VoIP

User Datagram Protocol (UDP)

Generally, there are two protocols available at the transport layer when transmitting information through an IP network. These are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Both protocols enable the transmission of information between the correct processes (or applications) on host computers. These processes are associated with unique port numbers (for example, the HTTP application is usually associated with port 80). TCP is a connection oriented protocol; that is, it establishes a communications path prior to transmitting data. It handles sequencing and error detection, ensuring that a reliable stream of data is received by the destination application. Voice is a real-time application, and mechanisms must be in place with ensure that information is received in the correct sequence, reliably and with predictable delay characteristics. Although TCP would address these requirements to a certain extent, there are some functions which are reserved for the layer above TCP. Therefore, for the transport layer, TCP is not used, and the alternative protocol, UDP, is commonly used. In common with IP, UDP is a connectionless protocol. UDP routes data to it's correct destination port, but does not attempt to perform any sequencing, or to ensure data reliability.