Overview and Principles of Internet Traffic Engineering

1.1. What is Internet Traffic Engineering?

One of the most significant functions performed in the Internet is the routing and forwarding 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 these routing and forwarding functions, to steer traffic through the network.¶

Internet traffic engineering is defined as that aspect of Internet network engineering dealing with the issues 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 [RFC2702], [AWD2].¶

It is the performance of the network as seen by end users of network services that is paramount. 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. This is accomplished by addressing traffic oriented performance requirements while utilizing network resources without waste and in a reliable way. Traffic oriented performance measures include delay, delay variation, packet loss, and throughput.¶

Internet TE responds to network events (such as link or node failures, reported or predicted congestion, planned maintenance, service degradation, planned changes in the traffic matrix, etc.). Aspects of capacity management respond at intervals ranging from days to years. Routing control functions operate at intervals ranging from milliseconds to days. Packet level processing functions operate at very fine levels of temporal resolution, ranging from picoseconds to milliseconds while reacting to the real-time statistical behavior of traffic.¶

Thus, the optimization aspects of TE can be viewed from a control perspective, and can be both pro-active and reactive. In the pro-active case, the TE control system takes preventive action to protect against predicted unfavorable future network states, for example, by engineering backup paths. It may also take action that will lead to a more desirable future network state. In the reactive case, the control system responds to correct issues and adapt to network events, such as routing after failure.¶

Another important objective of Internet TE is to facilitate reliable network operations [RFC2702]. Reliable network operations can be facilitated by providing mechanisms that enhance network integrity and by embracing policies emphasizing network survivability. This reduces the vulnerability of services to outages arising from errors, faults, and failures occurring within the network infrastructure.¶

The optimization aspects of TE can be achieved through capacity management and traffic management. 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. In this document, traffic management includes:¶

1. Nodal traffic control functions such as traffic conditioning, queue management, and scheduling.¶
2. Other functions that regulate the flow of traffic through the network or that arbitrate access to network resources between different packets or between different traffic flows.¶

One major challenge of Internet TE is the realization of automated control capabilities that adapt quickly and cost effectively to significant changes in network state, while still maintaining stability of the network. Performance evaluation can assess the effectiveness of TE methods, and the results of this evaluation can be used to identify existing problems, guide network re-optimization, and aid in the prediction of potential future problems. However, this process can also be time consuming and may not be suitable to act on short-lived changes in the network.¶

Performance evaluation can be achieved in many different ways. The most notable techniques include analytic methods, simulation, and empirical methods based on measurements.¶

Traffic engineering comes in two flavors:¶

• A background process that constantly monitors traffic and network conditions, and optimizes the use of resources to improve performance.¶
• A form of a pre-planned optimized traffic distribution that is considered optimal.¶

In the later case, any deviation from the optimum distribution (e.g., caused by a fiber cut) is reverted upon repair without further optimization. However, this form of TE relies upon the notion that the planned state of the network is optimal. Hence, in such a mode there are two levels of TE: the TE-planning task to enable optimum traffic distribution, and the routing and forwarding tasks that keep traffic flows attached to the pre-planned distribution.¶

As a general rule, TE concepts and mechanisms must be sufficiently specific and well-defined to address known requirements, but simultaneously flexible and extensible to accommodate unforeseen future demands (see Section 6.1).¶

1.2. Components of Traffic Engineering

As mentioned in Section 1.1, Internet traffic engineering provides performance optimization of IP networks while utilizing network resources economically and reliably. Such optimization is supported at the control/controller level and within the data/forwarding plane.¶

The key elements required in any TE solution are as follows:¶

1. Policy¶
2. Path steering¶
3. Resource management¶

Some TE solutions rely on these elements to a lesser or greater extent. Debate remains about whether a solution can truly be called TE if it does not include all of these elements. For the sake of this document, we assert that all TE solutions must include some aspects of all of these elements. Other solutions can be classed as "partial TE" and also fall in scope of this document.¶

Policy allows for the selection of paths (including next hops) based on information beyond basic reachability. Early definitions of routing policy, e.g., [RFC1102] and [RFC1104], discuss routing policy being applied to restrict access to network resources at an aggregate level. BGP is an example of a commonly used mechanism for applying such policies, see [RFC4271] and [RFC8955]. In the TE context, policy decisions are made within the control plane or by controllers in the management plane, and govern the selection of paths. Examples can be found in [RFC4655] and [RFC5394]. Standard TE solutions may cover the mechanisms to distribute and/or enforce polices, but specific policy definition is generally unspecified.¶

Path steering is the ability to forward packets using more information than just knowledge of the next hop. Examples of path steering include IPv4 source routes [RFC0791], RSVP-TE explicit routes [RFC3209], Segment Routing [RFC8402], and Service Function Chaining [RFC7665]. Path steering for TE can be supported via control plane protocols, by encoding in the data plane headers, or by a combination of the two. This includes when control is provided by a controller using a network-facing control protocol.¶

Resource management provides resource-aware control and forwarding. Examples of resources are bandwidth, buffers, and queues, all of which can be managed to control loss and latency.¶

• Resource reservation is the control aspect of resource management. It provides for domain-wide consensus about which network resources are used by a particular flow. This determination may be made at a very course or very fine level. Note that this consensus exists at the network control or controller level, not within the data plane. It may be composed purely of accounting/bookkeeping, but it typically includes an ability to admit, reject, or reclassify a flow based on policy. Such accounting can be done based on any combination of a static understanding of resource requirements, and the use of dynamic mechanisms to collect requirements (e.g., via [RFC3209]) and resource availability (e.g., via [RFC4203]).¶
• Resource allocation is the data plane aspect of resource management. It provides for the allocation of specific node and link resources to specific flows. Example resources include buffers, policing, and rate-shaping mechanisms that are typically supported via queuing. It also includes the matching of a flow (i.e., flow classification) to a particular set of allocated resources. The method of flow classification and granularity of resource management is technology specific. Examples include Diffserv with dropping and remarking [RFC4594], MPLS-TE [RFC3209], and GMPLS based label switched paths [RFC3945], as well as controller-based solutions [RFC8453]. This level of resource control, while optional, is important in networks that wish to support congestion management policies to control or regulate the offered traffic to deliver different levels of service and alleviate congestion problems, or those networks that wish to control the latency experienced by specific traffic flows.¶

1.3. Scope

The scope of this document is intra-domain TE. That is, TE within a given autonomous system in the Internet. This document discusses 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 describes and characterizes techniques already in use or in advanced development for Internet TE. The way these techniques fit together is discussed and scenarios in which they are useful will be identified.¶

Although the emphasis in this document is on intra-domain traffic engineering, an overview of the high level considerations pertaining to inter-domain TE is provided in Section 7. Inter-domain Internet TE is crucial to the performance enhancement of the global Internet infrastructure.¶

Whenever possible, relevant requirements from existing IETF documents and other sources are incorporated by reference.¶

1.4. Terminology

This section provides terminology which is useful for Internet TE. The definitions presented apply to this document. These terms may have other meanings elsewhere.¶

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.¶

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-based 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.¶

Hot-spot:

A network element or subsystem which is in a state of congestion.¶

Inter-domain traffic:

Traffic that originates in one Autonomous system and terminates in another.¶

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 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 (SLA):

A contract between a provider and a customer that guarantees specific levels of performance and reliability at a certain cost.¶

Service Level Objective (SLO):

A key element of an SLA between a provider and a customer. SLOs are agreed upon as a means of measuring the performance of the Service Provider and are outlined as a way of avoiding disputes between the two parties based on misunderstanding.¶

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.¶

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 together to accomplish traffic engineering objectives.¶

Traffic flow:

A stream of packets between two end-points that can be characterized in a certain way. A common classification for a traffic flow selects packets with the "five-tuple" of source and destination addresses, source and destination ports, and protocol ID.¶

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.¶

2.1. Context of Internet Traffic Engineering

The context of Internet traffic engineering includes:¶

1. A network domain context that defines the scope under consideration, and in particular the situations in which the TE problems occur. The network domain 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 TE 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 instantiated. The implementation and operational context includes planning, organization, and execution.¶

The context of Internet TE and the different problem scenarios are discussed in the following subsections.¶

2.2. Network Domain 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.¶

At the most basic level of abstraction, an IP network can be represented as a distributed dynamic system consisting of:¶

• a set of interconnected resources which provide transport services for IP traffic subject to certain constraints¶
• a demand system representing the offered load to be transported through the network¶
• 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 traffic demand is handled. Additionally, network resources may be equipped with traffic control mechanisms managing the way in which the demand is serviced. Traffic control mechanisms may be used to:¶

• control packet processing activities within a given resource¶
• arbitrate contention for access to the resource by different packets¶
• 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 carries 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 service provided by the network also depend 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.¶

Internet networks have two significant characteristics:¶

• they provide real-time services¶
• their operating environments are very dynamic.¶

The dynamic characteristics of IP and IP/MPLS 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 faults 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, for an interval of time, the arrival rate of packets exceed the output capacity of the resource. Congestion may result in some of the arriving packets being delayed or even dropped.¶

Congestion increases transit delay, delay variation, may lead to packet loss, and reduces the predictability of network services. Clearly, congestion is highly undesirable. Combating congestion at a reasonable cost is a major objective of Internet TE.¶

Efficient sharing of network resources by multiple traffic flows is a basic operational premise for the Internet. A fundamental challenge in network operation is to increase resource utilization while minimizing the possibility of congestion.¶

The Internet has to function in the presence of different classes of traffic with different service requirements. This requirement is clarified in [RFC2475] which also provides an architecture for Differentiated Services (Diffserv). That document describes how packets can be grouped into behavior aggregates such that each aggregate has a common set of behavioral characteristics or a common set of delivery requirements. 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 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:¶

  • integrity constraints such as packet loss¶
  • 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

There are several problems associated with operating a network described in the previous section. This section analyzes the problem context in relation to TE. The identification, abstraction, representation, and measurement of network features relevant to TE are significant issues.¶

A particular challenge is to formulate the problems that traffic engineering attempts to solve. For example:¶

• How to identify the requirements on the solution space?¶
• How to specify the desirable features of solutions?¶
• How to actually solve the problems?¶
• How to measure and characterize the effectiveness of solutions?¶

Another class of problems is how to measure and estimate relevant network state parameters. Effective TE 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 problem is how to characterize the state of the network and how to evaluate its performance. The performance evaluation problem is two-fold: one aspect relates to the evaluation of the system-level performance of the network; the other aspect relates to the evaluation of resource-level performance, which restricts attention to the performance analysis of individual network resources.¶

In this document, 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. Correspondingly, we refer to the TE 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 problem concerns how to effectively optimize network performance. Performance optimization may entail translating solutions for specific TE problems into network configurations. Optimization may also entail some degree of resource management control, routing control, and capacity augmentation.¶

2.4. Solution Context

The solution context for Internet TE involves analysis, evaluation of alternatives, and choice between alternative courses of action. Generally, the solution context is based on making inferences about the current or future state of the network, and making 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, derivation of solutions 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 TE.¶

• A set of policies, objectives, and requirements (which may be context dependent) for network performance evaluation and performance optimization.¶
• A collection of online and possibly offline tools and mechanisms for measurement, characterization, modeling, and control 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.¶
• A set of constraints on the operating environment, the network protocols, and the TE system itself.¶
• A set of quantitative and qualitative techniques and methodologies for abstracting, formulating, and solving TE problems.¶
• A set of administrative control parameters which may be manipulated through a configuration management system. Such system itself may include a configuration control subsystem, a configuration repository, a configuration accounting subsystem, and a configuration auditing subsystem.¶
• A set of guidelines for network performance evaluation, performance optimization, and performance improvement.¶

Determining traffic characteristics through measurement or estimation is very useful within the realm the TE solution space. Traffic estimates can be derived from customer subscription information, traffic projections, traffic models, and from actual measurements. The measurements may be performed at different levels, e.g., at the traffic-aggregate level or at the flow level. Measurements at the flow level or on small traffic aggregates may be performed at edge nodes, when traffic enters and leaves the network. Measurements for large traffic-aggregates may be performed within the core of the network.¶

To conduct performance studies and to support planning of existing and future networks, a routing analysis may be performed to determine the paths the routing protocols will choose for various traffic demands, and to ascertain the utilization of network resources as traffic is routed through the network. Routing analysis captures 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 exist) and over the underlying network infrastructure. A model of network topology is necessary to perform routing analysis. A network topology model may be extracted from:¶

• network architecture documents¶
• network designs¶
• information contained in router configuration files¶
• routing databases¶
• routing tables¶
• automated tools that discover and collate 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 interior gateway protocol (IGP) metrics. For path oriented technologies such as MPLS, routing can be further controlled by the manipulation of relevant TE parameters, resource parameters, and administrative policy constraints. Within the context of MPLS, the path of an explicitly routed label switched path (LSP) can be computed and established in various ways including:¶

• manually¶
• automatically, online using constraint-based routing processes implemented on label switching routers¶
• automatically, offline using constraint-based routing entities implemented on external TE support systems.¶

2.5. Implementation and Operational Context

The operational context of Internet TE is characterized by constant changes that 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 TE system and coordinating the activities to accomplish the desired TE objectives. Execution involves measuring and applying corrective or perfective actions to attain and maintain desired TE goals.¶

3.1. Components of the Traffic Engineering Process Model

The key components of the traffic engineering process model are as follows.¶

1. Measurement is crucial to the TE function. The operational state of a network can only be conclusively determined through measurement. Measurement is also critical to the optimization function because it provides feedback data which is used by TE control subsystems. This data is used to adaptively optimize network performance in response to events and stimuli originating within and outside the network. Measurement in support of the TE function can occur at different levels of abstraction. For example, measurement can be used to derive packet level characteristics, flow level characteristics, user or customer level characteristics, traffic aggregate characteristics, component level characteristics, and network wide characteristics.¶
2. Modeling, analysis, and simulation are important aspects of Internet TE. Modeling involves constructing an abstract or physical representation which depicts relevant traffic characteristics and network attributes. A network model is an abstract representation of the network which captures relevant network features, attributes, and characteristic. Network simulation tools are extremely useful for TE. Because of the complexity of realistic quantitative analysis of network behavior, certain aspects of network performance studies can only be conducted effectively using simulation.¶
3. Network performance optimization involves resolving network issues by transforming such issues into concepts that enable a solution, identification of a solution, and implementation of the solution. Network performance optimization can be corrective or perfective. In corrective optimization, the goal is to remedy a problem that has occurred or that is incipient. In perfective optimization, the goal is to improve network performance even when explicit problems do not exist and are not anticipated.¶

4.1. Time-Dependent Versus State-Dependent Versus Event-Dependent

Traffic engineering methodologies can be classified as time- dependent, state-dependent, or event-dependent. All TE schemes are considered to be dynamic in this document. Static TE implies that no TE methodology or algorithm is being applied - it is a feature of network planning, but lacks the reactive and flexible nature of TE.¶

In time-dependent TE, historical information based on periodic variations in traffic (such as time of day) is used to pre-program routing and other TE control mechanisms. Additionally, customer subscription or traffic projection may be used. Pre-programmed routing plans typically change on a relatively long time scale (e.g., daily). Time-dependent algorithms do not attempt to adapt to short-term variations in traffic or changing network conditions. An example of a time-dependent algorithm is a global centralized optimizer where the input to the system is a traffic matrix and multi-class QoS requirements as described [MR99]. Another example of such a methodology is the application of data mining to Internet traffic [AJ19] which enables the use of various machine learning algorithms to identify patterns within historically collected datasets about Internet traffic, and to extract information in order to guide decision-making, and to improve efficiency and productivity of operational processes.¶

State-dependent TE adapts the routing plans based on the current state of the network which provides additional information on variations in actual traffic (i.e., perturbations from regular variations) that could not be predicted using historical information. Constraint-based routing is an example of state-dependent TE operating in a relatively long time scale. An example operating in a relatively short timescale is a load-balancing algorithm described in [MATE]. The state of the network can be based on parameters flooded by the routers. Another approach is for a particular router performing adaptive TE to send probe packets along a path to gather the state of that path. [RFC6374] defines protocol extensions to collect performance measurements from MPLS networks. Another approach is for a management system to gather the relevant information directly from network elements using telemetry data collection "publication/subscription" techniques [RFC7923]. Timely gathering and distribution of state information is critical for adaptive TE. While time-dependent algorithms are suitable for predictable traffic variations, state-dependent algorithms may be applied to increase network efficiency and resilience to adapt to the prevailing network state.¶

Event-dependent TE methods can also be used for TE path selection. Event-dependent TE methods are distinct from time-dependent and state-dependent TE methods in the manner in which paths are selected. These algorithms are adaptive and distributed in nature and typically use learning models to find good paths for TE in a network. While state-dependent TE models typically use available-link-bandwidth (ALB) flooding for TE path selection, event-dependent TE methods do not require ALB flooding. Rather, event-dependent TE methods typically search out capacity by learning models, as in the success- to-the-top (STT) method. ALB flooding can be resource intensive, since it requires link bandwidth to carry LSAs, processor capacity to process LSAs, and the overhead can limit area/Autonomous System (AS) size. Modeling results suggest that event-dependent TE methods could lead to a reduction in ALB flooding overhead without loss of network throughput performance [I-D.ietf-tewg-qos-routing].¶

4.2. Offline Versus Online

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

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

4.3. Centralized Versus Distributed

Under centralized control there is a central authority which determines routing plans and perhaps other TE control parameters on behalf of each router. The central authority periodically collects network-state information from all routers, and sends routing information to the routers. The update cycle for information exchange in both directions is a critical parameter directly impacting the performance of the network being controlled. Centralized control may need high processing power and high bandwidth control channels.¶

Distributed control determines route selection by each router autonomously based on the router's view of the state of the network. The network state information may be obtained by the router using a probing method or distributed by other routers on a periodic basis using link state advertisements. Network state information may also be disseminated under exception conditions. Examples of protocol extensions used to advertise network link state information are defined in [RFC5305], [RFC6119], [RFC7471], [RFC8570], and [RFC8571]. See also Section 5.1.3.9.¶

4.4. Local Versus Global

Traffic engineering algorithms may require local and global network- state information.¶

Local information is the state of a portion of the domain. Examples include the bandwidth and packet loss rate of a particular path, or the state and capabilities of a network link. Local state information may be sufficient for certain instances of distributed control TE.¶

Global information is the state of the entire TE domain. Examples include a global traffic matrix, and loading information on each link throughout the domain of interest. Global state information is typically required with centralized control. Distributed TE systems may also need global information in some cases.¶

4.5. Prescriptive Versus Descriptive

TE systems may also be classified as prescriptive or descriptive.¶

Prescriptive traffic engineering evaluates alternatives and recommends a course of action. Prescriptive TE can be further categorized as either corrective or perfective. Corrective TE prescribes a course of action to address an existing or predicted anomaly. Perfective TE prescribes a course of action to evolve and improve network performance even when no anomalies are evident.¶

Descriptive traffic engineering, on the other hand, characterizes the state of the network and assesses the impact of various policies without recommending any particular course of action.¶

4.6. Open-Loop Versus Closed-Loop

Open-loop traffic engineering control is where control action does not use feedback information from the current network state. The control action may use its own local information for accounting purposes, however.¶

Closed-loop traffic engineering control is where control action utilizes feedback information from the network state. The feedback information may be in the form of historical information or current measurement.¶

4.7. Tactical versus Strategic

Tactical traffic engineering aims to address specific performance problems (such as hot-spots) that occur in the network from a tactical perspective, without consideration of overall strategic imperatives. Without proper planning and insights, tactical TE tends to be ad hoc in nature.¶

Strategic traffic engineering approaches the TE problem from a more organized and systematic perspective, taking into consideration the immediate and longer term consequences of specific policies and actions.¶

5.1. Overview of IETF Projects Related to Traffic Engineering

This subsection reviews a number of IETF activities pertinent to Internet traffic engineering.¶

5.2. Content Distribution

The Internet is dominated by client-server interactions, principally Web traffic although in the future, more sophisticated media servers may become dominant. The location and performance of major information servers has a significant impact on the traffic patterns within the Internet as well as on the perception of service quality by end users.¶

A number of dynamic load balancing techniques have been devised to improve the performance of replicated information servers. These techniques can cause spatial traffic characteristics to become more dynamic in the Internet because information servers can be dynamically picked based upon the location of the clients, the location of the servers, the relative utilization of the servers, the relative performance of different networks, and the relative performance of different parts of a network. This process of assignment of distributed servers to clients is called traffic directing. It is an application layer function.¶

Traffic directing schemes that allocate servers in multiple geographically dispersed locations to clients may require empirical network performance statistics to make more effective decisions. In the future, network measurement systems may need to provide this type of information.¶

When congestion exists in the network, traffic directing and traffic engineering systems should act in a coordinated manner. This topic is for further study.¶

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

6.1. Generic Non-functional Recommendations

The generic non-functional recommendations for Internet traffic engineering are listed in the paragraphs that follow. In a given context, some of these recommendations may be critical while others may be optional. Therefore, prioritization may be required during the development phase of a TE system to tailor it to a specific operational context.¶

Usability:

Usability is a human aspect of TE systems. It refers to the ease with which a TE system can be deployed and operated. In general, it is desirable to have a TE system that can be readily deployed in an existing network. It is also desirable to have a TE system that is easy to operate and maintain.¶

Automation:

Whenever feasible, a TE system should automate as many TE functions as possible to minimize the amount of human effort needed to analyze and control operational networks. Automation is particularly important in large-scale public networks because of the high cost of the human aspects of network operations and the high risk of network problems caused by human errors. Automation may entail the incorporation of automatic feedback and intelligence into some components of the TE system.¶

Scalability:

Public networks continue to grow rapidly with respect to network size and traffic volume. Therefore, to remain applicable as the network evolves, a TE system should be scalable. In particular, a TE system should remain functional as the network expands with regard to the number of routers and links, and with respect to the traffic volume. A TE system should have a scalable architecture, should not adversely impair other functions and processes in a network element, and should not consume too many network resources when collecting and distributing state information, or when exerting control.¶

Stability:

Stability is a very important consideration in TE systems that respond to changes in the state of the network. State-dependent TE methodologies typically include a trade-off between responsiveness and stability. It is strongly recommended that when a trade-off between responsiveness and stability is needed, it should be made in favor of stability (especially in public IP backbone networks).¶

Flexibility:

A TE system should allow for changes in optimization policy. In particular, a TE system should provide sufficient configuration options so that a network administrator can tailor the system to a particular environment. It may also be desirable to have both online and offline TE subsystems which can be independently enabled and disabled. TE systems that are used in multi-class networks should also have options to support class based performance evaluation and optimization.¶

Visibility:

Mechanisms should exist as part of the TE system to collect statistics from the network and to analyze these statistics to determine how well the network is functioning. Derived statistics such as traffic matrices, link utilization, latency, packet loss, and other performance measures of interest which are determined from network measurements can be used as indicators of prevailing network conditions. The capabilities of the various components of the routing system are other examples of status information which should be observable.¶

Simplicity:

A TE system should be as simple as possible. Simplicity in user interface does not necessarily imply that the TE system will use naive algorithms. When complex algorithms and internal structures are used, the user interface should hide such complexities from the network administrator as much as possible.¶

Interoperability:

Whenever feasible, TE systems and their components should be developed with open standards-based interfaces to allow interoperation with other systems and components.¶

Security:

Security is a critical consideration in TE systems. Such systems typically exert control over functional aspects of the network to achieve the desired performance objectives. Therefore, adequate measures must be taken to safeguard the integrity of the TE system. Adequate measures must also be taken to protect the network from vulnerabilities that originate from security breaches and other impairments within the TE system.¶

The remaining subsections of this section focus on some of the high-level functional recommendations for TE.¶

6.2. Routing Recommendations

Routing control is a significant aspect of Internet traffic engineering. Routing impacts many of the key performance measures associated with networks, such as throughput, delay, and utilization. Generally, it is very difficult to provide good service quality in a wide area network without effective routing control. A desirable TE routing system is one that takes traffic characteristics and network constraints into account during route selection while maintaining stability.¶

Shortest path first (SPF) IGPs are based on shortest path algorithms and have limited control capabilities for TE [RFC2702], [AWD2]. These limitations include:¶

1. Pure SPF protocols do not take network constraints and traffic characteristics into account during route selection. For example, IGPs always select the shortest paths based on link metrics assigned by administrators) so load sharing cannot be performed across paths of different costs. Using shortest paths to forward traffic may cause the following problems:¶

   • If traffic from a source to a destination exceeds the capacity of a link along the shortest path, the link (and hence the shortest path) becomes congested while a longer path between these two nodes may be under-utilized¶
   • The shortest paths from different sources can overlap at some links. If the total traffic from the sources exceeds the capacity of any of these links, congestion will occur.¶
   • Problems can also occur because traffic demand changes over time, but network topology and routing configuration cannot be changed as rapidly. This causes the network topology and routing configuration to become sub-optimal over time, which may result in persistent congestion problems.¶
2. The Equal-Cost Multi-Path (ECMP) capability of SPF IGPs supports sharing of traffic among equal cost paths between two nodes. However, ECMP attempts to divide the traffic as equally as possible among the equal cost shortest paths. Generally, ECMP does not support configurable load sharing ratios among equal cost paths. The result is that one of the paths may carry significantly more traffic than other paths because it may also carry traffic from other sources. This situation can result in congestion along the path that carries more traffic. Weighted ECMP (WECMP) (see, for example, [I-D.ietf-bess-evpn-unequal-lb]) provides some mitigation.¶
3. Modifying IGP metrics to control traffic routing tends to have network-wide effects. Consequently, undesirable and unanticipated traffic shifts can be triggered as a result. Work described in Section 8 may be capable of better control [FT00], [FT01].¶

Because of these limitations, new capabilities are needed to enhance the routing function in IP networks. Some of these capabilities are summarized below.¶

• Constraint-based routing computes routes to fulfill requirements subject to constraints. This can be useful in public IP backbones with complex topologies. Constraints may include bandwidth, hop count, delay, and administrative policy instruments such as resource class attributes [RFC2702], [RFC2386]. This makes it possible to select routes that satisfy a given set of requirements. Routes computed by constraint-based routing are not necessarily the shortest paths. Constraint-based routing works best with path-oriented technologies that support explicit routing, such as MPLS.¶

• Constraint-based routing can also be used as a way to distribute traffic onto the infrastructure, including for best effort traffic. For example, congestion problems caused by uneven traffic distribution may be avoided or reduced by knowing the reservable bandwidth attributes of the network links and by specifying the bandwidth requirements for path selection.¶

• A number of enhancements to the link state IGPs are needed to allow them to distribute additional state information required for constraint-based routing. The extensions to OSPF are described in [RFC3630], and to IS-IS in [RFC5305]. Some of the additional topology state information includes link attributes such as reservable bandwidth and link resource class attribute (an administratively specified property of the link). The resource class attribute concept is defined in [RFC2702]. The additional topology state information is carried in new TLVs and sub-TLVs in IS-IS, or in the Opaque LSA in OSPF [RFC5305], [RFC3630].¶

• An enhanced link-state IGP may flood information more frequently than a normal IGP. This is because even without changes in topology, changes in reservable bandwidth or link affinity can trigger the enhanced IGP to initiate flooding. A trade-off between the timeliness of the information flooded and the flooding frequency is typically implemented using a threshold based on the percentage change of the advertised resources to avoid excessive consumption of link bandwidth and computational resources, and to avoid instability in the TED.¶

• In a TE system, it is also desirable for the routing subsystem to make the load splitting ratio among multiple paths (with equal cost or different cost) configurable. This capability gives network administrators more flexibility in the control of traffic distribution across the network. It can be very useful for avoiding/relieving congestion in certain situations. Examples can be found in [XIAO] and [I-D.ietf-bess-evpn-unequal-lb].¶
• The routing system should also have the capability to control the routes of subsets of traffic without affecting the routes of other traffic if sufficient resources exist for this purpose. This capability allows a more refined control over the distribution of traffic across the network. For example, the ability to move traffic away from its original path to another path (without affecting other traffic paths) allows the traffic to be moved from resource-poor network segments to resource-rich segments. Path oriented technologies such as MPLS-TE inherently support this capability as discussed in [AWD2].¶
• Additionally, the routing subsystem should be able to select different paths for different classes of traffic (or for different traffic behavior aggregates) if the network supports multiple classes of service (different behavior aggregates).¶

6.3. Traffic Mapping Recommendations

Traffic mapping is the assignment of traffic workload onto (pre-established) paths to meet certain requirements. Thus, while constraint-based routing deals with path selection, traffic mapping deals with the assignment of traffic to established paths which may have been generated by constraint-based routing or by some other means. Traffic mapping can be performed by time-dependent or state- dependent mechanisms, as described in Section 4.1.¶

An important aspect of the traffic mapping function is the ability to establish multiple paths between an originating node and a destination node, and the capability to distribute the traffic between the two nodes across the paths according to some policies. A pre-condition for this scheme is the existence of flexible mechanisms to partition traffic and then assign the traffic partitions onto the parallel paths as noted in [RFC2702]. When traffic is assigned to multiple parallel paths, it is recommended that special care should be taken to ensure proper ordering of packets belonging to the same application (or traffic flow) at the destination node of the parallel paths.¶

Mechanisms that perform the traffic mapping functions should aim to map the traffic onto the network infrastructure to minimize congestion. If the total traffic load cannot be accommodated, or if the routing and mapping functions cannot react fast enough to changing traffic conditions, then a traffic mapping system may use short time scale congestion control mechanisms (such as queue management, scheduling, etc.) to mitigate congestion. Thus, mechanisms that perform the traffic mapping functions complement existing congestion control mechanisms. In an operational network, traffic should be mapped onto the infrastructure such that intra-class and inter-class resource contention are minimized (see Section 2).¶

When traffic mapping techniques that depend on dynamic state feedback (e.g., MATE [MATE] and such like) are used, special care must be taken to guarantee network stability.¶

6.4. Measurement Recommendations

The importance of measurement in TE has been discussed throughout this document. A TE system should include mechanisms to measure and collect statistics from the network to support the TE function. Additional capabilities may be needed to help in the analysis of the statistics. The actions of these mechanisms should not adversely affect the accuracy and integrity of the statistics collected. The mechanisms for statistical data acquisition should also be able to scale as the network evolves.¶

Traffic statistics may be classified according to long-term or short-term timescales. Long-term traffic statistics are very useful for traffic engineering. Long-term traffic statistics may periodicity record network workload (such as hourly, daily, and weekly variations in traffic profiles) as well as traffic trends. Aspects of the traffic statistics may also describe class of service characteristics for a network supporting multiple classes of service. Analysis of the long-term traffic statistics may yield other information such as busy hour characteristics, traffic growth patterns, persistent congestion problems, hot-spot, and imbalances in link utilization caused by routing anomalies.¶

A mechanism for constructing traffic matrices for both long-term and short-term traffic statistics should be in place. In multi-service IP networks, the traffic matrices may be constructed for different service classes. Each element of a traffic matrix represents a statistic about the traffic flow between a pair of abstract nodes. An abstract node may represent a router, a collection of routers, or a site in a VPN.¶

Traffic statistics should provide reasonable and reliable indicators of the current state of the network on the short-term scale. Some short term traffic statistics may reflect link utilization and link congestion status. Examples of congestion indicators include excessive packet delay, packet loss, and high resource utilization. Examples of mechanisms for distributing this kind of information include SNMP, probing tools, FTP, IGP link state advertisements, and NETCONF/RESTCONF, etc.¶

6.5. Policing, Planning, and Access Control

The recommendations in Section 6.2 and Section 6.3 may be sub-optimal or even ineffective if the amount of traffic flowing on a route or path exceeds the capacity of the resource on that route or path. Several approaches can be used to increase the performance of TE systems.¶

• The fundamental approach is some form of planning where traffic is steered onto paths so that it is distributed across the available resources. This planning may be centralized or distributed, and must be aware of the planned traffic volumes and available resources. However, this approach is only of value if the traffic is presented conformant to the planned traffic volumes.¶
• Traffic flows may be policed at the edges of a network. This is a simple way to check that the actual traffic volumes are consistent with the planned volumes. Some form of measurement (see Section 6.4) is used to determine the rate of arrival of traffic and excess traffic could be discarded. Alternatively, excess traffic could be forwarded as best-effort within the network. However, this approach is only completely effective if the planning is stringent and network-wide, and if a harsh approach is taken to disposing of excess traffic.¶
• Resource-based admission control is the process whereby network nodes decide whether to grant access to resources. The basis for the decision on a packet-by-packet basis is determination of the flow to which the packet belongs. This information is combined with policy instructions that have been locally configured, or installed through the management or control planes. The end result is that a packet may be allowed to access (or use) specific resources on the node if and only if the policy is conformed with for the flow to which the packet belongs.¶

Combining some element of all three of these measures is advisable to achieve a better TE system.¶

6.6. Network Survivability

Network survivability refers to the capability of a network to maintain service continuity in the presence of faults. This can be accomplished by promptly recovering from network impairments and maintaining the required QoS for existing services after recovery. Survivability is an issue of great concern within the Internet community due to the demand to carry mission critical traffic, real-time traffic, and other high priority traffic over the Internet. Survivability can be addressed at the device level by developing network elements that are more reliable; and at the network level by incorporating redundancy into the architecture, design, and operation of networks. It is recommended that a philosophy of robustness and survivability should be adopted in the architecture, design, and operation of TE that control IP networks (especially public IP networks). Because different contexts may demand different levels of survivability, the mechanisms developed to support network survivability should be flexible so that they can be tailored to different needs. A number of tools and techniques have been developed to enable network survivability including MPLS Fast Reroute [RFC4090], Topology Independent Loop-free Alternate Fast Re-route for Segment Routing [I-D.ietf-rtgwg-segment-routing-ti-lfa] RSVP-TE Extensions in Support of End-to-End GMPLS Recovery [RFC4872], and GMPLS Segment Recovery [RFC4873].¶

The impact of service outages varies significantly for different service classes depending on the duration of the outage which can vary from milliseconds (with minor service impact) to seconds (with possible call drops for IP telephony and session time-outs for connection oriented transactions) to minutes and hours (with potentially considerable social and business impact). Different duration outages have different impacts depending on the throughput of the traffic flows that are interrupted.¶

Failure protection and restoration capabilities are available in multiple layers as network technologies have continued to evolve. Optical networks are capable of providing dynamic ring and mesh restoration functionality at the wavelength level. At the SONET/SDH layer survivability capability is provided with Automatic Protection Switching (APS) as well as self-healing ring and mesh architectures. Similar functionality is provided by layer 2 technologies such as Ethernet.¶

Rerouting is used at the IP layer to restore service following link and node outages. Rerouting at the IP layer occurs after a period of routing convergence which may require seconds to minutes to complete. Path-oriented technologies such as MPLS ([RFC3469]) can be used to enhance the survivability of IP networks in a potentially cost effective manner.¶

An important aspect of multi-layer survivability is that technologies at different layers may provide protection and restoration capabilities at different granularities in terms of time scales and at different bandwidth granularity (from packet-level to wavelength level). Protection and restoration capabilities can also be sensitive to different service classes and different network utility models. Coordinating different protection and restoration capabilities across multiple layers in a cohesive manner to ensure network survivability is maintained at reasonable cost is a challenging task. Protection and restoration coordination across layers may not always be feasible, because networks at different layers may belong to different administrative domains.¶

The following paragraphs present some of the general recommendations for protection and restoration coordination.¶

• Protection and restoration capabilities from different layers should be coordinated to provide network survivability in a flexible and cost effective manner. Avoiding duplication of functions in different layers is one way to achieve the coordination. Escalation of alarms and other fault indicators from lower to higher layers may also be performed in a coordinated manner. The order of timing of restoration triggers from different layers is another way to coordinate multi-layer protection/restoration.¶
• Network capacity reserved in one layer to provide protection and restoration is not available to carry traffic in a higher layer: it is not visible as spare capacity in the higher layer. Placing protection/restoration functions in many layers may increase redundancy and robustness, but it can result in significant inefficiencies in network resource utilization. Careful planning is needed to balance the trade-off between the desire for survivability and the optimal use of resources.¶
• It is generally desirable to have protection and restoration schemes that are intrinsically bandwidth efficient.¶
• Failure notifications throughout the network should be timely and reliable if they are to be acted on as triggers for effective protection and restoration actions.¶
• Alarms and other fault monitoring and reporting capabilities should be provided at the right network layers so that the protection and restoration actions can be taken in those layers.¶

6.7. Multi-Layer Traffic Engineering

Networks are often arranged as layers. A layer relationship may represent the interaction between technologies (for example, an IP network operated over an optical network), or the relationship between different network operators (for example, a customer network operated over a service provider's network). Note that a multi-layer network does not imply the use of multiple technologies, although some form of encapsulation is often applied.¶

Multi-layer traffic engineering presents a number of challenges associated with scalability and confidentiality. These issues are addressed in [RFC7926] which discusses the sharing of information between domains through policy filters, aggregation, abstraction, and virtualization. That document also discusses how existing protocols can support this scenario with special reference to BGP-LS (see Section 5.1.3.10).¶

PCE (see Section 5.1.3.11) is also a useful tool for multi-layer networks as described in [RFC6805], [RFC8685], and [RFC5623]. Signaling techniques for multi-layer TE are described in [RFC6107].¶

See also Appendix A.3.1 for a discussion of how the overlay model has been important in the development of TE, and Section 6.6 for examination of multi-layer network survivability.¶

6.8. Traffic Engineering in Diffserv Environments

Increasing requirements to support multiple classes of traffic in the Internet, such as best effort and mission critical data, calls for IP networks to differentiate traffic according to some criteria and to give preferential treatment to certain types of traffic. Large numbers of flows can be aggregated into a few behavior aggregates based on some criteria based on common performance requirements in terms of packet loss ratio, delay, and jitter, or in terms of common fields within the IP packet headers.¶

Differentiated Services (Diffserv) [RFC2475] can be used to ensure that SLAs defined to differentiate between traffic flows are met. Classes of service (CoS) can be supported in a Diffserv environment by concatenating per-hop behaviors (PHBs) along the routing path. A PHB is the forwarding behavior that a packet receives at a Diffserv- compliant node, and it can be configured at each router. PHBs are delivered using buffer management and packet scheduling mechanisms and require that the ingress nodes use traffic classification, marking, policing, and shaping.¶

TE can complement Diffserv to improve utilization of network resources. TE can be operated on an aggregated basis across all service classes [RFC3270], or on a per service class basis. The former is used to provide better distribution of the traffic load over the network resources (see [RFC3270] for detailed mechanisms to support aggregate TE). The latter case is discussed below since it is specific to the Diffserv environment, with so called Diffserv-aware traffic engineering [RFC4124].¶

For some Diffserv networks, it may be desirable to control the performance of some service classes by enforcing relationships between the traffic workload contributed by each service class and the amount of network resources allocated or provisioned for that service class. Such relationships between demand and resource allocation can be enforced using a combination of, for example:¶

• TE mechanisms on a per service class basis that enforce the relationship between the amount of traffic contributed by a given service class and the resources allocated to that class.¶
• Mechanisms that dynamically adjust the resources allocated to a given service class to relate to the amount of traffic contributed by that service class.¶

It may also be desirable to limit the performance impact of high priority traffic on relatively low priority traffic. This can be achieved, for example, by controlling the percentage of high priority traffic that is routed through a given link. Another way to accomplish this is to increase link capacities appropriately so that lower priority traffic can still enjoy adequate service quality. When the ratio of traffic workload contributed by different service classes varies significantly from router to router, it may not be enough to rely on conventional IGP routing protocols or on TE mechanisms that are not sensitive to different service classes. Instead, it may be desirable to perform TE, especially routing control and mapping functions, on a per service class basis. One way to accomplish this in a domain that supports both MPLS and Diffserv is to define class specific LSPs and to map traffic from each class onto one or more LSPs that correspond to that service class. An LSP corresponding to a given service class can then be routed and protected/restored in a class dependent manner, according to specific policies.¶

Performing TE on a per class basis may require per-class parameters to be distributed. It is common to have some classes share some aggregate constraints (e.g., maximum bandwidth requirement) without enforcing the constraint on each individual class. These classes can be grouped into class-types, and per-class-type parameters can be distributed to improve scalability. This also allows better bandwidth sharing between classes in the same class-type. A class-type is a set of classes that satisfy the following two conditions:¶

• Classes in the same class-type have common aggregate requirements to satisfy required performance levels.¶
• There is no requirement to be enforced at the level of an individual class in the class-type. Note that it is, nevertheless, still possible to implement some priority policies for classes in the same class-type to permit preferential access to the class-type bandwidth through the use of preemption priorities.¶

See [RFC4124] for detailed requirements on Diffserv-aware TE.¶

6.9. Network Controllability

Offline and online (see Section 4.2) TE considerations are of limited utility if the network cannot be controlled effectively to implement the results of TE decisions and to achieve the desired network performance objectives.¶

Capacity augmentation is a coarse-grained solution to TE issues. However, it is simple and may be advantageous if bandwidth is abundant and cheap. However, bandwidth is not always abundant and cheap, and additional capacity might not always be the best solution. Adjustments of administrative weights and other parameters associated with routing protocols provide finer-grained control, but this approach is difficult to use and imprecise because of the the way the routing protocols interact occur across the network.¶

Control mechanisms can be manual (e.g., static configuration), partially-automated (e.g., scripts), or fully-automated (e.g., policy based management systems). Automated mechanisms are particularly useful in large scale networks. Multi-vendor interoperability can be facilitated by standardized management systems (e.g., YANG models) to support the control functions required to address TE objectives.¶

Network control functions should be secure, reliable, and stable as these are often needed to operate correctly in times of network impairments (e.g., during network congestion or security attacks).¶

A.1. Traffic Engineering in Classical Telephone Networks

This subsection presents a brief overview of traffic engineering in telephone networks which often relates to the way user traffic is steered from an originating node to the terminating node. This subsection presents a brief overview of this topic. A detailed description of the various routing strategies applied in telephone networks is included in the book by G. Ash [ASH2].¶

The early telephone network relied on static hierarchical routing, whereby routing patterns remained fixed independent of the state of the network or time of day. The hierarchy was intended to accommodate overflow traffic, improve network reliability via alternate routes, and prevent call looping by employing strict hierarchical rules. The network was typically over-provisioned since a given fixed route had to be dimensioned so that it could carry user traffic during a busy hour of any busy day. Hierarchical routing in the telephony network was found to be too rigid upon the advent of digital switches and stored program control which were able to manage more complicated TE rules.¶

Dynamic routing was introduced to alleviate the routing inflexibility in the static hierarchical routing so that the network would operate more efficiently. This resulted in significant economic gains [HUSS87]. Dynamic routing typically reduces the overall loss probability by 10 to 20 percent (compared to static hierarchical routing). Dynamic routing can also improve network resilience by recalculating routes on a per-call basis and periodically updating routes.¶

There are three main types of dynamic routing in the telephone network. They are time-dependent routing, state-dependent routing (SDR), and event dependent routing (EDR).¶

In time-dependent routing, regular variations in traffic loads (such as time of day or day of week) are exploited in pre-planned routing tables. In state-dependent routing, routing tables are updated online according to the current state of the network (e.g., traffic demand, utilization, etc.). In event dependent routing, routing changes are triggers by events (such as call setups encountering congested or blocked links) whereupon new paths are searched out using learning models. EDR methods are real-time adaptive, but they do not require global state information as does SDR. Examples of EDR schemes include the dynamic alternate routing (DAR) from BT, the state-and-time dependent routing (STR) from NTT, and the success-to- the-top (STT) routing from AT&T.¶

Dynamic non-hierarchical routing (DNHR) is an example of dynamic routing that was introduced in the AT&T toll network in the 1980's to respond to time-dependent information such as regular load variations as a function of time. Time-dependent information in terms of load may be divided into three timescales: hourly, weekly, and yearly. Correspondingly, three algorithms are defined to pre-plan the routing tables. The network design algorithm operates over a year-long interval while the demand servicing algorithm operates on a weekly basis to fine tune link sizes and routing tables to correct forecast errors on the yearly basis. At the smallest timescale, the routing algorithm is used to make limited adjustments based on daily traffic variations. Network design and demand servicing are computed using offline calculations. Typically, the calculations require extensive searches on possible routes. On the other hand, routing may need online calculations to handle crankback. DNHR adopts a "two-link" approach whereby a path can consist of two links at most. The routing algorithm presents an ordered list of route choices between an originating switch and a terminating switch. If a call overflows, a via switch (a tandem exchange between the originating switch and the terminating switch) would send a crankback signal to the originating switch. This switch would then select the next route, and so on, until there are no alternative routes available in which the call is blocked. Note that the concept of crankback found its way into GMPLS as [RFC4920].¶

A.2. Evolution of Traffic Engineering in Packet Networks

This subsection reviews related prior work that was intended to improve the performance of data networks. Indeed, optimization of the performance of data networks started in the early days of the ARPANET. Other early commercial networks such as SNA also recognized the importance of performance optimization and service differentiation.¶

In terms of traffic management, the Internet has been a best effort service environment until recently. In particular, very limited traffic management capabilities existed in IP networks to provide differentiated queue management and scheduling services to packets belonging to different classes.¶

In terms of routing control, the Internet has employed distributed protocols for intra-domain routing. These protocols are highly scalable and resilient. However, they are based on simple algorithms for path selection which have very limited functionality to allow flexible control of the path selection process.¶

In the following subsections, the evolution of practical traffic engineering mechanisms in IP networks and its predecessors are reviewed.¶

A.3. Development of Internet Traffic Engineering

B.1. Overview of ITU Activities Related to Traffic Engineering

This section provides an overview of prior work within the ITU-T pertaining to traffic engineering in traditional telecommunications networks.¶

ITU-T Recommendations E.600 [ITU-E600], E.701 [ITU-E701], and E.801 [ITU-E801] address TE issues in traditional telecommunications networks. Recommendation E.600 provides a vocabulary for describing TE concepts, while E.701 defines reference connections, Grade of Service (GoS), and traffic parameters for ISDN. Recommendation E.701 uses the concept of a reference connection to identify representative cases of different types of connections without describing the specifics of their actual realizations by different physical means. As defined in Recommendation E.600, "a connection is an association of resources providing means for communication between two or more devices in, or attached to, a telecommunication network." Also, E.600 defines "a resource as any set of physically or conceptually identifiable entities within a telecommunication network, the use of which can be unambiguously determined" [ITU-E600]. There can be different types of connections as the number and types of resources in a connection may vary.¶

Typically, different network segments are involved in the path of a connection. For example, a connection may be local, national, or international. The purposes of reference connections are to clarify and specify traffic performance issues at various interfaces between different network domains. Each domain may consist of one or more service provider networks.¶

Reference connections provide a basis to define grade of service (GoS) parameters related to TE within the ITU-T framework. As defined in E.600, "GoS refers to a number of traffic engineering variables which are used to provide a measure of the adequacy of a group of resources under specified conditions." These GoS variables may be probability of loss, dial tone, delay, etc. They are essential for network internal design and operation as well as for component performance specification.¶

GoS is different from quality of service (QoS) in the ITU framework. QoS is the performance perceivable by a telecommunication service user and expresses the user's degree of satisfaction of the service. QoS parameters focus on performance aspects observable at the service access points and network interfaces, rather than their causes within the network. GoS, on the other hand, is a set of network oriented measures which characterize the adequacy of a group of resources under specified conditions. For a network to be effective in serving its users, the values of both GoS and QoS parameters must be related, with GoS parameters typically making a major contribution to the QoS.¶

Recommendation E.600 stipulates that a set of GoS parameters must be selected and defined on an end-to-end basis for each major service category provided by a network to assist the network provider with improving efficiency and effectiveness of the network. Based on a selected set of reference connections, suitable target values are assigned to the selected GoS parameters under normal and high load conditions. These end-to-end GoS target values are then apportioned to individual resource components of the reference connections for dimensioning purposes.¶

C.1. RFC 3272

1.0 Introduction:

Edited in place in Section 1.¶

1.1 What is Internet Traffic Engineering?:

Edited in place in Section 1.1.¶

1.2 Scope:

Moved to Section 1.3.¶

1.3 Terminology:

Moved to Section 1.4 with some obsolete terms removed and a little editing.¶

2.0 Background:

Retained as Section 2 with some text removed.¶

2.1 Context of Internet Traffic Engineering:

Retained as Section 2.1.¶

2.2 Network Context:

Rewritten as Section 2.2.¶

2.3 Problem Context:

Rewritten as Section 2.3.¶

2.3.1 Congestion and its Ramifications:

Retained as Section 2.3.1.¶

2.4 Solution Context:

Edited as Section 2.4.¶

2.4.1 Combating the Congestion Problem:

Reformatted as Section 2.4.1.¶

2.5 Implementation and Operational Context:

Retained as Section 2.5.¶

3.0 Traffic Engineering Process Model:

Retained as Section 3.¶

3.1 Components of the Traffic Engineering Process Model:

Retained as Section 3.1.¶

3.2 Measurement:

Merged into Section 3.1.¶

3.3 Modeling, Analysis, and Simulation:

Merged into Section 3.1.¶

3.4 Optimization:

Merged into Section 3.1.¶

4.0 Historical Review and Recent Developments:

Retained as Section 5, but the very historic aspects moved to Appendix A.¶

4.1 Traffic Engineering in Classical Telephone Networks:

Moved to Appendix A.1.¶

4.2 Evolution of Traffic Engineering in the Internet:

Moved to Appendix A.2.¶

4.2.1 Adaptive Routing in ARPANET:

Moved to Appendix A.2.1.¶

4.2.2 Dynamic Routing in the Internet:

Moved to Appendix A.2.2.¶

4.2.3 ToS Routing:

Moved to Appendix A.2.3.¶

4.2.4 Equal Cost Multi-Path:

Moved to Appendix A.2.4.¶

4.2.5 Nimrod:

Moved to Appendix A.2.5.¶

4.3 Overlay Model:

Moved to Appendix A.3.1.¶

4.4 Constraint-Based Routing:

Retained as Section 5.1.3.1, but moved into Section 5.1.¶

4.5 Overview of Other IETF Projects Related to Traffic Engineering:

Retained as Section 5.1 with many new subsections.¶

4.5.1 Integrated Services:

Retained as Section 5.1.1.1.¶

4.5.2 RSVP:

Retained as Section 5.1.3.2 with some edits.¶

4.5.3 Differentiated Services:

Retained as Section 5.1.1.2.¶

4.5.4 MPLS:

Retained as Section 5.1.3.3.¶

4.5.5 IP Performance Metrics:

Retained as Section 5.1.3.6.¶

4.5.6 Flow Measurement:

Retained as Section 5.1.3.7 with some reformatting.¶

4.5.7 Endpoint Congestion Management:

Retained as Section 5.1.3.8.¶

4.6 Overview of ITU Activities Related to Traffic Engineering:

Moved to Appendix B.1.¶

4.7 Content Distribution:

Retained as Section 5.2.¶

5.0 Taxonomy of Traffic Engineering Systems:

Retained as Section 4.¶

5.1 Time-Dependent Versus State-Dependent:

Retained as Section 4.1.¶

5.2 Offline Versus Online:

Retained as Section 4.2.¶

5.3 Centralized Versus Distributed:

Retained as Section 4.3 with additions.¶

5.4 Local Versus Global:

Retained as Section 4.4.¶

5.5 Prescriptive Versus Descriptive:

Retained as Section 4.5 with additions.¶

5.6 Open-Loop Versus Closed-Loop:

Retained as Section 4.6.¶

5.7 Tactical vs Strategic:

Retained as Section 4.7.¶

6.0 Recommendations for Internet Traffic Engineering:

Retained as Section 6.¶

6.1 Generic Non-functional Recommendations:

Retained as Section 6.1.¶

6.2 Routing Recommendations:

Retained as Section 6.2 with edits.¶

6.3 Traffic Mapping Recommendations:

Retained as Section 6.3.¶

6.4 Measurement Recommendations:

Retained as Section 6.4.¶

6.5 Network Survivability:

Retained as Section 6.6.¶

6.5.1 Survivability in MPLS Based Networks:

Retained as Section 6.6.1.¶

6.5.2 Protection Option:

Retained as Section 6.6.2.¶

6.6 Traffic Engineering in Diffserv Environments:

Retained as Section 6.8 with edits.¶

6.7 Network Controllability:

Retained as Section 6.9.¶

7.0 Inter-Domain Considerations:

Retained as Section 7.¶

8.0 Overview of Contemporary TE Practices in Operational IP Networks:

Retained as Section 8.¶

9.0 Conclusion:

Removed.¶

10.0 Security Considerations:

Retained as Section 9 with considerable new text.¶

C.2. This Document

• Section 1: Based on Section 1 of RFC 3272.¶

  • Section 1.1: Based on Section 1.1 of RFC 3272.¶
  • Section 1.2: New for this document.¶
  • Section 1.3: Based on Section 1.2 of RFC 3272.¶
  • Section 1.4: Based on Section 1.3 of RFC 3272.¶

• Section 2: Based on Section 2. of RFC 3272.¶

  • Section 2.1: Based on Section 2.1 of RFC 3272.¶
  • Section 2.2: Based on Section 2.2 of RFC 3272.¶

  • Section 2.3: Based on Section 2.3 of RFC 3272.¶

    • Section 2.3.1: Based on Section 2.3.1 of RFC 3272.¶

  • Section 2.4: Based on Section 2.4 of RFC 3272.¶

    • Section 2.4.1: Based on Section 2.4.1 of RFC 327¶
  • Section 2.5: Based on Section 2.5 of RFC 3272.¶

• Section 3: Based on Section 3 of RFC 3272.¶

  • Section 3.1: Based on Sections 3.1, 3.2, 3.3, and 3.4 of RFC 3272.¶

• Section 4: Based on Section 5 of RFC 3272.¶

  • Section 4.1: Based on Section 5.1 of RFC 3272.¶
  • Section 4.2: Based on Section 5.2 of RFC 3272.¶

  • Section 4.3: Based on Section 5.3 of RFC 3272.¶

    • Section 4.3.1: New for this document.¶
    • Section 4.3.2: New for this document.¶
  • Section 4.4: Based on Section 5.4 of RFC 3272.¶

  • Section 4.5: Based on Section 5.5 of RFC 3272.¶

    • Section 4.5.1: New for this document.¶
  • Section 4.6: Based on Section 5.6 of RFC 3272.¶
  • Section 4.7: Based on Section 5.7 of RFC 3272.¶

• Section 5: Based on Section 4 of RFC 3272.¶

  • Section 5.1: Based on Section 4.5 of RFC 3272.¶

    • Section 5.1.1.1: Based on Section 4.5.1 of RFC 3272.¶
    • Section 5.1.1.2: Based on Section 4.5.3 of RFC 3272.¶
    • Section 5.1.1.3: New for this document.¶
    • Section 5.1.1.4: New for this document.¶
    • Section 5.1.1.5: New for this document.¶
    • Section 5.1.2.1: New for this document.¶
    • Section 5.1.2.2: New for this document.¶
    • Section 5.1.2.3: New for this document.¶

    • Section 5.1.3.1: Based on Section 4.4 of RFC 3272.¶

      • Section 5.1.3.1.1: New for this document.¶
    • Section 5.1.3.2: Based on Section 4.5.2 of RFC 3272.¶
    • Section 5.1.3.3: Based on Section 4.5.4 of RFC 3272.¶
    • Section 5.1.3.5: New for this document.¶
    • Section 5.1.3.6: Based on Section 4.5.5 of RFC 3272.¶
    • Section 5.1.3.7: Based on Section 4.5.6 of RFC 3272.¶
    • Section 5.1.3.8: Based on Section 4.5.7 of RFC 3272.¶
    • Section 5.1.3.9: New for this document.¶
    • Section 5.1.3.10: New for this document.¶
    • Section 5.1.3.11: New for this document.¶
    • Section 5.1.3.12: New for this document.¶
    • Section 5.1.3.14: New for this document.¶
    • Section 5.1.3.15: New for this document.¶
  • Section 5.2: Based on Section 4.7 of RFC 3272.¶

• Section 6: Based on Section 6 of RFC 3272.¶

  • Section 6.1: Based on Section 6.1 of RFC 3272.¶
  • Section 6.2: Based on Section 6.2 of RFC 3272.¶
  • Section 6.3: Based on Section 6.3 of RFC 3272.¶
  • Section 6.4: Based on Section 6.4 of RFC 3272.¶
  • Section 6.5: New for this document.¶

  • Section 6.6: Based on Section 6.5 of RFC 3272.¶

    • Section 6.6.1: Based on Section 6.5.1 of RFC 3272.¶
    • Section 6.6.2: Based on Section 6.5.2 of RFC 3272.¶
  • Section 6.7: New for this document.¶
  • Section 6.8: Based on Section 6.6. of RFC 3272.¶
  • Section 6.9: Based on Section 6.7 of RFC 3272.¶
• Section 7: Based on Section 7 of RFC 3272.¶
• Section 8: Based on Section 8 of RFC 3272.¶
• Section 9: Based on Section 10 of RFC 3272.¶

• Appendix A: New for this document.¶

  • Appendix A.1: Based on Section 4.1 of RFC 3272.¶

  • Appendix A.2: Based on Section 4.2 of RFC 3272.¶

    • Appendix A.2.1: Based on Section 4.2.1 of RFC 3272.¶
    • Appendix A.2.2: Based on Section 4.2.2 of RFC 3272.¶
    • Appendix A.2.3: Based on Section 4.2.3 of RFC 3272.¶
    • Appendix A.2.4: Based on Section 4.2.4 of RFC 3272.¶
    • Appendix A.2.5: Based on Section 4.2.5 of RFC 3272.¶

  • Appendix A.3: New for this document.¶

    • Appendix A.3.1: Based on Section 4.3 of RFC 3272.¶

• Appendix B: New for this document.¶

  • Appendix B.1: Based on Section 4.7 of RFC 3272.¶