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 models experienced in the networks. Each application has a set of unique characteristics in terms of the way it performs its transactions as well as the way transaction processing profile maps onto unique network resource requirements. In order to support Internet applications effectively, it is therefore important to understand and to characterize the application level transactions and also to investigate their scaling properties. Recent advances in high resolution traffic monitoring and analyzing capabilities have enabled us to build up realistic models for the TCP/IP protocol stack with diverse network applications. In this paper we report investigations of classical applications such as FTP, SMTP, and HTTP to evaluate end-to-end performance requirements and accordingly to assess end-user performance like Service Level Agreement (SLA) for WWW. 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.