This paper introduces a discrete-time model which captures the essential protocol features of the congestion control mechanism used by the TCP Reno protocol, subject to interference from other sources. Under this model, a single target session is modeled according to the TCP Reno mechanism, including fast retransmit and fast recovery. At the same time, other sources are modeled as a background process using a modified discrete batch Markov arrival process. In order to capture all the TCP Reno protocol features, two levels of Markov process modeling are used: a microscopic level, at the packet transmission time boundaries, and a macroscopic one, at the start of the new transmission windows. Several performance measures are derived, and numerical examples which demonstrate the protocol features are presented.