TCP/IP Transport Layer
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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. Later chapters examine the transport layer
in more detail and discuss more functions of the transport layer.
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.
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.
Same-Layer and Adjacent-Layer Interactions
The example in 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, and the higher layer 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
Table 1-3 Summary: Same-Layer and Adjacent-Layer Interactions