Workshop Note: Artificial Intelligence

Network Routing Problems

Steven Willmott*and Boi Faltingst

May 18, 1999

for

Abstract

Rapidly growing network usage, the deployment of heterogeneous tech-nologies and diverse service requirements are all factors fuelling the drivetowards the improvement of routing techniques. This paper aims to givean overview of network routing problems, the current state of technologyand previous Artificial Intelligence contributions to the problem area.

It is hoped that this will help stimulate discussion at the AiDIN99workshop held at AAAI99 in Orlando, Florida.

1 IntroductionCommunications networks require complex resource management mechanismsto stay operational. Network routing techniques are the key element in thisresource allocation. A large number of routing technologies have been devel-oped and many are still in service. The diversity of deployed networks and,above all, the rapid pace of technological change means that there remains acontinuing need for new solutions. Three of the major driving forces behind therequirements for new routing techniques are:

The need for Quality of Service guarantees,

Newly deregulated telecommunication industries,

Explosive growth in network size and usage.

* Laboratoire dIntelligence Artificielle (LIA), Department Informatique (DI), Swiss FederalInstitute of Technology (EPFL), IN (Ecublens), CH-1015 Lausanne, Switzerland. e-maih(willmott~lia.di.epfl.ch).

tLaboratoire dIntelligence Artificielle (LIA), Department Informatique (DI), Swiss FederalInstitute of Technology (EPFL), IN (Ecublens), CH-1015 Lausanne, Switzerland. e-mail:(faltings~lia.di.epfl.ch).

AiDIN99 Working Notes 10 Orlando, Florida, July 1999

From: AAAI Technical Report WS-99-03. Compilation copyright 1999, AAAI (www.aaai.org). All rights reserved.

Despite numerous predictions of a bandwidth glut ([Smith, 1999] among oth-ers), resources used still need to be carefully managed. The increased volumesof data flowing across modern networks mean that mismanagement can veryquickly result in bottlenecks and potentially catastrophic cell loss. Above all,the complexity of todays network technology and the operational strain it is putunder generates more need than ever for innovative and "intelligent" networkmanagement solutions.

Routing problems have been widely studied in Artificial Intelligence andComputer Science in an abstract context. The graph theoretic nature of theunderlying problem has often been helpful in abstracting away technical detail.Unfortunately the recent rapid pace of technological advancement and the in-creasing obscurity of networking terminology is making harder for researcherswithout the relevant background to make progress in the area. This informalreview gives a basic outline of the problems, key network technologies and solu-tion techniques in the area. The final third of the paper then goes on to outlinesome of Artificial Intelligence (AI) contributions to the study of routing.

Note that this review is exclusively focused on routing problems and we havetried to avoid digressing into related areas (such as network topology design,call scheduling, connection acceptance control capacity planning, etc.). Workscontaining good reviews for the applications of Artificial Intelligence techniquesto network problems in general include: [Liebowitz, J. and Prerau, D. S., 1995],[Kumar and Venkataram, 1997], and [Weihmayer and Velthuijsen, 1998].

It should also be noted that this review is a preliminary version and far frombeing comprehensive. The classifications used to divide up the technologies andproblems are also not the only possible groupings and hence should be treatedonly as a guideline. We hope, however, that the interested reader will be ableto find more detailed information in the referenced material.

2 A Generic Problem Description

Perhaps the easiest way to understand the issues in a domain is to relate themto a generic problem definition. The definition given in Section 2.2 is a slightvariation on the one given to encourage discussion at the AiDIN workshop.Following sections will then relate characteristics of the most important networktypes to this model.

2.1 Terminology

Before beginning it is perhaps helpful to clarify some of the terminology whichwill be used throughout this paper:

Routing mechanism, routing algorithm, routing method and rout-ing protocol: these terms are all very similar but have slightly differentmeanings:

AiDIN99 Working Notes 11 Orlando, Florida, July 1999

- Routing method, routing algorithm: these terms refer to the compu-tational method applied to finding a route or several routes. In thispaper the two terms are treated as interchangeable.

- Routing mechanism: a routing mechanism is the apparatus deployedin the network for routing calls which may include: the routing algo-rithms, strategies for decomposing the network into more manageablepieces (e.g. a routing hierarchy) and methods for updating networkstate information.

- Routing protocol: a routing protocol was originally taken to meanthe protocol used in a network to exchange network state informationused for routing (e.g. RIP messages, hello messages) but is oftentaken to mean something similar to a routing mechanism.

Demand: a demand is taken to mean a request by a user or system fora data transmission service, be it the sending of a single packet, estab-lishment of a connection, transfer of a file or establishment of a multicasttree.

Note that in some approaches the routing algorithm is strictly distinct formthe routing mechanism: the algorithm generates routing solutions, the routingmechanism applies the generated routes when forwarding packets or allocatingcircuits.

2.2 Problem Definition

The problem to be solved is defined in terms of the available infrastructure andthe required results. The available infrastructure is a communications networkcomprising:

10; 100; 1000; 10,000; 100,000, 1,000,000, 10,000,000 or 100,000,000 nodes(e.g.: touters, switches).

Communication links connecting these nodes such that:

- The links are evenly / unevenly distributed throughout the network(i.e. varying distributions of links of entering and leaving nodes).

- All links have equal / very different capacity, delay, cell loss and othertransport characteristics.

- The network graph is densely / sparsely connected (i.e. varying theratio of links to nodes across the network).

Traffic in the network is:

- Connection-oriented (or at least individual flows can be identifiedand routed): resources are allocated on a demand-by-demand basisat set-up time.

- Connection-less: traffic is managed on a packet-by-packet basis.

AiDIN99 Working Notesl 2

Orlando, Florida, July 1999

Given this generic infrastructure a routing mechanism must ensure that datacan travel through the network between arbitrary end points (or arbitrary setsof endpoints in the case of multicast). The resulting network should be able tosupport some subset of the following:

Heavily and lightly loaded networks,

Overload situations,

Fluctuation of traffic patterns (over the long and short term),

Prevention of routing oscillations and loops,

Fast response to resource demands (although this may not mean that route finding process itself has to be on-line and real time),

Quality of Service (QoS) guarantees (or at least the high probability good service),

The ability to plan for traffic which is booked on the network ahead of theactual demands start time (introduces a temporal aspect),

Function in a multi-provider environment where some network state in-formation may not be available outside provider domains.

Multicast and/or point-to-point traffic.

3 Todays Networks

The generic problem covers many types of networks (indeed in some cases therequirements may be contradictory). This section groups together certain setsof features to identify combinations which are common in todays networks.There are two main classes of networks: circuit-switched and packet switched.Circuit switched networks are descended from telephone networks and discussedin Section 3.1. Packet switched networks (discussed in Section 3.2) are furthersub-divided into two classes: those operating in a connection-less transfer mode(such as IP based Internet) and those operating in a connection-oriented transfermode (such as ATM networks).1

1Another class of routing problems is in Optical networks where routing involves as-signing wavelengths to each path so that paths sharing an edge have non-interfering(different) wavelengths. The goal is minimise the number of wavelengths assignedin the network. Optical networks will not be discussed further in this paper, see[ Ramaswami, R. and Sivarajan K. , 1998] for an overview.

AiDIN99 Working Notes 13 Orlando, Florida, July 1999

3.1 Circuit Switched Networks

Telephone networks have been in use for approximately a hundred years anda large majority of the deployed network infrastructure is still based on thePublic Switched Telephone Service (PSTN). The evolution from telephone callsto carry voice to connections carrying data has left the underlying principlesessentially unchanged. The first generation of ISDN (Narrowband ISDN), forexample, still uses similar multiplexing techniques to the Time Division Multi-plexing (TDM) used in telephone networks.

On receipt of a demand for communication between two nodes A and B inthe network, a circuit switched network sets up a dedicated circuit between thetwo nodes. This circuit (variously known as a call or connection) is only removedwhen the communication ends. During the duration of the communic