TCP Sockets

What are TCP Sockets?

A TCP socket is defined as an endpoint for communication. A socket consists of the pair <IP Address,Port>. For our purposes, a port will be defined as an integer number between 1024 and 65535. This is because all port numbers smaller than 1024 are considered well-known -- for example, telnet uses port 23, http uses 80, ftp uses 21, and so on. On Unix machines, the file /etc/services contains a list of services provided by that machine, along with their well-known ports

A TCP connection consists of a pair of sockets. Sockets are distinguished by client and server sockets. A server listens on a port, waiting for incoming requests from clients.

For example, a web server listens at port 80 for incoming request from clients (web browsers). When a client wishes to make a connection with a server socket, the client is assigned a port from the local host. Suppose that client X (at IP address 146.86.3.15) wishes to browse a web page on the server 146.86.5.20. If the port the local host assigned client X is port 12345, the connection between the client and the server is uniquely identified by the socket pair

            <146.86.3.15 : 12345, 146.86.5.20 : 80> 

TCP Clients and Servers

  socket()
  connect()
  send() and/or recv()
  close()

  socket()
  bind()
  listen()
  accept()
  send() and/or recv()
  close()

Socket Address Structures

  struct sockaddr_in{
    short sin_family;
    unsigned short sin_port;
    struct in_addr sin_addr;
    char sin_zero[8];
  };

  struct in_addr {
    unsigned long s_addr;
  };

  struct sockaddr_in sin;
  sin.sin_family = AF_INET;
  sin.sin_port = htons(9999)
  sin.sin_addr.s_addr = inet_addr("128.227.224.3");

TCP Library Calls

socket:

  int socket(int family, int type, int protocol);

  int s;
  s = socket(AF_INET, SOCK_STREAM, 0);

connect:

  int connect(int socket, struct sockaddr_in * address, int address_length);

  int c;
  struct sockaddr_in server_address;

  s = socket(...);
  server_address.??? = ...;
  c = connect(s, &server_address, sizeof(sockaddr_in));

bind:

  bind(int socket, struct sockaddr_in *address, int address_length);

The address of the sockaddr_in structure is passed into the bind call, so that the socket will now be ready to communicate with remote hosts. The third parameter passed to bind is the length of the sockaddr_in structure.

Example:

  struct sockaddr_in sin;
  int s;

  s = socket(AF_INET, SOCK_STREAM, 0);
  sin.sin_family = AF_INET;
  sin.sin_port = htons(9999);
  sin.sin_addr.s_addr = INADDR_ANY;
  bind(s, (struct sockaddr *)&sin, sizeof(sin));
 
  /* s is now a usable TCP socket. Source port is 9999 */

listen:

  listen(int socket, int backlog);

accept:

  int accept(int socket, struct sockaddr_in * address, int * addr_len);

send and recv:

  int send(int socket, char * message, int msg_len, int flags);
  int recv(int socket, char *buffer, int buf_len, int flags);

A client-server echo program

Testing the client-server echo program

To test the echo client-server application, start the server by entering (in Unix)

./EchoServer <port number>

a) Using telnet: in a separate window, enter the command:

telnet localhost <port number>

Now, everything you enter on that window will be echoed back to you. To quit, open another terminal on tanner and kill both applications (EchoServer and telnet).

a) Using the echo client: in a separate window, start the echo client by entering (in Unix)

./EchoClient localhost <port number>

Now, everything you enter on that window will be echoed back to you. Quit the client with CTRL-D (this is the end-of-file character in Unix).

Exercise