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    08-Jul-2015

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<ul><li> 1. Internet Applications: File Transfer Protocol (FTP)</li></ul> <p> 2. Data transfer before Internet </p> <ul><li>Magnetic media like tapes and disks: </li></ul> <ul><li><ul><li>An application transferred data on magnetic media </li></ul></li></ul> <ul><li><ul><li>The medium was physically moved from one computer to another; </li></ul></li></ul> <ul><li><ul><li>Drawback: SLOW </li></ul></li></ul> <ul><li>Fax: </li></ul> <ul><li><ul><li>Use the telephone lines; </li></ul></li></ul> <ul><li><ul><li>A fax machine consists of a printer, a scanner, a dial-up modem, and a dedicated computer; </li></ul></li></ul> <ul><li><ul><li>Drawbacks: requires a dedicated machine and a fax transmission is as expensive as a phone conversation. </li></ul></li></ul> <p> 3. The Internet can be used to transfer data </p> <ul><li>Benefits: </li></ul> <ul><li><ul><li>Efficient: Internet is designed for sending digital data; </li></ul></li></ul> <ul><li><ul><li>Less expensive than fax: Internet access is billed a flat rate; </li></ul></li></ul> <ul><li><ul><li>Can transfer more types of data than fax, includingaudio and video.</li></ul></li></ul> <p> 4. File Transfer Protocol (FTP) </p> <ul><li>A general-purpose protocol that can be used to copy an arbitrary file from one computer to another; </li></ul> <ul><li>one of the oldest network application---predates TCP and IP; </li></ul> <ul><li>Later versions were built on top of TCP/IP; </li></ul> <ul><li>Among the most heavily used applications: </li></ul> <ul><li><ul><li>FTP generated as much as 1/3 of the traffic on the Internet </li></ul></li></ul> <ul><li><ul><li>Was exceeded only by WWW(in 1995).</li></ul></li></ul> <p> 5. Issues in designing FTP </p> <ul><li><ul><li>Must transfer an arbitrary file (size, name,..) </li></ul></li></ul> <ul><li><ul><li>Must accommodate multiple file types; </li></ul></li></ul> <ul><li><ul><li>Must connect heterogeneous computers. May have to deal with different: </li></ul></li></ul> <ul><li><ul><li><ul><li>Data encodings; </li></ul></li></ul></li></ul> <ul><li><ul><li><ul><li>File names; </li></ul></li></ul></li></ul> <ul><li><ul><li><ul><li>File protections; </li></ul></li></ul></li></ul> <p> 6. FTP Commands </p> <ul><li>FTP is an interactive protocol: it responds to each command a user enters; signals when it is ready to execute another command; </li></ul> <ul><li>Examples of FTP commands: </li></ul> <ul><li><ul><li>Open---connect to a remote computer; </li></ul></li></ul> <ul><li><ul><li>Get---retrieve a file from the remote computer; </li></ul></li></ul> <ul><li><ul><li>Put---sends a file to the remote computer; </li></ul></li></ul> <ul><li><ul><li>Bye---terminate the connection and leave FTP. </li></ul></li></ul> <p> 7. Transfer Modes </p> <ul><li>FTP defines two types of transfer: textualand binary; </li></ul> <ul><li>Textual: is used for text files; </li></ul> <ul><li><ul><li>most textfiles are encoded in ASCII or EBCDIC </li></ul></li></ul> <ul><li><ul><li>ftp can translate from the local to remote character set when transferring a file; </li></ul></li></ul> <ul><li>Binary: used for all other files (audio, image, numbers, ) </li></ul> <ul><li><ul><li>Files are copied exactly; </li></ul></li></ul> <ul><li><ul><li>The resulting copy might be meaningless because FTP does not convert values to the local representation; </li></ul></li></ul> <p> 8. Connections, authorizations and file permissions </p> <ul><li>The remote system has to verify that the user is authorized to access files: </li></ul> <ul><li><ul><li>The user has to provide a login name and a password; </li></ul></li></ul> <ul><li><ul><li>If the user is authorized he/she may start transferring files; </li></ul></li></ul> <ul><li>What if the user does not have an account? </li></ul> <ul><li><ul><li>System administrator can configure FTP to support anonymous FTP; </li></ul></li></ul> <ul><li><ul><li>Login name anonymous and password guest (or e-mail address) allows a user access to public files.</li></ul></li></ul> <p> 9. A browser can use FTP</p> <ul><li>A WWW browser can be used to FTPinstead of a dedicatedinterface; </li></ul> <ul><li>A browser uses FTP as the transfer protocol, when the URL starts with ftp (instead of http) </li></ul> <ul><li>EX: </li></ul> <ul><li><ul><li>ftp://ftp.acunix.albany.edu/as7656/temp --- instructsthe browser to get file as7656/temp from machineftp.acunix.edu </li></ul></li></ul> <ul><li><ul><li>ftp://ftp.acunix.edu/as7656 --- displays all files in the directory as7656 </li></ul></li></ul> <p> 10. FTP uses the client-server paradigm: </p> <ul><li><ul><li>Local application (or browser) is the client </li></ul></li></ul> <ul><li><ul><li>Remote FTP program is the server; </li></ul></li></ul> <ul><li><ul><li>The FTP server authorizes the connection, locates the file, anduses TCP to send it. </li></ul></li></ul>

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