An empirical study of Internet services on IP over ATM links is conducted in this paper, in order to obtain a stochastic model of TCP flow in terms of network parameters (burstiness and throughput). We note that the link usage is primarily determined by short TCP connections, and so is the QoS perceived by the end user, which can be measured by the connection throughput. The impact of the connection establishment phase in such user-perceived throughput is striking. On the other hand, the TCP slow start dominates the flow lifetime, since only a few packets (less than 10 with 80% probability) are transmitted per connection. Thus, the influence of the TCP dynamics are critical for user-perceived QoS. Besides this throughput characteristics, we note that connections introduce very low burstiness per connection at the network layer.