The global Internet has seen tremendous growth in terms of nodes and user base as well as of types of applications. One of the most important consequences of this growth is related to an increased complexity of the traffic experienced in these networks. Each application has a set of unique characteristics in terms of its performance characteristics, its transactions as well as the way the transaction processing profile maps onto unique network resource requirements. In order to support Internet applications effectively, it is therefore important to understand and to characterise the application level transactions. Recent advances in high resolution traffic monitoring and analysis capabilities have enabled us to build up realistic models for diverse network applications. In this paper we report investigations of classical applications such as FTP, SMTP, and HTTP to evaluate end-to-end network performance requirements. Our results show the presence of a robust correlation structure in the traffic streams that has a fundamental bearing on the user perceived quality of the applications.