Lecture 11: Reliable Transport -- TCP

User Datagram Protocol

The User Datagram Protocol (UDP) provides an alternative, connectionless, transport service to TCP for applications where reliable stream service is not needed. UDP datagrams can be droppped, duplicated or delivered out of order, exactly as for IP.

The UDP transport service adds to IP the ability to deliver a datagram to a specified destination process using a port abstraction, in an analogous way to that used by TCP.

UDP segments (also commonly called datagrams, see later) have a minimal (8-byte) header. Data transfer with UDP has no initial connection overhead, and (obviously) no waiting for ACK segments as in TCP. Some typical UDP-based services include DNS, streaming multimedia and "Voice over IP" applications. It's also worth noting that an application can elect to use UDP where it's prepared to implement its own reliability -- some proprietory streaming protocols do this, for example.

NB: UDP communications, at the programming level, is based on sockets, as for TCP. However instead of reading from, and writing to, a socket in a stream-based model as for TCP, a UDP socket supports the operations send and receive, which are based on packet-sized chunks of data.

The tutorial for this lecture is Tutorial #11.
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