TCP/IP and UDP/IP communications

There are two communication protocols that one can use for socket programming: datagram communication and stream communication

Datagram communication (UDP):

The datagram communication protocol, known as UDP (user datagram protocol), is a connectionless protocol, meaning that each time you send datagrams, you also need to send the local socket descriptor and the receiving socket's address. As you can tell, additional data must be sent each time a communication is made. In other words, “a datagram is an independent, self-contained message sent over the network whose arrival, arrival time, and content are not guaranteed.”

Stream Communication or TCP (Transmission Control Protocol):

The stream communication protocol is known as TCP (transmission control protocol). Unlike UDP, TCP is a connection-oriented protocol.

To do communication over the TCP protocol, a connection must first be established between the pair of sockets. While one of the sockets listens for a connection request (server), the other asks for a connection (client). Once two sockets have been connected, they can be used to transfer data in both (or either one of the) directions.

Now, you might ask what protocol you should use -- UDP or TCP? This depends on the client/server application you are writing. The following discussion shows the differences between the UDP and TCP protocols; this might help you decide which protocol you should use.

In UDP, as you have read above, every time you send a datagram, you should send the local descriptor and the socket address of the receiving socket along with it. Since TCP is a connection-oriented protocol, on the other hand, a connection must be established before communications between the pair of sockets start. So, there is a connection setup time in TCP.

In UDP, there is a size limit of 64 kilobytes on datagrams you can send to a specified location, while in TCP there is no limit. Once a connection is established, the pair of sockets behaves like streams: All available data are read immediately in the same order in which they are received.

UDP is an unreliable protocol -- there is no guarantee that the datagrams you have sent will be received in the same order by the receiving socket. On the other hand, TCP is a reliable protocol; it is guaranteed that the packets you send will be received in the order in which they were sent.

In short, TCP is useful for implementing network services -- such as remote login (rlogin, telnet) and file transfer (FTP) -- which require data of indefinite length to be transferred. UDP is less complex and incurs fewer overheads. It is often used in implementing client/server applications in distributed systems built over local area networks.