If there’s one word you will get used to seeing a lot as you go through this Guide, it is this one: protocol. You will see reference to networking protocols, internetworking protocols, high-level protocols, low-level protocols, protocol stacks, protocol suites, sub-protocols, and so on. Clearly protocols are important, yet many reference works and standards use the term over and over again without ever explaining it. One reason for this may be because the term is somewhat vague and can have many meanings, which can make it difficult to grasp.

In some cases, understanding a technical term is easier if we go back to look at how the term is used in plain English. In the real world, a protocol often refers to a code of conduct, or a form of etiquette observed by diplomats. These people must follow certain rules of ceremony and form to ensure that they communicate effectively, and without coming into conflict. They also must understand what is expected of them when they interact with representatives from other nations, to make sure that, for example, they do not offend due to unfamiliarity with local customs. Even we “normal people” follow protocols of various sorts, which are sort of the “unwritten rules of society”.

This may seem to have little to do with networking, but in fact, this is a pretty good high-level description of what networking protocols are about. They define a language and a set of rules and procedures that enable devices and systems to communicate. Obviously, computers do not have “local customs”, and they hardly have to worry about committing a “faux pas” that might cause another computer to take offense. What networking protocols concern themselves with is ensuring that all the devices on a network or internetwork are in agreement about how various actions must be performed in the total communication process.

So, a protocol is basically a way of ensuring that devices are able to talk to each other effectively. In most cases, an individual protocol describes how communication is accomplished between one particular software or hardware element in two or more devices. In the context of the OSI Reference Model, a protocol is formally defined as a set of rules governing communication between entities at the same Reference Model layer. For example, the Transmission Control Protocol (TCP) is responsible for a specific set of functions on TCP/IP networks. Each host on a TCP/IP network has a TCP implementation, and they all communicate with each other logically at layer four of the OSI model.

While OSI Reference Model definitions are sometimes overly theoretical in nature, this particular one is rather accurate in assessing protocols in real-world networking. If something doesn’t specify a means of communication, it arguably isn’t a protocol.