Definition: A protocol is a set of formal rules that describe how devices or software can exchange information with one another across a computer or communications network.
In classical Greek, the word protocollon (literally, "first leaf") referred to the first sheet of paper or papyrus in a scroll, which was actually glued to the wooden scroll itself. By custom, this sheet was used to describe the contents of the entire scroll. When books replaced scrolls, the protocollon continued as a table of contents page glued into the front of the book.
For a long time, the word protocol was used primarily to refer to the etiquette of diplomacy and formal arrangements of affairs of state: seating arrangements, how to address dignitaries and so on. Later, the term became a name for a type of treaty or international agreement. Perhaps the best known of those in recent memory are the Montreal and Kyoto protocols, which are environmental agreements on greenhouse gases and global warming.
But information technology has co-opted the term, as it has so many others, giving it new meaning in an entirely different context. As first applied to technology in the 1950s, protocols were rules governing communication between electronic devices such as radios and telephones.
As electronic communication grew and computers came into widespread use, computing protocols were created to control the design of and interaction among various types of networks.
IT protocols today describe any set of rules that allow different machines or pieces of software to co-ordinate with one another unambiguously. The use of communication and computing protocols requires a common message format and an accepted set of commands that all parties to a communications exchange will understand. Thus protocols ensure that electronic communication transactions follow predictable, logical sequences.
A protocol is an agreed-upon set of rules typically used by network designers and developers to resolve a particular communications challenge. A protocol must be generally accepted as an industry standard before it can be widely used. A protocol becomes a standard when a standards development organisation or other respected group recognises and codifies it.
For example, Ethernet is a LAN protocol that forms the underlying transport vehicle used by several upper-level communication protocols, including TCP/IP. Originally developed by Xerox, Digital Equipment and Intel, Ethernet has since become a formal standard, accepted by the Institute of Electrical and Electronic Engineers Inc. as IEEE 802.3.
A frame-based networking technology, Ethernet defines wiring and signalling for the physical layer as well as data packet formats and protocols for the data link layer as described in the International Standards Organisation's seven-layer Open Systems Interconnection reference model. Ethernet specifies how data is broken up into discrete packets, how the network will be accessed and how the data is to be transmitted. The protocol also specifies how it will interact with both higher- and lower-level protocols.
Just as standards build upon one another, most protocols depend on other, related protocols to work properly in a broader context. This arrangement is often called a hierarchical protocol stack.
For example, low-level protocols such as Ethernet define electrical and physical standards, the order in which bits and bytes are interpreted and the transmission and error-detection/correction systems used in the bit stream.
Higher-level protocols deal with the way data is formatted, including the syntax of messages, dialogues between terminals and host computers, which character sets are used, how messages are properly sequenced and more. The complete protocol stack supports applications such as Web browsing or end-to-end telephone calls between voice-over-IP telephones.
Today, the Internet, the Web and other private and public networks simply couldn't function without the existence and acceptance of scores of specific protocols.