In this paper,we propose a new scheme for the on demand use of capacity in OBS networks, combining a two-way reservation protocol and an assembly scheme process that incorporates an aggressive and forward resource reservation mechanism.The key idea is to tune the assembly timer to be equal to the time associated with the establishment of the end-to-endconnection (round-trip-time) and, thus, synchronize the resource reservation with the assembly process. In this way, upon the arrival of the first packet in the queue, reservation of resources may start simultaneously based on an aggressive prediction of the burst length.