The Network Layer for a protocol stack is responsible for getting a message from the source machine to the destination machine.However,there are often several programs running on the source machine and several programs running on the destination machine. The Transport layer for a protocol stack is responsible for getting the message to the right program on the destination machine. On the Internet there is one main protocol for the Network layer called IP(Internet Protocol).However,on the Internet there are two alternative protocols for the Transport layer called TCP (Transmission Control Protocol) and UDP(User Datagram Protocol). TCP is used by more of the important applications because it generates reliable delivery. Reliable delivery means that TCP ensures that all parts of a message will eventually reach the destination program on the destination machine undamaged. Unfortunately, in reality parts of a message sometimes will not reach the destination or will reach the destination corrupted. Thus, TCP has to have some means of detecting and recovering from the lost or damaged message parts (the TCP message parts are officially called segments,but we will call them packets which is a more message-like name).This means of recovering from errors is called error control.
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