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. FTP Commands for Minimum Implementation
User Cmd
FTP Cmd
arguments 비고
open
<hostname>
Connect to FTP server
[user]
USER
<username>
User name
PASS
<password>
Password
nlist
NLST
[<pathname>]
Name list (ls)
ls, dir
LIST
Directory list (ls –l)
pwd
PWD
Print working directory cd
CWD
<pathname>
Change working directory
lcd
Change local working directory
type
[type] ascii
[type] binary
TYPE A I
FTP data type ?
A – NVT-ASCII text
I – binary data
get
RETR
Retrieve: download the file
put
STOR
Store: upload the file
passive
Passive mode on or off (toggle switch)
If passive mode on, send PASV for data transfer
Otherwise, send PORT
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
h1,h1,h3,h4: IPv4 address
p1,p2: port number
close
QUIT
Close FTP session
bye, quit
Close FTP session and exit
Jin Pyo Hong, HUFS
Jin Pyo Hong, HUFS

4. FTP Replies Code Description 1yz
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
close
bye, quit
QUIT
Jin Pyo Hong, HUFS
221 Service closing

7. 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

8. Data Transfer: get, out, ls - 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
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

9. State Transition Diagram
CWD, QUIT, PORT, PWD, TYPE
Error
1yz
3yz
Send
cmd
2yz
Begin
Wait
for reply
Success
4yz
5yz
Failure
Jin Pyo Hong, HUFS

10. State Transition Diagram
LIST, NLST, RETR, STOR
Error
3yz
Send
cmd
2yz
Begin
Wait
for reply
Success
1yz
4yz
5yz
Failure
Jin Pyo Hong, HUFS

11. State Transition Diagram
Login Sequence: USER, PASS
Send
USER
Begin
User ID
sent 1
Error 2 3
4, 5
Send
PASS
1,3
Wait
for reply
Success
Enter
passwd 2
4, 5
Failure
Jin Pyo Hong, HUFS

12. Reference
RFC959, File Transfer Protocol (FTP)
Jin Pyo Hong, HUFS