Transcript Chapter 5 Socket API

Chapter 5 Socket API
• 1980, a group at UC Berkeley create a
interface that applications use to
communicate.
• Known as socket API or socket interface
• System is known as Berkeley UNIX or BSD
UNIC
Specifying a protocol interface
• Two broad approaches
– Define functions specifically to support TCP/IP
communication
– Define functions that support network communication
in general, and use parameters to make TCP/IP
communication a special case.
• Because the designer at Berkeley wanted to
accommodate multiple sets of communication
protocol, they used the second approach.
• In fact, they provides for generality far beyond
TCP/IP. They allowed for multiple families of
protocols, with all TCP/IP protocols represented as
single family(family PF_INET)
• The socket API provides generalized functions
that support network communication using many
possible protocols as a single protocol family. The
calls allow the programmer to specify the type of
service required rather than the name of specific
protocol.
Socket abstraction
• File descriptors
– Open function create a file descriptor
– The OS maintains a separate file descriptor as
an array of pointers to internal data structure
– Application program only need to remember
the descriptor and use it
• Socket API adds a new abstraction for network
communication, the socket.
• Each active socket is identified by a small integer
called its socket descriptor
• OS provides a separate system function, socket,
that applications call to create a socket;
• Application only use open to crate file descriptor
• Once the socket has been created, an application
must make additional system calls to specify the
details of its exact use.
System data structure for sockets
Operating system
Descriptor table
( one per process)
0:
Family: PF_INET
1:
Service:SOCK_STREAM
2:
Local IP:
3:
Remote IP:
4:
Local Port:
Remote Port:
Using sockets
• A socket used by a server to wait for an
incoming connection is called a passive
socket
• A socket used by a client to initiate a
connection is called an active socket.
• The sockets are created the same way
initially
Endpoint address
• When a socket is created, it does not contain
detailed information about how it will be used.
(port number, IP address)
• ICP/IP protocols define a communication endpoint
to consist of an IP address and a protocol port
number.
• It allows each protocol family to specify endpoints
however it likes
• TCP/IP protocols all use a single address
representation, with address family AF_INET
Generic address structure
• Struct sockaddr{
• u_char sa_len; // total length
• U_short sa_family; // type of address
• char sa_data[14]; // value of address
• }