Socket programming Goal learn how to build clientserver

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

Socket programming with TCP Socket: a door between application process and endend-transport protocol (UDP or TCP) TCP service: reliable transfer of bytes from one process to another controlled by application developer controlled by operating system process socket TCP with buffers, variables host or server Internet socket TCP with buffers, variables controlled by application developer controlled by operating system host or server 2

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 client establishes TCP connection to server 3

Socket programming with TCP r When contacted by client, server TCP creates new socket for server process to communicate with client m allows server to talk with multiple clients m source port number used (how to distinguish data from different clients? ) client process client TCP socket ay w ee ake r h t dsh han byte pipe application viewpoint TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server welcoming TCP socket server connection TCP socket server process 4

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. 5

Socket programming with TCP Example client-server app: 1) client reads line from standard input (in. From. User stream) , sends to server via socket (out. To. Server 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 (in. From. Server stream) Client process client TCP socket 6

Client/server socket interaction: TCP Server Client (running on hostid) create socket, port=x, for incoming request: welcome. Socket = Server. Socket() TCP wait for incoming connection request connection. Socket = welcome. Socket. accept() read request from connection. Socket write reply to connection. Socket close connection. Socket setup create socket, connect to hostid, port=x client. Socket = Socket() send request using client. Socket read reply from client. Socket close client. Socket 7

Example: Java client (TCP) import java. io. *; import java. net. *; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modified. Sentence; Create input stream Create client socket, connect to server Create output stream attached to socket Buffered. Reader in. From. User = new Buffered. Reader(new Input. Stream. Reader(System. in)); Socket client. Socket = new Socket("hostname", 6789); Data. Output. Stream out. To. Server = new Data. Output. Stream(client. Socket. get. Output. Stream()); 8

Example: Java client (TCP), cont. Create input stream attached to socket Buffered. Reader in. From. Server = new Buffered. Reader(new Input. Stream. Reader(client. Socket. get. Input. Stream())); sentence = in. From. User. read. Line(); Send line to server out. To. Server. write. Bytes(sentence + 'n'); Read line from server modified. Sentence = in. From. Server. read. Line(); System. out. println("FROM SERVER: " + modified. Sentence); client. Socket. close(); } } 9

Example: Java server (TCP) import java. io. *; import java. net. *; class TCPServer { Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket public static void main(String argv[]) throws Exception { String client. Sentence; String capitalized. Sentence; Server. Socket welcome. Socket = new Server. Socket(6789); while(true) { Socket connection. Socket = welcome. Socket. accept(); Buffered. Reader in. From. Client = new Buffered. Reader(new Input. Stream. Reader(connection. Socket. get. Input. Stream())); 10

Example: Java server (TCP), cont Create output stream, attached to socket Data. Output. Stream out. To. Client = new Data. Output. Stream(connection. Socket. get. Output. Stream()); Read in line from socket client. Sentence = in. From. Client. read. Line(); capitalized. Sentence = client. Sentence. to. Upper. Case() + 'n'; Write out line to socket out. To. Client. write. Bytes(capitalized. Sentence); } } } End of while loop, loop back and wait for another client connection 11

Socket programming with UDP: no “connection” between client and server r no handshaking r sender explicitly attaches IP address and port of destination to each packet r server must extract IP address, port of sender from received packet to return uppercased sentence to sender application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server UDP: transmitted data may be received out of order, or lost 12

Client/server socket interaction: UDP Server (running on hostid) create socket, port=x, for incoming request: server. Socket = Datagram. Socket() read request from server. Socket write reply to server. Socket specifying client host address, port number Client create socket, client. Socket = Datagram. Socket() Create, address (hostid, port=x, send datagram request using client. Socket read reply from client. Socket close client. Socket 13

Example: Java client (UDP) Client process Input: receives packet (TCP received “byte stream”) Output: sends packet (TCP sent “byte stream”) client UDP socket 14

Example: Java client (UDP) import java. io. *; import java. net. *; Create input stream Create client socket Translate hostname to IP address using DNS class UDPClient { public static void main(String args[]) throws Exception { Buffered. Reader in. From. User = new Buffered. Reader(new Input. Stream. Reader(System. in)); Datagram. Socket client. Socket = new Datagram. Socket(); Inet. Address IPAddress = Inet. Address. get. By. Name("hostname"); byte[] send. Data = new byte[1024]; byte[] receive. Data = new byte[1024]; String sentence = in. From. User. read. Line(); send. Data = sentence. get. Bytes(); 15

Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port Send datagram to server Datagram. Packet send. Packet = new Datagram. Packet(send. Data, send. Data. length, IPAddress, 9876); client. Socket. send(send. Packet); Datagram. Packet receive. Packet = new Datagram. Packet(receive. Data, receive. Data. length); Read datagram from server client. Socket. receive(receive. Packet); String modified. Sentence = new String(receive. Packet. get. Data()); System. out. println("FROM SERVER: " + modified. Sentence); client. Socket. close(); } } 16

Example: Java server (UDP) import java. io. *; import java. net. *; Create datagram socket at port 9876 class UDPServer { public static void main(String args[]) throws Exception { Datagram. Socket server. Socket = new Datagram. Socket(9876); byte[] receive. Data = new byte[1024]; byte[] send. Data = new byte[1024]; while(true) { Create space for received datagram Receive datagram Datagram. Packet receive. Packet = new Datagram. Packet(receive. Data, receive. Data. length); server. Socket. receive(receive. Packet); 17

Example: Java server (UDP), cont String sentence = new String(receive. Packet. get. Data()); Get IP addr port #, of sender Inet. Address IPAddress = receive. Packet. get. Address(); int port = receive. Packet. get. Port(); String capitalized. Sentence = sentence. to. Upper. Case(); send. Data = capitalized. Sentence. get. Bytes(); Create datagram to send to client Write out datagram to socket } Datagram. Packet send. Packet = new Datagram. Packet(send. Data, send. Data. length, IPAddress, port); server. Socket. send(send. Packet); } } End of while loop, loop back and wait for another datagram 18

Socket programming: references C-language tutorial (audio/slides): r “Unix Network Programming” (J. Kurose), http: //manic. cs. umass. edu/courses 2. html Java-tutorials: r “All About Sockets” (Sun tutorial), http: //java. sun. com/docs/books/tutorial/networking/so ckets r “Socket Programming in Java: a tutorial” (Java World), http: //www. javaworld. com/javaworld/jw-12 -1996/jw-12 sockets. html 19