Basics of Sockets

Till now, you must have gained the basic idea about networking and python. If you have missed the last few tutorials, we advice you to go through them, before starting with sockets. Now the real fun begins when we start to mix these things up. So, let's get started with sockets and see what they are and why we use them.

To understand what sockets are, let's start with the Internet Connection. The Internet Connection basically connects two points across the internet for data sharing and other stuff. One process from computer C1 can communicate to a process from computer C2, over an internet connection. It has following properties:

• Reliable: It means until the cables connecting two computers are safe, data will be transfered safely.
• Point-to-Point: Connection is established between 2 points.
• Full-Duplex: It means transfer of information can occur in both ways i.e. from client to server as well as server to client simultaneously(at the same time).

Sockets are the endpoints of a bidirectional, point-to-point communication channel. Given an internet connection, say between client(a browser) and the server(say studytonight.com), we will have two sockets. A Client Socket and a Server Socket.

Socket acts on two parts: IP Address + Port Number

Vaguely speaking, when you clicked on the link that brought you to Studytonight website, your browser did something like the following:

#a socket object is created for communication
clientsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

# now connect to the web server on port 80
# - the normal http port
clientsocket.connect(("www.studytonight.com", 80))

This happened on the client side. When the client tries to connect with the server, a random port is assigned by the operating system for the connection. This random port is called Ephermal Port. In the above illustration, 1300 is an ephermal port on the source(client) machine. The client socket is short lived, i.e as soon as the data exchange ends it closes. Now what happens on server is a bit different from client. So,let's see:

serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

#bind the socket to a public host and a well-known port
serversocket.bind((socket.gethostname(), 80))

#become a server socket and listen for connections
serversocket.listen(5)

For now, just focus on Connect and Bind methods.

Connect is used by the client socket to start a connection with the server. This request is fulfilled by bind method of the server socket. If you are having problem with the code, don't worry. Every bit of it will be explained separately with examples.

Before concluding, let's see some differences between the client and the server sockets:

• Unlike client sockets, server sockets are not short lived. For example: you might need youtube.com for a single request but youtube.com has to be up 24*7 for any request which it might receive from users across the globe.
• Unlike client socket which uses Ephermal Port for connection, server socket requires a standard or well defined port for connection like: Port 80 for Normal HTTP Connection, Port 23 for Telnet etc.