Communicating Systems with UML 2: Modeling and Analysis of Network Protocols

Chapter 4

Non-reliable Transmission Mediums

4.1. Introduction

Chapter 3 presented a simple chat application, but that application assumes the existence of a perfect transmission medium, i.e. a medium in which there are neither losses nor unexpected delays or message disordering. However, this is not currently the case with the Internet.

Nowadays, applications use the TCP (Transmission Control Protocol) [RFC 93], a protocol to ensure the correct reception of data. However, this solution is not applicable for real-time multimedia data1. Most applications dealing with this kind of problem implement their own solutions but another possibility is the creation of a generic transport protocol that will make these problems transparent for all the applications.

Below, we propose a simple transport model and protocol to solve the losses and delay problem generated in the medium. To do so, we will divide our model into two parts:

1. The definition of an imperfect transport medium. This transport medium can lose and/or delay transmitted packets2. Let us take into account that the packet disordering problem is a natural consequence of a variable delay. This medium can be used to test if the proposed protocol behaves correctly in such cases. In particular, the medium must be dynamically configured, i.e. the defined loss rate and maximal delay limit should be able to be dynamically defined by a supervising user.

2. The proposal of a transport protocol solving the problem of losing and delaying ...