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. The CCNA 200-301 Official Cert Guide, Volume 2, Second Edition includes a chapter, “Introduction to TCP/IP Transport and Applications,” which examines the transport layer in more detail.
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.
For example, in Figure 1-5, 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.
Figure 1-7 TCP Error-Recovery Services as Provided to HTTP
Figure 1-7 shows web server Larry sending a web page to web browser Bob, using three separate messages. Note that this figure shows the same HTTP headers as Figure 1-6, but it also shows a TCP header. The TCP header shows a sequence number (SEQ) with each message. In this example, the network has a problem, and the network fails to deliver the TCP message (called a segment) with sequence number 2. When Bob receives messages with sequence numbers 1 and 3, but does not receive a message with sequence number 2, Bob realizes that message 2 was lost. That realization by Bob’s TCP logic causes Bob to send a TCP segment back to Larry, asking Larry to send message 2 again.
Figure 1-7 also demonstrates a function called adjacent-layer interaction, which refers to the concepts of how adjacent layers in a networking model, on the same computer, work together. In this example, the higher-layer protocol (HTTP) wants error recovery, so it uses the next lower-layer protocol (TCP) to perform the service of error recovery; the lower layer provides a service to the layer above it.
Figure 1-7 also shows an example of a similar function called same-layer interaction. When a particular layer on one computer wants to communicate with the same layer on another computer, the two computers use headers to hold the information that they want to communicate. For example, in Figure 1-7, Larry set the sequence numbers to 1, 2, and 3 so that Bob could notice when some of the data did not arrive. Larry’s TCP process created that TCP header with the sequence number; Bob’s TCP process received and reacted to the TCP segments.
Table 1-3 summarizes the key points about how adjacent layers work together on a single computer and how one layer on one computer works with the same networking layer on another computer.
Table 1-3 Summary: Same-Layer and Adjacent-Layer Interactions
| Concept | Description |
| Same-layer interaction on different computers | The two computers use a protocol to communicate with the same layer on another computer. The protocol defines a header that communicates what each computer wants to do. |
| Adjacent-layer interaction on the same computer | On a single computer, one lower layer provides a service to the layer just above. The software or hardware that implements the higher layer requests that the next lower layer perform the needed function. |
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