XTP is a transport protocol. A protocol is an agreement between the communicating parties on how communication is to proceed. For traditional transport layer connections, it is an error free point-to-point channel to provide reliable, cost-effective data transport from the source machine to the destination machine. The XTP is a new protocol with higher data rates, lower bit error rates, user defined service reliability, support multi-communication and latency controls, more suitable for today or the near future as modern networks continue to develop. Compared with traditional point-to-point communications, the XTP can provide efficient multicast data delivery, and the XTP also provides a powerful mechanism for group communication.  Multicasting represents a powerful mechanism for group communication. The multicast service provides a set of hosts to participate in real-time data transfer via a multicast call. This report describes the design and implementation of m by n multicasting in the Sandia XTP.