Information Technology Reference
In-Depth Information
UDP
UDP is a much simpler protocol than TCP and is useful in situations in which 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 UDP 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
that they support.
Table7-1
Internet Protocol/Application Mapping (with Common Port Numbers)
Application Protocols
WWW browser HTTP (TCP port 80)
The Hypertext Transfer Protocol (HTTP) is used by Web browsers and servers to
transfer the files that make up web pages.
File transfer FTP (TCP ports 20 and 21)
The File Transfer Protocol (FTP) provides a way to move files between computer
systems. Telnet allows virtual terminal emulation.
Terminal emulation Telnet (TCP port 23)
The Telnet protocol provides terminal emulation services over a reliable TCP stream.
The Telnet protocol also specifies how a client and server should negotiate the use of
certain features and options.
Electronic mail SMTP (TCP port 25), POP3 (TCP port 110), IMAP4 (TCP
port 143)
The Simple Mail Transfer Protocol (SMTP) is used to transfer electronic mail
between mail servers, and is used by mail clients to send mail. Mail clients do not
generally use SMTP to receive mail. Instead, they use either the Post Office Protocol
version 3 (POP3) or the Internet Message Access Protocol (IMAP); this will be
discussed in greater detail later in this chapter.
Network
management
SNMP (UDP port 161)
The Simple Network Management Protocol (SNMP) is a network management
protocol used for reporting anomalous network conditions and setting network
threshold values.
Distributed file
services
NFS, XDR, RPC (UDP port 111), X Windows (UDP ports
6000-6063)
X Windows 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.
