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.