The Internet—the largest computer network in the world today, is constructed from several protocols and protocol suites that work together to allow users (like you and I) to communicate across the globe. A protocol is simply a rule, or a collection of rules and conventions, that a device (such as your computer) follows in order to communicate with other devices around the world (which follow those same rules). A protocol suite is simply a collection of these rules, which work together to allow complex applications on networking devices (for example, web browsers on your computer) to communicate with billions of other devices around the world, through an assortment of networking equipment and media:
In this chapter, we will discuss two protocol suites in particular that have largely influenced the internet as we know it today:
- The Open Systems Interconnection (OSI) reference model
- The Transmission Control Protocol/Internet Protocol (TCP/IP) suite
Although these two protocol suites possess significant differences between them, they both serve as important blocks in the foundation of the internet, and, as such, they both continue to exist as important concepts that budding Network Engineers and System Administrators must understand and appreciate if they wish to become exceptional in their careers.
By understanding these two protocol suites, professionals add an important tool to their arsenal of network troubleshooting weaponry; namely, a systematic, step-by-step approach to be followed in the diagnostic processing of any networking issue, which both simplifies and speeds up the process of pinpointing the root cause of an issue and the rectification of the situation. These suites allow both equipment vendors and Network Engineers to segment the operation of a network into several discrete modular parts or layers, and deal with each layer individually. This allows us to focus on a single part of a system at a time, thus greatly simplifying the development and troubleshooting of networking equipment.
To illustrate this concept in a real-life scenario, consider the following situation—you're a System Administrator in a small IT firm. It's 4 o'clock on a Friday evening and you're excited to clock out and start your weekend. Suddenly, your Syslog Server starts sending emails to all the administrators in your team, complaining about a reachability issue regarding a particular server in your datacenter. Your co-workers immediately begin to panic, knowing that several employees have already left and that they'll likely be working late on a Friday evening. However, since you've mastered your protocol suites, you immediately locate the server and begin troubleshooting the issue from the Physical Layer upwards, quickly locating a disconnected cable to the server and saving your team a lot of troubleshooting time and stress:
For the rest of this chapter, we will first discuss the OSI reference model, delving into a bit of its history and the combination of factors and entities that led to its development and subsequent publication in 1984, before discussing each of the seven layers of the developed model in detail, explaining the purpose of each of the layers and illustrating how each of the layers interact to effect communication between devices across a network. We will then introduce the TCP/IP protocol suite, comparing and contrasting it to the OSI reference model, and similarly explaining and illustrating how each of the layers plays a vital role in transmitting messages across a network. Lastly, we will conclude this chapter with a set of practice questions, which will allow you to test how much information you've retained about the content we've covered in this chapter.
