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FTP Client Application

FTP Client Application. CSC 8560 Brian Jorgage 4/27/2004. Basic Approach. Install FTP Server locally Observe interaction between command-line FTP and FTP server Read RFC 959 Iterative Development – “Build a little, test a little”. Major Design Issues.

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FTP Client Application

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  1. FTP Client Application CSC 8560 Brian Jorgage 4/27/2004

  2. Basic Approach • Install FTP Server locally • Observe interaction between command-line FTP and FTP server • Read RFC 959 • Iterative Development – “Build a little, test a little”

  3. Major Design Issues • Application built in Visual Basic 6.0 • Winsock control used for socket connections • Synchronization between client and server processes is critical • Need to understand state machine for Winsock control – based on TCP

  4. Login • Login accomplished via USER and PASS commands sent over control connection • Beginning of synchronization issues • USER <userid> sent over control line generates 331 response code from server • On receipt of 331 client sends PASS <pw> • Initially these were implemented as distinct command buttons but later the synch logic was built in • Present operation has Winsock.connect kick off whole process

  5. FTP Sockets • RFC specifies one control connection and one data connection for FTP – this requires 2 instances of Winsock control • Commands sent over control connection (e.g. login, dir, get, put) • Data sent over data connection (actual bits and bytes during file transfer) • RFC specifies that PORT command sent from client will send connect information to server then Server will initiate the connection • PORT command sends parameters h1, h2, h3, h4, p1, p2 • H1 thru H4 are just IP address of client • P1 and P2 are data connection port # converted to mod 256 (e.g. port 4440 = 17 * 256 + 88) – thus P1=17, P2=88 • Unable to get this working – TCP state is “listening” but connect request never arrives from server • Alternate method is to send PASV command to server and wait for response • Server sends an available port number and then client initiates the connection • Second major synchronization issue – return code from server is 227 • On receipt of 227 the data connection is established

  6. Get and Put • FTP Get command implemented as RETR command • File transfer is not complete until server sends 226 code • FTP Put command implemented as STOR • 3rd synch issue – handshake sequence occurs as follows: • PASV sent and operation code is set on client (1=LIST, 2=RETR, 3=STOR) • Server responds w/227 • Client initiates data connection then branches on operation code to send LIST, RETR or STOR • Server responds w/150 • If operation is Put then parse file and send over data connection

  7. Text mode vs Binary mode • Text files and binary files handled differently in FTP • Text files require “TYPE A” command • Default mode in FTP is text mode • Images and executables need to be sent in binary mode • Binary files require “TYPE I” command

  8. Navigation • Server-side navigation accomplished via CWD command • Client process responds to double-click event and parses out directory name then sends “CWD <dirname>” • Move up to parent directory accomplished via CDUP command • Client-side navigation accomplished via CommonDialog control accessed by “Browse” button • DIR sent as “LIST” command • Server response on data connection is displayed on ListBox control

  9. Managing Connections • Managing the state of the data and control lines turned out to be an issue • Client needs to close the data connection after file is sent • This is detected by the SendComplete event on Winsock for the data connection • On SendComplete the data connection is closed by client process • Data connection also needs to be closed on completion of LIST • If Client process exits before both sockets are closed the application will hang • Needed to implement logic to avoid this • FTP command to disconnect is QUIT • Client process checks state of both lines before exiting • If either line is still open client sends a warning message

  10. Raw FTP commands • Raw FTP commands collected together under a menu selection • This allows Get to be implemented as PASV + RETR menu selections • Put can be called as PASV + STOR menu selections • Also Dir can be called as PASV + LIST • Text mode is “TYPE A” menu • Binary mode is “TYPE I” menu • PORT command is PORT menu – as stated before this connection method does not work • Additional commands SYST, STAT, NOOP, HELP added to menu • STOU (store unique) is a variation on STOR (i.e. PUT) that guarantees unique filenames on host – menu option is there but does not work

  11. Remaining Issues • Cerberus FTP server sends files in 8K chunks • Each chunk generates a message box • Message box was used during development but when it was taken out file transfer was incomplete • It seems that the server is closing the connection before the last chunk of data is received • When msg box is left in it forces the correct sequence to occur • Other features to add are Abort, Delete, Mkdir, Rename • Determine how to implement connection as PORT command and then retire the use of PASV – default behavior should use PORT

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