RFC 3272
Overview and Principles of Internet Traffic Engineering
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
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
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.....................................711.0 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.
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
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.
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.
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).
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. - Baseline analysis: A study conducted to serve as a baseline for comparison to the actual behavior of the network. - 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. - Bottleneck: A network element whose input traffic rate tends to be greater than its output rate. - Congestion: A state of a network resource in which the traffic incident on the resource exceeds its output capacity over an interval of time. - Congestion avoidance: An approach to congestion management that attempts to obviate the occurrence of congestion. - Congestion control: An approach to congestion management that attempts to remedy congestion problems that have already occurred. - 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.
- Demand side congestion management:
A congestion management scheme that addresses congestion
problems by regulating or conditioning offered load.
- 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.
- Egress traffic:
Traffic exiting a network or network element.
- Hot-spot:
A network element or subsystem which is in a state of
congestion.
- Ingress traffic:
Traffic entering a network or network element.
- Inter-domain traffic:
Traffic that originates in one Autonomous system and
terminates in another.
- 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.
- Metric:
A parameter defined in terms of standard units of
measurement.
- Measurement Methodology:
A repeatable measurement technique used to derive one or
more metrics of interest.
- Network Survivability:
The capability to provide a prescribed level of QoS for
existing services after a given number of failures occur
within the network.
- Offline traffic engineering:
A traffic engineering system that exists outside of the
network.
- Online traffic engineering:
A traffic engineering system that exists within the network,
typically implemented on or as adjuncts to operational
network elements.
- Performance measures:
Metrics that provide quantitative or qualitative measures of
the performance of systems or subsystems of interest.
- Performance management:
A systematic approach to improving effectiveness in the
accomplishment of specific networking goals related to
performance improvement.
- Performance Metric:
A performance parameter defined in terms of standard units
of measurement.
- Provisioning:
The process of assigning or configuring network resources to
meet certain requests.
- QoS routing:
Class of routing systems that selects paths to be used by a
flow based on the QoS requirements of the flow.
- Service Level Agreement:
A contract between a provider and a customer that guarantees
specific levels of performance and reliability at a certain
cost.
- Stability:
An operational state in which a network does not oscillate
in a disruptive manner from one mode to another mode.
- Supply side congestion management:
A congestion management scheme that provisions additional
network resources to address existing and/or anticipated
congestion problems.
- Transit traffic:
Traffic whose origin and destination are both outside of the
network under consideration.
- Traffic characteristic:
A description of the temporal behavior or a description of
the attributes of a given traffic flow or traffic aggregate.
- Traffic engineering system:
A collection of objects, mechanisms, and protocols that are
used conjunctively to accomplish traffic engineering
objectives.
- 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.
- 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.
- 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.
- 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.
- 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.
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.
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.
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.
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.
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.
(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
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 - 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.
- 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. - 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-
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 - 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. - 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 - 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
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. - 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.
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