Connection-Oriented and Connectionless Services

Layers can offer different types of services to the layers above them:

• Connection-oriented
• Connectionless.

In the Connection-oriented service, the service user first establishes a connection, uses the connection and then releases the connection. The essential aspect of a connection is that it acts like a tube: the sender pushes objects (bits) in at one end, and the receiver takes them out in the same order at the other end.

In the Connectionless service, each message carries the full destination address and each one is routed through the system independent of all the others. Normally, when two messages are sent to the same destination, the first one sent will be the first one to arrive. However, it is possible that the first one sent. It can be delayed so that the second one arrives first. With a connection-oriented service this is impossible.