TCP/IP Transport Layer overview

 Although many TCP/IP application layer protocols exist, the TCP/IP transport layer includes a smaller number of protocols. The two most commonly used transport layer protocols are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). Transport layer protocols provide services to the application layer protocols that reside one layer higher in the TCP/IP model. How does a transport layer protocol provide a service to a higher-layer protocol? This section introduces that general concept by focusing on a single service provided by TCP: error recovery.

TCP Error Recovery Basics

TCP Error Recovery Basics 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, like the error-recovery service provided to application layer protocols by TCP. Bob and Larry used HTTP to transfer the home page from web server Larry to Bob’s web browser. But what would have happened if Bob’s HTTP GET request had been lost in transit through the TCP/IP network? Or, what would have happened if Larry’s response, which included the contents of the home page, had been lost? Well, as you might expect, in either case, the page would not have shown up in Bob’s browser. TCP/IP needs a mechanism to guarantee delivery of data across a network. Because many application layer protocols probably want a way to guarantee delivery of data across a network, the creators of TCP included an error-recovery feature. To recover from errors, TCP uses the concept of acknowledgments. Figure 1-7 outlines the basic idea behind how TCP notices lost data and asks the sender to try again.