Socket Programming*

1
Socket Programming

• T.A. Mustafa MISIR (http//cse.yeditepe.edu.tr/mm
  isir/s2008/cse471)
• Modified version of a socket programming
  related presentation by Vijayanand Bharadwaj _at_
  West Virginia University based on Computer
  Networking A Top-Down Approach Featuring the
  Internet, James Kurose and Keith Ross.

2
Overview

• Sockets What are they?
• Sockets How do they work?
• Sockets How to use them?
• Sockets Sample Programs

3
Sockets What are they?

• Network computing involves working with programs
  running on remote hosts
• Need to communicate with those programs
• Some terms
• Host a node consisting of all hardware and
  software on which programs run.
• Process a running program
• Multiple processes can run on one host

4
Sockets What are they?

• Processes on same host communicate through
  inter-process communication, defined by the
  operating system.
• How do processes on different hosts communicate?

5
Sockets What are they?

• Processes on remote hosts use messages to
  communicate.
• Client process process that initiates
  communication
• Server process process that waits to be
  contacted
• Clients and Servers use sockets to communicate

6
Sockets What are they?

• Process sends/receives messages to/from its
  socket
• Socket is analogous to door, electrical wall
  socket
• sending process shoves message out door
• sending process relies on transport
  infrastructure on other side of door which brings
  message to socket at receiving process

7
Sockets What are they?
user
application
Internet
8
Sockets What are they?

• Applications interface with the transport layer
  using sockets.
• This interface is implemented through an
  Application Programming Interface (API)
• i.e. applications can programmatically create and
  use sockets.
• Thus physically, a socket is a software
  interface. More formally
• -- a host-local, application-created,
  OS-controlled interface (a door) into which
  application process can both send and receive
  messages to/from another application process.

9
TCP vs. UDP
10
TCP vs. UDP (cont.)
11
Sockets Sample Programs

• Socket programming with
• TCP
• UDP
• Java Socket API
• Object-Oriented encapsulates most of the details
  into objects.
• Simple compared to the BSD Sockets API

12
Sockets Sample Programs

• Socket programming with
• TCP
• UDP
• Java Socket API
• Object-Oriented encapsulates most of the details
  into objects.
• Simple compared to the BSD Sockets API

13
Socket-programming with TCP

• TCP service reliable transfer of bytes from one
  process to another

controlled by application developer
controlled by application developer
controlled by operating system
controlled by operating system
internet
host or server
host or server
14
Socket-programming with TCP

• Client must contact server
• Server process must first be running
• Server must have created socket (door) that
  welcomes clients contact
• Client contacts server by
• Creating client-local TCP socket
• Specifying IP address, port number of server
  process
• When client creates socket client TCP
  establishes connection to server TCP

15
Socket-programming with TCP

• When contacted by client, server TCP creates new
  socket for server process to communicate with
  client
• allows server to talk with multiple clients
• source port numbers used to distinguish clients

16
Stream jargon

• A stream is a sequence of characters that flow
  into or out of a process.
• An input stream is attached to some input source
  for the process, eg, keyboard or socket.
• An output stream is attached to an output source,
  eg, monitor or socket.

17
Socket programming with TCP

• Example client-server application
• 1) Client reads line from standard input
  (inFromUser stream) , sends to server via socket
  (outToServer stream)
• 2) Server reads line from socket
• 3) Server converts line to uppercase, sends back
  to client
• 4) Client reads, prints modified line from
  socket (inFromServer stream)

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Socket programming with TCP
Client process
client TCP socket
19
Client/server socket interaction TCP
Server (running on hostid)
Client
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Example Java client (TCP)
import java.io. import java.net. class
TCPClient public static void main(String
argv) throws Exception String
sentence String modifiedSentence
BufferedReader inFromUser new
BufferedReader(new InputStreamReader(System.in))
Socket clientSocket new
Socket("hostname", 6789)
DataOutputStream outToServer new
DataOutputStream(clientSocket.getOutputStream())

Create input stream
Create client socket, connect to server
Create output stream attached to socket
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Example Java client (TCP), cont.
Create input stream attached to socket
BufferedReader inFromServer
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()))
sentence inFromUser.readLine()
outToServer.writeBytes(sentence '\n')
modifiedSentence inFromServer.readLine()
System.out.println("FROM SERVER "
modifiedSentence) clientSocket.close()

Send line to server
Read line from server
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Example Java server (TCP)
import java.io. import java.net. class
TCPServer public static void main(String
argv) throws Exception String
clientSentence String capitalizedSentence
ServerSocket welcomeSocket new
ServerSocket(6789) while(true)
Socket connectionSocket
welcomeSocket.accept()
BufferedReader inFromClient new
BufferedReader(new
InputStreamReader(connectionSocket.getInputStream(
)))
Create welcoming socket at port 6789
Wait, on welcoming socket for contact by client
Create input stream, attached to socket
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Example Java server (TCP), cont
DataOutputStream outToClient
new DataOutputStream(connectionSocket.get
OutputStream()) clientSentence
inFromClient.readLine()
capitalizedSentence clientSentence.toUpperCase()
'\n' outToClient.writeBytes(capit
alizedSentence)
Create output stream, attached to socket
Read in line from socket
Write out line to socket
End of while loop, loop back and wait for another
client connection
24
Socket programming with UDP

• UDP no connection between client and server
• no handshaking
• sender explicitly attaches IP address and port of
  destination to each packet
• server must extract IP address, port of sender
  from received packet
• UDP transmitted data may be received out of
  order, or lost

25
Client/server socket interaction UDP
Server (running on hostid)
26
Example Java client (UDP)
Client process
Input receives packet (TCP received byte
stream)
Output sends packet (TCP sent byte stream)
client UDP socket
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Example Java client (UDP)
import java.io. import java.net. class
UDPClient public static void main(String
args) throws Exception
BufferedReader inFromUser new
BufferedReader(new InputStreamReader(System.in))
DatagramSocket clientSocket new
DatagramSocket() InetAddress IPAddress
InetAddress.getByName("hostname")
byte sendData new byte1024 byte
receiveData new byte1024 String
sentence inFromUser.readLine() sendData
sentence.getBytes()
Create input stream
Create client socket
Translate hostname to IP address using DNS
28
Example Java client (UDP), cont.
Create datagram with data-to-send, length, IP
addr, port
DatagramPacket sendPacket new
DatagramPacket(sendData, sendData.length,
IPAddress, 9876) clientSocket.send(send
Packet) DatagramPacket receivePacket
new DatagramPacket(receiveData,
receiveData.length) clientSocket.receiv
e(receivePacket) String
modifiedSentence new
String(receivePacket.getData())
System.out.println("FROM SERVER"
modifiedSentence) clientSocket.close()

Send datagram to server
Read datagram from server
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Example Java server (UDP)
import java.io. import java.net. class
UDPServer public static void main(String
args) throws Exception
DatagramSocket serverSocket new
DatagramSocket(9876) byte
receiveData new byte1024 byte
sendData new byte1024 while(true)
DatagramPacket
receivePacket new
DatagramPacket(receiveData, receiveData.length)
serverSocket.receive(receivePacket)
Create datagram socket at port 9876
Create space for received datagram
Receive datagram
30
Example Java server (UDP), cont
String sentence new
String(receivePacket.getData())
InetAddress IPAddress receivePacket.getAddress()
int port receivePacket.getPort()
String
capitalizedSentence sentence.toUpperCase()
sendData capitalizedSentence.getBytes()
DatagramPacket sendPacket
new DatagramPacket(sendData,
sendData.length, IPAddress,
port) serverSocket.send(s
endPacket)
Get IP addr port , of sender
Create datagram to send to client
Write out datagram to socket
End of while loop, loop back and wait for another
datagram
31
Points to Remember

• A host has an IP addresses
• Can have multiple IP addresses. Actually each
  interface has an IP Address.
• Many applications can run on a host.
• Each application is identified by a port number
• An application can be made up of many processes.

32
Points to Remember

• Processes communicate using Sockets
• One process can have multiple sockets
• Example. TCPServer.java
• All sockets share the same port number !!
• But why no BIND EXCEPTION !!!!
• We will see next section !!
• Multiple processes can share a single socket !!
• Example. An application where multiple processes
  take turns communicating using the same socket
• Application developer needs to synchronize access
  to Socket.

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Points to Remember

• An example Multi-Threaded Web Server
• The original process listens on one socket (
  welcomeSocket) and creates multiple dedicated
  sockets for each client that contacts it.
  Original socket bound to port 80.
• Creates a process to deal with each client and
  assigns it the newly created socket.
• All sockets are bound to port 80 !!
• Very Similar Example is the KKMultiServer.java
  and KKMultiServerThread.java on SUNs official
  Java tutorial web site
• http//java.sun.com/docs/books/tutorial/networking
  /sockets/clientServer.html

34
Points to Remember

• Try A New Example ...
• Modify TCPServer.java
• Modify TCPClient.java
• Compile
• Launch TCPServer
• Launch TCPClient

35
Points to Remember

• PROMPTgtjava TCPServer
• Server Socket's IP Address localhost/127.0.0.1
• Server Socket's Port Number 6789
• Connection Socket's IP Address /127.0.0.1
• Connection Socket's Port Number 6789
• Connection Socket's Remote IP Address
  /127.0.0.1
• Connection Socket's Remote Port Number 1796
• PROMPTgtjava TCPClient
• Client Socket's IP Address /127.0.0.1
• Client Socket's Port Number 1796
• Hello There!
• FROM SERVER HELLO THERE!
• Same Port Numbers Used by both Sockets!

36
Points to Remember

• Run TCPServer Again from another prompt !
• BIND EXCEPTION !!!
• PROMPTgtjava TCPServer
• Exception in thread "main" java.net.BindException
  Address already in use JVM_Bind
• at java.net.PlainSocketImpl.socketBind(Nat
  ive Method)
• at java.net.PlainSocketImpl.bind(Unknown
  Source)
• at java.net.ServerSocket.bind(Unknown
  Source)
• at java.net.ServerSocket.ltinitgt(Unknown
  Source)
• at java.net.ServerSocket.ltinitgt(Unknown
  Source)
• at TCPServer.main(TCPServer.java11)

37
WHY IS THIS BEHAVIOR ???

• When Transport Layer handles Sockets it manages
  them by assigning UNIQUE IDENTIFIERS to each
  socket it creates.
• Thus sockets can share port numbers since
  Transport Layer uses the port number as one piece
  of the socket identifier
• However when the application layer tries to bind
  more than one socket to a port, transport throws
  an EXCEPTION !!!