TCPlIP Applications

TCPlIP Applications

A number of applications have been standardized to operate on top of TCP. We
mention three of the most common here.
The Simple Mail Transfer Protocol (SMTP) provides a basic electronic mail
facility. It provides a mechanism for transferring messages among separate hosts.
Features of SMTP include mailing lists, return receipts, and forwarding. The SMTP
protocol does not specify the way in which messages are to be created; some local
editing or native electronic mail facility is required. Once a message is created,
SMTP accepts the message and makes use ofTCP to send it to an SMTP module on
another host. The target SMTP module will make use of a local electronic mail
package to store the incoming message in a user's mailbox.
The File Transfer Protocol (FTP) is used to send files from one system to
another under user command. Both text and binary files are accommodated, and the
protocol provides features for controlling user access. When a user wishes to engage
in file transfer, FTP sets up aTCP connection to the target system for the exchange of
control messages. This connection allows user ID and password to be transmitted and
allows the user to specify the file and file actions desired. Once a file transfer is
approved, a second TCP connection is set up for the data transfer. The file is transferred
over the data connection, without the overhead of any headers or control
information at the application level. When the transfer is complete, the control connection
is used to signal the completion and to accept new file transfer commands.
TELNET provides a remote logon capability, which enables a user at a terminal
or personal computer to logon to a remote computer and function as if directly
connected to that computer. The protocol was designed to work with simple scrollmode
terminals. TELNET is actually implemented in two modules: User TELNET
interacts with the terminal I/O module to communicate with a local terminal. It
converts the characteristics of real terminals to the network standard and vice
versa. ServerTELNET interacts with an application, acting as a surrogate terminal
handler so that remote terminals appear as local to the application. Terminal traffic
between User and Server TELNET is carried on a TCP connection.