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Internet Applications: File Transfer Protocol (FTP)
CSI103: Introduction to the Internet
Data transfer before Internet Magnetic media like tapes and disks:
An application transferred data on magnetic media The medium was physically moved from one
computer to another; Drawback: SLOW
Fax: Use the telephone lines; A fax machine consists of a printer, a scanner, a
dial-up modem, and a dedicated computer; Drawbacks: requires a dedicated machine and a fax
transmission is as expensive as a phone conversation.
CSI103: Introduction to the Internet
The Internet can be used to transfer data Benefits:
Efficient: Internet is designed for sending digital data;
Less expensive than fax: Internet access is billed a flat rate;
Can transfer more types of data than fax, including audio and video.
CSI103: Introduction to the Internet
File Transfer Protocol (FTP) A general-purpose protocol that can be used
to copy an arbitrary file from one computer to another;
one of the oldest network application---predates TCP and IP;
Later versions were built on top of TCP/IP; Among the most heavily used applications:
FTP generated as much as 1/3 of the traffic on the Internet
Was exceeded only by WWW (in 1995).
CSI103: Introduction to the Internet
Issues in designing FTP
Must transfer an arbitrary file (size, name,..)
Must accommodate multiple file types; Must connect heterogeneous computers.
May have to deal with different: Data encodings; File names; File protections;
CSI103: Introduction to the Internet
FTP Commands FTP is an interactive protocol: it responds to
each command a user enters; signals when it is ready to execute another command;
Examples of FTP commands: Open---connect to a remote computer; Get---retrieve a file from the remote computer; Put---sends a file to the remote computer; Bye---terminate the connection and leave FTP.
CSI103: Introduction to the Internet
Transfer Modes FTP defines two types of transfer: textual
and binary; Textual: is used for text files;
most text files are encoded in ASCII or EBCDIC ftp can translate from the local to remote character
set when transferring a file; Binary: used for all other files (audio, image,
numbers, …) Files are copied exactly; The resulting copy might be meaningless because
FTP does not convert values to the local representation;
CSI103: Introduction to the Internet
Connections, authorizations and file permissions The remote system has to verify that the user
is authorized to access files: The user has to provide a login name and a
password; If the user is authorized he/she may start
transferring files; What if the user does not have an account?
System administrator can configure FTP to support anonymous FTP;
Login name anonymous and password guest (or e-mail address) allows a user access to public files.
CSI103: Introduction to the Internet
A browser can use FTP A WWW browser can be used to FTP instead
of a dedicated interface; A browser uses FTP as the transfer protocol,
when the URL starts with ftp (instead of http) EX:
ftp://ftp.acunix.albany.edu/as7656/temp --- instructs the browser to get file “as7656/temp” from machine ftp.acunix.edu
ftp://ftp.acunix.edu/as7656 --- displays all files in the directory “as7656”
CSI103: Introduction to the Internet
FTP uses the client-server paradigm:
Local application (or browser) is the client Remote FTP program is the server; The FTP server authorizes the connection,
locates the file, and uses TCP to send it.
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