We address the issue of providing differentiated services to IP packets over an optical burst switched core network, and we introduce a new approach for assembling packets into a burst. In this technique, a composite burst is created by combining packets of different classes into the same burst. The packets are placed from the head of the burst to the tail of the burst in order of decreasing class. The performance of this approach is enhanced by using a burst segmentation technique in which, during burst contention, only the packets in the tail of a burst are dropped. We describe a generalized model for burst assembly and burst scheduling, and we propose several composite burst assembly methods. We observe that having multiple classes of packets in a burst performs better than having a single class of packets in a burst.