A protocol is a set of rules. Internet protocols are sets of rules governing communication within and between computers on a network. Protocol specifications define the format of the messages that are exchanged. A letter sent through the postal system also uses protocols. Part of the protocol specifies where the delivery address on the envelope needs to be written. If the delivery address is written in the wrong place, the letter cannot be delivered.

Timing is crucial for the reliable delivery of packets. Protocols require messages to arrive within certain time intervals so that computers do not wait indefinitely for messages that might have been lost. Systems maintain one or more timers during the transmission of data. Protocols also initiate alternative actions if the network does not meet the timing rules.

These are the main functions of protocols:

Devices and computers connected to the Internet use a protocol suite called TCP/IP to communicate with each other. The information is transmitted most often via two protocols, TCP and UDP, as shown in the figure.

In the design of a network, you must determine the protocols that are going to be used. Some protocols are proprietary and only work on specific equipment, while other protocols are open standard and work on a variety of equipment.