1. File Transfer & Access (FTP, TFTP, NFS)
Chapter 25

2. File Access & Transfer
Many network systems provide file access on remote machines
Lots of designs; each optimized for some goals
Lower overall cost; use centralized file server
Data archive
Data sharing by multiple users, programs or sites

3. On-line Shared Access Two distinct forms of file sharing
On-line access
Multiple programs can access single file concurrently
Changes take affect immediately; available to all
Whole-file copying
Program obtains local copy to access a file
Often used for read-only data
If must be modified, changed copy is transferred back

4. File sharing is usually easy to use
Advantage: no change to application programs
Disadvantages may be less obvious
Network/Remote machine down: application may not work
Remote could be overloaded or network congested
Application appears unreliable
Implementing integrated, transparent access may be difficult
File names may be hard to map
Mechanism must handle notions of ownership, authorization, and access protection
Different file representations may make it difficult or impossible to implement all operations on all files

5. Sharing by File Transfer
Alternative to transparent, integrated, on-line access
User obtains local copy; operates on the copy
Transfer mechanism not integrated w/file system
Special client program used to transfer files
Specify remote machine & any login information
Client contacts server and asks for copy of file
After transfer, user terminates client
Have efficient usage of file once have copy

6. Whole-file transfer can be difficult
Client & server must agree on:
Ownership, protection, authorization, data format
Exact detail of differences in representation techniques depends on the systems involved
Information can be lost during translations

7. FTP – File Transfer Protocol
The major TCP/IP file transfer software
File transfer one of most used TCP/IP applications
Accounts for much network traffic
Early ARPANET transfer protocols evolved into current standard
Other functions offered beyond file transfer:
Interactive interface with remote server
File format specification
Authentication control

8. FTP Process Model
FTP servers allow concurrent access by multiple clients
Clients use TCP to connect
Have usual master/slave functions
Except, slave not perform all necessary computation
Slave handles control connection from the client
Carries commands telling which file to transfer
Separate processes handle data transfer connection
Also uses TCP as the transport protocol

9. Figure 25.1

10. TCP Port Number Assignment
Multiple connections require multiple ports
Control connection:
Client: random, locally assigned port
Server: uses well-known port (21)
For data transfer:
Client: unused port; server process: FTP data transfer port (20)
Server cannot accept connection from any process
Control connection used to communicate port number
Client control process gets port for data transfer; creates transfer process to listen; sends number to server over control connection; waits for server to connect to the port
Use NVT protocol for data on the control connection

11. Anonymous FTP FTP contains access authorization facilities
Strict enforcement keeps arbitrary users away
Anonymous FTP used to access public files
Client does not need an account or password
Uses login name of anonymous; password guest
Have access, but only to public files

12. ftp> open ftp.cs.purdue.edu
Connected to lucan.cs.purdue.edu.
220 lucan.cs.purdue.edu FTP server (Version wu-2.6.2(1) Mon Feb 28 15:45:56 EST
2005) ready.
User (lucan.cs.purdue.edu:(none)): anonymous
331 Guest login ok, send your complete address as password.
Password:
230-
Purdue University
Department of Computer Sciences
Access is allowed all day. Local time is Thu Aug 3 12:46:
All transfers are logged with your host name and address.
If you don't like this policy, disconnect now!
If your FTP client crashes or hangs shortly after login, try using a
dash (-) as the first character of your password. This will turn off
the informational messages which may be confusing your ftp client.
Report any problems to
230 User ftp logged in. Access restrictions apply.

13. Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 14
drwx Jan OLD
d--x--s--x Jun bin
d--x--s--x Jun dev
d--x--s--x Apr
d--x--s--x Jun lib
drwxr-sr-x May 2 14:00 pub
d--x--s--x Apr usr
226 Transfer complete.

14. ftp> help
Commands may be abbreviated. Commands are:
! debug mdir put size
$ dir mget pwd status
account direct mkdir quit struct
append disconnect mls quote system
ascii form mode recv sunique
bell get modtime reget tenex
binary glob mput rstatus trace
bye hash newer rhelp type
case help nmap rename user
cd idle nlist reset umask
cdup image ntrans restart verbose
chmod lcd open rmdir win
close ls prompt runique ?
cr macdef proxy send
delete mdelete sendport site
ftp> quit
221-You have transferred 0 bytes in 0 files.
221-Total traffic for this session was 1476 bytes in 1 transfers.
221-Thank you for using the FTP service on lucan.cs.purdue.edu.
221 Goodbye.

15. TFTP
FTP is the most general file transfer protocol in the TCP/IP suite
Also most complex & difficult to program
Many applications do not need full functionality
Has second file transfer protocol
Provides inexpensive, unsophisticated service
Trivial File Transfer Protocol (TFTP)
Intended for applications with less complex interactions between the client and the server

16. Secure File Transfer Original FTP has password mechanism
Password and data are unencrypted
Other protocols have added security
Secure Sockets Layer FTP (SSL-FTP)
Uses secure socket mechanism
Secure File Transfer Program (sftp)
Alternative to FTP; uses SSH tunnel
Secure Copy (scp)
Uses same syntax as Unix copy command
Uses SSH

17. Summary Two forms of remote data access FTP uses whole-file copying
Shared on-line access
FTP uses whole-file copying
Provides transfer and user interface facilities
TFTP provides simple alternative for FTP
Are secure alternatives to FTP