System information

Upper-Layer Protocols
Book Title
7-98
• Window—Specifies the size of the sender’s receive window (that is, 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.
• Options—Specifies various TCP options.
• Data—Contains upper-layer information.
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 allows packet integrity checking. The UDP checksum is optional.
Upper-Layer Protocols
The Internet Protocol suite includes many upper-layer protocols, representing a wide variety of
applications, including network management, file transfer, distributed file services, terminal
emulation, and electronic mail. Table 7-1 maps the best-known Internet upper-layer protocols to the
applications they support.
Table 7-1 Internet Protocol/Application Mapping
File Transfer Protocol (FTP) provides a way to move files between computer systems. Telnet allows
virtual terminal emulation. The Simple Network Management Protocol (SNMP) is a network
management protocol used for reporting anomalous network conditions and setting network
threshold values. X Window is a popular protocol that permits intelligent terminals to communicate
with remote computers as if they were directly attached. Network file system (NFS), external data
representation (XDR), and remote-procedure call (RPC) combine to allow transparent access to
remote network resources. The Simple Mail Transfer Protocol (SMTP) provides an electronic mail
transport mechanism. These and other network applications use the services of TCP/IP and other
lower-layer Internet protocols to provide users with basic network services.
Application Protocols
File transfer FTP
Terminal emulation Telnet
Electronic mail SMTP
Network management SNMP
Distributed file services NFS, XDR, RPC, X Window