines "a
 resource as any set of physically or conceptually identifiable
 entities within a telecommunication network, the use of which can be
 unambiguously determined" [ITU-E600].  There can be different types
 of connections as the number and types of resources in a connection
 may vary.
 Typically, different network segments are involved in the path of a
 connection.  For example, a connection may be local, national, or
 international.  The purposes of reference connections are to clarify
 and specify traffic performance issues at various interfaces between
 different network domains.  Each domain may consist of one or more
 service provider networks.
 Reference connections provide a basis to define grade of service
 (GoS) parameters related to traffic engineering within the ITU-T
 framework.  As defined in E.600, "GoS refers to a number of traffic
 engineering variables which are used to provide a measure of the
 adequacy of a group of resources under specified conditions."  These
 GoS variables may be probability of loss, dial tone, delay, etc.
 They are essential for network internal design and operation as well
 as for component performance specification.
 GoS is different from quality of service (QoS) in the ITU framework.
 QoS is the performance perceivable by a telecommunication service
 user and expresses the user's degree of satisfaction of the service.
 QoS parameters focus on performance aspects observable at the service

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 access points and network interfaces, rather than their causes within
 the network.  GoS, on the other hand, is a set of network oriented
 measures which characterize the adequacy of a group of resources
 under specified conditions.  For a network to be effective in serving
 its users, the values of both GoS and QoS parameters must be related,
 with GoS parameters typically making a major contribution to the QoS.
 Recommendation E.600 stipulates that a set of GoS parameters must be
 selected and defined on an end-to-end basis for each major service
 category provided by a network to assist the network provider with
 improving efficiency and effectiveness of the network.  Based on a
 selected set of reference connections, suitable target values are
 assigned to the selected GoS parameters under normal and high load
 conditions.  These end-to-end GoS target values are then apportioned
 to individual resource components of the reference connections for
 dimensioning purposes.

4.7 Content Distribution

 The Internet is dominated by client-server interactions, especially
 Web traffic (in the future, more sophisticated media servers may
 become dominant).  The location and performance of major information
 servers has a significant impact on the traffic patterns within the
 Internet as well as on the perception of service quality by end
 users.
 A number of dynamic load balancing techniques have been devised to
 improve the performance of replicated information servers.  These
 techniques can cause spatial traffic characteristics to become more
 dynamic in the Internet because information servers can be
 dynamically picked based upon the location of the clients, the
 location of the servers, the relative utilization of the servers, the
 relative performance of different networks, and the relative
 performance of different parts of a network.  This process of
 assignment of distributed servers to clients is called Traffic
 Directing.  It functions at the application layer.
 Traffic Directing schemes that allocate servers in multiple
 geographically dispersed locations to clients may require empirical
 network performance statistics to make more effective decisions.  In
 the future, network measurement systems may need to provide this type
 of information.  The exact parameters needed are not yet defined.
 When congestion exists in the network, Traffic Directing and Traffic
 Engineering systems should act in a coordinated manner.  This topic
 is for further study.

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 The issues related to location and replication of information
 servers, particularly web servers, are important for Internet traffic
 engineering because these servers contribute a substantial proportion
 of Internet traffic.

5.0 Taxonomy of Traffic Engineering Systems

 This section presents a short taxonomy of traffic engineering
 systems.  A taxonomy of traffic engineering systems can be
 constructed based on traffic engineering styles and views as listed
 below:
  1. Time-dependent vs State-dependent vs Event-dependent
  2. Offline vs Online
  3. Centralized vs Distributed
  4. Local vs Global Information
  5. Prescriptive vs Descriptive
  6. Open Loop vs Closed Loop
  7. Tactical vs Strategic
 These classification systems are described in greater detail in the
 following subsections of this document.

5.1 Time-Dependent Versus State-Dependent Versus Event Dependent

 Traffic engineering methodologies can be classified as time-
 dependent, or state-dependent, or event-dependent.  All TE schemes
 are considered to be dynamic in this document.  Static TE implies
 that no traffic engineering methodology or algorithm is being
 applied.
 In the time-dependent TE, historical information based on periodic
 variations in traffic, (such as time of day), is used to pre-program
 routing plans and other TE control mechanisms.  Additionally,
 customer subscription or traffic projection may be used.  Pre-
 programmed routing plans typically change on a relatively long time
 scale (e.g., diurnal).  Time-dependent algorithms do not attempt to
 adapt to random variations in traffic or changing network conditions.
 An example of a time-dependent algorithm is a global centralized
 optimizer where the input to the system is a traffic matrix and
 multi-class QoS requirements as described [MR99].
 State-dependent TE adapts the routing plans for packets based on the
 current state of the network.  The current state of the network
 provides additional information on variations in actual traffic
 (i.e., perturbations from regular variations) that could not be
 predicted using historical information.  Constraint-based routing is

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 an example of state-dependent TE operating in a relatively long time
 scale.  An example operating in a relatively short time scale is a
 load-balancing algorithm described in [MATE].
 The state of the network can be based on parameters such as
 utilization, packet delay, packet loss, etc.  These parameters can be
 obtained in several ways.  For example, each router may flood these
 parameters periodically or by means of some kind of trigger to other
 routers.  Another approach is for a particular router performing
 adaptive TE to send probe packets along a path to gather the state of
 that path.  Still another approach is for a management system to
 gather relevant information from network elements.
 Expeditious and accurate gathering and distribution of state
 information is critical for adaptive TE due to the dynamic nature of
 network conditions.  State-dependent algorithms may be applied to
 increase network efficiency and resilience.  Time-dependent
 algorithms are more suitable for predictable traffic variations.  On
 the other hand, state-dependent algorithms are more suitable for
 adapting to the prevailing network state.
 Event-dependent TE methods can also be used for TE path selection.
 Event-dependent TE methods are distinct from time-dependent and
 state-dependent TE methods in the manner in which paths are selected.
 These algorithms are adaptive and distributed in nature and typically
 use learning models to find good paths for TE in a network.  While
 state-dependent TE models typically use available-link-bandwidth
 (ALB) flooding for TE path selection, event-dependent TE methods do
 not require ALB flooding.  Rather, event-dependent TE methods
 typically search out capacity by learning models, as in the success-
 to-the-top (STT) method.  ALB flooding can be resource intensive,
 since it requires link bandwidth to carry LSAs, processor capacity to
 process LSAs, and the overhead can limit area/autonomous system (AS)
 size.  Modeling results suggest that event-dependent TE methods could
 lead to a reduction in ALB flooding overhead without loss of network
 throughput performance [ASH3].

5.2 Offline Versus Online

 Traffic engineering requires the computation of routing plans.  The
 computation may be performed offline or online.  The computation can
 be done offline for scenarios where routing plans need not be
 executed in real-time.  For example, routing plans computed from
 forecast information may be computed offline.  Typically, offline
 computation is also used to perform extensive searches on multi-
 dimensional solution spaces.

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 Online computation is required when the routing plans must adapt to
 changing network conditions as in state-dependent algorithms.  Unlike
 offline computation (which can be computationally demanding), online
 computation is geared toward relative simple and fast calculations to
 select routes, fine-tune the allocations of resources, and perform
 load balancing.

5.3 Centralized Versus Distributed

 Centralized control has a central authority which determines routing
 plans and perhaps other TE control parameters on behalf of each
 router.  The central authority collects the network-state information
 from all routers periodically and returns the routing information to
 the routers.  The routing update cycle is a critical parameter
 directly impacting the performance of the network being controlled.
 Centralized control may need high processing power and high bandwidth
 control channels.
 Distributed control determines route selection by each router
 autonomously based on the routers view of the state of the network.
 The network state information may be obtained by the router using a
 probing method or distributed by other routers on a periodic basis
 using link state advertisements.  Network state information may also
 be disseminated under exceptional conditions.

5.4 Local Versus Global

 Traffic engineering algorithms may require local or global network-
 state information.
 Local information pertains to the state of a portion of the domain.
 Examples include the bandwidth and packet loss rate of a particular
 path.  Local state information may be sufficient for certain
 instances of distributed-controlled TEs.
 Global information pertains to the state of the entire domain
 undergoing traffic engineering.  Examples include a global traffic
 matrix and loading information on each link throughout the domain of
 interest.  Global state information is typically required with
 centralized control.  Distributed TE systems may also need global
 information in some cases.

5.5 Prescriptive Versus Descriptive

 TE systems may also be classified as prescriptive or descriptive.

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 Prescriptive traffic engineering evaluates alternatives and
 recommends a course of action.  Prescriptive traffic engineering can
 be further categorized as either corrective or perfective.
 Corrective TE prescribes a course of action to address an existing or
 predicted anomaly.  Perfective TE prescribes a course of action to
 evolve and improve network performance even when no anomalies are
 evident.
 Descriptive traffic engineering, on the other hand, characterizes the
 state of the network and assesses the impact of various policies
 without recommending any particular course of action.

5.6 Open-Loop Versus Closed-Loop

 Open-loop traffic engineering control is where control action does
 not use feedback information from the current network state.  The
 control action may use its own local information for accounting
 purposes, however.
 Closed-loop traffic engineering control is where control action
 utilizes feedback information from the network state.  The feedback
 information may be in the form of historical information or current
 measurement.

5.7 Tactical vs Strategic

 Tactical traffic engineering aims to address specific performance
 problems (such as hot-spots) that occur in the network from a
 tactical perspective, without consideration of overall strategic
 imperatives.  Without proper planning and insights, tactical TE tends
 to be ad hoc in nature.
 Strategic traffic engineering approaches the TE problem from a more
 organized and systematic perspective, taking into consideration the
 immediate and longer term consequences of specific policies and
 actions.

6.0 Recommendations for Internet Traffic Engineering

 This section describes high level recommendations for traffic
 engineering in the Internet.  These recommendations are presented in
 general terms.
 The recommendations describe the capabilities needed to solve a
 traffic engineering problem or to achieve a traffic engineering
 objective.  Broadly speaking, these recommendations can be
 categorized as either functional and non-functional recommendations.

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 Functional recommendations for Internet traffic engineering describe
 the functions that a traffic engineering system should perform.
 These functions are needed to realize traffic engineering objectives
 by addressing traffic engineering problems.
 Non-functional recommendations for Internet traffic engineering
 relate to the quality attributes or state characteristics of a
 traffic engineering system.  These recommendations may contain
 conflicting assertions and may sometimes be difficult to quantify
 precisely.

6.1 Generic Non-functional Recommendations

 The generic non-functional recommendations for Internet traffic
 engineering include: usability, automation, scalability, stability,
 visibility, simplicity, efficiency, reliability, correctness,
 maintainability, extensibility, interoperability, and security.  In a
 given context, some of these recommendations may be critical while
 others may be optional.  Therefore, prioritization may be required
 during the development phase of a traffic engineering system (or
 components thereof) to tailor it to a specific operational context.
 In the following paragraphs, some of the aspects of the non-
 functional recommendations for Internet traffic engineering are
 summarized.
 Usability: Usability is a human factor aspect of traffic engineering
 systems.  Usability refers to the ease with which a traffic
 engineering system can be deployed and operated.  In general, it is
 desirable to have a TE system that can be readily deployed in an
 existing network.  It is also desirable to have a TE system that is
 easy to operate and maintain.
 Automation: Whenever feasible, a traffic engineering system should
 automate as many traffic engineering functions as possible to
 minimize the amount of human effort needed to control and analyze
 operational networks.  Automation is particularly imperative in large
 scale public networks because of the high cost of the human aspects
 of network operations and the high risk of network problems caused by
 human errors.  Automation may entail the incorporation of automatic
 feedback and intelligence into some components of the traffic
 engineering system.
 Scalability: Contemporary public networks are growing very fast with
 respect to network size and traffic volume.  Therefore, a TE system
 should be scalable to remain applicable as the network evolves.  In
 particular, a TE system should remain functional as the network
 expands with regard to the number of routers and links, and with

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 respect to the traffic volume.  A TE system should have a scalable
 architecture, should not adversely impair other functions and
 processes in a network element, and should not consume too much
 network resources when collecting and distributing state information
 or when exerting control.
 Stability: Stability is a very important consideration in traffic
 engineering systems that respond to changes in the state of the
 network.  State-dependent traffic engineering methodologies typically
 mandate a tradeoff between responsiveness and stability.  It is
 strongly recommended that when tradeoffs are warranted between
 responsiveness and stability, that the tradeoff should be made in
 favor of stability (especially in public IP backbone networks).
 Flexibility: A TE system should be flexible to allow for changes in
 optimization policy.  In particular, a TE system should provide
 sufficient configuration options so that a network administrator can
 tailor the TE system to a particular environment.  It may also be
 desirable to have both online and offline TE subsystems which can be
 independently enabled and disabled.  TE systems that are used in
 multi-class networks should also have options to support class based
 performance evaluation and optimization.
 Visibility: As part of the TE system, mechanisms should exist to
 collect statistics from the network and to analyze these statistics
 to determine how well the network is functioning.  Derived statistics
 such as traffic matrices, link utilization, latency, packet loss, and
 other performance measures of interest which are determined from
 network measurements can be used as indicators of prevailing network
 conditions.  Other examples of status information which should be
 observed include existing functional routing information
 (additionally, in the context of MPLS existing LSP routes), etc.
 Simplicity: Generally, a TE system should be as simple as possible.
 More importantly, the TE system should be relatively easy to use
 (i.e., clean, convenient, and intuitive user interfaces).  Simplicity
 in user interface does not necessarily imply that the TE system will
 use naive algorithms.  When complex algorithms and internal
 structures are used, such complexities should be hidden as much as
 possible from the network administrator through the user interface.
 Interoperability: Whenever feasible, traffic engineering systems and
 their components should be developed with open standards based
 interfaces to allow interoperation with other systems and components.
 Security: Security is a critical consideration in traffic engineering
 systems.  Such traffic engineering systems typically exert control
 over certain functional aspects of the network to achieve the desired

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 performance objectives.  Therefore, adequate measures must be taken
 to safeguard the integrity of the traffic engineering system.
 Adequate measures must also be taken to protect the network from
 vulnerabilities that originate from security breaches and other
 impairments within the traffic engineering system.
 The remainder of this section will focus on some of the high level
 functional recommendations for traffic engineering.

6.2 Routing Recommendations

 Routing control is a significant aspect of Internet traffic
 engineering.  Routing impacts many of the key performance measures
 associated with networks, such as throughput, delay, and utilization.
 Generally, it is very difficult to provide good service quality in a
 wide area network without effective routing control.  A desirable
 routing system is one that takes traffic characteristics and network
 constraints into account during route selection while maintaining
 stability.
 Traditional shortest path first (SPF) interior gateway protocols are
 based on shortest path algorithms and have limited control
 capabilities for traffic engineering [RFC-2702, AWD2].  These
 limitations include :
 1. The well known issues with pure SPF protocols, which do not take
    network constraints and traffic characteristics into account
    during route selection.  For example, since IGPs always use the
    shortest paths (based on administratively assigned link metrics)
    to forward traffic, load sharing cannot be accomplished among
    paths of different costs.  Using shortest paths to forward traffic
    conserves network resources, but may cause the following problems:
    1) If traffic from a source to a destination exceeds the capacity
    of a link along the shortest path, the link (hence the shortest
    path) becomes congested while a longer path between these two
    nodes may be under-utilized; 2) the shortest paths from different
    sources can overlap at some links.  If the total traffic from the
    sources exceeds the capacity of any of these links, congestion
    will occur.  Problems can also occur because traffic demand
    changes over time but network topology and routing configuration
    cannot be changed as rapidly.  This causes the network topology
    and routing configuration to become sub-optimal over time, which
    may result in persistent congestion problems.
 2. The Equal-Cost Multi-Path (ECMP) capability of SPF IGPs supports
    sharing of traffic among equal cost paths between two nodes.
    However, ECMP attempts to divide the traffic as equally as
    possible among the equal cost shortest paths.  Generally, ECMP

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    does not support configurable load sharing ratios among equal cost
    paths.  The result is that one of the paths may carry
    significantly more traffic than other paths because it may also
    carry traffic from other sources.  This situation can result in
    congestion along the path that carries more traffic.
 3. Modifying IGP metrics to control traffic routing tends to have
    network-wide effect.  Consequently, undesirable and unanticipated
    traffic shifts can be triggered as a result.  Recent work
    described in Section 8.0 may be capable of better control [FT00,
    FT01].
 Because of these limitations, new capabilities are needed to enhance
 the routing function in IP networks.  Some of these capabilities have
 been described elsewhere and are summarized below.
 Constraint-based routing is desirable to evolve the routing
 architecture of IP networks, especially public IP backbones with
 complex topologies [RFC-2702].  Constraint-based routing computes
 routes to fulfill requirements subject to constraints.  Constraints
 may include bandwidth, hop count, delay, and administrative policy
 instruments such as resource class attributes [RFC-2702, RFC-2386].
 This makes it possible to select routes that satisfy a given set of
 requirements subject to network and administrative policy
 constraints.  Routes computed through constraint-based routing are
 not necessarily the shortest paths.  Constraint-based routing works
 best with path oriented technologies that support explicit routing,
 such as MPLS.
 Constraint-based routing can also be used as a way to redistribute
 traffic onto the infrastructure (even for best effort traffic).  For
 example, if the bandwidth requirements for path selection and
 reservable bandwidth attributes of network links are appropriately
 defined and configured, then congestion problems caused by uneven
 traffic distribution may be avoided or reduced.  In this way, the
 performance and efficiency of the network can be improved.
 A number of enhancements are needed to conventional link state IGPs,
 such as OSPF and IS-IS, to allow them to distribute additional state
 information required for constraint-based routing.  These extensions
 to OSPF were described in [KATZ] and to IS-IS in [SMIT].
 Essentially, these enhancements require the propagation of additional
 information in link state advertisements.  Specifically, in addition
 to normal link-state information, an enhanced IGP is required to
 propagate topology state information needed for constraint-based
 routing.  Some of the additional topology state information include
 link attributes such as reservable bandwidth and link resource class
 attribute (an administratively specified property of the link).  The

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 resource class attribute concept was defined in [RFC-2702].  The
 additional topology state information is carried in new TLVs and
 sub-TLVs in IS-IS, or in the Opaque LSA in OSPF [SMIT, KATZ].
 An enhanced link-state IGP may flood information more frequently than
 a normal IGP.  This is because even without changes in topology,
 changes in reservable bandwidth or link affinity can trigger the
 enhanced IGP to initiate flooding.  A tradeoff is typically required
 between the timeliness of the information flooded and the flooding
 frequency to avoid excessive consumption of link bandwidth and
 computational resources, and more importantly, to avoid instability.
 In a TE system, it is also desirable for the routing subsystem to
 make the load splitting ratio among multiple paths (with equal cost
 or different cost) configurable.  This capability gives network
 administrators more flexibility in the control of traffic
 distribution across the network.  It can be very useful for
 avoiding/relieving congestion in certain situations.  Examples can be
 found in [XIAO].
 The routing system should also have the capability to control the
 routes of subsets of traffic without affecting the routes of other
 traffic if sufficient resources exist for this purpose.  This
 capability allows a more refined control over the distribution of
 traffic across the network.  For example, the ability to move traffic
 from a source to a destination away from its original path to another
 path (without affecting other traffic paths) allows traffic to be
 moved from resource-poor network segments to resource-rich segments.
 Path oriented technologies such as MPLS inherently support this
 capability as discussed in [AWD2].
 Additionally, the routing subsystem should be able to select
 different paths for different classes of traffic (or for different
 traffic behavior aggregates) if the network supports multiple classes
 of service (different behavior aggregates).

6.3 Traffic Mapping Recommendations

 Traffic mapping pertains to the assignment of traffic workload onto
 pre-established paths to meet certain requirements.  Thus, while
 constraint-based routing deals with path selection, traffic mapping
 deals with the assignment of traffic to established paths which may
 have been selected by constraint-based routing or by some other
 means.  Traffic mapping can be performed by time-dependent or state-
 dependent mechanisms, as described in Section 5.1.

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 An important aspect of the traffic mapping function is the ability to
 establish multiple paths between an originating node and a
 destination node, and the capability to distribute the traffic
 between the two nodes across the paths according to some policies.  A
 pre-condition for this scheme is the existence of flexible mechanisms
 to partition traffic and then assign the traffic partitions onto the
 parallel paths.  This requirement was noted in [RFC-2702].  When
 traffic is assigned to multiple parallel paths, it is recommended
 that special care should be taken to ensure proper ordering of
 packets belonging to the same application (or micro-flow) at the
 destination node of the parallel paths.
 As a general rule, mechanisms that perform the traffic mapping
 functions should aim to map the traffic onto the network
 infrastructure to minimize congestion.  If the total traffic load
 cannot be accommodated, or if the routing and mapping functions
 cannot react fast enough to changing traffic conditions, then a
 traffic mapping system may rely on short time scale congestion
 control mechanisms (such as queue management, scheduling, etc.) to
 mitigate congestion.  Thus, mechanisms that perform the traffic
 mapping functions should complement existing congestion control
 mechanisms.  In an operational network, it is generally desirable to
 map the traffic onto the infrastructure such that intra-class and
 inter-class resource contention are minimized.
 When traffic mapping techniques that depend on dynamic state feedback
 (e.g., MATE and such like) are used, special care must be taken to
 guarantee network stability.

6.4 Measurement Recommendations

 The importance of measurement in traffic engineering has been
 discussed throughout this document.  Mechanisms should be provided to
 measure and collect statistics from the network to support the
 traffic engineering function.  Additional capabilities may be needed
 to help in the analysis of the statistics.  The actions of these
 mechanisms should not adversely affect the accuracy and integrity of
 the statistics collected.  The mechanisms for statistical data
 acquisition should also be able to scale as the network evolves.
 Traffic statistics may be classified according to long-term or
 short-term time scales.  Long-term time scale traffic statistics are
 very useful for traffic engineering.  Long-term time scale traffic
 statistics may capture or reflect periodicity in network workload
 (such as hourly, daily, and weekly variations in traffic profiles) as
 well as traffic trends.  Aspects of the monitored traffic statistics
 may also depict class of service characteristics for a network
 supporting multiple classes of service.  Analysis of the long-term

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 traffic statistics MAY yield secondary statistics such as busy hour
 characteristics, traffic growth patterns, persistent congestion
 problems, hot-spot, and imbalances in link utilization caused by
 routing anomalies.
 A mechanism for constructing traffic matrices for both long-term and
 short-term traffic statistics should be in place.  In multi-service
 IP networks, the traffic matrices may be constructed for different
 service classes.  Each element of a traffic matrix represents a
 statistic of traffic flow between a pair of abstract nodes.  An
 abstract node may represent a router, a collection of routers, or a
 site in a VPN.
 Measured traffic statistics should provide reasonable and reliable
 indicators of the current state of the network on the short-term
 scale.  Some short term traffic statistics may reflect link
 utilization and link congestion status.  Examples of congestion
 indicators include excessive packet delay, packet loss, and high
 resource utilization.  Examples of mechanisms for distributing this
 kind of information include SNMP, probing techniques, FTP, IGP link
 state advertisements, etc.

6.5 Network Survivability

 Network survivability refers to the capability of a network to
 maintain service continuity in the presence of faults.  This can be
 accomplished by promptly recovering from network impairments and
 maintaining the required QoS for existing services after recovery.
 Survivability has become an issue of great concern within the
 Internet community due to the increasing demands to carry mission
 critical traffic, real-time traffic, and other high priority traffic
 over the Internet.  Survivability can be addressed at the device
 level by developing network elements that are more reliable; and at
 the network level by incorporating redundancy into the architecture,
 design, and operation of networks.  It is recommended that a
 philosophy of robustness and survivability should be adopted in the
 architecture, design, and operation of traffic engineering that
 control IP networks (especially public IP networks).  Because
 different contexts may demand different levels of survivability, the
 mechanisms developed to support network survivability should be
 flexible so that they can be tailored to different needs.
 Failure protection and restoration capabilities have become available
 from multiple layers as network technologies have continued to
 improve.  At the bottom of the layered stack, optical networks are
 now capable of providing dynamic ring and mesh restoration
 functionality at the wavelength level as well as traditional
 protection functionality.  At the SONET/SDH layer survivability

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 capability is provided with Automatic Protection Switching (APS) as
 well as self-healing ring and mesh architectures.  Similar
 functionality is provided by layer 2 technologies such as ATM
 (generally with slower mean restoration times).  Rerouting is
 traditionally used at the IP layer to restore service following link
 and node outages.  Rerouting at the IP layer occurs after a period of
 routing convergence which may require seconds to minutes to complete.
 Some new developments in the MPLS context make it possible to achieve
 recovery at the IP layer prior to convergence [SHAR].
 To support advanced survivability requirements, path-oriented
 technologies such a MPLS can be used to enhance the survivability of
 IP networks in a potentially cost effective manner.  The advantages
 of path oriented technologies such as MPLS for IP restoration becomes
 even more evident when class based protection and restoration
 capabilities are required.
 Recently, a common suite of control plane protocols has been proposed
 for both MPLS and optical transport networks under the acronym
 Multi-protocol Lambda Switching [AWD1].  This new paradigm of Multi-
 protocol Lambda Switching will support even more sophisticated mesh
 restoration capabilities at the optical layer for the emerging IP
 over WDM network architectures.
 Another important aspect regarding multi-layer survivability is that
 technologies at different layers provide protection and restoration
 capabilities at different temporal granularities (in terms of time
 scales) and at different bandwidth granularity (from packet-level to
 wavelength level).  Protection and restoration capabilities can also
 be sensitive to different service classes and different network
 utility models.
 The impact of service outages varies significantly for different
 service classes depending upon the effective duration of the outage.
 The duration of an outage can vary from milliseconds (with minor
 service impact) to seconds (with possible call drops for IP telephony
 and session time-outs for connection oriented transactions) to
 minutes and hours (with potentially considerable social and business
 impact).
 Coordinating different protection and restoration capabilities across
 multiple layers in a cohesive manner to ensure network survivability
 is maintained at reasonable cost is a challenging task.  Protection
 and restoration coordination across layers may not always be
 feasible, because networks at different layers may belong to
 different administrative domains.

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 The following paragraphs present some of the general recommendations
 for protection and restoration coordination.
  1. Protection and restoration capabilities from different layers

should be coordinated whenever feasible and appropriate to provide

 network survivability in a flexible and cost effective manner.
 Minimization of function duplication across layers is one way to
 achieve the coordination.  Escalation of alarms and other fault
 indicators from lower to higher layers may also be performed in a
 coordinated manner.  A temporal order of restoration trigger timing
 at different layers is another way to coordinate multi-layer
 protection/restoration.
  1. Spare capacity at higher layers is often regarded as working

traffic at lower layers. Placing protection/restoration functions in

 many layers may increase redundancy and robustness, but it should not
 result in significant and avoidable inefficiencies in network
 resource utilization.
  1. It is generally desirable to have protection and restoration

schemes that are bandwidth efficient.

  1. Failure notification throughout the network should be timely and

reliable.

  1. Alarms and other fault monitoring and reporting capabilities

should be provided at appropriate layers.

6.5.1 Survivability in MPLS Based Networks

 MPLS is an important emerging technology that enhances IP networks in
 terms of features, capabilities, and services.  Because MPLS is
 path-oriented, it can potentially provide faster and more predictable
 protection and restoration capabilities than conventional hop by hop
 routed IP systems.  This subsection describes some of the basic
 aspects and recommendations for MPLS networks regarding protection
 and restoration.  See [SHAR] for a more comprehensive discussion on
 MPLS based recovery.
 Protection types for MPLS networks can be categorized as link
 protection, node protection, path protection, and segment protection.
  1. Link Protection: The objective for link protection is to protect

an LSP from a given link failure. Under link protection, the path

    of the protection or backup LSP (the secondary LSP) is disjoint
    from the path of the working or operational LSP at the particular
    link over which protection is required.  When the protected link
    fails, traffic on the working LSP is switched over to the

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    protection LSP at the head-end of the failed link.  This is a
    local repair method which can be fast.  It might be more
    appropriate in situations where some network elements along a
    given path are less reliable than others.
  1. Node Protection: The objective of LSP node protection is to

protect an LSP from a given node failure. Under node protection,

    the path of the protection LSP is disjoint from the path of the
    working LSP at the particular node to be protected.  The secondary
    path is also disjoint from the primary path at all links
    associated with the node to be protected.  When the node fails,
    traffic on the working LSP is switched over to the protection LSP
    at the upstream LSR directly connected to the failed node.
  1. Path Protection: The goal of LSP path protection is to protect an

LSP from failure at any point along its routed path. Under path

    protection, the path of the protection LSP is completely disjoint
    from the path of the working LSP.  The advantage of path
    protection is that the backup LSP protects the working LSP from
    all possible link and node failures along the path, except for
    failures that might occur at the ingress and egress LSRs, or for
    correlated failures that might impact both working and backup
    paths simultaneously.  Additionally, since the path selection is
    end-to-end, path protection might be more efficient in terms of
    resource usage than link or node protection.  However, path
    protection may be slower than link and node protection in general.
  1. Segment Protection: An MPLS domain may be partitioned into

multiple protection domains whereby a failure in a protection

    domain is rectified within that domain.  In cases where an LSP
    traverses multiple protection domains, a protection mechanism
    within a domain only needs to protect the segment of the LSP that
    lies within the domain.  Segment protection will generally be
    faster than path protection because recovery generally occurs
    closer to the fault.

6.5.2 Protection Option

 Another issue to consider is the concept of protection options.  The
 protection option uses the notation m:n protection, where m is the
 number of protection LSPs used to protect n working LSPs.  Feasible
 protection options follow.
  1. 1:1: one working LSP is protected/restored by one protection LSP.
  1. 1:n: one protection LSP is used to protect/restore n working LSPs.

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  1. n:1: one working LSP is protected/restored by n protection LSPs,

possibly with configurable load splitting ratio. When more than

    one protection LSP is used, it may be desirable to share the
    traffic across the protection LSPs when the working LSP fails to
    satisfy the bandwidth requirement of the traffic trunk associated
    with the working LSP.  This may be especially useful when it is
    not feasible to find one path that can satisfy the bandwidth
    requirement of the primary LSP.
  1. 1+1: traffic is sent concurrently on both the working LSP and the

protection LSP. In this case, the egress LSR selects one of the

    two LSPs based on a local traffic integrity decision process,
    which compares the traffic received from both the working and the
    protection LSP and identifies discrepancies.  It is unlikely that
    this option would be used extensively in IP networks due to its
    resource utilization inefficiency.  However, if bandwidth becomes
    plentiful and cheap, then this option might become quite viable
    and attractive in IP networks.

6.6 Traffic Engineering in Diffserv Environments

 This section provides an overview of the traffic engineering features
 and recommendations that are specifically pertinent to Differentiated
 Services (Diffserv) [RFC-2475] capable IP networks.
 Increasing requirements to support multiple classes of traffic, such
 as best effort and mission critical data, in the Internet calls for
 IP networks to differentiate traffic according to some criteria, and
 to accord preferential treatment to certain types of traffic.  Large
 numbers of flows can be aggregated into a few behavior aggregates
 based on some criteria in terms of common performance requirements in
 terms of packet loss ratio, delay, and jitter; or in terms of common
 fields within the IP packet headers.
 As Diffserv evolves and becomes deployed in operational networks,
 traffic engineering will be critical to ensuring that SLAs defined
 within a given Diffserv service model are met.  Classes of service
 (CoS) can be supported in a Diffserv environment by concatenating
 per-hop behaviors (PHBs) along the routing path, using service
 provisioning mechanisms, and by appropriately configuring edge
 functionality such as traffic classification, marking, policing, and
 shaping.  PHB is the forwarding behavior that a packet receives at a
 DS node (a Diffserv-compliant node).  This is accomplished by means
 of buffer management and packet scheduling mechanisms.  In this
 context, packets belonging to a class are those that are members of a
 corresponding ordering aggregate.

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 Traffic engineering can be used as a compliment to Diffserv
 mechanisms to improve utilization of network resources, but not as a
 necessary element in general.  When traffic engineering is used, it
 can be operated on an aggregated basis across all service classes
 [RFC-3270] or on a per service class basis.  The former is used to
 provide better distribution of the aggregate traffic load over the
 network resources.  (See [RFC-3270] for detailed mechanisms to
 support aggregate traffic engineering.)  The latter case is discussed
 below since it is specific to the Diffserv environment, with so
 called Diffserv-aware traffic engineering [DIFF_TE].
 For some Diffserv networks, it may be desirable to control the
 performance of some service classes by enforcing certain
 relationships between the traffic workload contributed by each
 service class and the amount of network resources allocated or
 provisioned for that service class.  Such relationships between
 demand and resource allocation can be enforced using a combination
 of, for example: (1) traffic engineering mechanisms on a per service
 class basis that enforce the desired relationship between the amount
 of traffic contributed by a given service class and the resources
 allocated to that class, and (2) mechanisms that dynamically adjust
 the resources allocated to a given service class to relate to the
 amount of traffic contributed by that service class.
 It may also be desirable to limit the performance impact of high
 priority traffic on relatively low priority traffic.  This can be
 achieved by, for example, controlling the percentage of high priority
 traffic that is routed through a given link.  Another way to
 accomplish this is to increase link capacities appropriately so that
 lower priority traffic can still enjoy adequate service quality.
 When the ratio of traffic workload contributed by different service
 classes vary significantly from router to router, it may not suffice
 to rely exclusively on conventional IGP routing protocols or on
 traffic engineering mechanisms that are insensitive to different
 service classes.  Instead, it may be desirable to perform traffic
 engineering, especially routing control and mapping functions, on a
 per service class basis.  One way to accomplish this in a domain that
 supports both MPLS and Diffserv is to define class specific LSPs and
 to map traffic from each class onto one or more LSPs that correspond
 to that service class.  An LSP corresponding to a given service class
 can then be routed and protected/restored in a class dependent
 manner, according to specific policies.
 Performing traffic engineering on a per class basis may require
 certain per-class parameters to be distributed.  Note that it is
 common to have some classes share some aggregate constraint (e.g.,
 maximum bandwidth requirement) without enforcing the constraint on
 each individual class.  These classes then can be grouped into a

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 class-type and per-class-type parameters can be distributed instead
 to improve scalability.  It also allows better bandwidth sharing
 between classes in the same class-type.  A class-type is a set of
 classes that satisfy the following two conditions:
 1) Classes in the same class-type have common aggregate requirements
 to satisfy required performance levels.
 2) There is no requirement to be enforced at the level of individual
 class in the class-type.  Note that it is still possible,
 nevertheless, to implement some priority policies for classes in the
 same class-type to permit preferential access to the class-type
 bandwidth through the use of preemption priorities.
 An example of the class-type can be a low-loss class-type that
 includes both AF1-based and AF2-based Ordering Aggregates.  With such
 a class-type, one may implement some priority policy which assigns
 higher preemption priority to AF1-based traffic trunks over AF2-based
 ones, vice versa, or the same priority.
 See [DIFF-TE] for detailed requirements on Diffserv-aware traffic
 engineering.

6.7 Network Controllability

 Off-line (and on-line) traffic engineering considerations would be of
 limited utility if the network could not be controlled effectively to
 implement the results of TE decisions and to achieve desired network
 performance objectives.  Capacity augmentation is a coarse grained
 solution to traffic engineering issues.  However, it is simple and
 may be advantageous if bandwidth is abundant and cheap or if the
 current or expected network workload demands it.  However, bandwidth
 is not always abundant and cheap, and the workload may not always
 demand additional capacity.  Adjustments of administrative weights
 and other parameters associated with routing protocols provide finer
 grained control, but is difficult to use and imprecise because of the
 routing interactions that occur across the network.  In certain
 network contexts, more flexible, finer grained approaches which
 provide more precise control over the mapping of traffic to routes
 and over the selection and placement of routes may be appropriate and
 useful.
 Control mechanisms can be manual (e.g., administrative
 configuration), partially-automated (e.g., scripts) or fully-
 automated (e.g., policy based management systems).  Automated
 mechanisms are particularly required in large scale networks.
 Multi-vendor interoperability can be facilitated by developing and
 deploying standardized management

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 systems (e.g., standard MIBs) and policies (PIBs) to support the
 control functions required to address traffic engineering objectives
 such as load distribution and protection/restoration.
 Network control functions should be secure, reliable, and stable as
 these are often needed to operate correctly in times of network
 impairments (e.g., during network congestion or security attacks).

7.0 Inter-Domain Considerations

 Inter-domain traffic engineering is concerned with the performance
 optimization for traffic that originates in one administrative domain
 and terminates in a different one.
 Traffic exchange between autonomous systems in the Internet occurs
 through exterior gateway protocols.  Currently, BGP [BGP4] is the
 standard exterior gateway protocol for the Internet.  BGP provides a
 number of attributes and capabilities (e.g., route filtering) that
 can be used for inter-domain traffic engineering.  More specifically,
 BGP permits the control of routing information and traffic exchange
 between Autonomous Systems (AS's) in the Internet.  BGP incorporates
 a sequential decision process which calculates the degree of
 preference for various routes to a given destination network.  There
 are two fundamental aspects to inter-domain traffic engineering using
 BGP:
  1. Route Redistribution: controlling the import and export of routes

between AS's, and controlling the redistribution of routes between

    BGP and other protocols within an AS.
  1. Best path selection: selecting the best path when there are

multiple candidate paths to a given destination network. Best

    path selection is performed by the BGP decision process based on a
    sequential procedure, taking a number of different considerations
    into account.  Ultimately, best path selection under BGP boils
    down to selecting preferred exit points out of an AS towards
    specific destination networks.  The BGP path selection process can
    be influenced by manipulating the attributes associated with the
    BGP decision process.  These attributes include: NEXT-HOP, WEIGHT
    (Cisco proprietary which is also implemented by some other
    vendors), LOCAL-PREFERENCE, AS-PATH, ROUTE-ORIGIN, MULTI-EXIT-
    DESCRIMINATOR (MED), IGP METRIC, etc.
 Route-maps provide the flexibility to implement complex BGP policies
 based on pre-configured logical conditions.  In particular, Route-
 maps can be used to control import and export policies for incoming
 and outgoing routes, control the redistribution of routes between BGP
 and other protocols, and influence the selection of best paths by

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 manipulating the attributes associated with the BGP decision process.
 Very complex logical expressions that implement various types of
 policies can be implemented using a combination of Route-maps, BGP-
 attributes, Access-lists, and Community attributes.
 When looking at possible strategies for inter-domain TE with BGP, it
 must be noted that the outbound traffic exit point is controllable,
 whereas the interconnection point where inbound traffic is received
 from an EBGP peer typically is not, unless a special arrangement is
 made with the peer sending the traffic.  Therefore, it is up to each
 individual network to implement sound TE strategies that deal with
 the efficient delivery of outbound traffic from one's customers to
 one's peering points.  The vast majority of TE policy is based upon a
 "closest exit" strategy, which offloads interdomain traffic at the
 nearest outbound peer point towards the destination autonomous
 system.  Most methods of manipulating the point at which inbound
 traffic enters a network from an EBGP peer (inconsistent route
 announcements between peering points, AS pre-pending, and sending
 MEDs) are either ineffective, or not accepted in the peering
 community.
 Inter-domain TE with BGP is generally effective, but it is usually
 applied in a trial-and-error fashion.  A systematic approach for
 inter-domain traffic engineering is yet to be devised.
 Inter-domain TE is inherently more difficult than intra-domain TE
 under the current Internet architecture.  The reasons for this are
 both technical and administrative.  Technically, while topology and
 link state information are helpful for mapping traffic more
 effectively, BGP does not propagate such information across domain
 boundaries for stability and scalability reasons.  Administratively,
 there are differences in operating costs and network capacities
 between domains.  Generally, what may be considered a good solution
 in one domain may not necessarily be a good solution in another
 domain.  Moreover, it would generally be considered inadvisable for
 one domain to permit another domain to influence the routing and
 management of traffic in its network.
 MPLS TE-tunnels (explicit LSPs) can potentially add a degree of
 flexibility in the selection of exit points for inter-domain routing.
 The concept of relative and absolute metrics can be applied to this
 purpose.  The idea is that if BGP attributes are defined such that
 the BGP decision process depends on IGP metrics to select exit points
 for inter-domain traffic, then some inter-domain traffic destined to
 a given peer network can be made to prefer a specific exit point by
 establishing a TE-tunnel between the router making the selection to
 the peering point via a TE-tunnel and assigning the TE-tunnel a
 metric which is smaller than the IGP cost to all other peering

Awduche, et. al. Informational [Page 58] RFC 3272 Overview and Principles of Internet TE May 2002

 points.  If a peer accepts and processes MEDs, then a similar MPLS
 TE-tunnel based scheme can be applied to cause certain entrance
 points to be preferred by setting MED to be an IGP cost, which has
 been modified by the tunnel metric.
 Similar to intra-domain TE, inter-domain TE is best accomplished when
 a traffic matrix can be derived to depict the volume of traffic from
 one autonomous system to another.
 Generally, redistribution of inter-domain traffic requires
 coordination between peering partners.  An export policy in one
 domain that results in load redistribution across peer points with
 another domain can significantly affect the local traffic matrix
 inside the domain of the peering partner.  This, in turn, will affect
 the intra-domain TE due to changes in the spatial distribution of
 traffic.  Therefore, it is mutually beneficial for peering partners
 to coordinate with each other before attempting any policy changes
 that may result in significant shifts in inter-domain traffic.  In
 certain contexts, this coordination can be quite challenging due to
 technical and non- technical reasons.
 It is a matter of speculation as to whether MPLS, or similar
 technologies, can be extended to allow selection of constrained paths
 across domain boundaries.

8.0 Overview of Contemporary TE Practices in Operational IP Networks

 This section provides an overview of some contemporary traffic
 engineering practices in IP networks.  The focus is primarily on the
 aspects that pertain to the control of the routing function in
 operational contexts.  The intent here is to provide an overview of
 the commonly used practices.  The discussion is not intended to be
 exhaustive.
 Currently, service providers apply many of the traffic engineering
 mechanisms discussed in this document to optimize the performance of
 their IP networks.  These techniques include capacity planning for
 long time scales, routing control using IGP metrics and MPLS for
 medium time scales, the overlay model also for medium time scales,
 and traffic management mechanisms for short time scale.
 When a service provider plans to build an IP network, or expand the
 capacity of an existing network, effective capacity planning should
 be an important component of the process.  Such plans may take the
 following aspects into account: location of new nodes if any,
 existing and predicted traffic patterns, costs, link capacity,
 topology, routing design, and survivability.

Awduche, et. al. Informational [Page 59] RFC 3272 Overview and Principles of Internet TE May 2002

 Performance optimization of operational networks is usually an
 ongoing process in which traffic statistics, performance parameters,
 and fault indicators are continually collected from the network.
 This empirical data is then analyzed and used to trigger various
 traffic engineering mechanisms.  Tools that perform what-if analysis
 can also be used to assist the TE process by allowing various
 scenarios to be reviewed before a new set of configurations are
 implemented in the operational network.
 Traditionally, intra-domain real-time TE with IGP is done by
 increasing the OSPF or IS-IS metric of a congested link until enough
 traffic has been diverted from that link.  This approach has some
 limitations as discussed in Section 6.2.  Recently, some new intra-
 domain TE approaches/tools have been proposed
 [RR94][FT00][FT01][WANG].  Such approaches/tools take traffic matrix,
 network topology, and network performance objective(s) as input, and
 produce some link metrics and possibly some unequal load-sharing
 ratios to be set at the head-end routers of some ECMPs as output.
 These new progresses open new possibility for intra-domain TE with
 IGP to be done in a more systematic way.
 The overlay model (IP over ATM or IP over Frame relay) is another
 approach which is commonly used in practice [AWD2].  The IP over ATM
 technique is no longer viewed favorably due to recent advances in
 MPLS and router hardware technology.
 Deployment of MPLS for traffic engineering applications has commenced
 in some service provider networks.  One operational scenario is to
 deploy MPLS in conjunction with an IGP (IS-IS-TE or OSPF-TE) that
 supports the traffic engineering extensions, in conjunction with
 constraint-based routing for explicit route computations, and a
 signaling protocol (e.g., RSVP-TE or CRLDP) for LSP instantiation.
 In contemporary MPLS traffic engineering contexts, network
 administrators specify and configure link attributes and resource
 constraints such as maximum reservable bandwidth and resource class
 attributes for links (interfaces) within the MPLS domain.  A link
 state protocol that supports TE extensions (IS-IS-TE or OSPF-TE) is
 used to propagate information about network topology and link
 attribute to all routers in the routing area.  Network administrators
 also specify all the LSPs that are to originate each router.  For
 each LSP, the network administrator specifies the destination node
 and the attributes of the LSP which indicate the requirements that to
 be satisfied during the path selection process.  Each router then
 uses a local constraint-based routing process to compute explicit
 paths for all LSPs originating from it.  Subsequently, a signaling

Awduche, et. al. Informational [Page 60] RFC 3272 Overview and Principles of Internet TE May 2002

 protocol is used to instantiate the LSPs.  By assigning proper
 bandwidth values to links and LSPs, congestion caused by uneven
 traffic distribution can generally be avoided or mitigated.
 The bandwidth attributes of LSPs used for traffic engineering can be
 updated periodically.  The basic concept is that the bandwidth
 assigned to an LSP should relate in some manner to the bandwidth
 requirements of traffic that actually flows through the LSP.  The
 traffic attribute of an LSP can be modified to accommodate traffic
 growth and persistent traffic shifts.  If network congestion occurs
 due to some unexpected events, existing LSPs can be rerouted to
 alleviate the situation or network administrator can configure new
 LSPs to divert some traffic to alternative paths.  The reservable
 bandwidth of the congested links can also be reduced to force some
 LSPs to be rerouted to other paths.
 In an MPLS domain, a traffic matrix can also be estimated by
 monitoring the traffic on LSPs.  Such traffic statistics can be used
 for a variety of purposes including network planning and network
 optimization.  Current practice suggests that deploying an MPLS
 network consisting of hundreds of routers and thousands of LSPs is
 feasible.  In summary, recent deployment experience suggests that
 MPLS approach is very effective for traffic engineering in IP
 networks [XIAO].
 As mentioned previously in Section 7.0, one usually has no direct
 control over the distribution of inbound traffic.  Therefore, the
 main goal of contemporary inter-domain TE is to optimize the
 distribution of outbound traffic between multiple inter-domain links.
 When operating a global network, maintaining the ability to operate
 the network in a regional fashion where desired, while continuing to
 take advantage of the benefits of a global network, also becomes an
 important objective.
 Inter-domain TE with BGP usually begins with the placement of
 multiple peering interconnection points in locations that have high
 peer density, are in close proximity to originating/terminating
 traffic locations on one's own network, and are lowest in cost.
 There are generally several locations in each region of the world
 where the vast majority of major networks congregate and
 interconnect.  Some location-decision problems that arise in
 association with inter-domain routing are discussed in [AWD5].
 Once the locations of the interconnects are determined, and circuits
 are implemented, one decides how best to handle the routes heard from
 the peer, as well as how to propagate the peers' routes within one's
 own network.  One way to engineer outbound traffic flows on a network
 with many EBGP peers is to create a hierarchy of peers.  Generally,

Awduche, et. al. Informational [Page 61] RFC 3272 Overview and Principles of Internet TE May 2002

 the Local Preferences of all peers are set to the same value so that
 the shortest AS paths will be chosen to forward traffic.  Then, by
 over-writing the inbound MED metric (Multi-exit-discriminator metric,
 also referred to as "BGP metric".  Both terms are used
 interchangeably in this document) with BGP metrics to routes received
 at different peers, the hierarchy can be formed.  For example, all
 Local Preferences can be set to 200, preferred private peers can be
 assigned a BGP metric of 50, the rest of the private peers can be
 assigned a BGP metric of 100, and public peers can be assigned a BGP
 metric of 600.  "Preferred" peers might be defined as those peers
 with whom the most available capacity exists, whose customer base is
 larger in comparison to other peers, whose interconnection costs are
 the lowest, and with whom upgrading existing capacity is the easiest.
 In a network with low utilization at the edge, this works well.  The
 same concept could be applied to a network with higher edge
 utilization by creating more levels of BGP metrics between peers,
 allowing for more granularity in selecting the exit points for
 traffic bound for a dual homed customer on a peer's network.
 By only replacing inbound MED metrics with BGP metrics, only equal
 AS-Path length routes' exit points are being changed.  (The BGP
 decision considers Local Preference first, then AS-Path length, and
 then BGP metric).  For example, assume a network has two possible
 egress points, peer A and peer B.  Each peer has 40% of the
 Internet's routes exclusively on its network, while the remaining 20%
 of the Internet's routes are from customers who dual home between A
 and B.  Assume that both peers have a Local Preference of 200 and a
 BGP metric of 100.  If the link to peer A is congested, increasing
 its BGP metric while leaving the Local Preference at 200 will ensure
 that the 20% of total routes belonging to dual homed customers will
 prefer peer B as the exit point.  The previous example would be used
 in a situation where all exit points to a given peer were close to
 congestion levels, and traffic needed to be shifted away from that
 peer entirely.
 When there are multiple exit points to a given peer, and only one of
 them is congested, it is not necessary to shift traffic away from the
 peer entirely, but only from the one congested circuit.  This can be
 achieved by using passive IGP-metrics, AS-path filtering, or prefix
 filtering.
 Occasionally, more drastic changes are needed, for example, in
 dealing with a "problem peer" who is difficult to work with on
 upgrades or is charging high prices for connectivity to their
 network.  In that case, the Local Preference to that peer can be
 reduced below the level of other peers.  This effectively reduces the
 amount of traffic sent to that peer to only originating traffic

Awduche, et. al. Informational [Page 62] RFC 3272 Overview and Principles of Internet TE May 2002

 (assuming no transit providers are involved).  This type of change
 can affect a large amount of traffic, and is only used after other
 methods have failed to provide the desired results.
 Although it is not much of an issue in regional networks, the
 propagation of a peer's routes back through the network must be
 considered when a network is peering on a global scale.  Sometimes,
 business considerations can influence the choice of BGP policies in a
 given context.  For example, it may be imprudent, from a business
 perspective, to operate a global network and provide full access to
 the global customer base to a small network in a particular country.
 However, for the purpose of providing one's own customers with
 quality service in a particular region, good connectivity to that
 in-country network may still be necessary.  This can be achieved by
 assigning a set of communities at the edge of the network, which have
 a known behavior when routes tagged with those communities are
 propagating back through the core.  Routes heard from local peers
 will be prevented from propagating back to the global network,
 whereas routes learned from larger peers may be allowed to propagate
 freely throughout the entire global network.  By implementing a
 flexible community strategy, the benefits of using a single global AS
 Number (ASN) can be realized, while the benefits of operating
 regional networks can also be taken advantage of.  An alternative to
 doing this is to use different ASNs in different regions, with the
 consequence that the AS path length for routes announced by that
 service provider will increase.

9.0 Conclusion

 This document described principles for traffic engineering in the
 Internet.  It presented an overview of some of the basic issues
 surrounding traffic engineering in IP networks.  The context of TE
 was described, a TE process models and a taxonomy of TE styles were
 presented.  A brief historical review of pertinent developments
 related to traffic engineering was provided.  A survey of
 contemporary TE techniques in operational networks was presented.
 Additionally, the document specified a set of generic requirements,
 recommendations, and options for Internet traffic engineering.

10.0 Security Considerations

 This document does not introduce new security issues.

11.0 Acknowledgments

 The authors would like to thank Jim Boyle for inputs on the
 recommendations section, Francois Le Faucheur for inputs on Diffserv
 aspects, Blaine Christian for inputs on measurement, Gerald Ash for

Awduche, et. al. Informational [Page 63] RFC 3272 Overview and Principles of Internet TE May 2002

 inputs on routing in telephone networks and for text on event-
 dependent TE methods, Steven Wright for inputs on network
 controllability, and Jonathan Aufderheide for inputs on inter-domain
 TE with BGP.  Special thanks to Randy Bush for proposing the TE
 taxonomy based on "tactical vs strategic" methods.  The subsection
 describing an "Overview of ITU Activities Related to Traffic
 Engineering" was adapted from a contribution by Waisum Lai.  Useful
 feedback and pointers to relevant materials were provided by J. Noel
 Chiappa.  Additional comments were provided by Glenn Grotefeld during
 the working last call process.  Finally, the authors would like to
 thank Ed Kern, the TEWG co-chair, for his comments and support.

12.0 References

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             McGraw Hill, 1998.
 [ASH3]      Ash, J., "TE & QoS Methods for IP-, ATM-, & TDM-Based
             Networks", Work in Progress, March 2001.
 [AWD1]      D. Awduche and Y. Rekhter, "Multiprocotol Lambda
             Switching:  Combining MPLS Traffic Engineering Control
             with Optical Crossconnects", IEEE Communications
             Magazine, March 2001.
 [AWD2]      D. Awduche, "MPLS and Traffic Engineering in IP
             Networks", IEEE Communications Magazine, Dec. 1999.
 [AWD5]      D. Awduche et al, "An Approach to Optimal Peering Between
             Autonomous Systems in the Internet", International
             Conference on Computer Communications and Networks
             (ICCCN'98), Oct. 1998.
 [CRUZ]      R. L. Cruz, "A Calculus for Network Delay, Part II:
             Network Analysis", IEEE Transactions on Information
             Theory, vol. 37, pp.  132-141, 1991.
 [DIFF-TE]   Le Faucheur, F., Nadeau, T., Tatham, M., Telkamp, T.,
             Cooper, D., Boyle, J., Lai, W., Fang, L., Ash, J., Hicks,
             P., Chui, A., Townsend, W. and D. Skalecki, "Requirements
             for support of Diff-Serv-aware MPLS Traffic Engineering",
             Work in Progress, May 2001.
 [ELW95]     A. Elwalid, D. Mitra and R.H. Wentworth, "A New Approach
             for Allocating Buffers and Bandwidth to Heterogeneous,
             Regulated Traffic in an ATM Node", IEEE IEEE Journal on
             Selected Areas in Communications, 13:6, pp. 1115-1127,
             Aug. 1995.

Awduche, et. al. Informational [Page 64] RFC 3272 Overview and Principles of Internet TE May 2002

 [FGLR]      A. Feldmann, A. Greenberg, C. Lund, N. Reingold, and J.
             Rexford, "NetScope: Traffic Engineering for IP Networks",
             IEEE Network Magazine, 2000.
 [FLJA93]    S. Floyd and V. Jacobson, "Random Early Detection
             Gateways for Congestion Avoidance", IEEE/ACM Transactions
             on Networking, Vol. 1 Nov. 4., p. 387-413, Aug. 1993.
 [FLOY94]    S. Floyd, "TCP and Explicit Congestion Notification", ACM
             Computer Communication Review, V. 24, No. 5, p. 10-23,
             Oct. 1994.
 [FT00]      B. Fortz and M. Thorup, "Internet Traffic Engineering by
             Optimizing OSPF Weights", IEEE INFOCOM 2000, Mar. 2000.
 [FT01]      B. Fortz and M. Thorup, "Optimizing OSPF/IS-IS Weights in
             a Changing World",
             www.research.att.com/~mthorup/PAPERS/papers.html.
 [HUSS87]    B.R. Hurley, C.J.R. Seidl and W.F. Sewel, "A Survey of
             Dynamic Routing Methods for Circuit-Switched Traffic",
             IEEE Communication Magazine, Sep. 1987.
 [ITU-E600]  ITU-T Recommendation E.600, "Terms and Definitions of
             Traffic Engineering", Mar. 1993.
 [ITU-E701]  ITU-T Recommendation E.701, "Reference Connections for
             Traffic Engineering", Oct. 1993.
 [ITU-E801]  ITU-T Recommendation E.801, "Framework for Service
             Quality Agreement", Oct. 1996.
 [JAM]       Jamoussi, B., Editior, Andersson, L., Collon, R. and R.
             Dantu, "Constraint-Based LSP Setup using LDP", RFC 3212,
             January 2002.
 [KATZ]      Katz, D., Yeung, D. and K. Kompella, "Traffic Engineering
             Extensions to OSPF", Work in Progress, February 2001.
 [LNO96]     T. Lakshman, A. Neidhardt, and T. Ott, "The Drop from
             Front Strategy in TCP over ATM and its Interworking with
             other Control Features", Proc. INFOCOM'96, p. 1242-1250,
             1996.
 [MA]        Q. Ma, "Quality of Service Routing in Integrated Services
             Networks", PhD Dissertation, CMU-CS-98-138, CMU, 1998.

Awduche, et. al. Informational [Page 65] RFC 3272 Overview and Principles of Internet TE May 2002

 [MATE]      A. Elwalid, C. Jin, S. Low, and I. Widjaja, "MATE: MPLS
             Adaptive Traffic Engineering", Proc. INFOCOM'01, Apr.
             2001.
 [MCQ80]     J.M. McQuillan, I. Richer, and E.C. Rosen, "The New
             Routing Algorithm for the ARPANET", IEEE.  Trans. on
             Communications, vol. 28, no. 5, pp. 711-719, May 1980.
 [MR99]      D. Mitra and K.G. Ramakrishnan, "A Case Study of
             Multiservice, Multipriority Traffic Engineering Design
             for Data Networks", Proc. Globecom'99, Dec 1999.
 [RFC-1458]  Braudes, R. and S. Zabele, "Requirements for Multicast
             Protocols", RFC 1458, May 1993.
 [RFC-1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
             (BGP-4)", RFC 1771, March 1995.
 [RFC-1812]  Baker, F., "Requirements for IP Version 4 Routers", STD
             4, RFC 1812, June 1995.
 [RFC-1992]  Castineyra, I., Chiappa, N. and M. Steenstrup, "The
             Nimrod Routing Architecture", RFC 1992, August 1996.
 [RFC-1997]  Chandra, R., Traina, P. and T. Li, "BGP Community
             Attributes", RFC 1997, August 1996.
 [RFC-1998]  Chen, E. and T. Bates, "An Application of the BGP
             Community Attribute in Multi-home Routing", RFC 1998,
             August 1996.
 [RFC-2205]  Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
             Jamin, "Resource Reservation Protocol (RSVP) - Version 1
             Functional Specification", RFC 2205, September 1997.
 [RFC-2211]  Wroclawski, J., "Specification of the Controlled-Load
             Network Element Service", RFC 2211, September 1997.
 [RFC-2212]  Shenker, S., Partridge, C. and R. Guerin, "Specification
             of Guaranteed Quality of Service", RFC 2212, September
             1997.

Awduche, et. al. Informational [Page 66] RFC 3272 Overview and Principles of Internet TE May 2002

 [RFC-2215]  Shenker, S. and J. Wroclawski, "General Characterization
             Parameters for Integrated Service Network Elements", RFC
             2215, September 1997.
 [RFC-2216]  Shenker, S. and J. Wroclawski, "Network Element Service
             Specification Template", RFC 2216, September 1997.
 [RFC-2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, July 1997.
 [RFC-2330]  Paxson, V., Almes, G., Mahdavi, J. and M. Mathis,
             "Framework for IP Performance Metrics", RFC 2330, May
             1998.
 [RFC-2386]  Crawley, E., Nair, R., Rajagopalan, B. and H. Sandick, "A
             Framework for QoS-based Routing in the Internet", RFC
             2386, August 1998.
 [RFC-2474]  Nichols, K., Blake, S., Baker, F. and D. Black,
             "Definition of the Differentiated Services Field (DS
             Field) in the IPv4 and IPv6 Headers", RFC 2474, December
             1998.
 [RFC-2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
             and W. Weiss, "An Architecture for Differentiated
             Services", RFC 2475, December 1998.
 [RFC-2597]  Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
             "Assured Forwarding PHB Group", RFC 2597, June 1999.
 [RFC-2678]  Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring
             Connectivity", RFC 2678, September 1999.
 [RFC-2679]  Almes, G., Kalidindi, S. and M. Zekauskas, "A One-way
             Delay Metric for IPPM", RFC 2679, September 1999.
 [RFC-2680]  Almes, G., Kalidindi, S. and M. Zekauskas, "A One-way
             Packet Loss Metric for IPPM", RFC 2680, September 1999.
 [RFC-2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
             McManus, "Requirements for Traffic Engineering over
             MPLS", RFC 2702, September 1999.
 [RFC-2722]  Brownlee, N., Mills, C. and G. Ruth, "Traffic Flow
             Measurement: Architecture", RFC 2722, October 1999.

Awduche, et. al. Informational [Page 67] RFC 3272 Overview and Principles of Internet TE May 2002

 [RFC-2753]  Yavatkar, R., Pendarakis, D. and R. Guerin, "A Framework
             for Policy-based Admission Control", RFC 2753, January
             2000.
 [RFC-2961]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.
             and S. Molendini, "RSVP Refresh Overhead Reduction
             Extensions", RFC 2961, April 2000.
 [RFC-2998]  Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
             Speer, M., Braden, R., Davie, B., Wroclawski, J. and E.
             Felstaine, "A Framework for Integrated Services Operation
             over Diffserv Networks", RFC 2998, November 2000.
 [RFC-3031]  Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol
             Label Switching Architecture", RFC 3031, January 2001.
 [RFC-3086]  Nichols, K. and B. Carpenter, "Definition of
             Differentiated Services Per Domain Behaviors and Rules
             for their Specification", RFC 3086, April 2001.
 [RFC-3124]  Balakrishnan, H. and S. Seshan, "The Congestion Manager",
             RFC 3124, June 2001.
 [RFC-3209]  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.
 [RFC-3210]  Awduche, D., Hannan, A. and X. Xiao, "Applicability
             Statement for Extensions to RSVP for LSP-Tunnels", RFC
             3210, December 2001.
 [RFC-3213]  Ash, J., Girish, M., Gray, E., Jamoussi, B. and G.
             Wright, "Applicability Statement for CR-LDP", RFC 3213,
             January 2002.
 [RFC-3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaahanen,
             P., Krishnan, R., Cheval, P. and J. Heinanen, "Multi-
             Protocol Label Switching (MPLS) Support of Differentiated
             Services", RFC 3270, April 2002.
 [RR94]      M.A. Rodrigues and K.G. Ramakrishnan, "Optimal Routing in
             Shortest Path Networks", ITS'94, Rio de Janeiro, Brazil.
 [SHAR]      Sharma, V., Crane, B., Owens, K., Huang, C., Hellstrand,
             F., Weil, J., Anderson, L., Jamoussi, B., Cain, B.,
             Civanlar, S. and A. Chui, "Framework for MPLS Based
             Recovery", Work in Progress.

Awduche, et. al. Informational [Page 68] RFC 3272 Overview and Principles of Internet TE May 2002

 [SLDC98]    B. Suter, T. Lakshman, D. Stiliadis, and A. Choudhury,
             "Design Considerations for Supporting TCP with Per-flow
             Queueing", Proc. INFOCOM'98, p. 299-306, 1998.
 [SMIT]      Smit, H. and T. Li, "IS-IS extensions for Traffic
             Engineering", Work in Progress.
 [WANG]      Y. Wang, Z. Wang, L. Zhang, "Internet traffic engineering
             without full mesh overlaying", Proceedings of
             INFOCOM'2001, April 2001.
 [XIAO]      X. Xiao, A. Hannan, B. Bailey, L. Ni, "Traffic
             Engineering with MPLS in the Internet", IEEE Network
             magazine, Mar. 2000.
 [YARE95]    C. Yang and A. Reddy, "A Taxonomy for Congestion Control
             Algorithms in Packet Switching Networks", IEEE Network
             Magazine, p.  34-45, 1995.

Awduche, et. al. Informational [Page 69] RFC 3272 Overview and Principles of Internet TE May 2002

13.0 Authors' Addresses

 Daniel O. Awduche
 Movaz Networks
 7926 Jones Branch Drive, Suite 615
 McLean, VA 22102
 Phone: 703-298-5291
 EMail: awduche@movaz.com
 Angela Chiu
 Celion Networks
 1 Sheila Dr., Suite 2
 Tinton Falls, NJ 07724
 Phone: 732-747-9987
 EMail: angela.chiu@celion.com
 Anwar Elwalid
 Lucent Technologies
 Murray Hill, NJ 07974
 Phone: 908 582-7589
 EMail: anwar@lucent.com
 Indra Widjaja
 Bell Labs, Lucent Technologies
 600 Mountain Avenue
 Murray Hill, NJ 07974
 Phone: 908 582-0435
 EMail: iwidjaja@research.bell-labs.com
 XiPeng Xiao
 Redback Networks
 300 Holger Way
 San Jose, CA 95134
 Phone: 408-750-5217
 EMail: xipeng@redback.com

Awduche, et. al. Informational [Page 70] RFC 3272 Overview and Principles of Internet TE May 2002

14.0 Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Awduche, et. al. Informational [Page 71]

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