Internet DRAFT - draft-dt-teas-rfc3272bis
draft-dt-teas-rfc3272bis
TEAS Working Group A. Farrel, Ed.
Internet-Draft Old Dog Consulting
Obsoletes: 3272 (if approved) May 13, 2020
Intended status: Informational
Expires: November 14, 2020
Overview and Principles of Internet Traffic Engineering
draft-dt-teas-rfc3272bis-11
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, and to provide
a common basis for the development of traffic engineering
capabilities for the Internet. The principles, architectures, and
methodologies for performance evaluation and performance optimization
of operational networks are discussed throughout this document.
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
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Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. What is Internet Traffic Engineering? . . . . . . . . . . 4
1.2. Components of Traffic Engineering . . . . . . . . . . . . 6
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1. Context of Internet Traffic Engineering . . . . . . . . . 12
2.2. Network Context . . . . . . . . . . . . . . . . . . . . . 12
2.3. Problem Context . . . . . . . . . . . . . . . . . . . . . 15
2.3.1. Congestion and its Ramifications . . . . . . . . . . 16
2.4. Solution Context . . . . . . . . . . . . . . . . . . . . 16
2.4.1. Combating the Congestion Problem . . . . . . . . . . 19
2.5. Implementation and Operational Context . . . . . . . . . 22
3. Traffic Engineering Process Models . . . . . . . . . . . . . 22
3.1. Components of the Traffic Engineering Process Model . . . 23
4. Review of TE Techniques . . . . . . . . . . . . . . . . . . . 24
4.1. Overview of IETF Projects Related to Traffic Engineering 24
4.1.1. Constraint-Based Routing . . . . . . . . . . . . . . 24
4.1.2. Integrated Services . . . . . . . . . . . . . . . . . 25
4.1.3. RSVP . . . . . . . . . . . . . . . . . . . . . . . . 25
4.1.4. Differentiated Services . . . . . . . . . . . . . . . 26
4.1.5. MPLS . . . . . . . . . . . . . . . . . . . . . . . . 27
4.1.6. Generalized MPLS . . . . . . . . . . . . . . . . . . 28
4.1.7. IP Performance Metrics . . . . . . . . . . . . . . . 29
4.1.8. Flow Measurement . . . . . . . . . . . . . . . . . . 29
4.1.9. Endpoint Congestion Management . . . . . . . . . . . 30
4.1.10. TE Extensions to the IGPs . . . . . . . . . . . . . . 30
4.1.11. Link-State BGP . . . . . . . . . . . . . . . . . . . 30
4.1.12. Path Computation Element . . . . . . . . . . . . . . 31
4.1.13. Application-Layer Traffic Optimization . . . . . . . 31
4.1.14. Segment Routing with MPLS encapsuation (SR-MPLS) . . 31
4.1.15. Network Virtualization and Abstraction . . . . . . . 32
4.1.16. Deterministic Networking . . . . . . . . . . . . . . 33
4.1.17. Network TE State Definition and Presentation . . . . 33
4.1.18. System Management and Control Interfaces . . . . . . 34
4.2. Content Distribution . . . . . . . . . . . . . . . . . . 34
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5. Taxonomy of Traffic Engineering Systems . . . . . . . . . . . 35
5.1. Time-Dependent Versus State-Dependent Versus Event
Dependent . . . . . . . . . . . . . . . . . . . . . . . . 35
5.2. Offline Versus Online . . . . . . . . . . . . . . . . . . 37
5.3. Centralized Versus Distributed . . . . . . . . . . . . . 37
5.3.1. Hybrid Systems . . . . . . . . . . . . . . . . . . . 38
5.3.2. Considerations for Software Defined Networking . . . 38
5.4. Local Versus Global . . . . . . . . . . . . . . . . . . . 38
5.5. Prescriptive Versus Descriptive . . . . . . . . . . . . . 38
5.5.1. Intent-Based Networking . . . . . . . . . . . . . . . 38
5.6. Open-Loop Versus Closed-Loop . . . . . . . . . . . . . . 39
5.7. Tactical vs Strategic . . . . . . . . . . . . . . . . . . 39
6. Recommendations for Internet Traffic Engineering . . . . . . 39
6.1. Generic Non-functional Recommendations . . . . . . . . . 40
6.2. Routing Recommendations . . . . . . . . . . . . . . . . . 42
6.3. Traffic Mapping Recommendations . . . . . . . . . . . . . 44
6.4. Measurement Recommendations . . . . . . . . . . . . . . . 45
6.5. Network Survivability . . . . . . . . . . . . . . . . . . 46
6.5.1. Survivability in MPLS Based Networks . . . . . . . . 48
6.5.2. Protection Option . . . . . . . . . . . . . . . . . . 49
6.6. Traffic Engineering in Diffserv Environments . . . . . . 50
6.7. Network Controllability . . . . . . . . . . . . . . . . . 52
7. Inter-Domain Considerations . . . . . . . . . . . . . . . . . 53
8. Overview of Contemporary TE Practices in Operational IP
Networks . . . . . . . . . . . . . . . . . . . . . . . . . . 55
9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 59
10. Security Considerations . . . . . . . . . . . . . . . . . . . 59
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 59
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 59
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 61
14. Informative References . . . . . . . . . . . . . . . . . . . 62
Appendix A. Historic Overview . . . . . . . . . . . . . . . . . 70
A.1. Traffic Engineering in Classical Telephone Networks . . . 70
A.2. Evolution of Traffic Engineering in Packet Networks . . . 72
A.2.1. Adaptive Routing in the ARPANET . . . . . . . . . . . 72
A.2.2. Dynamic Routing in the Internet . . . . . . . . . . . 73
A.2.3. ToS Routing . . . . . . . . . . . . . . . . . . . . . 74
A.2.4. Equal Cost Multi-Path . . . . . . . . . . . . . . . . 74
A.2.5. Nimrod . . . . . . . . . . . . . . . . . . . . . . . 75
A.3. Development of Internet Traffic Engineering . . . . . . . 75
A.3.1. Overlay Model . . . . . . . . . . . . . . . . . . . . 75
Appendix B. Overview of Traffic Engineering Related Work in
Other SDOs . . . . . . . . . . . . . . . . . . . . . 75
B.1. Overview of ITU Activities Related to Traffic Engineering 75
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 77
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1. Introduction
This memo describes the principles of Internet traffic engineering.
The objective of the document is to articulate the general issues and
principles for Internet traffic engineering; and where appropriate to
provide recommendations, guidelines, and options for the development
of online and offline Internet traffic engineering capabilities and
support systems.
This document can aid service providers in devising and implementing
traffic engineering solutions for their networks. Networking
hardware and software vendors will also find this document helpful in
the development of mechanisms and support systems for the Internet
environment that support the traffic engineering function.
This document provides a terminology for describing and understanding
common Internet traffic engineering concepts. This document also
provides a taxonomy of known traffic engineering styles. In this
context, a traffic engineering style abstracts important aspects from
a traffic engineering methodology. Traffic engineering styles can be
viewed in different ways depending upon the specific context in which
they are used and the specific purpose which they serve. The
combination of styles and views results in a natural taxonomy of
traffic engineering systems.
Even though Internet traffic engineering is most effective when
applied end-to-end, the focus of this document is traffic engineering
within a given domain (such as an autonomous system). However,
because a preponderance of Internet traffic tends to originate in one
autonomous system and terminate in another, this document provides an
overview of aspects pertaining to inter-domain traffic engineering.
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 traffic
engineering and to include references to the latest relevant work in
the IETF. It is worth noting around three fifths of the RFCs
referenced in this document post-date the publication of RFC 3272.
1.1. What is Internet Traffic Engineering?
The Internet exists in order to transfer information from source
nodes to destination nodes. Accordingly, one of the most significant
functions performed by the Internet is the routing of traffic from
ingress nodes to egress nodes. Therefore, one of the most
distinctive functions performed by Internet traffic engineering is
the control and optimization of the routing function, to steer
traffic through the network.
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Internet traffic engineering is defined as that aspect of Internet
network engineering dealing with the issue of performance evaluation
and performance optimization of operational IP networks. Traffic
Engineering encompasses the application of technology and scientific
principles to the measurement, characterization, modeling, and
control of Internet traffic [RFC2702], [AWD2].
Ultimately, it is the performance of the network as seen by end users
of network services that is truly paramount. The characteristics
visible to end users are the emergent properties of the network,
which are the characteristics of the network when viewed as a whole.
A central goal of the service provider, therefore, is to enhance the
emergent properties of the network while taking economic
considerations into account. This is accomplished by addressing
traffic oriented performance requirements, while utilizing network
resources economically and reliably. Traffic oriented performance
measures include delay, delay variation, packet loss, and throughput.
Internet traffic engineering responds to network events. Aspects of
capacity management respond at intervals ranging from days to years.
Routing control functions operate at intervals ranging from
milliseconds to days. Packet level processing functions operate at
very fine levels of temporal resolution, ranging from picoseconds to
milliseconds while responding to the real-time statistical behavior
of traffic.
Thus, the optimization aspects of traffic engineering can be viewed
from a control perspective and can be pro-active and/or reactive. In
the pro-active case, the traffic engineering control system takes
preventive action to obviate predicted unfavorable future network
states such as e.g. engineering a backup path. It may also take
perfective action to induce a more desirable state in the future. In
the reactive case, the control system responds correctively and
perhaps adaptively to events that have already transpired in the
network, such as routing after failure.
Another important objective of Internet traffic engineering 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 results in a minimization of the vulnerability
of the network to service outages arising from errors, faults, and
failures occurring within the infrastructure.
The optimization aspects of traffic engineering can be achieved
through capacity management and traffic management. As used in this
document, capacity management includes capacity planning, routing
control, and resource management. Network resources of particular
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interest include link bandwidth, buffer space, and computational
resources. Likewise, as used in this document, traffic management
includes (1) nodal traffic control functions such as traffic
conditioning, queue management, scheduling, and (2) other functions
that regulate traffic flow through the network or that arbitrate
access to network resources between different packets or between
different traffic streams.
One major challenge of Internet traffic engineering is the
realization of automated control capabilities that adapt quickly and
cost effectively to significant changes in a network's state, while
still maintaining stability of the network. Results from performance
evaluation assessing the effectiveness of traffic engineering methods
can be used to identify existing problems, guide network re-
optimization, and aid in the prediction of potential future problems.
However this process can also be time consuming and may not be
suitable to act on sudden, ephemeral changes in the network.
Performance evaluation can be achieved in many different ways. The
most notable techniques include analytical methods, simulation, and
empirical methods based on measurements.
In genaral, traffic engineering comes in two flavors. Either as a
background process that constantly monitors traffic and optimize the
usage of resources to improve performance, or in form of a pre-
planned optimized traffic distribution that is considered optimal.
In the later case, any deviation from the optimum distribution (e.g.,
caused by a fiber cut) is reverted upon repair without further
optimization. However, this form of traffic engineering heavily
relies upon the notion that the planned state of the network is
indeed optimal. Hence, in such a mode there are two levels of
traffic engineering: the TE-planning task to enable an optimum
traffic distribution, and the routing task keeping traffic flows
attached to the pre-planned distribution
As a general rule, traffic engineering concepts and mechanisms must
be sufficiently specific and well-defined to address known
requirements, but simultaneously flexible and extensible to
accommodate unforeseen future demands.
1.2. Components of Traffic Engineering
As mentioned in Section 1.1, Internet Traffic Engineering provides
performance optimization of operational IP networks while utilizing
network resources economically and reliably. Such optimization is
supported at the control/controller level and within the data/
forwarding plane.
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The key elements required in any TE solution are:
1. Policy
2. Path steering
3. Resource management
Policy allows for the selection of next hops and paths based on
information beyond basic reachability. Early definitions of routing
policy, e.g., [RFC1102] and [RFC1104], discuss routing policy being
applied to restrict access to network resources at an aggregate
level. BGP is an example of a commonly used mechanism for applying
such policies, see [RFC4271] and [RFC5575]. In the Traffic
Engineering context, policy decisions are made within the control
plane or by controllers, and govern the selection of paths. Examples
of such can be found in [RFC4655] and [RFC5394]. Standard TE
solutions may cover the mechanisms to distribute and/or enforce
polices, but specific policy definition is generally unspecified.
Path steering is the ability to forward packets using information
beyond the next hop. Examples of path steering include IPv4 source
routes [RFC0791], RSVP-TE explicit routes [RFC3209], and Segment
Routing [RFC8402]. Path steering for TE can be supported via control
plane protocols or by encoding in the data plane headers or any
combination of the two. This includes when control is provided via a
controller and some southbound, i.e., controller to router, control
protocol.
Resource management provides resource aware control and, in some
cases, forwarding. Examples of resources are bandwidth, buffers and
queues, which in turn can be managed to control loss and latency.
Resources reservation is the control aspect of resource management.
It provides for network domain-wide consensus on which network
(including node and link) resources are to be used by a particular
flow. This determination may be done on a very course or very fine
level. Note that this consensus exists at the network control or
controller level, not the data plane level. It may be purely
composed of accounting/bookkeeping. It typically includes an ability
to admit, reject or reclassify a flow based on policy. Such
accounting can be done based on a static understanding of resource
requirements, or using dynamic mechanisms to collect requirements
(e.g., via [RFC3209]) and resource availability (e.g., via
[RFC4203]), or any combination of the two.
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
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rate-shaping mechanisms which are typically supported via queuing.
It also includes the matching of a flow, i.e., flow classification,
to a particular set of allocated resources. The method for flow
classification and granularity of resource management is technology
specific. Examples include DiffServ with dropping and remarking
[RFC4594], MPLS-TE [RFC3209] and GMPLS [RFC3945] based LSPs, and
controller-based solutions implementing [RFC8453]. This level of
resource control, while optional, is important in networks that wish
to support congestion management policies to control or regulate the
offered traffic to deliver different levels of service and alleviate
congestion problems, or those networks that wish to control latencies
experienced by specific traffic flows.
1.3. Scope
The scope of this document is intra-domain traffic engineering; that
is, traffic engineering within a given autonomous system in the
Internet. This document will discuss concepts pertaining to intra-
domain traffic control, including such issues as routing control,
micro and macro resource allocation, and the control coordination
problems that arise consequently.
This document describes and characterize techniques already in use or
in advanced development for Internet traffic engineering. The way
these techniques fit together will be discussed and scenarios in
which they are useful will be identified.
While this document considers various intra-domain traffic
engineering approaches, it focuses more on traffic engineering with
MPLS and GMPLS. Traffic engineering based upon manipulation of IGP
metrics is not addressed in detail. This topic may be addressed by
other working group documents.
Although the emphasis is on intra-domain traffic engineering, in
Section 7, an overview of the high level considerations pertaining to
inter-domain traffic engineering will be provided. Inter-domain
Internet traffic engineering is crucial to the performance
enhancement of the global Internet infrastructure.
Whenever possible, relevant requirements from existing IETF documents
and other sources will be incorporated by reference.
1.4. Terminology
This subsection provides terminology which is useful for Internet
traffic engineering. The definitions presented apply to this
document. These terms may have other meanings elsewhere.
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Baseline analysis: A study conducted to serve as a baseline for
comparison to the actual behavior of the network.
Busy hour: A one hour period within a specified interval of time
(typically 24 hours) in which the traffic load in a network or
sub-network is greatest.
Bottleneck: A network element whose input traffic rate tends to be
greater than its output rate.
Congestion: A state of a network resource in which the traffic
incident on the resource exceeds its output capacity over an
interval of time.
Congestion avoidance: An approach to congestion management that
attempts to obviate the occurrence of congestion.
Congestion control: An approach to congestion management that
attempts to remedy congestion problems that have already occurred.
Constraint-based routing: A class of routing protocols that take
specified traffic attributes, network constraints, and policy
constraints into account when making routing decisions.
Constraint-based routing is applicable to traffic aggregates as
well as flows. It is a generalization of QoS routing.
Demand side congestion management: A congestion management scheme
that addresses congestion problems by regulating or conditioning
offered load.
Effective bandwidth: The minimum amount of bandwidth that can be
assigned to a flow or traffic aggregate in order to deliver
'acceptable service quality' to the flow or traffic aggregate.
Egress traffic: Traffic exiting a network or network element.
Hot-spot: A network element or subsystem which is in a state of
congestion.
Ingress traffic: Traffic entering a network or network element.
Inter-domain traffic: Traffic that originates in one Autonomous
system and terminates in another.
Loss network: A network that does not provide adequate buffering for
traffic, so that traffic entering a busy resource within the
network will be dropped rather than queued.
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Metric: A parameter defined in terms of standard units of
measurement.
Measurement methodology: A repeatable measurement technique used to
derive one or more metrics of interest.
Network survivability: The capability to provide a prescribed level
of QoS for existing services after a given number of failures
occur within the network.
Offline traffic engineering: A traffic engineering system that
exists outside of the network.
Online traffic engineering: A traffic engineering system that exists
within the network, typically implemented on or as adjuncts to
operational network elements.
Performance measures: Metrics that provide quantitative or
qualitative measures of the performance of systems or subsystems
of interest.
Performance management: A systematic approach to improving
effectiveness in the accomplishment of specific networking goals
related to performance improvement.
Performance metric: A performance parameter defined in terms of
standard units of measurement.
Provisioning: The process of assigning or configuring network
resources to meet certain requests.
QoS routing: Class of routing systems that selects paths to be used
by a flow based on the QoS requirements of the flow.
Service Level Agreement (SLA): A contract between a provider and a
customer that guarantees specific levels of performance and
reliability at a certain cost.
Service Level Objective (SLO): A key element of an SLA between a
provider and a customer. SLOs are agreed upon as a means of
measuring the performance of the Service Provider and are outlined
as a way of avoiding disputes between the two parties based on
misunderstanding.
Stability: An operational state in which a network does not
oscillate in a disruptive manner from one mode to another mode.
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Supply-side congestion management: A congestion management scheme
that provisions additional network resources to address existing
and/or anticipated congestion problems.
Transit traffic: Traffic whose origin and destination are both
outside of the network under consideration.
Traffic characteristic: A description of the temporal behavior or a
description of the attributes of a given traffic flow or traffic
aggregate.
Traffic engineering system: A collection of objects, mechanisms, and
protocols that are used conjunctively to accomplish traffic
engineering objectives.
Traffic flow: A stream of packets between two end-points that can be
characterized in a certain way. A micro-flow has a more specific
definition A micro-flow is a stream of packets with the same
source and destination addresses, source and destination ports,
and protocol ID.
Traffic intensity: A measure of traffic loading with respect to a
resource capacity over a specified period of time. In classical
telephony systems, traffic intensity is measured in units of
Erlangs.
Traffic matrix: A representation of the traffic demand between a set
of origin and destination abstract nodes. An abstract node can
consist of one or more network elements.
Traffic monitoring: The process of observing traffic characteristics
at a given point in a network and collecting the traffic
information for analysis and further action.
Traffic trunk: An aggregation of traffic flows belonging to the same
class which are forwarded through a common path. A traffic trunk
may be characterized by an ingress and egress node, and a set of
attributes which determine its behavioral characteristics and
requirements from the network.
2. Background
The Internet must convey IP packets from ingress nodes to egress
nodes efficiently, expeditiously, and economically. Furthermore, in
a multiclass service environment (e.g., Diffserv capable networks),
the resource sharing parameters of the network must be appropriately
determined and configured according to prevailing policies and
service models to resolve resource contention issues arising from
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mutual interference between packets traversing through the network.
Thus, consideration must be given to resolving competition for
network resources between traffic streams belonging to the same
service class (intra-class contention resolution) and traffic streams
belonging to different classes (inter-class contention resolution).
2.1. Context of Internet Traffic Engineering
The context of Internet traffic engineering pertains to the scenarios
where traffic engineering is used. A traffic engineering methodology
establishes appropriate rules to resolve traffic performance issues
occurring in a specific context. The context of Internet traffic
engineering includes:
1. A network context defining the universe of discourse, and in
particular the situations in which the traffic engineering
problems occur. The network context includes network structure,
network policies, network characteristics, network constraints,
network quality attributes, and network optimization criteria.
2. A problem context defining the general and concrete issues that
traffic engineering addresses. The problem context includes
identification, abstraction of relevant features, representation,
formulation, specification of the requirements on the solution
space, and specification of the desirable features of acceptable
solutions.
3. A solution context suggesting how to address the issues
identified by the problem context. The solution context includes
analysis, evaluation of alternatives, prescription, and
resolution.
4. An implementation and operational context in which the solutions
are methodologically instantiated. The implementation and
operational context includes planning, organization, and
execution.
The context of Internet traffic engineering and the different problem
scenarios are discussed in the following subsections.
2.2. Network Context
IP networks range in size from small clusters of routers situated
within a given location, to thousands of interconnected routers,
switches, and other components distributed all over the world.
Conceptually, at the most basic level of abstraction, an IP network
can be represented as a distributed dynamical system consisting of:
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o a set of interconnected resources which provide transport services
for IP traffic subject to certain constraints
o a demand system representing the offered load to be transported
through the network
o a response system consisting of network processes, protocols, and
related mechanisms which facilitate the movement of traffic
through the network (see also [AWD2]).
The network elements and resources may have specific characteristics
restricting the manner in which the demand is handled. Additionally,
network resources may be equipped with traffic control mechanisms
superintending the way in which the demand is serviced. Traffic
control mechanisms may, for example, be used to:
o control various packet processing activities within a given
resource
o arbitrate contention for access to the resource by different
packets
o regulate traffic behavior through the resource.
A configuration management and provisioning system may allow the
settings of the traffic control mechanisms to be manipulated by
external or internal entities in order to exercise control over the
way in which the network elements respond to internal and external
stimuli.
The details of how the network provides transport services for
packets are specified in the policies of the network administrators
and are installed through network configuration management and policy
based provisioning systems. Generally, the types of services
provided by the network also depends upon the technology and
characteristics of the network elements and protocols, the prevailing
service and utility models, and the ability of the network
administrators to translate policies into network configurations.
Contemporary Internet networks have three significant
characteristics:
o they provide real-time services
o they have become mission critical
o their operating environments are very dynamic.
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The dynamic characteristics of IP and IP/MPLS networks can be
attributed in part to fluctuations in demand, to the interaction
between various network protocols and processes, to the rapid
evolution of the infrastructure which demands the constant inclusion
of new technologies and new network elements, and to transient and
persistent impairments which occur within the system.
Packets contend for the use of network resources as they are conveyed
through the network. A network resource is considered to be
congested if, for an interval of time, the arrival rate of packets
exceed the output capacity of the resource. Congestion may result in
some of the arrival packets being delayed or even dropped.
Congestion increases transit delays, delay variation, packet loss,
and reduces the predictability of network services. Clearly,
congestion is highly undesirable.
Combating congestion at a reasonable cost is a major objective of
Internet traffic engineering.
Efficient sharing of network resources by multiple traffic streams is
a basic operatoinal premise for packet switched networks in general
and for the Internet in particular. A fundamental challenge in
network operation, especially in a large scale public IP network, is
to increase the efficiency of resource utilization while minimizing
the possibility of congestion.
The Internet will have to function in the presence of different
classes of traffic with different service requirements. RFC 2475
provides an architecture for Differentiated Services and makes this
requirement clear [RFC2475]. The RFC allows packets to 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 and QoS constraints.
Capacity constraints can be expressed statistically as peak rates,
mean rates, burst sizes, or as some deterministic notion of effective
bandwidth. QoS requirements can be expressed in terms of:
o integrity constraints such as packet loss
o in terms of temporal constraints such as timing restrictions for
the delivery of each packet (delay) and timing restrictions for
the delivery of consecutive packets belonging to the same traffic
stream (delayvariation).
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2.3. Problem Context
There are several large problems in association with operating a
network described by the simple model of the previous subsection.
This subsection analyze the problem context in relation to traffic
engineering.
The identification, abstraction, representation, and measurement of
network features relevant to traffic engineering are significant
issues.
A particular challenge is to explicitly formulate the problems that
traffic engineering attempts to solve. For example:
o how to identify the requirements on the solution space
o how to specify the desirable features of good solutions
o how to actually solve the problems
o how to measure and characterize the effectiveness of the
solutions.
Another class of problems is how to measure and estimate relevant
network state parameters. Effective traffic engineering relies on a
good estimate of the offered traffic load as well as a view of the
underlying topology and associated resource constraints. A network-
wide view of the topology is also a must for offline planning.
Still another class of problems is how to characterize the state of
the network and how to evaluate its performance under a variety of
scenarios. The performance evaluation problem is two- fold. One
aspect of this problem relates to the evaluation of the system-level
performance of the network. The other aspect relates to the
evaluation of the resource-level performance, which restricts
attention to the performance analysis of individual network
resources.
In this 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
shall refer to the traffic engineering schemes dealing with network
performance optimization at the systems level as "macro-TE" and the
schemes that optimize at the individual resource level as "micro-TE."
Under certain circumstances, the system-level performance can be
derived from the resource-level performance using appropriate rules
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of composition, depending upon the particular performance measures of
interest.
Another fundamental class of problems concerns how to effectively
optimize network performance. Performance optimization may entail
translating solutions to specific traffic engineering problems into
network configurations. Optimization may also entail some degree of
resource management control, routing control, and/or capacity
augmentation.
As noted previously, congestion is an undesirable phenomena in
operational networks. Therefore, the next subsection addresses the
issue of congestion and its ramifications within the problem context
of Internet traffic engineering.
2.3.1. Congestion and its Ramifications
Congestion is one of the most significant problems in an operational
IP context. A network element is said to be congested if it
experiences sustained overload over an interval of time. Congestion
almost always results in degradation of service quality to end users.
Congestion control schemes can include demand-side policies and
supply-side policies. Demand-side policies may restrict access to
congested resources and/or dynamically regulate the demand to
alleviate the overload situation. Supply-side policies may expand or
augment network capacity to better accommodate offered traffic.
Supply-side policies may also re-allocate network resources by
redistributing traffic over the infrastructure. Traffic
redistribution and resource re-allocation serve to increase the
'effective capacity' seen by the demand.
The emphasis of this 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. 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 traffic engineering involves
analysis, evaluation of alternatives, and choice between alternative
courses of action. Generally the solution context is based on making
reasonable inferences about the current or future state of the
network, and subsequently making appropriate decisions that may
involve a preference between alternative sets of action. More
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specifically, the solution context demands reasonable estimates of
traffic workload, characterization of network state, deriving
solutions to traffic engineering problems which may be implicitly or
explicitly formulated, and possibly instantiating a set of control
actions. Control actions may involve the manipulation of parameters
associated with routing, control over tactical capacity acquisition,
and control over the traffic management functions.
The following list of instruments may be applicable to the solution
context of Internet traffic engineering.
o A set of policies, objectives, and requirements (which may be
context dependent) for network performance evaluation and
performance optimization.
o A collection of online and possibly offline tools and mechanisms
for measurement, characterization, modeling, and control of
Internet traffic and control over the placement and allocation of
network resources, as well as control over the mapping or
distribution of traffic onto the infrastructure.
o A set of constraints on the operating environment, the network
protocols, and the traffic engineering system itself.
o A set of quantitative and qualitative techniques and methodologies
for abstracting, formulating, and solving traffic engineering
problems.
o A set of administrative control parameters which may be
manipulated through a Configuration Management (CM) system. The
CM system itself may include a configuration control subsystem, a
configuration repository, a configuration accounting subsystem,
and a configuration auditing subsystem.
o A set of guidelines for network performance evaluation,
performance optimization, and performance improvement.
Determining traffic characteristics through measurement and/or
estimation is very useful within the realm the traffic engineering
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. the traffic-aggregate level or at the flow
level. Measuring at different levels is done in order to aquire
traffic statistics at more or less detail. Measurements at the flow
level or on small traffic aggregates may be performed at edge nodes,
when traffic enters and leaves the network. Measurements for large
traffic-aggregates may be performed within the core of the network.
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To conduct performance studies and to support planning of existing
and future networks, a routing analysis may be performed to determine
the paths the routing protocols will choose for various traffic
demands, and to ascertain the utilization of network resources as
traffic is routed through the network. The routing analysis should
capture the selection of paths through the network, the assignment of
traffic across multiple feasible routes, and the multiplexing of IP
traffic over traffic trunks (if such constructs exists) and over the
underlying network infrastructure. A network topology model is a
necessity for routing analysis. A network topology model may be
extracted from:
o network architecture documents
o network designs
o information contained in router configuration files
o routing databases
o routing tables
o automated tools that discover and depict network topology
information.
Topology information may also be derived from servers that monitor
network state, and from servers that perform provisioning functions.
Routing in operational IP networks can be administratively controlled
at various levels of abstraction including the manipulation of BGP
attributes and IGP metrics. For path oriented technologies such as
MPLS, routing can be further controlled by the manipulation of
relevant traffic engineering parameters, resource parameters, and
administrative policy constraints. Within the context of MPLS, the
path of an explicitly routed label switched path (LSP) can be
computed and established in various ways including:
o manually
o automatically online using constraint-based routing processes
implemented on label switching routers
o automatically offline using constraint-based routing entities
implemented on external traffic engineering support systems.
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2.4.1. Combating the Congestion Problem
Minimizing congestion is a significant aspect of Internet traffic
engineering. This subsection gives an overview of the general
approaches that have been used or proposed to combat congestion
problems.
Congestion management policies can be categorized based upon the
following criteria (see e.g., [YARE95] for a more detailed taxonomy
of congestion control schemes):
o Response time scale which can be characterized as long, medium, or
short
o reactive versus preventive which relates to congestion control and
congestion avoidance
o supply side versus demand side congestion management schemes.
These aspects are discussed in the following paragraphs.
1. Congestion Management based on Response Time Scales
* Long (weeks to months): Expanding network capacity by adding
new equipement, 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 forcasts of future traffic development and
traffic distribution. These upgrades are typically carried
out over weeks or months, or maybe even years.
* Medium (minutes to days): Several control policies fall within
the medium timescale category. Examples include:
a. Adjusting IGP and/or BGP parameters to route traffic away
or towards certain segments of the network
b. Setting up and/or adjusting some explicitly routed LSPs in
MPLS networks to route some traffic trunks away from
possibly congested resources or toward possibly more
favorable routes
c. Re-configuring the logical topology of the network to make
it correlate more closely with the spatial traffic
distribution using for example some underlying path-
oriented technology such as MPLS LSPs or optical channel
trails.
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Many of these adaptive medium time scale response schemes rely
on a measurement system. The measurement system monitors
changes in traffic distribution, traffic shifts, and network
resource utilization. The measurement system then provides
feedback to the online and/or offline traffic engineering
mechanisms and tools which employ this feedback information to
trigger certain control actions to occur within the network.
The traffic engineering mechanisms and tools can be
implemented in a distributed or centralized fashion, and may
have a hierarchical or flat structure. The comparative merits
of distributed and centralized control structures for networks
are well known. A centralized scheme may have global
visibility into the network state and may produce potentially
more optimal solutions. However, centralized schemes are
prone to single points of failure and may not scale as well as
distributed schemes. Moreover, the information utilized by a
centralized scheme may be stale and may not reflect the actual
state of the network. It is not an objective of this memo to
make a recommendation between distributed and centralized
schemes. This is a choice that network administrators must
make based on their specific needs.
* Short (picoseconds to minutes): This category includes packet
level processing functions and events on the order of several
round trip times. It includes router mechanisms such as
passive and active buffer management. These mechanisms are
used to control congestion and/or signal congestion to end
systems so that they can adaptively regulate the rate at which
traffic is injected into the network. One of the most popular
active queue management schemes, especially for TCP traffic,
is Random Early Detection (RED) [FLJA93]. RED supports
congestion avoidance by controlling the average queue size.
During congestion (but before the queue is filled), the RED
scheme chooses arriving packets to "mark" according to a
probabilistic algorithm which takes into account the average
queue size. For a router that does not utilize explicit
congestion notification (ECN) see e.g., [FLOY94], the marked
packets can simply be dropped to signal the inception of
congestion to end systems. On the other hand, if the router
supports ECN, then it can set the ECN field in the packet
header. Several variations of RED have been proposed to
support different drop precedence levels in multi-class
environments [RFC2597], e.g., RED with In and Out (RIO) and
Weighted RED. There is general consensus that RED provides
congestion avoidance performance which is not worse than
traditional Tail-Drop (TD) queue management (drop arriving
packets only when the queue is full). Importantly, however,
RED reduces the possibility of global synchronization and
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improves fairness among different TCP sessions. However, RED
by itself can not prevent congestion and unfairness caused by
sources unresponsive to RED, e.g., UDP traffic and some
misbehaved greedy connections. Other schemes have been
proposed to improve the performance and fairness in the
presence of unresponsive traffic. Some of these schemes were
proposed as theoretical frameworks and are typically not
available in existing commercial products. Two such schemes
are Longest Queue Drop (LQD) and Dynamic Soft Partitioning
with Random Drop (RND) [SLDC98].
2. Congestion Management: Reactive versus Preventive Schemes
* Reactive: Reactive (recovery) congestion management policies
react to existing congestion problems to improve it. All the
policies described in the long and medium time scales above
can be categorized as being reactive especially if the
policies are based on monitoring and identifying existing
congestion problems, and on the initiation of relevant actions
to ease a situation.
* Preventive: Preventive (predictive/avoidance) policies take
proactive action to prevent congestion based on estimates and
predictions of future potential congestion problems. Some of
the policies described in the long and medium time scales fall
into this category. They do not necessarily respond
immediately to existing congestion problems. Instead
forecasts of traffic demand and workload distribution are
considered and action may be taken to prevent potential
congestion problems in the future. The schemes described in
the short time scale (e.g., RED and its variations, ECN, LQD,
and RND) are also used for congestion avoidance since dropping
or marking packets before queues actually overflow would
trigger corresponding TCP sources to slow down.
3. Congestion Management: Supply-Side versus Demand-Side Schemes
* Supply-side: Supply-side congestion management policies
increase the effective capacity available to traffic in order
to control or reduce congestion. This can be accomplished by
increasing capacity. Another way to accomplish this is to
minimize congestion by having a relatively balanced
distribution of traffic over the network. For example,
capacity planning should aim to provide a physical topology
and associated link bandwidths that match estimated traffic
workload and traffic distribution. This may be based on
forecasting and subject to budgetary or other constraints. If
actual traffic distribution does not match the topology
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derived from capacity panning, then the traffic can be mapped
onto the existing topology using routing control mechanisms,
using path oriented technologies (e.g., MPLS LSPs and optical
channel trails) to modify the logical topology, or by using
some other load redistribution mechanisms.
* Demand-side: Demand-side congestion management policies
control or regulate the offered traffic to alleviate
congestion problems. For example, some of the short time
scale mechanisms described earlier (such as RED and its
variations, ECN, LQD, and RND) as well as policing and rate-
shaping mechanisms attempt to regulate the offered load in
various ways. Tariffs may also be applied as a demand side
instrument. To date, however, tariffs have not been used as a
means of demand-side congestion management within the
Internet.
In summary, a variety of mechanisms can be used to address congestion
problems in IP networks. These mechanisms may operate at multiple
time-scales and at multiple traffic aggregation levels.
2.5. Implementation and Operational Context
The operational context of Internet traffic engineering is
characterized by constant changes which occur at multiple levels of
abstraction. The implementation context demands effective planning,
organization, and execution. The planning aspects may involve
determining prior sets of actions to achieve desired objectives.
Organizing involves arranging and assigning responsibility to the
various components of the traffic engineering system and coordinating
the activities to accomplish the desired TE objectives. Execution
involves measuring and applying corrective or perfective actions to
attain and maintain desired TE goals.
3. 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 a traffic engineer, or more generally a traffic
engineering system, must perform to optimize the performance of an
operational network (see also [RFC2702], AWD2]). This process model
may be enacted explicitly or implicitly, by an automaton and/or by a
human.
The traffic engineering process model is iterative [AWD2]. The four
phases of the process model described below are repeated continually.
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o Define the relevant control policies that govern the operation of
the network.
o A feedback mechanism involving the acquisition of measurement data
from the operational network.
o Analyze the network state and to characterize traffic workload.
Performance analysis may be proactive and/or reactive. Proactive
performance analysis identifies potential problems that do not
exist, but could manifest in the future. Reactive performance
analysis identifies existing problems, determines their cause
through diagnosis, and valuates alternative approaches to remedy
the problem, if necessary.
o Performance optimization of the network. It involves a decision
process which selects and implements a set of actions from a set
of alternatives. Optimization actions may include the use of
appropriate techniques to either control the offered traffic or to
control the distribution of traffic across the network.
3.1. Components of the Traffic Engineering Process Model
The key components of the traffic engineering process model are:
1. Measurement is crucial to the traffic engineering function. The
operational state of a network can be conclusively determined
only through measurement. Measurement is also critical to the
optimization function because it provides feedback data which is
used by traffic engineering control subsystems. This data is
used to adaptively optimize network performance in response to
events and stimuli originating within and outside the network.
Measurement in support of the TE function can occur at different
levels of abstraction. For example, measurement can be used to
derive packet level characteristics, flow level characteristics,
user or customer level characteristics, traffic aggregate
characteristics, component level characteristics, and network
wide characteristics.
2. Modeling, analysis, and simulation are important aspects of
Internet traffic engineering. Modeling involves constructing an
abstract or physical representation which depicts relevant
traffic characteristics and network attributes. A network model
is an abstract representation of the network which captures
relevant network features, attributes, and characteristic.
Network simulation tools are extremely useful for traffic
engineering. Because of the complexity of realistic quantitative
analysis of network behavior, certain aspects of network
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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. Review of TE Techniques
This section briefly reviews different traffic engineering approaches
proposed and implemented in telecommunications and computer networks.
The discussion is not intended to be comprehensive. It is primarily
intended to illuminate pre-existing perspectives and prior art
concerning traffic engineering in the Internet and in legacy
telecommunications networks. A historic overview is provided in
Appendix A.
4.1. Overview of IETF Projects Related to Traffic Engineering
This subsection reviews a number of IETF activities pertinent to
Internet traffic engineering. These activities are primarily
intended to evolve the IP architecture to support new service
definitions which allow preferential or differentiated treatment to
be accorded to certain types of traffic.
4.1.1. 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 setting,
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 impose
restrictions on the solution space of the routing function. Path
oriented technologies such as MPLS have made constraint-based routing
feasible and attractive in public IP networks.
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The concept of constraint-based routing within the context of MPLS
traffic engineering requirements in IP networks was first described
in [RFC2702] and led to developments such as MPLS-TE [RFC3209] as
described in Section 4.1.5.
Unlike QoS routing (for example, see [RFC2386] and [MA]) which
generally addresses the issue of routing individual traffic flows to
satisfy prescribed flow based QoS requirements subject to network
resource availability, constraint-based routing is applicable to
traffic aggregates as well as flows and may be subject to a wide
variety of constraints which may include policy restrictions.
4.1.2. Integrated Services
The IETF Integrated Services working group developed the integrated
services (Intserv) model. This model requires resources, such as
bandwidth and buffers, to be reserved a priori for a given traffic
flow to ensure that the quality of service requested by the traffic
flow is satisfied. The integrated services model includes additional
components beyond those used in the 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 model has been
scalability [RFC2998], especially in large public IP networks which
may potentially have millions of active micro-flows in transit
concurrently.
A notable feature of the Integrated Services model is that it
requires explicit signaling of QoS requirements from end systems to
routers [RFC2753]. The Resource Reservation Protocol (RSVP) performs
this signaling function and is a critical component of the Integrated
Services model. RSVP is described next.
4.1.3. 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 for
applications to communicate QoS requirements to the network and for
the network to reserve relevant resources to satisfy the QoS
requirements [RFC2205].
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Under RSVP, the sender or source node sends a PATH message to the
receiver with the same source and destination addresses as the
traffic which the sender will generate. The PATH message contains:
(1) a sender Tspec specifying the characteristics of the traffic, (2)
a sender Template specifying the format of the traffic, and (3) an
optional Adspec which is used to support the concept of One Pass With
Advertising (OPWA) [RFC2205]. Every intermediate router along the
path forwards the PATH Message to the next hop determined by the
routing protocol. Upon receiving a PATH Message, the receiver
responds with a RESV message which includes a flow descriptor used to
request resource reservations. The RESV message travels to the
sender or source node in the opposite direction along the path that
the PATH message traversed. Every intermediate router along the path
can reject or accept the reservation request of the RESV message. If
the request is rejected, the rejecting router will send an error
message to the receiver and the signaling process will terminate. If
the request is accepted, link bandwidth and buffer space are
allocated for the flow and the related flow state information is
installed in the router.
One of the issues with the original RSVP specification was
Scalability. This is because reservations were required for micro-
flows, so that the amount of state maintained by network elements
tends to increase linearly with the number of micro-flows. These
issues are described in [RFC2961].
Recently, RSVP has been modified and extended in several ways to
mitigate the scaling problems. As a result, it is becoming a
versatile signaling protocol for the Internet. For example, RSVP has
been extended to reserve resources for aggregation of flows, to set
up MPLS explicit label switched paths, and to perform other signaling
functions within the Internet. There are also a number of proposals
to reduce the amount of refresh messages required to maintain
established RSVP sessions [RFC2961].
A number of IETF working groups have been engaged in activities
related to the RSVP protocol. These include the original RSVP
working group, the MPLS working group, the Resource Allocation
Protocol working group, and the Policy Framework working group.
4.1.4. Differentiated Services
The goal of the Differentiated Services (Diffserv) effort within the
IETF is 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 such as link bandwidth and buffer space
[RFC2475]. One of the primary motivations for the Diffserv effort
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was to devise alternative mechanisms for service differentiation in
the Internet that mitigate the scalability issues encountered with
the Intserv model.
The IETF Diffserv working group has defined a Differentiated Services
field in the IP header (DS field). The DS field consists of six bits
of the part of the IP header formerly known as the TOS octet. The DS
field is used to indicate the forwarding treatment that a packet
should receive at a node [RFC2474]. The Diffserv working group has
also standardized a number 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 of network services to receive Differentiated
Services from its Internet Service Provider (ISP), it may be
necessary for the user to have a Service Level Agreement (SLA) with
the ISP. An SLA may explicitly or implicitly specify a Traffic
Conditioning Agreement (TCA) which defines classifier rules as well
as metering, marking, discarding, and shaping rules.
Packets are 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 according to existing agreements between the domains.
Differentiated Services 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 and the amount of state
information is proportional to the number of classes rather than to
the number of application flows.
It should be obvious from the previous discussion that the Diffserv
model essentially deals with traffic management issues on a per hop
basis. The Diffserv control model consists of a collection of micro-
TE control mechanisms. Other traffic engineering 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 has been introduced to better
capture the notion of differentiated services across a complete
domain [RFC3086].
4.1.5. MPLS
MPLS is an advanced forwarding scheme which also includes extensions
to conventional IP control plane protocols. MPLS extends the
Internet routing model and enhances packet forwarding and path
control [RFC3031].
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At the ingress to an MPLS domain, Label Switching Routers (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. 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
(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) is the path between an ingress LSRs and an
egress LSRs 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 LDP to set up LSPs.
MPLS is a very powerful technology for Internet traffic engineering
because it supports explicit LSPs which allow constraint-based
routing to be implemented efficiently in IP networks [AWD2]. The
requirements for traffic engineering over MPLS are described in
[RFC2702]. Extensions to RSVP to support instantiation of explicit
LSP are discussed in [RFC3209].
4.1.6. Generalized MPLS
GMPLS extends MPLS control protocols to encompass time-division
(e.g., SONET/SDH, PDH, G.709), wavelength (lambdas), and spatial
switching (e.g., incoming port or fiber to outgoing port or fiber) as
well as continuing to support packet switching. GMPLS provides a
common set of control protocols for all of these layers (including
some technology-specific extensions) each of which has a diverse data
or forwarding plane. GMPLS covers both the signaling and the routing
part of that control plane and is based on the Traffic Engineering
extensions to MPLS (see Section 4.1.5).
In GMPLS, the original MPLS architecture is extended to include LSRs
whose forwarding planes rely on circuit 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.
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4.1.7. IP Performance Metrics
The IETF IP Performance Metrics (IPPM) working group has been
developing 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, and
independent testing groups to provide users and service providers
with a common understanding of the performance and reliability of the
Internet component 'clouds' they use/provide [RFC2330]. The criteria
for performance metrics developed by the IPPM WG 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 WG are useful
for specifying Service Level Agreements (SLAs). SLAs are sets of
service level objectives negotiated between users and service
providers, wherein each objective is a combination of one or more
performance metrics, possibly subject to certain constraints.
4.1.8. Flow Measurement
The IETF Real Time Flow Measurement (RTFM) working group has produced
an architecture document defining a method to specify traffic flows
as well as a number of components for flow measurement (meters, meter
readers, manager) [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, a flow measurement
system can be very useful in the following contexts:
o understanding the behavior of existing networks
o planning for network development and expansion
o quantification of network performance
o verifying the quality of network service
o attribution of network usage to users.
A flow measurement system consists of meters, meter readers, and
managers. A meter observes packets passing through a measurement
point, classifies them into certain groups, accumulates certain 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 a user
application, a host, a network, a group of networks, etc. A meter
reader gathers usage data from various meters so it can be made
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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 specification, meter control
parameters, and sampling techniques. The instructions received by a
meter reader from a manager include the address of the meter whose
date is to be collected, the frequency of data collection, and the
types of flows to be collected.
4.1.9. Endpoint Congestion Management
[RFC3124] is intended to provide a set of congestion control
mechanisms that transport protocols can use. It is also intended to
develop mechanisms for unifying congestion control across a subset of
an endpoint's active unicast connections (called a 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.
4.1.10. TE Extensions to the IGPs
TBD
4.1.11. Link-State BGP
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
traffic engineering information. This is information typically
distributed by IGP routing protocols within the network (see
Section 4.1.10.
The Border Gateway Protocol (BGP) Section 7 is one of the essential
routing protocols that glue the Internet together. BGP Link State
(BGP-LS) [RFC7752] is a mechanism by which link-state and traffic
engineering 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, and is
subject to policy control.
Information collected by BGP-LS can be used to construct the Traffic
Engineering Database (TED, see Section 4.1.17) for use by the Path
Computation Element (PCE, see Section 4.1.12), or may be used by
Application-Layer Traffic Optimization (ALTO) servers (see
Section 4.1.13).
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4.1.12. Path Computation Element
Constraint-based path computation is a fundamental building block for
traffic engineering 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) [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 traffic engineering
system operating on the Traffic Engineering Database (TED, see
Section 4.1.17) with delegated responsibility for determining paths
in MPLS, GMPLS, or Segment Routing 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 traffic engineering systems,
such as the Application of the Path Computation Element Architecture
[RFC6805], the Applicability of a Stateful Path Computation Element
[RFC8051], Abstraction and Control of TE Networks (ACTN)
Section 4.1.15, Centralized Network Control [RFC8283], and Software
Defined Networking (SDN) Section 5.3.2.
4.1.13. Application-Layer Traffic Optimization
TBD
4.1.14. Segment Routing with MPLS encapsuation (SR-MPLS)
Segment Routing (SR) leverages the source routing and tunneling
paradigms: The path packet takes is defined at the ingress and
tunneled to the egress.
A node steers a packet through a controlled set of instructions,
called segments, by prepending the packet with an SR header, label
stack in MPLS case.
A segment can represent any instruction, topological or service-
based, thanks to the MPLS architecture [RFC3031]. Labels cand be
looked up in a global context (platform wide) as well as in some
other context (see "context labels" in section 3 of [RFC5331]).
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4.1.14.1. Base Segment Routing Identifier Types
Segments are identified by Segment Identifiers (SIDs). There are
four types of SID that are relevant for traffic engineering.
Prefix SID: Uses SR Global Block (SRGB), must be unique within the
routing domain SRGB, and is advertised by an IGP. The Prefix-SID
can be configured as an absolute value or an index.
Node SID: A Node SID is a prefix SID with the 'N' (node) bit set, it
is associated with a host prefix (/32 or /128) that identifies the
node. More than 1 Node SID can be configured per node.
Adjacency SID: An Adjacency SID is locally significant (by default).
It can be made globally significant through use of the 'L' flag.
It identifies unidirectional adjacency. In most implementations
Adjacency SIDs are automatically allocated for each adjacency.
They are always encoded as an absolute (not indexed) value.
Binding SID: A Binding SID has two purposes
1. Mapping Server in ISIS
ISIS:The SID/Label Binding TLV is used to advertise
prefixes to SID/Label mappings. This functionality is
called the Segment Routing Mapping Server (SRMS). The
behavior of the SRMS is defined in [RFC8661]
2. Cross-connect (label to FEC mapping)
This is fundamental for multi-domain/multi-layer operation.
The Binding SID identifies a new (could be SR or
hierarchical, at another OSI Layer) path available at the
anchor point. Is always local to the originator (must not
be at the top of the stack), must be looked up in the
context of the nodal SID. It could be provisioned through
Netconf/Restconf, PCEP, BGP, or the CLI.
4.1.15. Network Virtualization and Abstraction
One of the main drivers for Software-Defined Networking (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 [RFC3945] and the Path Computation Element
(PCE) [RFC4655]. One of the advantages of SDN is its logically
centralized control regime that allows a global view of the
underlying networks. Centralized control in SDN helps improve
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network resource utilization compared with distributed network
control.
Abstraction and Control of TE networks (ACTN) [RFC8453] defines an
hierarchical SDN architecture which describes the functional entities
and methods for the coordination of resources across multiple
domains, to provide end-to-end traffic engineered services. ACTN
facilitates end-to-end connections and provides them to the user.
ACTN is focused on aspects like abstraction, virtualization and
presentation. In particular it deals with:
o Abstraction of the underlying network resources and how they are
provided to higher-layer applications and customers.
o Virtualization of underlying resources, whose selection criterion
is the allocation of those resources for the customer,
application, or service. The creation of a virtualized
environment allowis operators to view and control multi-domain
networks as a single virtualized network.
o Presentation to customers of networks as a virtual network via
open and programmable interfaces.
The ACTN managed infrastructure are traffic engineered network
resources, which may include statistical packet bandwidth, physical
forwarding plane sources (such as wavelengths and time slots),
forwarding and cross connect capabilities. The ACTN type of network
virtualization provides customers and applications (tenants) to
utilise and independently control allocated virtual network resources
as if resources as if they were physically their own resource. The
ACTN network is "sliced", with tenants being given a different
partial and abstracted topology view of the physical underlying
network.
4.1.16. Deterministic Networking
TBD
4.1.17. Network TE State Definition and Presentation
The network states that are relevant to the traffic engineering need
to be stored in the system and presented to the user. The Traffic
Engineering Database (TED) is a collection of all TE information
about all TE nodes and TE links in the network, which is an essential
component of a TE system, such as MPLS-TE [RFC2702] and GMPLS
[RFC3945]. In order to formally define the data in the TED and to
present the data to the user with high usability, the data modeling
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language YANG [RFC7950] can be used as described in
[I-D.ietf-teas-yang-te-topo].
4.1.18. System Management and Control Interfaces
The traffic engineering control system needs to have a management
interface that is human-friendly and a control interfaces that is
programable for automation. The Network Configuration Protocol
(NETCONF) [RFC6241] or the RESTCONF Protocol [RFC8040] provide
programmable interfaces that are also human-friendly. These
protocols use XML or 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, 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) [RFC5440]
is another protocol that has evolved to be an option for the TE
system control interface. The messages of PCEP are TLV-based, not
defined by a data modeling language such as YANG.
4.2. Content Distribution
The Internet is dominated by client-server interactions, especially
Web traffic (in the future, more sophisticated media servers may
become dominant). The location and performance of major information
servers has a significant impact on the traffic patterns within the
Internet as well as on the perception of service quality by end
users.
A number of dynamic load balancing techniques have been devised to
improve the performance of replicated information servers. These
techniques can cause spatial traffic characteristics to become more
dynamic in the Internet because information servers can be
dynamically picked based upon the location of the clients, the
location of the servers, the relative utilization of the servers, the
relative performance of different networks, and the relative
performance of different parts of a network. This process of
assignment of distributed servers to clients is called Traffic
Directing. It is an application layer function.
Traffic Directing schemes that allocate servers in multiple
geographically dispersed locations to clients may require empirical
network performance statistics to make more effective decisions. In
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the future, network measurement systems may need to provide this type
of information. The exact parameters needed are not yet defined.
When congestion exists in the network, Traffic Directing and Traffic
Engineering systems should act in a coordinated manner. This topic
is for further study.
The issues related to location and replication of information
servers, particularly web servers, are important for Internet traffic
engineering because these servers contribute a substantial proportion
of Internet traffic.
5. Taxonomy of Traffic Engineering Systems
This section presents a short taxonomy of traffic engineering
systems. A taxonomy of traffic engineering systems can be
constructed based on traffic engineering styles and views as listed
below:
o Time-dependent vs State-dependent vs Event-dependent
o Offline vs Online
o Centralized vs Distributed
o Local vs Global Information
o Prescriptive vs Descriptive
o Open Loop vs Closed Loop
o Tactical vs Strategic
These classification systems are described in greater detail in the
following subsections of this document.
5.1. Time-Dependent Versus State-Dependent Versus Event Dependent
Traffic engineering methodologies can be classified as time-
dependent, or state-dependent, or event-dependent. All TE schemes
are considered to be dynamic in this document. Static TE implies
that no traffic engineering methodology or algorithm is being
applied.
In the time-dependent TE, historical information based on periodic
variations in traffic, (such as time of day), is used to pre-program
routing plans and other TE control mechanisms. Additionally,
customer subscription or traffic projection may be used. Pre-
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programmed routing plans typically change on a relatively long time
scale (e.g., diurnal). Time-dependent algorithms do not attempt to
adapt to random variations in traffic or changing network conditions.
An example of a time-dependent algorithm is a global centralized
optimizer where the input to the system is a traffic matrix and
multi-class QoS requirements as described [MR99]. Another example of
such a methodology is the application of data mining to Internet
traffic [AJ19]. Data mining enables the use of various machine
learning algorithms to identify patterns within historically
collected datasets about Internet traffic, and to extract information
in order to guide decision-making, and to improve efficiency and
productivity of operational processes.
State-dependent TE adapts the routing plans for packets based on the
current state of the network. The current state of the network
provides additional information on variations in actual traffic
(i.e., perturbations from regular variations) that could not be
predicted using historical information. Constraint-based routing is
an example of state-dependent TE operating in a relatively long time
scale. An example operating in a relatively short timescale is a
load-balancing algorithm described in [MATE].
The state of the network can be based on parameters such as
utilization, packet delay, packet loss, etc. These parameters can be
obtained in several ways. For example, each router may flood these
parameters periodically or by means of some kind of trigger to other
routers. Another approach is for a particular router performing
adaptive TE to send probe packets along a path to gather the state of
that path. [RFC6374] defines protocol extensions to collect
performance measurements from MPLS networks. Another approach is for
a management system to gather the relevant information directly from
network elements using telemetry data collection "publication/
subscription" techniques [RFC7923].
Expeditious and accurate gathering and distribution of state
information is critical for adaptive TE due to the dynamic nature of
network conditions. State-dependent algorithms may be applied to
increase network efficiency and resilience. Time-dependent
algorithms are more suitable for predictable traffic variations. On
the other hand, state-dependent algorithms are more suitable for
adapting to the prevailing network state.
Event-dependent TE methods can also be used for TE path selection.
Event-dependent TE methods are distinct from time-dependent and
state-dependent TE methods in the manner in which paths are selected.
These algorithms are adaptive and distributed in nature and typically
use learning models to find good paths for TE in a network. While
state-dependent TE models typically use available-link-bandwidth
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(ALB) flooding for TE path selection, event-dependent TE methods do
not require ALB flooding. Rather, event-dependent TE methods
typically search out capacity by learning models, as in the success-
to-the-top (STT) method. ALB flooding can be resource intensive,
since it requires link bandwidth to carry LSAs, processor capacity to
process LSAs, and the overhead can limit area/Autonomous System (AS)
size. Modeling results suggest that event-dependent TE methods could
lead to a reduction in ALB flooding overhead without loss of network
throughput performance [I-D.ietf-tewg-qos-routing].
5.2. Offline Versus Online
Traffic engineering requires the computation of routing plans. The
computation may be performed offline or online. The computation can
be done offline for scenarios where routing plans need not be
executed in real-time. For example, routing plans computed from
forecast information may be computed offline. Typically, offline
computation is also used to perform extensive searches on multi-
dimensional solution spaces.
Online computation is required when the routing plans must adapt to
changing network conditions as in state-dependent algorithms. Unlike
offline computation (which can be computationally demanding), online
computation is geared toward relative simple and fast calculations to
select routes, fine-tune the allocations of resources, and perform
load balancing.
5.3. Centralized Versus Distributed
Centralized control has a central authority which determines routing
plans and perhaps other TE control parameters on behalf of each
router. The central authority collects the network-state information
from all routers periodically and returns the routing information to
the routers. The routing update cycle 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 routers view of the state of the network.
The network state information may be obtained by the router using a
probing method or distributed by other routers on a periodic basis
using link state advertisements. Network state information may also
be disseminated under exceptional conditions. Examples of protocol
extensions used to advertise network link state information are
defined in [RFC5305], [RFC6119], [RFC7471], [RFC7810], and [RFC8571].
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5.3.1. Hybrid Systems
TBD
5.3.2. Considerations for Software Defined Networking
TBD
5.4. Local Versus Global
Traffic engineering algorithms may require local or global network-
state information.
Local information pertains to the state of a portion of the domain.
Examples include the bandwidth and packet loss rate of a particular
path. Local state information may be sufficient for certain
instances of distributed-controlled TEs.
Global information pertains to the state of the entire domain
undergoing traffic engineering. Examples include a global traffic
matrix and loading information on each link throughout the domain of
interest. Global state information is typically required with
centralized control. Distributed TE systems may also need global
information in some cases.
5.5. Prescriptive Versus Descriptive
TE systems may also be classified as prescriptive or descriptive.
Prescriptive traffic engineering evaluates alternatives and
recommends a course of action. Prescriptive traffic engineering 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.
5.5.1. Intent-Based Networking
TBD
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5.6. Open-Loop Versus Closed-Loop
Open-loop traffic engineering control is where control action does
not use feedback information from the current network state. The
control action may use its own local information for accounting
purposes, however.
Closed-loop traffic engineering control is where control action
utilizes feedback information from the network state. The feedback
information may be in the form of historical information or current
measurement.
5.7. Tactical vs Strategic
Tactical traffic engineering aims to address specific performance
problems (such as hot-spots) that occur in the network from a
tactical perspective, without consideration of overall strategic
imperatives. Without proper planning and insights, tactical TE tends
to be ad hoc in nature.
Strategic traffic engineering approaches the TE problem from a more
organized and systematic perspective, taking into consideration the
immediate and longer term consequences of specific policies and
actions.
6. Recommendations for Internet Traffic Engineering
This section describes high-level recommendations for traffic
engineering in the Internet. These recommendations are presented in
general terms.
The recommendations describe the capabilities needed to solve a
traffic engineering problem or to achieve a traffic engineering
objective. Broadly speaking, these recommendations can be
categorized as either functional or non-functional recommendations.
Functional recommendations for Internet traffic engineering describe
the functions that a traffic engineering system should perform.
These functions are needed to realize traffic engineering objectives
by addressing traffic engineering problems.
Non-functional recommendations for Internet traffic engineering
relate to the quality attributes or state characteristics of a
traffic engineering system. These recommendations may contain
conflicting assertions and may sometimes be difficult to quantify
precisely.
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6.1. Generic Non-functional Recommendations
The generic non-functional recommendations for Internet traffic
engineering include: usability, automation, scalability, stability,
visibility, simplicity, efficiency, reliability, correctness,
maintainability, extensibility, interoperability, and security. 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 traffic engineering system (or
components thereof) to tailor it to a specific operational context.
In the following paragraphs, some of the aspects of the non-
functional recommendations for Internet traffic engineering are
summarized.
Usability: Usability is a human factor aspect of traffic engineering
systems. Usability refers to the ease with which a traffic
engineering system can be deployed and operated. In general, it is
desirable to have a TE system that can be readily deployed in an
existing network. It is also desirable to have a TE system that is
easy to operate and maintain.
Automation: Whenever feasible, a traffic engineering system should
automate as many traffic engineering functions as possible to
minimize the amount of human effort needed to control and analyze
operational networks. Automation is particularly imperative in large
scale public networks because of the high cost of the human aspects
of network operations and the high risk of network problems caused by
human errors. Automation may entail the incorporation of automatic
feedback and intelligence into some components of the traffic
engineering system.
Scalability: Contemporary public networks are growing very fast with
respect to network size and traffic volume. Therefore, a TE system
should be scalable to remain applicable as the network evolves. In
particular, a TE system should remain functional as the network
expands with regard to the number of routers and links, and with
respect to the traffic volume. A TE system should have a scalable
architecture, should not adversely impair other functions and
processes in a network element, and should not consume too much
network resources when collecting and distributing state information
or when exerting control.
Stability: Stability is a very important consideration in traffic
engineering systems that respond to changes in the state of the
network. State-dependent traffic engineering methodologies typically
mandate a tradeoff between responsiveness and stability. It is
strongly recommended that when tradeoffs are warranted between
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responsiveness and stability, that the tradeoff should be made in
favor of stability (especially in public IP backbone networks).
Flexibility: A TE system should be flexible to allow for changes in
optimization policy. In particular, a TE system should provide
sufficient configuration options so that a network administrator can
tailor the TE system to a particular environment. It may also be
desirable to have both online and offline TE subsystems which can be
independently enabled and disabled. TE systems that are used in
multi-class networks should also have options to support class based
performance evaluation and optimization.
Visibility: As part of the TE system, mechanisms should exist to
collect statistics from the network and to analyze these statistics
to determine how well the network is functioning. Derived statistics
such as traffic matrices, link utilization, latency, packet loss, and
other performance measures of interest which are determined from
network measurements can be used as indicators of prevailing network
conditions. Other examples of status information which should be
observed include existing functional routing information
(additionally, in the context of MPLS existing LSP routes), etc.
Simplicity: Generally, a TE system should be as simple as possible.
More importantly, the TE system should be relatively easy to use
(i.e., clean, convenient, and intuitive user interfaces). Simplicity
in user interface does not necessarily imply that the TE system will
use naive algorithms. When complex algorithms and internal
structures are used, such complexities should be hidden as much as
possible from the network administrator through the user interface.
Interoperability: Whenever feasible, traffic engineering systems and
their components should be developed with open standards based
interfaces to allow interoperation with other systems and components.
Security: Security is a critical consideration in traffic engineering
systems. Such traffic engineering systems typically exert control
over certain functional aspects of the network to achieve the desired
performance objectives. Therefore, adequate measures must be taken
to safeguard the integrity of the traffic engineering system.
Adequate measures must also be taken to protect the network from
vulnerabilities that originate from security breaches and other
impairments within the traffic engineering system.
The remainder of this section will focus on some of the high-level
functional recommendations for traffic engineering.
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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
routing system is one that takes traffic characteristics and network
constraints into account during route selection while maintaining
stability.
Traditional shortest path first (SPF) interior gateway protocols are
based on shortest path algorithms and have limited control
capabilities for traffic engineering [RFC2702], [AWD2]. These
limitations include:
1. The well known issues with pure SPF protocols, which do not take
network constraints and traffic characteristics into account
during route selection. For example, since IGPs always use the
shortest paths (based on administratively assigned link metrics)
to forward traffic, load sharing cannot be accomplished among
paths of different costs. Using shortest paths to forward
traffic conserves network resources, but may cause the following
problems: 1) If traffic from a source to a destination exceeds
the capacity of a link along the shortest path, the link (hence
the shortest path) becomes congested while a longer path between
these two nodes may be under-utilized; 2) the shortest paths from
different sources can overlap at some links. If the total
traffic from the sources exceeds the capacity of any of these
links, congestion will occur. Problems can also occur because
traffic demand changes over time but network topology and routing
configuration cannot be changed as rapidly. This causes the
network topology and routing configuration to become sub-optimal
over time, which may result in persistent congestion problems.
2. The Equal-Cost Multi-Path (ECMP) capability of SPF IGPs supports
sharing of traffic among equal cost paths between two nodes.
However, ECMP attempts to divide the traffic as equally as
possible among the equal cost shortest paths. Generally, ECMP
does not support configurable load sharing ratios among equal
cost paths. The result is that one of the paths may carry
significantly more traffic than other paths because it may also
carry traffic from other sources. This situation can result in
congestion along the path that carries more traffic.
3. Modifying IGP metrics to control traffic routing tends to have
network-wide effect. Consequently, undesirable and unanticipated
traffic shifts can be triggered as a result. Recent work
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described in Section 8 may be capable of better control [FT00],
[FT01].
Because of these limitations, new capabilities are needed to enhance
the routing function in IP networks. Some of these capabilities have
been described elsewhere and are summarized below.
Constraint-based routing is desirable to evolve the routing
architecture of IP networks, especially public IP backbones with
complex topologies [RFC2702]. Constraint-based routing computes
routes to fulfill requirements subject to constraints. Constraints
may include bandwidth, hop count, delay, and administrative policy
instruments such as resource class attributes [RFC2702], [RFC2386].
This makes it possible to select routes that satisfy a given set of
requirements subject to network and administrative policy
constraints. Routes computed through 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 redistribute
traffic onto the infrastructure (even for best effort traffic). For
example, if the bandwidth requirements for path selection and
reservable bandwidth attributes of network links are appropriately
defined and configured, then congestion problems caused by uneven
traffic distribution may be avoided or reduced. In this way, the
performance and efficiency of the network can be improved.
A number of enhancements are needed to conventional link state IGPs,
such as OSPF and IS-IS, to allow them to distribute additional state
information required for constraint-based routing. These extensions
to OSPF were described in [RFC3630] and to IS-IS in [RFC5305].
Essentially, these enhancements require the propagation of additional
information in link state advertisements. Specifically, in addition
to normal link-state information, an enhanced IGP is required to
propagate topology state information needed for constraint-based
routing. Some of the additional topology state information include
link attributes such as reservable bandwidth and link resource class
attribute (an administratively specified property of the link). The
resource class attribute concept was defined in [RFC2702]. The
additional topology state information is carried in new TLVs and sub-
TLVs in IS-IS, or in the Opaque LSA in OSPF [RFC5305], [RFC3630].
An enhanced link-state IGP may flood information more frequently than
a normal IGP. This is because even without changes in topology,
changes in reservable bandwidth or link affinity can trigger the
enhanced IGP to initiate flooding. A tradeoff is typically required
between the timeliness of the information flooded and the flooding
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frequency to avoid excessive consumption of link bandwidth and
computational resources, and more importantly, to avoid instability.
In a TE system, it is also desirable for the routing subsystem to
make the load splitting ratio among multiple paths (with equal cost
or different cost) configurable. This capability gives network
administrators more flexibility in the control of traffic
distribution across the network. It can be very useful for avoiding/
relieving congestion in certain situations. Examples can be found in
[XIAO].
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
from a source to a destination away from its original path to another
path (without affecting other traffic paths) allows traffic to be
moved from resource-poor network segments to resource-rich segments.
Path oriented technologies such as MPLS 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 pertains to the assignment of traffic workload onto
pre-established paths to meet certain requirements. Thus, while
constraint-based routing deals with path selection, traffic mapping
deals with the assignment of traffic to established paths which may
have been selected 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 5.1.
An important aspect of the traffic mapping function is the ability to
establish multiple paths between an originating node and a
destination node, and the capability to distribute the traffic
between the two nodes across the paths according to some policies. A
pre-condition for this scheme is the existence of flexible mechanisms
to partition traffic and then assign the traffic partitions onto the
parallel paths. This requirement was noted in [RFC2702]. When
traffic is assigned to multiple parallel paths, it is recommended
that special care should be taken to ensure proper ordering of
packets belonging to the same application (or micro-flow) at the
destination node of the parallel paths.
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As a general rule, 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 rely on short time scale congestion
control mechanisms (such as queue management, scheduling, etc.) to
mitigate congestion. Thus, mechanisms that perform the traffic
mapping functions should complement existing congestion control
mechanisms. In an operational network, it is generally desirable to
map the traffic onto the infrastructure such that intra-class and
inter-class resource contention are minimized.
When traffic mapping techniques that depend on dynamic state feedback
(e.g., MATE and such like) are used, special care must be taken to
guarantee network stability.
6.4. Measurement Recommendations
The importance of measurement in traffic engineering has been
discussed throughout this document. Mechanisms should be provided to
measure and collect statistics from the network to support the
traffic engineering 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 timescale traffic statistics are very
useful for traffic engineering. Long-term time scale traffic
statistics may capture or reflect periodicity in network workload
(such as hourly, daily, and weekly variations in traffic profiles) as
well as traffic trends. Aspects of the monitored traffic statistics
may also depict class of service characteristics for a network
supporting multiple classes of service. Analysis of the long-term
traffic statistics may yield secondary statistics such as busy hour
characteristics, traffic growth patterns, persistent congestion
problems, hot-spot, and imbalances in link utilization caused by
routing anomalies.
A mechanism for constructing traffic matrices for both long-term and
short-term traffic statistics should be in place. In multi-service
IP networks, the traffic matrices may be constructed for different
service classes. Each element of a traffic matrix represents a
statistic of 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.
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Measured traffic statistics should provide reasonable and reliable
indicators of the current state of the network on the short-term
scale. Some short term traffic statistics may reflect link
utilization and link congestion status. Examples of congestion
indicators include excessive packet delay, packet loss, and high
resource utilization. Examples of mechanisms for distributing this
kind of information include SNMP, probing techniques, FTP, IGP link
state advertisements, etc.
6.5. 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 has become an issue of great concern within the
Internet community due to the increasing demands to carry mission
critical traffic, real-time traffic, and other high priority traffic
over the Internet. Survivability can be addressed at the device
level by developing network elements that are more reliable; and at
the network level by incorporating redundancy into the architecture,
design, and operation of networks. It is recommended that a
philosophy of robustness and survivability should be adopted in the
architecture, design, and operation of traffic engineering that
control IP networks (especially public IP networks). Because
different contexts may demand different levels of survivability, the
mechanisms developed to support network survivability should be
flexible so that they can be tailored to different needs. A number
of tools and techniques have been developed to enable network
survivability including MPLS Fast Reroute [RFC4090], RSVP-TE
Extensions in Support of End-to-End Generalized Multi-Protocol Label
Switching (GMPLS) Recovery [RFC4872], and GMPLS Segment Recovery
[RFC4873].
Failure protection and restoration capabilities have become available
from multiple layers as network technologies have continued to
improve. At the bottom of the layered stack, optical networks are
now capable of providing dynamic ring and mesh restoration
functionality at the wavelength level as well as traditional
protection functionality. At the SONET/SDH layer survivability
capability is provided with Automatic Protection Switching (APS) as
well as self-healing ring and mesh architectures. Similar
functionality is provided by layer 2 technologies such as ATM
(generally with slower mean restoration times). Rerouting is
traditionally used at the IP layer to restore service following link
and node outages. Rerouting at the IP layer occurs after a period of
routing convergence which may require seconds to minutes to complete.
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Some new developments in the MPLS context make it possible to achieve
recovery at the IP layer prior to convergence [RFC3469].
To support advanced survivability requirements, path-oriented
technologies such a MPLS can be used to enhance the survivability of
IP networks in a potentially cost effective manner. The advantages
of path oriented technologies such as MPLS for IP restoration becomes
even more evident when class based protection and restoration
capabilities are required.
Recently, a common suite of control plane protocols has been proposed
for both MPLS and optical transport networks under the acronym Multi-
protocol Lambda Switching [AWD1]. This new paradigm of Multi-
protocol Lambda Switching will support even more sophisticated mesh
restoration capabilities at the optical layer for the emerging IP
over WDM network architectures.
Another important aspect regarding multi-layer survivability is that
technologies at different layers provide protection and restoration
capabilities at different temporal granularities (in terms of time
scales) and at different bandwidth granularity (from packet-level to
wavelength level). Protection and restoration capabilities can also
be sensitive to different service classes and different network
utility models.
The impact of service outages varies significantly for different
service classes depending upon the effective duration of the outage.
The duration of an outage can vary from milliseconds (with minor
service impact) to seconds (with possible call drops for IP telephony
and session time-outs for connection oriented transactions) to
minutes and hours (with potentially considerable social and business
impact).
Coordinating different protection and restoration capabilities across
multiple layers in a cohesive manner to ensure network survivability
is maintained at reasonable cost is a challenging task. Protection
and restoration coordination across layers may not always be
feasible, because networks at different layers may belong to
different administrative domains.
The following paragraphs present some of the general recommendations
for protection and restoration coordination.
o Protection and restoration capabilities from different layers
should be coordinated whenever feasible and appropriate to provide
network survivability in a flexible and cost effective manner.
Minimization of function duplication across layers is one way to
achieve the coordination. Escalation of alarms and other fault
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indicators from lower to higher layers may also be performed in a
coordinated manner. A temporal order of restoration trigger
timing at different layers is another way to coordinate multi-
layer protection/restoration.
o Spare capacity at higher layers is often regarded as working
traffic at lower layers. Placing protection/restoration functions
in many layers may increase redundancy and robustness, but it
should not result in significant and avoidable inefficiencies in
network resource utilization.
o It is generally desirable to have protection and restoration
schemes that are bandwidth efficient.
o Failure notification throughout the network should be timely and
reliable.
o Alarms and other fault monitoring and reporting capabilities
should be provided at appropriate layers.
6.5.1. Survivability in MPLS Based Networks
MPLS is an important emerging technology that enhances IP networks in
terms of features, capabilities, and services. Because MPLS is path-
oriented, it can potentially provide faster and more predictable
protection and restoration capabilities than conventional hop by hop
routed IP systems. This subsection describes some of the basic
aspects and recommendations for MPLS networks regarding protection
and restoration. See [RFC3469] for a more comprehensive discussion
on MPLS based recovery.
Protection types for MPLS networks can be categorized as link
protection, node protection, path protection, and segment protection.
o Link Protection: The objective for link protection is to protect
an LSP from a given link failure. Under link protection, the path
of the protection or backup LSP (the secondary LSP) is disjoint
from the path of the working or operational LSP at the particular
link over which protection is required. When the protected link
fails, traffic on the working LSP is switched over to the
protection LSP at the head-end of the failed link. This is a
local repair method which can be fast. It might be more
appropriate in situations where some network elements along a
given path are less reliable than others.
o Node Protection: The objective of LSP node protection is to
protect an LSP from a given node failure. Under node protection,
the path of the protection LSP is disjoint from the path of the
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working LSP at the particular node to be protected. The secondary
path is also disjoint from the primary path at all links
associated with the node to be protected. When the node fails,
traffic on the working LSP is switched over to the protection LSP
at the upstream LSR directly connected to the failed node.
o Path Protection: The goal of LSP path protection is to protect an
LSP from failure at any point 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 that might occur at the ingress and egress LSRs, or for
correlated failures that might impact both working and backup
paths simultaneously. Additionally, since the path selection is
end-to-end, path protection might be more efficient in terms of
resource usage than link or node protection. However, path
protection may be slower than link and node protection in general.
o Segment Protection: An MPLS domain may be partitioned into
multiple protection domains whereby a failure in a protection
domain is rectified within that 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 path protection because recovery generally occurs
closer to the fault.
6.5.2. Protection Option
Another issue to consider is the concept of protection options. The
protection option uses the notation m:n protection, where m is the
number of protection LSPs used to protect n working LSPs. Feasible
protection options follow.
o 1:1: one working LSP is protected/restored by one protection LSP.
o 1:n: one protection LSP is used to protect/restore n working LSPs.
o n:1: one working LSP is protected/restored by n protection LSPs,
possibly with configurable load splitting ratio. When more than
one protection LSP is used, it may be desirable to share the
traffic across the protection LSPs when the working LSP fails to
satisfy the bandwidth requirement of the traffic trunk associated
with the working LSP. This may be especially useful when it is
not feasible to find one path that can satisfy the bandwidth
requirement of the primary LSP.
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o 1+1: traffic is sent concurrently on both the working LSP and the
protection LSP. In this case, the egress LSR selects one of the
two LSPs based on a local traffic integrity decision process,
which compares the traffic received from both the working and the
protection LSP and identifies discrepancies. It is unlikely that
this option would be used extensively in IP networks due to its
resource utilization inefficiency. However, if bandwidth becomes
plentiful and cheap, then this option might become quite viable
and attractive in IP networks.
6.6. Traffic Engineering in Diffserv Environments
This section provides an overview of the traffic engineering features
and recommendations that are specifically pertinent to Differentiated
Services (Diffserv) [RFC2475] capable IP networks.
Increasing requirements to support multiple classes of traffic, such
as best effort and mission critical data, in the Internet calls for
IP networks to differentiate traffic according to some criteria, and
to accord preferential treatment to certain types of traffic. Large
numbers of flows can be aggregated into a few behavior aggregates
based on some criteria in terms of common performance requirements in
terms of packet loss ratio, delay, and jitter; or in terms of common
fields within the IP packet headers.
As Diffserv evolves and becomes deployed in operational networks,
traffic engineering will be critical to ensuring that SLAs defined
within a given Diffserv service model are met. Classes of service
(CoS) can be supported in a Diffserv environment by concatenating
per-hop behaviors (PHBs) along the routing path, using service
provisioning mechanisms, and by appropriately configuring edge
functionality such as traffic classification, marking, policing, and
shaping. PHB is the forwarding behavior that a packet receives at a
DS node (a Diffserv-compliant node). This is accomplished by means
of buffer management and packet scheduling mechanisms. In this
context, packets belonging to a class are those that are members of a
corresponding ordering aggregate.
Traffic engineering can be used as a compliment to Diffserv
mechanisms to improve utilization of network resources, but not as a
necessary element in general. When traffic engineering is used, it
can be operated on an aggregated basis across all service classes
[RFC3270] or on a per service class basis. The former is used to
provide better distribution of the aggregate traffic load over the
network resources. (See [RFC3270] for detailed mechanisms to support
aggregate traffic engineering.) The latter case is discussed below
since it is specific to the Diffserv environment, with so called
Diffserv-aware traffic engineering [RFC4124].
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For some Diffserv networks, it may be desirable to control the
performance of some service classes by enforcing certain
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:
o traffic engineering mechanisms on a per service class basis that
enforce the desired relationship between the amount of traffic
contributed by a given service class and the resources allocated
to that class
o 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 by, for example, 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 vary significantly from router to router, it may not suffice
to rely exclusively on conventional IGP routing protocols or on
traffic engineering mechanisms that are insensitive to different
service classes. Instead, it may be desirable to perform traffic
engineering, especially routing control and mapping functions, on a
per service class basis. One way to accomplish this in a domain that
supports both MPLS and Diffserv is to define class specific LSPs and
to map traffic from each class onto one or more LSPs that correspond
to that service class. An LSP corresponding to a given service class
can then be routed and protected/restored in a class dependent
manner, according to specific policies.
Performing traffic engineering on a per class basis may require
certain per-class parameters to be distributed. Note that it is
common to have some classes share some aggregate constraint (e.g.,
maximum bandwidth requirement) without enforcing the constraint on
each individual class. These classes then can be grouped into a
class-type and per-class-type parameters can be distributed instead
to improve scalability. It also allows better bandwidth sharing
between classes in the same class-type. A class-type is a set of
classes that satisfy the following two conditions:
o Classes in the same class-type have common aggregate requirements
to satisfy required performance levels.
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o There is no requirement to be enforced at the level of individual
class in the class-type. Note that it is still possible,
nevertheless, to implement some priority policies for classes in
the same class-type to permit preferential access to the class-
type bandwidth through the use of preemption priorities.
An example of the class-type can be a low-loss class-type that
includes both AF1-based and AF2-based Ordering Aggregates. With such
a class-type, one may implement some priority policy which assigns
higher preemption priority to AF1-based traffic trunks over AF2-based
ones, vice versa, or the same priority.
See [RFC4124] for detailed requirements on Diffserv-aware traffic
engineering.
6.7. Network Controllability
Off-line (and on-line) traffic engineering considerations would be of
limited utility if the network could not be controlled effectively to
implement the results of TE decisions and to achieve desired network
performance objectives. Capacity augmentation is a coarse grained
solution to traffic engineering issues. However, it is simple and
may be advantageous if bandwidth is abundant and cheap or if the
current or expected network workload demands it. However, bandwidth
is not always abundant and cheap, and the workload may not always
demand additional capacity. Adjustments of administrative weights
and other parameters associated with routing protocols provide finer
grained control, but is difficult to use and imprecise because of the
routing interactions that occur across the network. In certain
network contexts, more flexible, finer grained approaches which
provide more precise control over the mapping of traffic to routes
and over the selection and placement of routes may be appropriate and
useful.
Control mechanisms can be manual (e.g., administrative
configuration), partially-automated (e.g., scripts) or fully-
automated (e.g., policy based management systems). Automated
mechanisms are particularly required in large scale networks. Multi-
vendor interoperability can be facilitated by developing and
deploying standardized management systems (e.g., standard MIBs) and
policies (PIBs) to support the control functions required to address
traffic engineering objectives such as load distribution and
protection/restoration.
Network control functions should be secure, reliable, and stable as
these are often needed to operate correctly in times of network
impairments (e.g., during network congestion or security attacks).
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7. Inter-Domain Considerations
Inter-domain traffic engineering is concerned with the performance
optimization for traffic that originates in one administrative domain
and terminates in a different one.
Traffic exchange between autonomous systems in the Internet occurs
through exterior gateway protocols. Currently, BGP [RFC4271] is the
standard exterior gateway protocol for the Internet. BGP provides a
number of attributes and capabilities (e.g., route filtering) that
can be used for inter-domain traffic engineering. More specifically,
BGP permits the control of routing information and traffic exchange
between Autonomous Systems (ASes) in the Internet. BGP incorporates
a sequential decision process which calculates the degree of
preference for various routes to a given destination network. There
are two fundamental aspects to inter-domain traffic engineering using
BGP:
o Route Redistribution: controlling the import and export of routes
between AS's, and controlling the redistribution of routes between
BGP and other protocols within an AS.
o Best path selection: selecting the best path when there are
multiple candidate paths to a given destination network. Best
path selection is performed by the BGP decision process based on a
sequential procedure, taking a number of different considerations
into account. Ultimately, best path selection under BGP boils
down to selecting preferred exit points out of an AS towards
specific destination networks. The BGP path selection process can
be influenced by manipulating the attributes associated with the
BGP decision process. These attributes include: NEXT-HOP, WEIGHT
(Cisco proprietary which is also implemented by some other
vendors), LOCAL-PREFERENCE, AS-PATH, ROUTE-ORIGIN, MULTI-EXIT-
DESCRIMINATOR (MED), IGP METRIC, etc.
Route-maps provide the flexibility to implement complex BGP policies
based on pre-configured logical conditions. In particular, Route-
maps can be used to control import and export policies for incoming
and outgoing routes, control the redistribution of routes between BGP
and other protocols, and influence the selection of best paths by
manipulating the attributes associated with the BGP decision process.
Very complex logical expressions that implement various types of
policies can be implemented using a combination of Route-maps, BGP-
attributes, Access-lists, and Community attributes.
When looking at possible strategies for inter-domain TE with BGP, it
must be noted that the outbound traffic exit point is controllable,
whereas the interconnection point where inbound traffic is received
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from an EBGP peer typically is not, unless a special arrangement is
made with the peer sending the traffic. Therefore, it is up to each
individual network to implement sound TE strategies that deal with
the efficient delivery of outbound traffic from one's customers to
one's peering points. The vast majority of TE policy is based upon a
"closest exit" strategy, which offloads interdomain traffic at the
nearest outbound peer point towards the destination autonomous
system. Most methods of manipulating the point at which inbound
traffic enters a network from an EBGP peer (inconsistent route
announcements between peering points, AS pre-pending, and sending
MEDs) are either 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. A systematic approach for
inter-domain traffic engineering is yet to be devised.
Inter-domain TE is inherently more difficult than intra-domain TE
under the current Internet architecture. The reasons for this are
both technical and administrative. Technically, while topology and
link state information are helpful for mapping traffic more
effectively, BGP does not propagate such information across domain
boundaries for stability and scalability reasons. Administratively,
there are differences in operating costs and network capacities
between domains. Generally, what may be considered a good solution
in one domain may not necessarily be a good solution in another
domain. Moreover, it would generally be considered inadvisable for
one domain to permit another domain to influence the routing and
management of traffic in its network.
MPLS TE-tunnels (explicit LSPs) can potentially add a degree of
flexibility in the selection of exit points for inter-domain routing.
The concept of relative and absolute metrics can be applied to this
purpose. The idea is that 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 between the router making the selection to
the peering point via a TE-tunnel and assigning the TE-tunnel a
metric which is smaller than the IGP cost to all other peering
points. If a peer accepts and processes MEDs, then a similar MPLS
TE-tunnel based scheme can be applied to cause certain entrance
points to be preferred by setting MED to be an IGP cost, which has
been modified by the tunnel metric.
Similar 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 autonomous system to another.
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Generally, redistribution of inter-domain traffic requires
coordination between peering partners. An export policy in one
domain that results in load redistribution across peer points with
another domain can significantly affect the local traffic matrix
inside the domain of the peering partner. This, in turn, will affect
the intra-domain TE due to changes in the spatial distribution of
traffic. Therefore, it is mutually beneficial for peering partners
to coordinate with each other before attempting any policy changes
that may result in significant shifts in inter-domain traffic. In
certain contexts, this coordination can be quite challenging due to
technical and non- technical reasons.
It is a matter of speculation as to whether MPLS, or similar
technologies, can be extended to allow selection of constrained paths
across domain boundaries.
8. Overview of Contemporary TE Practices in Operational IP Networks
This section provides an overview of some contemporary traffic
engineering practices in IP networks. The focus is primarily on the
aspects that pertain to the control of the routing function in
operational contexts. The intent here is to provide an overview of
the commonly used practices. The discussion is not intended to be
exhaustive.
Currently, service providers apply many of the traffic engineering
mechanisms discussed in this document to optimize the performance of
their IP networks. These techniques include capacity planning for
long timescales, routing control using IGP metrics and MPLS for
medium timescales, the overlay model also for medium timescales, and
traffic management mechanisms for short timescale.
When a service provider plans to build an IP network, or expand the
capacity of an existing network, effective capacity planning should
be an important component of the process. Such plans may take the
following aspects into account: location of new nodes if any,
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 then analyzed and used to trigger various
traffic engineering mechanisms. Tools that perform what-if analysis
can also be used to assist the TE process by allowing various
scenarios to be reviewed before a new set of configurations are
implemented in the operational network.
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Traditionally, intra-domain real-time TE with IGP is done by
increasing the OSPF or IS-IS metric of a congested link until enough
traffic has been diverted from that link. This approach has some
limitations as discussed in Section 6.2. Recently, some new intra-
domain TE approaches/tools have been proposed [RR94] [FT00] [FT01]
[WANG]. Such approaches/tools take traffic matrix, network topology,
and network performance objectives as input, and produce some link
metrics and possibly some unequal load-sharing ratios to be set at
the head-end routers of some ECMPs as output. These new progresses
open new possibility for intra-domain TE with IGP to be done in a
more systematic way.
The overlay model (IP over ATM, or IP over Frame Relay) is another
approach which was commonly used [AWD2], but has been replaced by
MPLS and router hardware technology.
Deployment of MPLS for traffic engineering applications has commenced
in some service provider networks. One operational scenario is to
deploy MPLS in conjunction with an IGP (IS-IS-TE or OSPF-TE) that
supports the traffic engineering extensions, in conjunction with
constraint-based routing for explicit route computations, and a
signaling protocol (e.g., RSVP-TE) for LSP instantiation.
In contemporary MPLS traffic engineering contexts, network
administrators specify and configure link attributes and resource
constraints such as maximum reservable bandwidth and resource class
attributes for links (interfaces) within the MPLS domain. A link
state protocol that supports TE extensions (IS-IS-TE or OSPF-TE) is
used to propagate information about network topology and link
attribute to all routers in the routing area. Network administrators
also specify all the LSPs that are to originate each router. For
each LSP, the network administrator specifies the destination node
and the attributes of the LSP which indicate the requirements that to
be satisfied during the path selection process. Each router then
uses a local constraint-based routing process to compute explicit
paths for all LSPs originating from it. Subsequently, a signaling
protocol is used to instantiate the LSPs. By assigning proper
bandwidth values to links and LSPs, congestion caused by uneven
traffic distribution can generally be avoided or mitigated.
The bandwidth attributes of LSPs used for traffic engineering can be
updated periodically. The basic concept is that the bandwidth
assigned to an LSP should relate in some manner to the bandwidth
requirements of traffic that actually flows through the LSP. The
traffic attribute of an LSP can be modified to accommodate traffic
growth and persistent traffic shifts. If network congestion occurs
due to some unexpected events, existing LSPs can be rerouted to
alleviate the situation or network administrator can configure new
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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.
In an MPLS domain, a traffic matrix 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. Current practice suggests that deploying an MPLS
network consisting of hundreds of routers and thousands of LSPs is
feasible. In summary, recent deployment experience suggests that
MPLS approach is very effective for traffic engineering in IP
networks [XIAO].
As mentioned previously in Section 7, one usually has no direct
control over the distribution of inbound traffic. Therefore, the
main goal of contemporary inter-domain TE is to optimize the
distribution of outbound traffic between multiple inter-domain links.
When operating a global network, maintaining the ability to operate
the network in a regional fashion where desired, while continuing to
take advantage of the benefits of a global network, also becomes an
important objective.
Inter-domain TE with BGP usually begins with the placement of
multiple peering interconnection points in locations that have high
peer density, are in close proximity to originating/terminating
traffic locations on one's own network, and are lowest in cost.
There are generally several locations in each region of the world
where the vast majority of major networks congregate and
interconnect. Some location-decision problems that arise in
association with inter-domain routing are discussed in [AWD5].
Once the locations of the interconnects are determined, and circuits
are implemented, one decides how best to handle the routes heard from
the peer, as well as how to propagate the peers' routes within one's
own network. One way to engineer outbound traffic flows on a network
with many EBGP peers is to create a hierarchy of peers. Generally,
the Local Preferences of all peers are set to the same value so that
the shortest AS paths will be chosen to forward traffic. Then, by
over-writing the inbound MED metric (Multi-exit-discriminator metric,
also referred to as "BGP metric". Both terms are used
interchangeably in this document) with BGP metrics to routes received
at different peers, the hierarchy can be formed. For example, all
Local Preferences can be set to 200, preferred private peers can be
assigned a BGP metric of 50, the rest of the private peers can be
assigned a BGP metric of 100, and public peers can be assigned a BGP
metric of 600. "Preferred" peers might be defined as those peers
with whom the most available capacity exists, whose customer base is
larger in comparison to other peers, whose interconnection costs are
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the lowest, and with whom upgrading existing capacity is the easiest.
In a network with low utilization at the edge, this works well. The
same concept could be applied to a network with higher edge
utilization by creating more levels of BGP metrics between peers,
allowing for more granularity in selecting the exit points for
traffic bound for a dual homed customer on a peer's network.
By only replacing inbound MED metrics with BGP metrics, only equal
AS-Path length routes' exit points are being changed. (The BGP
decision considers Local Preference first, then AS-Path length, and
then BGP metric). For example, assume a network has two possible
egress points, peer A and peer B. Each peer has 40% of the
Internet's routes exclusively on its network, while the remaining 20%
of the Internet's routes are from customers who dual home between A
and B. Assume that both peers have a Local Preference of 200 and a
BGP metric of 100. If the link to peer A is congested, increasing
its BGP metric while leaving the Local Preference at 200 will ensure
that the 20% of total routes belonging to dual homed customers will
prefer peer B as the exit point. The previous example would be used
in a situation where all exit points to a given peer were close to
congestion levels, and traffic needed to be shifted away from that
peer entirely.
When there are multiple exit points to 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 circuit. This can be
achieved by using passive IGP-metrics, AS-path filtering, or prefix
filtering.
Occasionally, more drastic changes are needed, for example, in
dealing 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 Local Preference to that peer can be
reduced below the level of other peers. This effectively reduces the
amount of traffic sent to that peer to only originating traffic
(assuming no transit providers are involved). This type of change
can affect a large amount of traffic, and is only used after other
methods have failed to provide the desired results.
Although it is not much of an issue in regional networks, the
propagation of a peer's routes back through the network must be
considered when a network is peering on a global scale. Sometimes,
business considerations can influence the choice of BGP policies in a
given context. For example, it may be imprudent, from a business
perspective, to operate a global network and provide full access to
the global customer base to a small network in a particular country.
However, for the purpose of providing one's own customers with
quality service in a particular region, good connectivity to that in-
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country network may still be necessary. This can be achieved by
assigning a set of communities at the edge of the network, which have
a known behavior when routes tagged with those communities are
propagating back through the core. Routes heard from local peers
will be prevented from propagating back to the global network,
whereas routes learned from larger peers may be allowed to propagate
freely throughout the entire global network. By implementing a
flexible community strategy, the benefits of using a single global AS
Number (ASN) can be realized, while the benefits of operating
regional networks can also be taken advantage of. An alternative to
doing this is to use different ASNs in different regions, with the
consequence that the AS path length for routes announced by that
service provider will increase.
9. Conclusion
This document described principles for traffic engineering in the
Internet. It presented an overview of some of the basic issues
surrounding traffic engineering in IP networks. The context of TE
was described, a TE process models and a taxonomy of TE styles were
presented. A brief historical review of pertinent developments
related to traffic engineering was provided. A survey of
contemporary TE techniques in operational networks was presented.
Additionally, the document specified a set of generic requirements,
recommendations, and options for Internet traffic engineering.
10. Security Considerations
This document does not introduce new security issues.
11. IANA Considerations
This draft makes no requests for IANA action.
12. Acknowledgments
Much of the text in this document is derived from RFC 3272. The
authors of 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 orginal authors should be
considered as Contributors to this work. They were:
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Daniel O. Awduche
Movaz Networks
7926 Jones Branch Drive, Suite 615
McLean, VA 22102
Phone: 703-298-5291
EMail: awduche@movaz.com
Angela Chiu
Celion Networks
1 Sheila Dr., Suite 2
Tinton Falls, NJ 07724
Phone: 732-747-9987
EMail: angela.chiu@celion.com
Anwar Elwalid
Lucent Technologies
Murray Hill, NJ 07974
Phone: 908 582-7589
EMail: anwar@lucent.com
Indra Widjaja
Bell Labs, Lucent Technologies
600 Mountain Avenue
Murray Hill, NJ 07974
Phone: 908 582-0435
EMail: iwidjaja@research.bell-labs.com
XiPeng Xiao
Redback Networks
300 Holger Way
San Jose, CA 95134
Phone: 408-750-5217
EMail: xipeng@redback.com
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.
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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 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 authors of this document would also like to thank TBD.
13. Contributors
The following people contributed substantive text to this document:
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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
14. Informative References
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approach for predicting the daily Internet data traffic of
a smart university", Article Journal of Big Data, 2019,
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[ASH2] Ash, J., "Dynamic Routing in Telecommunications Networks",
Book McGraw Hill, 1998.
[AWD1] Awduche, D. and Y. Rekhter, "Multiprocotol Lambda
Switching - Combining MPLS Traffic Engineering Control
with Optical Crossconnects", Article IEEE Communications
Magazine, March 2001.
[AWD2] Awduche, D., "MPLS and Traffic Engineering in IP
Networks", Article IEEE Communications Magazine, December
1999.
[AWD5] Awduche, D., "An Approach to Optimal Peering Between
Autonomous Systems in the Internet", Paper International
Conference on Computer Communications and Networks
(ICCCN'98), October 1998.
[FLJA93] Floyd, S. and V. Jacobson, "Random Early Detection
Gateways for Congestion Avoidance", Article IEEE/ACM
Transactions on Networking, Vol. 1, p. 387-413, November
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[FLOY94] Floyd, S., "TCP and Explicit Congestion Notification",
Article ACM Computer Communication Review, V. 24, No. 5,
p. 10-23, October 1994.
[FT00] Fortz, B. and M. Thorup, "Internet Traffic Engineering by
Optimizing OSPF Weights", Article IEEE INFOCOM 2000, March
2000.
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[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>.
[HUSS87] Hurley, B., Seidl, C., and W. Sewel, "A Survey of Dynamic
Routing Methods for Circuit-Switched Traffic",
Article IEEE Communication Magazine, September 1987.
[I-D.ietf-teas-yang-te-topo]
Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
O. Dios, "YANG Data Model for Traffic Engineering (TE)
Topologies", draft-ietf-teas-yang-te-topo-22 (work in
progress), June 2019.
[I-D.ietf-tewg-qos-routing]
Ash, G., "Traffic Engineering & QoS Methods for IP-, ATM-,
& Based Multiservice Networks", draft-ietf-tewg-qos-
routing-04 (work in progress), October 2001.
[ITU-E600]
"Terms and Definitions of Traffic Engineering",
Recommendation ITU-T Recommendation E.600, March 1993.
[ITU-E701]
"Reference Connections for Traffic Engineering",
Recommendation ITU-T Recommendation E.701, October 1993.
[ITU-E801]
"Framework for Service Quality Agreement",
Recommendation ITU-T Recommendation E.801, October 1996.
[MA] Ma, Q., "Quality of Service Routing in Integrated Services
Networks", Ph.D. PhD Dissertation, CMU-CS-98-138, CMU,
1998.
[MATE] Elwalid, A., Jin, C., Low, S., and I. Widjaja, "MATE -
MPLS Adaptive Traffic Engineering",
Proceedings INFOCOM'01, April 2001.
[MCQ80] McQuillan, J., Richer, I., and E. Rosen, "The New Routing
Algorithm for the ARPANET", Transaction IEEE Transactions
on Communications, vol. 28, no. 5, p. 711-719, May 1980.
[MR99] Mitra, D. and K. Ramakrishnan, "A Case Study of
Multiservice, Multipriority Traffic Engineering Design for
Data Networks", Proceedings Globecom'99, December 1999.
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[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>.
[RFC1992] Castineyra, I., Chiappa, N., and M. Steenstrup, "The
Nimrod Routing Architecture", RFC 1992,
DOI 10.17487/RFC1992, August 1996,
<https://www.rfc-editor.org/info/rfc1992>.
[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>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[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>.
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[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>.
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[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., 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>.
[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>.
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[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>.
[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., 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., 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>.
[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>.
[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>.
[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>.
[RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
and D. McPherson, "Dissemination of Flow Specification
Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,
<https://www.rfc-editor.org/info/rfc5575>.
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[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[RFC7810] Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
RFC 7810, DOI 10.17487/RFC7810, May 2016,
<https://www.rfc-editor.org/info/rfc7810>.
[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>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
[RFC8661] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., and S. Litkowski, "Segment Routing MPLS
Interworking with LDP", RFC 8661, DOI 10.17487/RFC8661,
December 2019, <https://www.rfc-editor.org/info/rfc8661>.
[RR94] Rodrigues, M. and K. Ramakrishnan, "Optimal Routing in
Shortest Path Networks", Proceedings ITS'94, Rio de
Janeiro, Brazil, 1994.
[SLDC98] Suter, B., Lakshman, T., Stiliadis, D., and A. Choudhury,
"Design Considerations for Supporting TCP with Per-flow
Queueing", Proceedings INFOCOM'98, p. 299-306, 1998.
[WANG] Wang, Y., Wang, Z., and L. Zhang, "Internet traffic
engineering without full mesh overlaying",
Proceedings INFOCOM'2001, April 2001.
[XIAO] Xiao, X., Hannan, A., Bailey, B., and L. Ni, "Traffic
Engineering with MPLS in the Internet", Article IEEE
Network Magazine, March 2000.
[YARE95] Yang, C. and A. Reddy, "A Taxonomy for Congestion Control
Algorithms in Packet Switching Networks", Article IEEE
Network Magazine, p. 34-45, 1995.
Appendix A. Historic Overview
A.1. Traffic Engineering in Classical Telephone Networks
This subsection presents a brief overview of traffic engineering in
telephone networks which often relates to the way user traffic is
steered from an originating node to the terminating node. This
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subsection presents a brief overview of this topic. A detailed
description of the various routing strategies applied in telephone
networks is included in the book by G. Ash [ASH2].
The early telephone network relied on static hierarchical routing,
whereby routing patterns remained fixed independent of the state of
the network or time of day. The hierarchy was intended to
accommodate overflow traffic, improve network reliability via
alternate routes, and prevent call looping by employing strict
hierarchical rules. The network was typically over-provisioned since
a given fixed route had to be dimensioned so that it could carry user
traffic during a busy hour of any busy day. Hierarchical routing in
the telephony network was found to be too rigid upon the advent of
digital switches and stored program control which were able to manage
more complicated traffic engineering rules.
Dynamic routing was introduced to alleviate the routing inflexibility
in the static hierarchical routing so that the network would operate
more efficiently. This resulted in significant economic gains
[HUSS87]. Dynamic routing typically reduces the overall loss
probability by 10 to 20 percent (compared to static hierarchical
routing). Dynamic routing can also improve network resilience by
recalculating routes on a per-call basis and periodically updating
routes.
There are three main types of dynamic routing in the telephone
network. They are time-dependent routing, state-dependent routing
(SDR), and event dependent routing (EDR).
In time-dependent routing, regular variations in traffic loads (such
as time of day or day of week) are exploited in pre-planned routing
tables. In state-dependent routing, routing tables are updated
online according to the current state of the network (e.g., traffic
demand, utilization, etc.). In event dependent routing, routing
changes are incepted by events (such as call setups encountering
congested or blocked links) whereupon new paths are searched out
using learning models. EDR methods are real-time adaptive, but they
do not require global state information as does SDR. Examples of EDR
schemes include the dynamic alternate routing (DAR) from BT, the
state-and-time dependent routing (STR) from NTT, and the success-to-
the-top (STT) routing from AT&T.
Dynamic non-hierarchical routing (DNHR) is an example of dynamic
routing that was introduced in the AT&T toll network in the 1980's to
respond to time-dependent information such as regular load variations
as a function of time. Time-dependent information in terms of load
may be divided into three timescales: hourly, weekly, and yearly.
Correspondingly, three algorithms are defined to pre-plan the routing
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tables. The network design algorithm operates over a year-long
interval while the demand servicing algorithm operates on a weekly
basis to fine tune link sizes and routing tables to correct forecast
errors on the yearly basis. At the smallest timescale, the routing
algorithm is used to make limited adjustments based on daily traffic
variations. Network design and demand servicing are computed using
offline calculations. Typically, the calculations require extensive
searches on possible routes. On the other hand, routing may need
online calculations to handle crankback. DNHR adopts a "two-link"
approach whereby a path can consist of two links at most. The
routing algorithm presents an ordered list of route choices between
an originating switch and a terminating switch. If a call overflows,
a via switch (a tandem exchange between the originating switch and
the terminating switch) would send a crankback signal to the
originating switch. This switch would then select the next route,
and so on, until there are no alternative routes available in which
the call is blocked.
A.2. Evolution of Traffic Engineering in Packet Networks
This subsection reviews related prior work that was intended to
improve the performance of data networks. Indeed, optimization of
the performance of data networks started in the early days of the
ARPANET. Other early commercial networks such as SNA also recognized
the importance of performance optimization and service
differentiation.
In terms of traffic management, the Internet has been a best effort
service environment until recently. In particular, very limited
traffic management capabilities existed in IP networks to provide
differentiated queue management and scheduling services to packets
belonging to different classes.
In terms of routing control, the Internet has employed distributed
protocols for intra-domain routing. These protocols are highly
scalable and resilient. However, they are based on simple algorithms
for path selection which have very limited functionality to allow
flexible control of the path selection process.
In the following subsections, the evolution of practical traffic
engineering mechanisms in IP networks and its predecessors are
reviewed.
A.2.1. Adaptive Routing in the ARPANET
The early ARPANET recognized the importance of adaptive routing where
routing decisions were based on the current state of the network
[MCQ80]. Early minimum delay routing approaches forwarded each
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packet to its destination along a path for which the total estimated
transit time was the smallest. Each node maintained a table of
network delays, representing the estimated delay that a packet would
experience along a given path toward its destination. The minimum
delay table was periodically transmitted by a node to its neighbors.
The shortest path, in terms of hop count, was also propagated to give
the connectivity information.
One drawback to this approach is that dynamic link metrics tend to
create "traffic magnets" causing congestion to be shifted from one
location of a network to another location, resulting in oscillation
and network instability.
A.2.2. Dynamic Routing in the Internet
The Internet evolved from the ARPANET and adopted dynamic routing
algorithms with distributed control to determine the paths that
packets should take en-route to their destinations. The routing
algorithms are adaptations of shortest path algorithms where costs
are based on link metrics. The link metric can be based on static or
dynamic quantities. The link metric based on static quantities may
be assigned administratively according to local criteria. The link
metric based on dynamic quantities may be a function of a network
congestion measure such as delay or packet loss.
It was apparent early that static link metric assignment was
inadequate because it can easily lead to unfavorable scenarios in
which some links become congested while others remain lightly loaded.
One of the many reasons for the inadequacy of static link metrics is
that link metric assignment was often done without considering the
traffic matrix in the network. Also, the routing protocols did not
take traffic attributes and capacity constraints into account when
making routing decisions. This results in traffic concentration
being localized in subsets of the network infrastructure and
potentially causing congestion. Even if link metrics are assigned in
accordance with the traffic matrix, unbalanced loads in the network
can still occur due to a number factors including:
o Resources may not be deployed in the most optimal locations from a
routing perspective.
o Forecasting errors in traffic volume and/or traffic distribution.
o Dynamics in traffic matrix due to the temporal nature of traffic
patterns, BGP policy change from peers, etc.
The inadequacy of the legacy Internet interior gateway routing system
is one of the factors motivating the interest in path oriented
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technology with explicit routing and constraint-based routing
capability such as MPLS.
A.2.3. ToS Routing
Type-of-Service (ToS) routing involves different routes going to the
same destination with selection dependent upon the ToS field of an IP
packet [RFC2474]. The ToS classes may be classified as low delay and
high throughput. Each link is associated with multiple link costs
and each link cost is used to compute routes for a particular ToS. A
separate shortest path tree is computed for each ToS. The shortest
path algorithm must be run for each ToS resulting in very expensive
computation. Classical ToS-based routing is now outdated as the IP
header field has been replaced by a Diffserv field. Effective
traffic engineering is difficult to perform in classical ToS-based
routing because each class still relies exclusively on shortest path
routing which results in localization of traffic concentration within
the network.
A.2.4. Equal Cost Multi-Path
Equal Cost Multi-Path (ECMP) is another technique that attempts to
address the deficiency in the Shortest Path First (SPF) interior
gateway routing systems [RFC2328]. In the classical SPF algorithm,
if two or more shortest paths exist to a given destination, the
algorithm will choose one of them. The algorithm is modified
slightly in ECMP so that if two or more equal cost shortest paths
exist between two nodes, the traffic between the nodes is distributed
among the multiple equal-cost paths. Traffic distribution across the
equal-cost paths is usually performed in one of two ways: (1) packet-
based in a round-robin fashion, or (2) flow-based using hashing on
source and destination IP addresses and possibly other fields of the
IP header. The first approach can easily cause out- of-order packets
while the second approach is dependent upon the number and
distribution of flows. Flow-based load sharing may be unpredictable
in an enterprise network where the number of flows is relatively
small and less heterogeneous (for example, hashing may not be
uniform), but it is generally effective in core public networks where
the number of flows is large and heterogeneous.
In ECMP, link costs are static and bandwidth constraints are not
considered, so ECMP attempts to distribute the traffic as equally as
possible among the equal-cost paths independent of the congestion
status of each path. As a result, given two equal-cost paths, it is
possible that one of the paths will be more congested than the other.
Another drawback of ECMP is that load sharing cannot be achieved on
multiple paths which have non-identical costs.
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A.2.5. Nimrod
Nimrod was a routing system developed to provide heterogeneous
service specific routing in the Internet, while taking multiple
constraints into account [RFC1992]. Essentially, Nimrod was a link
state routing protocol to support path oriented packet forwarding.
It used the concept of maps to represent network connectivity and
services at multiple levels of abstraction. Mechanisms allowed
restriction of the distribution of routing information.
Even though Nimrod did not enjoy deployment in the public Internet, a
number of key concepts incorporated into the Nimrod architecture,
such as explicit routing which allows selection of paths at
originating nodes, are beginning to find applications in some recent
constraint-based routing initiatives.
A.3. Development of Internet Traffic Engineering
A.3.1. Overlay Model
In the overlay model, a virtual-circuit network, such as Sonet/SDH,
OTN, or WDM, provides virtual-circuit connectivity between routers
that are located at the edges of a virtual-circuit cloud. In this
mode, two routers that are connected through a virtual circuit see a
direct adjacency between themselves independent of the physical route
taken by the virtual circuit through the ATM, frame relay, or WDM
network. Thus, the overlay model essentially decouples the logical
topology that routers see from the physical topology that the ATM,
frame relay, or WDM network manages. The overlay model based on ATM
or frame relay enables a network administrator or an automaton to
employ traffic engineering concepts to perform path optimization by
re-configuring or rearranging the virtual circuits so that a virtual
circuit on a congested or sub-optimal physical link can be re-routed
to a less congested or more optimal one. In the overlay model,
traffic engineering is also employed to establish relationships
between the traffic management parameters (e.g., PCR, SCR, and MBS
for ATM) of the virtual-circuit technology and the actual traffic
that traverses each circuit. These relationships can be established
based upon known or projected traffic profiles, and some other
factors.
Appendix B. Overview of Traffic Engineering Related Work in Other SDOs
B.1. Overview of ITU Activities Related to Traffic Engineering
This section provides an overview of prior work within the ITU-T
pertaining to traffic engineering in traditional telecommunications
networks.
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ITU-T Recommendations E.600 [ITU-E600], E.701 [ITU-E701], and E.801
[ITU-E801] address traffic engineering issues in traditional
telecommunications networks. Recommendation E.600 provides a
vocabulary for describing traffic engineering concepts, while E.701
defines reference connections, Grade of Service (GOS), and traffic
parameters for ISDN. Recommendation E.701 uses the concept of a
reference connection to identify representative cases of different
types of connections without describing the specifics of their actual
realizations by different physical means. As defined in
Recommendation E.600, "a connection is an association of resources
providing means for communication between two or more devices in, or
attached to, a telecommunication network." Also, E.600 defines "a
resource as any set of physically or conceptually identifiable
entities within a telecommunication network, the use of which can be
unambiguously determined" [ITU-E600]. There can be different types
of connections as the number and types of resources in a connection
may vary.
Typically, different network segments are involved in the path of a
connection. For example, a connection may be local, national, or
international. The purposes of reference connections are to clarify
and specify traffic performance issues at various interfaces between
different network domains. Each domain may consist of one or more
service provider networks.
Reference connections provide a basis to define grade of service
(GoS) parameters related to traffic engineering within the ITU-T
framework. As defined in E.600, "GoS refers to a number of traffic
engineering variables which are used to provide a measure of the
adequacy of a group of resources under specified conditions." These
GoS variables may be probability of loss, dial tone, delay, etc.
They are essential for network internal design and operation as well
as for component performance specification.
GoS is different from quality of service (QoS) in the ITU framework.
QoS is the performance perceivable by a telecommunication service
user and expresses the user's degree of satisfaction of the service.
QoS parameters focus on performance aspects observable at the service
access points and network interfaces, rather than their causes within
the network. GoS, on the other hand, is a set of network oriented
measures which characterize the adequacy of a group of resources
under specified conditions. For a network to be effective in serving
its users, the values of both GoS and QoS parameters must be related,
with GoS parameters typically making a major contribution to the QoS.
Recommendation E.600 stipulates that a set of GoS parameters must be
selected and defined on an end-to-end basis for each major service
category provided by a network to assist the network provider with
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improving efficiency and effectiveness of the network. Based on a
selected set of reference connections, suitable target values are
assigned to the selected GoS parameters under normal and high load
conditions. These end-to-end GoS target values are then apportioned
to individual resource components of the reference connections for
dimensioning purposes.
Author's Address
Adrian Farrel (editor)
Old Dog Consulting
Email: adrian@olddog.co.uk
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