Connection-Oriented and Connectionless Network Services

Transport protocols are characterized as either connection-oriented or connectionless:

• Connection-oriented services must first establish a connection between the two end-points (sending/receiving) before passing any data traffic between them. An example of a connection-oriented service is Frame Relay, where a VC (virtual connection) is required between both end-points before data traffic can be exchanged. Connection-oriented service involves three phases:

  - Connection establishment

  - Data transfer

  - Connection termination

  During connection establishment, the end-points can reserve resources and negotiate traffic parameters for the connection; for example, to ensure Quality of Service (QoS), further discussed in Chapter 13, "Quality of Service Introduction."

• Connectionless services can send data without requiring an established connection. Connection-oriented services provide some level of delivery guarantee, whereas connectionless services do not. An example of a connectionless service is any IP service, such as the Internet. No established connection is made between a web browsing user and the home page being viewed.

During data transfer, most connection-oriented services will monitor for lost packets and handle resending them. The protocol also is responsible for putting the packets in the right sequence before passing the data up the protocol stack.

Connection-oriented network services have more overhead than connectionless ones. Connection-oriented services negotiate a connection, transfer data, and tear down the connection, whereas a connectionless transfer can send the data without the additional overhead required to create and tear down connections.