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1 Introduction

Topological information and the dissemination

of such information are important issues when

discussing the architecture of IP networks capa-

ble of delivering differentiated QoS. The pur-

pose of this paper is to describe a network level

model representing the topological aspects of

such networks. It is based on efforts carried out

within the TrafHan project.

Section 2 provides a description of networks

exhibiting a varying degree of QoS support,

ranging from no support (Best Effort) via sup-

port per traffic class (DiffServ or DiffServ-over-

MPLS) to support per traffic flow (IntServ).

Four major limitations related to the Best Effort

service are identified and used as the basis for

the assessment of the others. The current model

describing topology in terms of subnetworks,

topological links and connection point groups

is introduced and so is the Traffic Trunk, an

abstract representation of traffic in DiffServ-

over-MPLS networks.

The modelling methodology utilized is found

in Section 3. The description of the network

resources build on the entities of the generic

network architecture defined in [G.805]. The

methodology is an enhanced version of the Ref-

erence Model for Open Distributed Processing

[RM-ODP], adapted to fit with the modelling

requirements for telecom networks.

The Enterprise Viewpoint, constituting the

requirements part of the resulting model, is

described in Section 4 in terms of common

community policies and actions related to the

resource types involved.

In Section 5 the deletion of a topological link is

presented as an example in terms of Enterprise

(requirements), Information (static behaviour),

and Computational (dynamic behaviour) View-

point descriptions. This constitutes the commu-

nication protocol neutral part of the model. The

protocol specific part belongs to the implementa-

tion and is not addressed. It would have been

documented in the Engineering Viewpoint based

on the elements of the specific communication

protocol chosen.

2 Network Characteristics

In this section, we will describe the network

topology being the basis for the formal model.

Best Effort networks as well as networks offer-

ing services with quantifiable QoS are discussed.

Rather than providing a full description, the

emphasis has been on describing properties that

are candidates for being visible as part of the

model.

The functionality described in this section is

more extensive than that exposed by the current

model. This is because the level of detail of the

final model is to be decided later in the TrafHan

project.

2.1 Best Effort IP Networks

IP networks are located at the network layer in

the OSI model. The core element of an IP net-

work is the router. It consists of two main func-

tions, the routing function and the forwarding

function, confer Figure 1.

The forwarding function transfers packets

between the input- and output ports on the basis

of the information in the routing table. The

entries of the routing table, also known as next

hop information, are calculated by the routing

function taking into account the destination

address and the routing algorithm chosen. It does

so by comparing the appropriate part of the des-

Modelling the Topology of IP Networks

TERJE HENRIKSEN, ANNE-GRETHE KÅRÅSEN

AND STÅLE WOLLAND

Anne-Grethe Kåråsen (42) is
Research Scientist, R&D Kjeller.
She is working in the Internet
Network Architecture group with
special interest in layer 1-3 net-
work management and control.

anne-grethe.karasen
@telenor.com

Terje Henriksen (59) is
Research Scientist at Telenor
R&D. He has been involved in
standardization and R&D pro-
jects within the area of network
level modelling since 1991; from
1996 to 2001 as the rapporteur
for Question 18 of SG4 within
the ITU-T. He is working in the
Internet Network Architecture
group.

terje-fredrik.henriksen
@telenor.com

Routing

Forwarding

Incoming

routing

information

Incoming

traffic

(packets)

Outgoing

routing

information

Outgoing

traffic

(packets)

Control

Figure 1 The Best Effort IP router

Telektronikk 2/3.2001