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.

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 and or connection.

Importance

The widespread use and expansion of communications protocols is both a prerequisite for 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, and 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)

Protocol testing

In general, protocol testers work by capturing the information exchanged between a Device Under Test (DUT) and a reference device known to operate properly. In the example of a manufacturer producing a new keyboard for a personal computer, the Device Under Test would be the keyboard and the reference device, the PC. The information exchanged between the two devices is governed by rules set out in a technical specification called a "communication protocol". Both the nature of the communication and the actual data exchanged are defined by the specification. Since communication protocols are state-dependent (what should happen next depends on what previously happened), specifications are complex and the documents describing them can be hundreds of pages.

The captured information is decoded from raw digital form into a human-readable format that permits users of the protocol tester to easily review the exchanged information. Protocol testers vary in their abilities to display data in multiple views, automatically detect errors, determine the root causes of errors, generate timing diagrams, etc.

Some protocol testers can also generate traffic and thus act as the reference device. Such testers generate protocol-correct traffic for functional testing, and may also have the ability to deliberately introduce errors to test for the DUT's ability to deal with error conditions.

Protocol testing is an essential step towards commercialization of standards-based products. It helps to ensure that products from different manufacturers will operate together properly ("interoperate") and so satisfy customer expectations. This type of testing is of particular importance for new emerging communication technologies.

Manufacturers of protocol test equipment include Absolute Analysis, which has been providing protocol test solutions to developers since 1991.

See also

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