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- 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
- 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. The Internet can be used to transfer data
- 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, includingaudio and video.
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).
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;
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.
7. Transfer Modes
- FTP defines two types of transfer: textualand binary;
- Textual: is used for text files;
- most textfiles 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. 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. A browser can use FTP
- A WWW browser can be used to FTPinstead of a dedicatedinterface;
- A browser uses FTP as the transfer protocol, when the URL starts with ftp (instead of http)
- ftp://ftp.acunix.albany.edu/as7656/temp --- instructsthe browser to get file as7656/temp from machineftp.acunix.edu
- ftp://ftp.acunix.edu/as7656 --- displays all files in the directory as7656
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, anduses TCP to send it.