Practical TCP/IP and Ethernet Networking- P31: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 132 Practical TCP IP and Ethernet Networking Checksum 16 bits This is the 16-bit one s complement of the one s complement sum of a pseudo header of information from the IP header the UDP header and the data padded with 0 bytes at the end if necessary to make a multiple of two bytes. The pseudo header conceptually prefixed to the UDP header contains the source address the destination address the protocol and the UDP length. As in the case of TCP this header is used for computational purposes only and is NOT transmitted. This information gives protection against misrouted datagrams. This checksum procedure is the same as is used in TCP. The Pseudo Header used during UDP Checksum Computation 12 Octets 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 SOURCE IP ADDRESS DESTINATION IP ADDRESi 00000000 00010001 UDP LENGTH All Zero s PROTO Figure UDP pseudo header format If the computed checksum is zero it is transmitted as all ones the equivalent in one s complements arithmetic . An all zero transmitted checksum value means that the transmitter generated no checksum for debugging or for higher level protocols that don t care . UDP is numbered protocol 17 21 octal when used with the Internet protocol. 8 Application layer protocols Objectives When you have completed study of this chapter you should have a basic understanding of the application and operation of the following application layer protocols FTP TFTP TELNET RLOGIN NFS DNS WINS SNMP SMTP POP3 HTTP BOOTP DHCP Introduction This chapter examines the process application layer of the TCP IP model. Protocols at this layer act as intermediaries between some user application external to the TCP IP communication stack and the lower-level protocols such as TCP or UDP. An example is SMTP which acts as an interface between an e-mail client or server and TCP. Note that the list of protocols supplied here is by no means complete as new protocols are developed all the time. Using a .
