1. Tips on FTP Implementation

2. FTP Model User Interface User Server PI User PI File System Server DTP
Commands
User
Interface
User
FTP Commands
Server PI
User PI
FTP Replies
Port L
File
System
Server
DTP
User
DTP
File
System
Data
Connection
Port L-1
FTP Server
FTP Client
PI: Protocol Interpreter
DTP: Data Transfer Process
Jin Pyo Hong, HUFS

3. Commands: Minimum Implementation
User Cmd
arguments
FTP Cmd
Meaning
open
<hostname>
Connect to FTP server
[user]
<username>
USER
User name
<password>
PASS
Password
system
SYST
Show the type of OS running on the remote machine
nlist
[<pathname>]
NLST
<pathname>
Name list (ls)
ls, dir
LIST
Directory list (ls –l)
pwd
PWD
Print working directory cd
CWD
Change working directory
lcd
Change local working directory
[type] ascii
[type] binary
TYPE A I
A – NVT-ASCII text
I – binary data
get
RETR
Retrieve: download the file
put
STOR
Store: upload the file
PASV
Set server into passive mode. Server will reply 227 response with its IP and port
PORT
h1,h2,h3,h4,p1,p2
Data port for FTP client (Set server into active mode)
h1,h1,h3,h4: IPv4 address, p1,p2: port number
close
QUIT
Close FTP session
bye, quit
Close FTP session and exit
passive
Passive mode on or off (toggle switch)
If passive mode on, send PASV for data transfer
Otherwise, send PORT (active mode, default)
type
FTP data type ?
debug
Debug mode on or off (toggle switch
Jin Pyo Hong, HUFS
Jin Pyo Hong, HUFS

4. FTP Replies Code Description 1yz 2yz 3yz 4yz 5yz
Positive preliminary reply
The command is being acted upon; expect a final reply code before sending another command
2yz
Positive completion reply
The command was successfully executed; a new command may be sent
3yz
Positive intermediate reply
The command was accepted, but the final result is being delayed because other information needs to be supplied from the client
4yz
Transient negative completion reply
The command failed, but the condition is temporary; the user agent should try again
5yz
Permanent negative completion reply
The command failed and will always fail if given again; the command should not be attempted again
Jin Pyo Hong, HUFS

5. FTP Response Codes
125 Data connection already open; transfer starting.
150 File status okay; about to open data connection.
200 Command okay.
212 Directory status.
213 File status.
220 Service ready for new user.
221 Service closing control connection.
225 Data connection open; no transfer in progress.
226 Closing data connection.
227 Entering Passive Mode (203,253,69,113,201,131)
230 User logged in, proceed.
250 Requested file action okay, completed.
331 User name okay, need password.
350 Requested file action pending further information.
421 Service not available, closing control connection.
425 Can't open data connection.
426 Connection closed; transfer aborted.
450 Requested file action not taken.
File unavailable (e.g., file busy).
500 Syntax error, command unrecognized.
502 Command not implemented.
530 Not logged in.
550 Requested action not taken.
File unavailable (e.g., file not found, no access).
Jin Pyo Hong, HUFS

6. Open and Closing FTP Sessions
User
Interface
User PI
Server PI
open <host>
or from argv
설계: State transition diagram
- Event: User command, Reply msg
Action: send FTP command …
* reply msg에 대한 state transition도
포함해야
Establish Control Connection
220 Service ready
<user name>
USER <user name>
<password>
331 Need passwd
State transition table을 만든다면,
if-then-else를 쓰지 않고,
table-driven programming할
수 있지 않을까?
PASS <password>
230 User logged in
Data Transfer Session
close
bye, quit
QUIT
Jin Pyo Hong, HUFS
221 Service closing

7. Data Transfer: get, out, ls - active mode (passive mode off)
Server PI
Server
DTP
User
Interface
User
DTP
User PI
User logged in
How to know the listening socket address?
- IP addr: control connection socket에서
/* getsockname(controlfd, …); */
- port: 0
- bind(listenfd, …);
/* kernel이 사용하지 않는 port를 선택 */
- listen(listenfd, …);
- getsockname(listenfd, …);
- send PORT command
- datafd = accept(listenfd, … );
get/put/ls <file>
PORT h1,h2,h3,h4,p1,p2
200 Command Successful
RETR/STOR/LIST <file>
Establish data connection
150 Opening data connection
Data Transfer
Close connection
226 Transfer Complete
Jin Pyo Hong, HUFS

8. Data Transfer: get, put, ls - passive mode on
Server PI
Server
DTP
User
Interface
User
DTP
User PI
get/put/ls <file>
IP addr, port of server DTP
(server’s listening socket addr)
PASV
227 Entering Passive Mode (203,253,69,113,201,131)
RETR/STOR/LIST <file>
Establish data connection
150 Opening data connection
Data Transfer
Close connection
226 Transfer Complete
Jin Pyo Hong, HUFS

9. FTP command and Response
Jin Pyo Hong, HUFS

10. More Specific Diagram Login sequence Jin Pyo Hong, HUFS Wait for
S reply
Wait for
I reply
Wait for
P reply
Wait for
S reply
, 3
4, 5 F
Login sequence
Jin Pyo Hong, HUFS

11. State Transition Diagram
open, user 3
PASS
Wait for
I reply
Wait for
S reply 2
NULL 2
NULL
open
Open control Connection
user
USER
Wait for
User Cmd
Jin Pyo Hong, HUFS

12. State Transition Diagram
pwd, cd, ascii, binary, system, close 2
Success
1, 3
Error
pwd, cd, ascii, binary, system, close
PWD, CWD, TYPE A, TYPE I, SYST, QUIT
Wait
For S reply
Wait for
User Cmd
4, 5
Failure
Jin Pyo Hong, HUFS
Jin Pyo Hong, HUFS

13. State Transition Diagram
ls, dir, nlist, get, put
ls, dir, nlist
Type A 2
PASV/ PORT
Wait for
User Cmd
Wait for
S reply - 1
Wait for
S reply - 2
get, put
PASV / PORT 2
Connect/Listen
(LIST/NLIST/RETR/STOR) 2
Close Data Connection 2
Close Data Connection
Data
Transfer
Wait for
P reply 1
Pre Reply
Jin Pyo Hong, HUFS
Jin Pyo Hong, HUFS

14. Event State Table States Events Wait for user cmd (U_CMD) S reply
wait for S reply-3
(S_REPLY3)
P reply
(P_REPLY)
Data transfer
(DATA)
I reply
(I_REPLY)
From User Interface
open
Open control connection
USER
I_REPLY
user
nlist, ls, dir
TYPE A
 S_REPLY1
get, put
PASV/PORT
 S_REPLY2
pwd,, cd, ascii, binary,
system
PWD, CWD,
TYPE A, TYPE I
SYST
 S_REPLY
close, bye, quit
QUIT
Close control connection
 S_REPLY3
lcd, type, passive, debug
Local action
 U_CMD
From FTP Server
1yz
Error
Pre. Reply
DATA
2yz
Ignore
Success
 S_REPLY2
Connect or listen
LIST/NLIST/RETR/STOR
 P_REPLY
success
U_CMD
or Exit (bye, quit)
Close data connection if not closed
Password does not required
3yz
PASS
S_REPLY
4yz, 5yz
failure