Tips for the assignment
1
Socket Programming using TCP
Socket: a door between application process and endend-transport protocol (UDP or TCP)
TCP service: reliable transfer of bytes from one process
to another
controlled by
application
developer
controlled by
operating
system
process
process
socket
TCP with
buffers,
variables
host or
server
Internet
socket
TCP with
buffers,
variables
host or
server
controlled by
application
developer
controlled by
operating
system
2
3
Client Sockets & Server Sockets
client
server
Client
socket
Welcome
socket
time
Client
socket
bytes
Connection
socket
4
Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
Socket()
wait for incoming
connection request
connectionSocket =
accept()
read request from
connectionSocket
write reply to
connectionSocket
close
connectionSocket
TCP
connection setup
create socket,
connect to hostid, port=x
clientSocket =
Socket()
send request using
clientSocket
read reply from
clientSocket
close
clientSocket
5
Application 2-5
Simple Server Pseudo-Code
socket
bind
listen
loop "forever"
{
accept /* by creating new socket */
/* process the client’s request */
loop until done
{
receive/send
}
closesocket(newsocket)
}
6
Simple Client Pseudo-Code
socket
connect
/* process the client’s request */
loop until done
{
send/receive
}
closesocket(socket)
7
Proper Ordering of Send and Recv
socket
socket
bind
connect
listen
/* process the clients request */
loop "forever"
loop until done
{
{
accept /* by creating new socket */
/* process the clients request */
loop until done
send/receive
}
closesocket(socket)
{
receive/send
}
closesocket(newsocket)
}
8
Test the TCP Client-Server Codes
 Run ServerWindows.c
 Compile ClientWindows.c, look for the
executable.
 Run ClientWindows.c from the command
prompt to connect to the server:
 ClientWindows
localhost 1234
 Alternatively, use IpConfig to find out what your IP
address is: (e.g. 130.123.123.111), then connect
to the server using:
• ClientWindows 130.123.123.111 1234
 Let’s see how to use the compiler’s debug
mode to trace the exchange of messages.
9
10
FTP (Multiple Clients)
TCP Control Socket
C
DIR
Port 127,0,0,1,6,11
Listening
Socket
After file transfer
TCP Control Socket
S
Quit
TCP Active Data Socket
Server
 In TCP, the Server should be running already prior to a
Client connecting to it
11
12
FTP Operation
CLIENT
SERVER
Control connection
Data connection
Note that Ports 1216 & 1217 are ephemeral ports. Any port [1024, 65535] will work.
13
Source: Computer Networking and the Internet (5/e) by Fred Halsall
FTP Commands
COMMAND
DESCRIPTION
USER username
User name on the FTP server
PASS password
User’s password on the FTP server
SYST
Type of operating system requested
TYPE type
File type to be transferred: A (ASCII), I
(Image/Binary)
PORT n1,n2,n3,n4,n5,n6
Client IP address (n1-n4) and port number (n5,
n6)
RETR filename.type
Retrieve (get) a file
STOR filename.type
Store (put) a file
LIST filelist
List files or directories
QUIT
Log off from server
Source: Computer Networking and the Internet (5/e) by Fred Halsall
14
FTP Reply Codes
REPLY
DESCRIPTION
1yz
Positive reply, wait for another reply before sending a new command
2yz
Positive reply, a new command can be sent
3yz
Positive reply, another command is awaited
4yz
Negative reply, try again
5yz
Negative reply, do not retry
x0z
Syntax
x1z
Information
x2z
Control or data connection
x3z
Authentication
x4z
Unspecified
x5z
File status
Source: Computer Networking and the Internet (5/e) by Fred Halsall
15
Active FTP (or Standard )
Standard (or PORT or Active)
The Standard mode FTP client sends PORT commands to the FTP server.
These commands are sent over the FTP command channel when
establishing the FTP session.
Operation
Standard mode FTP clients first establish a connection to TCP port 21 on the
FTP server. This connection establishes the FTP command channel.
The client sends a PORT command over the FTP command channel when
the FTP client needs to send or receive data, such as a folder list or file.
The PORT command contains information about which port the FTP client
receives the data connection on. In Standard mode, the FTP server always
starts the data connection from TCP port 20.
The FTP server must open a new connection to the client when it sends or
receives data, and the FTP client requests this by using the PORT command
again.
16
Passive FTP (or PASV)
Passive (or PASV)
The Passive mode client sends PASV commands to the FTP Server.
Operation
Passive mode FTP clients also start by establishing a connection to TCP
port 21 on the FTP server to create the control channel. When the client
sends a PASV command over the command channel, the FTP server opens
an ephemeral port (between 1024 and 5000) and informs the FTP client to
connect to that port before requesting data transfer. As in Standard mode,
the FTP client must send a new PASV command prior to each new transfer,
and the FTP server will await a connection at a new port for each transfer.
17
p. 50, RFC 959
18
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
time
time
Passive open
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
19
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
20
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
21
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
22
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
23
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
24
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
25
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
26
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
SYST
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
27
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
SYST
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
28
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
get <filename.type>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
29
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
get <filename.type>
time
time
30
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
get <filename.type>
time
time
31
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
get <filename.type>
RETR <filename.type>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
32
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
get <filename.type>
RETR <filename.type>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
33
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
Server does an active
open to Port n5, n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
get <filename.type>
RETR <filename.type>
time
time
time
34
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
Server does an active
open to Port n5, n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
get <filename.type>
RETR <filename.type>
time
time
time
35
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
Server does an active
open to Port n5, n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
get <filename.type>
RETR <filename.type>
time
User prompted for a
conmand
time
36
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
Server does an active
open to Port n5, n6
RETR <filename.type>
QUIT
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
get <filename.type>
User prompted for a
conmand
37
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
Server does an active
open to Port n5, n6
RETR <filename.type>
QUIT
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
get <filename.type>
User prompted for a
conmand
38
ACTIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
20
Port
21
control connection
Data connection
Port
1120
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PORT n1-n6
Server does an active
open to Port n5, n6
RETR <filename.type>
QUIT
Server closes data connection
time
Source:
Networking
and the connection
Internet (5/e) by Fred
firstComputer
, then
control
Halsall
get <filename.type>
User prompted for a
conmand
39
40
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Data connection
Port
1120
Port
1121
time
time
Passive open
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
41
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
42
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
43
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
44
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
45
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
Data
control
connection
connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
46
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
47
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
48
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
SYST
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
49
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
SYST
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
50
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
51
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
52
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
dir
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
53
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
dir
time
time
54
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
dir
time
time
55
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
dir
Client does an active open to Port n5,n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
56
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
dir
Client does an active open to Port n5,n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
time
time
57
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
dir
Client does an active open to Port n5,n6
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
User is prompted for a command
58
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
dir
Client does an active open to Port n5,n6
QUIT
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
User is prompted for a command
59
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
dir
Client does an active open to Port n5,n6
QUIT
time
Source: Computer Networking and the Internet (5/e) by Fred
Halsall
time
User is prompted for a command
60
PASSIVE FTP Operation
SERVER
CLIENT
Data connection
control connection
Port
2024
Port
21
control connection
Port
1120
Data connection
Port
1121
Active open
Passive open
TCP control connection
to Port 21 established
USER <username>
PASS <password>
PASV
LIST
dir
Client does an active open to Port n5,n6
QUIT
Server closes
time data connection
Source:
Networking
and the connection
Internet (5/e) by Fred
firstComputer
, then
control
Halsall
time
User is prompted for a command
61
Security Issues
Firewall administrators may not want to use Passive
mode FTP servers because the FTP server can open
any ephemeral port number.
Many FTP servers are configured with an ephemeral
port range of 1024 through 65535.
Firewall configurations that allow full access to all
ephemeral ports for unsolicited connections may be
considered unsecured.
http://support.microsoft.com/kb/323446
63
How to change the Internet
Explorer FTP Client mode
 Start Internet Explorer.
 On the Tools menu, click Internet Options.
 Click the Advanced tab.
 Under Browsing, click to clear the Enable folder
view for FTP sites check box.
 Click to select the Use Passive FTP (for firewall
and DSL modem compatibility) check box.
Internet Explorer behaves as a Standard mode FTP client if you select
the Enable folder view for FTP sites check box, even if you also select
the Use Passive FTP check box. If you clear the Enable folder view for
FTP sites check box and then select the Use Passive FTP check box,
Internet Explorer behaves as a Passive mode FTP client.
66
The End
67