Internet Draft

Traffic Engineering Working Group W. Lai
Internet Draft AT&T
Document: <draft-wlai-tewg-cap-eng-00.txt> March 2000
Expiration Date: September 2000

Capacity Engineering of IP-based Networks with MPLS

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Abstract

Drawing from the experience and work done in ITU-T on traffic
engineering, the feasibility of extending telecommunications network
dimensioning methods to IP-based networks with MPLS is explored.
This version of the document is preliminary, reporting work in
progress.

1. Introduction

The routing protocols currently used in public IP networks are
typically traffic-insensitive in that they do not include network
resource utilization information in making routing decisions. As a
result of this lack of traffic control, traffic tends to converge
onto the same network segments, thereby causing unbalanced network
loading with subsets of network resources being congested and other
resources being underutilized. This shortcoming of network
operation, coupled with the phenomenal growth of Internet usage,
makes it very difficult to manage IP-based network performance.
Hence, current engineering practice sometimes resorts to over-
provisioning of network capacity.

As IP-based network services evolve from a single best-effort class
toward differentiation with multiple levels of quality of service

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(QoS), a network infrastructure that offers consistent and
predictable network performance is required. To achieve this,
proper control and provisioning of network resources would be
needed. Recently, the multiprotocol label switching (MPLS)
technology [1, 2] has been developed to overcome the above
limitations of existing destination-based routing protocols so that
more effective traffic engineering of IP-based networks can be
performed.

MPLS integrates the connection-oriented label-swapping operation
with connectionless network-layer routing, thus gaining much of the
control capabilities of a connection-oriented network such as ATM-
based B-ISDN. In particular, MPLS facilitates the establishment of
a set of paths satisfying certain constraints [3]. Such a kind of
path, called a constraint-based routing label-switched path (CRLSP),
is conceptually very similar to a virtual-path connection in an ATM
network for traffic engineering purposes. For example, a CRLSP
enables the implementation of connection control and resource
allocation functions on aggregated groups of traffic flows, instead
of on individual flows. This allows support for different classes
of service [3, 4] and fits well with the Differentiated Services
(DiffServ) architecture [5, 6].

Furthermore, the ability of MPLS to set up explicitly routed CRLSPs
permits source-node control, i.e., source routing, to operate
efficiently. This provides network administrators the capability to
steer traffic on desired paths. Such flexibility to direct the
traffic to follow engineered paths through a network offers a number
of advantages, e.g., load sharing across multiple paths, and
shifting traffic away from congested links. The ability to use
fixed preferred paths for routing traffic, so-called route pinning,
also gives the means to measure and capture the statistics of the
traffic associated with each source-destination node pair. As a
result, there is the potential to develop more refined measurement
technology so as to obtain a more accurate estimation and
characterization of the traffic demands for network planning
purposes.

The rest of this document is organized into sections describing
general aspects of traffic engineering, capacity engineering,
traffic demand characterization, performance objectives and
reference connections, and network dimensioning. Much of the
materials presented here are adapted and drawn from the experience
and work done in ITU-T on traffic engineering of telecommunications
networks.

2. Traffic Engineering of IP-based Networks

The issue of IP network traffic engineering has been addressed by
two recent documents [7, 8]. As defined in [7], Internet traffic
engineering is ôthat aspect of Internet network engineering that
deals with the issue of performance evaluation and performance

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optimization of operational IP networks.ö Since network congestion
is a primary cause of performance degradation, a major objective of
traffic engineering is to increase the efficiency of resource
utilization while minimizing the possibility of congestion through
capacity and traffic management. Reference [7] also includes a
discussion of requirements for routing, traffic mapping,
measurement, survivability, and other attributes, from the
perspective of traffic engineering.

The interactions between traffic and capacity management in
controlling a networkÆs response to traffic demands and network
failures are further explored in [8]. Real-time traffic management
functions such as routing control, path selection, and resource
management, ensures that performance objectives are met under all
conditions including load shifts and failures. Capacity management,
through control of network design, ensures that network provisioning
meets performance objectives for a given set of traffic demands at
minimum cost.

A global problem of network engineering is the choice and
implementation of a specific routing strategy for the selection of
paths for carrying traffic. Reference [8] provides an overview and
categorization of various routing methods, as well as routing table
management methods for different network types and their
interworking with each other. Also described are resource
management methods to achieve QoS objectives, such as connection
admission and bandwidth reservation.

Taking account of the provisioned network capacity, routing patterns
(i.e., path sets and rules for path selection) are designed for
different traffic streams between different source-destination node
pairs. Periodically or possibly on a real-time basis, these routing
patterns may be adjusted as necessary to correct service problems.
An iterative network design process encompassing both routing-
pattern design and capacity allocation is used to determine the
minimum network capacity required to meet the performance objectives
for given traffic demands.

Reference [8] also presents a set of traffic engineering operational
requirements. For example, network management controls such as call
gapping can be used to assure acceptable network performance in case
of overload or failures.

3. Capacity Engineering

According to ITU-T Recommendation E.600 [9], traffic engineering
includes measurements, forecasting, planning, dimensioning, and
performance monitoring. Traffic engineering has a goal of ensuring
trafficability performance objectives for telecommunications
services. Trafficability performance is defined in Recommendation
E.800 [10] as follows. For each individual network element or
functional subsystem, it is the ability of the element to meet a

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traffic demand of a given size and other characteristics, under
given internal conditions. These internal conditions may be, e.g.,
any combination of faulty and not faulty parts within the element.

Thus, trafficability performance has a direct impact on the
accessibility, retainability, and integrity of any given service
offered by a network; it is one of the major factors in QoS.
Service accessibility refers to the ability of a service to be
obtained, within specified tolerances and other given conditions,
when requested by the user. Service retainability refers to the
ability of a service, once obtained, to continue to be provided
under given conditions for a requested duration. Service integrity
refers to the degree to which a service is provided without
excessive impairments, once obtained.

Generally, trafficability performance is an attribute of network
performance and can be described in terms of measures such as losses
and delays. However, E.800 has not explicitly provided any specific
measures for trafficability performance.

In this document, the focus is primarily on the dimensioning aspect
of traffic engineering and its related tasks. In mapping given user
demands onto network resources, network dimensioning involves the
sizing of the network elements, such as links and buffers, so that
performance objectives can be met at minimum cost. Thus, two major
inputs to the dimensioning process are characterization of user
demands and specification of performance objectives. (While cost is
an important factor, the development of cost models will not be
dealt with in this document.) The term capacity engineering is used
here to cover this subset of traffic engineering tasks related to
dimensioning. These tasks are covered in more detail in the next
three sections.

4. Traffic Demand Characterization

Typically, dimensioning procedures are based on models that
approximate the statistical behavior of network traffic in large
populations of users. To allow straightforward characterization of
the traffic demands, these models necessarily adopt some simplifying
assumptions concerning the usually complicated traffic processes,
such as the arrival patterns of flows and the distribution of flow
sizes. For these assumptions to be relevant and applicable, they
must give rise to statistical patterns that closely approximate the
behavior of aggregate traffic flows in operational networks.
Traffic data are collected to validate these assumptions, with
modifications being made when needed. Additionally, traffic
measurements are used to estimate offered load and to provide
forecasting of future demands for capacity planning purposes.
Forecasting and planning may result in capacity augmentation or may
lead to the introduction of new technology and architecture.

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Thus, a first step in the dimensioning process is to develop user
demand models as input to characterize the offered load to the
network. In the context of an ATM-based B-ISDN, ITU-T
Recommendation E.716 [11] describes the characterization of user
demand as manifested at the user-network interface. Since a CRLSP
in MPLS is conceptually similar to a virtual-path connection in ATM
for traffic engineering purposes, the methodology of E.716, when
suitably modified and extended, may be applicable to MPLS-based IP
networks. Further investigation of this feasibility is needed.

To allow the characterization of traffic offered to an IP-based
network, user demands may be modeled as an arrival process of
demands for IP-based services of different types. As each such
service demand generates a set of flows, each service type may be
defined by a set of flow attributes and by a flow pattern as
follows. (1) Flow attributes are those attributes of the service
demand that identify the resources needed by the service demand in
the network. These may include, e.g., access channel rate,
communication configuration such as point-to-point or multipoint,
and traffic conditioning agreements as defined in DiffServ [6]. (2)
A flow pattern describes the packet arrival process in a flow
through a set of traffic variables. E.716 presents four approaches
for defining these traffic variables to describe the transient
nature of rate variations. For example, they may be related to the
burst structure of the packet flow, the number of packet arrivals in
time intervals of specified length, packet interarrival time, or the
number of packet arrivals exceeding a given rate. (Currently,
models that describe the self-similar nature of IP traffic have been
proposed in the literature. However, for capacity engineering
purposes, models that capture the burstiness characteristics of
source traffic will suffice.)

To recapitulate, a set of flow attributes and traffic variables may
be used together to characterize a particular service type by
setting appropriate values for the parameters. Depending on the
different values chosen for these attributes and variables, the
number of service types can potentially be very large. To minimize
the effort of traffic engineering, especially in the initial stage
of deployment, it may be desirable to limit the total number of
service types. For example, service types with similar values may
possibly be combined to form one representative type that captures
the salient features essential for dimensioning.

5. Performance Objectives and Reference Connections

Performance objectives can be viewed from two perspectives: the user
and the network service provider. Quality of service (QoS), which
is performance perceivable by a user of a service, expresses the
user's degree of satisfaction of the service [9, 10]. Thus, QoS
parameters focus on performance effects that are observable at the
service access points and network interfaces, rather than their
causes within the network. Different service types usually have

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different QoS requirements. This allows a network provider to
provide different treatment to different service types, to gain
higher resource utilization.

Grade of service (GoS) is a number of traffic engineering parameters
to provide a measure of adequacy of a group of resources under
specified conditions [9]. These GoS parameters may be probability
of blocking, probability of delay, etc. They are essential for
network internal design and operation, as well as specification of
component performance. The latter is related to the trafficability
performance described previously.

Based on a given set of QoS requirements, a set of GoS parameters
are selected and defined on an end-to-end basis within the network
boundary, for each major service category provided by a network.
The selected GoS parameters are specified in such a way that the GoS
can be derived at well-defined reference points, i.e., traffic
significant points. This is to allow the partitioning of end-to-end
GoS objectives to obtain the GoS objectives for each network stage
or component, on the basis of some well-defined reference
connections.

As defined in E.600, for traffic engineering purposes, a connection
is an association of resources providing means for communication
between two or more devices in, or attached to, a telecommunication
network. There can be different types of connections as the number
and types of resources in a connection may vary. Therefore, the
concept of a reference connection is used to identify representative
cases of the different types of connections without involving the
specifics of their actual realizations by different physical means.

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 for
clarifying and specifying traffic performance issues at various
interfaces between different network domains. Each domain may
consist of one or more service provider networks. Recommendation
E.651 [12] specifies reference connections for IP-access networks.
Other reference connections are to be specified.

From the QoS objectives, a set of end-to-end GoS parameters and
their objectives for different reference connections are derived.
For example, end-to-end connection blocking probability and end-to-
end packet transfer delay may be relevant GoS parameters. The GoS
objectives should be specified with reference to traffic load
conditions, such as under normal and high load conditions. The end-
to-end GoS objectives are then apportioned to individual resource
components of the reference connections for dimensioning purposes.
In an operational network, to ensure that the GoS objectives have
been met, performance measurements and performance monitoring are
required.

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6. Network Dimensioning

As discussed previously, network dimensioning involves the optimal
(e.g., minimum cost) sizing of the network elements to accommodate
given traffic demands, while meeting performance objectives. In
using MPLS, this means the dimensioning of a set of pre-selected
CRLSPs for carrying traffic, and mapping the logical network of
CRLSPs onto a physical network of links with capacity constraints.
The dimensioning process also determines the link capacity
parameters or thresholds associated with the use of some bandwidth
reservation scheme for service protection. Service protection
controls the GoS for certain service types by restricting access to
bandwidth, or by giving priority access to one type of traffic over
another. Such methods are essential, e.g., to guarantee a minimum
amount of resources for connections with expected short duration, to
improve the blocking probabilities for connections with high
bandwidth requirements, or to maintain network stability by
preventing GoS degradation in case of a local overload.

In performing the task of dimensioning, it is assumed that a network
topology, both at the logical and physical levels, has been defined.
This is because the layout of a network is usually influenced by
other factors, such as the network providerÆs policy/administrative
constraints, or considerations of an existing network. Also, it is
assumed that the network is available, i.e., it does not consider
network equipment in a failure state.

Routing deals with the selection of network paths for connection
requests. To simplify the dimensioning process, a fixed routing-
pattern with pre-determined paths for different traffic streams is
usually assumed. As described previously, the process of routing-
pattern design and dimensioning is iterated until an optimal design
is reached.

The series of ITU-T Recommendations E.735-7 [13, 14, 15] presents a
set of general principles and methods for dimensioning ATM-based B-
ISDNs. The notion of Equivalent Cell Rate (ECR) has been used
effectively therein for dimensioning purposes. The ECR captures the
effects of expected traffic mix, cell-level control mechanisms,
priority scheduling, bandwidth and buffer capacity limitations,
thereby characterizing the estimated amount of resources that needs
to be allocated to a connection to satisfy the specified cell-level
GoS objectives. Borrowing from this technique, it may be useful to
define some Equivalent Bandwidth parameter for dimensioning IP-based
networks. Further studies are needed for its definition.

Assuming that connection blocking probability is the only GoS
parameter at the connection level, and using ECR to account for
cell-level performance, E.737 presents several iterative methods for
dimensioning. To reduce computational complexity, approximation
methods based on the independence assumption and the principle of
decomposition may be developed. For example, if the capacity of a
link can be considered as well delimited, independent of the traffic

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carried by other links, then the global end-to-end decision on
connection admission can be decomposed into local decisions. These
techniques may be applicable to the dimensioning of CRLSPs in an
MPLS-based network.

7. Security Considerations

Security considerations are not addressed in this version of the
draft.

8. References

[1] E.C. Rosen, A. Viswanathan, and R. Callon, ôMultiprotocol Label
Switching Architecture,ö Internet-Draft, Work in Progress, August
1999.

[2] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow, and
A. Viswanathan, ôA Framework for Multiprotocol Label Switching,ö
Internet-Draft, Work in Progress, September 1999.

[3] D. Awduche, J. Malcolm, J. Agogbua, M. OÆDell, and J. McManus,
ôRequirements for Internet Traffic Engineering Over MPLS,ö RFC 2702,
September 1999.

[4] T. Li and Y. Rekhter, ôA Provider Architecture for
Differentiated Services and Traffic Engineering (PASTE),ö RFC 2430,
October 1998.

[5] K. Nichols, S. Blake, F. Baker, and D. Black, ôDefinition of
the Differentiated Services Field (DS Field) in the IPv4 and IPv6
Headers,ö RFC 2474, December 1998.

[6] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, and W.
Weiss, ôAn Architecture for Differentiated Services,ö RFC 2475,
December 1998.

[7] D.O. Awduche, A. Chiu, A. Elwalid, I. Widjaja, and X. Xiao, ôA
Framework for Internet Traffic Engineering,ö Internet-Draft, Work in
Progress, January 2000.

[8] ITU-T Draft Recommendation E.TE, ôTraffic Engineering and QoS
Methods for IP-, ATM-, and TDM-Based Multiservice Networks,ö March
2000 (Contact: G. Ash).

[9] ITU-T Recommendation E.600, ôTerms and Definitions of Traffic
Engineering,ö March 1993.

[10] ITU-T Recommendation E.800, ôTerms and Definitions Related to
Quality of Service and Network
Performance Including Dependability,ö August 1994.

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[11] ITU-T Recommendation E.716, ôUser Demand Modelling in
Broadband-ISDN,ö October 1996.

[12] ITU-T Draft Recommendation E.651, ôReference Connections for
Traffic Engineering of IP Access Networks,ö to be approved.

[13] ITU-T Recommendation E.735, ôFramework for Traffic Control and
Dimensioning in B-ISDN,ö May 1997.

[14] ITU-T Recommendation E.736, ôMethods for Cell Level Traffic
Control in B-ISDN,ö May 1997.

[15] ITU-T Recommendation E.737, ôDimensioning Methods for B-ISDN,ö
May 1997.

10. Acknowledgments

The review and comments of Gerald Ash is much appreciated.

11. Author's Addresses

Wai Sum Lai
AT&T Labs
Room D5-3D18
200 Laurel Avenue
Middletown, New Jersey 07748, USA
Phone: 732-420-3712
Email: wlai@att.com

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