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Thanks for your understanding!
Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide
TCP/IP is most often studied in terms of its layer-based architecture and the protocols that it provides at those different layers. And we're certainly going to do that, don't worry. These protocols, however, represent the technical details of how TCP/IP works. They are of interest to us as students of technology, but are normally hidden from users who do not need to see the “guts” of how TCP/IP works to know that it works. Before proceeding to these details, I think it might be instructive to take a “bigger picture” look at what TCP/IP does.
In the section describing the OSI Reference Model I mentioned that the theoretical operation of the model is based on the concept of one layer providing services to the layers above it. TCP/IP covers many layers of the OSI model, and so it collectively provides services of this sort as well in many ways. Conceptually, we can divide TCP/IP services into two groups: services provided to other protocols and services provided to end users directly.
The first group of services consists of the core functions implemented by the main TCP/IP protocols such as IP, TCP and UDP. These services are designed to actually accomplish the internetworking functions of the protocol suite. For example, at the network layer, IP provides functions such as addressing, delivery, and datagram packaging, fragmentation and reassembly. At the transport layer, TCP and UDP are concerned with encapsulating user data and managing connections between devices. Other protocols provide routing and management functionality. Higher-layer protocols use these services, allowing them to concentrate on what they are intended to accomplish.
The other general types of service provided by TCP/IP are end-user services. These facilitate the operation of the applications that users run to make use of the power of the Internet and other TCP/IP networks. For example, the World Wide Web (WWW) is arguably the most important Internet application. WWW services are provided through the Hypertext Transfer Protocol (HTTP), a TCP/IP application layer protocol. HTTP in turn uses services provided by lower-level protocols. All of these details are of course hidden from the end users, which is entirely on purpose!