Socket Programming

When computers talk to each other over a network, they use sockets.

What Is a Socket?

A socket is the interface between an application and the network.

It allows an application to:

• Send data to another host
• Receive data from another host
• Choose the communication style (reliable vs unreliable, connection-oriented vs connectionless)

Each machine has:

• An IP address → identifies the host
• Ports (0–65535) → identify applications/processes
• A socket → binds IP + Port + protocol

Think of:

• IP address → Apartment building
• Port → Mailbox
• Application → Person receiving mail
• Network → Postal service

How Network Socket Work

TCP vs UDP — The Two Core Socket Types

TCP (Transmission Control Protocol)

• Reliable delivery
• In-order transmission
• Connection-oriented
• Bidirectional communication

Used in:

• Chat applications
• File transfer (FTP)
• Email (SMTP)
• Web communication (HTTP runs on TCP)

Best when reliability matters.

UDP (User Datagram Protocol)

• No guaranteed delivery
• No order guarantee
• No connection setup
• Faster, lightweight

Used in:

• Streaming
• Broadcasting
• Real-time systems

Best when speed matters more than reliability.

TCP vs UDP

TCP Connection Lifecycle

In TCP:

Server (Passive Participant)

1. Creates a ServerSocket
2. Listens on a port
3. Accepts connection requests
4. Creates a new socket per client

Client (Active Participant)

1. Creates a Socket
2. Connects to server
3. Exchanges data

Important concept:

• The server keeps listening while handling multiple clients.
• Multi-threading is required to prevent blocking.

Blocking and Concurrency

Socket operations are blocking by default.

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If a server waits on one client, others cannot connect.

Solution:

• Multi-threading
• One thread per connection

This concept is foundational in scalable backend systems.

Java Socket Programming

Java provides networking support via:

java.net

Core classes:

For TCP:

• Socket (client-side)
• ServerSocket (server-side)

For UDP:

• DatagramSocket
• DatagramPacket

For Address Resolution:

• InetAddress

Socket I/O in Java

TCP sockets use:

• InputStream
• OutputStream

Data is sent as a stream of bytes.

Common operations:

• read()
• write()
• close()

UDP Programming Model

Instead:

• Sender creates a DatagramPacket
• Attaches destination IP + port
• Sends via DatagramSocket
• Receiver extracts data from received packet

No handshake.
No connection.
No guarantees.

TCP vs HTTP

Important clarification:

• TCP = Transport layer protocol
• HTTP = Application layer protocol (runs on TCP)

All web communication ultimately relies on socket communication underneath.

TCP vs HTTP

WebSockets

WebSockets provide:

• Persistent, bidirectional communication
• Built on top of TCP
• Used in real-time applications

Again — underneath, it’s still socket communication.

Why This Matters

Socket programming is:

• The most fundamental form of client-server communication
• The backbone of distributed systems
• The underlying mechanism behind:
• REST APIs
• Microservices
• Web services
• Cloud communication

Even high-level frameworks ultimately depend on socket-level communication.