Protocol (computing)

For other senses of this word, see protocol.

In computing, a protocol is a convention or standard that controls or enables the connection, communication, and data transfer between two computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax, semantics, and synchronization of communication. Protocols may be implemented by hardware, software, or a combination of the two. At the lowest level, a protocol defines the behavior of a hardware connection.

Meaning: Set of rules.

Typical properties

It is difficult to generalize about protocols because they vary so greatly in purpose and sophistication. Most protocols specify one or more of the following properties:

• Detection of the underlying physical connection (wired or wireless), or the existence of the other endpoint or node
• Handshaking
• Negotiation of various connection characteristics
• How to start and end a message
• How to format a message
• What to do with corrupted or improperly formatted messages (error correction)
• How to detect unexpected loss of the connection, and what to do next
• Termination of the session or connection.

Importance

The widespread use and expansion of communications protocols is both a prerequisite to the Internet, and a major contributor to its power and success. The pair of Internet Protocol (or IP) and Transmission Control Protocol (or TCP) are the most important of these, and the term TCP/IP refers to a collection (or protocol suite) of its most used protocols. Most of the Internet's communication protocols are described in the RFC documents of the Internet Engineering Task Force (or IETF).

The protocols in human communication are separate rules about appearance, speaking, listening and understanding. All these rules, also called protocols of conversation, represent different layers of communication. They work together to help people successfully communicate. The need for protocols also applies to network devices. Computers have no way of learning protocols, so network engineers have written rules for communication that must be strictly followed for successful host-to-host communication. These rules apply to different layers of sophistication such as which physical connections to use, how hosts listen, how to interrupt, how to say good-bye,in short how to communicate, what language to use and many others. These rules, or protocols, that work together to ensure successful communication are groups into what is known as a protocol suite.

Object-oriented programming has extended the use of the term to include the programming protocols available for connections and communication between objects.

Generally, only the simplest protocols are used alone. Most protocols, especially in the context of communications or networking, are layered together into protocol stacks where the various tasks listed above are divided among different protocols in the stack.

Whereas the protocol stack denotes a specific combination of protocols that work together, a reference model is a software architecture that lists each layer and the services each should offer. The classic seven-layer reference model is the OSI model, which is used for conceptualizing protocol stacks and peer entities. This reference model also provides an opportunity to teach more general software engineering concepts like hiding, modularity, and delegation of tasks. This model has endured in spite of the demise of many of its protocols (and protocol stacks) originally sanctioned by the ISO. The OSI model is not the only reference model however.

Common protocols

• IP (Internet Protocol)
• UDP (User Datagram Protocol)
• TCP (Transmission Control Protocol)
• DHCP (Dynamic Host Configuration Protocol)
• HTTP (Hypertext Transfer Protocol)
• FTP (File Transfer Protocol)
• Telnet (Telnet Remote Protocol)
• SSH (Secure Shell Remote Protocol)
• POP3 (Post Office Protocol 3)
• SMTP (Simple Mail Transfer Protocol)
• IMAP (Internet Message Access Protocol)

See also

• Internet protocol suite
• Communications protocol
• List of network protocols
• Application programming interface
• Calling convention