Wednesday, 1 February 2017

Python Network Programming

Python Network Programming

Python provides two levels of access to network services. At a low level, we can access the basic socket support in the underlying operating system, which would allows to implement clients and servers for both connection-oriented and connectionless protocols.
Python also has libraries that would provide higher-level access to specific application-level network protocols, such as FTP, HTTP, and so on.
What is Sockets?
Sockets are the endpoints of a bidirectional communications channel. Sockets may communicate within a process, between processes on the same machine, or between processes on different continents.
Sockets would be implemented over a number of different channel types such as Unix domain sockets, TCP, UDP, and so on. The socket library provides specific classes for handling the common transports as well as a generic interface for handling the rest.
Socket Module
To create a socket, we must use the socket.socket() function available in socket module, which has the general syntax that are given as follows:
s = socket.socket (socket_family, socket_type, protocol=0)
The parameters can be described as follows
socket_family - This is either AF_UNIX or AF_INET.
socket_type - This is either SOCK_STREAM or SOCK_DGRAM.
protocol - This is usually left out, defaulting to 0.
Server Socket Methods
s.bind() - This method is used to binds the address (hostname, port number pair) to socket.
s.listen() - This method is used to sets up and start TCP listener.
s.accept() - This method passively accept TCP client connection, waiting until connection arrives (blocking).
Client Socket Methods
s.connect()This method actively initiates TCP server connection.

General Socket Methods

s.recv() - This method receives TCP message
s.send() - This method transmits TCP message
s.recvfrom() - This method receives UDP message
s.sendto() - This method transmits UDP message
s.close() - This method closes socket
socket.gethostname() – This method would returns the hostname.
A Simple Server
To write Internet servers, we use the socket function available in socket module to create a socket object. A socket object is used to call other functions to setup a socket server.
Call the bind(hostname, port) function to specify a port for the service on the given host.
Next, call the accept method of the returned object. This method waits until a client connects to the port specified, and then returns a connection object that represents the connection to that client.
A Simple Client
Write a very simple client program which opens a connection to a given port 12345 and given host. This is very simple to create a socket client using Python's socket module function.
The socket.connect(hostname, port) opens a TCP connection to hostname on the port. Once the socket is open, read from it like any IO object. When done, remember to close it, as to close a file.

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