FTP uses two TCP ports, one for control commands and one for data transfers. It supports both active and passive modes for negotiating the data connection. TFTP is a simpler file transfer protocol that uses UDP and does not support features like directories or error recovery. Both protocols support different data types, file structures, and transmission modes for transferring files between 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)
ClientServer
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.
ClientServer
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

15. 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

16. TFTP
• Typically used in short distance, low noise
environments
• Server is usually implement in firmware for
updating things like routers, bios...

17. 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

18. Transfer Modes
• netascii (8-bit ASCII)
• raw binary
• mail (obsolete, not used)

19. TFTP Transfers
• client send a RRQ or WRQ to server
• server sends ACK 0
• client sends DAT1
• repeat until done

20. 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 |
-----------------------------------------------------