FTP uses two TCP ports, one for control commands and one for data transfers. It uses a command-response protocol over the control port with US-ASCII formatting. There are two main connection modes for FTP - active and passive. Active mode has the client connect to the server's command port to initiate the session, while passive mode has the client open random ports that the server connects to. FTP supports different file structures, data types, and transmission modes to handle file transfers across heterogeneous systems.

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) Client Server 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. Client Server 20 21 1026 1027 1 2 2024 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