At present, the unicast transmission mode is usually used over Internet.  The unicast transmission mode allows a single transmitter to transmit data to a single receiver. 
Unlike the unicast transmission mode, the multicast transmission mode allows a single transmitter to transmit data to multiple receivers.  The multicast transmission mode has been researched and developed in various research institutions.  An Internet protocol (IP) multicast transmission mode is the multicast transmission mode that has been developed for common use and is being used at present.  The IP multicast transmission mode is network-level multicast in which a transmitter marks the address of a group, to which a receiver belongs, in the header of a data transmission packet instead of marking the address of the receiver and transmits multicast data to the router of a local group.  Accordingly, the IP multicast transmission mode is advantageous in minimizing the waste of network resources caused by repetition of data transmission. 
Since not an actual host address but a group address according to a D-class IP address is marked on the header of a transmission packet in the IP multicast transmission mode, a special router for supporting the transmission of multicast data packets is required.  In the meantime, most of present Internet routers do not support the transmission of multicast data packets.  Accordingly, to realize IP multicast, all routers on a data transmission path must be replaced with routers supporting the multicast data packet transmission.  Accordingly, it is impossible to actually apply the IP multicast transmission mode to the Internet. 
To overcome this problem, another IP multicast transmission mode, in which packets encapsulated based on the concept of tunneling are transmitted, is used. 
In the IP multicast transmission mode using tunneling, a tunnel among multicast routers is set.  Thereafter, when a multicast router supporting multicast routes a data packet, the multicast router adds the IP addresses of both ends of the tunnel to the front of the header of the data packet.  In other words, when the multicast data packet passes through usual routers which do not support multicast, the multicast data packet is routed in the same manner as unicast packets, based on the IP addresses of both ends of the tunnel.  Accordingly, the multicast transmission mode using tunneling can be implemented in the present Internet environment.  However, the IP multicast transmission mode using tunneling can be applied to only a very small number of receivers having a router supporting multicast.  Moreover, since information on final receivers cannot be obtained, it is impossible to bill them for commercialized multicast services. 
Multicast transmission modes researched at present can be largely divided into network-level multicast and application-level multicast. 
In the network-level multicast transmission mode, a router is provided with an additional function of performing distributed transmission in hardware.  In order to apply the network-level multicast transmission mode to the Internet, all of the routers on the Internet must be replaced with routers having the additional function.  Consequently, it is impossible to actually apply the network-level multicast transmission mode to the internet. 
In the application-level multicast transmission mode, a primary receiver receiving data from a transmitter transmits the data to a secondary receiver.  In the case where an unspecified number of users simultaneously request access and thus the number of connection paths rapidly increases, it is difficult for a transmitter to manage the connection paths.  Accordingly, the number of users that are allowed for simultaneous access is limited.  In addition, in the case of disconnection of an upper receiver, maintenance of connection of a large number of lower receivers and the reliability of data transmission cannot be secured.