We model the TCP/IP network with non persistent sessions on three levels: Packet level, Session level and System level. On the packet level we character- ize the packet arrival process at the bottleneck queue and we calculate the packet loss probability using the fixed point approach. In particular, we study by simulation under which conditions the multiplexed traffic is close to Poisson. On the session level, using the fluid model approach and conditioning on the number of loss instants, we calculate the average transfer time of the TCP flows as a function of the packet loss probability and the parameters of the TCP protocol. We show that the expected latency conditioned on the fact that a packet has been lost is not monotone with respect to the file size. Finally, on the system level we apply the M/G/ model to obtain the distribution of the number of active sessions. All analytical results are confirmed by NS simulations.