lec-9 - What's next. Network Applications File Transfer...

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23 Aug 2008  BITS C481 (Computer Networks) 1 What’s next… Network Applications File Transfer Mail Transfer
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23 Aug 2008  BITS C481 (Computer Networks) 2 FTP: the file transfer protocol Transfer file to/from remote host Client/server model client: side that initiates transfer (either to/from remote) server: remote host ftp: RFC 959 ftp server: port 21 file transfer FTP server FTP user interface FTP client local file system remote file system user  at host
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23 Aug 2008  BITS C481 (Computer Networks) 3 FTP: separate control, data connections FTP client contacts FTP server at port 21, specifying TCP as transport protocol Client obtains authorization over control connection Client browses remote directory by sending commands over control connection. When server receives file transfer command, server opens 2 nd TCP connection (for file) to client After transferring one file, server closes data connection. Server opens another TCP data connection to transfer another file. Control connection: “out of band” FTP server maintains “state”: current directory, earlier authentication FTP client FTP server TCP control connection port 21 TCP data connection port 20
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23 Aug 2008  BITS C481 (Computer Networks) 4 FTP commands, responses Sample commands: sent as ASCII text over control channel USER username PASS password LIST return list of file in current directory RETR filename retrieves (gets) file STOR filename stores (puts) file onto remote host Sample return codes status code and phrase (as in HTTP) 331 Username OK, password required 125 data connection already open; transfer starting 425 Can’t open data connection 452 Error writing file
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lec-9 - What's next. Network Applications File Transfer...

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