Each and every computer networks provide some services to their users, that they implement using some certain rules or protocols about legal message exchanges.
All computer networks provide some services to the hosts and processes using them.
After the telephone system, the connection-oriented service is modelled.
As in telephone system, if you want to talk to someone, for example, your friend, then you have to do the following things one by one:
Similarly, to use a connection-oriented network service, the service users have to do the following things one by one:
The essential aspect of a connection is that it acts like a tube, that is, the sender pushes the objects in at one end, and the receiver takes them out in the same order at the other end.
Each and every computer networks have some highly-specialized rules for what messages may be sent under the certain circumstances, for example, file transfer, whenever any message is sent from the source to the destination, then the destination is required to send an acknowledgement back indicating the correct receipt of the message.
Now, under some other circumstances, for example, digital telephony, no such acknowledgement is expected.
The set of rules through which particular computer communicate is called as a protocol.