We present a model of TCP flows suited to Internet backbone traffic, where links are usually not congested. We characterize the traffic using information on flows, i.e., arrival time, size, and duration. The major contribution of this paper is a model capturing the variation of the transmission rate during a TCP flow solely based on the above flow parameters. This model accounts for the dynamics of TCP congestion window and for the Timeout mechanism. It is independent of the packet loss rate and the round-trip time of the connection. We then model the traffic on a backbone link by aggregating TCP flows. Our model is easy to compute, and we show via simulations that it gives a good approximation of Internet backbone traffic.