Trying out HTTP (client side) for yourself

1. 1. Telnet to your favorite Web server: Trying out HTTP (client side) for yourself Opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. Anything typed in sent to port 80 at cis.poly.edu telnet cis.poly.edu 80 • 2. Type in a GET HTTP request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server GET /~ross/ HTTP/1.1 Host: cis.poly.edu 3. Look at response message sent by HTTP server! CPSC 441 - Application Layer

2. Try SMTP interaction for yourself: • telnet servername 25 • see 220 reply from server • enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send email without using email client (reader) CPSC 441 - Application Layer

3. 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 Socket programming Goal: learn how to build client/server application that communicate using sockets 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 CPSC 441 - Application Layer

4. 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

5. 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

6. 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 (UCP 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

7. 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 (more in Chap 3) TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint Socket programming with TCP CPSC 441 - Application Layer

8. Socket Programming in C CPSC 441 - Application Layer

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

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

11. 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

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

13. 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 };

14. Address Structure • Declare address structure • struct sockaddr_in sockAdd; • Set family • sockAdd.sin_family = AF_INET; • Set IP address • sockAdd.sin_addr.s_addr = inet_pton(“127.0.0.1”)‏ • Set port • sockAdd.sin_port = htons(9999);

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

16. accept() • int s = accept(sock, &name, &namelen); • s: new socket for communication with client • sock: the listening socket • name: struct sockaddr, address of client • namelen: sizeof(name): size of client address structure • accept is blocking: waits for connection before returning

17. 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

18. 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

19. Socket Programming in Java CPSC 441 - Application Layer

20. 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

21. 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

22. 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

23. 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

24. 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

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

26. 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