traffic+engineering+rexford-te
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IEEE Communications Magazine • October 2002
118
Traffic Engineering with
Traditional IP Routing Protocols
0163-6804/02/$17.00 © 2002 IEEE
ABSTRACT
Traffic engineering involves adapting the
routing of traffic to network conditions, with the
joint goals of good user performance and effi-
cient use of network resources. In this article we
describe an approach to intradomain traffic
engineering that works within the existing
deployed base of interior gateway protocols,
such as Open Shortest Path First and Intermedi-
ate System-Intermediate System. We explain
how to adapt the configuration of link weights,
based on a networkwide view of the traffic and
topology within a domain. In addition, we sum-
marize the results of several studies of tech-
niques for optimizing OSPF/IS-IS weights to the
prevailing traffic. The article argues that tradi-
tional shortest path routing protocols are sur-
prisingly effective for engineering the flow of
traffic in large IP networks.
INTRODUCTION
In some sense, IP networks manage themselves.
A host implementing the Transmission Control
Protocol (TCP) adjusts its sending rate to the
bandwidth available on the path to the destina-
tion, and routers react to changes in the net-
work topology by computing new paths. This
has made the Internet an extremely robust
communication network, even in the face of
rapid growth and occasional failures. However,
these mechanisms do not ensure that the net-
work runs efficiently. For example, a particular
link may be congested despite the presence of
underutilized links in other parts of the net-
work, or a voice-over-IP call may travel over a
route with high propagation delay when a low-
latency path is available. Improving user perfor-
mance and making more efficient use of
network resources requires adapting the routing
of traffic to the prevailing demands. This task is
referred to as traffic engineering [1]. In this arti-
cle we focus on engineering the flow of traffic
within a single autonomous system (AS), such
as a company, university campus, or Internet
service provider (ISP).
INTRADOMAIN TRAFFIC ENGINEERING
Traffic engineering depends on having a set of
performance objectives that guide the selection
of paths, as well as effective mechanisms for the
routers to select paths that satisfy these objec-
tives. Most large IP networks run interior gate-
way protocols (IGPs) such as Open Shortest
Path First (OSPF) or Intermediate System-Inter-
mediate System (IS-IS) that select paths based
on static link weights. These weights are typically
configured by the network operators. Routers
use these protocols to exchange link weights and
construct a complete view of the topology inside
the AS, as shown in Fig. 1. Then each router
computes shortest paths (where the length of a
path is the sum of the weights on the links) and
creates a table that controls the forwarding of
each IP packet to the next hop in its route.
On the surface, shortest path routing does
not seem flexible enough to support traffic engi-
neering in a network supporting a diverse set of
applications. First, these IGPs are limited to
routing scenarios that can be specified with a
single integer weight on each link. However, we
argue that link weights suffice to specify near-
optimal routing for large real-world networks.
Second, in their basic forms, the OSPF and IS-IS
protocols do not adapt the link weights in
response to changes in traffic or failures of net-
work elements, and the path selection process
does not directly incorporate any performance
objectives. Recent standards activity has pro-
posed traffic engineering extensions to OSPF
and IS-IS to incorporate traffic load in the link
state advertisements and path selection deci-
sions. However, these extensions require modifi-
cations to the routers to collect and disseminate
the traffic statistics and establish paths based on
the load metrics. Instead, we argue that it is
often possible to select static link weights that
are resilient to traffic fluctuations and link fail-
ures, allowing the use of the traditional incarna-
tions of OSPF and IS-IS.
The example in Fig. 2 shows how to control
the distribution of traffic in a network by tuning
the IGP weights. All three diagrams concern the
Bernard Fortz, Université Catholique de Louvain
Jennifer Rexford and Mikkel Thorup, AT&T Labs-Research
TOPICS IN INTERNET TECHNOLOGY
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