In the following tutorial we will deal with the basics of socket programming over TCP/IP networks. Also, we will create our own client/server program to create a remote calculator.

This tutorial focuses on socket programming in Python. Those more familiar with Java might find this client-server tutorial more useful.

Sockets

Sockets are just the endpoints of a two-way communication link in a network. Socket helps in the communication of two processes/programs on a network (eg. Internet).

Bidirectional inter process communication

The programs can communicate by reading/writing via their sockets.

A socket comprises of:

• IP Address: IP Address is an identification label given to a device/machine connected to a network. It acts as the address of the machine in the network, eg. 192.168.11.22 . Based on the size of label, IP address can be classified as:

IPv4: 32-bit address

IPv6: 128-bit address

• Port number: A port is a virtual endpoint of communication link through which the information passes. It is application-specific ie, different applications can communicate through different ports in a same computer. A port number is a 16-bit unsigned integer (0 to 65535) which uniquely identifies a port. Some port numbers are reserved eg port 80 - HTTP.  We can assign any port number to an application provided it is not reserved and not used by any other application.

The combination of both is also termed as a socket address. Hence using socket address we can connect to any remote machine.

Every socket follows some protocol (set of rules) for transferring the data. Based on the type of protocol used sockets can be classified as:

• Datagram Sockets:  Datagram Sockets are connectionless-oriented and unreliable. Uses UDP (User Datagram Protocol) protocol. They are basically used in audio/video streaming.

• Stream Sockets: Stream Sockets are connection-oriented and guarantees reliable and in order data transmission. Uses TCP (Transmission Control Protocol) protocol. Used in areas where reliability cannot be compromised.

• Raw Sockets: Raw sockets do not use any transport protocol but data is directly transmitted over IP protocol. They are used by ICMP (Internet Control Message Protocol) application layer protocol.

Following code shows how to create a socket:

Client/Server Model

Sockets as we know are used in bidirectional network communication and a good example of it is client/server model. It is architecture for communication between two machines where one requests for information and the other (waiting for requests) generates the desired response.

Client:  Client is a machine which generates the requests.

Server: Server is a machine which is generates a response to the corresponding requests. It is always listening for requests.

Consider an example, a website

A website is hosted on a machine (server) which listens continuously for HTTP requests. All the webpages are stored in the server. An webpage is sent as a response to a request.

Normal users who log into the website are actually clients which send HTTP request to access a webpage.

Online Calculator

Let’s build an online expression evaluator using client-server architecture. Client will send the expression to the server and the server will give the desired output.

Server

Server will be waiting for the clients to connect and send the expressions. Once a request arrives, it will process the expression and return the desired output back to the client.

Client

Client will initiate a connection to the server. Once the connection is established then client can send the expression to the server as requests.

Instructions. Here is how you may execute the examples provided above: