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There are two major transport protocols used on the Internet:

• User Datagram Protocol (UDP)
• Transport Control Protocol (TCP)

User Datagram Protocol (UDP)

A Connectionless Protocol. Provides for communication that is not guaranteed between two applications on the Internet. It acts rather like the postal service, where letters and packages are sent, but delivery is not guaranteed, and multiple pieces of mail are independent of each other, and may arrive in a different order from the order in which they were sent.

UDP uses independent, self-contained packets of data called datagrams.

Some applications don't need reliable, guaranteed delivery of messages. A time service, or the ping service are examples.

Transport Control Protocol (TCP)

A Connection-oriented Protocol. Provides for reliable communication between two applications on the Internet. It is analogous to making a telephone call: One side initiates the connection and the other side answers; messages are sent back and forth over the connection, and they arrive in the same order in which they were sent. The protocol guarantees the messages sent actually arrive, or else an error is reported.

Most application level protocols (such as HTTP, ftp, telnet, etc.) rely on the TCP protocol to ensure that the data sent back and forth actually arrives, and in the proper order.