A protocol is a rule which guides how an activity should be performed. Formerly used mainly in the diplomatic and government fields of endeavor to denote unwritten guidlines, by the turn of the twenty first century it had come into wide use in the computer and communications fields.
In international law and international relations, a protocol is a treaty or international agreement that supplements a previous treaty or international agreement. A protocol can amend the previous treaty, or add additional provisions. Parties to the earlier agreement are not required to adopt the protocol; sometimes this is made clearer by calling it an 'optional protocol', especially where many parties to the first agreement do not support the protocol.
Some examples: the United Nations Framework Convention on Climate Change (UNFCCC) established a framework for the development of binding greenhouse gas emission limits, while the Kyoto Protocol contained the limits later agreed upon.
When used in computing, a protocol is a convention or standard that controls or enables the connection, communication, and data transfer between two computing devices. Protocols may be implemented by hardware, software, or a combination of the two. At the lowest level, a protocol defines a hardware connection.
The wide-spread use and expansion of protocols is both a prerequisite to the Internet, and a major contributor to its power and success. The Transmission Control Protocol (or TCP) was the first of these, and the term TCP-IP refers to an important collection of the most used protocols. Most communication protocols are established in the RFC documents of the Internet Engineering Task Force or IETF. See also Communications protocol for more information.
Object-oriented programming has extended the use of the term to include the protocol used for connections and communication between objects.