Optical Burst Switching (OBS) has been proposed to increase the resource utilization achieved by Optical Circuit Switching (OCS), which is impaired by its coarse granularity. A crucial step toward OBS maturity deals with the feature of lossless guaranteed transfers. No contention resolution mechanism, even combined with efficient pro-active mechanisms has been able to provide such a guarantee so far. In this paper, we investigate further the ldquooffset based priorityrdquo and the ldquostreamline effectrdquo to identify flow isolation patterns. We then propose a specific medium access protocol in order to preserve the isolation of transit bursts over ingress bursts. The isolation patterns are translated into an MILP optimization OBS model that performs routing and wavelength assignment (RWA-OBS). Using the routes obtained with RWA-OBS guarantees asynchronous loss-less transfers. It preserves the statistical multiplexing potential and the asynchronous nature of OBS, and thus addresses the drawbacks of WR-OBS and SOBS which were recently introduced in polymorphous OBS (POBS) (Qiao et al. 2006). Experimental results show the benefits of RWA-OBS over WR-OBS in terms of grade of service, while sidestepping the synchronization issues of SOBS with similar grade of service.