Sockets

Sockets

Both forms of communication (UDP and TCP) use the socket abstraction, which provides an endpoint for ommunication between processes. Sockets originate from BSD UNIX but are also present in most other versions of UNIX, including Linux as well as Windows and the Macintosh  OS. Interprocess communication consists of transmitting a message between a socket in one process and a socket in another process, is shown in the following figure.

For a process to receive messages, its socket must be bound to a local port and one of the Internet addresses of the computer on which it runs. Messages sent to a particular Internet address and port number can be received only by

a process whose socket is associated with that Internet address and port number. Processes may use the same socket for sending and receiving messages. Each computer has a large number (216) of possible port numbers for use by local processes for receiving messages. Any process may make use of multiple ports to receive messages, but a process cannot share ports with other processes on the same computer. However, any number of processes may send messages to the same port. Each socket is associated with a particular protocol – either UDP or TCP.

Java API for Internet addresses • As the IP packets underlying UDP and TCP are sent to Internet addresses, Java provides a class, InetAddress, that represents Internet addresses. Users of this class refer to computers by Domain Name System (DNS) hostnames. For example, instances of InetAddress that contain Internet addresses can be created by calling a static method of InetAddress, giving a DNS hostname as the argument. The method uses the DNS to get the corresponding Internet address. For example, to get an object representing the Internet address of the host whose DNS name is bruno.dcs.qmul.ac.uk, use:

InetAddress aComputer = InetAddress.getByName("bruno.dcs.qmul.ac.uk");

This method can throw an UnknownHostException. Note that the user of the class does not need to state the explicit value of an Internet address. In fact, the class encapsulates the details of the representation of Internet addresses. Thus the interface for this class is not dependent on the number of bytes needed to represent Internet addresses – 4 bytes in IPv4 and 16 bytes in IPv6.