Protocols for Transporting Data

There are many protocols for communicating between applications. Protocols are sets of rules that govern how two entities should interact. In this chapter, we focus on Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). .NET's TCP and UDP networking capabilities are defined in the System.Net.Sockets namespace.

Transmission Control Protocol (TCP) is a connection-oriented communication protocol which guarantees that sent packets will arrive at the intended receiver undamaged and in the correct sequence. TCP allows protocols like HTTP (Chapter 21) to send information across a network as simply and reliably as writing to a file on a local computer. If packets of information don't arrive at the recipient, TCP ensures that the packets are sent again. If the packets arrive out of order, TCP reassembles them in the correct order transparently to the receiving application. If duplicate packets arrive, TCP discards them.

Applications that do not require TCP's reliable end-to-end transmission guaranty typically use the connectionless User Datagram Protocol (UDP). UDP incurs the minimum overhead necessary to communicate between applications. UDP makes no guarantees that packets, called datagrams, will reach their destination or arrive in their original order.

There are benefits to using UDP over TCP. UDP has little overhead because UDP datagrams do not need to carry the information that TCP packets carry to ensure reliability. UDP also reduces network traffic relative to TCP due to the absence of handshaking, retransmissions, etc.

Unreliable communication is acceptable in many situations. First, reliability is not necessary for some applications, so the overhead imposed by a protocol that guarantees reliability can be avoided. Second, some applications, such as streaming audio and video, can tolerate occasional datagram loss. This usually results in a small pause (or "hiccup") in the audio or video being played. If the same application were run over TCP, a lost segment could cause a significant pause, since the protocol would wait until the lost segment was retransmitted and delivered correctly before continuing. Finally, applications that need to implement their own reliability mechanisms different from those provided by TCP can build such mechanisms over UDP.