Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 25
Introduction to Transport Layer Protocols

Since IP provides for a connectionless delivery service of TCP (Transmission Control Protocol) data, TCP provides application programs access to the network, using a reliable connection–oriented transport–layer service. This protocol is responsible for establishing sessions between user processes on the internet, and also ensures reliable communications between two or more processes. The functions that it provides are to:

1.  Listen for incoming session establishment requests

2.  Request a session to another network station

3.  Send and receive data reliably using sequence numbers and acknowledgments

4.  Gracefully close a session

The User Datagram Protocol (UDP) provides application programs access to the network using an unreliable connectionless transport–layer service. It allows the transfer of data between source and destination stations without having to establish a session before data is transferred. This protocol also does not use the end–to–end error checking and correction that TCP uses. With UDP, transport–layer functionality is there, but the overhead is low. It is primarily used for those applications that do not require the robustness of the TCP protocol; for example, mail, broadcast messages, naming service, and network management.

The Real Time Protocol (RTP) and the Real Time Control Protocol (RTCP) allow for real–time applications to truly exist on an IP network. RTP resides at the transport layer and works alongside the TCP protocol, and is a replacement for the TCP protocol for real–time applications. RTCP is the protocol that provides feedback to the RTP application and lets the application know how things are going on the network. The protocols are actually frameworks more than protocols and are usually included in the application itself rather than residing as a separate protocol that has an interface.

Data is not the only information that is being passed around on the Internet. Multimedia applications such as voice and video are moving from experimental status to emerging. However, voice and video cannot simply be placed on a connectionless, packet switched network. They need some help, and RTP, along with RTCP, provides this help. This in conjunction with RSVP is paving the way for real–time applications on the Internet.