What is UDF

The multimedia applications generally support one-tomany group communication. Multicasting decreases the communication costs for applications, which send the same data to multiple receivers. Table 1 summarizes the types of the communication among the hosts. Currently, there is an increasing need for scalable and efficient group communication. Theoretically, multicasting is optimal for such purposes. Therefore, this technology is an emerging media dissemination technology, instead of the traditional unicast communication. It has two important types: the networklevel, namely IP-multicast, and the Application-Layer, host-multicast. In the former one, the data packets are delivered by the IP protocol, from one host to many hosts that are member of a multicast group. The routers run an IP-multicast routing protocol in order to construct a multicast tree. Along this tree, the data is forwarded to each host. Special IP addresses (224.0.0.0 - 239.255.255.255 address range) are used, which do not belong to hosts, but rather define multicast channels. In the case of Application-Layer Multicast (ALM), the hosts use unicast IP delivery, and the routers do not play any special role. Reliability is one of the most important features of all multimedia applications, independently from the multicast technology in use. This requirement is especially critical in the case of multicast, where the large volume of data is to be transferred, and correction or resending of lost data is even more difficult in time. In the multicast technology, the maintenance of the group membership information is also an important question from the point of view of the robustness of the so-called multicast delivery tree. The root of the tree is the sender, the leaves are the receivers, and the intermediate nodes are the routers in case of the IP-multicast. In the following sections, the reliability properties of different multicast technologies are overviewed.