Overview of the TCP/IP Networking Model – Introduction to Networking – 200-301 Study Guide

Overview of the TCP/IP Networking Model

The TCP/IP model both defines and references a large collection of protocols that allow computers to communicate. To define a protocol, TCP/IP uses documents called Requests For Comments (RFC). (You can find these RFCs using any online search engine.) Each layer broadly defines a set of functions that helps create a working communication system, and each RFC gives the specifics about an option to implement one or more of the functions at some layer of the model.

The TCP/IP model also avoids repeating work already done by some other standards body or vendor consortium by simply referring to standards or protocols created by those groups. For example, the Institute of Electrical and Electronics Engineers (IEEE) defines Ethernet LANs; the TCP/IP model does not define Ethernet in RFCs, but refers to IEEE Ethernet as an option.

To help people understand a networking model, each model breaks the functions into a small number of categories called layers. Each layer includes protocols and standards that relate to that category of functions, as shown in Figure 1-4.

Figure 1-4 The TCP/IP Networking Model

Note

The network layer, shown as the middle layer of the TCP/IP model in Figure 1-4, may also be called the Internet layer in reference to its primary protocol, the Internet Protocol (IP).

On a single computer, different components implement different protocols and standards from different layers. Imagine buying a new computer and connecting it to either your home network or network at work using a cable or wireless. You would expect that you can open a web browser and connect to a website without much more effort. To make all that work, the network hardware in your computer—an integrated network interface card (NIC)—implements some physical layer standards to support physical communications. That NIC also supports the related data-link standards. The OS on the computer implements protocols from the network and transport layers. Finally, that web browser implements some application layer protocols (for instance, HTTP or HTTPS.)

More generally, the TCP/IP model in Figure 1-4 shows the more common terms and layers used when people talk about TCP/IP today. The physical layer focuses on how to transmit bits over each link. The data-link layer focuses on the rules that control the use of the physical link, analogous to how we need standards for roads, cars, and traffic signals. The network layer focuses on delivering data over the entire path from the original sending computer to the final destination computer, analogous to how a national postal service arranges for unique postal addresses and a system to deliver mail to all those addresses. The top two layers focus more on the applications that need to send and receive data—for instance, how to identify data, how to ask for the data to be sent, and how to recover the data if lost in transmission.

Many of you will have already heard of several TCP/IP protocols, like the examples listed in Table 1-2. Most of the protocols and standards in this table will be explained in more detail as you work through this book. Following the table, this section takes a closer look at the layers of the TCP/IP model.

Table 1-2 ^{TCP/IP Architectural Model and Example Protocols}