| So far, we have actually discussed only unreliable and connectionless multicast transmissions based on UDP. This type of transmission is generally the most frequently used application of multicast, mainly because it is much easier to handle. Nevertheless, there are application cases for connection-oriented and reliable multicast communication, and so extensive interesting research work is undertaken in this field. Because the tasks involved in the reliable and connection-oriented transmission of multicast data correspond mainly to the tasks of a transport protocol and these work on top of the IP Multicast, a layer-3 service, the protocols developed so far are normally called multicast transport protocols. The most important tasks of a transport protocol, including connection management, flow control, error correction, and congestion control, are relatively complex and expensive for unicast communication, and point-to-multipoint communication adds special problems to this situation. For example, consider the sender implosion problem, which occurs when many receivers return acknowledgements for received data packets to the sender, overloading the sender with an enormous data volume. We will not discuss multicast transport protocols any further at this point, because there is currently no protocol used as a standard under Linux. We do list a few protocols and research projects here. Some of these protocols have been implemented and evaluated. However, none of these protocols is especially suited for all multicast applications; each one has specific benefits and drawbacks. Real-Time Transport Protocol (RTP) for real-time and multimedia applications. Scalable Reliable Multicast (SRM) is currently used by the White Board tool. Uniform Reliable Group Communication Protocol (URGC) supports reliable and in-order communication. Muse an application-specific protocol for multicast news. Multicast File Transfer Protocol (MFTP) works much like the File Transfer Protocol (FTP). Local Group Concept (LGC) uses a hierarchy of local groups to prevent sender implosion.
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