1. Sub Topic : Introduction to File Transfer Protocol
(FTP)
.
ftp protocol with video 1

2. Recap
In the previous class, you have learnt about
• Post Office Protocol
• Installation and configuration of POP3 connector
• POP and commands and process
• Limitations of POP
• Internet Mail Access Protocol (IMAP)
• Comparison of POP and IMAP
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3. Objectives
On completion of this class, you would be able to
understand about
• FTP’s connections
• FTP commands and responses
• FTP in action
• FTP Connections
• Transfer Modes
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4. Recap of Mail Protocols
BHARATHI SANTOSH
SMTP SMTP POP3 or user
user
agent IMAP agent
sender’s mail receiver’s mail
server server Fig .1
• SMTP: delivery/storage to receiver’s mail server
• Mail access protocols: retrieval from server
– POP: Post Office Protocol [RFC 1939]
• authorization and download
– IMAP: Internet Mail Access Protocol [RFC 1730]
• manipulation of stored messages on server
– HTTP: Hotmail , Yahoo! Mail, etc
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5. File Transfer Protocol (FTP)
• A general-purpose protocol that can be used to copy an
arbitrary file from one computer to another
• one of the oldest network application---predates TCP
and IP
• Later versions were built on top of TCP/IP
• Among the most heavily used applications
– FTP generated as much as 1/3 of the traffic on the
Internet
– Was exceeded only by WWW (in 1995)
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6. File Transfer Protocol
• Provides reliable transfer of files
• Between different hosts running on different platforms
MS Windows, Apple Mac, Sun,
BSD UNIX, SCO UNIX, Linux
• Using different file structures
• Supports ASCII and binary content
Fig .2
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7. FTP Standards
• 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
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8. Control and Data Connections
• Control functions (commands) and reply codes are
transferred over the control connection
• All data transfer takes place over the data connection
• The control connection must be “up” while data transfer
takes place
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9. Control Connection
• The control connection is the “well known” service
• The control connection uses the TELNET protocol
• Commands and replies are all line oriented text (default
is ASCII)
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10. Standard Connection Model
Control
A
A Data B
B
Fig .3
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11. Alternative Connection Model
Control Control
A
B
B Data C
C
Fig .4
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12. Transfer Modes
• FTP defines two types of transfer
– textual and binary
• Textual: is used for text files
– most text files are encoded in ASCII or EBCDIC
– ftp can translate from the local to remote character
set when transferring a file
• Binary: used for all other files (audio, image, numbers,
…)
– Files are copied exactly
– The resulting copy might be meaningless because
FTP does not convert values to the local
representation
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13. FTP: File Transfer Protocol
FTP file transfer
FTP FTP
user client server
interface
local file remote file
user
system system
at host
Fig .5
• transfer file to/from remote host
• client/server model
– client: side that initiates transfer (either to/from remote)
– server: remote host, serves the file
• ftp: RFC 959
• ftp server: port 21 (well known)
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14. Separate Control & Data Connections
• ftp client contacts server at
port 21, specifying TCP as
the transport protocol TCP control connection
port 21
• Parallel TCP connections
– control: exchange commands,
TCP data connection
responses between client and FTP port 20 FTP
server client server
– data: file data to/from server
Fig .6
• ftp server maintains “state”
– current directory
– earlier authentication
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15. FTP Client Commands
Command Description
get filename Retrieve file from server
mget filename* Retrieve multiple files from server
put filename Copy local file to server
mput filename* Copy multiple local files to server
open server Begin login to server
bye / close / exit Logoff server
ls / dir List files in current remote dir on server
lcd Change local directory
cd Change remote directory
rhelp / remotehelp
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Table 1

16. FTP Commands
Command Description
Abor Abort previous FTP command and any
data transfer
List [filelist ] List files or directories (ls / dir)
Pass password Password on server
Port n1,n2,n3,n4,n5,n6 Client IP and port number
Retr filename Retrieve (get) filename
Stor filename Store (put) filename
Noop Do nothing (check if server is alive)
Table 2
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17. FTP Response Format
Reply Description
1yz Positive preliminary reply.The action is being started but
expect another reply before sending another cmd
2yz Positive completion reply. A new cmd can be sent
3yz Positive intermediate reply. The cmd has been accepted but
another cmd must be sent
4yz Transient negative completion reply. The requested action
did not take place but can be sent later
5yz Permanent negative completion reply. Cmd not accepted and
should not be reissued
x0z Syntax errors
x1z Information
x2z Connections. Replies referring to the control or data
connections
x3z Authentication and accounting
x4z Unspecified
x5z Filesystem status Table 3
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18. Example FTP Responses
• 120 Service will be ready shortly
• 200 Command OK
• 230 User login OK
• 331 User name OK; password is needed
• 421 Service not available
• 503 Bad sequence of commands
• 530 User not logged in
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19. Connections, Authorizations and File permissions
• The remote system has to verify that the user is
authorized to access files
– The user has to provide a login name and a password
– If the user is authorized he/she may start transferring
files
• What if the user does not have an account?
– System administrator can configure FTP to support
anonymous FTP
– Login name anonymous and password guest (or e-
mail address) allows a user access to public files
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20. FTP uses the client-server paradigm
– Local application (or
browser) is the client
– Remote FTP program
is the server
– The FTP server
authorizes the
connection, locates
the file, and uses TCP
Fig .8
to send it
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21. 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
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22. 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
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23. 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
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24. 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
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25. 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
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26. A Browser can use FTP
• A W W W browser can be used for FTP instead of a
dedicated interface
• A browser uses FTP as the transfer protocol, when the
URL starts with ftp (instead of http)
• Example
– ftp://ftp.acunix.albany.edu/as7656/temp --- instructs
the browser to get file “as7656/temp” from machine
ftp.acunix.edu
– ftp://ftp.acunix.edu/as7656 --- displays all files in the
directory “as7656”
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27. Summary
In this class, you have learnt about
• File Transfer Protocol
• Commands & Responses
• FTP Transfer Modes
• Transaction Phase
• Transmission Modes
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28. Quiz
1) FTP uses 2 TCP ports : one for data transfer and the
other for____
A) Control
2) Control connection uses ___ protocol
A) Telnet
3) The FTP command used to retrieve a file from the
remote computer is ___
A) Get
4) The remote system verifies ____ and ___ to
authorize user to access files
A) login Name (or user id) and Password
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29. Frequently Asked Questions
1. Explain file transfer protocol (FTP)
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ftp protocol with video