wdiff rfc9522.original rfc9522.txt

TEAS Working Group

Internet Engineering Task Force (IETF) A. Farrel, Ed.
Internet-Draft
Request for Comments: 9522 Old Dog Consulting
Obsoletes: 3272 (if approved) 12 August 2023
Intended status: Informational
Expires: 13 February January 2024
Category: Informational
ISSN: 2070-1721

Overview and Principles of Internet Traffic Engineering
draft-ietf-teas-rfc3272bis-27

Abstract

This document 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 the networks that support IP networking, networking and to provide a
common basis for the development of traffic engineering traffic-engineering capabilities
for the Internet. The principles, architectures, and methodologies
for performance evaluation and performance optimization of
operational networks are also discussed.

This work was first published as RFC 3272 in May 2002. This document
obsoletes RFC 3272 by making a complete update to bring the text in
line with best current practices for Internet traffic engineering and
to include references to the latest relevant work in the IETF.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents not an Internet Standards Track specification; it is
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Standard; see Section 2 of RFC 7841.

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This Internet-Draft will expire on 13 February 2024.
https://www.rfc-editor.org/info/rfc9522.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. What is Internet Traffic Engineering? . . . . . . . . . . 4
1.2. Components of Traffic Engineering . . . . . . . . . . . . 6
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1. Context of Internet Traffic Engineering . . . . . . . . . 12
2.2. Network Domain Context . . . . . . . . . . . . . . . . . 13
2.3. Problem Context . . . . . . . . . . . . . . . . . . . . . 15
2.3.1. Congestion and its Its Ramifications . . . . . . . . . . 16
2.4. Solution Context . . . . . . . . . . . . . . . . . . . . 17
2.4.1. Combating the Congestion Problem . . . . . . . . . . 19
2.5. Implementation and Operational Context . . . . . . . . . 22
3. Traffic Engineering Traffic-Engineering Process Models . . . . . . . . . . . . . 22
3.1. Components of the Traffic Engineering Traffic-Engineering Process Model . . . 23
4. Taxonomy of Traffic Engineering Traffic-Engineering Systems . . . . . . . . . . . 23
4.1. Time-Dependent Versus versus State-Dependent Versus versus
Event-Dependent . . . . . . . . . . . . . . . . . . . . . 24
4.2. Offline Versus versus Online . . . . . . . . . . . . . . . . . . 25
4.3. Centralized Versus versus Distributed . . . . . . . . . . . . . 26
4.3.1. Hybrid Systems . . . . . . . . . . . . . . . . . . . 26
4.3.2. Considerations for Software Defined Software-Defined Networking . . . 27
4.4. Local Versus versus Global . . . . . . . . . . . . . . . . . . . 27
4.5. Prescriptive Versus versus Descriptive . . . . . . . . . . . . . 28
4.5.1. Intent-Based Networking . . . . . . . . . . . . . . . 28
4.6. Open-Loop Versus versus Closed-Loop . . . . . . . . . . . . . . 29
4.7. Tactical versus Strategic . . . . . . . . . . . . . . . . 29
5. Review of TE Techniques . . . . . . . . . . . . . . . . . . . 29
5.1. Overview of IETF Projects Related to Traffic Engineering . . . . . . . . . . . . . . . . . . . . . . . 30
5.1.1. IETF TE Mechanisms . . . . . . . . . . . . . . . . . 30
5.1.2. IETF Approaches Relying on TE Mechanisms . . . . . . 35
5.1.3. IETF Techniques Used by TE Mechanisms . . . . . . . . 37
5.2. Content Distribution . . . . . . . . . . . . . . . . . . 49
6. Recommendations for Internet Traffic Engineering . . . . . . 49
6.1. Generic Non-functional Recommendations . . . . . . . . . 50
6.2. Routing Recommendations . . . . . . . . . . . . . . . . . 52
6.3. Traffic Mapping Recommendations . . . . . . . . . . . . . 54
6.4. Measurement Recommendations . . . . . . . . . . . . . . . 55
6.5. Policing, Planning, and Access Control . . . . . . . . . 56
6.6. Network Survivability . . . . . . . . . . . . . . . . . . 57
6.6.1. Survivability in MPLS Based MPLS-Based Networks . . . . . . . . 59
6.6.2. Protection Options . . . . . . . . . . . . . . . . . 60
6.7. Multi-Layer Traffic Engineering . . . . . . . . . . . . . 61
6.8. Traffic Engineering in Diffserv Environments . . . . . . 61
6.9. Network Controllability . . . . . . . . . . . . . . . . . 63
7. Inter-Domain Considerations . . . . . . . . . . . . . . . . . 64
8. Overview of Contemporary TE Practices in Operational IP
Networks . . . . . . . . . . . . . . . . . . . . . . . . 65
9. Security Considerations . . . . . . . . . . . . . . . . . . . 68
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 69
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 69
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 71
13. Informative References . . . . . . . . . . . . . . . . . . . 72
Appendix A. Summary of Changes Since since RFC 3272 . . . . . . . . . 89
A.1. RFC 3272 . . . . . . . . . . . . . . . . . . . . . . . . 89
A.2. This Document . . . . . . . . . . . . . . . . . . . . . . 92
Acknowledgments
Contributors
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 95

1. Introduction

This document describes the principles of Internet traffic
engineering (TE). The objective of the document is to articulate the
general issues and principles for Internet TE, and TE and, where appropriate appropriate,
to provide recommendations, guidelines, and options for the
development of preplanned (offline) and dynamic (online) Internet TE
capabilities and support systems.

Even though Internet TE is most effective when applied end-to-end,
the focus of this document is TE within a given domain (such as an
autonomous system).
Autonomous System (AS)). However, because a preponderance of
Internet traffic tends to originate in one autonomous system AS and terminate in
another, this document also provides an overview of aspects
pertaining to inter-domain TE.

This document provides a terminology and a taxonomy for describing and
understanding common Internet TE concepts.

This work was first published as [RFC3272] in May 2002. This
document obsoletes [RFC3272] by making a complete update to bring the
text in line with best current practices for Internet TE and to
include references to the latest relevant work in the IETF. It is
worth noting around three fifths three-fifths of the RFCs referenced in this
document post-date postdate the publication of RFC 3272. [RFC3272]. Appendix A provides
a summary of changes between RFC 3272 [RFC3272] and this document.

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], [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 traffic-oriented
performance requirements while utilizing network resources without
excessive waste and in a reliable way. Traffic oriented 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 network 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 Packet-level processing
functions operate at very fine levels of temporal resolution (up to
milliseconds) while reacting to statistical measures of the real-time
behavior of traffic.

Thus, the optimization aspects of TE can be viewed from a control
perspective,
perspective and can be both proactive and reactive. In the proactive
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 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 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, reoptimization,
and aid in the prediction of potential future problems. However,
this process can also be time consuming 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,
conditions and optimizes the use of resources to improve
performance.

* A form of a pre-planned traffic distribution that is considered
optimal.

In the latter 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 in such a mode:

* The TE-planning task to enable optimum traffic distribution, and the distribution.

* 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, 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
"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, policies; see [RFC4271] and [RFC8955]. In the TE context,
policy decisions are made within the control plane or by controllers
in the management plane, plane and govern the selection of paths. Examples
can be found in [RFC4655] and [RFC5394]. TE solutions may cover the
mechanisms to distribute and/or enforce policies, but definition of
specific policies is left to the network operator.

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 (SR) [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 coarse or very fine level. Note that this consensus exists at
the network control or controller level, level but not within the data
plane. It may be composed purely of accounting/
bookkeeping, 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, requirements and
the use of dynamic mechanisms to collect requirements (e.g., via
RSVP-TE [RFC3209]) and resource availability (e.g., via OSPF
extensions for GMPLS [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
Resource allocation 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 GMPLS-based Label Switched
Paths (LSPs) [RFC3945], as well as controller-based solutions
[RFC8453]. This level of resource control, while optional, is
important in networks that wish to support network congestion
management policies to control or regulate the offered traffic to
deliver different levels of service and alleviate network congestion problems, or those
problems. It is also important in networks that wish to control the
latency experienced by specific traffic flows.

1.3. Scope

The scope of this document is intra-domain TE because this is the
practical level of TE technology that exists in the Internet at the
time of writing. That is, it this document describes TE within a given autonomous
system
AS 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 are
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 world-wide
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 that is useful for Internet TE.
The definitions presented apply to this document. These terms may
have other meanings elsewhere.

Busy hour: A one hour 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. A small amount of congestion may be beneficial
to ensure that network resources are run at full capacity, and
this may be particularly true at the network edge where it is
desirable to ensure that user traffic is served as much as
possible. Within the network, if congestion is allowed to build
(such as when input traffic exceeds output traffic in a sustained
way)
way), it will have a negative effect on user traffic.

Congestion avoidance: An approach to congestion management that
attempts to obviate the occurrence of congestion. Chiefly It is chiefly
relevant to network congestion congestion, although it may form a part of demand-
side
demand-side congestion management.

Congestion response: 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 takes
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
the offered load.

Effective bandwidth: The minimum amount of bandwidth that can be
assigned to a flow or traffic aggregate in order to deliver
'acceptable
"acceptable service quality' quality" to the flow or traffic aggregate.
See [KELLY] for a more mathematical definition.

Egress node: The device (router) at which traffic leaves a network
toward a destination (host, server, etc.) or to another network.

End-to-end: This term is context-dependent and often applies to the
life of a traffic flow from original source to final destination.
In contrast, edge-to-edge is often used to describe the traffic
flow from the entry to of a domain or network, network to the exit from of that
domain or network. In However, in some contexts, however, for example contexts (for example, where
there is a service interface between a network and the client of
that network, network or where a path traverses multiple domains under the
control of a single process, process), end-to-end is used to refer to the
full operation of the service that may be composed of concatenated
edge-to-edge operations. Thus, in the context of TE, the term
end-to-end
"end-to-end" may refer to the full TE path, path but not to the complete
path of the traffic from source application to ultimate
destination.

Hot-spot:

Hotspot: A network element or subsystem which that is in a considerably
higher state of congestion than others.

Ingress node: The device (router) at which traffic enters a network
from a source (host) or from another network.

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 congestion: Congestion within the network at a specific node
or a specific link that is sufficiently extreme that it results in
unacceptable queuing delay or packet loss. Network congestion can
negatively impact end-to-end or edge-to-edge traffic flows, so TE
schemes may be deployed to balance traffic in the network and
deliver congestion avoidance.

Network survivability: The capability to provide a prescribed level
of QoS for existing services after a given number of failures
occur within the network.

Offered load: The offered Offered load or offered is also sometimes called "offered traffic load
load". It is a measure of the amount of traffic being presented
to be carried across a network compared to the capacity of the
network to carry it. This term derives from queuing theory theory, and
an offered load of 1 indicates that the network can carry, but
only just manage to carry, all of the traffic presented to it.

Offline traffic engineering: A traffic engineering system that
exists outside of the network.

Online traffic engineering: A traffic engineering 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.

Quality of Service (QoS): QoS ([RFC3198]) [RFC3198] refers to the mechanisms
used within a network to achieve specific goals for the delivery
of traffic for a particular service according to the parameters
specified in a Service Level Agreement. "Quality" is
characterized by service availability, delay, jitter, throughput throughput,
and packet loss ratio. At a network resource level, "Quality of
Service" refers to a set of capabilities that allow a service
provider to prioritize traffic, control bandwidth, and network
latency.

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 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

Traffic-engineering system: A collection of objects, mechanisms, and
protocols that are used together to accomplish traffic engineering traffic-engineering
objectives.

Traffic flow: A stream of packets between two end-points endpoints that can be
characterized in a certain way. A common classification for a
traffic flow selects packets with the "five-tuple" five-tuple of source and
destination addresses, source and destination ports, and protocol
ID. Flows may be very small and transient, ranging to very large.
The TE techniques described in this document are likely to be more
effective when applied to large flows. Traffic flows may be
aggregated and treated as a single unit in some forms of TE TE,
making it possible to apply TE to the smaller flows that comprise
the aggregate.

Traffic mapping: Traffic mapping is the assignment of traffic
workload onto (pre- established) (pre-established) paths to meet certain
requirements.

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

Workload: The workload or traffic workload Workload is also sometimes called "traffic workload". It
is an evaluation of the amount of work that must be done in a
network in order to facilitate the traffic demand. Colloquially,
it is the answer to, "How busy is the network?"

2. Background

The Internet aims to convey IP packets from ingress nodes to egress
nodes efficiently, expeditiously, and economically. Furthermore, in
a multiclass multi-class service environment (e.g., Diffserv capable networks - networks;
see Section 5.1.1.2), the resource sharing 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
the network. Thus, consideration must be given to resolving
competition for network resources between traffic flows belonging to
the same service class (intra-class contention resolution) and
traffic flows belonging to different classes (inter-class contention
resolution).

2.1. Context of Internet Traffic Engineering

The context of Internet traffic engineering includes the following
sub-contexts:

1. A network domain context that defines the scope under
consideration, and
consideration and, in particular particular, the situations in which the TE
problems occur. The network domain context includes network
structure, policies, characteristics, constraints, quality
attributes, and 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, 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 that 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 that facilitate the movement of traffic through
the network (see also [AWD2]). [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. 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 They provide real-time services services.

* their 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 that demands the constant inclusion of new
technologies and new network elements, and to the transient and
persistent faults which that 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
exceeds the output capacity of the resource. Network congestion may
result in some of the arriving packets being delayed or even dropped.

Network congestion increases transit delay and delay variation, may
lead to packet loss, and reduces the predictability of network
services. Clearly, while congestion may be a useful tool at ingress
edge nodes, network congestion is highly undesirable. Combating
network congestion at a reasonable cost is a major objective of
Internet TE TE, although it may need to be traded with other objectives
to keep the costs reasonable.

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 the architecture for Differentiated Services (Diffserv)
[RFC2475]. 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 constraints, such as packet loss

- temporal constraints 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). variation)

2.3. Problem Context

There are several problems associated with operating a network like
those 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? space

* How to specify the desirable features of solutions? solutions

* How to actually solve the problems? problems

* How to measure and characterize the effectiveness of solutions? 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. Offline planning requires a
full view of the topology of the network or partial network that is
being planned.

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; network, and 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.

2.3.1. Congestion and its Its Ramifications

Network 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. Although
congestion at the edge of the network may be beneficial in ensuring
that the network delivers as much traffic as possible, network
congestion almost always results in degradation of service quality to
end users. Congestion avoidance and response schemes can include
demand-side policies and supply-side policies. Demand-side policies
may restrict access to congested resources 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 reallocate network resources
by redistributing traffic over the infrastructure. Traffic
redistribution and resource re-allocation reallocation serve to increase the
'effective capacity'
effective capacity of the network.

The emphasis of this document 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. end systems. That is, the aspects that are
considered in this document 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 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, 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 that may be implicitly or explicitly formulated, and
possibly instantiating instantiation of 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. 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 and, in some cases cases, possibly offline tools
and mechanisms for measurement, characterization, modeling, and
control of 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 that may be manipulated
through a configuration management system. Such a system may, itself, may
itself 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 of 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 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,
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 such as the link state link-state database of an interior
gateway protocol Interior
Gateway Protocol (IGP)

* routing tables

* automated tools that discover and collate network topology
information.
information

Topology information may also be derived from servers that monitor
network state, state and from servers that perform provisioning functions.

Routing in operational IP networks can be administratively controlled
at various levels of abstraction abstraction, including the manipulation of BGP
attributes and 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) LSP can be computed and established in various ways
ways, including:

* manually

* automatically, automatically and online using constraint-based routing processes
implemented on label switching routers Label Switching Routers (LSRs)

* automatically, automatically and offline using constraint-based routing entities
implemented on external TE support systems. 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.

Congestion management policies can be categorized based upon the
following criteria (see [YARE95] for a more detailed taxonomy of
congestion control schemes):

1. Congestion Management Based on Response Timescales

* Long (weeks to months): Expanding network capacity by adding
new equipment, routers, and links takes time and is
comparatively costly. Capacity planning needs to take this
into consideration. Network capacity is expanded based on
estimates or forecasts of future traffic development and
traffic distribution. These upgrades are typically carried
out over weeks or weeks, months, or maybe even years.

* Medium (minutes to days): Several control policies fall within
the medium timescale category. Examples include:

a. Adjusting routing protocol parameters to route traffic
away from or towards certain segments of the network.

b. Setting up or adjusting explicitly routed LSPs in MPLS
networks to route traffic trunks away from possibly
congested resources or toward possibly more favorable
routes.

c. Re-configuring Reconfiguring the logical topology of the network to make
it correlate more closely with the spatial traffic
distribution using, for example, an underlying path-
oriented technology such as MPLS LSPs or optical channel
trails.

When these schemes are adaptive, they rely on measurement
systems. A measurement system monitors changes in traffic
distribution, traffic loads, and network resource utilization
and then provides feedback to the online or offline TE
mechanisms and tools so that they can trigger control actions
within the network. The TE mechanisms and tools can be
implemented in a distributed or centralized fashion. A
centralized scheme may have full visibility into the network
state and may produce 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
might not reflect the actual state of the network. It is not
an objective of this document to make a recommendation between
distributed and centralized schemes: schemes; that is a choice that
network administrators must make based on their specific
needs.

* Short (minutes or less): This category includes packet level packet-level
processing functions and events that are recorded on the order
of several round-trip times. It also includes router
mechanisms such as passive and active buffer management. All
of these mechanisms are used to control congestion or signal
congestion to end systems so that they can adaptively regulate
the rate at which traffic is injected into the network. A
well-known active queue management scheme, especially for
responsive traffic such as TCP, is Random Early Detection
(RED) [FLJA93]. During congestion (but before the queue is
filled), the RED scheme chooses arriving packets to "mark"
according to a probabilistic algorithm which that takes into account
the average queue size. A router that does not utilize explicit congestion notification
Explicit Congestion Notification (ECN) [RFC3168] can simply
drop marked packets to alleviate congestion and implicitly
notify the receiver about the congestion. On the other hand,
if the router and the end hosts support ECN, they can set the
ECN field in the packet header, and the end host can act on
this information. Several variations of RED have been
proposed to support different drop precedence levels in
multi-class multi-
class environments [RFC2597]. RED provides congestion
avoidance which that is better than or equivalent to traditional Tail-Drop (TD)
queue management (drop arriving packets only when the queue is
full). Importantly, RED reduces the possibility of
retransmission bursts becoming synchronized within the network, network
and improves fairness among different responsive traffic
sessions. However, RED by itself cannot prevent congestion
and unfairness caused by sources unresponsive to RED, e.g.,
some misbehaved greedy connections. Other schemes have been
proposed to improve performance and fairness in the presence
of unresponsive traffic. Some of those schemes (such as
Longest Queue Drop (LQD) and Dynamic Soft Partitioning with
Random Drop (RND) [SLDC98]) were proposed as theoretical
frameworks and are typically not available in existing
commercial products, while others (such as Approximate Fairness Through Differential Fair
Dropping (AFD)
[AFD03] [AFD03]) have seen some implementation. Advice
on the use of Active Queue Management (AQM) schemes is
provided in [RFC7567]. [RFC7567] recommends self-tuning AQM
algorithms like those that the IETF has published in
[RFC8290], [RFC8033], [RFC8034], and [RFC9332], but RED is
still appropriate for links with stable bandwidth, if
configured carefully.

2. Reactive Versus versus Preventive Congestion Management Schemes

* Reactive (recovery) congestion management policies react to
existing congestion problems. All the policies described
above for the short and medium timescales can be categorized
as being reactive. They are based on monitoring and
identifying congestion problems that exist in the network, network and
on the initiation of relevant actions to ease a situation.
Reactive congestion management schemes may also be preventive.

* Preventive (predictive/avoidance) policies take proactive
action to prevent congestion based on estimates and
predictions of future congestion problems (e.g., traffic
matrix forecasts). Some of the policies described for the
long and medium timescales fall into this category.
Preventive policies 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 future congestion problems.
The schemes described for the short timescale can also be used
for congestion avoidance because dropping or marking packets
before queues actually overflow would trigger corresponding
responsive traffic sources to slow down. Preventive
congestion management schemes may also be reactive.

3. Supply-Side Versus versus Demand-Side Congestion Management Schemes

* Supply-side congestion management policies increase the
effective capacity available to traffic in order to control or
reduce congestion. This can be accomplished by increasing
capacity or by balancing distribution of traffic over the
network. Capacity planning aims to provide a physical
topology and associated link bandwidths that match or exceed
estimated traffic workload and traffic distribution distribution, subject
to traffic forecasts and budgetary or other (or other) constraints. If
the actual traffic distribution does not fit the topology
derived from capacity planning, then the traffic can be mapped
onto the topology by using routing control mechanisms, by
applying
path oriented path-oriented technologies (e.g., MPLS LSPs and
optical channel trails) to modify the logical topology, topology or by
employing some other load redistribution mechanisms.

* Demand-side congestion management policies control or regulate
the offered traffic to alleviate congestion problems. For
example, some of the short timescale mechanisms described
earlier as well as policing and rate-shaping mechanisms
attempt to regulate the offered load in various ways.

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. Traffic Engineering Traffic-Engineering Process Models

This section describes a generic process model that captures the
high-level practical aspects of Internet traffic engineering in an
operational context. The process model is described as a sequence of
actions that must be carried out to optimize the performance of an
operational network (see also [RFC2702], [RFC2702] and [AWD2]). This process
model may be enacted explicitly or implicitly, by a software process
or by a human.

The TE process model is iterative [AWD2]. The four phases of the
process model described below are repeated as a continual sequence.

* sequence:

1. Define the relevant control policies that govern the operation of
the network.

2. Acquire measurement data from the operational network.

3. Analyze the network state and characterize the traffic workload.
Proactive analysis identifies potential problems that could
manifest in the future. Reactive analysis identifies existing
problems and determines their causes.

4. Optimize the performance of the network. This involves a
decision process which that selects and implements a set of actions
from a set of alternatives given the results of the three
previous steps. Optimization actions may include the use of
techniques to control the offered traffic and to control the
distribution of traffic across the network.

3.1. Components of the Traffic Engineering Traffic-Engineering Process Model

The key components of the traffic engineering traffic-engineering process model are as
follows.
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 that 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 packet-level
characteristics, flow level flow-level characteristics, user user- or customer customer-
level characteristics, traffic aggregate traffic-aggregate characteristics,
component level
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 that depicts relevant traffic
characteristics and network attributes. A network model is an
abstract representation of the network which that captures relevant
network features, attributes, and characteristics. 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. Taxonomy of Traffic Engineering Traffic-Engineering Systems

This section presents a short taxonomy of traffic engineering traffic-engineering systems
constructed based on TE styles and views as listed below and
described in greater detail in the following subsections of this
document.
document:

* Time-dependent Time-Dependent versus State-dependent State-Dependent versus Event-dependent Event-Dependent

* Offline versus Online

* Centralized versus Distributed

* Local versus Global Information

* Prescriptive versus Descriptive

* Open Loop Open-Loop versus Closed Loop Closed-Loop

* Tactical versus Strategic

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

Traffic engineering

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, 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 timescale (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 centralized optimizer
where the input to the system is a traffic matrix and multi-class QoS
requirements as described in [MR99]. Another example of such a
methodology is the application of data mining to Internet traffic
[AJ19]
[AJ19], which enables the use of various machine learning algorithms
to identify patterns within historically collected datasets about
Internet traffic, traffic and to extract information in order to guide
decision-making,
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 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 timescale. An example of 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" 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
needed to increase network efficiency and to provide resilience to
adapt to changes in 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 they
typically use learning models to find good paths for TE in a network.
While state-dependent TE models typically use available-link-
bandwidth (ALB) [E.360.1] flooding [E.360.1] 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 [RFC6601]. ALB
flooding can be resource intensive, since it requires link bandwidth
to carry routing protocol link state advertisements, link-state advertisements and processor
capacity to process those advertisements, and advertisements; in addition, the overhead
of the ALB advertisements and their processing can limit area/Autonomous System
(AS) size. the size of
the area and AS. 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]. [TE-QoS-ROUTING].

A fully functional TE system is likely to use all aspects of time-
dependent, state-dependent, and event-dependent methodologies as
described in Section 4.3.1.

4.2. Offline Versus 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 relatively simple and fast calculations
to select routes, fine-tune the allocations of resources, and perform
load balancing.

4.3. Centralized Versus versus Distributed

Under centralized control control, there is a central authority which that
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, 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 link-state advertisements. Network state information may also
be disseminated under exception conditions. Examples of protocol
extensions used to advertise network link state link-state information are
defined in [RFC5305], [RFC6119], [RFC7471], [RFC8570], and [RFC8571].
See also Section 5.1.3.9.

4.3.1. Hybrid Systems

In practice, most TE systems will be a hybrid of central and
distributed control. For example, a popular MPLS approach to TE is
to use a central controller based on an active, stateful Path
Computation Element (PCE), (PCE) but to use routing and signaling protocols
to make local decisions at routers within the network. Local
decisions may be able to respond more quickly to network events, events but
may result in conflicts with decisions made by other routers.

Network operations for TE systems may also use a hybrid of offline
and online computation. TE paths may be precomputed based on stable-
state network information and planned traffic demands, demands but may then be
modified in the active network depending on variations in network
state and traffic load. Furthermore, responses to network events may
be precomputed offline to allow rapid reactions without further
computation,
computation or may be derived online depending on the nature of the
events.

Lastly, note that a fully functional TE system is likely to use all
aspects of time-dependent, state-dependent, and event-dependent
methodologies as described in Section 4.1.

4.3.2. Considerations for Software Defined Software-Defined Networking

As discussed in Section 5.1.2.2, one of the main drivers for SDN
Software-Defined Networking (SDN) is a decoupling of the network
control plane from the data plane [RFC7149]. However, SDN may also
combine centralized control of
resources, resources and facilitate application-to-network application-
to-network interaction via an
application programming interface (API) Application Programming Interface
(API), such as the one described in [RFC8040]. Combining these
features provides a flexible network architecture that can adapt to
the network requirements of a variety of higher-layer applications, a
concept often referred to as the "programmable network" [RFC7426].

The centralized control aspect of SDN helps improve network resource
utilization compared with distributed network control, where local
policy may often override network-wide optimization goals. In an SDN
environment, the data plane forwards traffic to its desired
destination. However, before traffic reaches the data plane, the
logically centralized SDN control plane often determines the path the
application traffic will take in the network. Therefore, the SDN
control plane needs to be aware of the underlying network topology,
capabilities
capabilities, and current node and link resource state.

Using a PCE-based SDN control framework [RFC7491], the available
network topology may be discovered by running a passive instance of
OSPF or IS-IS, or via BGP-LS [RFC7752], BGP Link State (BGP-LS) [RFC9552]), to generate
a Traffic Engineering Database (TED, see (TED) (see Section 5.1.3.14). The PCE
is used to compute a path (see Section 5.1.3.11) based on the TED and
available bandwidth, and further path optimization may be based on
requested objective functions [RFC5541]. When a suitable path has
been
computed computed, the programming of the explicit network path may be
either performed using either a signaling protocol that traverses the length
of the path [RFC3209] or performed per-hop with each node being
directly programmed [RFC8283] by the SDN controller.

By utilizing a centralized approach to network control, additional
network benefits are also available, including Global Concurrent
Optimization (GCO) [RFC5557]. A GCO path computation request will
simultaneously use the network topology and a set of new path
signaling requests, along with their respective constraints, for
optimal placement in the network. Correspondingly, a GCO-based
computation may be applied to recompute existing network paths to
groom traffic and to mitigate congestion.

4.4. Local Versus versus Global

Traffic engineering

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, 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, 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 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.

4.5.1. Intent-Based Networking

One way to express a service request is through "intent". Intent-
Based Networking aims to produce networks that are simpler to manage
and operate, requiring only minimal intervention. Intent is defined
in [RFC9315] as a follows:

| A set of operational goals (that a network should meet) and
| outcomes (that a network is supposed to deliver), deliver) defined in a
| declarative manner without specifying how to achieve or implement
| them.

Intent provides data and functional abstraction so that users and
operators do not need to be concerned with low-level device
configuration or the mechanisms used to achieve a given intent. This
approach can be conceptually easier for a user, user but may be less
expressive in terms of constraints and guidelines.

Intent-Based Networking is applicable to TE because many of the high-
level objectives may be expressed as "intent." For intent (for example, load
balancing, delivery of services, and robustness against failures. failures).
The intent is converted by the management system into TE actions
within the network.

4.6. Open-Loop Versus versus Closed-Loop

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

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

4.7. Tactical versus Strategic

Tactical traffic engineering aims to address specific performance
problems (such as hot-spots) hotspots) 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 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. Review of TE Techniques

This section briefly reviews different TE-related approaches proposed
and implemented in telecommunications and computer networks using
IETF protocols and architectures. These approaches are organized
into three categories:

* TE mechanisms which that adhere to the definition provided in
Section 1.2. 1.2

* Approaches that rely upon those TE mechanisms. mechanisms

* Techniques that are used by those TE mechanisms and approaches. approaches

The discussion is not intended to be comprehensive. It is primarily
intended to illuminate existing approaches to TE in the Internet. A
historic overview of TE in telecommunications networks was provided
in Section 4 of [RFC3272], and Section 4.6 of that document presented
an outline of some early approaches to TE conducted in other
standards bodies. It is out of the scope of this document to provide
an analysis of the history of TE or an inventory of TE-related
efforts conducted by other SDOs. Standards Development Organizations
(SDOs).

5.1. Overview of IETF Projects Related to Traffic Engineering

This subsection reviews a number of IETF activities pertinent to
Internet traffic engineering. Some of these technologies are widely
deployed, others are mature but have seen less deployment, and some
are unproven or are still under development.

5.1.1. IETF TE Mechanisms

5.1.1.1. Integrated Services

The IETF developed the Integrated Services (Intserv) model that
requires resources, such as bandwidth and buffers, to be reserved a
priori for a given traffic flow to ensure that the quality of service QoS requested by
the traffic flow is satisfied. The Integrated Services Intserv model includes additional
components beyond those used in the best-
effort best-effort model such as packet
classifiers, packet schedulers, and admission control. A packet
classifier is used to identify flows that are to receive a certain
level of service. A packet scheduler handles the scheduling of
service to different packet flows to ensure that QoS commitments are
met. Admission control is used to determine whether a router has the
necessary resources to accept a new flow.

The main issue with the Integrated Services Intserv model has been scalability [RFC2998],
especially in large public IP networks which that may potentially have
millions of active traffic flows in transit concurrently. Pre-Congestion Pre-
Congestion Notification (PCN) [RFC5559] solves the scaling problems
of Intserv by using measurement-based admission control (and flow
termination to handle failures) between edge-nodes. edge nodes. Nodes between
the edges of the internetwork have no per-flow
operations operations, and the
edge nodes can use the Resource Reservation Protocol (RSVP) per-flow
or per-aggregate.

A notable feature of the Integrated Services Intserv model is that it requires explicit
signaling of QoS requirements from end systems to routers [RFC2753].
RSVP performs this signaling function and is a critical component of
the Integrated Services Intserv model. RSVP is described in Section 5.1.3.2.

5.1.1.2. Differentiated Services

The goal of Differentiated Services (Diffserv) within the IETF was to
devise scalable mechanisms for categorization of traffic into
behavior aggregates, which ultimately allows each behavior aggregate
to be treated differently, especially when there is a shortage of
resources
resources, such as link bandwidth and buffer space [RFC2475]. One of
the primary motivations for Diffserv was to devise alternative
mechanisms for service differentiation in the Internet that mitigate
the scalability issues encountered with the Intserv model.

Diffserv uses the Differentiated Services field in the IP header (the
DS field) consisting of six bits in what was formerly known as the
Type of Service (TOS) octet. The DS field is used to indicate the
forwarding treatment that a packet should receive at a transit node
[RFC2474]. Diffserv includes the concept of Per-Hop Behavior (PHB)
groups. Using the PHBs, several classes of services can be defined
using different classification, policing, shaping, and scheduling
rules.

For an end-user end user of network services to utilize Differentiated
Services Diffserv provided by
its Internet Service Provider (ISP), it may be necessary for the user
to have an SLA with the ISP. An SLA may explicitly or implicitly
specify a Traffic Conditioning Agreement (TCA) which that defines
classifier rules as well as metering, marking, discarding, and
shaping rules.

Packets are classified, classified and possibly policed and shaped at the ingress
to a Diffserv network. When a packet traverses the boundary between
different Diffserv domains, the DS field of the packet may be
re-marked re-
marked according to existing agreements between the domains.

Differentiated Services

Diffserv allows only a finite number of service classes to be
specified by the DS field. The main advantage of the Diffserv
approach relative to the Intserv model is scalability. Resources are
allocated on a per-class basis basis, and the amount of state information
is proportional to the number of classes rather than to the number of
application flows.

Once the network has been planned and the packets have been marked at
the network edge, the Diffserv model deals with traffic management
issues on a per hop per-hop basis. The Diffserv control model consists of a
collection of micro-TE control mechanisms. Other TE capabilities,
such as capacity management (including routing control), are also
required in order to deliver acceptable service quality in Diffserv
networks. The concept of Per Domain Behaviors "Per-Domain Behaviors" has been introduced
to better capture the notion of Differentiated Services Diffserv across a complete domain
[RFC3086].

Diffserv procedures can also be applied in an MPLS context. See
Section 6.8 for more information.

5.1.1.3. Segment Routing SR Policy

SR Policy [RFC9256] is an evolution of Segment Routing SR (see Section 5.1.3.12) to
enhance the TE capabilities of SR. It is a framework that enables
instantiation of an ordered list of segments on a node for
implementing a source routing policy with a specific intent for
traffic steering from that node.

An SR Policy is identified through the tuple <head-end, <headend, color, end-
point>.
endpoint>. The head-end headend is the IP address of the node where the
policy is instantiated. The endpoint is the IP address of the
destination of the policy. The color is an index that associates the
SR Policy with an intent (e.g., low latency).

The head-end headend node is notified of SR Policies and associated SR paths
via configuration or by extensions to protocols such as PCEP the Path
Computation Element Communication Protocol (PCEP) [RFC8664] or BGP [I-D.ietf-idr-segment-routing-te-policy].
[SR-TE-POLICY]. Each SR path consists of a Segment-List segment list (an SR
source-routed path), and the
head-end headend uses the endpoint and color
parameters to classify packets to match the SR policy Policy and so
determine along which path to forward them. If an SR Policy is
associated with a set of SR paths, each is associated with a weight
for weighted load balancing. Furthermore, multiple SR Policies may
be associated with a set of SR paths to allow multiple traffic flows
to be placed on the same paths.

An SR Binding SID (BSID) may also be associated with each candidate
path associated with an SR Policy, Policy or with the SR Policy itself. The
head-end
headend node installs a BSID-keyed entry in the forwarding plane and
assigns it the action of steering packets that match the entry to the
selected path of the SR Policy. This steering can be done in various
ways:

SID Steering: Incoming packets have an active SID Segment Identifier
(SID) matching a local BSID at the head-end.

* headend.

Per-destination Steering: Incoming packets match a BGP/Service
route
route, which indicates an SR Policy.

Per-flow Steering: Incoming packets match a forwarding array (for
example, the classic 5-tuple) 5-tuple), which indicates an SR Policy.

Policy-based Steering: Incoming packets match a routing policy policy,
which directs them to an SR Policy.

5.1.1.4. Layer 4 Transport-Based TE

In addition to IP-based TE mechanisms, layer Layer 4 transport-based TE
approaches can be considered in specific deployment contexts (e.g.,
data centers, centers and multi-homing). For example, the 3GPP defines the
Access Traffic Steering, Switching, and Splitting (ATSSS) [ATSSS]
service functions as follows. follows:

Access Traffic Steering: This is the selection of an access network
for a new flow and the transfer of the traffic of that flow over
the selected access network.

Access Traffic Switching: This is the migration of all packets of an
ongoing flow from one access network to another access network.
Only one access network is in use at a time.

Access Traffic Splitting: This is about forwarding the packets of a
flow across multiple access networks simultaneously.

The control plane is used to provide hosts and specific network
devices with a set of policies that specify which flows are eligible
to use the ATSSS service. The traffic that matches an ATSSS policy
can be distributed among the available access networks following one
of the following four modes. modes:

Active-Standby: The traffic is forwarded via a specific access
(called "active access") and switched to another access (called
"standby access") when the active access is unavailable.

Priority-based: Network accesses are assigned priority levels that
indicate which network access is to be used first. The traffic
associated with the matching flow will be steered onto the network
access with the highest priority until congestion is detected,
then detected.
Then, the overflow will be forwarded over the next highest
priority access.

Load-Balancing: The traffic is distributed among the available
access networks following a distribution ratio (e.g., 75% - to 25%).

Smallest Delay: The traffic is forwarded via the access that
presents the smallest round-trip-time round-trip time (RTT).

For resource management purposes, hosts and network devices support
means such as congestion control, RTT measurement, and packet
scheduling.

For TCP traffic, Multipath TCP [RFC8684] and the 0-RTT Convert
Protocol [RFC8803] are used to provide the ATSSS service.

Multipath QUIC [I-D.ietf-quic-multipath] [QUIC-MULTIPATH] and Proxying UDP in HTTP [RFC9298]
are used to provide the ATSSS service for UDP traffic. Note that
QUIC [RFC9000] natively support supports the switching and steering functions.
Indeed, QUIC supports a connection migration procedure that allows
peers to change their layer Layer 4 transport coordinates (IP addresses,
port numbers) without breaking the underlying QUIC connection.

Extensions to the Datagram Congestion Control Protocol (MP-DCCP) (DCCP)
[RFC4340] to support multipath operations are defined in
[I-D.ietf-tsvwg-multipath-dccp].
[MULTIPATH-DCCP].

5.1.1.5. Deterministic Networking

Deterministic Networking (DetNet) [RFC8655] is an architecture for
applications with critical timing and reliability requirements. The
layered architecture particularly focuses on developing DetNet
service capabilities in the data plane [RFC8938]. The DetNet service
sub-layer provides a set of Packet Replication, Elimination, and
Ordering Functions (PREOF) to provide end-to-end service assurance.
The DetNet forwarding sub-layer provides corresponding forwarding
assurance (low packet loss, bounded latency, and in-order delivery)
functions using resource allocations and explicit route mechanisms.

The separation into two sub-layers allows a greater flexibility to
adapt DetNet capability over a number of TE data plane mechanisms mechanisms,
such as IP, MPLS, and Segment Routing. SR. More importantly importantly, it interconnects IEEE
802.1 Time Sensitive Networking (TSN) [RFC9023] deployed in Industry
Control and Automation Systems (ICAS).

DetNet can be seen as a specialized branch of TE, since it sets up
explicit optimized paths with allocation of resources as requested.
A DetNet application can express its QoS attributes or traffic
behavior using any combination of DetNet functions described in sub-
layers. They are then distributed and provisioned using well-
established control and provisioning mechanisms adopted for traffic
engineering.

In DetNet, a considerable amount of state information is required to
maintain per-flow queuing disciplines and resource reservation for a
large number of individual flows. This can be quite challenging for
network operations during network events events, such as faults, change in
traffic volume volume, or re-provisioning. reprovisioning. Therefore, DetNet recommends
support for aggregated flows, flows; however, it still requires a large
amount of control signaling to establish and maintain DetNet flows.

Note that DetNet might suffer from some of the scalability concerns
described for Intserv in Section 5.1.1.1, but the scope of DetNet's
deployment scenarios is smaller and so therefore less exposed to scaling
issues.

5.1.2. IETF Approaches Relying on TE Mechanisms

5.1.2.1. Application-Layer Traffic Optimization

This document describes various TE mechanisms available in the
network. However, in general, distributed applications in general and, in
particular,
(particularly, bandwidth-greedy P2P applications that are used, for
example, used for
file sharing, for example) cannot directly use those techniques. As
per [RFC5693], applications could greatly improve traffic
distribution and quality by cooperating with external services that
are aware of the network topology. Addressing the Application-Layer
Traffic Optimization (ALTO) problem means, on the one hand, deploying
an ALTO service to provide applications with information regarding
the underlying network (e.g., basic network location structure and
preferences of network paths) and, on the other hand, enhancing
applications in order to use such information to perform better-than-
random selection of the endpoints with which they establish
connections.

The basic function of ALTO is based on abstract maps of a network.
These maps provide a simplified view, yet enough information about a
network for applications to effectively utilize them. Additional
services are built on top of the maps. [RFC7285] describes a
protocol implementing the ALTO services as an information-publishing
interface that allows a network to publish its network information to
network applications. This information can include network node
locations, groups of node-to-node connectivity arranged by cost
according to configurable granularities, and end-host properties.
The information published by the ALTO Protocol should benefit both
the network and the applications. The ALTO Protocol uses a REST-ful
design and encodes its requests and responses using JSON [RFC8259]
with a modular design by dividing ALTO information publication into
multiple ALTO services (e.g., the Map service, Service, the Map-Filtering
Service, the Endpoint Property Service, and the Endpoint Cost
Service).

[RFC8189] defines a new service that allows an ALTO Client to
retrieve several cost metrics in a single request for an ALTO
filtered cost map and endpoint cost map. [RFC8896] extends the ALTO
cost information service so that applications decide not only 'where' "where"
to connect, connect but also 'when'. "when". This is useful for applications that
need to perform bulk data transfer and would like to schedule these
transfers during an off-peak hour, for example. [RFC9439] introduces
network performance metrics, including network delay, jitter, packet
loss rate, hop count, and bandwidth. The ALTO server may derive and
aggregate such performance metrics from BGP-LS (see
Section 5.1.3.10)
or 5.1.3.10), IGP-TE (see Section 5.1.3.9), or management tools, tools
and then expose the information to allow applications to determine 'where'
"where" to connect based on network performance criteria. The ALTO WG
Working Group is evaluating the use of network TE properties while
making application decisions for new use cases such as Edge edge computing
and Datacenter data-center interconnect.

5.1.2.2. Network Virtualization and Abstraction

One of the main drivers for Software Defined Networking (SDN) SDN [RFC7149] is a decoupling of the
network control plane from the data plane. This separation has been
achieved for TE networks with the development of MPLS/GMPLS MPLS and GMPLS (see Section
Sections 5.1.3.3 and Section 5.1.3.5) 5.1.3.5, respectively) and the PCE (Section (see
Section 5.1.3.11). One of the advantages of SDN is its logically
centralized control regime that allows a full view of the underlying
networks. Centralized control in SDN helps improve network resource
utilization compared with distributed network control.

Abstraction and Control of TE Networks (ACTN) [RFC8453] defines a
hierarchical SDN architecture which that describes the functional entities
and methods for the coordination of resources across multiple
domains, to provide composite traffic-engineered services. ACTN
facilitates composed, multi-domain connections and provides them to
the user. ACTN is focused on:

* Abstraction of the underlying network resources and how they are
provided to higher-layer applications and customers.

* Virtualization of underlying resources for use by the customer,
application, or service. The creation of a virtualized
environment allows operators to view and control multi-domain
networks as a single virtualized network.

* Presentation to customers of networks as a virtual network via
open and programmable interfaces.

The ACTN managed infrastructure is built from traffic-engineered
network resources, which may include statistical packet bandwidth,
physical forwarding plane forwarding-plane sources (such as wavelengths and time
slots), and forwarding and cross-connect capabilities. The type of
network virtualization seen in ACTN allows customers and applications
(tenants) to utilize and independently control allocated virtual
network resources as if they were physically their own resource. The
ACTN network is "sliced", sliced, with tenants being given a different partial
and abstracted topology view of the physical underlying network.

5.1.2.3. Network Slicing

An IETF Network Slice is a logical network topology connecting a
number of endpoints using a set of shared or dedicated network
resources [I-D.ietf-teas-ietf-network-slices]. [NETWORK-SLICES]. The resources are used to satisfy
specific Service Level Objectives (SLOs) SLOs specified by the consumer.

IETF network slices Network Slices are not, of themselves, TE constructs. However,
a network operator that offers IETF network slices Network Slices is likely to use
many TE tools in order to manage their network and provide the
services.

IETF Network Slices are defined such that they are independent of the
underlying infrastructure connectivity and technologies used. From a
customer's perspective, an IETF Network Slice looks like a VPN
connectivity matrix with additional information about the level of
service that the customer requires between the endpoints. From an
operator's perspective, the IETF Network Slice looks like a set of
routing or tunneling instructions with the network resource
reservations necessary to provide the required service levels as
specified by the SLOs. The concept of an IETF network slice Network Slice is
consistent with an enhanced VPN (VPN+) [I-D.ietf-teas-enhanced-vpn]. [ENHANCED-VPN].

5.1.3. IETF Techniques Used by TE Mechanisms

5.1.3.1. Constraint-Based Routing

Constraint-based routing refers to a class of routing systems that
compute routes through a network subject to the satisfaction of a set
of constraints and requirements. In the most general case,
constraint-based routing may also seek to optimize overall network
performance while minimizing costs.

The constraints and requirements may be imposed by the network itself
or by administrative policies. Constraints may include bandwidth,
hop count, delay, and policy instruments such as resource class
attributes. Constraints may also include domain-specific attributes
of certain network technologies and contexts which that impose restrictions
on the solution space of the routing function. Path
oriented Path-oriented
technologies such as MPLS have made constraint-based routing feasible
and attractive in public IP networks.

The concept of constraint-based routing within the context of MPLS TE MPLS-TE
requirements in IP networks was first described in [RFC2702] and led
to developments such as MPLS-TE [RFC3209] as described in
Section 5.1.3.3.

Unlike QoS-based routing (for example, see [RFC2386], [MA], and
[I-D.ietf-idr-performance-routing]) which
[PERFORMANCE-ROUTING]) that generally addresses the issue of routing
individual traffic flows to satisfy prescribed flow-
based flow-based QoS
requirements subject to network resource availability,
constraint-based constraint-
based routing is applicable to traffic aggregates as well as flows
and may be subject to a wide variety of constraints which that may include
policy restrictions.

5.1.3.1.1. IGP Flexible Algorithms (Flex-Algos)

The traditional normal approach to routing in an IGP network relies on the IGPs
deriving "shortest paths" over the network based solely on the IGP
metric assigned to the links. Such an approach is often limited:
traffic may tend to converge toward the destination, possibly causing
congestion;
congestion, and it is not possible to steer traffic onto paths
depending on the end-to-end qualities demanded by the applications.

To overcome this limitation, various sorts of TE have been widely
deployed (as described in this document), where the TE component is
responsible for computing the path based on additional metrics and/or
constraints. Such paths (or tunnels) need to be installed in the
routers' forwarding tables in addition to, or as a replacement for for,
the original paths computed by IGPs. The main drawback drawbacks of these TE
approaches is are the additional complexity of protocols and management, management
and the state that may need to be maintained within the network.

IGP flexible algorithms (flex-algos) Flexible Algorithms [RFC9350] allow IGPs to construct constraint-based constraint-
based paths over the network by computing
constraint- based constraint-based next hops.
The intent of flex-algos Flexible Algorithms is to reduce TE complexity by
letting an IGP perform some basic TE computation capabilities. Flex-algo
Flexible Algorithm includes a set of extensions to the IGPs that
enable a router to send TLVs that:

* describe a set of constraints on the topology

* identify calculation-type

* describe a metric-type that is to be used to compute the best
paths through the constrained topology. topology

A given combination of calculation-type, metric-type, and constraints
is known as a "Flexible Flexible Algorithm Definition" (or FAD). Definition (FAD). A router that
sends such a set of TLVs also assigns a specific identifier (the
Flexible Algorithm) to the specified combination of calculation-type,
metric-type, and constraints.

There are two use cases for flex-algo: Flexible Algorithm: in IP networks
[I-D.ietf-lsr-ip-flexalgo]
[RFC9502] and in Segment Routing SR networks [RFC9350]. In the first case, flex-algo Flexible
Algorithm computes paths to an IPv4 or IPv6
address, address; in the second
case, flex-algo Flexible Algorithms computes paths to a prefix Prefix SID (see
Section 5.1.3.12).

Examples of where flex-algo Flexible Algorithms can be useful include:

* Expansion of the function of IP Performance performance metrics [RFC5664]
where specific constraint-based routing (flex-algo) (Flexible Algorithm) can
be instantiated within the network based on the results of
performance measurement.

* The formation of an 'underlay' "underlay" network using flex-algo, Flexible Algorithms,
and the realization of an 'overlay' "overlay" network using TE techniques.
This approach can leverage the nested combination of flex-algo Flexible
Algorithm and TE extensions for IGP (see Section 5.1.3.9).

* Flex-algo Flexible Algorithms in SR-MPLS (Section 5.1.3.12) can be used as a
base to easily build a TE-like topology without TE components on
routers or the use of a PCE (see Section 5.1.3.11).

* The support for network slices [I-D.ietf-teas-ietf-network-slices] [NETWORK-SLICES] where the SLOs of
a particular IETF network slice Network Slice can be guaranteed by a flex-algo, Flexible
Algorithm or where a Filtered Topology
[I-D.ietf-teas-ietf-network-slices] [NETWORK-SLICES] can be
created as a TE-like topology using a flex-algo. Flexible Algorithm.

5.1.3.2. RSVP

RSVP is a soft-state signaling protocol [RFC2205]. It supports
receiver-initiated establishment of resource reservations for both
multicast and unicast flows. RSVP was originally developed as a
signaling protocol within the Integrated Services framework (see
Section 5.1.1.1) for applications to communicate QoS requirements to
the network and for the network to reserve relevant resources to
satisfy the QoS requirements [RFC2205].

In RSVP, the traffic sender or source node sends a PATH Path message to
the traffic receiver with the same source and destination addresses
as the traffic which that the sender will generate. The PATH Path message
contains: (1) a

* A sender traffic specification describing the characteristics of
the traffic, (2) a traffic

* A sender template specifying the format of the traffic, and (3) an traffic

* An optional advertisement specification which that is used to support
the concept of One Pass With Advertising (OPWA) [RFC2205]. [RFC2205]

Every intermediate router along the path forwards the PATH Path message to
the next hop determined by the routing protocol. Upon receiving a PATH
Path message, the receiver responds with a RESV Resv message which that includes
a flow descriptor used to request resource reservations. The RESV Resv
message travels to the sender or source node in the opposite
direction along the path that the PATH Path message traversed. Every
intermediate router along the path can reject or accept the
reservation request of the RESV Resv message. If the request is rejected,
the rejecting router will send an error message to the receiver receiver, and
the signaling process will terminate. If the request is accepted,
link bandwidth and buffer space are allocated for the flow flow, and the
related flow state information is installed in the router.

One of the issues with the original RSVP specification [RFC2205] was
scalability. This was because reservations were required for micro-
flows, so that the amount of state maintained by network elements
tended to increase linearly with the number of traffic flows. These
issues are described in [RFC2961] [RFC2961], which also modifies and extends
RSVP to mitigate the scaling problems to make RSVP a versatile
signaling protocol for the Internet. For example, RSVP has been
extended to reserve resources for aggregation of flows [RFC3175], to
set up MPLS explicit label switched paths LSPs (see Section 5.1.3.3), and to perform other
signaling functions within the Internet. [RFC2961] also describes a
mechanism to reduce the amount of Refresh messages required to
maintain established RSVP sessions.

5.1.3.3. Multiprotocol Label Switching (MPLS) MPLS

MPLS is a forwarding scheme which that also includes extensions to
conventional IP control plane protocols. MPLS extends the Internet
routing model and enhances packet forwarding and path control
[RFC3031].

At the ingress to an MPLS domain, Label Switching Routers (LSRs) LSRs classify IP packets into
Forwarding Equivalence Classes (FECs) based on a variety of factors,
including, e.g., a combination of the information carried in the IP
header of the packets and the local routing information maintained by
the LSRs. An MPLS label stack entry is then prepended to each packet
according to their forwarding
equivalence classes. FECs. The MPLS label stack entry is 32 bits long
and contains a 20-bit label field.

An LSR makes forwarding decisions by using the label prepended to
packets as the index into a local next hop label forwarding entry Next Hop Label Forwarding Entry
(NHLFE). The packet is then processed as specified in the NHLFE.
The incoming label may be replaced by an outgoing label (label swap),
and the packet may be forwarded to the next LSR. Before a packet
leaves an MPLS domain, its MPLS label may be removed (label pop). A
Label Switched Path (LSP) An
LSP is the path between an ingress LSR and an egress LSR through
which a labeled packet traverses. The path of an explicit LSP is
defined at the originating (ingress) node of the LSP. MPLS can use a
signaling protocol such as RSVP or the Label Distribution Protocol
(LDP) to set up LSPs.

MPLS is a powerful technology for Internet TE because it supports
explicit LSPs which that allow constraint-based routing to be implemented
efficiently in IP networks [AWD2]. The requirements for TE over MPLS
are described in [RFC2702]. Extensions to RSVP to support
instantiation of explicit LSP are discussed in [RFC3209] and
Section 5.1.3.4.

5.1.3.4. RSVP-TE

RSVP-TE is a protocol extension of RSVP (Section 5.1.3.2) for traffic
engineering. The base specification is found in [RFC3209]. RSVP-TE
enables the establishment of traffic-engineered MPLS LSPs (TE LSPs),
using loose or strict paths, paths and taking into consideration network
constraints such as available bandwidth. The extension supports
signaling LSPs on explicit paths that could be administratively
specified,
specified or computed by a suitable entity (such as a PCE,
Section 5.1.3.11) based on QoS and policy requirements, taking into
consideration the prevailing network state as advertised by the IGP
extension for IS-IS in [RFC5305], for OSPFV2 OSPFv2 in [RFC3630], and for
OSPFv3 in [RFC5329]. RSVP-TE enables the reservation of resources
(for example, bandwidth) along the path.

RSVP-TE includes the ability to preempt LSPs based on priorities, priorities and
uses link affinities to include or exclude links from the LSPs. The
protocol is further extended to support Fast Reroute (FRR) [RFC4090],
Diffserv [RFC4124], and bidirectional LSPs [RFC7551]. RSVP-TE
extensions for support for GMPLS (see Section 5.1.3.5) are specified
in [RFC3473].

Requirements for point-to-multipoint (P2MP) MPLS TE MPLS-TE LSPs are
documented in [RFC4461], and signaling protocol extensions for
setting up P2MP MPLS TE MPLS-TE LSPs via RSVP-TE are defined in [RFC4875] [RFC4875],
where a P2MP LSP comprise comprises multiple source-to-leaf (S2L) sub-LSPs.
To determine the paths for P2MP LSPs, selection of the branch points
(based on capabilities, network state, and policies) is key [RFC5671]

RSVP-TE has evolved to provide real time real-time dynamic metrics for path
selection for low latency low-latency paths using extensions to IS-IS [RFC8570]
and OSPF [RFC7471] based on STAMP performance measurements for the Simple
Two-Way Active Measurement Protocol (STAMP) [RFC8972] and TWAMP [RFC5357]
performance measurements. the Two-Way
Active Measurement Protocol (TWAMP) [RFC5357].

RSVP-TE has historically been used when bandwidth was constrained, constrained;
however, as bandwidth has increased, RSVP-TE has developed into a
bandwidth management tool to provide bandwidth efficiency and
proactive resource management.

5.1.3.5. Generalized MPLS (GMPLS)

GMPLS extends MPLS control protocols to encompass time-division
(e.g., Synchronous Optical Network / Synchronous Digital Hierarchy
(SONET/SDH), Plesiochronous Digital Hierarchy (PDH), and Optical
Transport Network (OTN)), wavelength (lambdas), and spatial switching
(e.g., incoming port or fiber to outgoing port or fiber) as well as
continuing and
continues to support packet switching. GMPLS provides a common set
of control protocols for all of these layers (including some
technology-specific extensions) extensions), each of which has a distinct data or
forwarding plane. GMPLS covers both the signaling and the routing
part of that control plane and is based on the TE extensions to MPLS
(see Section 5.1.3.4).

In GMPLS [RFC3945], the original MPLS architecture is extended to
include LSRs whose forwarding planes rely on circuit switching, switching and
therefore cannot forward data based on the information carried in
either packet or cell headers. Specifically, such LSRs include
devices where the switching is based on time slots, wavelengths, or
physical ports. These additions impact basic LSP properties: how
labels are requested and communicated, the unidirectional nature of
MPLS LSPs, how errors are propagated, and information provided for
synchronizing the ingress and egress LSRs [RFC3473].

5.1.3.6. IP Performance Metrics (IPPM)

The IETF IP Performance Metrics (IPPM) working group Working Group has developed a
set of standard metrics that can be used to monitor the quality,
performance, and reliability of Internet services. These metrics can
be applied by network operators, end-users, end users, and independent testing
groups to provide users and service providers with a common
understanding of the performance and reliability of the Internet
component 'clouds' clouds they use/provide [RFC2330]. The criteria for
performance metrics developed by the IPPM working group Working Group are described
in [RFC2330]. Examples of performance metrics include one-way packet
loss [RFC7680], one-way delay [RFC7679], and connectivity measures
between two nodes [RFC2678]. Other metrics include second-order
measures of packet loss and delay.

Some of the performance metrics specified by the IPPM working group Working Group
are useful for specifying SLAs. SLAs are sets of service level
objectives SLOs negotiated
between users and service providers, wherein each objective is a
combination of one or more performance metrics, possibly subject to
certain constraints.

The IPPM working group Working Group also designs measurement techniques and
protocols to obtain thwse these metrics.

5.1.3.7. Flow Measurement

The IETF Real Time Flow Measurement (RTFM) working group Working Group produced an
architecture that defines a method to specify traffic flows as well
as a number of components for flow measurement (meters, meter
readers, manager) and managers) [RFC2722]. A flow measurement system enables
network traffic flows to be measured and analyzed at the flow level
for a variety of purposes. As noted in RFC 2722, [RFC2722], a flow measurement
system can be very useful in the following contexts:

* understanding the behavior of existing networks

* planning for network development and expansion

* quantification of network performance

* verifying the quality of network service

* attribution of network usage to users. users

A flow measurement system consists of meters, meter readers, and
managers. A meter observes packets passing through a measurement
point, classifies them into groups, accumulates usage data (such as
the number of packets and bytes for each group), and stores the usage
data in a flow table. A group may represent any collection of user
applications, hosts, networks, etc. A meter reader gathers usage
data from various meters so it can be made available for analysis. A
manager is responsible for configuring and controlling meters and
meter readers. The instructions received by a meter from a manager
include flow specifications, meter control parameters, and sampling
techniques. The instructions received by a meter reader from a
manager include the address of the meter whose data are to be
collected, the frequency of data collection, and the types of flows
to be collected.

IP Flow Information Export (IPFIX) [RFC5470] defines an architecture
that is very similar to the RTFM architecture and includes Metering,
Exporting, and Collecting Processes. [RFC5472] describes the
applicability of IPFIX and makes a comparison with RTFM, pointing out
that, architecturally, while RTM talks about devices, IPFIX deals
with processed processes to clarify that multiple of those processes may be co-
located on the same machine. The IPFIX protocol [RFC7011] is widely
implemented.

5.1.3.8. Endpoint Congestion Management

[RFC3124] provides a set of congestion control mechanisms for the use
of transport protocols. It also allows the development of mechanisms
for unifying congestion control across a subset of an endpoint's
active unicast connections (called a congestion group). "congestion group"). A
congestion manager continuously monitors the state of the path for
each congestion group under its control. The manager uses that
information to instruct a scheduler on how to partition bandwidth
among the connections of that congestion group.

The concepts described in [RFC3124] and the lessons that can be
learned from that work found a home in HTTP/2 [RFC9113] and QUIC
[RFC9000], while [RFC9040] describes TCP control block
interdependence which that is a core construct underpinning the congestion
manager defined in [RFC3124].

5.1.3.9. TE Extensions to the IGPs

[RFC5305] describes the extensions to the Intermediate System to
Intermediate System (IS-IS) protocol to support TE, similarly TE. Similarly,
[RFC3630] specifies TE extensions for OSPFv2, and [RFC5329] has the
same description for OSPFv3.

IS-IS and OSPF share the common concept of TE extensions to
distribute TE parameters parameters, such as link type and ID, local and remote
IP addresses, TE metric, maximum bandwidth, maximum reservable
bandwidth and
bandwidth, unreserved bandwidth, and admin group. The information
distributed by the IGPs in this way can be used to build a view of
the state and capabilities of a TE network (see Section 5.1.3.14).

The difference between IS-IS and OSPF is in the details of how they
encode and transmit the TE parameters:

* IS-IS uses the Extended IS Reachability TLV (type 22), the
Extended IP Reachability TLV (type 135), and the TE Router Traffic
Engineering router ID TLV (type 134). These TLVs use specific Sub-TLVs
sub-TLVs described in [RFC8570] to carry for the TE parameters.

* OSPFv2 uses Opaque LSA [RFC5250] type 10 10, and OSPFv3 uses the
Intra-Area-TE-LSA. In both OSPF cases, two top-level TLVs are
used (Router Address and Link TLVs), and these use Sub-TLVs sub-TLVs to
carry the TE parameters (as defined in [RFC7471] for OSPFv2 and
[RFC5329] for OSPFv3. OSPFv3).

5.1.3.10. BGP Link-State - Link State

In a number of environments, a component external to a network is
called upon to perform computations based on the network topology and
current state of the connections within the network, including TE
information. This is information typically distributed by IGP
routing protocols within the network (see Section 5.1.3.9).

The Border Gateway Protocol (BGP)

BGP (see also Section 7) is one of the essential routing protocols
that glue glues the Internet together. BGP
Link State (BGP-LS) [RFC7752] BGP-LS [RFC9552] is a mechanism by
which link-state and TE information can be collected from networks
and shared with external components using the BGP routing protocol.
The mechanism is applicable to physical and virtual IGP links, links and is
subject to policy control.

Information collected by BGP-LS can be used, for example, to
construct the TED (Section 5.1.3.14) for use by the Path Computation
Element (PCE, see PCE (see
Section 5.1.3.11), 5.1.3.11) or may be used by Application-
Layer Traffic Optimization (ALTO) ALTO servers (see
Section 5.1.2.1).

5.1.3.11. Path Computation Element

Constraint-based path computation is a fundamental building block for
TE in MPLS and GMPLS networks. Path computation in large, multi-
domain networks is complex and may require special computational
components and cooperation between the elements in different domains.
The Path Computation Element (PCE) PCE [RFC4655] is an entity (component, application, or network
node) that is capable of computing a network path or route based on a
network graph and applying computational constraints.

Thus, a PCE can provide a central component in a TE system operating
on the TE Database (TED, see TED (see Section 5.1.3.14) with delegated responsibility for
determining paths in MPLS, GMPLS, or Segment
Routing SR networks. The PCE uses the
Path Computation Element Communication Protocol (PCEP) [RFC5440] to
communicate with Path Computation Clients (PCCs), such as MPLS LSRs,
to answer their requests for computed paths or to instruct them to
initiate new paths [RFC8281] and maintain state about paths already
installed in the network [RFC8231].

PCEs form key components of a number of TE systems. More information
about the applicability of PCE PCEs can be found in [RFC8051], while
[RFC6805] describes the application of PCE PCEs to determining paths
across multiple domains. PCE PCEs also has have potential use uses in
Abstraction and Control of TE Networks (ACTN) (see Section 5.1.2.2),
Centralized Network Control [RFC8283], and Software Defined Networking (SDN) SDN (see Section 4.3.2).

5.1.3.12. Segment Routing

The Segment Routing (SR)

The SR architecture [RFC8402] leverages the source routing and
tunneling paradigms. The path a packet takes is defined at the ingress
ingress, and the packet is tunneled to the egress.

In a protocol realization, an ingress node steers a packet using a
set of instructions, called segments, "segments", that are included in an SR
header prepended to the packet: a label stack in MPLS case, and a
series of 128-bit segment identifiers SIDs in the IPv6 case.

Segments are identified by Segment Identifiers (SIDs). SIDs. There are four types of SID SIDs that
are relevant for TE.

* Prefix SID: A SID that is unique within the routing domain and is
used to identify a prefix.

* Node SID: A Prefix SID with the 'N' "N" bit set to identify a node.

* Adjacency SID: Identifies a unidirectional adjacency.

* Binding SID: A Binding SID has two purposes:

1. To advertise the mappings of prefixes to SIDs/Labels. SIDs/Labels

2. To advertise a path available for a Forwarding Equivalence
Class.
Class (FEC)

A segment can represent any instruction, topological or service-
based. SIDs can be looked up in a global context (domain wide) (domain-wide) as
well as in some other context contexts (see, for example, "context labels" in
Section 3 of [RFC5331]).

The application of "policy" policy to Segment Routing SR can make SR into a TE
mechanism mechanism, as
described in Section 5.1.1.3.

5.1.3.13. Tree Engineering for Bit Index Explicit Replication Tree Engineering

Bit Index Explicit Replication (BIER) [RFC8279] specifies an
encapsulation for multicast forwarding that can be used on MPLS or
Ethernet transports. A mechanism known as Tree Engineering for Bit
Index Explicit Replication (BIER-TE) [RFC9262] provides a component
that could be used to build a traffic-engineered multicast system.
BIER-TE does not of itself offer full traffic engineering, and the
abbreviation "TE" does not, in this case, refer to traffic
engineering.

In BIER-TE, path steering is supported via the definition of a
bitstring attached to each packet that determines how the packet is
forwarded and replicated within the network. Thus, this bitstring
steers the traffic within the network and forms an element of a
traffic engineering
traffic-engineering system. A central controller that is aware of
the capabilities and state of the network as well as the demands of
the various traffic flows, flows is able to select multicast paths that take
account of the available resources and demands. This
controller, therefore, Therefore, this
controller is responsible for the policy elements of traffic
engineering.

Resource management has implications for the forwarding plane beyond
the steering of packets defined for BIER-TE. These include the
allocation of buffers to meet the requirements of admitted traffic, traffic
and may include policing and/or rate-shaping mechanisms achieved via
various forms of queuing. 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
problems. It is also important in networks that wish to control
latencies experienced by specific traffic flows.

5.1.3.14. Network TE State Definition and Presentation

The network states that are relevant to TE need to be stored in the
system and presented to the user. The Traffic Engineering Database
(TED) TED is a collection of all TE
information about all TE nodes and TE links in the network. It is an
essential component of a TE system, such as MPLS-TE [RFC2702] or
GMPLS [RFC3945]. In order to formally define the data in the TED and
to present the data to the user, the data modeling language YANG
[RFC7950] can be used as described in [RFC8795].

5.1.3.15. System Management and Control Interfaces

The TE control system needs to have a management interface that is
human-friendly and a control interface that is programmable for
automation. The Network Configuration Protocol (NETCONF) [RFC6241]
or
and the RESTCONF Protocol protocol [RFC8040] provide programmable interfaces
that are also human-friendly. These protocols use XML XML- or JSON JSON-
encoded messages. When message compactness or protocol bandwidth
consumption needs to be optimized for the control interface, other
protocols, such as Group Communication for the Constrained
Application Protocol (CoAP) [RFC7390] or gRPC [GRPC], are available,
especially when the protocol messages are encoded in a binary format.
Along with any of these protocols, the data modeling language YANG
[RFC7950] can be used to formally and precisely define the interface
data.

The Path Computation Element Communication Protocol (PCEP)

PCEP [RFC5440] is another protocol that has evolved to be an option
for the TE system control interface. The messages of PCEP messages are TLV based;
they are TLV-based, not defined by a data modeling data-modeling language such as YANG.

5.2. Content Distribution

The Internet is dominated by client-server interactions, principally
Web
web traffic and multimedia streams, although in the future, more
sophisticated media servers may become dominant. The location and
performance of major information servers has have 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. "traffic
directing". It is an application layer application-layer function.

Traffic directing

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 traffic-directing and traffic 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. Recommendations for Internet Traffic Engineering

This section describes high-level recommendations for traffic
engineering in the Internet in general terms.

The recommendations describe the capabilities needed to solve a TE
problem or to achieve a TE objective. Broadly speaking, these
recommendations can be categorized as either functional or non-
functional recommendations. recommendations:

* Functional recommendations describe the functions that a traffic traffic-
engineering system should perform. These functions are needed to
realize TE objectives by addressing traffic engineering traffic-engineering problems.

* Non-functional recommendations relate to the quality attributes or
state characteristics of a TE system. These recommendations may
contain conflicting assertions and may sometimes be difficult to
quantify precisely.

The subsections that follow first summarize the non-functional
requirements,
requirements and then detail the functional requirements.

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.

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 additionally benefit from feedback from the network that
indicates the state of network resources and the current load in
the network. Further, placing intelligence into components of the
TE system could enable automation to be more dynamic and
responsive to changes in the network.

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 that 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.

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

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, links and with
respect to the number of flows and 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, information or when exerting control.

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.

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.

Stability: Stability refers to the resistance of the network to
oscillate (flap) in a disruptive manner from one state to another,
which may result in traffic being routed first one way and then
another without satisfactory resolution of the underlying TE
issues,
issues and with continued changes that do not settle down.
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).

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.

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 (such as traffic matrices, link utilization,
latency, packet loss, and other performance measures of interest
which interest)
that 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 that should be observable.

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 Path First (SPF) IGPs are based on shortest path algorithms
and have limited control capabilities for TE [RFC2702], [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. Note that link
metrics are assigned following a range of operator-selected
policies that might reflect preference for the use of some links
over others, and others; therefore, "shortest" might not, therefore, not be purely a measure
of distance. 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 Multipath (ECMP) capability of SPF IGPs supports
sharing of traffic among equal-cost paths. 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 load-sharing ratios among equal cost 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]) [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], [FT00]
[FT01].

Because of these limitations, capabilities are needed to enhance the
routing function in IP networks. Some of these capabilities are
summarized below. 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 instruments, such as
resource class attributes [RFC2702], [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 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 link-state IGPs allow them to
distribute additional state information required for constraint-
based routing. The extensions to OSPF are described in [RFC3630],
and the extensions to IS-IS are described in [RFC5305]. Some of
the additional topology state information includes link attributes
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, IS-IS [RFC5305] 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, 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 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]. [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 Path-
oriented technologies technologies, such as MPLS-TE 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 that 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.

Two important aspect aspects of the traffic mapping function is are the ability
to establish multiple paths between an originating node and a
destination node, node and the capability to distribute the traffic
between the two nodes across the
those paths according to configured policies. A precondition for
this scheme is the existence of flexible mechanisms to partition
traffic and then assign the traffic partitions onto the parallel
paths (described as "parallel traffic trunks" 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 timescale 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 MPLS Adaptive Traffic Engineering (MATE) [MATE] and suchlike)
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 periodically
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, hotspots, 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
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 link-state advertisements, and
NETCONF/RESTCONF, etc.

6.5. Policing, Planning, and Access Control

The recommendations in Section Sections 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. 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, distributed and
must be aware of the planned traffic volumes and available
resources. However, this approach is only of value if the traffic
that is presented conforms to the planned traffic volumes.

* Traffic flows may be policed at the edges of a network. This is a
simple way to ensure 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, 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 the determination of
the flow to which the packet belongs. This information is
combined with policy instructions that have been locally
configured,
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 if, and only if if, the flow to which
the packet belongs conforms to the policy.

Combining some element elements 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 high-priority traffic over the Internet.
Survivability can be addressed at the device level by developing
network elements that are more reliable; 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 used to 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 survivability, including MPLS Fast
Reroute [RFC4090], Topology Independent Loop-free Alternate Fast Re-route
Reroute for Segment Routing [I-D.ietf-rtgwg-segment-routing-ti-lfa] [SR-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 outage, which can
vary from milliseconds (with minor service impact) to seconds (with
possible call drops for IP telephony and session timeouts for
connection-oriented transactions) to minutes and hours (with
potentially considerable social and business impact). Outages of
different durations have different impacts depending on the nature 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
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 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 convergence, which may require seconds to minutes to
complete. Path-oriented technologies such as MPLS ([RFC3469]) [RFC3469] can be
used to enhance the survivability of IP networks in a potentially cost-
effective
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 timescales and at
different bandwidth granularity granularities (from the level of packets to that
of wavelengths). 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

Some of the general recommendations for protection and restoration coordination.
coordination are as follows:

* 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 tradeoff 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.

6.6.1. Survivability in MPLS Based MPLS-Based Networks

Because MPLS is path-oriented, it has the potential to provide faster
and more predictable protection and restoration capabilities than
conventional hop-by-hop routed IP systems. Protection types for MPLS
networks can be divided into four categories.

* categories:

Link Protection: The objective of link protection is to protect an
LSP from the failure of a given link. Under link protection, a
protection or backup LSP (the secondary LSP) follows a path that
is disjoint from the path of the working or operational LSP (the
primary LSP) at the particular link where link protection is
required. When the protected link fails, traffic on the working
LSP is switched to the protection LSP at the head-end headend of the failed
link. As a local repair method, link protection can be fast.
This form of protection may be most appropriate in situations
where some network elements along a given path are known to be
less reliable than others.

Node Protection: The objective of node protection is to protect an
LSP from the failure of a given node. Under node protection, the
secondary LSP follows a path that is disjoint from the path of the
primary LSP at the particular node where node protection is
required. The secondary LSP is also disjoint from the primary LSP
at all links attached to the node to be protected. When the
protected node fails, traffic on the working LSP is switched over
to the protection LSP at the upstream LSR directly connected to
the failed node. Node protection covers a slightly larger part of
the network compared to link protection, protection but is otherwise
fundamentally the same.

Path Protection: The goal of LSP path protection (or end-to-end
protection) is to protect an LSP from any failure along its routed
path. Under path protection, the path of the protection LSP is
completely disjoint from the path of the working LSP. The
advantage of path protection is that the backup LSP protects the
working LSP from all possible link and node failures along the
path, except for failures of ingress or egress LSR. Additionally,
path protection may be more efficient in terms of resource usage
than link or node protection applied at every hop along the path.
However, path protection may be slower than link and node
protection because the fault notifications have to be propagated
further.

Segment Protection: An MPLS domain may be partitioned into multiple
subdomains (protection domains). Path protection is applied to
the path of each LSP as it crosses the domain from its ingress to
the domain to where it egresses the domain. In cases where an LSP
traverses multiple protection domains, a protection mechanism
within a domain only needs to protect the segment of the LSP that
lies within the domain. Segment protection will generally be
faster than end-to-end path protection because recovery generally
occurs closer to the fault fault, and the notification doesn't have to
propagate as far.

See [RFC3469] and [RFC6372] for a more comprehensive discussion of
MPLS based
MPLS-based recovery.

6.6.2. Protection Options

Another issue to consider is the concept of protection options. We
use notation such as "m:n protection", where m is the number of
protection LSPs used to protect n working LSPs. In all cases except
1+1 protection, the resources associated with the protection LSPs can
be used to carry preemptable best-effort traffic when the working LSP
is functioning correctly.

1:1 protection: One working LSP is protected/restored by one
protection LSP. Traffic is sent only on the protected LSP until
the protection/restoration event switches the traffic to the
protection LSP.

1:n protection: One protection LSP is used to protect/restore n
working LSPs. Traffic is sent only on the n protected working
LSPs until the protection/restoration event switches the traffic
from one failed LSP to the protection LSP. Only one failed LSP
can be restored at any time.

n:1 protection: One working LSP is protected/restored by n
protection LSPs, possibly with load splitting across the
protection LSPs. This may be especially useful when it is not
feasible to find one path for the backup that can satisfy the
bandwidth requirement of the primary LSP.

1+1 protection: Traffic is sent concurrently on both the working LSP
and a protection LSP. The egress LSR selects one of the two LSPs
based on local policy (usually based on traffic integrity). When
a fault disrupts the traffic on one LSP, the egress switches to
receive traffic from the other LSP. This approach is expensive in
how it consumes network but recovers from failures most rapidly.

6.7. Multi-Layer Traffic Engineering

Networks are often implemented as layers. A layer relationship may
represent the interaction between technologies (for example, an IP
network operated over an optical network), 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] [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 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 best-effort and mission critical mission-critical data, call 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, 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 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 buffer-management and packet scheduling 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], [RFC3270] or on a per-service class 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 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 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 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, 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. class type. A class- class
type is a set of classes that satisfy the following two conditions:

* Classes in the same class-type 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. class type. Note that it is,
nevertheless, still possible to implement some priority policies
for classes in the same class-type class type to permit preferential access
to the class-type 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, may be applied through creating parallel links
that form part of an ECMP scheme, 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 way the routing protocols interactions occur interact across the network.

Control mechanisms can be manual (e.g., static configuration),
partially-automated
partially automated (e.g., scripts), or fully-automated fully automated (e.g., policy
based
policy-based management systems). Automated mechanisms are
particularly useful in large-scale networks. Multi-vendor
interoperability can be facilitated by standardized management tools
(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 attacks).

7. Inter-Domain Considerations

Inter-domain TE is concerned with performance optimization for
traffic that originates in one administrative domain and terminates
in a different one.

BGP [RFC4271] is the standard exterior gateway protocol used to
exchange routing information between autonomous systems (ASes) ASes in the Internet. BGP
includes a decision process that calculates the preference for routes
to a given destination network. There are two fundamental aspects to
inter-domain TE using BGP:

Route Propagation: Controlling the import and export of routes
between ASes, ASes and controlling the redistribution of routes between
BGP and other protocols within an AS.

Best-path selection: Selecting the best path when there are multiple
candidate paths to a given destination network. This is performed
by the BGP decision process, selecting which selects the preferred exit
points out of an AS towards toward specific destination networks by taking
a number of different considerations into account. The BGP path
selection process can be influenced by manipulating the attributes
associated with the process, including NEXT_HOP, LOCAL_PREF,
AS_PATH, ORIGIN, MULTI_EXIT_DISC (MED), IGP metric, etc.

Most BGP implementations provide constructs that facilitate the
implementation of complex BGP policies based on pre-configured
logical conditions. These can be used to control import and export
of incoming and outgoing routes, control redistribution of routes
between BGP and other protocols, and influence the selection of best
paths by manipulating the attributes (either standardized, standardized or local to
the implementation) associated with the BGP decision process.

When considering inter-domain TE with BGP, note that the outbound
traffic exit point is controllable, whereas the interconnection point
where inbound traffic is received typically is not. Therefore, it is
up to each individual network to implement TE strategies that deal
with the efficient delivery of outbound traffic from its customers to
its peering points. The vast majority of TE policy is based on a
"closest exit" strategy, which offloads inter-domain traffic at the
nearest outbound peering point towards the destination AS. Most
methods of manipulating the point at which inbound traffic enters are
either ineffective, ineffective or not accepted in the peering community.

Inter-domain TE with BGP is generally effective, but it is usually
applied in a trial-and-error fashion because a TE system usually only
has a view of the available network resources within one domain (an
AS in this case). A systematic approach for inter-domain TE requires
cooperation between the domains. Further, what may be considered a
good solution in one domain may not necessarily be a good solution in
another. Moreover, it is generally considered inadvisable for one
domain to permit a control process from another domain to influence
the routing and management of traffic in its network.

MPLS TE-tunnels

MPLS-TE tunnels (LSPs) can add a degree of flexibility in the
selection of exit points for inter-domain routing by applying the
concept of relative and absolute metrics. If BGP attributes are
defined such that the BGP decision process depends on IGP metrics to
select exit points for inter-domain traffic, then some inter-domain
traffic destined to a given peer network can be made to prefer a
specific exit point by establishing a TE-tunnel TE tunnel between the router
making the selection and the peering point via a TE-tunnel TE tunnel and
assigning the TE-tunnel TE tunnel a metric which that is smaller than the IGP cost to
all other peering points. RSVP-TE protocol extensions for inter-
domain MPLS and GMPLS are described in [RFC5151].

Similarly to intra-domain TE, inter-domain TE is best accomplished
when a traffic matrix can be derived to depict the volume of traffic
from one AS to another.

Layer 4 multipath transport protocols are designed to move traffic
between domains and to allow some influence over the selection of the
paths. To be truly effective, these protocols would require
visibility of paths and network conditions in other domains, and but that
information may not be available, might not be complete, and is not
necessarily trustworthy.

8. Overview of Contemporary TE Practices in Operational IP Networks

This section provides an overview of some TE practices in IP
networks. The focus is on aspects of control of the routing function
in operational contexts. The intent here is to provide an overview
of the commonly used practices: practices; the discussion is not intended to be
exhaustive.

Service providers apply many of the TE mechanisms described in this
document to optimize the performance of their IP networks, although
others choose to not use any of them. These techniques include
capacity planning planning, including adding ECMP options options, for long
timescales; routing control using IGP metrics and MPLS, as well as
path planning and path control using MPLS and Segment Routing SR for medium
timescales; and traffic management mechanisms for short timescale. timescales.

* Capacity planning is an important component of how a service
provider plans an effective IP network. These plans may take the
following aspects into account: location of any new links or
nodes, WECMP algorithms, existing and predicted traffic patterns,
costs, link capacity, topology, routing design, and survivability.

* Performance optimization of operational networks is usually an
ongoing process in which traffic statistics, performance
parameters, and fault indicators are continually collected from
the network. This empirical data is analyzed and used to trigger
TE mechanisms. Tools that perform what-if analysis can also be
used to assist the TE process by reviewing scenarios before a new
set of configurations are implemented in the operational network.

* Real-time intra-domain TE using the IGP is done by increasing the
OSPF or IS-IS metric of a congested link until enough traffic has
been diverted away from that link. This approach has some
limitations as discussed in Section 6.2. Intra-domain TE
approaches ([RR94] [RR94] [FT00] [FT01] [WANG]) [WANG] take traffic matrix,
network topology, and network performance objectives as input, input and
produce link metrics and load-sharing ratios. These processes
open the possibility for intra-domain TE with IGP to be done in a
more systematic way.

Administrators of MPLS-TE networks specify and configure link
attributes and resource constraints such as maximum reservable
bandwidth and resource class attributes for the links in the domain.
A link state link-state IGP that supports TE extensions (IS-IS-TE or OSPF-TE) is
used to propagate information about network topology and link
attributes to all routers in the domain. Network administrators
specify the LSPs that are to originate at each router. For each LSP,
the network administrator specifies the destination node and the
attributes of the LSP which that indicate the requirements that are to be
satisfied during the path selection process. The attributes may
include an explicit path for the LSP to follow, or the originating
router may use a local constraint-based routing process to compute
the path of the LSP. RSVP-TE is used as a signaling protocol to
instantiate the LSPs. By assigning proper bandwidth values to links
and LSPs, congestion caused by uneven traffic distribution can be
avoided or mitigated.

The bandwidth attributes of an LSP relate to the bandwidth
requirements of traffic that flows through the LSP. The traffic
attribute of an LSP can be modified to accommodate persistent shifts
in demand (traffic growth or reduction). If network congestion
occurs due to unexpected events, existing LSPs can be rerouted to
alleviate the situation, or the network administrator can configure
new LSPs to divert some traffic to alternative paths. The reservable
bandwidth of the congested links can also be reduced to force some
LSPs to be rerouted to other paths. A traffic matrix in an MPLS
domain can also be estimated by monitoring the traffic on LSPs. Such
traffic statistics can be used for a variety of purposes including
network planning and network optimization.

Network management and planning systems have evolved and assumed a
lot of the responsibility for determining traffic paths in TE
networks. This allows a network-wide view of resources, resources and
facilitates coordination of the use of resources for all traffic
flows in the network. Initial solutions using a PCE to perform path
computation on behalf of network routers have given way to an
approach that follows the SDN architecture. A stateful PCE is able
to track all of the LSPs in the network and can redistribute them to
make better use of the available resources. Such a PCE can form part
of a network orchestrator that uses PCEP or some other configuration
and management interface to instruct the signaling protocol or
directly program the routers.

Segment Routing

SR leverages a centralized TE controller and either an MPLS or IPv6
forwarding plane, plane but does not need to use a signaling protocol or
management plane protocol to reserve resources in the routers. All
resource reservation is logical within the controller, controller and is not
distributed to the routers. Packets are steered through the network
using Segment Routing, SR, and this may have configuration and operational scaling
benefits.

As mentioned in Section 7, there is usually no direct control over
the distribution of inbound traffic to a domain. Therefore, the main
goal of inter-domain TE is to optimize the distribution of outbound
traffic between multiple inter-domain links. When operating a
geographically widespread network (such as for a multi-national or
global network provider), maintaining the ability to operate the
network in a regional fashion where desired, while continuing to take
advantage of the benefits of a globally interconnected network, also
becomes an important objective.

Inter-domain TE with BGP begins with the placement of multiple
peering interconnection points that are in close proximity to traffic
sources/destination,
sources/destinations and offer lowest-cost paths across the network
between the peering points and the sources/destinations. Some
location-decision problems that arise in association with inter-
domain routing are discussed in [AWD5].

Once the locations of the peering interconnects have been determined
and implemented, the network operator decides how best to handle the
routes advertised by the peer, as well as how to propagate the peer's
routes within their network. One way to engineer outbound traffic
flows in a network with many peering interconnects is to create a
hierarchy of peers. Generally, the shortest AS paths will be chosen
to forward traffic traffic, but BGP metrics can be used to prefer some peers
and so favor particular paths. Preferred peers are those peers
attached through peering interconnects with the most available
capacity. Changes may be needed, for example, to deal with a
"problem peer" who is difficult to work with on upgrades or is
charging high prices for connectivity to their network. In that
case, the peer may be given a reduced preference. This type of
change can affect a large amount of traffic, traffic and is only used after
other methods have failed to provide the desired results.

When there are multiple exit points toward a given peer, and only one
of them is congested, it is not necessary to shift traffic away from
the peer entirely, but only from the one congested connections. connection. This
can be achieved by using passive IGP metrics, AS_PATH filtering, or
prefix filtering.

9. Security Considerations

In general, TE mechanisms are security-neutral, security neutral, and this document
does not introduce new security issues.

Network security is, of course, an important issue, and TE mechanisms
can have benefits and drawbacks. drawbacks:

* TE may use tunnels which that can slightly help protect traffic from
inspection and which, that, in some cases, can be secured using
encryption.

* TE puts traffic onto predictable paths within the network that may
make it easier to find and attack.

* TE often increases the complexity of operation and management of
the network network, which may lead to errors that compromise security.

* TE enables traffic to be steered onto more secure links or to more
secure parts of the network.

* TE can be used to steer traffic through network nodes that are
able to provide additional security functions.

The consequences of attacks on the control and management protocols
used to operate TE networks can be significant: traffic

* Traffic can be hijacked to pass through specific nodes that
perform inspection, inspection or even to be delivered to the wrong place; traffic place.

* Traffic can be steered onto paths that deliver quality that is
below the desired quality; and,
networks quality.

* Networks can be congested or have resources on key links consumed.

Thus, it is important to use adequate protection mechanisms, such as
authentication, on all protocols used to deliver TE.

Certain aspects of a network may be deduced from the details of the
TE paths that are used. For example, the link connectivity of the
network,
network and the quality and load on individual links may be inferred
from knowing the paths of traffic and the requirements they place on
the network (for example, by seeing the control messages or through
path- trace
path-trace techniques). Such knowledge can be used to launch
targeted attacks (for example, taking down critical links) or can
reveal commercially sensitive information (for example, whether a
network is close to capacity). Network Therefore, network operators may, therefore, may
choose techniques that mask or hide information from within the
network.

External control interfaces that are introduced to provide additional
control and management of TE systems (see Section 5.1.2) provide
flexibility to management and to customers, but they do so at the
risk of exposing the internals of a network to potentially malicious
actors. The protocols used at these interfaces must be secured to
protect against snooping and modification, and use of the interfaces
must be authenticated.

10. IANA Considerations

This draft makes document has no requests for IANA action. actions.

11. Acknowledgments

Much of the text in this document is derived from RFC 3272. The
editor and contributors to this document would like to express their
gratitude to all involved in that work. Although the source text has
been edited in the production of this document, the original authors
should be considered as Contributors to this work. They were:

Daniel O. Awduche
Movaz Networks

Angela Chiu
Celion Networks

Anwar Elwalid
Lucent Technologies

Indra Widjaja
Bell Labs, Lucent Technologies

XiPeng Xiao
Redback Networks

The acknowledgements in RFC3272 were as below. All people who helped
in the production of that document also need to be thanked for the
carry-over into this new document.

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

The early versions of this document were produced by the TEAS Working
Group's RFC3272bis Design Team. The full list of members of this
team is:

Acee Lindem
Adrian Farrel
Aijun Wang
Daniele Ceccarelli
Dieter Beller
Jeff Tantsura
Julien Meuric
Liu Hua
Loa Andersson
Luis Miguel Contreras
Martin Horneffer
Tarek Saad
Xufeng Liu

The production of this document includes a fix to the original text
resulting from an Errata Report by Jean-Michel Grimaldi.

The editor of this document would also like to thank Dhruv Dhody,
Gyan Mishra, Joel Halpern, Dave Taht, John Scudder, Rich Salz, Behcet
Sarikaya, Bob Briscoe, Erik Kline, Jim Guichard, Martin Duke, and
Roman Danyliw, for review comments.

This work is partially supported by the European Commission under
Horizon 2020 grant agreement number 101015857 Secured autonomic
traffic management for a Tera of SDN flows (Teraflow).

12. Contributors

The following people contributed substantive text to this document:

Gert Grammel
EMail: ggrammel@juniper.net

Loa Andersson
EMail: loa@pi.nu

Xufeng Liu
EMail: xufeng.liu.ietf@gmail.com

Lou Berger
EMail: lberger@labn.net

Jeff Tantsura
EMail: jefftant.ietf@gmail.com

Daniel King
EMail: daniel@olddog.co.uk

Boris Hassanov
EMail: bhassanov@yandex-team.ru

Kiran Makhijani
Email: kiranm@futurewei.com

Dhruv Dhody
Email: dhruv.ietf@gmail.com

Mohamed Boucadair
Email: mohamed.boucadair@orange.com

13. Informative References

[AFD03] Pan, R., Breslau, L., Prabhakar, B., and S. Shenker,
"Approximate fairness through differential dropping",
Article ACM
SIGCOMM Computer Communication Review, Volume 33, Issue 2, April 2003, pp
Pages 23-39, DOI 10.1145/956981.956985, April 2003,
<https://dl.acm.org/doi/10.1145/956981.956985>.

[AJ19] Adekitan, A., Abolade, J., and O. Shobayo, "Data mining
approach for predicting the daily Internet data traffic of
a smart university", Article Journal of Big Data, 2019, Volume 6, Number
1, Page 1, 1998, DOI 10.1186/s40537-019-0176-5, February 2019,
<https://journalofbigdata.springeropen.com/track/
pdf/10.1186/s40537-019-0176-5.pdf>.

[ATSSS] 3GPP, "Study on access traffic steering, switch and
splitting support in the 5G System (5GS) architecture",
Specification 3GPP Technical Report 23.793
Release 16, 3GPP TR 23.793, December 2018,
<https://www.3gpp.org/ftp//Specs/
archive/23_series/23.793/23793-g00.zip>.

[AWD2] Awduche, D., "MPLS and Traffic Engineering traffic engineering in IP
Networks", Article
networks", IEEE Communications Magazine, Volume 37, Issue
12, Pages 42-47, DOI 10.1109/35.809383, December 1999,
<https://ieeexplore.ieee.org/document/809383>.

[AWD5] Awduche, D., "An Approach approach to Optimal Peering Between
Autonomous Systems optimal peering between
autonomous systems in the Internet", Paper Proceedings 7th
International Conference on Computer Communications and
Networks
(ICCCN'98), (Cat. No. 98EX226),
DOI 10.1109/ICCCN.1998.998795, October 1998,
<https://ieeexplore.ieee.org/document/998795>.

[E.360.1] International Telecommunication Union - Telecommunication
Standardization Sector, "E.360.1: Framework ITU-T, "Framework for QoS routing and related traffic
engineering methods for IP-, ATM-, and TDM-based
multiservice networks",
Recommendation ITU-T Recommendation E.360.1, 16 May
2002, <https://www.itu.int/rec/T-REC-E.360.1-200205-I/en>.

[ENHANCED-VPN]
Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
Framework for NRP-based Enhanced Virtual Private Network",
Work in Progress, Internet-Draft, draft-ietf-teas-
enhanced-vpn-17, 25 December 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
enhanced-vpn-17>.

[Err309] RFC Errata, Erratum ID 309, RFC 3272,
<https://www.rfc-editor.org/errata/eid309>.

[EVPN-UNEQUAL-LB]
Malhotra, N., Ed., Sajassi, A., Rabadan, J., Drake, J.,
Lingala, A., and S. Thoria, "Weighted Multi-Path
Procedures for EVPN Multi-Homing", Work in Progress,
December 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-bess-evpn-unequal-lb-21>.

[FLJA93] Floyd, S. and V. Jacobson, "Random Early Detection
Gateways for Congestion Avoidance", Article IEEE/ACM Transactions
on Networking, Vol. Volume 1, p. 387-413, November Issue 4, Pages 397-413,
DOI 10.1109/90.251892, August 1993,
<https://www.icir.org/floyd/papers/early.twocolumn.pdf>.

[FT00] Fortz, B. and M. Thorup, "Internet Traffic Engineering by
Optimizing OSPF Weights", Article Proceedings IEEE INFOCOM 2000,
DOI 10.1109/INFCOM.2000.832225, March 2000,
<https://www.cs.cornell.edu/courses/cs619/2004fa/
documents/ospf_opt.pdf>.

[FT01] Fortz, B. and M. Thorup, "Optimizing OSPF/IS-IS Weights in
a Changing World", n.d.,
<http://www.research.att.com/~mthorup/PAPERS/papers.html>. IEEE Journal on Selected Areas in
Communications, DOI 10.1109/JSAC.2002.1003042, May 2002,
<https://ieeexplore.ieee.org/document/1003042>.

[GRPC] Cloud Native Computing Foundation, "gPPC: gRPC Authors, "gRPC: A high performance, open source
universal RPC framework", n.d., <https://grpc.io>.

[I-D.ietf-bess-evpn-unequal-lb]
Malhotra, N., Sajassi,

[KELLY] Kelly, F., "Notes on effective bandwidths", Oxford
University Press, 1996.

[MA] Ma, Q., "Quality-of-Service Routing in Integrated Services
Networks", Ph.D. Dissertation, Carnegie Mellon University,
CMU-CS-98-138, January 1998,
<https://apps.dtic.mil/sti/pdfs/ADA352299.pdf>.

[MATE] Elwalid, A., Rabadan, J., Drake, J.,
Lingala, Jin, C., Low, S., and I. Widjaja, "MATE: MPLS
Adaptive Traffic Engineering", Proceedings IEEE INFOCOM
2001, Conference on Computer Communications, Twentieth
Annual Joint Conference of the IEEE Computer and
Communications Society (Cat. No. 01CH37213),
DOI 10.1109/INFCOM.2001.916625, August 2002,
<https://www.yumpu.com/en/document/view/35140398/mate-
mpls-adaptive-traffic-engineering-infocom-ieee-xplore/8>.

[MR99] Mitra, D. and K.G. Ramakrishnan, "A case study of
multiservice, multipriority traffic engineering design for
data networks", Seamless Interconnection for Universal
Services, Global Telecommunications Conference,
GLOBECOM'99, (Cat. No. 99CH37042),
DOI 10.1109/GLOCOM.1999.830281, December 1999,
<https://ieeexplore.ieee.org/document/830281>.

[MULTIPATH-DCCP]
Amend, M., Ed., Brunstrom, A., Kassler, A., Rakocevic, V.,
and S. Thoria, "Weighted Multi-Path
Procedures Johnson, "DCCP Extensions for EVPN Multi-Homing", Multipath Operation
with Multiple Addresses", Work in Progress,
Internet-Draft, draft-ietf-bess-evpn-unequal-lb-18, 1 June Internet-
Draft, draft-ietf-tsvwg-multipath-dccp-11, 12 October
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
bess-evpn-unequal-lb-18>.

[I-D.ietf-idr-performance-routing]
tsvwg-multipath-dccp-11>.

[NETWORK-SLICES]
Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S.,
Makhijani, K., Contreras, L. M., and J. Tantsura, "A
Framework for Network Slices in Networks Built from IETF
Technologies", Work in Progress, Internet-Draft, draft-
ietf-teas-ietf-network-slices-25, 14 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
ietf-network-slices-25>.

[PERFORMANCE-ROUTING]
Xu, X., Hegde, S., Talaulikar, K., Boucadair, M., and C.
Jacquenet, "Performance-based BGP Routing Mechanism", Work
in Progress, Internet-Draft, draft-ietf-idr-performance-
routing-03, 22 December 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
performance-routing-03>.

[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-segment-
routing-te-policy-23, 25 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
segment-routing-te-policy-23>.

[I-D.ietf-lsr-ip-flexalgo]
Britto, W., Hegde, S., Kaneriya, P., Shetty, R., Bonica,
R., and P. Psenak, "IGP Flexible Algorithms (Flex-
Algorithm) In IP Networks", Work in Progress, Internet-
Draft, draft-ietf-lsr-ip-flexalgo-17, 24 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-
flexalgo-17>.

[I-D.ietf-quic-multipath]

[QUIC-MULTIPATH]
Liu, Y., Ed., Ma, Y., Ed., De Coninck, Q., Ed.,
Bonaventure, O., Huitema, C., and M. Kühlewind, Ed.,
"Multipath Extension for QUIC", Work in Progress,
Internet-Draft, draft-ietf-quic-multipath-05,
10 July 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-quic-multipath-05>.

[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
11, 30 June 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-rtgwg-segment-routing-ti-lfa-11>.

[I-D.ietf-teas-enhanced-vpn]
Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
Framework for Enhanced Virtual Private Network (VPN+)",
Work in Progress, Internet-Draft, draft-ietf-teas-
enhanced-vpn-14, 28 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
enhanced-vpn-14>.

[I-D.ietf-teas-ietf-network-slices]
Farrel, A., Drake, J., Rokui, R., Homma, S., Makhijani,
K., Contreras, L. M., and J. Tantsura, "A Framework for
IETF Network Slices", Work in Progress, Internet-Draft,
draft-ietf-teas-ietf-network-slices-22, 24 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
ietf-network-slices-22>.

[I-D.ietf-tewg-qos-routing]
Ash, G., "Traffic Engineering & QoS Methods for IP-, ATM-,
& Based Multiservice Networks", Work in Progress,
Internet-Draft, draft-ietf-tewg-qos-routing-04, 15 draft-ietf-quic-multipath-06, 23 October
2001, <https://datatracker.ietf.org/doc/html/draft-ietf-
tewg-qos-routing-04>.

[I-D.ietf-tsvwg-multipath-dccp]
Amend, M., Brunstrom, A., Kassler, A., Rakocevic, V., and
S. Johnson, "DCCP Extensions for Multipath Operation with
Multiple Addresses", Work in Progress, Internet-Draft,
draft-ietf-tsvwg-multipath-dccp-10, 26 July
2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
multipath-dccp-10>.

[KELLY] Kelly, F., "Notes on effective bandwidths. In Stochastic
Networks: Theory and Applications", Book Oxford University
Press, 1996.

[MA] Ma, Q., "Quality of Service Routing in Integrated Services
Networks", Ph.D. PhD Dissertation, CMU-CS-98-138, CMU,
1998, <https://apps.dtic.mil/sti/pdfs/ADA352299.pdf>.

[MATE] Elwalid, A., Jin, C., Low, S., and I. Widjaja, "MATE -
MPLS Adaptive Traffic Engineering",
Proceedings INFOCOM'01, April 2001,
<https://www.yumpu.com/en/document/view/35140398/mate-
mpls-adaptive-traffic-engineering-infocom-ieee-xplore/8>.

[MR99] Mitra, D. and K.G. Ramakrishnan, "A Case Study of
Multiservice, Multipriority Traffic Engineering Design for
Data Networks", Proceedings Globecom'99, December 1999,
<https://ieeexplore.ieee.org/document/830281>. <https://datatracker.ietf.org/doc/html/draft-ietf-
quic-multipath-06>.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC1102] Clark, D., "Policy routing in Internet protocols",
RFC 1102, DOI 10.17487/RFC1102, May 1989,
<https://www.rfc-editor.org/info/rfc1102>.

[RFC1104] Braun, H., "Models of policy based routing", RFC 1104,
DOI 10.17487/RFC1104, June 1989,
<https://www.rfc-editor.org/info/rfc1104>.

[RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, DOI 10.17487/RFC2205,
September 1997, <https://www.rfc-editor.org/info/rfc2205>.

[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330,
DOI 10.17487/RFC2330, May 1998,
<https://www.rfc-editor.org/info/rfc2330>.

[RFC2386] Crawley, E., Nair, R., Rajagopalan, B., and H. Sandick, "A
Framework for QoS-based Routing in the Internet",
RFC 2386, DOI 10.17487/RFC2386, August 1998,
<https://www.rfc-editor.org/info/rfc2386>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.

[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.

[RFC2678] Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring
Connectivity", RFC 2678, DOI 10.17487/RFC2678, September
1999, <https://www.rfc-editor.org/info/rfc2678>.

[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, DOI 10.17487/RFC2702, September 1999,
<https://www.rfc-editor.org/info/rfc2702>.

[RFC2722] Brownlee, N., Mills, C., and G. Ruth, "Traffic Flow
Measurement: Architecture", RFC 2722,
DOI 10.17487/RFC2722, October 1999,
<https://www.rfc-editor.org/info/rfc2722>.

[RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
for Policy-based Admission Control", RFC 2753,
DOI 10.17487/RFC2753, January 2000,
<https://www.rfc-editor.org/info/rfc2753>.

[RFC2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
and S. Molendini, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, DOI 10.17487/RFC2961, April 2001,
<https://www.rfc-editor.org/info/rfc2961>.

[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
Felstaine, "A Framework for Integrated Services Operation
over Diffserv Networks", RFC 2998, DOI 10.17487/RFC2998,
November 2000, <https://www.rfc-editor.org/info/rfc2998>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.

[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, DOI 10.17487/RFC3086,
April 2001, <https://www.rfc-editor.org/info/rfc3086>.

[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
RFC 3124, DOI 10.17487/RFC3124, June 2001,
<https://www.rfc-editor.org/info/rfc3124>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.

[RFC3175] Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
"Aggregation of RSVP for IPv4 and IPv6 Reservations",
RFC 3175, DOI 10.17487/RFC3175, September 2001,
<https://www.rfc-editor.org/info/rfc3175>.

[RFC3198] Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
J., and S. Waldbusser, "Terminology for Policy-Based
Management", RFC 3198, DOI 10.17487/RFC3198, November
2001, <https://www.rfc-editor.org/info/rfc3198>.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.

[RFC3270] Le Faucheur, F., Ed., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen,
"Multi-Protocol Label Switching (MPLS) Support of
Differentiated Services", RFC 3270, DOI 10.17487/RFC3270,
May 2002, <https://www.rfc-editor.org/info/rfc3270>.

[RFC3272] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X.
Xiao, "Overview and Principles of Internet Traffic
Engineering", RFC 3272, DOI 10.17487/RFC3272, May 2002,
<https://www.rfc-editor.org/info/rfc3272>.

[RFC3469] Sharma, V., Ed. and F. Hellstrand, Ed., "Framework for
Multi-Protocol Label Switching (MPLS)-based Recovery",
RFC 3469, DOI 10.17487/RFC3469, February 2003,
<https://www.rfc-editor.org/info/rfc3469>.

[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
DOI 10.17487/RFC3473, January 2003,
<https://www.rfc-editor.org/info/rfc3473>.

[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.

[RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945,
DOI 10.17487/RFC3945, October 2004,
<https://www.rfc-editor.org/info/rfc3945>.

[RFC4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
DOI 10.17487/RFC4090, May 2005,
<https://www.rfc-editor.org/info/rfc4090>.

[RFC4124] Le Faucheur, F., Ed., "Protocol Extensions for Support of
Diffserv-aware MPLS Traffic Engineering", RFC 4124,
DOI 10.17487/RFC4124, June 2005,
<https://www.rfc-editor.org/info/rfc4124>.

[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.

[RFC4461] Yasukawa, S., Ed., "Signaling Requirements for Point-to-
Multipoint Traffic-Engineered MPLS Label Switched Paths
(LSPs)", RFC 4461, DOI 10.17487/RFC4461, April 2006,
<https://www.rfc-editor.org/info/rfc4461>.

[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.

[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.

[RFC4872] Lang, J.P., Ed., Rekhter, Y., Ed., and D. Papadimitriou,
Ed., "RSVP-TE Extensions in Support of End-to-End
Generalized Multi-Protocol Label Switching (GMPLS)
Recovery", RFC 4872, DOI 10.17487/RFC4872, May 2007,
<https://www.rfc-editor.org/info/rfc4872>.

[RFC4873] Berger, L., Bryskin, I., Papadimitriou, D., and A. Farrel,
"GMPLS Segment Recovery", RFC 4873, DOI 10.17487/RFC4873,
May 2007, <https://www.rfc-editor.org/info/rfc4873>.

[RFC4875] Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
Yasukawa, Ed., "Extensions to Resource Reservation
Protocol - Traffic Engineering (RSVP-TE) for Point-to-
Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
DOI 10.17487/RFC4875, May 2007,
<https://www.rfc-editor.org/info/rfc4875>.

[RFC5151] Farrel, A., Ed., Ayyangar, A., and JP. Vasseur, "Inter-
Domain MPLS and GMPLS Traffic Engineering -- Resource
Reservation Protocol-Traffic Engineering (RSVP-TE)
Extensions", RFC 5151, DOI 10.17487/RFC5151, February
2008, <https://www.rfc-editor.org/info/rfc5151>.

[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <https://www.rfc-editor.org/info/rfc5250>.

[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.

[RFC5329] Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
"Traffic Engineering Extensions to OSPF Version 3",
RFC 5329, DOI 10.17487/RFC5329, September 2008,
<https://www.rfc-editor.org/info/rfc5329>.

[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC5331, August 2008,
<https://www.rfc-editor.org/info/rfc5331>.

[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<https://www.rfc-editor.org/info/rfc5357>.

[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
DOI 10.17487/RFC5394, December 2008,
<https://www.rfc-editor.org/info/rfc5394>.

[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.

[RFC5470] Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek,
"Architecture for IP Flow Information Export", RFC 5470,
DOI 10.17487/RFC5470, March 2009,
<https://www.rfc-editor.org/info/rfc5470>.

[RFC5472] Zseby, T., Boschi, E., Brownlee, N., and B. Claise, "IP
Flow Information Export (IPFIX) Applicability", RFC 5472,
DOI 10.17487/RFC5472, March 2009,
<https://www.rfc-editor.org/info/rfc5472>.

[RFC5541] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
Objective Functions in the Path Computation Element
Communication Protocol (PCEP)", RFC 5541,
DOI 10.17487/RFC5541, June 2009,
<https://www.rfc-editor.org/info/rfc5541>.

[RFC5557] Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
Computation Element Communication Protocol (PCEP)
Requirements and Protocol Extensions in Support of Global
Concurrent Optimization", RFC 5557, DOI 10.17487/RFC5557,
July 2009, <https://www.rfc-editor.org/info/rfc5557>.

[RFC5559] Eardley, P., Ed., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, DOI 10.17487/RFC5559, June 2009,
<https://www.rfc-editor.org/info/rfc5559>.

[RFC5623] Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
"Framework for PCE-Based Inter-Layer MPLS and GMPLS
Traffic Engineering", RFC 5623, DOI 10.17487/RFC5623,
September 2009, <https://www.rfc-editor.org/info/rfc5623>.

[RFC5664] Halevy, B., Welch, B., and J. Zelenka, "Object-Based
Parallel NFS (pNFS) Operations", RFC 5664,
DOI 10.17487/RFC5664, January 2010,
<https://www.rfc-editor.org/info/rfc5664>.

[RFC5671] Yasukawa, S. and A. Farrel, Ed., "Applicability of the
Path Computation Element (PCE) to Point-to-Multipoint
(P2MP) MPLS and GMPLS Traffic Engineering (TE)", RFC 5671,
DOI 10.17487/RFC5671, October 2009,
<https://www.rfc-editor.org/info/rfc5671>.

[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
DOI 10.17487/RFC5693, October 2009,
<https://www.rfc-editor.org/info/rfc5693>.

[RFC6107] Shiomoto, K., Ed. and A. Farrel, Ed., "Procedures for
Dynamically Signaled Hierarchical Label Switched Paths",
RFC 6107, DOI 10.17487/RFC6107, February 2011,
<https://www.rfc-editor.org/info/rfc6107>.

[RFC6119] Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
February 2011, <https://www.rfc-editor.org/info/rfc6119>.

[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.

[RFC6372] Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
Profile (MPLS-TP) Survivability Framework", RFC 6372,
DOI 10.17487/RFC6372, September 2011,
<https://www.rfc-editor.org/info/rfc6372>.

[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011,
<https://www.rfc-editor.org/info/rfc6374>.

[RFC6601] Ash, G., Ed. and D. McDysan, "Generic Connection Admission
Control (GCAC) Algorithm Specification for IP/MPLS
Networks", RFC 6601, DOI 10.17487/RFC6601, April 2012,
<https://www.rfc-editor.org/info/rfc6601>.

[RFC6805] King, D., Ed. and A. Farrel, Ed., "The Application of the
Path Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
DOI 10.17487/RFC6805, November 2012,
<https://www.rfc-editor.org/info/rfc6805>.

[RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
"Specification of the IP Flow Information Export (IPFIX)
Protocol for the Exchange of Flow Information", STD 77,
RFC 7011, DOI 10.17487/RFC7011, September 2013,
<https://www.rfc-editor.org/info/rfc7011>.

[RFC7149] Boucadair, M. and C. Jacquenet, "Software-Defined
Networking: A Perspective from within a Service Provider
Environment", RFC 7149, DOI 10.17487/RFC7149, March 2014,
<https://www.rfc-editor.org/info/rfc7149>.

[RFC7285] Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
"Application-Layer Traffic Optimization (ALTO) Protocol",
RFC 7285, DOI 10.17487/RFC7285, September 2014,
<https://www.rfc-editor.org/info/rfc7285>.

[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<https://www.rfc-editor.org/info/rfc7390>.

[RFC7426] Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
Defined Networking (SDN): Layers and Architecture
Terminology", RFC 7426, DOI 10.17487/RFC7426, January
2015, <https://www.rfc-editor.org/info/rfc7426>.

[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.

[RFC7491] King, D. and A. Farrel, "A PCE-Based Architecture for
Application-Based Network Operations", RFC 7491,
DOI 10.17487/RFC7491, March 2015,
<https://www.rfc-editor.org/info/rfc7491>.

[RFC7551] Zhang, F., Ed., Jing, R., and R. Gandhi, Ed., "RSVP-TE
Extensions for Associated Bidirectional Label Switched
Paths (LSPs)", RFC 7551, DOI 10.17487/RFC7551, May 2015,
<https://www.rfc-editor.org/info/rfc7551>.

[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.

[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.

[RFC7679] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Delay Metric for IP Performance Metrics
(IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
2016, <https://www.rfc-editor.org/info/rfc7679>.

[RFC7680] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Loss Metric for IP Performance Metrics
(IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
2016, <https://www.rfc-editor.org/info/rfc7680>.

[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.

[RFC7923] Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
for Subscription to YANG Datastores", RFC 7923,
DOI 10.17487/RFC7923, June 2016,
<https://www.rfc-editor.org/info/rfc7923>.

[RFC7926] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
Ceccarelli, D., and X. Zhang, "Problem Statement and
Architecture for Information Exchange between
Interconnected Traffic-Engineered Networks", BCP 206,
RFC 7926, DOI 10.17487/RFC7926, July 2016,
<https://www.rfc-editor.org/info/rfc7926>.

[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.

[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<https://www.rfc-editor.org/info/rfc8033>.

[RFC8034] White, G. and R. Pan, "Active Queue Management (AQM) Based
on Proportional Integral Controller Enhanced (PIE) for
Data-Over-Cable Service Interface Specifications (DOCSIS)
Cable Modems", RFC 8034, DOI 10.17487/RFC8034, February
2017, <https://www.rfc-editor.org/info/rfc8034>.

[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.

[RFC8051] Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
Stateful Path Computation Element (PCE)", RFC 8051,
DOI 10.17487/RFC8051, January 2017,
<https://www.rfc-editor.org/info/rfc8051>.

[RFC8189] Randriamasy, S., Roome, W., and N. Schwan, "Multi-Cost
Application-Layer Traffic Optimization (ALTO)", RFC 8189,
DOI 10.17487/RFC8189, October 2017,
<https://www.rfc-editor.org/info/rfc8189>.

[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.

[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.

[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.

[RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for PCE-Initiated LSP Setup in a Stateful PCE
Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
<https://www.rfc-editor.org/info/rfc8281>.

[RFC8283] Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An
Architecture for Use of PCE and the PCE Communication
Protocol (PCEP) in a Network with Central Control",
RFC 8283, DOI 10.17487/RFC8283, December 2017,
<https://www.rfc-editor.org/info/rfc8283>.

[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>.

[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.

[RFC8453] Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
Abstraction and Control of TE Networks (ACTN)", RFC 8453,
DOI 10.17487/RFC8453, August 2018,
<https://www.rfc-editor.org/info/rfc8453>.

[RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
2019, <https://www.rfc-editor.org/info/rfc8570>.

[RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
IGP Traffic Engineering Performance Metric Extensions",
RFC 8571, DOI 10.17487/RFC8571, March 2019,
<https://www.rfc-editor.org/info/rfc8571>.

[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.

[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.

[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.

[RFC8685] Zhang, F., Zhao, Q., Gonzalez de Dios, O., Casellas, R.,
and D. King, "Path Computation Element Communication
Protocol (PCEP) Extensions for the Hierarchical Path
Computation Element (H-PCE) Architecture", RFC 8685,
DOI 10.17487/RFC8685, December 2019,
<https://www.rfc-editor.org/info/rfc8685>.

[RFC8795] Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
O. Gonzalez de Dios, "YANG Data Model for Traffic
Engineering (TE) Topologies", RFC 8795,
DOI 10.17487/RFC8795, August 2020,
<https://www.rfc-editor.org/info/rfc8795>.

[RFC8803] Bonaventure, O., Ed., Boucadair, M., Ed., Gundavelli, S.,
Seo, S., and B. Hesmans, "0-RTT TCP Convert Protocol",
RFC 8803, DOI 10.17487/RFC8803, July 2020,
<https://www.rfc-editor.org/info/rfc8803>.

[RFC8896] Randriamasy, S., Yang, R., Wu, Q., Deng, L., and N.
Schwan, "Application-Layer Traffic Optimization (ALTO)
Cost Calendar", RFC 8896, DOI 10.17487/RFC8896, November
2020, <https://www.rfc-editor.org/info/rfc8896>.

[RFC8938] Varga, B., Ed., Farkas, J., Berger, L., Malis, A., and S.
Bryant, "Deterministic Networking (DetNet) Data Plane
Framework", RFC 8938, DOI 10.17487/RFC8938, November 2020,
<https://www.rfc-editor.org/info/rfc8938>.

[RFC8955] Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
Bacher, "Dissemination of Flow Specification Rules",
RFC 8955, DOI 10.17487/RFC8955, December 2020,
<https://www.rfc-editor.org/info/rfc8955>.

[RFC8972] Mirsky, G., Min, X., Nydell, H., Foote, R., Masputra, A.,
and E. Ruffini, "Simple Two-Way Active Measurement
Protocol Optional Extensions", RFC 8972,
DOI 10.17487/RFC8972, January 2021,
<https://www.rfc-editor.org/info/rfc8972>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[RFC9023] Varga, B., Ed., Farkas, J., Malis, A., and S. Bryant,
"Deterministic Networking (DetNet) Data Plane: IP over
IEEE 802.1 Time-Sensitive Networking (TSN)", RFC 9023,
DOI 10.17487/RFC9023, June 2021,
<https://www.rfc-editor.org/info/rfc9023>.

[RFC9040] Touch, J., Welzl, M., and S. Islam, "TCP Control Block
Interdependence", RFC 9040, DOI 10.17487/RFC9040, July
2021, <https://www.rfc-editor.org/info/rfc9040>.

[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.

[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.

[RFC9262] Eckert, T., Ed., Menth, M., and G. Cauchie, "Tree
Engineering for Bit Index Explicit Replication (BIER-TE)",
RFC 9262, DOI 10.17487/RFC9262, October 2022,
<https://www.rfc-editor.org/info/rfc9262>.

[RFC9298] Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
DOI 10.17487/RFC9298, August 2022,
<https://www.rfc-editor.org/info/rfc9298>.

[RFC9315] Clemm, A., Ciavaglia, L., Granville, L. Z., and J.
Tantsura, "Intent-Based Networking - Concepts and
Definitions", RFC 9315, DOI 10.17487/RFC9315, October
2022, <https://www.rfc-editor.org/info/rfc9315>.

[RFC9332] De Schepper, K., Briscoe, B., Ed., and G. White, "Dual-
Queue Coupled Active Queue Management (AQM) for Low
Latency, Low Loss, and Scalable Throughput (L4S)",
RFC 9332, DOI 10.17487/RFC9332, January 2023,
<https://www.rfc-editor.org/info/rfc9332>.

[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.

[RFC9439] Wu, Q., Yang, Y., Lee, Y., Dhody, D., Randriamasy, S., and
L. Contreras, "Application-Layer Traffic Optimization
(ALTO) Performance Cost Metrics", RFC 9439,
DOI 10.17487/RFC9439, August 2023,
<https://www.rfc-editor.org/info/rfc9439>.

[RFC9502] Britto, W., Hegde, S., Kaneriya, P., Shetty, R., Bonica,
R., and P. Psenak, "IGP Flexible Algorithm in IP
Networks", RFC 9502, DOI 10.17487/RFC9502, November 2023,
<https://www.rfc-editor.org/info/rfc9502>.

[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.

[RR94] Rodrigues, M.A. M. and K.G. Ramakrishnan, "Optimal Routing routing in
Shortest Path Data Networks", Proceedings ITS'94, Rio de
Janeiro, Brazil, 1994,
shortest-path data networks", Bell Labs Technical Journal,
Volume 6, Issue 1, Pages 117-138, DOI 10.1002/bltj.2267,
August 2002,
<https://onlinelibrary.wiley.com/doi/abs/10.1002/
bltj.2267>.

[SLDC98] Suter, B., Lakshman, T., T.V., Stiliadis, D., and A. A.K.
Choudhury, "Design Considerations considerations for Supporting supporting TCP with Per-flow
Queueing",
per-flow queueing", Proceedings INFOCOM'98, p. 299-306, IEEE INFOCOM '98,
DOI 10.1109/INFCOM.1998.659666, April 1998,
<https://ieeexplore.ieee.org/document/659666>.

[SR-TE-POLICY]
Previdi, S., Filsfils, C., Talaulikar, K., Ed., Mattes,
P., and D. Jain, "Advertising Segment Routing Policies in
BGP", Work in Progress, Internet-Draft, draft-ietf-idr-
segment-routing-te-policy-26, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
segment-routing-te-policy-26>.

[SR-TI-LFA]
Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
13, 16 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-13>.

[TE-QoS-ROUTING]
Ash, G., "Traffic Engineering & QoS Methods for IP-, ATM-,
& Based Multiservice Networks", Work in Progress,
Internet-Draft, draft-ietf-tewg-qos-routing-04, October
2001, <https://datatracker.ietf.org/doc/html/draft-ietf-
tewg-qos-routing-04>.

[WANG] Wang, Y., Wang, Z., and L. Zhang, "Internet traffic
engineering without full mesh overlaying", Proceedings INFOCOM'2001,
IEEE INFOCOM 2001, DOI 10.1109/INFCOM.2001.916782, April
2001, <https://ieeexplore.ieee.org/document/916782>.

[XIAO] Xiao, X., Hannan, A., Bailey, B., and L. Ni, "Traffic
Engineering with MPLS in the Internet", Article IEEE
Network Magazine, Network,
Volume 14, Issue 2, Pages 28-33, DOI 10.1109/65.826369,
March 2000,
<https://courses.cs.washington.edu/courses/cse561/02au/
papers/xiao-mpls-net00.pdf>.

[YARE95] Yang, C. and A. Reddy, "A Taxonomy for Congestion Control
Algorithms in Packet Switching Networks", Article IEEE
Network Magazine, p. Network,
Pages 34-45, DOI 10.1109/65.397042, August 1995,
<http://www.cs.uccs.edu/~zbo/teaching/CS522/Projects/
Taxonomy_Network1995.pdf>.
<https://ieeexplore.ieee.org/document/397042>.

Appendix A. Summary of Changes Since since RFC 3272

The changes to this document since RFC 3272 [RFC3272] are substantial and not
easily summarized as section-by-section changes. The material in the
document has been moved around considerably, some of it removed, and
new text added.

The approach taken here is to list the table of content contents of both the
previous RFC [RFC3272] and
this document saying, respectively, where the text has been placed
and where the text came from.

A.1. RFC 3272

* Section 1.0 Introduction: ("Introduction"): Edited in place in Section 1.

- Section 1.1 What ("What is Internet Traffic Engineering?: Engineering?"): Edited
in place in Section 1.1.

- Section 1.2 Scope: ("Scope"): Moved to Section 1.3.

- Section 1.3 Terminology: ("Terminology"): Moved to Section 1.4 with some
obsolete terms removed and a little editing.

* Section 2.0 Background: ("Background"): Retained as Section 2 with some text
removed.

- Section 2.1 Context ("Context of Internet Traffic Engineering: Engineering"):
Retained as Section 2.1.

- Section 2.2 Network Context: ("Network Context"): Rewritten as Section 2.2.

- Section 2.3 Problem Context: ("Problem Context"): Rewritten as Section 2.3.

o Section 2.3.1 Congestion ("Congestion and its Ramifications: Ramifications"): Retained
as Section 2.3.1.

- Section 2.4 Solution Context: ("Solution Context"): Edited as Section 2.4.

o Section 2.4.1 Combating ("Combating the Congestion Problem: Problem"):
Reformatted as Section 2.4.1.

- Section 2.5 Implementation ("Implementation and Operational Context: Context"):
Retained as Section 2.5.

* Section 3.0 Traffic ("Traffic Engineering Process Model: Model"): Retained as
Section 3.

- Section 3.1 Components ("Components of the Traffic Engineering Process Model:
Model"): Retained as Section 3.1.

- Section 3.2 Measurement: ("Measurement"): Merged into Section 3.1.

- Section 3.3 Modeling, ("Modeling, Analysis, and Simulation: Simulation"): Merged into
Section 3.1.

- Section 3.4 Optimization: ("Optimization"): Merged into Section 3.1.

* Section 4.0 Historical ("Historical Review and Recent Developments: Developments"):
Retained as Section 5, but the very historic aspects have been
deleted.

- Section 4.1 Traffic ("Traffic Engineering in Classical Telephone Networks:
Networks"): Deleted.

- Section 4.2 Evolution ("Evolution of Traffic Engineering in the Internet:
Internet"): Deleted.

- Section 4.3 Overlay Model: ("Overlay Model"): Deleted.

- Section 4.4 Constraint-Based Routing: ("Constraint-Based Routing"): Retained as
Section 5.1.3.1, but moved into Section 5.1.

- Section 4.5 Overview ("Overview of Other IETF Projects Related to
Traffic
Engineering: Engineering"): Retained as Section 5.1 with many new
subsections.

o Section 4.5.1 Integrated Services: ("Integrated Services"): Retained as
Section 5.1.1.1.

o Section 4.5.2 RSVP: ("RSVP"): Retained as Section 5.1.3.2 with
some edits.

o Section 4.5.3 Differentiated Services: ("Differentiated Services"): Retained as
Section 5.1.1.2.

o Section 4.5.4 MPLS: ("MPLS"): Retained as Section 5.1.3.3.

o Section 4.5.5 IP ("IP Performance Metrics: Metrics"): Retained as
Section 5.1.3.6.

o Section 4.5.6 Flow Measurement: ("Flow Measurement"): Retained as
Section 5.1.3.7 with some reformatting.

o Section 4.5.7 Endpoint ("Endpoint Congestion Management: Management"): Retained
as Section 5.1.3.8.

- Section 4.6 Overview ("Overview of ITU Activities Related to Traffic Engineering: De
leted.
Engineering"): Deleted.

- Section 4.7 Content Distribution: ("Content Distribution"): Retained as Section 5.2.

* Section 5.0 Taxonomy ("Taxonomy of Traffic Engineering Systems: Systems"): Retained
as Section 4.

- Section 5.1 Time-Dependent ("Time-Dependent Versus State-Dependent: State-Dependent"): Retained
as Section 4.1.

- Section 5.2 Offline ("Offline Versus Online: Online"): Retained as Section 4.2.

- Section 5.3 Centralized ("Centralized Versus Distributed: Distributed"): Retained as
Section 4.3 with additions.

- Section 5.4 Local ("Local Versus Global: Global"): Retained as Section 4.4.

- Section 5.5 Prescriptive ("Prescriptive Versus Descriptive: Descriptive"): Retained as
Section 4.5 with additions.

- Section 5.6 Open-Loop ("Open-Loop Versus Closed-Loop: Closed-Loop"): Retained as
Section 4.6.

- Section 5.7 Tactical ("Tactical vs Strategic: Strategic"): Retained as Section 4.7.

* Section 6.0 Recommendations ("Recommendations for Internet Traffic Engineering: Engineering"):
Retained as Section 6.

- Section 6.1 Generic ("Generic Non-functional Recommendations: Recommendations"):
Retained as Section 6.1.

- Section 6.2 Routing Recommendations: ("Routing Recommendations"): Retained as
Section 6.2 with edits.

- Section 6.3 Traffic ("Traffic Mapping Recommendations: Recommendations"): Retained as
Section 6.3.

- Section 6.4 Measurement Recommendations: ("Measurement Recommendations"): Retained as
Section 6.4.

- Section 6.5 Network Survivability: ("Network Survivability"): Retained as Section 6.6.

o Section 6.5.1 Survivability ("Survivability in MPLS Based Networks: Networks"):
Retained as Section 6.6.1.

o Section 6.5.2 Protection Option: ("Protection Option"): Retained as
Section 6.6.2.

- Section 6.6 Traffic ("Traffic Engineering in Diffserv Environments: Environments"):
Retained as Section 6.8 with edits.

- Section 6.7 Network Controllability: ("Network Controllability"): Retained as
Section 6.9.

* Section 7.0 Inter-Domain Considerations: ("Inter-Domain Considerations"): Retained as
Section 7.

* Section 8.0 Overview ("Overview of Contemporary TE Practices in Operational
IP
Networks: Networks"): Retained as Section 8.

* Section 9.0 Conclusion: ("Conclusion"): Removed.

* Section 10.0 Security Considerations: ("Security Considerations"): Retained as Section 9
with considerable new text.

A.2. This Document

* Section 1: Based on Section 1 of RFC 3272. [RFC3272].

- Section 1.1: Based on Section 1.1 of RFC 3272. [RFC3272].

- Section 1.2: New for this document.

- Section 1.3: Based on Section 1.2 of RFC 3272. [RFC3272].

- Section 1.4: Based on Section 1.3 of RFC 3272. [RFC3272].

* Section 2: Based on Section 2. 2 of RFC 3272. [RFC3272].

- Section 2.1: Based on Section 2.1 of RFC 3272. [RFC3272].

- Section 2.2: Based on Section 2.2 of RFC 3272. [RFC3272].

- Section 2.3: Based on Section 2.3 of RFC 3272. [RFC3272].

o Section 2.3.1: Based on Section 2.3.1 of RFC 3272. [RFC3272].

- Section 2.4: Based on Section 2.4 of RFC 3272. [RFC3272].

o Section 2.4.1: Based on Section 2.4.1 of RFC 327 [RFC3272].

- Section 2.5: Based on Section 2.5 of RFC 3272. [RFC3272].

* Section 3: Based on Section 3 of RFC 3272. [RFC3272].

- Section 3.1: Based on Sections 3.1, 3.2, 3.3, and 3.4 of RFC
3272.
[RFC3272].

* Section 4: Based on Section 5 of RFC 3272. [RFC3272].

- Section 4.1: Based on Section 5.1 of RFC 3272. [RFC3272].

- Section 4.2: Based on Section 5.2 of RFC 3272. [RFC3272].

- Section 4.3: Based on Section 5.3 of RFC 3272. [RFC3272].

o Section 4.3.1: New for this document.

o Section 4.3.2: New for this document.

- Section 4.4: Based on Section 5.4 of RFC 3272. [RFC3272].

- Section 4.5: Based on Section 5.5 of RFC 3272. [RFC3272].

o Section 4.5.1: New for this document.

- Section 4.6: Based on Section 5.6 of RFC 3272. [RFC3272].

- Section 4.7: Based on Section 5.7 of RFC 3272. [RFC3272].

* Section 5: Based on Section 4 of RFC 3272. [RFC3272].

- Section 5.1: Based on Section 4.5 of RFC 3272. [RFC3272].

o Section 5.1.1.1: Based on Section 4.5.1 of RFC 3272. [RFC3272].

o Section 5.1.1.2: Based on Section 4.5.3 of RFC 3272. [RFC3272].

o Section 5.1.1.3: New for this document.

o Section 5.1.1.4: New for this document.

o Section 5.1.1.5: New for this document.

o Section 5.1.2.1: New for this document.

o Section 5.1.2.2: New for this document.

o Section 5.1.2.3: New for this document.

o Section 5.1.3.1: Based on Section 4.4 of RFC 3272. [RFC3272].

+ Section 5.1.3.1.1: New for this document.

o Section 5.1.3.2: Based on Section 4.5.2 of RFC 3272. [RFC3272].

o Section 5.1.3.3: Based on Section 4.5.4 of RFC 3272. [RFC3272].

o Section 5.1.3.4: New for this document.

o Section 5.1.3.5: New for this document.

o Section 5.1.3.6: Based on Section 4.5.5 of RFC 3272. [RFC3272].

o Section 5.1.3.7: Based on Section 4.5.6 of RFC 3272. [RFC3272].

o Section 5.1.3.8: Based on Section 4.5.7 of RFC 3272. [RFC3272].

o Section 5.1.3.9: New for this document.

o Section 5.1.3.10: New for this document.

o Section 5.1.3.11: New for this document.

o Section 5.1.3.12: New for this document.

o Section 5.1.3.13: New for this document.

o Section 5.1.3.14: New for this document.

o Section 5.1.3.15: New for this document.

- Section 5.2: Based on Section 4.7 of RFC 3272. [RFC3272].

* Section 6: Based on Section 6 of RFC 3272. [RFC3272].

- Section 6.1: Based on Section 6.1 of RFC 3272. [RFC3272].

- Section 6.2: Based on Section 6.2 of RFC 3272. [RFC3272].

- Section 6.3: Based on Section 6.3 of RFC 3272. [RFC3272].

- Section 6.4: Based on Section 6.4 of RFC 3272. [RFC3272].

- Section 6.5: New for this document.

- Section 6.6: Based on Section 6.5 of RFC 3272. [RFC3272].

o Section 6.6.1: Based on Section 6.5.1 of RFC 3272. [RFC3272].

o Section 6.6.2: Based on Section 6.5.2 of RFC 3272. [RFC3272].

- Section 6.7: New for this document.

- Section 6.8: Based on Section 6.6. 6.6 of RFC 3272. [RFC3272].

- Section 6.9: Based on Section 6.7 of RFC 3272. [RFC3272].

* Section 7: Based on Section 7 of RFC 3272. [RFC3272].

* Section 8: Based on Section 8 of RFC 3272. [RFC3272].

* Section 9: Based on Section 10 of RFC 3272. [RFC3272].

Acknowledgments

Much of the text in this document is derived from [RFC3272]. The
editor and contributors to this document would like to express their
gratitude to all involved in that work. Although the source text has
been edited in the production of this document, the original authors
should be considered as contributors to this work. They were:

Daniel O. Awduche
Movaz Networks

Angela Chiu
Celion Networks

Anwar Elwalid
Lucent Technologies

Indra Widjaja
Bell Labs, Lucent Technologies

XiPeng Xiao
Redback Networks

The acknowledgements in [RFC3272] were as below. All people who
helped in the production of that document also need to be thanked for
the carry-over into this new document.

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

The early draft versions of this document were produced by the TEAS
Working Group's RFC3272bis Design Team. The full list of members of
this team is:

Acee Lindem
Adrian Farrel
Aijun Wang
Daniele Ceccarelli
Dieter Beller
Jeff Tantsura
Julien Meuric
Liu Hua
Loa Andersson
Luis Miguel Contreras
Martin Horneffer
Tarek Saad
Xufeng Liu

The production of this document includes a fix to the original text
resulting from an errata report #309 [Err309] by Jean-Michel
Grimaldi.

This work is partially supported by the European Commission under
Horizon 2020 grant agreement number 101015857 Secured autonomic
traffic management for a Tera of SDN flows (Teraflow).

Contributors

The following people contributed substantive text to this document:

Gert Grammel
Email: ggrammel@juniper.net

Loa Andersson
Email: loa@pi.nu

Xufeng Liu
Email: xufeng.liu.ietf@gmail.com

Lou Berger
Email: lberger@labn.net

Jeff Tantsura
Email: jefftant.ietf@gmail.com

Daniel King
Email: daniel@olddog.co.uk

Boris Hassanov
Email: bhassanov@yandex-team.ru

Kiran Makhijani
Email: kiranm@futurewei.com

Dhruv Dhody
Email: dhruv.ietf@gmail.com

Mohamed Boucadair
Email: mohamed.boucadair@orange.com

Author's Address

Adrian Farrel (editor)
Old Dog Consulting
Email: adrian@olddog.co.uk