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Internet Applications: File Transfer Protocol (FTP)

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Presentation on theme: "Internet Applications: File Transfer Protocol (FTP)"— Presentation transcript:

1 Internet Applications: File Transfer Protocol (FTP)

2 Spring 2002Computer Networks Applications 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.

3 Spring 2002Computer Networks Applications 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.

4 Spring 2002Computer Networks Applications 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).

5 Spring 2002Computer Networks Applications 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;

6 Spring 2002Computer Networks Applications 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.

7 Spring 2002Computer Networks Applications 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;

8 Spring 2002Computer Networks Applications 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.

9 Spring 2002Computer Networks Applications 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: --- instructs the browser to get file “pub/comer/example” from machine --- displays all files in the directory “pub/comer/”

10 Spring 2002Computer Networks Applications 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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