Network Programming in Java: Internet protocols (IP, UDP, TCP) by Alan MOUHLI

Author:Alan MOUHLI
Language: eng
Format: mobi
Publisher: Alan MOUHLI
Published: 2016-04-05T21:00:00+00:00

import java.util.*; import java.io.*; import java.net.*;

public class TimeSender implements AutoCloseable {

private final DatagramSocket socket; private final DatagramPacket packet;

public TimeSender(int port) throws SocketException {

this.socket = new DatagramSocket();

this.packet = new DatagramPacket(new byte[0], 0); // Packet with empty array

this.packet.setPort(port);

}

public void sendTimePacket(InetAddress a) throws IOException

{

// We reuse the same datagram packet but modify its content and addressee

byte[] timeData = new

Date().toString().getBytes();

packet.setData(timeData);

packet.setAddress(a);

// this.packet.setPort is useless since the port does not change

socket.send(packet); // May throw an IOException }

public static InetAddress getSuccessor(InetAddress a) {

// Exercise: write the implementation of getSuccessor(InetAddress) returning the successor of an IP address.

// Example: the successor of 10.1.2.3 is 10.1.2.4 (for last byte 0 and 255 are forbidden)

}

public void close() throws IOException

{

socket.close();

}

public static void main(String[] args) throws IOException {

if (args.length < 3) throw new

IllegalArgumentException("Not enough arguments"); InetAddress firstA =

InetAddress.getByName(args[0]); // Start address InetAddress lastA =

InetAddress.getByName(args[1]); // Stop address int port = Integer.parseInt(args[2]); try (TimeSender ts = new TimeSender(port)) {

for (InetAddress currentA = firstA; !

currentA.equals(lastA); currentA = getSuccessor(currentA)) ts.sendTimePacket(currentA); }

}

}

Some tips for the exchange of packets

 Recycle DatagramPacket already created (by changing the hosted data)

 Do not create unnecessary DatagramSocket, release them when no longer needed

 Always check that the receipt size is sufficient

classic mistake: send k byte packet and receive on the same package without changing table or size; it is then limited to k bytes for reception

 Do not forget to use the size of the data received: the end of the table is not exploitable

 Check the IP address / transmitter port of a received packet (even if it makes only a small safety)

Mechanism of pseudo-connection

 Possibility of pseudo-connect two UDP sockets with void

DatagramSocket.connect (InetAddress addr, int port): automatic filtering of packets sent and received

 Checking the connection withisConnected boolean ()

 Disconnection with void disconnect ()

Some tests with Netcat

Netcat: simple application to communicate with UDP (and TCP)

 We run the server on port UDP N listening on all interfaces (wildcard address): nc -l -u N

 It runs a client connecting to the port N of the machine:

n -u N MachineName

 The client sends each line of standard input in a UDP datagram: the server receives and displays it on its standard output

 server datagram sending to the customer also possible

Java example: receive datagrams on a port with a deadline

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