tailieunhanh - The Illustrated Network- P7

The Illustrated Network- P7:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 1 Protocols and Layers 29 the network layer looks at the control information usually in a header in the data it receives from the data link layer and passes the remainder up to the transport layer for further processing. This is called encapsulation because one layer has no idea what the structure or meaning of the PDU is at other layers. The PDU has several more or less official names for the structure at each layer. The exception to this general rule is the data link layer which adds both a header and a trailer to the data it receives from the network layer. The general flow of encapsulation in TCP IP is shown in Figure . Note that on the transmission media itself or communications link there are only bits and that some extra bits are added by the communication link for its own purposes. Each PDU at the other layers is labeled as data for its layer and the headers are abbreviated by layer name. The exception is the second layer the data link layer which shows a header and trailer added at that level of encapsulation. Although the intermediate nodes are not shown these network devices will only process the data at most through the first three layers. In other words there is no transport layer to which to pass network-layer PDUs on these systems for data communications management is another issue . Device A Device B Data from Application Data to Application Application Layer Data Application Layer Data Trl Transport Layer Data Network Layer Data THj NH Transport Layer Data Network Layer Data Data Link Layer Data Trl 010101010101011100101010101010101011110 THj Data Link Layer Data NH Hdr I 110 I 010101010101011100101010101010101011110 Transmission Media FIGURE TCP IP encapsulation and headers. The unstructured stream of bits represents frames with distinct content. 110 I 30 PART I Networking Basics THE LAYERS OF TCP IP TCP IP is mature and stable and is the only protocol stack used on the Internet. This book is all about networking with TCP IP but .