The IETF is currently focused on Differentiated Services (Diffserv) as the architecture to provide Quality of Service in IP networks (IP-QoS). The Diffserv architecture consists of two key components: traffic conditioning at the edge and Per Hop Behavior (PHB) at the core. Traffic Conditioners are realized through various building blocks such as classifier, meter, marker, policer etc. This paper focuses on an empirical study of pre-Diffserv Internet traffic. We use the results of the study to understand design choices for some of the Diffserv architecture building blocks.The primary contribution is an investigation into the usage of the Type-of-Service (TOS) field [1] marking to understand if its usage justifies the IETF Diffserv WG's standardization of Class Selector PHB based on backwards compatibility with the TOS octet. A second contribution is a detailed examination of Internet and Intranet traffic traces to understand issues related to packet classification in Diffserv edge routers. In particular, we study the need for classifiers that maintain Per-flow State and utilize techniques based on inspection of Layer-7 header/ payload.