Berkeley Sockets

• Generic:
  • support for multiple protocol families.
  • address representation independence
• Uses existing I/O programming interface as much as possible.

Socket

• A socket is an abstract representation of a communication endpoint.
• Sockets work with Unix I/O services just like files, pipes & FIFOs.
• Sockets (obviously) have special needs:
  • establishing a connection
  • specifying communication endpoint addresses

Unix Descriptor Table

Socket Descriptor Data Structure

Creating a Socket

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

• family specifies the protocol family (PF_INET for TCP/IP).
• type specifies the type of service (SOCK_STREAM, SOCK_DGRAM).
• protocol specifies the specific protocol (usually 0 which means the default).

socket()

• The socket() system call returns a socket descriptor (small integer) or a -1 on error.
• socket() allocates resources needed for a communication endpoint - but it does not deal with endpoint addressing.

Specifying an Endpoint Address

• Remember that the sockets API is generic.
• There must be a generic way to specify endpoint addresses.
• TCP/IP requires an IP address and a port number for each endpoint address.
• Other protocol suites (families) may use other schemes.

Generic socket addresses

	struct sockaddr {
		u_short	   sa_family; 
		char	   sa_data[14];
	};

• sa_family specifies the address type.
• sa_data specifies the address value.

sockaddr

• An address that will allow me to use sockets to communicate with my kids.
• address type AF_DAVESKIDS
• address values:
  • Andrea 1
  • Jeff 2
  • Robert 3
  • Emily 4
  • Mom 5
  • Dad 6
  • Dog 7

AF_DAVESKIDS

• Initializing a sockaddr structure to point to Robert:

  	struct sockaddr robster;
  
  	robster.sa_family = AF_DAVESKIDS;
  
  	robster.sa_data[0] = 3;
  

AF_INET

• For AF_DAVESKIDS we only needed 1 byte to specify the address.
• For AF_INET we need:
  • 16 bit port number
  • 32 bit IP address

struct sockaddr_in

struct sockaddr_in {

		u_short	       sin_family;

		u_short	   	   sin_port;

 		struct in_addr  sin_addr; 

		char	            sin_zero[8];

	};

struct in_addr

struct in_addr {

	 u_long s_addr;	 /* IP ADDRESS */

};


in_addr just provides a name for the 'C' type associated with IP addresses.

Network Byte Order


All values stored in a sockaddr_in must be in network byte order.

sin_port  a TCP/IP port number.
sin_addr  an IP address.




TCP/IP Addresses


We don't need to deal with sockaddr structures since we will only deal with one protocol family.

We can always use sockaddr_in structures.

The C functions that make up the sockets API expect structures of type sockaddr.



Assigning an address to a socket


The bind() system call is used to assign an address to an existing socket.

int bind( int sockfd, struct sockaddr *myaddr, int addrlen);


bind returns 0 if successfull or -1 on error.


bind()


calling bind() assigns the address specified by the sockaddr structure to the socket descriptor.

You can give bind() a sockaddr_in structure:
   bind( mysock, (struct sockaddr*) &myaddr, sizeof(myaddr) );



bind() Example

int mysock;

struct sockaddr_in myaddr;

  mysock = socket(PF_INET,SOCK_STREAM,0);

  myaddr.sin_family = AF_INET;

  myaddr.sin_port = htons( portnum );

  myaddr.sin_addr = htonl( ipaddress);

  bind(mysock, &myaddr, sizeof(myaddr));



Uses for bind()


There are 3 uses for bind():

Server would like to bind to a well known address (port number).

Client can bind to a specific port.

Client can ask the O.S. to assign any available port number.



Port schmort - who cares ?


Clients typically don't care what port they are assigned.

When you call bind you can tell it to assign you any available port:
	myaddr.port = htons(0);



What is my IP address ?

How can you find out what your IP address is so you can tell bind() ?

There is no realistic way for you to know the right IP address to give bind() - what if the computer has multiple network interfaces?

specify the IP address as: INADDR_ANY, this tells the OS to take care of things.


Other socket system calls


General Use

 
read()
 
write()
 
close()

Connection-oriented (TCP)

connect()
listen()
accept()


Connectionless (UDP)

send()
recv()