https://www.digitalocean.com/community/tutorials/understanding-sockets

Introduction

Sockets are a way to enable inter-process communication between programs running on a server, or between programs running on separate servers. Communication between servers relies on network sockets, which use the Internet Protocol (IP) to encapsulate and handle sending and receiving data.

Network sockets on both clients and servers are referred to by their socket address. An address is a unique combination of a transport protocol like the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP), an IP address, and a port number.

In this tutorial you will learn about the following different types of sockets that are used for inter-process communication:

• Stream sockets, which use TCP as their underlying transport protocol
• Datagram sockets, which use UDP as their underlying transport protocol
• Unix Domain Sockets, which use local files to send and receive data instead of network interfaces and IP packets.

In each section of this tutorial you will also learn how to enumerate the respective socket types on a Linux system. You’ll examine each type of socket using a variety of command line tools.

Prerequisites

The examples in this tutorial were validated on an Ubuntu 20.04 server. You can follow this tutorial using most modern Linux distributions on a local computer or remote server, as long as you have the equivalent version of each of the required tools for your distribution installed.

To get started using Ubuntu 20.04, you will need one server that has been configured by following our Initial Server Setup for Ubuntu 20.04 guide.

You will also need a few other packages in order to examine sockets on your system. Ensure that your local package cache is up to date using the apt update command:

Then install the required packages using this command:

The iproute2 package contains the ss utility, which is what we’ll use to inspect sockets. We’ll use the netcat-openbsd package to install netcat. Note that netcat is abbreviated to nc when it is invoked on the command line. Finally, we’ll use the socat package to create example sockets.

What is a Stream Socket?

Stream sockets are connection oriented, which means that packets sent to and received from a network socket are delivered by the host operating system in order for processing by an application. Network based stream sockets typically use the Transmission Control Protocol (TCP) to encapsulate and transmit data over a network interface.

TCP is designed to be a reliable network protocol that relies on a stateful connection. Data that is sent by a program using a TCP-based stream socket will be successfully received by a remote system (assuming there are no routing, firewall, or other connectivity issues). TCP packets can arrive on a physical network interface in any order. In the event that packets arrive out of order, the network adapter and host operating system will ensure that they are reassembled in the correct sequence for processing by an application.