Internet Applications: File Transfer Protocol (FTP)

1. Internet Applications: File Transfer Protocol (FTP)

2. 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. Computer Networks Applications

3. 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. Computer Networks Applications

4. 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). Computer Networks Applications

5. 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; Computer Networks Applications

6. 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. Computer Networks Applications

7. 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; Computer Networks Applications

8. 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. Computer Networks Applications

9. 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.cs.purdue.edu/pub/comer/example --- instructs the browser to get file “pub/comer/example” from machine ftp.cs.purdue.edu • ftp://ftp.cs.purdue.edu/pub/comer/ --- displays all files in the directory “pub/comer/” Computer Networks Applications

10. 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. Computer Networks Applications