Since more and more unicast real-time transactions like VoIP (Voice over IP) are carried on the packet based network, guaranteed good QoS of these transactions becomes an important issue. Traditional RSVP (Resource Reservation Protocol) is aimed at multicast real-time transactions such as VoD (Video on Demand). It does not fit some features of unicast transactions. In this paper, we present a new way to reserve resource on packet based network. Our scheme adds back-track capability into the process of resource reservation thus increasing the successful rate of setting up a Resource Reservation Path. Besides, our method is different from RSVP in that it is sender-oriented. So it can notify the session sender of abnormal events in the reservation process as soon as possible, fulfilling the need of some unicast real-time transactions such as VoIP.