Multicast is a type of network traffic which lies between unicast (send one receive one type of network communication) and broadcast (send once receive all type of network communication). Multicast is can be defined as "send once receive many" type of network communication.
Multicast type of network communication allows a server to send single copies of data packet as streams and later distribute those packets only to computers interested and request that traffic. The computers those are not interested and not request that traffic will not receive a copy. Multicast is has many advantages for network applications like Online TVs, Video conferencing, Virtual classroom applications, Stock exchange applications etc.
Multicast removes some disadvantages of unicast and broadcast type of network communication for streaming type of network communication. Please note that multicast is not an alternative for unicast. But multicast sent to all computers is broadcast.
Advantages of multicast type of network communication
- Multicast removes the overload burden of the server (sending multicast) in sending copies to all computers as unicast (sending data packets one by one to all computers).
- Multicast reduces the CPU load on client computers by identifying and discarding the data packets the clients are not interested at the datalink layer itself.
- Multicast removes the overload burden of client computers who are not interested in that multicast traffic in processing unwanted data packets.
- Multicast type of network communication is highly bandwidth saving. For example; data size of many video-streaming type of traffic is very high. Multicast type of network communication allows huge save in bandwidth requirement by delivering a single stream to multiple recipients.
- Multicast reduces the number of streaming servers required. If heavy video streaming is done by unicast type of network communication, more server hardware and other resources are needed.
Disadvantages of multicast type of network communication
- Since multicast is streaming type of network communication, UDP (User Datagram Protocol) is used as the transport layer protocol. There is no reliabilty, flow control or error recovery functions in UDP.
- Since TCP window mechanism is not availabe in UDP, possibility of congestion is there in multicast.
- There is no reliability in multicast and reliability must be maintained by multicast applications.
- There are chances for generation of duplicate packets in multicast.
- Possibility of out-of-sequence delivery is more in multicast.