In this paper we present a signaling protocol for QoS differentiation suitable for optical burst switching networks. The proposed protocol is a two-way reservation scheme that employs delayed and in-advance reservation of resources. In this scheme delayed reservations may be relaxed, introducing a reservation duration parameter that is negotiated during call setup phase. This feature allows bursts to reserve resources beyond their actual size to increase their successful forwarding probability and is used to provide QoS differentiation. The proposed signaling protocol offers a low blocking probability for bursts that can tolerate the round-trip delay required for the reservations. We present the main features of the protocol and describe in detail timing considerations regarding the call setup and the reservation process. We also describe several methods for choosing the protocol parameters so as to optimize performance and present corresponding evaluation results. Furthermore, we compare the performance of the proposed protocol against that of two other typical reservation protocols, a Tell-and-Wait and a Tell-and-Go protocol.