1 CS: 4244 Internet Programming Sockets Programming Dr. Eli Tilevich January 29, 2007

2 Chapter 2 Application Layer Computer Networking: A Top Down Approach Featuring the Internet, 3 rd 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. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

3 Sockets r process sends/receives messages to/from its socket r socket analogous to door  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 process TCP with buffers, variables socket host or server process TCP with buffers, variables socket host or server Internet controlled by OS controlled by app developer r API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later)

4 Socket programming Socket API r introduced in BSD4.1 UNIX, 1981 r explicitly created, used, released by apps r client/server paradigm r two types of transport service via socket API:  unreliable datagram  reliable, byte stream- oriented 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 socket Goal: learn how to build client/server application that communicate using sockets

5 Socket-programming using TCP Socket: a door between application process and end- end-transport protocol (UCP or TCP) TCP service: reliable transfer of bytes from one process to another process TCP with buffers, variables socket controlled by application developer controlled by operating system host or server process TCP with buffers, variables socket controlled by application developer controlled by operating system host or server internet

6 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-local 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  allows server to talk with multiple clients  source port numbers used to distinguish clients TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint

7 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, e.g., keyboard or socket. r An output stream is attached to an output source, e.g., monitor or socket.

8 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

9 Client/server socket interaction: TCP 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

10 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

11 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

12 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

13 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

14 TCP/IP Sockets in Java: Practical Guide for Programmers r Kenneth L. Calvert r Michael J. Donahoo

15 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection Server starts by getting ready to receive client connections…

16 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection ServerSocket servSock = new ServerSocket(servPort);

17 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection for (;;) { Socket clntSock = servSock.accept();

18 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection Server is now blocked waiting for connection from a client

19 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection Later, a client decides to talk to the server…

20 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection Socket socket = new Socket(server, servPort);

21 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection OutputStream out = socket.getOutputStream(); out.write(byteBuffer);

22 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection Socket clntSock = servSock.accept();

23 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Communicate 3. Close the connection Server 1. Create a TCP socket 2. Repeatedly: a. Accept new connection b. Communicate c. Close the connection InputStream in = clntSock.getInputStream(); recvMsgSize = in.read(byteBuffer);

24 TCP Client/Server Interaction Client 1. Create a TCP socket 2. Establish connection 3. Communicate 4. Close the connection Server 1. Create a TCP socket 2. Bind socket to a port 3. Set socket to listen 4. Repeatedly: a. Accept new connection b. Communicate c. Close the connection close(sock); close(clntSocket)

25 TCP Tidbits Client out.write(“Hello Bob”) in.read() -> “Hi Jane” Server in.read() -> “Hello ” in.read() -> “Bob” out.write(“Hi ”) out.write(“Jane”) Client knows server address and port No correlation between send() and recv()

26 Closing a Connection close() used to delimit communication Analogous to EOF Client out.write(string) while (not received entire string) in.read(buffer) out.write(buffer) close(socket) Server in.read(buffer) while(client has not closed connection) out.write(buffer) in.read(buffer) close(client socket)

27 TCP Tidbits Client out.write(“Hello Bob”) in.read() -> “Hi Jane” Server in.read() -> “Hello ” in.read() -> “Bob” out.write(“Hi ”) out.write(“Jane”) Client knows server address and port No correlation between send() and recv()

28 Closing a Connection close() used to delimit communication Analogous to EOF Client out.write(string) while (not received entire string) in.read(buffer) out.write(buffer) close(socket) Server in.read(buffer) while(client has not closed connection) out.write(buffer) in.read(buffer) close(client socket)

29 Constructing Messages …beyond simple strings

30 TCP/IP Byte Transport r TCP/IP protocols transports bytes r Application protocol provides semantics Application TCP/IP byte stream Application TCP/IP byte stream Here are some bytes. I don’t know what they mean. I’ll pass these to the app. It knows what to do.

31 Application Protocol r Encode information in bytes r Sender and receiver must agree on semantics r Data encoding  Primitive types: strings, integers, and etc.  Composed types: message with fields

32 Primitive Types r String  Character encoding: ASCII, Unicode, UTF  Delimit: length vs. termination character Mom\n

33 r Integer  Strings of character encoded decimal digits Advantage:1. Human readable 2. Arbitrary size Disadvantage:1. Inefficient 2. Arithmetic manipulation Primitive Types ‘1’‘7’‘9’‘9’‘8’‘7’‘0’\n

34 Primitive Types r Integer  Native representation  Network byte order (Big-Endian) Use for multi-byte, binary data exchange htonl(), htons(), ntohl(), ntohs() byte two’s-complement integer 23, Big-Endian Little-Endian

35 Message Composition r Message composed of fields  Fixed-length fields  Variable-length fields integershort Mike12\n