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Communication protocol

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A communication protocol is a formal system of rules. These rules allow two or more entities in a communications system to transmit information. Without these rules, different machines could not understand each other. Protocols define the syntax, which is the structure of the messages. They also define semantics, which is the actual meaning of the data. Furthermore, they handle synchronization to keep devices in time. They even include methods for error recovery if data is lost.

Internet layering
Internet layering

To understand how they work, think of a protocol as a language. Some experts say protocols are to communication what algorithms are to computation. In digital systems, these rules are often expressed through algorithms and data structures. When a message is sent, it is usually divided into two parts. The first part is called the header. This area contains fields relevant to the protocol's operation. The second part is the payload. This is where the actual message or data lives.

Message flows
Message flows

Communication often happens through a layered approach. Instead of using one single protocol, systems use a protocol suite. A protocol suite is a group of cooperating protocols. When these are implemented in software, they are called a protocol stack. Each layer in the stack handles one specific part of the problem. For example, one layer might handle application functions. Another might handle transport or network interface functions. To send a message, the system selects a protocol from each layer. It then extends the message with a protocol selector for each level.

Protocol and Software layering
Protocol and Software layering

There are two primary types of protocols based on how they represent content. The first type is the text-based protocol. These use human-readable formats like ASCII or UTF-8. They might use structured formats like XML or JSON. Text-based protocols are great for humans to inspect during debugging. Examples include FTP, SMTP, and early versions of HTTP. The second type is the binary protocol. These use all available values in a byte. They are designed for machines rather than humans. Binary protocols are very terse, which makes them fast and efficient. Modern standards like HTTP/3 use binary protocols.

History shows how these rules evolved through many brilliant minds. The term "protocol" first appeared in a 1967 memorandum for the NPL network. This was written by Roger Scantlebury and Keith Bartlett under Donald Davies. In 1969, Bob Kahn wrote the 1822 protocol for the ARPANET. Later, in 1970, Steve Crocker and Jon Postel developed the Network Control Program. This was an early example of the protocol layering concept. Louis Pouzin later designed the CYCLADES network in the early 1970s. His team implemented the end-to-end principle, making hosts responsible for reliable data delivery.

The 1970s were a busy time for protocol development. Bob Kahn and Vint Cerf worked together on the Transmission Control Program, or TCP. Cerf, Yogen Dalal, and Carl Sunshine wrote its specification in December 1974. This design was originally monolithic, meaning it was one single piece. Later, TCP was redesigned into a modular stack called TCP/IP. This modular version was installed on the ARPANET in January 1983. By 1989, a complete Internet protocol suite was developed. This laid the foundation for the modern Internet we use today.

Today, many different organizations manage these important standards. The Internet Engineering Task Force, or IETF, publishes Internet protocols. The IEEE handles rules for wired and wireless networking. The International Organization for Standardization, or ISO, manages other types. For telephone networks, the ITU-T handles telecommunications protocols. There was once a period of competition between different models. Engineers debated whether the OSI model or the Internet protocol suite was better. Eventually, the Internet protocol suite became the core component of the emerging Internet.

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