If two networks with the same capacity, A and B respectively, are connected to a third network D with the same capacity, packets from A and B cannot flow to network D at full speed.
When cars from two lanes merge into a single lane, the traffic slows down or stops completely (traffic jam!).
On the Internet, datagrams are discarded!
Though routers have memory and can store data during temporary congestion, if datagrams continue to arrive faster than they can leave, the router discards them until congestion clears.
Packet switching systems, like those used in the Internet, need additional communication software to ensure that data is delivered
Transmission Control Protocol (TCP) is the second major communication protocol (after IP)