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File Transfer Protocol (FTP)
Chuan-Ming Liu Computer Science and Information Engineering National Taipei University of Technology Fall 2007, TAIWAN
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File Transfer Protocol (FTP) Chuan-Ming Liu Computer Science and - - PowerPoint PPT Presentation
File Transfer Protocol (FTP) Chuan-Ming Liu Computer Science and - - PowerPoint PPT Presentation
File Transfer Protocol (FTP) Chuan-Ming Liu Computer Science and Information Engineering National Taipei University of Technology Fall 2007 , TAIWAN 1 Contents CONNECTIONS COMMUNICATION COMMAND PROCESSING FILE TRANSFER SIMPLE FTP LAB
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File Transfer Protocol (1)
Transferring files from one computer to another Using two connections between hosts
One for data transfer One for control information
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File Transfer Protocol (2)
Three components for clients: user interface, client control process, and client data transfer process Two components for servers: server control process and server data transfer process
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File Transfer Protocol (3)
FTP uses the services of TCP FTP uses the services of TCP The well The well-
- known port 21 is used for the
known port 21 is used for the control connection control connection The well The well-
- known port 20 for the data
known port 20 for the data connection connection
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The control connection remains connected during the entire interactive FTP session; the data connection is opened and then closed for each file transferred.
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Contents
CONNECTIONS COMMUNICATION COMMAND PROCESSING FILE TRANSFER SIMPLE FTP – LAB 3
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Control Connection
Created in the same ways as other applications
Server issues a passive open on port 21 and waits Clients uses an ephemeral port and issues an active open
Remains open during the entire process Needs minimize delay for service
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Data Connection
Uses the port 20 usually at the server side Creation of a data connection includes
Client issues a passive open using an ephemeral port Client sends this port number to the server using PORT command Server receives the port number and issues an active open using the well-known port
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Contents
CONNECTIONS COMMUNICATION COMMAND PROCESSING FILE TRANSFER SIMPLE FTP – LAB 3
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Communication
FTP client and server run on different computers using different OS, character sets, and file formats. FTP must take this heterogeneity compatible Communication over
Control connection Data connection
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Communication over Control Connection
Uses the NVT ASCII character set Achieves the communication by commands and responses Each command or response is a short line terminated with a two-character (CR+LF) end-of-line token
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NVT
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Using the Control Connection
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Communication over Data Connection
Transfer files through the data connection The client must define
File type Data structure Transmission mode
Before sending the file through the data connection, we prepare for transmission through control connection
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File Type
ASCII file
default format for transferring text file
EBCDIC file Image file
default format for transferring binary files Mostly used to transfer binary files such as compiled programs
Nonprint v.s. TELNET attribute
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Data Structure
File structure (default)
Continuous stream of bytes
Record structure
Divided into records (or structs in C) Used only with text files
Page structure
Divided into pages each having a page number and header Can be stored or accessed randomly or sequentially
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Transmission Mode
Stream mode
Default mode Data are delivered from FTP to TCP as a continuous stream of bytes
Block mode Compressed Mode
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Using the data connection
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Contents
CONNECTIONS COMMUNICATION COMMAND PROCESSING FILE TRANSFER SIMPLE FTP – LAB 3
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Command Processing
FTP uses the control connection to communication. Commands are sent from the client to the server and the responses are sent back from the server to the client.
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Command Processing
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Commands
In the form of ASCII uppercase May be followed by an argument Six groups:
Access commands File management commands Data format commands Port defining commands File transferring commands Miscellaneous commands
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Access Commands
include
USER user-id PASS user-passwd ACCT account to be changed REIN QUIT ABOR
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File Management Commands
includes
CWD dir CDUP DELE file LIST dir NLIST dir MKD dir PWD RMD dir
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Data Formatting Commands
includes
TYPE A(ASCII), E(EBCDIC), I (Image), N (Nonprint), or T(TELNET) STRU F (File), R (Record), or P (page) MODE S (Stream), B (Block), or C (Compressed)
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Port Defining Commands
Define the port number for the data connection on the client site
Selects an port number and sends it to the server using a passive open Asks the server to choose a port number and send the port number in the response and then the client creates an active open using that port
Two commands
PORT (6-digits id) PASV
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File Transfer Commands
include
RETR file(s) STOR file(s) APPE file(s) STOU file(s) ALLO file(s) REST file(s) STAT file(s)
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Miscellaneous Commands
include
HELP NOOP SITE SYST
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Responses
Each FTP command generates at least one response Each response has two parts:
3-digit number: defines the code Text: defined needed parameters or extra explanations
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Contents
CONNECTIONS COMMUNICATION COMMAND PROCESSING FILE TRANSFER SIMPLE FTP – LAB 3
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File Transfer
One of the three things
Retrieving a file (RETR) Storing a file (STOR) Listing directory or file names (LIST) Note that, FTP treats a list of directory or file name as a file and sends it over the data connection.
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File transfer
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Example of Retrieving a List
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Example of storing an image (binary) file
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Contents
CONNECTIONS COMMUNICATION COMMAND PROCESSING FILE TRANSFER SIMPLE FTP – LAB 3
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LAB 3 – Simple FTP Client
Implement a simple FTP client Start your ftp service on the client side by typing sftp To connect to a server, use open command
- pen hostname (or IP)
Exit the service by giving the bye command
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Commands to be Implemented
Two categories for the other commands
Commands on the remote machine ls, cd, get, mget, put, mput Commands on the local machine lls, lcd
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General Commands
USER id PASS password PORT h1,h2,h3,h4,p1,p2
PORT 140,124,182,140,4,200 IP:140.124.182.140 Port:4*256+200=1224
TYPE mode
AASCII IBinary
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General Commands
LIST RETR path
download
STOR path
upload
QUIT PASV
Return h1,h2,h3,h4,p1,p4
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Login & Logout Scenario
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Some Details
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Basic Commands (1/5)
USER <SP> <username> <CRLF>
The username is required by the server for access to its file system.
PASS <SP> <password> <CRLF>
The argument field is a TELNET string identifying the user's password.
QUIT <CRLF>
This command terminates a USER and if file transfer is not in progress, the server closes the TELNET connection.
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Basic Commands (2/5)
PORT <SP> <Host-port> <CRLF>
The argument is a HOST-PORT specification for the data port to be used in data connection. A port command would be: PORT h1,h2,h3,h4,p1,p2 where, h1 is the high order 8 bits of the internet host address.
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Basic Commands (3/5)
PASV <CRLF>
This command requests the server to "listen" on a data port (which may not be its default data port) and to wait for a connection rather than initiate one upon receipt of a transfer command. Server replies a code : 227 Entering Passive Mode. h1,h2,h3,h4,p1,p2
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Basic Commands (4/5)
TYPE <SP> <type code> <CRLF>
The argument specifies the representation type, for example:
A - ASCII I - Image
RETR <SP> <pathname> <CRLF>
This command causes the server to transfer a copy of the file, specified in the pathname, to the server or user at the other end of the data connection.
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Basic Commands (5/5)
STOR <SP> <pathname> <CRLF> This command causes the server to accept the data transferred via the data connection and to store the data as a file at the server site. CWD <SP> <pathname> <CRLF> This command allows the user to work with a different directory or dataset for file storage or retrieval without altering his login or accounting information. LIST [<SP> <pathname>] <CRLF> This command causes a list to be sent from the server to the client. PWD <CRLF> Return the current location of direction
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The open command
Client Server USER username Reply a code PASS password Reply a code
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The bye command
QUIT Reply a code Client Server Reply a code
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The ls command
Client Server PASV Reply a code Connect to the data connection socket Transmit the information of direction by data connection socket LIST <pathname> Rely a code Rely a code Close the data connection socket
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The cd command
Client Server CWD pathname Reply a code PWD Reply a code
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The got command
Client Server PASV Reply a code Connect to the data connection socket Reply a code RETR filename Reply a code Transmit the file though data connection socket Close the data connection socket
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The put command
Client Server PASV Reply a code Connect to the data connection socket Reply a code STOR filename Reply a code Transmit the file though data connection socket Close the data connection socket