DoD did not want conversation to be broken off. DoD wanted connection to remain intact as long as source and destination were functioning. A flexible architecture was needed.The architecture later became known as TCP/IP Reference model,after its 2 primary protocols.
The Internet Layer
Packet switching n/w based on a connectionless internetwork layer.This layer called the Internet Layer.
The analogy is with the (snail) mail system. Most of them will be delivered to the correct address in the destination country.
The Internet layer defines an official packet format and Protocol called IP(Internet Protocol). The job of internet layer is to deliver IP packets where they are supposed to go.Packet routing is the major issue,as is avoiding congestion. TCP/IP internet layer is similar in functionality to OSI n/w layer.