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Socket Programming

Socket Programming

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Socket Programming

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  1. Socket Programming

  2. Outline: • Socket programming. • Socket programming with TCP. • Socket programming with UDP. • Evaluation.

  3. 1. Socket Programming Goal: Learn how to build client/server application that communicate using sockets.

  4. 1.Socket Programming Socket: - A host-local, application-created, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another application processes. - A door between application process and end-end-transport protocol (UCP or TCP)

  5. 1.Socket Programming Socket Types: 1. Datagram socketconnectionlessUDP 2. Stream socket connection oriented TCP 3. Raw socket Socket API: • introduced in BSD4.1 UNIX, 1981 • explicitly created, used, released by apps • client/server paradigm • two types of transport service via socket API: • unreliable datagram - reliable, byte stream-oriented

  6. process process TCP with buffers, variables TCP with buffers, variables socket socket 2. Socket-programming using TCP TCP service: Reliable transfer of bytesfrom one process to another. controlled by application developer controlled by application developer controlled by operating system internet host or server host or server

  7. 2. Socket-programming using TCP Client must contact server • server process must first be running • server must have created socket (door) that welcomes client’s 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 • 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

  8. create socket, connect to hostid, port=x create socket, port=x, for incoming request: clientSocket = new Socket(host,x) welcomeSocket = ServerSocket(x) TCP connection setup wait for incoming connection request connectionSocket = welcomeSocket.accept() send request using clientSocket read request from connectionSocket write reply to connectionSocket read reply from clientSocket close connectionSocket close clientSocket 2. Socket-programming using TCP Server (running on hostid) Client

  9. 2. Socket-programming using TCP Stream: • A stream is a sequence of characters that flow into or out of a process. • An input streamis attached to some input source for the process, e.g., keyboard or socket. • An output stream is attached to an output source, e.g., monitor or socket. Client process client TCP socket

  10. 2. Socket-programming using TCP Example client-server app: 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)

  11. 2. Socket-programming using TCPJava Client import*; import*; class TCPClient{ public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; BufferedReaderinFromUser = new BufferedReader(new InputStreamReader(; Socket clientSocket = new Socket("hostname", 6789); DataOutputStreamoutToServer = new DataOutputStream(clientSocket.getOutputStream()); Create input stream Create client socket, connect to server Create output stream attached to socket

  12. 2. Socket-programming using TCPJava Client (Cont) BufferedReaderinFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); outToServer.writeBytes(sentence + '\n'); modifiedSentence= inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); outToServer.close(); clientSocket.close(); } } Create input stream attached to socket Send line to server Read line from server

  13. 2. Socket-programming using TCPJava Server import*; import*; class TCPServer{ public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocketwelcomeSocket= new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReaderinFromClient = 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

  14. 2. Socket-programming using TCPJava Server(Cont) Crete output stream Attached to socket DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } } } Read in line from socket Write out line to socket End of while loop, loop back and wait for another client connection

  15. 3. Socket-programming using 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 UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server

  16. Client create socket, port=x, for incoming request: serverSocket = DatagramSocket() create socket, clientSocket = DatagramSocket() Create, address (hostid, port=x, send datagram request using clientSocket read request from serverSocket write reply to serverSocket specifying client host address, port number read reply from clientSocket close clientSocket 3. Socket-programming using UDP Server (running on hostid)

  17. 3. Socket-programming using UDP Client process Input: receives packet (recall thatTCP received “byte stream”) Output: sends packet (recall that TCP sent “byte stream”) client UDP socket

  18. 3. Socket-programming using UDPJava Client import*; import*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReaderinFromUser = new BufferedReader(new InputStreamReader(; DatagramSocketclientSocket = new DatagramSocket(); InetAddressIPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine(); sendData = sentence.getBytes(); Create input stream Create client socket Translate hostname to IP address using DNS

  19. 3. Socket-programming using UDPJava Client(Cont) Create datagram with data-to-send, length, IP addr, port DatagramPacketsendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacketreceivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } } Send datagram to server Read datagram from server

  20. 3. Socket-programming using UDPJava Server import*; import*; class UDPServer{ public static void main(String args[]) throws Exception { DatagramSocketserverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacketreceivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket); Create datagram socket at port 9876 Create space for received datagram Receive datagram

  21. 3. Socket-programming using UDPJava Server (Cont) String sentence = new String(receivePacket.getData()); InetAddressIPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); DatagramPacketsendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } } } 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

  22. 4. Evaluation 1. Modify the sever code (the one in the slides), so that the server process counts the number of characters in the line and returns it to the client. The client will print the number on its monitor. 2. Write a client process that will communicate with a server that you will also write. Your client will open a TCP socket to your server and send a message to your server containing your name (X). The server will accept connection from your client and will send a message (“Hello X”) back to your client, where X is replaced by your name. The client will print the message.

  23. 3. Team up with someone from the class. You should get your client and their server processes to inter-operate with each other (or vice versa). Your client should send a message contains your name and your partner name and their server should eventually print out the message after changing it to lower case. 4. Repeat problem 1 using UDP sockets

  24. References: Computer Networking: A Top Down Approach Featuring the Internet, 6th edition. Jim Kurose, Keith RossAddison-Wesley, July 2013.