This layer enhances the IP layer by providing a mechanism by which the datagram can be directed to the appropriate APPLICATION LAYER.

  This layer can also provide reliable streaming services to these applications using it.

  The two transport protocols are

1. User Datagram Protocol (UDP).
2. Transmission Control Protocol (TCP).

 Multiplexing and de-multiplexing the data from many applications to and from the IP layer and directing the data to the correct application is one of the responsibilities of the Transport Layer.

 Both UDP and TCP use port addresses to deliver the information to the correct application layer service.

 A port is a 16-bit address. A number of these have been previously allocated for the most common services. These ports are called as well known ports (0-255).

 Look at a listing of the well-known port numbers.

 A socket is typically used with TCP/IP. A socket is a concatenation of the IP address and the port number.

 A socket can uniquely identify the application layer service.

 Note that well-known port numbers are used only by server level application layer services.

 Client program services select a unique port number that has not been already used on their machine. This is done to ensure that no two sessions use the same socket!

Back to Main