As you can see from the explanations of how HTTP, TCP, IP, and Ethernet do their jobs, when sending data, each layer adds its own header (and for data-link protocols, also a trailer) to the data supplied by the higher layer. The term encapsulation refers to the process of putting headers (and sometimes trailers) around some data.
Many of the examples in this chapter show the encapsulation process. For example, web server Larry encapsulated the contents of the home page inside an HTTP header in Figure 1-6. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure 1-7. IP encapsulated the TCP headers and the data inside an IP header in Figure 1-10. Finally, the Ethernet link layer encapsulated the IP packets inside both a header and a trailer in Figure 1-11.
The process by which a TCP/IP host sends data can be viewed as a five-step process. The first four steps relate to the encapsulation performed by the four TCP/IP layers, and the last step is the actual physical transmission of the data by the host. In fact, if you use the five-layer TCP/IP model, one step corresponds to the role of each layer. The steps are summarized in the following list:
Step 1. Create and encapsulate the application data with any required application layer headers. For example, the HTTP OK message can be returned in an HTTP header, followed by part of the contents of a web page.
Step 2. Encapsulate the data supplied by the application layer inside a transport layer header. For end-user applications, a TCP or UDP header is typically used.
Step 3. Encapsulate the data supplied by the transport layer inside a network layer (IP) header. IP defines the IP addresses that uniquely identify each computer.
Step 4. Encapsulate the data supplied by the network layer inside a data-link layer header and trailer. This layer uses both a header and a trailer.
Step 5. Transmit the bits. The physical layer encodes a signal onto the medium to transmit the frame.
The numbers in Figure 1-12 correspond to the five steps in this list, graphically showing the same concepts. Note that because the application layer often does not need to add a header, the figure does not show a specific application layer header, but the application layer will also at times add a header as well.
One reason this chapter takes the time to show the encapsulation steps in detail has to do with terminology. When talking and writing about networking, people use segment, packet, and frame to refer to the messages shown in Figure 1-13 and the related list. Each term has a specific meaning, referring to the headers (and possibly trailers) defined by a particular layer and the data encapsulated following that header. Each term, however, refers to a different layer: segment for the transport layer, packet for the network layer, and frame for the link layer. Figure 1-13 shows each layer along with the associated term.
Figure 1-12 Five Steps of Data Encapsulation: TCP/IP
Figure 1-13 Perspectives on Encapsulation and “Data”*
* The letters LH and LT stand for link header and link trailer, respectively, and refer to the data-link layer header and trailer.
Figure 1-13 also shows the encapsulated data as simply “data.” When you are focusing on the work done by a particular layer, the encapsulated data typically is unimportant. For example, an IP packet can indeed have a TCP header after the IP header, an HTTP header after the TCP header, and data for a web page after the HTTP header. However, when discussing IP, you probably just care about the IP header, so everything after the IP header is just called data. So, when you are drawing IP packets, everything after the IP header is typically shown simply as data.
Note
You will also see the generic term protocol data unit, or PDU, used to refer to any message defined by a protocol. A TCP segment, IP packet, and Ethernet frame are all PDUs, for instance.
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