UDP Basics in Java

Lecture 02

The UDP protocol

The UDP protocol is similar to the postal services.

A UDP socket allows to send and receive packets from/to any other reachable UDP socket.

There is no notion of connection!

Principe UDP

Guaranties of UDP

IP in Java

Java provides a very limited access to IP protocol:

IP Address

Represented in Java by InetAddress with two sub-classes Inet4Address and Inet6Address.

We will always work with InetAddress.

InetAddress

Created using the factory method InetAddress.getByName(String host).

InetAddress addr1 = InetAddress.getByName("192.168.0.1");
InetAddress addr2 = InetAddress.getByName("FE80:0:0:0:0202:B3FF:FE1E:8329");
InetAddress addr3 = InetAddress.getByName("www.google.fr");
                                    
System.out.println(addr1+"\n"+addr2+"\n"+addr3);

/192.168.0.1
/fe80:0:0:0:202:b3ff:fe1e:8329
www.google.fr/64.233.166.94

This third example requires a DNS resolution.

InetAddress = address (numerical) + [ name ]

InetSocketAddress

Socket Address = IP Address + port number

Several constructors:

InetAddress inetAddress = InetAddress.getByName("www.google.com");
InetSocketAddress insa1 = new InetSocketAddress(inetAddress,7);
                                    
InetSocketAddress insa2 = new  InetSocketAddress("www.u-pem.fr",7);

InetSocketAddress insa3 = new InetSocketAddress("192.168.0.1",1025);

InetSocketAddress insa4 = new  InetSocketAddress(7777);

System.out.println(insa1+"\n"+insa2+"\n"+insa3+"\n"+insa4);

www.google.com/173.194.67.147:7
www.u-pem.fr/193.50.159.151:7
/192.168.0.1:1025
0.0.0.0/0.0.0.0:7777

DatagramChannel

UDP sockets are represented by the class DatagramChannel.

They are created using the factory method:
DatagramChannel.open

DatagramChannel dc = DatagramChannel.open();

Not yet bound (no bind) = no socket address

DatagramChannel

Binding is done using datagramChannel.bind.

dc.bind(null);

Binds to a random available port

Sending a packet

To send a packet, we need data (ByteBuffer in read-mode) and socket address of the recipient (InetSocketAddress)

We use the method send of DatagramChannel:

datagramChannel.send(ByteBuffer buff, SocketAddress dest);

This method is blocking: it returns when buff.remaining() data have been flushed to the system (to be sent).

DatagramChannel dc; 
...
ByteBuffer buff = StandardCharsets.UTF_8.encode("Hello world €");
InetSocketAddress dest= new InetSocketAddress("gaspard.univ-mlv.fr",7);
dc.send(buff,dest);

Receiving a packet

To receive a packet, we need a ByteBuffer ready to store incoming data (write-mode).

SocketAddress datagramChannel.receive(ByteBuffer buff);

This method is blocking: it returns when data of a packet has been read and stored into buff.
The method will return the socket address of the sender.

DatagramChannel dc;
...
ByteBuffer buff = ByteBuffer.allocate(1024);
InetSocketAddress exp=(InetSocketAddress) dc.receive(buff);
buff.flip();
System.out.println("Received "+ buff.remaining() + " bytes from "+ exp);

Don't forget the flip !

If the packet size is greater than the the buffer size, the extra data is lost without any notification.

Focus on the client side (0/11)

Focus on the client side (1/11)

Focus on the client side (2/11)

Focus on the client side (3/11)

Focus on the client side (4/11)

Focus on the client side (5/11)

Focus on the client side (6/11)

Focus on the client side (7/11)

Focus on the client side (8/11)

Focus on the client side (9/11)

Focus on the client side (10/11)

Focus on the client side (11/11)

Freeing resources

The creation with open and the binding with bind reserve system resources
(descriptor, system buffers, port)

You must free these resources

either with datagramChannel.close();

or with try-with-resources

try(DatagramChannel dc = DatagramChannel.open()){
    ...
}