Connection-oriented:

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 at the other end.
A circuit is another name for a connection with associated resources, such as a fixed bandwidth.
Reliable connection-oriented service has two minor variations: message sequences and byte streams. In the former variant, the message boundaries are preserved. When two 1024-byte messages are sent, they arrive as two distinct 1024- byte messages, never as one 2048-byte message.

In the latter, the connection is simply a stream of bytes, with no message boundaries. When 2048 bytes
arrive at the receiver, there is no way to tell if they were sent as one 2048-byte message, two 1024-byte messages, or 2048 1-byte messages.

Connectionless Service:

Each message (letter) carries the full destination address, and each one is routed through the intermediate nodes inside the system independent of all the subsequent messages.
A packet is a message at the network layer. When the intermediate nodes receive a message in full before sending it on to the next node, this is called store-and-forward s witching. The alternative, in which the onward transmission of a message at a node starts before it is completely received by the node, is called cut-through s witching.

Voice over IP: For some applications, the transit delays introduced by acknowledgements are
unacceptable. One such application is digitized voice traffic for voice over IP. It is less disruptive for
telephone users to hear a bit of noise on the line from time to time than to experience a delay waiting for acknowledgments.
Unreliable (meaning not acknowledged) connectionless service is often called datagram service. The six different types of service are