rfc2702.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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.

Abstract

   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
   characteristics.

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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   scenarios:

   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
   therein).

   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
   enhanced.

   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