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File Transfer Protocol

File Transfer Protocol. CS-328 Dick Steflik. FTP. RFC 959 uses two TCP Ports one for control one for data transfers command-response protocol control port uses telnet protocol to negotiate session US-ASCII <crlf> is end-of-line character. Active Mode FTP.

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File Transfer Protocol

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  1. File Transfer Protocol CS-328 Dick Steflik

  2. FTP • RFC 959 • uses two TCP Ports • one for control • one for data transfers • command-response protocol • control port uses telnet protocol to negotiate session • US-ASCII • <crlf> is end-of-line character

  3. Active Mode FTP • Client connect from a random unprivileged port (n > 1023) to the servers command port (21) and sends port command to tell server to connect to n+1 then listens on the next higher unprivileged port (n+1) for server responses. The server connects from it’s data port (20) to the client data port (n+1) Server Client 20 21 1026 1027 1 2 3 4

  4. Passive Mode FTP • Client opens two random unprivileged ports ( n > 1023 and n+1; ex 1026 and 1027) and connects the first port (n) to server command port 21 and issues a pasv command (server sends port to use for data); client connects to servers specified data port, server completes connection. Server Client 20 2024 21 1026 1027 1 2 3 4

  5. Transfer Files in a Heterogeneous Host Environment • Due to multiple hardware types and operating systems file are converted to four environmentally neutral data type for transport and the converted to local types at the destination • ASCII A NVT-ASCII • EBCDIC E EBCDIC Text • IMAGE I Raw binary, series of octets • LOCAL L Raw binary using a variable byte size • Client responsibility to tell server data type to use • Default data type, unless otherwise specified is ASCII

  6. File Structures • Operating System store files in different structures • FTP defined file structures for transporting files • File F Unstructured, sequence of bytes • Record R Series of records • Page P Series of data blocks (pages) • Default file structure is File (F) • File Structure specified using STRU command

  7. Transmission Modes • Mode is used to specify additional coding or sequencing performed on data • independent of data type and file structure • Stream S stream of bytes, if record structure EOF sent as record indication; if file eof indicated by closing stream • Block B file sent as sequence of blocks preceded by header info allows restart of an interruped transfer • Compressed C data compressed using run length encoding

  8. FTP Commands • USER R User name, userid for access control • PASS O Password for access control • ACCT O Account info • CWD O Change working directory • CDUP O Change to parent directory • SMNT O Structure mount, mount a different file system • QUIT R informs server that client wants out • REIN O restarts session at authentication phase • PORT R Host addr and data port to use

  9. FTP Commands (more) • PASV O Passive; informs server that client will contact to set up data connections, ask server to sent port info • TYPE R Data type, type of subsequent transfers • STRU R File structure • MODE R Transfer mode • RETR R Retrieve, download the file from server • STOR R Store, upload the specified to server • STOU O Store unique, same as store but server picks unique file name

  10. FTP Commands (more) • APPE O Append, upload file to server, if file name exists, append the upload • ALLO O Allocate, sometimes used to preallocate space • REST O Restart, restart an interrupted transfer • RNFR O Rename file from filename • RNTO O Rename file to • ABOR O Abort, ask server to abort last command • DELE O Delete specified file • RMD O Remove directory • MKD O Make directory

  11. FTP Commands (more) • PWD O Print working directory • LIST O Request directory listing • NLST O Request just a file name list • SITE O Site parameters, allow client to specify site specific options and parameters • SYST O request server operating system • STAT O Request server to send status of current xfr • HELP O general and command specific • NOOP R ask server to send a positive reply

  12. FTP Responses • Each command generates a server response • 3 digit code, text, <crlf> • use 3 digit code as driver for GUI Clients or programatic implementations • use text for Command line clients

  13. Responses • 1yz - Positive preliminary reply - command is being acted upon; expect a final reply code before sending another command • 2yz - Positive completion reply - command was successfully executed; new command may be sent • 3yz - Positive intermediate reply - command was accepted, but the final result is being delayed because other information needs to be supplied from the client; reply is used for sequencing command groups • 4yz - Transient negative completion reply - command failed, but the condition is temporary • 5yz - Permanent negative completion reply - command failed and will always fail if given again; the command should not be attempted again

  14. Response • x0z - Refers to command syntax • x1z - Indicates information returned by commands requesting information such as status or help • x2z - Refers to the state of the control or data connections • x3z - The reply is associated with the login process and accounting procedures • x4z - Reserved for future use • x5z - Refers to the state of the requested file transfer or other file system command

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