it can also be used to identify an initial sequence number to be used in the upcoming transmission. Acknowledgment number Contains the sequence number of the next byte of data the sender of the packet expects to receive. Data offset Indicates the number of 32-bit words in the TCP header. Reserved Reserved for future use. Flags Carries a variety of control information. Window Specifies the size of the sender's receive window (that is, the buffer space available for incoming data). Checksum Indicates whether the header was damaged in transit. Urgent pointer Points to the first urgent data byte in the packet. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets X.25, Internets, Intranets, and Extranets Options Specifies various TCP options. Data Contains upper-layer information. User Datagram Protocol (UDP) UDP is a much simpler protocol than TCP and is useful in situations where the reliability mechanisms of TCP are not necessary. The UDP header has only four fields: source port, destination port, length, and UDP checksum. The source and destination port fields serve the same functions as they do in the TCP header. The length field specifies the length of the UDP header and data, and the checksum field provides packet integrity checking. The UDP checksum is optional. Figure 7-26 shows the UDP header. Upper-Layer Protocols The IP suite includes many upper-layer protocols representing a wide vari- ety of applications, including network management, file transfer, distrib- uted file services, terminal emulation, and electronic mail. The best-known Internet upper-layer protocols support certain applications. The File Transfer Protocol (FTP) provides a way to move files between computer systems. Telnet enables virtual terminal emulation. The Simple Network Management Protocol (SNMP) is a network management protoc