In this paper we study the stability of the Open Box Transport Protocol (OBP), an explicit congestion control protocol that provides information to the end systems about the current state of the network path. To support the study we deduced the OBP transfer function, which modulates the OBP behavior at equilibrium. In the stability evaluation we use Bode and Nyquist diagrams. The OBP stability characteristics, discussed in this work, are only related with the equilibrium phase, in other words when all the network capacity is in use. The results show that the OBP is stable for the tested scenarios and the transfer function does not have unstable poles. We also present an evaluation process that helps to identify the OBP performance. We show that the OBP reaches high utilization of the bottleneck channel and has fairness skills.