Lecture 9: Socket Programming Interface

The TCP Connection (Revisited)

...the client process opens a TCP connection to a server, at port number xx...

In a major change from previous years, we will use the Java programming language for our examples. Previously we used C, the standard system-level programming language on Unix systems. You should be aware that the concepts we will introduce in today's lecture were originally (like most Internet protocols) developed in a Unix environment, and still retain some of the flavour of Unix -- that is, when you occasionally wonder why the designers took a particular approach, the answer is quite likely "...that's how it's done in Unix..

The Socket Abstraction

Sockets (and the TCP software to which they form the interface) are implemented in all modern operating systems as system calls; that is, they form part of the operating system kernel. In modern object-oriented languages such as Java (and others), a socket class is implemented "on top of" these basic system-level services.

TCP sockets in Java are different for server and client processes. For a client process, a socket is created as follows:

Socket myclientsocket = new Socket("ironbark", 79);

An Example Client.

import java.io.*;
import java.net.*;
class FingerClient {
    public static void main(String argv[])
	throws Exception
    {
        String user = "pscott";
        String response;
        Socket clientSock =
	    new Socket("ironbark", 79);
        DataOutputStream toserver =
            new DataOutputStream(
                clientSock.getOutputStream());
        DataInputStream serverReply =
            new DataInputStream(
                clientSock.getInputStream());
        toserver.writeBytes(user + "\n");
        response = serverReply.readLine();
        System.out.println("Finger Reply: "
	    + response);
        clientSock.close();
    }
}

Other Socket Operations

getInetAddress()

Returns the IP address of the system that this socket is connected to.

getPort()

Returns the port number that the socket is connected to on the remote system.

getLocalPort()

Returns the port number on the local system. For a client socket, this will be the random (?) port number allocated when the initial TCP connection was established.

Server Sockets in Java

import java.io.*;
import java.net.*;
class SillyServer {
    public static void main(String argv[])
	throws Exception
    {
        String incoming;
        String outgoing;
        ServerSocket servSock =
	    new ServerSocket(7277);
        
        while(true) {
            Socket connectedSocket =
		servSock.accept();
            DataInputStream fromClient =
                new DataInputStream(
                    connectedSocket.getInputStream());
            DataOutputStream toClient =
                new DataOutputStream(
                    connectedSocket.getOutputStream());
            incoming = fromClient.readLine();
            outgoing = incoming.toUpperCase() + "\n";
            toClient.writeBytes(outgoing);
            connectedSocket.close();
        }
    }
}

Multi-Threaded Servers

Whilst the Java code for a multi-threaded server is too complex to include into the lecture, the basic concept is that the accept() method returns a new, connected, socket. A program can start up a new thread of execution which handles communications using the connected socket, whilst allowing the "main program" to again go back to "accepting connections".

Conceptual Conundrum -- how can the newly-created program thread perform communications on (eg) port 7277 at the same time as the server is "waiting for connections" at the same port number? The answer to this is very subtle. In fact, an active TCP connection is characterised by a 4-tuple, consisting of the IP addresses of each of the computers involved, plus both of the port numbers (both client and server) in use. Because this 4-tuple is unique for every TCP connection (Exercise: why?), it's possible to have many server threads serving client requests simultaneously (or, more correctly, concurrently)

Sockets in Other Languages

#!/usr/local/bin/perl
use IO::Socket::INET;

$sock = IO::Socket::INET->new(PeerAddr => 'ironbark',
                              PeerPort => 'finger(79)',
                              Proto    => 'tcp');
print $sock "pscott\n";

while(<$sock>) {
    print;
}