What is a socket?

A socket is an endpoint of a network connection, through which an application sends and receives data over the network.

More simply: a socket is a pairing of an address and a port through which a program communicates over the network.

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#What a socket actually is

In practice, a socket is uniquely identified by the set:

IP address + port + protocol (TCP or UDP)

Example:

192.168.1.10 : 443 / TCP

This means:

• a device with IP 192.168.1.10
• an application listening on port 443
• using the TCP protocol

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#Why sockets are needed

Sockets are needed so that:

• applications can exchange data over the network
• the operating system understands:
  • which process to pass incoming data to
• multiple simultaneous connections can be supported

Without sockets:

• the OS could not connect the network to applications

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#How sockets are used in a connection

#TCP

In TCP, a connection is defined by a pair of sockets:

• the client socket
• the server socket

A full TCP connection:

(client IP, client port) ↔ (server IP, server port)

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#UDP

In UDP:

• there is no persistent connection
• a socket simply:
  • receives
  • sends datagrams

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#Types of sockets (at a basic level)

• TCP socket - reliable connection, a data stream
• UDP socket - message delivery without guarantees

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#What is important to remember for the interview

• a socket is not a port by itself
• a socket is the interface between an application and the transport layer
• one port can serve many sockets (different clients)
• sockets exist on the side of the OS and applications

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#Relationship to the OSI layers

• the application layer - uses sockets
• the transport layer - implements sockets (TCP/UDP)
• the network layer - delivers packets