Network Working Group                                          D. Awduche
Request for Comments: 2702                                     J. Malcolm
Category: Informational                                        J. Agogbua
                                                                M. O'Dell
                                                               J. McManus
                                                     UUNET (MCI Worldcom)
                                                           September 1999

             Requirements for Traffic Engineering Over MPLS

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   This document presents a set of requirements for Traffic Engineering
   over Multiprotocol Label Switching (MPLS). It identifies the
   functional capabilities required to implement policies that
   facilitate efficient and reliable network operations in an MPLS
   domain. These capabilities can be used to optimize the utilization of
   network resources and to enhance traffic oriented performance

Table of Contents

   1.0   Introduction .............................................  2
   1.1   Terminology ..............................................  3
   1.2   Document Organization ....................................  3
   2.0   Traffic Engineering ......................................  4
   2.1   Traffic Engineering Performance Objectives ...............  4
   2.2   Traffic and Resource Control .............................  6
   2.3   Limitations of Current IGP Control Mechanisms ............  6
   3.0   MPLS and Traffic Engineering .............................  7
   3.1   Induced MPLS Graph .......................................  9
   3.2   The Fundamental Problem of Traffic Engineering Over MPLS .  9
   4.0   Augmented Capabilities for Traffic Engineering Over MPLS . 10
   5.0   Traffic Trunk Attributes and Characteristics   ........... 10
   5.1   Bidirectional Traffic Trunks ............................. 11
   5.2   Basic Operations on Traffic Trunks ....................... 12
   5.3   Accounting and Performance Monitoring .................... 12

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   5.4   Basic Attributes of Traffic Trunks ....................... 13
   5.5   Traffic Parameter Attributes  ............................ 14
   5.6   Generic Path Selection and Management Attributes ......... 14
   5.6.1 Administratively Specified Explicit Paths ................ 15
   5.6.2 Hierarchy of Preference Rules for Multi-paths ............ 15
   5.6.3 Resource Class Affinity Attributes ....................... 16
   5.6.4 Adaptivity Attribute ..................................... 17
   5.6.5 Load Distribution Across Parallel Traffic Trunks ......... 18
   5.7   Priority Attribute ....................................... 18
   5.8   Preemption Attribute ..................................... 18
   5.9   Resilience Attribute ..................................... 19
   5.10  Policing Attribute  ...................................... 20
   6.0   Resource Attributes ...................................... 21
   6.1   Maximum Allocation Multiplier ............................ 21
   6.2   Resource Class Attribute  ................................ 22
   7.0   Constraint-Based Routing  ................................ 22
   7.1   Basic Features of Constraint-Based Routing ............... 23
   7.2   Implementation Considerations ............................ 24
   8.0   Conclusion   ............................................. 25
   9.0   Security Considerations .................................. 26
   10.0  References   ............................................. 26
   11.0  Acknowledgments .......................................... 27
   12.0  Authors' Addresses ....................................... 28
   13.0  Full Copyright Statement ................................. 29

1.0 Introduction

   Multiprotocol Label Switching (MPLS) [1,2] integrates a label
   swapping framework with network layer routing. The basic idea
   involves assigning short fixed length labels to  packets at the
   ingress to an MPLS cloud (based on the concept of forwarding
   equivalence classes [1,2]). Throughout the interior of the MPLS
   domain, the labels attached to packets are used to make forwarding
   decisions  (usually without recourse to the original packet headers).

   A set of powerful constructs to address many critical issues in the
   emerging differentiated services Internet can be devised from this
   relatively simple paradigm.  One of the most significant initial
   applications of MPLS will be in Traffic Engineering. The importance
   of this application is already well-recognized (see [1,2,3]).

   This manuscript is exclusively focused on the Traffic Engineering
   applications of MPLS. Specifically, the goal of this document is to
   highlight the issues and requirements for Traffic Engineering in a
   large Internet backbone. The expectation is that the MPLS
   specifications, or implementations derived therefrom, will address

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   the realization of these objectives.  A description of the basic
   capabilities and functionality required of an MPLS implementation to
   accommodate the requirements is also presented.

   It should be noted that even though the focus is on Internet
   backbones, the capabilities described in this document are equally
   applicable to Traffic Engineering in enterprise networks. In general,
   the capabilities can  be applied to any label switched network under
   a single technical administration in which at least two paths exist
   between two nodes.

   Some recent manuscripts have focused on the considerations pertaining
   to Traffic Engineering and Traffic management under MPLS, most
   notably the works of Li and Rekhter [3], and others.  In [3], an
   architecture is proposed which employs MPLS and RSVP to provide
   scalable differentiated services and Traffic Engineering in the
   Internet.  The present manuscript complements the aforementioned and
   similar efforts.  It reflects the authors' operational experience in
   managing a large Internet backbone.

1.1 Terminology

   The reader is assumed to be familiar with the MPLS terminology as
   defined in [1].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [11].

1.2 Document Organization

   The remainder of this document is organized as follows: Section 2
   discusses the basic functions of Traffic Engineering in the Internet.
   Section 3, provides an overview of the traffic Engineering potentials
   of MPLS. Sections 1 to 3 are essentially background material. Section
   4 presents an overview of the fundamental requirements for Traffic
   Engineering over MPLS. Section 5 describes the desirable attributes
   and characteristics of traffic trunks which are pertinent to Traffic
   Engineering. Section 6 presents a set of attributes which can be
   associated with resources to constrain the routability of traffic
   trunks and LSPs through them. Section 7 advocates the introduction of
   a "constraint-based routing" framework in MPLS domains.  Finally,
   Section 8 contains concluding remarks.

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2.0 Traffic Engineering

   This section describes the basic functions of Traffic Engineering in
   an Autonomous System in the contemporary Internet. The limitations of
   current IGPs with respect to traffic and resource control are
   highlighted. This section serves as motivation for the requirements
   on MPLS.

   Traffic Engineering (TE) is concerned with performance optimization
   of operational networks. In general, it encompasses the application
   of technology and scientific principles to the measurement, modeling,
   characterization, and control of Internet traffic, and the
   application of such knowledge and techniques to achieve specific
   performance objectives. The aspects of Traffic Engineering that are
   of interest concerning MPLS are measurement and control.

   A major goal of Internet Traffic Engineering is to facilitate
   efficient and reliable network operations while simultaneously
   optimizing network resource utilization and traffic performance.
   Traffic Engineering has become an indispensable function in many
   large Autonomous Systems because of the high cost of network assets
   and the commercial and competitive nature of the Internet. These
   factors emphasize the need for maximal operational efficiency.

2.1 Traffic Engineering Performance Objectives

   The key performance objectives associated with traffic engineering
   can be classified as being either:

    1. traffic oriented or

    2. resource oriented.

   Traffic oriented performance objectives include the aspects that
   enhance the QoS of traffic streams. In a single class, best effort
   Internet service model, the key traffic oriented performance
   objectives include: minimization of packet loss, minimization of
   delay, maximization of throughput, and enforcement of service level
   agreements. Under a single class best effort Internet service model,
   minimization of packet loss is one of the most important traffic
   oriented performance objectives. Statistically bounded traffic
   oriented performance objectives (such as peak to peak packet delay
   variation, loss ratio, and maximum packet transfer delay) might
   become useful in the forthcoming differentiated services Internet.

   Resource oriented performance objectives include the aspects
   pertaining to the optimization of resource utilization. Efficient
   management of network resources is the vehicle for the attainment of

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   resource oriented performance objectives. In particular, it is
   generally desirable to ensure that subsets of network resources do
   not become over utilized and congested while other subsets along
   alternate feasible paths remain underutilized. Bandwidth is a crucial
   resource in contemporary networks.  Therefore, a central function of
   Traffic Engineering is to efficiently manage bandwidth resources.

   Minimizing congestion is a primary traffic and resource oriented
   performance objective.  The interest here is on congestion problems
   that are prolonged rather than on transient congestion resulting from
   instantaneous bursts.  Congestion typically manifests under two

   1. When network resources are insufficient or inadequate to
      accommodate offered load.

   2. When traffic streams are inefficiently mapped onto available
      resources; causing subsets of network resources to become
      over-utilized while others remain underutilized.

   The first type of congestion problem can be addressed by either: (i)
   expansion of capacity, or (ii) application of classical congestion
   control techniques, or (iii) both. Classical congestion control
   techniques attempt to regulate the demand so that the traffic fits
   onto available resources. Classical techniques for congestion control
   include: rate limiting, window flow control, router queue management,
   schedule-based control, and others; (see [8] and the references

   The second type of congestion problems, namely those resulting from
   inefficient resource allocation, can usually be addressed through
   Traffic Engineering.

   In general, congestion resulting from inefficient resource allocation
   can be reduced by adopting load balancing policies. The objective of
   such strategies is to minimize maximum congestion or alternatively to
   minimize maximum resource utilization, through efficient resource
   allocation. When congestion is minimized through efficient resource
   allocation, packet loss decreases, transit delay decreases, and
   aggregate throughput increases. Thereby, the perception of network
   service quality experienced by end users becomes significantly

   Clearly, load balancing is an important network performance
   optimization policy. Nevertheless, the capabilities provided for
   Traffic Engineering should be flexible enough so that network
   administrators can implement other policies which take into account
   the prevailing cost structure and the utility or revenue model.

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2.2 Traffic and Resource Control

   Performance optimization of operational networks is fundamentally a
   control problem. In the traffic engineering process model, the
   Traffic Engineer, or a suitable automaton, acts as the controller in
   an adaptive feedback control system. This system includes a set of
   interconnected network elements, a network performance monitoring
   system, and a set of network configuration management tools. The
   Traffic Engineer formulates a control policy, observes the state of
   the network through the monitoring system, characterizes the traffic,
   and applies control actions to drive the network to a desired state,
   in accordance with the control policy.  This can be accomplished
   reactively by taking action in response to the current state of the
   network, or pro-actively by using forecasting techniques to
   anticipate future trends and applying action to obviate the predicted
   undesirable future states.

   Ideally, control actions should involve:

   1. Modification of traffic management parameters,

   2. Modification of parameters associated with routing, and

   3. Modification of attributes and constraints associated with

   The level of manual intervention involved in the traffic engineering
   process should be minimized whenever possible.  This can be
   accomplished by automating aspects of the control actions described
   above, in a distributed and scalable fashion.

2.3 Limitations of Current IGP Control Mechanisms

   This subsection reviews some of the well known limitations of current
   IGPs with regard to Traffic Engineering.

   The control capabilities offered by existing Internet interior
   gateway protocols are not adequate for Traffic Engineering.  This
   makes it difficult to actualize effective policies to address network
   performance problems.  Indeed, IGPs based on shortest path algorithms
   contribute significantly to congestion problems in Autonomous Systems
   within the Internet. SPF algorithms generally optimize based on a
   simple additive metric. These protocols are topology driven, so
   bandwidth availability and traffic characteristics are not factors
   considered in routing decisions. Consequently, congestion frequently
   occurs when:

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   1. the shortest paths of multiple traffic streams converge on
      specific links or router interfaces, or

   2. a given traffic stream is routed through a link or router
      interface which does not have enough bandwidth to accommodate

   These scenarios manifest even when feasible alternate paths with
   excess capacity exist. It is this aspect of congestion problems (-- a
   symptom of suboptimal resource allocation) that Traffic Engineering
   aims to vigorously obviate.  Equal cost path load sharing can be used
   to address the second cause for congestion listed above with some
   degree of success, however it is generally not helpful in alleviating
   congestion due to the first cause listed above and particularly not
   in large networks with dense topology.

   A popular approach to circumvent the inadequacies of current IGPs is
   through the use of an overlay model, such as IP over ATM or IP over
   frame relay. The overlay model extends the design space by enabling
   arbitrary virtual topologies to be provisioned atop the network's
   physical topology. The virtual topology is constructed from virtual
   circuits which appear as physical links to the IGP routing protocols.
   The overlay model provides additional important services to support
   traffic and resource control, including: (1) constraint-based routing
   at the VC level, (2) support for administratively configurable
   explicit VC paths, (3) path compression, (4) call admission control
   functions, (5) traffic shaping and traffic policing functions, and
   (6) survivability of VCs. These capabilities enable the actualization
   of a variety of Traffic Engineering policies. For example, virtual
   circuits can easily be rerouted to move traffic from over-utilized
   resources onto relatively underutilized ones.

   For Traffic Engineering in large dense networks, it is desirable to
   equip MPLS with a level of functionality at least commensurate with
   current overlay models. Fortunately, this can be done in a fairly
   straight forward manner.

3.0  MPLS and Traffic Engineering

   This section provides an overview of the applicability of MPLS to
   Traffic Engineering. Subsequent sections discuss the set of
   capabilities required to meet the Traffic Engineering requirements.

   MPLS is strategically significant for Traffic Engineering because it
   can potentially provide most of the functionality available from the
   overlay model, in an integrated manner, and at a lower cost than the
   currently competing alternatives. Equally importantly, MPLS offers

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   the possibility to automate aspects of the Traffic Engineering
   function. This last consideration requires further investigation and
   is beyond the scope of this manuscript.

   A note on terminology: The concept of MPLS traffic trunks is used
   extensively in the remainder of this document. According to Li and
   Rekhter [3], a traffic trunk is an aggregation of traffic flows of
   the same class which are placed inside a Label Switched Path.
   Essentially, a traffic trunk is an abstract representation of traffic
   to which specific characteristics can be associated. It is useful to
   view traffic trunks as objects that can be routed; that is, the path
   through which a traffic trunk traverses can be changed. In this
   respect, traffic trunks are similar to virtual circuits in ATM and
   Frame Relay networks.  It is important, however, to emphasize that
   there is a fundamental distinction between a traffic trunk and the
   path, and indeed the LSP, through which it traverses. An LSP is a
   specification of the label switched path through which the traffic
   traverses. In practice, the terms LSP and traffic trunk are often
   used synonymously. Additional characteristics of traffic trunks as
   used in this manuscript are summarized in section 5.0.

   The attractiveness of  MPLS for Traffic Engineering can be attributed
   to the following factors: (1) explicit label switched paths which are
   not constrained by the destination based forwarding paradigm can be
   easily created through manual administrative action or through
   automated action by the underlying protocols, (2) LSPs can
   potentially be efficiently maintained, (3) traffic trunks can be
   instantiated and mapped onto LSPs, (4) a set of attributes can be
   associated with traffic trunks which modulate their behavioral
   characteristics, (5) a set of attributes can be associated with
   resources which constrain the placement of LSPs and traffic trunks
   across them, (6) MPLS allows for both traffic aggregation and
   disaggregation whereas classical destination only based IP forwarding
   permits only aggregation, (7) it is relatively easy to integrate a
   "constraint-based routing" framework with MPLS, (8) a good
   implementation of MPLS can offer significantly lower overhead than
   competing alternatives for Traffic Engineering.

   Additionally, through explicit label switched paths, MPLS permits a
   quasi circuit switching capability to be superimposed on the current
   Internet routing model.  Many of the existing proposals for Traffic
   Engineering over MPLS focus only on the potential to create explicit
   LSPs. Although this capability is fundamental for Traffic
   Engineering, it is not really sufficient.  Additional augmentations
   are required to foster the actualization of policies leading to
   performance optimization of large operational networks. Some of the
   necessary augmentations are described in this manuscript.

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3.1 Induced MPLS Graph

   This subsection introduces the concept of an "induced MPLS graph"
   which is central to Traffic Engineering in MPLS domains. An induced
   MPLS graph is analogous to a virtual topology in an overlay model. It
   is logically mapped onto the physical network through the selection
   of LSPs for traffic trunks.

   An induced MPLS graph consists of a set of LSRs which comprise the
   nodes of the graph and a set of LSPs which provide logical point to
   point connectivity between the LSRs, and hence serve as the links of
   the induced graph. it may be possible to construct hierarchical
   induced MPLS graphs based on the concept of label stacks (see [1]).

   Induced MPLS graphs are important because the basic problem of
   bandwidth management in an MPLS domain is the issue of how to
   efficiently map an induced MPLS graph onto the physical network
   topology.  The induced MPLS graph abstraction is formalized below.

   Let G = (V, E, c) be a capacitated graph depicting the physical
   topology of the network. Here, V is the set of nodes in the network
   and E is the set of links; that is, for v and w in V, the object
   (v,w) is in E if v and w are directly connected under G. The
   parameter "c" is a set of capacity and other constraints associated
   with E and V. We will refer to G as the "base" network topology.

   Let H = (U, F, d) be  the induced MPLS graph, where U is a subset of
   V representing the set of LSRs in the network, or more precisely the
   set of LSRs that are the endpoints of at least one LSP. Here, F is
   the set of LSPs, so that for x and y in U, the object (x, y) is in F
   if there is an LSP with x and y as endpoints. The parameter "d" is
   the set of demands and restrictions associated with F. Evidently, H
   is a directed graph. It can be seen that H depends on the
   transitivity characteristics of G.

3.2 The Fundamental Problem of Traffic Engineering Over MPLS

   There are basically three fundamental problems that relate to Traffic
   Engineering over MPLS.

   - The first problem concerns how to map packets onto forwarding
     equivalence classes.

   - The second problem concerns how to map forwarding equivalence
     classes onto traffic trunks.

   - The third problem concerns how to map traffic trunks onto the
     physical network topology through label switched paths.

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   This document is not focusing on the first two problems listed.
   (even-though they are quite important). Instead, the remainder of
   this manuscript will focus on the capabilities that permit the third
   mapping function to be performed in a manner resulting in efficient
   and reliable network operations. This is really the problem of
   mapping an induced MPLS graph (H) onto the "base" network topology

4.0 Augmented  Capabilities for Traffic Engineering Over MPLS

   The previous sections reviewed the basic functions of Traffic
   Engineering in the contemporary Internet. The applicability of MPLS
   to that activity was also discussed. The remaining sections of this
   manuscript describe the functional capabilities required to fully
   support Traffic Engineering over MPLS in large networks.

   The proposed capabilities consist of:

   1. A set of attributes associated with traffic trunks which
      collectively specify their behavioral characteristics.

   2. A set of attributes associated with resources which constrain
      the placement of traffic trunks through them. These can also be
      viewed as topology attribute constraints.

   3. A "constraint-based routing" framework which is used to select
      paths for traffic trunks subject to constraints imposed by items
      1) and 2) above. The constraint-based routing framework does not
      have to be part of MPLS. However, the two need to be tightly
      integrated together.

   The attributes associated with traffic trunks and resources, as well
   as parameters associated with routing, collectively represent the
   control variables which can be modified either through administrative
   action or through automated agents to drive the network to a desired

   In an operational network, it is highly desirable that these
   attributes can be dynamically modified online by an operator without
   adversely disrupting network operations.

5.0 Traffic Trunk Attributes and Characteristics

   This section describes the desirable attributes which can be
   associated with traffic trunks to influence their behavioral

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   First, the basic properties of traffic trunks (as used in this
   manuscript) are summarized below:

    - A traffic trunk is an *aggregate* of traffic flows belonging
      to the same class. In some contexts, it may be desirable to
      relax this definition and allow traffic trunks to include
      multi-class traffic aggregates.

    - In a single class service model, such as the current Internet,
      a traffic trunk could encapsulate all of the traffic between an
      ingress LSR and an egress LSR, or subsets thereof.

    - Traffic trunks are routable objects (similar to ATM VCs).

    - A traffic trunk is distinct from the LSP through which it
      traverses. In operational contexts, a traffic trunk can be
      moved from one path onto another.

    - A traffic trunk is unidirectional.

   In practice, a traffic trunk can be characterized by its ingress and
   egress LSRs, the forwarding equivalence class which is mapped onto
   it, and a set of attributes which determine its behavioral

   Two basic issues are of particular significance: (1) parameterization
   of traffic trunks and (2) path placement and maintenance rules for
   traffic trunks.

5.1 Bidirectional Traffic Trunks

   Although traffic trunks are conceptually unidirectional, in many
   practical contexts, it is useful to  simultaneously instantiate two
   traffic trunks with the same endpoints, but which carry packets in
   opposite directions. The two traffic trunks are logically coupled
   together.  One trunk, called the forward trunk, carries traffic from
   an originating node to a destination node. The other trunk, called
   the backward trunk, carries traffic from the destination node to the
   originating node. We refer to the amalgamation of two such traffic
   trunks as one bidirectional traffic trunk (BTT) if the following two
   conditions hold:

   - Both traffic trunks are instantiated through an atomic action at
     one LSR, called the originator node, or through an atomic action
     at a network management station.

   - Neither of the composite traffic trunks can exist without the
     other. That is, both are instantiated and destroyed together.

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   The topological properties of BTTs should also be considered. A BTT
   can be topologically symmetric or topologically asymmetric.  A BTT is
   said to be "topologically symmetric" if its constituent traffic
   trunks are routed through the same physical path, even though they
   operate in opposite directions. If, however, the component traffic
   trunks are routed through different physical paths, then the BTT is
   said to be "topologically asymmetric."

   It should be noted that bidirectional traffic trunks are merely an
   administrative convenience. In practice, most traffic engineering
   functions can be implemented using only unidirectional traffic

5.2 Basic Operations on Traffic Trunks

   The basic operations on traffic trunks significant to Traffic
   Engineering purposes are summarized below.

   - Establish: To create an instance of a traffic trunk.

   - Activate: To cause a traffic trunk to start passing traffic.
     The establishment and activation of a traffic trunk are
     logically separate events. They may, however, be implemented
     or invoked as one atomic action.

   - Deactivate: To cause a traffic trunk to stop passing traffic.

   - Modify Attributes: To cause the attributes of a traffic trunk
     to be modified.

   - Reroute: To cause a traffic trunk to change its route. This
     can be done through administrative action or automatically
     by the underlying protocols.

   - Destroy: To remove an instance of a traffic trunk from the
     network and reclaim all resources allocated to it. Such
     resources include label space and possibly available bandwidth.

   The above are considered the basic operations on traffic trunks.
   Additional operations are also possible such as policing and traffic

5.3 Accounting and Performance Monitoring

   Accounting and performance monitoring capabilities are very important
   to the billing and traffic characterization functions.  Performance
   statistics obtained from accounting and performance monitoring

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   systems can be used for traffic characterization, performance
   optimization, and capacity planning within the Traffic Engineering

   The capability to obtain statistics at the traffic trunk level is so
   important that it should be considered an essential requirement for
   Traffic Engineering over MPLS.

5.4 Basic Traffic Engineering Attributes of Traffic Trunks

   An attribute of a traffic trunk is a parameter assigned to it which
   influences its behavioral characteristics.

   Attributes can be explicitly assigned to traffic trunks through
   administration action or they can be implicitly assigned by the
   underlying protocols when packets are classified and mapped into
   equivalence classes at the ingress to an MPLS domain. Regardless of
   how the attributes were originally assigned, for Traffic Engineering
   purposes, it should be possible to administratively modify such

   The basic attributes of traffic trunks  particularly significant for
   Traffic Engineering are itemized below.

   - Traffic parameter attributes

   - Generic Path selection and maintenance attributes

   - Priority attribute

   - Preemption attribute

   - Resilience attribute

   - Policing  attribute

   The combination of traffic parameters and policing attributes is
   analogous to usage parameter control in ATM networks. Most of the
   attributes listed above have analogs in well established
   technologies.  Consequently, it should be relatively straight forward
   to map the traffic trunk attributes onto many existing switching and
   routing architectures.

   Priority and preemption can be regarded as relational attributes
   because they express certain binary relations between traffic trunks.
   Conceptually, these binary relations determine the manner in which
   traffic trunks interact with each other as they compete for network
   resources during path establishment and path maintenance.

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5.5 Traffic parameter attributes

   Traffic parameters can be used to capture the characteristics of the
   traffic streams (or more precisely the forwarding equivalence class)
   to be transported through the traffic trunk. Such characteristics may
   include peak rates, average rates, permissible burst size, etc.  From
   a traffic engineering perspective, the traffic parameters are
   significant because they indicate the resource requirements of the
   traffic trunk. This is useful for resource allocation and congestion
   avoidance through anticipatory policies.

   For the purpose of bandwidth allocation, a single canonical value of
   bandwidth requirements can be computed from a traffic trunk's traffic
   parameters.  Techniques for performing these computations are well
   known. One example of this is the theory of effective bandwidth.

5.6 Generic Path Selection and Management Attributes

   Generic path selection and management attributes define the rules for
   selecting the route taken by a traffic trunk as well as the rules for
   maintenance of paths that are already established.

   Paths can be computed automatically by the underlying routing
   protocols or they can be defined administratively by a network
   operator. If there are no resource requirements or restrictions
   associated with a traffic trunk, then a topology driven protocol can
   be used to select its path. However, if resource requirements or
   policy restrictions exist, then a constraint-based routing scheme
   should be used for path selection.

   In Section 7, a constraint-based routing framework which can
   automatically compute paths subject to a set of constraints is
   described.  Issues pertaining to explicit paths instantiated through
   administrative action are discussed in Section 5.6.1 below.

   Path management concerns all aspects pertaining to the maintenance of
   paths traversed by traffic trunks.  In some operational contexts, it
   is desirable that an MPLS implementation can dynamically reconfigure
   itself, to adapt to some notion of change in "system state."
   Adaptivity and resilience are aspects of dynamic path management.

   To guide the path selection and management process, a set of
   attributes are required. The basic attributes and behavioral
   characteristics associated with traffic trunk path selection and
   management are described in the remainder of this sub-section.

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5.6.1 Administratively Specified Explicit Paths

   An administratively specified explicit path for a traffic trunk is
   one which is configured through operator action. An administratively
   specified path can be completely specified or partially specified. A
   path is completely specified if all of the required hops between the
   endpoints are indicated. A path is partially specified if only a
   subset of intermediate hops are indicated. In this case, the
   underlying protocols are required to complete the path. Due to
   operator errors, an administratively specified path can be
   inconsistent or illogical. The underlying protocols should be able to
   detect such inconsistencies and provide appropriate feedback.

   A "path preference rule" attribute should be associated with
   administratively specified explicit paths.  A path preference rule
   attribute is a binary variable which  indicates whether the
   administratively configured explicit path is "mandatory" or "non-

   If an administratively specified explicit path is selected with a
   "mandatory attribute, then that path (and only that path) must be
   used. If a mandatory path is topological infeasible (e.g. the two
   endpoints are topologically partitioned), or if the path cannot be
   instantiated because the available resources are inadequate, then the
   path setup process fails. In other words, if a path is specified as
   mandatory, then an alternate path cannot be used regardless of
   prevailing circumstance.  A mandatory path which is successfully
   instantiated is also implicitly pinned. Once the path is instantiated
   it cannot be changed except through deletion and instantiation of a
   new path.

   However, if an administratively specified explicit path is selected
   with a "non-mandatory" preference rule attribute value, then the path
   should be used if feasible.  Otherwise, an alternate path can be
   chosen instead by the underlying protocols.

5.6.2 Hierarchy of Preference Rules For Multi-Paths

   In some practical contexts, it can be useful to have the ability to
   administratively specify a set of candidate explicit paths for a
   given traffic trunk and define a hierarchy of preference relations on
   the paths. During path establishment, the preference rules are
   applied to select a suitable path from the candidate list. Also,
   under failure scenarios the preference rules are applied to select an
   alternate path from the candidate list.

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5.6.3 Resource Class Affinity Attributes

   Resource class affinity attributes associated with a traffic trunk
   can be used to specify the class of resources (see Section 6) which
   are to be explicitly included or excluded from the path of the
   traffic trunk. These are policy attributes which can be used to
   impose additional constraints on the path traversed by a given
   traffic trunk.  Resource class affinity attributes for a traffic can
   be specified as a sequence of tuples:

    <resource-class, affinity>; <resource-class, affinity>; ..

   The resource-class parameter identifies a resource class for which an
   affinity relationship is defined with respect to the traffic trunk.
   The affinity parameter indicates the affinity relationship; that is,
   whether members of the resource class are to be included or excluded
   from the path of the traffic trunk. Specifically, the affinity
   parameter may be a binary variable which takes one of the following
   values: (1) explicit inclusion, and (2) explicit exclusion.

   If the affinity attribute is a binary variable, it may be possible to
   use Boolean expressions to specify the resource class affinities
   associated with a given traffic trunk.

   If no resource class affinity attributes are specified, then a "don't
   care" affinity relationship is assumed to hold between the traffic
   trunk and all resources. That is, there is no requirement to
   explicitly include or exclude any resources from the traffic trunk's
   path. This should be the default in practice.

   Resource class affinity attributes are very useful and powerful
   constructs because they can be used to implement a variety of
   policies. For example, they can be used to contain certain traffic
   trunks within specific topological regions of the network.

   A "constraint-based routing" framework (see section 7.0) can be used
   to compute an explicit path for a traffic trunk subject to resource
   class affinity constraints in the following manner:

   1. For explicit inclusion, prune all resources not belonging
      to the specified classes prior to performing path computation.

   2. For explicit exclusion, prune all resources  belonging to the
      specified classes before performing path placement computations.

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5.6.4 Adaptivity Attribute

   Network characteristics and state change over time. For example, new
   resources become available, failed resources become reactivated, and
   allocated resources become deallocated. In general, sometimes more
   efficient paths become available.  Therefore, from a Traffic
   Engineering perspective, it is necessary to have administrative
   control parameters that can be used to specify how traffic trunks
   respond to this dynamism. In some scenarios, it might be desirable to
   dynamically change the paths of certain traffic trunks in response to
   changes in network state. This process is called re-optimization.  In
   other scenarios, re-optimization might be very undesirable.

   An Adaptivity attribute is a part of the path maintenance parameters
   associated with traffic trunks. The adaptivity attribute associated
   with a traffic trunk indicates whether the trunk is subject to re-
   optimization.  That is, an adaptivity attribute is a binary variable
   which takes one of the following values: (1) permit re-optimization
   and (2) disable re-optimization.

   If re-optimization is enabled, then a traffic trunk can be rerouted
   through different paths by the underlying protocols in response to
   changes in network state (primarily changes in resource
   availability). Conversely, if re-optimization is disabled, then the
   traffic trunk is "pinned" to its established path and cannot be
   rerouted in response to changes in network state.

   Stability is a major concern when re-optimization is permitted. To
   promote stability, an MPLS implementation should not be too reactive
   to the evolutionary dynamics of the network. At the same time, it
   must adapt fast enough so that optimal use can be made of network
   assets. This implies that the frequency of re-optimization should be
   administratively configurable to allow for tuning.

   It is to be noted that re-optimization is distinct from resilience. A
   different attribute is used to specify the resilience characteristics
   of a traffic trunk (see section 5.9).  In practice, it would seem
   reasonable to expect traffic trunks subject to re-optimization to be
   implicitly resilient to failures along their paths. However, a
   traffic trunk which is not subject to re-optimization and whose path
   is not administratively specified with a "mandatory" attribute can
   also be required to be resilient to link and node failures along its
   established path

   Formally, it can be stated that adaptivity to state evolution through
   re-optimization implies resilience to failures, whereas resilience to
   failures does not imply general adaptivity through re-optimization to
   state changes.

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5.6.5 Load Distribution Across Parallel Traffic Trunks

   Load distribution across multiple parallel traffic trunks between two
   nodes is an important consideration.  In many practical contexts, the
   aggregate traffic between two nodes may be such that no single link
   (hence no single path) can carry the load. However, the aggregate
   flow might be less than the maximum permissible flow across a "min-
   cut" that partitions the two nodes. In this case, the only feasible
   solution is to appropriately divide the aggregate traffic into sub-
   streams and route the sub-streams through multiple paths between the
   two nodes.

   In an MPLS domain, this problem can be addressed by instantiating
   multiple traffic trunks between the two nodes, such that each traffic
   trunk carries a proportion of the aggregate traffic. Therefore, a
   flexible means of load assignment to multiple parallel traffic trunks
   carrying traffic between a pair of nodes is required.

   Specifically, from an operational perspective, in situations where
   parallel traffic trunks are warranted,  it would be useful to have
   some attribute that can be used to indicate the relative proportion
   of traffic to be carried by each traffic trunk. The underlying
   protocols will then map the load onto the traffic trunks according to
   the specified proportions. It is also, generally desirable to
   maintain packet ordering between packets belong to the same micro-
   flow (same source address, destination address, and port number).

5.7 Priority attribute

   The priority attribute defines the relative importance of traffic
   trunks.  If a constraint-based routing framework is used with MPLS,
   then priorities become very important because they can be used to
   determine the order in which path selection is done for traffic
   trunks at connection establishment and under fault scenarios.

   Priorities are also important in implementations  permitting
   preemption because they can be used to impose a partial order on the
   set of traffic trunks according to which preemptive policies can be

5.8 Preemption attribute

   The preemption attribute determines whether a traffic trunk can
   preempt another traffic trunk from a given path, and whether another
   traffic trunk can preempt a specific traffic trunk.  Preemption is
   useful for both traffic oriented and resource oriented performance

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   objectives. Preemption can used to assure that high priority traffic
   trunks can always be routed through relatively favorable paths within
   a differentiated services environment.

   Preemption can also be used to implement various prioritized
   restoration policies following fault events.

   The preemption attribute can be used to specify four preempt modes
   for a traffic trunk: (1) preemptor enabled, (2) non-preemptor, (3)
   preemptable, and (4) non-preemptable. A preemptor enabled traffic
   trunk can preempt lower priority traffic trunks designated as
   preemptable. A traffic specified as non-preemptable cannot be
   preempted by any other trunks, regardless of relative priorities. A
   traffic trunk designated as preemptable can be preempted by higher
   priority trunks which are preemptor enabled.

   It is trivial to see that some of the preempt modes are mutually
   exclusive. Using the numbering scheme depicted above, the feasible
   preempt mode combinations for a given traffic trunk are as follows:
   (1, 3), (1, 4), (2, 3), and (2, 4). The (2, 4) combination should be
   the default.

   A traffic trunk, say "A", can preempt another traffic trunk, say "B",
   only if *all* of the following five conditions hold: (i) "A" has a
   relatively higher priority than "B", (ii) "A" contends for a resource
   utilized by "B", (iii) the resource cannot concurrently accommodate
   "A" and "B" based on certain decision criteria, (iv) "A" is preemptor
   enabled, and (v) "B" is preemptable.

   Preemption is not considered a mandatory attribute under the current
   best effort Internet service model although it is useful. However, in
   a differentiated services scenario, the need for preemption becomes
   more compelling. Moreover, in the emerging optical internetworking
   architectures, where some protection and restoration functions may be
   migrated from the optical layer to data network elements (such as
   gigabit and terabit label switching routers) to reduce costs,
   preemptive strategies can be used to reduce the restoration time for
   high priority traffic trunks under fault conditions.

5.9 Resilience Attribute

   The resilience attribute determines the behavior of a traffic trunk
   under fault conditions. That is, when a fault occurs along the path
   through which the traffic trunk traverses. The following basic
   problems need to be addressed under such circumstances: (1) fault
   detection, (2) failure notification, (3) recovery and service
   restoration. Obviously, an MPLS implementation will have to
   incorporate mechanisms to address these issues.

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   Many recovery policies can be specified for traffic trunks whose
   established paths are impacted by faults. The following are examples
   of feasible schemes:

   1. Do not reroute the traffic trunk. For example, a survivability
      scheme may already be in place, provisioned through an
      alternate mechanism, which guarantees service continuity
      under failure scenarios without the need to reroute traffic
      trunks. An example of such an alternate scheme (certainly
      many others exist), is a situation whereby multiple parallel
      label switched paths are provisioned between two nodes, and
      function in a manner such that failure of one LSP causes the
      traffic trunk placed on it to be mapped onto the remaining LSPs
      according to some well defined policy.

   2. Reroute through a feasible path with enough resources. If none
      exists, then do not reroute.

   3. Reroute through any available path regardless of resource

   4. Many other schemes are possible including some which might be
      combinations of the above.

   A "basic" resilience attribute indicates the recovery procedure to be
   applied to traffic trunks whose paths are impacted by faults.
   Specifically, a "basic" resilience attribute is a binary variable
   which determines whether the target traffic trunk is to be rerouted
   when segments of its path fail. "Extended" resilience attributes can
   be used to specify detailed actions to be taken under fault
   scenarios.  For example, an extended resilience attribute might
   specify a set of alternate paths to use under fault conditions, as
   well as the rules that govern the relative preference of each
   specified path.

   Resilience attributes mandate close interaction between MPLS and

5.10 Policing attribute

   The policing attribute determines the actions that should be taken by
   the underlying protocols when a traffic trunk becomes non-compliant.
   That is, when a traffic trunk exceeds its contract as specified in
   the traffic parameters.  Generally, policing attributes can indicate
   whether a non-conformant traffic trunk is to be rate limited, tagged,
   or simply forwarded without any policing action.  If policing is
   used, then adaptations of established algorithms such as the ATM
   Forum's GCRA [11] can be used to perform this function.

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   Policing is necessary in many operational scenarios, but is quite
   undesirable in some others. In general, it is usually desirable to
   police at the ingress to a network (to enforce compliance with
   service level agreements) and to minimize policing within the core,
   except when capacity constraints dictate otherwise.

   Therefore, from a Traffic Engineering perspective, it is necessary to
   be able to administratively enable or disable traffic policing for
   each traffic trunk.

6.0 Resource Attributes

   Resource attributes are part of the topology state parameters, which
   are used to constrain the routing of traffic trunks through specific

6.1 Maximum Allocation Multiplier

   The maximum allocation multiplier (MAM) of a resource is an
   administratively configurable attribute which determines the
   proportion of the resource that is available for allocation to
   traffic trunks.  This attribute is mostly applicable to link
   bandwidth. However,  it can also be applied to buffer resources on
   LSRs. The concept of MAM is analogous to the concepts of subscription
   and booking factors in frame relay and ATM networks.

   The values of the MAM can be chosen so that a resource can be under-
   allocated or over-allocated. A resource is said  to be under-
   allocated if the aggregate demands of all traffic trunks (as
   expressed in the trunk traffic parameters) that can be allocated to
   it are always less than the capacity of the resource. A resource is
   said to be over-allocated if the aggregate demands of all traffic
   trunks allocated to it can exceed the capacity of the resource.

   Under-allocation can be used to bound the utilization of resources.
   However,the situation under MPLS is more complex than in circuit
   switched schemes because under MPLS, some flows can be routed via
   conventional hop by hop protocols (also via explicit paths) without
   consideration for resource constraints.

   Over-allocation can be used to take advantage of the statistical
   characteristics of traffic in order to implement more efficient
   resource allocation policies. In particular, over-allocation can be
   used in situations where the peak demands of traffic trunks do not
   coincide in time.

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6.2 Resource Class Attribute

   Resource class attributes are administratively assigned parameters
   which express some notion of "class" for resources. Resource class
   attributes can be viewed as "colors" assigned to resources such that
   the set of resources with the same "color" conceptually belong to the
   same class. Resource class attributes can be used to implement a
   variety of policies. The key resources of interest here are links.
   When applied to links, the resource class attribute effectively
   becomes  an aspect of the "link state" parameters.

   The concept of resource class attributes is a powerful abstraction.
   From a Traffic Engineering perspective, it can be used to implement
   many policies with regard to both traffic and resource oriented
   performance optimization. Specifically, resource class attributes can
   be used to:

   1. Apply uniform policies to a set of resources that do not need
      to be in the same topological region.

   2. Specify the relative preference of sets of resources for
      path placement of traffic trunks.

   3. Explicitly restrict the placement of traffic trunks
      to  specific subsets of resources.

   4. Implement generalized inclusion / exclusion policies.

   5. Enforce traffic locality containment policies. That is,
      policies    that seek to contain local traffic within
      specific topological regions of the network.

   Additionally, resource class attributes can be used for
   identification purposes.

   In general, a resource can be assigned more than one resource class
   attribute. For example, all of the OC-48 links in a given network may
   be assigned a distinguished resource class attribute. The subsets of
   OC-48 links which exist with a given abstraction domain of the
   network may be assigned additional resource class attributes in order
   to implement specific containment policies, or to architect the
   network in a certain manner.

7.0 Constraint-Based Routing

   This section discusses the issues pertaining to constraint-based
   routing in MPLS domains. In contemporary terminology, constraint-
   based routing is often referred to as "QoS Routing" see [5,6,7,10].

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   This document uses the term "constraint-based routing" however,
   because it better captures the functionality envisioned, which
   generally encompasses QoS routing as a subset.

   constraint-based routing enables a demand driven, resource
   reservation aware, routing paradigm to co-exist with current topology
   driven hop by hop Internet interior gateway protocols.

   A constraint-based routing framework uses the following as input:

    - The attributes associated with traffic trunks.

    - The attributes associated with resources.

    - Other topology state information.

   Based on this information, a constraint-based routing process on each
   node automatically computes explicit routes for each traffic trunk
   originating from the node. In this case, an explicit route for each
   traffic trunk is a specification of a label switched path that
   satisfies the demand requirements expressed in the trunk's
   attributes, subject to constraints imposed by resource availability,
   administrative policy, and other topology state information.

   A constraint-based routing framework can greatly reduce the level of
   manual configuration and intervention required to actualize Traffic
   Engineering policies.

   In practice, the Traffic Engineer, an operator, or even an automaton
   will specify the endpoints of a traffic trunk and assign a set of
   attributes to the trunk which encapsulate the performance
   expectations and behavioral characteristics of the trunk. The
   constraint-based routing framework is then expected to find a
   feasible path to satisfy the expectations.  If necessary, the Traffic
   Engineer or a traffic engineering support system can then use
   administratively configured explicit routes to perform fine grained

7.1 Basic Features of Constraint-Based Routing

   A constraint-based routing framework should at least have the
   capability to automatically obtain a basic feasible solution to the
   traffic trunk path placement problem.

   In general, the constraint-based routing problem is known to be
   intractable for most realistic constraints. However, in practice, a
   very simple well known heuristic (see e.g. [9]) can be used to find a
   feasible path if one exists:

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    - First prune resources that do not satisfy the requirements of
      the traffic trunk attributes.

    - Next, run a shortest path algorithm on the residual graph.

   Clearly, if a feasible path exists for a single traffic trunk, then
   the above simple procedure will find it. Additional rules can be
   specified to break ties and perform further optimizations.  In
   general, ties should be broken so that congestion is minimized.  When
   multiple traffic trunks are to be routed, however, it can be shown
   that the above algorithm may not always find a mapping, even when a
   feasible mapping exists.

7.2 Implementation Considerations

   Many commercial implementations of frame relay and ATM switches
   already support some notion of constraint-based routing. For such
   devices or for the novel MPLS centric contraptions devised therefrom,
   it should be relatively easy to extend the current constraint-based
   routing implementations to accommodate the peculiar requirements of

   For routers that use topology driven hop by hop IGPs, constraint-
   based routing can be incorporated in at least one of two ways:

   1. By extending the current IGP protocols such as OSPF and IS-IS to
      support constraint-based routing. Effort is already underway to
      provide such extensions to OSPF (see [5,7]).

   2. By adding a constraint-based routing process to each router which
      can co-exist with current IGPs. This scenario is depicted
      in Figure 1.

        |          Management Interface            |
            |                 |                 |
     ------------     ------------------    --------------
    |    MPLS    |<->| Constraint-Based |  | Conventional |
    |            |   | Routing Process  |  | IGP Process  |
     ------------     ------------------    --------------
                           |                  |
             -----------------------    --------------
            | Resource  Attribute   |  | Link State   |
            | Availability Database |  | Database     |
             -----------------------    --------------

    Figure 1. Constraint-Based Routing Process on Layer 3 LSR

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   There are many important details associated with implementing
   constraint-based routing on Layer 3 devices which we do not discuss
   here. These include the following:

   - Mechanisms for exchange of topology state information
     (resource availability information, link state information,
     resource attribute information) between constraint-based
     routing processes.

   - Mechanisms for maintenance of topology state information.

   - Interaction between constraint-based routing processes and
     conventional IGP processes.

   - Mechanisms to accommodate the adaptivity requirements of
     traffic trunks.

   - Mechanisms to accommodate the resilience and survivability
     requirements of traffic trunks.

   In summary, constraint-based routing assists in performance
   optimization of operational networks by automatically finding
   feasible paths that satisfy a set of constraints for traffic trunks.
   It can drastically reduce the amount of administrative explicit path
   configuration and manual intervention required to achieve Traffic
   Engineering objectives.

8.0 Conclusion

   This manuscript presented a set of requirements for Traffic
   Engineering over MPLS. Many capabilities were described aimed at
   enhancing the applicability of MPLS to Traffic Engineering in the

   It should be noted that some of the issues described here can be
   addressed by incorporating a minimal set of building blocks into
   MPLS, and then using a network management superstructure to extend
   the functionality in order to realize the requirements. Also, the
   constraint-based routing framework does not have to be part of the
   core MPLS specifications. However, MPLS does require some interaction
   with a constraint-based routing framework in order to meet the

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9.0 Security Considerations

   This document does not introduce new security issues beyond those
   inherent in MPLS and may use the same mechanisms proposed for this
   technology. It is, however, specifically important that manipulation
   of administratively configurable parameters be executed in a secure
   manner by authorized entities.

10.0 References

   [1]  Rosen, E., Viswanathan, A. and R. Callon, "A Proposed
        Architecture for MPLS", Work in Progress.

   [2]  Callon, R., Doolan, P., Feldman, N., Fredette, A., Swallow, G.
        and A. Viswanathan, "A Framework for Multiprotocol Label
        Switching", Work in Progress.

   [3]  Li, T. and Y. Rekhter, "Provider Architecture for Differentiated
        Services and Traffic Engineering (PASTE)", RFC 2430, October

   [4]  Rekhter, Y., Davie, B., Katz, D., Rosen, E. and  G. Swallow,
        "Cisco Systems' Tag Switching Architecture - Overview", RFC
        2105, February 1997.

   [5]  Zhang, Z., Sanchez, C., Salkewicz, B. and E. Crawley "Quality of
        Service Extensions to OSPF", Work in Progress.

   [6]  Crawley, E., Nair, F., Rajagopalan, B. and H. Sandick, "A
        Framework for QoS Based Routing in the Internet", RFC 2386,
        August 1998.

   [7]  Guerin, R., Kamat, S., Orda, A., Przygienda, T. and D. Williams,
        "QoS Routing Mechanisms and OSPF Extensions", RFC 2676, August

   [8]  C. Yang and A. Reddy, "A Taxonomy for Congestion Control
        Algorithms in Packet Switching Networks," IEEE Network Magazine,
        Volume 9, Number 5, July/August 1995.

   [9]  W. Lee, M. Hluchyi, and P. Humblet, "Routing Subject to Quality
        of Service Constraints in Integrated Communication Networks,"
        IEEE Network, July 1995, pp 46-55.

   [10] ATM Forum, "Traffic Management Specification: Version 4.0" April

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11.0 Acknowledgments

   The authors would like to thank Yakov Rekhter for his review of an
   earlier draft of this document. The authors would also like to thank
   Louis Mamakos and Bill Barns for their helpful suggestions, and
   Curtis Villamizar for providing some useful feedback.

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12.0 Authors' Addresses

   Daniel O. Awduche
   UUNET (MCI Worldcom)
   3060 Williams Drive
   Fairfax, VA 22031

   Phone: +1 703-208-5277
   EMail: awduche@uu.net

   Joe Malcolm
   UUNET  (MCI Worldcom)
   3060 Williams Drive
   Fairfax, VA 22031

   Phone: +1 703-206-5895
   EMail: jmalcolm@uu.net

   Johnson Agogbua
   UUNET  (MCI Worldcom)
   3060 Williams Drive
   Fairfax, VA 22031

   Phone: +1 703-206-5794
   EMail: ja@uu.net

   Mike O'Dell
   UUNET  (MCI Worldcom)
   3060 Williams Drive
   Fairfax, VA 22031

   Phone: +1 703-206-5890
   EMail: mo@uu.net

   Jim McManus
   UUNET  (MCI Worldcom)
   3060 Williams Drive
   Fairfax, VA 22031

   Phone: +1 703-206-5607
   EMail: jmcmanus@uu.net

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13.0  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  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

   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


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

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