Socket-level Programming

TCP/IP

The TCP/IP protocol was devised through a long-running DARPA project. This worked by implementation followed by RFCs (Request For Comment). TCP/IP is the principal Unix networking protocol. TCP/IP = Transmission Control Protocol/Internet Protocol.

TCP/IP stack

IP datagrams

The IP layer supplies a checksum that includes its own header. The header includes the source and destination addresses.

The IP layer handles routing through an Internet. It is also responsible for breaking up large datagrams into smaller ones for transmission and reassembling them at the other end.

UDP

TCP

Internet adddresses

The address is a 32 bit integer which gives the IP address. This encodes a network ID and more addressing. The network ID falls into various classes according to the size of the network address.

Network address

The University of Canberra is registered as a Class B network, so we have a 16 bit network address with 16 bits left to identify each machine.

Subnet address

Host address

Total address

Symbolic names

Programming interface in C

Address conversion

struct in_addr {
    unsigned long int s_addr;
}

struct hostent {
   char    *h_name;        /* official name of host */
   char    **h_aliases;    /* alias list */
   int     h_addrtype;     /* host address type */
   int     h_length;       /* length of address */
   char    **h_addr_list;  /* list of addresses */
}

Port addresses

Certain of these ports are ``well known''. They are listed in the file /etc/services. For example,

• ftp: 21/tcp
• time: 37/tcp
• time: 37/udp
• finger 79/tcp

The function ``getservbyname'' can be used to find the port for a service that is registered in /etc/services.

struct servent *getservbyname(const char *name, const char *proto);

struct servent {
   char    *s_name;        /* official service name */
   char    **s_aliases;    /* alias list */
   int     s_port;         /* port number */
   char    *s_proto;       /* protocol to use */
}

Berkeley sockets

If you are a server, you need to be able to create a port and listen at it. When a message comes in you need to be able to read and write to it.

Berkeley sockets are the BSD Unix system calls for this. They are part of the BSD Unix kernel. They have also been adopted by the PC world. They form the lowest practical level of doing client/server on both Windows and Unix.

Data representation

     5 *16777216 + 6 * 65536 

The Sun RPC uses a format known as XDR, which just happens to be the format that doesn't require any conversions for Suns. However, if two 386s are communicating then each of them will have to keep swapping bytes both on receipt and send.

The OSF DCE uses native format, with the receiving machine swapping bytes if needed. This section describes the Unix BSD networking API for IP as in WR Stevens ``Unix Network Programming.''

Byte ordering

• htonl - host to network, long int
• htons - host to network, short int
• ntohl - network to host, long int
• ntohs - network to host, short int

Addresses

Sockets

A socket is created using the call ``socket''. It returns an integer that is like a file descriptor: it is an index into a table and ``reads'' and ``writes'' to the network use this ``socket file descriptor''.

Two processes wishing to communicate over a network create a socket each. These are similar to two ends of a pipe - but the actual pipe does not yet exist.

Connection oriented (TCP)

It then ``listens'' on this socket to ``accept'' any incoming messages.

The other process (client) establishes a network connection to it, and then the two exchange messages.

As many messages as needed may be sent along this channel, in either direction.

Server:

create endpoint (socket())
bind address (bind())
specify queue (listen())
wait for conection (accept())
transfer data (read() write())

Client:

create endpoint (socket())
connect to server (connect())
transfer data (read() write())

TCP time client

• tcptime 139.130.4.6
• tcptime 192.76.144.75
• tcptime 146.169.22.37

TCP time server

Connectionless (UDP)

When bind is called it binds to a new port - it cannot bind to one already in use. If you specify the port as zero the system gives you a currently unused port.

Because of this extra task on each message send, the processes do not use read/write but recvfrom/sendto. These functions take as parameters the socket to write to, and the address of the service on the remote machine.

Server:

create endpoint (socket())
bind address (bind())
transfer data (sendto() recvfrom())

Client:

create endpoint (socket())
bind address (bind())
connect to server (connect())
transfer data (sendto() recvfrom())

Time client (UDP)

/* UDP client for time */

#include 
#include 
#include 
#include 
#include 

#define SIZE 1400
char buf[SIZE];

#define TIME_PORT 13

int main(int argc,
         char *argv[])
{
  int sockfd;
  int nread;
  struct sockaddr_in serv_addr,
                   client_addr;
  int len;

  if (argc != 2) {
    fprintf(stderr,
          "usage: %s IPaddr\n",
          argv[0]);
    exit(1);
  }
  if ((sockfd =
       socket(AF_INET,
              SOCK_DGRAM, 0)) 
       < 0) {
    perror(NULL);
    exit(2);
  }
  client_addr.sin_family =
             AF_INET;
  client_addr.sin_addr.s_addr =
             htonl(INADDR_ANY);
  client_addr.sin_port =
             htons(0);

  serv_addr.sin_family =
            AF_INET;
  serv_addr.sin_addr.s_addr =
            inet_addr(argv[1]);
  serv_addr.sin_port =  
            htons(TIME_PORT);


  if (bind(sockfd,
           &client_addr, 
           sizeof(client_addr))
      < 0) {
    perror(NULL);
    close(sockfd);
    exit(3);
  }

  len = sizeof(serv_addr);
  sendto(sockfd, buf, 1, 0,
         &serv_addr, len);
  nread = recvfrom(sockfd, buf,
                  SIZE, 0,
                  &client_addr,
                  &len);
  write(1, buf, nread);

  close(sockfd);
  exit(0);
}

Socket controls

In addition, ``getsockopt'' and ``setsockopt'' can be used for more specific socket control:

• broadcast - allowed for datagrams
• keepalive - periodic transmits on connected sockets

Timeout algorithms should adjust the time according to the curent trip time in some manner. They can be implemented using timer signals.

Server slaves

In Unix this is fairly easy: after the accept() has succedeed, the server should fork off a new process to handle the client. Here is the TCP time server, capable of handling many clients at once:

Server listening on multiple sockets

The select() call lets the kernel do this work. The call takes a number of file descriptors. The process is suspended. When I/O is ready on one of these, a wakeup is done, and the process can continue. This is cheaper than busy polling.

Internet superserver

When a request comes through, a server is created to handle it. To make it simpler for the server, instead of having to deal with ports, inetd will remap these onto stdin and stdout so that the server just reads/writes to stdin/stdout.

Sockets in Java

Client

import java.io.*;
impoert java.net.*;

public class SocketTest {

  public static void main(String argv[]) {
    try {
      Socket t = new Socket("java.sun.com", 13);
      DataInputStream is =
	new DataInputStream(t.getInputStream());
      boolean more = true;
      while (mnore) {
	String str = is.readLine();
	if (str == null)
	  more = false;
	else
	  System.out.println(str);
	}
      }
    } catch(IOException e) {
      System.out.println("Error" + e);
    }
  }
}

Server

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

public class EchoServer {
  public static void main(String argv[]) {
    try {
      ServerSocket s = new ServerSocket(8189);
      Socket incoming = s.accept();
      DataInputStream in =
	new DataInputStream(incoming.getInputStream());
      PrintStream out =
	new PrintStream(incoming.getOutputStream());
      out.println("Hello. Enter BYE to exit");

      boolean done = false;
      while ( ! done) {
	String str = in.readLine();
	if (str == null) 
	  done = true;
	else {
	  out.println("Echo: " + str);
	  if (str.trim().equals("BYE"))
	    done = true;
        }
      incoming.close();
    } catch(Exception e) {
      System.out.println(e);
    }
  }
}

Sockets in Perl

Perl has a C-like interface to sockets. It also has a higher level one using the IO::Socket module. This example is a client to fetch documents from a Web server.

#!/usr/bin/perl -w
use IO::Socket;
unless (@ARGV > 1) { die "usage: $0 host document ..." }
$host = shift(@ARGV);
$EOL = "\015\012";
$BLANK = $EOL x 2;
foreach $document ( @ARGV ) {
    $remote = IO::Socket::INET->new( Proto => "tcp",
                                     PeerAddr  => $host,
                                     PeerPort  => "http(80)",
                                    );
    unless ($remote) { die "cannot connect to http daemon on $host" }
    $remote->autoflush(1);
    print $remote "GET $document HTTP/1.0" . $BLANK;
    while ( <$remote> ) { print }
    close $remote;
}

Here is a server that will execute some commands and return a result

#!/usr/bin/perl -w
use IO::Socket;
use Net::hostent;              # for OO version of gethostbyaddr

$PORT = 9000;                  # pick something not in use

$server = IO::Socket::INET->new( Proto     => 'tcp',
                                 LocalPort => $PORT,
                                 Listen    => SOMAXCONN,
                                 Reuse     => 1);

die "can't setup server" unless $server;
print "[Server $0 accepting clients]\n";

while ($client = $server->accept()) {
    $client->autoflush(1);
    print $client "Welcome to $0; type help for command list.\n";
    $hostinfo = gethostbyaddr($client->peeraddr);
    printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost;
    print $client "Command? ";
    while (<$client>) {
        next unless /\S/;       # blank line
        if (/quit|exit/i) {
            last;
        } elsif (/date|time/i) {
            printf $client "%s\n", scalar localtime;
        } elsif (/who/i ) {
            print  $client `who 2>&1`;
        } elsif (/cookie/i ) {
            print  $client `/usr/games/fortune 2>&1`;
        } elsif (/motd/i ) {
           print  $client `cat /etc/motd 2>&1`;
        } else {
           print $client "Commands: quit date who cookie motd\n";
        }
     } continue {
        print $client "Command? ";
     }
     close $client;
}