tailieunhanh - The Illustrated Network- P21

The Illustrated Network- P21: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 6 IPv4 and IPv6 Headers 169 FIGURE Capture of IPv4 header fields. The frame is broken out to show the content and meaning of every field in the IPv4 header. Note that the DF Don t Fragment bit is set on the packet. fields in the IPv4 header. The source and destination addresses are listed first. Although we ll see that they are not the first fields in the header they are definitely the fields that most frequently are of interest. Ethereal interprets a field in the IPv4 header called the Type of Service TOS field according to something called Differentiated Services DiffServ . DiffServ is only one way to interpret these fields. The figure shows that there are three things indicated by the 8 bits in the TOS field Differentiate Services Code Point DSCP The default is zero which means this packet does not require special handling by any router or host other than IP s normal best-effort service. Explicit-Congestion-Notification Capable Transport ECT This bit is set by devices when the transport is able to provide an indication of network congestion to network-attached devices. The value of zero shows that Ethernet is not an ECT so packets cannot tell devices when the LAN is congested. ECN Congestion Explicit ECT-CE On transport that can report congestion this bit is set when some predefined criteria for network congestion is met. This is often a percentage of output buffer fullness. On Ethernet this bit is always zero. 170 PART II Core Protocols We ll say a little more about DSCP and quality of service QOS in a later chapter. However the incomplete support for and variations in QOS implementations rule out QOS or DSCP as a topic for an entire chapter. There are also four flag bits shown in the figure. The two most important are the bits that indicate this packet content is not to be fragmented the DF bit is set to 1 and that there are no more frames carrying pieces of this packet s payload the More Fragments bit is set to 0 . In the following we talk about .