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

Src Dst

IP Data

TCP Header

Type:TCP

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)

HelloClientSocket 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)

HelloServerServerSocket 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.