TCP

TCP (Transmission Control Protocol) is a reliable, connection-oriented protocol in the transport layer of the OSI model (Layer 4) and Internet Protocol Suite (TCP/IP stack). It enables reliable communication between two endpoints (e.g., client and server) over an unreliable network like the Internet.

Features of TCP

Feature	Description
Connection-oriented	TCP establishes a connection using a 3-way handshake (SYN → SYN-ACK → ACK) before sending data.
Reliable delivery	Guarantees all packets are delivered in order and without corruption (using acknowledgments and retransmissions).
Error checking	Uses checksums to detect errors in data transmission.
Flow control	Uses sliding window protocol to prevent overwhelming the receiver.
Congestion control	Avoids network congestion via algorithms like TCP Tahoe, Reno, Cubic, etc.
Stream-oriented	Treats data as a continuous stream (not message-based like UDP).

How TCP Works

1. Client sends a request to server (e.g., HTTP over TCP).
2. TCP handshake ensures both sides are ready to communicate.
3. Data is split into segments, numbered, and sent.
4. Receiver acknowledges each received packet.
5. Missing packets are retransmitted.
6. Connection is gracefully closed using FIN-ACK sequence.

Example Architecture Using TCP

Scenario:

You’re designing a basic web system:

• Frontend (React/Browser)
• Backend API (Node.js)
• Database (PostgreSQL)

All these components use TCP under the hood.

System Design Diagram (simplified):

[Client Browser]
     |
     |  HTTP over TCP
     v
[Node.js Backend]
     |
     |  TCP (pg driver)
     v
[PostgreSQL DB]

Explanation:

1. Client ↔ Backend (HTTP over TCP):

   • The browser opens a TCP connection to the server (e.g., port 443 for HTTPS).
   • TCP ensures the request and response are delivered reliably.

2. Backend ↔ Database (TCP Socket):

   • The backend uses a TCP connection to communicate with the database.
   • The Node.js pg driver uses TCP to send SQL queries and receive results.

3. Reliability is critical. If any packets are dropped (common on WiFi or mobile), TCP retransmits them without your app needing to handle it manually.

Example of TCP with Node.js

TCP Server (Node.js)

const net = require("net");

const server = net.createServer((socket) => {
  console.log("Client connected");

  socket.on("data", (data) => {
    console.log(`Received: ${data}`);
    socket.write("Hello from server!");
  });

  socket.on("end", () => {
    console.log("Client disconnected");
  });
});

server.listen(3000, () => {
  console.log("TCP server listening on port 3000");
});

TCP Client (Node.js)

const net = require("net");

const client = net.createConnection({ port: 3000 }, () => {
  console.log("Connected to server");
  client.write("Hello from client!");
});

client.on("data", (data) => {
  console.log(`Server says: ${data}`);
  client.end();
});

client.on("end", () => {
  console.log("Disconnected from server");
});

When to Use TCP in System Design

• Web apps (HTTP, HTTPS)
• Database connections
• Microservice communication via HTTP/gRPC
• File transfer protocols (FTP, SFTP)

Advantages & Disadvantages of TCP

Advantage	Disadvantage
Reliable & ordered delivery	Slower than UDP due to overhead
Built-in congestion & flow control	More complex than connectionless protocols
Suited for critical apps (e.g., web, DB)	Not ideal for low-latency needs (e.g., gaming, streaming)