1. File Transfer on the Internet Trivial File Transfer Protocol (TFTP) File Transfer Protocol (FTP)

2. TFTP: Trivial file transfer protocol
LAN
file transfer
TFTP
server
TFTP
client
TFTP
client
remote file
system
Used to transfer a file to/from remote host
Intended to be used when bootstrapping diskless systems with BOOTP or DHCP
System Components
TFTP Server – UDP port 69
TFTP Client
TFTP Protocol

3. TFTP Commands Mode IP Header UDP Header20 8 2 N 1 M 1 IP
Header
UDP
Header
Opcode
(1=RRQ)
(2=WRQ)
filename
mode 2
0-512
Mode
ASCII: Each line ends with CRLF
octet: binary
Opcode
(3=data)
block
number
data 2
Opcode
(4=ACK)
block
number 2 N
Opcode
(5=error)
error
number
Error message

4. TFTP File Transfer Employs a “stop-and-wait” protocol
Server sends a block, waits for ACK, then sends the next block
Server must to handle duplicate/lost packets.
EOF: When the client receives a block that has less than 512 bytes, it knows this is the last block.

5. FTP: the file transfer protocol
user
interface
client
file transfer
FTP
server
user
at host
local file
system
remote file
system
Problem:
Want to transfer file to/from remote host
System Components
FTP Server – TCP port 21
FTP Client
FTP Protocol -- RFC 959

6. FTP: separate control, data connections
FTP client contacts FTP server at port 21, specifying TCP as transport protocol
Client obtains authorization over control connection
Client browses remote directory by sending commands over control connection.
When server receives a command for a file transfer, the server opens a TCP data connection to client
Uses the Port command
After transferring one file, server closes connection.
FTP
client
server
TCP control connection
port 21
TCP data connection
port 20
Server opens a second TCP data connection to transfer another file.
Control connection: “out of band”
FTP server maintains “state”: current directory, earlier authentication

7. FTP commands, responses
Sample commands:
sent as ASCII text over control channel
USER username
PASS password
LIST return list of file in current directory
CWD change current directory
RETR filename retrieves (gets) file
STOR filename stores (puts) file onto remote host
PORT n1,n2,n3,n4,n5,n6
QUIT
Sample return codes
status code and phrase
331 Username OK, password required
125 data connection already open; transfer starting
425 Can’t open data connection
452 Error writing file

8. Example: RETRieving a file
Client
Server
SYN
SYN + ACK
ACK
220 FTP server ready
USER cakinlar
331 Password required
PASS cakinlar
230 User cakinlar logged in
PORT 127,0,0,1,4,150
220 PORT command successful
RETR a.c
150 Opening ASCII mode connection …
Server opens a TCP connection
to port 1174 of the client
and transfers the data
over the this data channel
226 Transfer complete