Client/Server In Java

Client/Server In Java An Introduction to TCP/IP and Sockets

IP: Internet Protocol • IP is a protocol for connecting networks • IP is a packet-based protocol • Packets can be dropped, garbled or re-ordered and duplicated: • IP promises nothing but best-effort delivery • IP usually runs over ethernet, but not always • IP can run over PPP

Addressing • Every packet has a source and a destination • A Host is identified by an IP address. • IP addresses are 4 bytes • Example: 129.123.12.43 • An IP can belong to only one host in the Internet • Routers use the IP addresses to decide where to direct packets

TCP • TCP is a stream-based protocol over IP • TCP promises a reliable transport • TCP takes care of packet ordering, packet loss and data corruption. • A TCP stream is a connection.

TCP over IP IP Header IP Data Src Dst Type:TCP TCP Header TCP Data SrcPort DstPort SeqNum Application Data

TCP Connections • A TCP connection is between a Client Host and a Server Host • The Server is passive; It just waits • The Client is active • More than one connection is allowed between to hosts • TCP connections are 4-tuples • {<src-host,src-port>,<dst-host,dst-port>}

Sockets • The Java interface with a TCP connection is a socket. • A socket is a Java object which has methods to connect, accept connections and transfer data. • Core Networking is in java.net.* • TCP Sockets come in two flavours: ServerSocket and Socket.

java.net.Socket • Socket(InetAddress address, int port) • Creates a stream socket and connects it to the specified port number at the specified IP address. • InputStream getInputStream() • Returns an input stream for this socket. • OutputStream getOutputStream() • Returns an output stream for this socket. • void close() • closes this socket (cuts the connection)

HelloClient Socket sock; try { InetAddress host = InetAddress.getByName(“antares.math.tau.ac.il”); sock = new Socket(host, 6789); InputStream instr = sock.getInputStream(); InputStreamReader inread = new InputStreamReader(instr); BufferedReader inp = new BufferedReader(inread); PrintStream out = new PrintStream(sock.getOutputStream()); String line; line = inp.readLine(); out.println(line); sock.close(); } catch (UnknownHostException e) { System.err.println("Unknown host: " + e.getMessage()); } catch (IOException e) { System.err.println("Error connecting: " + e.getMessage()); }

java.net.ServerSocket • ServerSocket(int port) • Creates a server socket on a specified port. • Socket accept() • Listens for a connection to be made to this socket and accepts it. • void close() • closes this socket (stops listening)

HelloServer ServerSocket sock; try { sock = new ServerSocket(9987); Socket client; client = sock.accept(); InputStream instr = client.getInputStream(); InputStreamReader inread = new InputStreamReader(instr); BufferedReader inp = new BufferedReader(inread); PrintStream out = new PrintStream(client.getOutputStream()); String line; line = inp.readLine(); out.println(line); client.close(); sock.close(); } catch (IOException e) { System.err.println(”Network Error: " + e.getMessage()); }

Domain Name Service • Translates Names to Addresses • Hierarchical • Hostname format: hostname.subdomain.domain.tld. • Reverse DNS: IP->Name • Special DNS domain: 4.3.2.1.in-addr.arpa

java.net.InetAddress • static InetAddress getByName(String name) • returns the address for “name”. Throws UnknownHostException if it’s unknown. • This class is the interface from Java to DNS

Multiple clients • One ServerSocket can accept multiple clients simultaneously • Use multithreading to handle them • One thread waits for “accept()” • Open a new thread for each connection.

Client/Server In Java