Connection-Oriented and Connectionless Service.

A layer can offers two different types of services to the layer immediately above it – connection-oriented and connectionless service.

In connection oriented service user establish a connection at first, communicates over the connection and at last release the connection. It is similar to a telephone system.
In connectionless service no connection is established beforehand. Instead, like in a postal system, every message caries its full destination address and each routed independently to its destination, then in connectionless service sometime it may happen that the first piece may arrive at the destination after the latter pieces. 

That means at the receiving end the order of delivery may not be same as the  order of transmission. In connection oriented service this never happens

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