tailieunhanh - The Illustrated Network- P31

The Illustrated Network- P31: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 10 User Datagram Protocol 269 1 byte_ 1 byte_ __1 byte_ 1 byte Source IPv6 Address Destination IPv6 Address UDP Upper Layer Protocol Length All 0 bytes Next Header FIGURE The UDP IPv6 pseudo-header. Use of the UDP checksum is not optional in IPv6. PORT NUMBERS Each application running above UDP and TCP and IP is indexed by its port number allowing for the multiplexing of the IP layer. Just as frames with different types of packets inside on Ethernet IPv4 is 0x0800 and IPv6 is 0x86DD are multiplexed onto a single LAN interface the individual IPv4 or IPv6 packets are multiplexed and distributed by the protocol number UDP is IP protocol number 17 and TCP is 6 . The port numbers in turn multiplex and distribute datagrams from applications allowing them to share a single UDP or TCP process which is usually integrated closely with the operating system. This function of frame Ethertype packet protocol and datagram port is shown in Figure . The figure shows how IPv4 data for DNS makes its way from frame through IPv4 through UDP to the DNS application listening on UDP port 53. Well-Known Ports Port numbers can run from 0 to 65353. Port numbers from 0 to 1023 are reserved for common TCP IP applications and are called well-known use of well-known ports allows client applications to easily locate the corresponding server application processes on other hosts. For example a client process wanting to contact a DNS 270 PART II Core Protocols Frame Header FIGURE UDP port multiplexing and distribution showing how a single IP layer IPv6 in this case can be used by multiple transport protocols and applications. process running on a server must send the datagram to some destination port. The well-known port number for DNS is 53 and that s where the server process should be listening for client requests. These ports are sometimes called privileged ports although a number of applications that formerly ran in privileged mode