2: Application Layer1 Chapter 2 Application Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.  This slide show has been modified by Gergana Miteva, Angela Murphy and Sonya Nikolova Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

2: Application Layer2 Socket programming Two types of client/server apps r Protocol implementation by the RFC rules r Proprietary implementation Two types of transport service m unreliable datagram m reliable, byte stream- oriented an application- created/owned, OS-controlled interface into which A process can both send and receive messages to/from another process socket Goal: learn how to build client/server application that communicate using sockets

2: Application Layer3 Sockets Socket: a “door” between application process and end-to-end-transport protocol (UDP or TCP) process kernel buffers, variables socket controlled by application developer controlled by operating system host or server process kernel buffers, variables socket controlled by application developer controlled by operating system host or server internet

2: Application Layer4 Stream jargon r A stream is a sequence of characters that flow into or out of a process. r An input stream is attached to some input source for the process, eg, keyboard or socket. r An output stream is attached to an output source, eg, monitor or socket.

2: Application Layer5 Languages and Platforms Socket API is available for many languages on many platforms: r C, Java, Perl, Python,… r Unix, Linux, Windows,… Socket Programs written in any language and running on any platform can communicate with each other!

2: Application Layer6 Transport Protocols: Review r TCP m connection-oriented service m guaranteed delivery m flow control m congestion control mechanism (message segmentation) r UDP m connectionless service m no flow and no congestion control m faster data delivery, but no guarantee

2: Application Layer7 Decisions r Before you write socket code, decide m Do you want a TCP-style reliable, full duplex, connection oriented channel? Or do you want a UDP-style, unreliable, message oriented channel? m Will the code you are writing be the client or the server? Client: you assume that there is a process already running on another machines that you need to connect to. Server: you will just start up and wait to be contacted

2: Application Layer8 Socket programming with TCP Client must contact server r server process must first be running r server must have created socket (door) that welcomes client’s contact Client contacts server by: r creating client TCP socket r specifying IP address, port number of server process r When client creates socket: client TCP establishes connection to server TCP r When contacted by client, server TCP creates new socket for server process to communicate with client m Frees up welcoming port m allows server to talk with multiple clients TCP provides reliable, stream transfer of bytes (“pipe”) between client and server application viewpoint

2: Application Layer9 Socket Programming with TCP Client Process Client socket Connection Socket Server Process bytes Welcoming Socket Three-way handshake

2: Application Layer10 Socket programming with 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) Client process client TCP socket

2: Application Layer11 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

2: Application Layer12 Example: Java client (TCP), cont. 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(); } Create input stream attached to socket Send line to server Read line from server

2: Application Layer13 Pseudo code TCP server Create socket (WelcomeSocket) Bind socket to a specific port where clients can contact you Server listens on WelcomeSocket for client to contact it Loop Accept new connection (connectSocket) Read and Write Data Into connectSocket to Communicate with client Close connectSocket End Loop Close WelcomeSocket

2: Application Layer14 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

2: Application Layer15 Example: Java server (TCP), cont DataOutputStream outToClient = new DataOutputStream (connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } Read in line from socket Create output stream, attached to socket Write out line to socket End of while loop, loop back and wait for another client connection

2: Application Layer16 Client/server socket interaction: TCP (Java) wait for incoming connection request connectionSocket = welcomeSocket.accept() create socket, port= x, for incoming request: welcomeSocket = ServerSocket() create socket, connect to hostid, port= x clientSocket = Socket() close connectionSocket read reply from clientSocket close clientSocket Server (running on hostid ) Client send request using clientSocket read request from connectionSocket write reply to connectionSocket TCP connection setup

2: Application Layer17 Socket programming with UDP UDP: very different mindset than TCP r no connection just independent messages sent r no handshaking r sender explicitly attaches IP address and port of destination r server must extract IP address, port of sender from received datagram to know who to respond to UDP: transmitted data may be received out of order, or lost application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server

2: Application Layer18 Example: Java client (UDP) Output: sends packet (TCP sent “byte stream”) Input: receives packet (TCP received “byte stream”) Client process client UDP socket

2: Application Layer19 Pseudo code UDP client Create socket Loop (Send Message To Well-known port of server)+ (Receive Message From Server) Close Socket

2: Application Layer20 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 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

2: Application Layer21 Example: Java client (UDP), cont. DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } Create datagram with data-to-send, length, IP addr, port Send datagram to server Read datagram from server

2: Application Layer22 Pseudo code UDP server Create socket Bind socket to a specific port where clients can contact you Loop (Receive UDP Message from client x)+ (Send UDP Reply to client x)* Close Socket

2: Application Layer23 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 byte[1024]; byte[] sendData = new byte[1024]; 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

2: Application Layer24 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(sendPacket); } Get IP addr port #, of sender Write out datagram to socket End of while loop, loop back and wait for another datagram Create datagram to send to client

2: Application Layer25 Client/server socket interaction: UDP close clientSocket Server (running on hostid ) read reply from clientSocket create socket, clientSocket = DatagramSocket() Client Create, address ( hostid, port=x, send datagram request using clientSocket create socket, port= x, for incoming request: serverSocket = DatagramSocket() read request from serverSocket write reply to serverSocket specifying client host address, port umber

2: Application Layer26 UDP Server vs Client r Server has a well-known port number r Client initiates contact with the server r Less difference between server and client code than in TCP m Both client and server bind to a UDP socket m Not accept for server and connect for client r Client send to the well-known server port; server extracts the client’s address from the datagram it receives

2: Application Layer27 TCP vs. UDP r TCP m “pipe” between the two processes m the pipe is logically connected to the destination m reliable byte stream channel r UDP m no welcoming socket, no pipe m destination address attached to bytes m unreliable transport service m receiving process must unravel each received packet for packet’s information bytes