The disadvantage of unicast is that each client that connects to the server receives a separate stream, which rapidly uses up network bandwidth. IP Multicast refers to the networking technique in which one computer sends a single copy of the data over the network and many computers receive that data. Unlike a broadcast, routers can control where a multicast travels on the network. When streaming multimedia over the network, the advantage to multicasting is that only a single copy of the data is sent across the network, which preserves network bandwidth. To use IP multicast on a network, the network routers must support the IP Multicast protocol.
Most routers now handle multicast.
Generally, each configures the data into packets, with each packet having a 'header' that identifies its contents. The protocol used is usually determined by the need to have reliable or unreliable communications. This protocol is used to guarantee sequenced, error-free transmission, but its very nature can cause delays and reduced throughput. This can be especially annoying when streaming audio and video. RTP is a derivative of UDP in which a time-stamp and sequence number is added to the packet header.