1. File Transfer Protocol
CS-328
Dick Steflik

2. FTP RFC 959 uses two TCP Ports command-response protocol
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

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 bytes string byte string byte RRQ/ | 01/02 | Filename | | Mode | | WRQ bytes 2 bytes n bytes DATA | | Block # | Data | bytes bytes ACK | | Block # | bytes 2 bytes string byte ERROR | | ErrorCode | ErrMsg | |