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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• 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)
ClientServer
20 21 1026 1027
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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.
ClientServer
20 21 1026 1027
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2024
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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
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
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
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
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
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
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
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
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
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
Trivial File Transfer Protocol
• RFC 1782
• UDP port 69
• Simple protocol, usually used to transfer configuration files
• Usually used for transferring boot file for diskless hosts (X-Stations) or updating NVRAM
TFTP
• Typically used in short distance, low noise environments
• Server is usually implement in firmware for updating things like routers, bios...
TFTP
• Because of its compact size:– no error recovery like TCP based FTP– no command structure like FTP– cannot list directories– transfers to server are to a single configured
directory
TFTP Transfers
• client send a RRQ or WRQ to server
• server sends ACK 0
• client sends DAT1
• repeat until done
TFTP Formats
• Type Op # Format without header 2 bytes string 1 byte string 1 byte ------------------------------------------------------ RRQ/ | 01/02 | Filename | 0 | Mode | 0 | WRQ ------------------------------------------------------- 2 bytes 2 bytes n bytes --------------------------------- DATA | 03 | Block # | Data | --------------------------------- 2 bytes 2 bytes ----------------------------- ACK | 04 | Block # | ----------------------------- 2 bytes 2 bytes string 1 byte ----------------------------------------------------- ERROR | 05 | ErrorCode | ErrMsg | 0 | -----------------------------------------------------