Berkeley
Socket API:
Developed in the
early 1980s at the University of California at Berkeley. There are communications tools that are built
on tool of Berkeley sockets. (E.g. RPC ).
It is an API.
Its implementation is usually requires kernel code. It is the defacto
standard for communications programming. There are higher level tools for programs that
span more than one machine. RPC, DCOM
and windows remoting are examples.
Used for
point-to-point communications between computers through an inter-systems
pipe. Namely can use the UNIX read,
write, close, select, etc. system calls.
Supports
broadcast. This is where the same
message may be delivered to multiple systems on a network without additional
overhead. Available on every UNIX system. Build for client/server development. That is having one system provide a service
to other systems.
e.g. Remote Procedure Call (RPC)
Types
of sockets :
Two types of socket one is TCP (Stream
socket) and second is UDP (Datagram socket).
TCP
(Stream socket)
Connection-oriented (includes establishment
+ termination),
Reliable,
in order delivery
At-most-once delivery, no duplicates E.g.,
ssh, http
UDP
(Datagram socket)
Connectionless (just data-transfer)
“Best-effort” delivery, possibly lower
variance in delay
E.g., IP Telephony, streaming audio.
Commmon
System Calls/functions used for TCP and IP
Socket() - creates a
socket for network communications.
Returns a file descriptor for the socket.
Prototype: int Socket (family, type, protocol)
Family-
IPv4 or IPv6
Type-
TCP or UDP
Protocol:
0 not used for internet socket.
bind()- assigns a socket
to an address. When a socket is created using socket(), it is only given a
protocol family, but not assigned an address. This association with an address
must be performed with the bind() system call before the socket can accept
connections to other hosts. bind() takes three arguments:
Prototype: int bind(int fd, const struct
sockaddr *my_addr, socklen_t addrlen);
fd, a descriptor representing the socket to
perform the bind on.
my_addr, a pointer to a sockaddr
structure representing the address to bind to.
addrlen, a socklen_t field specifying
the size of the sockaddr structure.
Bind() returns 0 on success and -1 if an
error occurs.Syntax: Bind(Fd,addr,addrlen)
Listen - After a socket
has been associated with an address, listen() prepares it for incoming
connections. However, this is only necessary for the stream-oriented
(connection-oriented) data modes, i.e., for socket types (SOCK_STREAM,
SOCK_SEQPACKET). listen() requires two arguments:
backlog, an integer representing the number of
pending connections that can be queued up at any one time. The operating system
usually places a cap on this value. Once a connection is accepted, it is
dequeued. On success, 0 is returned. If an error occurs, -1 is returned.
Prototype: int listen(int sockfd, int
backlog);
Backlog:
Maximum no of connection.
Connect - called by the
client process to establish a connection between it and a server process. Need the server's address and port number. The file descriptor from the socket call is
one of the arguments. Once the
connection is made, this file descriptor may be used for communications. Times
out after 75 seconds if no response.
Syntax: Connect(Fd,addr,addrlen)
Addr:
Pointer to a struct sockaddr
Addrlen:
Size of the struct in byte
Accept - When an
application is listening for stream-oriented connections from other hosts, it
is notified of such events (cf. select() function) and must initialize the
connection using the accept() function. The accept() function creates a new
socket for each connection and removes the connection from the listen queue. It
takes the following arguments:
The accept()
function returns the new socket descriptor for the accepted connection, or -1
if an error occurs. All further communication with the remote host now occurs
via this new socket. Datagram sockets do not require processing by accept()
since the receiver may immediately respond to the request using the listening
socket.
Prototype: int accept(int sockfd, struct
sockaddr *cliaddr, socklen_t *addrlen);
sockfd, the descriptor of
the listening socket that has the connection queued.
cliaddr, a pointer to a sockaddr structure to
receive the client's address information.
addrlen, a pointer to a socklen_t location
that specifies the size of the client address structure passed to accept().
When accept() returns, this location indicates how many bytes of the structure
were actually used.
write - to send data on the connection.
If connection broken your program will receive a SIGPIPE signal or an
error is returned.
read - to get data that was sent on the connection. It returns the number of bytes read. 0 is returned if the connection broken.
select
- used to determine if there is data available on a
socket or if there is a client queued up for a connection to a server.
close
- to de-allocate the socket.
