The TCP/IP application layer includes a relatively large number of protocols, with HTTP being only one of those. The TCP/IP transport layer consists of two main protocol options the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). To get a true appreciation for what TCP/IP transport layer protocols do, read post Fundamentals of TCP and UDP. However, in this post, you will learn about one of the key features of TCP, which enables us to cover some more general concepts about how networking models behave.
To appreciate what the transport layer protocols do, you must think about the layer above the transport layer, the application layer. Why? Well, each layer provides a service to the layer above it. For example, in Figure 2-2, Bob and Larry used HTTP to transfer the home page from Larry to Bob. But what would have happened if Bob’s HTTP get request was lost in transit through the TCP/IP network? Or, what would have happened if Larry’s response, which includes the contents of the home page, was lost? Well, the page would not show up in Bob’s browser, as you might expect.
So, TCP/IP needs a mechanism to guarantee delivery of data across a network. TCP provides that feature by using acknowledgments. Figure 2-3 outlines the basic acknowledgment logic.
