TCP Socket Programming

1. TCP Socket Programming

2. What is a socket? • An abstract interface provided to the application programmer • File descriptor, allows apps to read/write to the network • Allows to processes on remotely connected computers to talk to each other

3. Two types of sockets • SOCK_STREAM • TCP • connection oriented, bidirectional • reliable, in-order delivery • SOCK_DGRAM • UDP • no connection • unreliable delivery, no guarantee on the order • can send/receive

4. process process TCP with buffers, variables TCP with buffers, variables socket socket Socket-programming using TCP Socket: a door between application process and end-end-transport protocol (UDP or TCP) TCP service: reliable transfer of bytesfrom 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 CPSC 441 - Application Layer

5. 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 TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint Socket programming with TCP CPSC 441 - Application Layer

6. Ports • Used to address processes on a host • 0-1024 is usually reserved for known service FTP Server Web Server 21 80 Transport Layer Network Layer DLL/Physical

7. Socket Programming in C CPSC 441 - Application Layer

8. Socket Programming - Flow socket() socket() bind() connect()‏ listen() Server Client send() accept() recv() wait for connection request from next client recv() . . . send() close() close()

9. socket()‏ • int s_listen = socket(family, type, protocol); • family: AF_INET specifies Ipv4 • type: SOCK_STREAM, SOCK_DGRAM • protocol: 0 (pseudo, IP ). See /etc/protocols

10. bind() • bind(s_listen, localAdd, addLength) • Server specifies which port and address it will be listening to • s_listen: our listening socket descriptor • localAdd: socket address structure • addLength: length of localAdd

11. Address Structure struct sockaddr_in { u_char sin_len; // length of address u_char sin_family; // family of address u_short sin_port; // protocol port num struct in_addr sin_addr; // IP Addr char sin_zero[8]; // set to zero, used for padding };

12. Address Structure • Declare address structure • struct sockaddr_in sockAdd; • Set family • sockAdd.sin_family = AF_INET; • Set IP address (2 ways) //specify address to listen to • inet_pton(AF_INET, “127.0.0.1”, &sockAdd.sin_addr.s_addr)‏ //listen to any local address • sockAdd.sin_addr.s_addr = htonl(INADDR_ANY) • Set port • sockAdd.sin_port = htons(9999);

13. listen() • int status = listen(s_listen, queuelength); • status: -1 if error, 0 otherwise • s_listen: socket descriptor • queuelength: Number of clients that can “wait” for a connection • listen is non-blocking: returns immediately

14. accept() • int s_new = accept(s_listen, &clientAddress, &addLength); • s_new: new socket for communication with client • s_listen: the listening socket • clientAddress: struct sockaddr, address of client • addLength: size of client address structure • accept is blocking: waits for connection before returning

15. Talking • int send(int s_new, const void *buf, int len, int flags); • s_new – socket descriptor • buf – pointer to buffer • len – size of buffer • flags – can be safely set to 0 • int recv(int s_new, void *buf, int len, unsigned int flags); • similar to send • buf holds the data to be transferred

16. System calls - fork() • fork() is a C system call used to spawn child processes • Execution for both child and parent process continues at the next instruction • fork() returns • 0 if this is the child process • PID (>0) of the child process if this is the parent • <0 if fork() fails • Used to keep listening on socket and talking on another socket

17. Socket Programming in Java CPSC 441 - Application Layer

18. 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, e.g., keyboard or socket. An output stream is attached to an output source, e.g., monitor or socket. Stream jargon Client process client TCP socket CPSC 441 - Application Layer

19. 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 CPSC 441 - Application Layer

20. 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 CPSC 441 - Application Layer

21. 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 CPSC 441 - Application Layer

22. Example: Java server (TCP), cont DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } } } 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 CPSC 441 - Application Layer

23. A simple client – server example: Echo Server Demo

24. References • Socket Programming, Dan Rubinstein, http://www1.cs.columbia.edu/~danr/courses/6761/Fall00/intro/6761-1b-sockets.ppt • 15-441 Socket Programming, www.cs.cmu.edu/afs/cs/academic/class/15441-f01/www/lectures/lecture03.ppt • Network Programming, Geoff Kuenning, www.cs.hmc.edu/~geoff/classes/hmc.cs105.200701/slides/class21_net2.ppt • Socket Programming, Abhinav Jain, www.cs.purdue.edu/homes/jain8/cs422/pso3.ppt