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TCP vs UDP for System Design

Learn the differences between TCP and UDP from a System Design perspective. This guide explains connection-oriented vs connectionless communication, reliability, packet delivery, handshakes, performance, real-world use cases, and how Amazon, Netflix, WhatsApp, Zoom, gaming platforms, and banking systems use TCP and UDP.

Introduction

Whenever you:

  • Open Amazon
  • Login to your banking application
  • Watch Netflix
  • Join a Zoom meeting
  • Play an online game
  • Make a WhatsApp call

your device communicates using one of two transport layer protocols:

  • TCP (Transmission Control Protocol)
  • UDP (User Datagram Protocol)

Choosing the wrong protocol can significantly impact:

  • Application performance
  • Reliability
  • Latency
  • Scalability
  • User experience

Every Solution Architect should understand when to use TCP and when to use UDP.

Learning Objectives

After completing this article, you will understand:

  • What is TCP?
  • What is UDP?
  • TCP vs UDP
  • Three-Way Handshake
  • Packet Delivery
  • Reliability
  • Ordering
  • Flow Control
  • Congestion Control
  • Real-World Examples
  • Best Practices

OSI Network Stack

flowchart TD
    A[Application Layer]
    B[Transport Layer]
    C[Internet Layer]
    D[Network Interface Layer]

    A --> B
    B --> C
    C --> D

TCP and UDP operate at the Transport Layer.

Where TCP and UDP Fit

flowchart TD
    A[Browser]
    B[HTTP / HTTPS]
    C[TCP or UDP]
    D[IP]
    E[Ethernet / WiFi]

    A --> B
    B --> C
    C --> D
    D --> E

What is TCP?

TCP stands for

Transmission Control Protocol

TCP provides:

  • Reliable delivery
  • Ordered packets
  • Error detection
  • Flow control
  • Congestion control

Think of TCP as a registered courier service.

Every package is tracked.

Nothing is lost.

Nothing arrives out of order.

TCP Communication

flowchart LR
    A[Client]

    B[TCP Connection]

    C[Spring Boot Server]

    A --> B
    B --> C

TCP Three-Way Handshake

Before sending data, TCP establishes a connection.

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: SYN
    Server->>Client: SYN + ACK
    Client->>Server: ACK

    Note over Client,Server: Connection Established

This ensures both client and server are ready before exchanging data.

TCP Data Transfer

flowchart LR
    A[Packet 1]
    B[Packet 2]
    C[Packet 3]
    D[Packet 4]

    A --> B
    B --> C
    C --> D

Packets arrive in order.

Missing packets are retransmitted automatically.

TCP Characteristics

  • Connection Oriented
  • Reliable
  • Ordered Delivery
  • Error Checking
  • Retransmission
  • Congestion Control
  • Flow Control

Real-Time Banking Example

Customer transfers money.

flowchart TD
    A[Mobile Banking]

    B[TCP Connection]

    C[Payment API]

    D[Core Banking]

    E[(Database)]

    A --> B
    B --> C
    C --> D
    D --> E

If a payment packet is lost,

TCP retransmits it.

Money is never partially transferred.

What is UDP?

UDP stands for

User Datagram Protocol

UDP sends packets without creating a connection.

Think of UDP as sending postcards.

Once sent,

there is no guarantee they arrive.

UDP Communication

flowchart LR
    A[Client]

    B[UDP Packet]

    C[Server]

    A --> B
    B --> C

No handshake.

No acknowledgments.

No retransmissions.

UDP Packet Flow

flowchart LR
    A[Packet 1]
    B[Packet 2]
    C[Packet 3]
    D[Packet Lost]
    E[Packet 5]

    A --> B
    B --> C
    C --> D
    D --> E

If Packet 4 is lost,

UDP simply continues.

TCP vs UDP

Feature TCP UDP
Connection Yes No
Reliable Yes No
Ordered Delivery Yes No
Retransmission Yes No
Handshake Yes No
Speed Slower Faster
Latency Higher Lower
Packet Loss Recovered Ignored

Why TCP is Slower

TCP performs additional work.

flowchart TD
    A[Handshake]

    B[Packet Delivery]

    C[Acknowledgement]

    D[Retransmission]

    A --> B
    B --> C
    C --> D

This improves reliability but increases latency.

Why UDP is Faster

UDP simply sends packets.

flowchart LR
    A[Send Packet]

    B[Receive Packet]

    A --> B

No waiting.

No acknowledgments.

Minimal overhead.

Real-Time Video Streaming

Netflix Video

flowchart TD
    A[Netflix]

    B[CDN]

    C[Video Player]

    A --> B
    B --> C

Netflix streams primarily over HTTP/HTTPS, which runs on TCP, ensuring reliable delivery of video segments. Modern streaming protocols optimize buffering to maintain smooth playback.

Video Calls

Zoom

Google Meet

Microsoft Teams

flowchart LR
    A[Camera]

    B[UDP]

    C[Remote User]

    A --> B
    B --> C

For live audio and video,

a few lost packets are acceptable.

Low latency is more important than perfect reliability.

Online Gaming

flowchart LR
    A[Player]

    B[UDP]

    C[Game Server]

    A --> B
    B --> C

If one position update is lost,

the next update quickly replaces it.

Waiting for retransmission would make the game feel sluggish.

Banking System

flowchart TD
    A[Customer]

    B[HTTPS]

    C[TCP]

    D[Payment Service]

    E[(Database)]

    A --> B
    B --> C
    C --> D
    D --> E

Banking systems always prioritize reliability.

WhatsApp

WhatsApp uses different protocols depending on the workload.

flowchart TD
    A[WhatsApp]

    A --> B[Messages]
    A --> C[Voice Calls]
    A --> D[Video Calls]

    B --> E[TCP]
    C --> F[UDP]
    D --> F

Messages require reliable delivery.

Voice and video prioritize low latency.

Amazon

Shopping workflow

flowchart TD
    A[Customer]

    B[HTTPS]

    C[TCP]

    D[Order Service]

    A --> B
    B --> C
    C --> D

Every order must be delivered exactly once.

Congestion Control

TCP automatically reduces transmission speed during network congestion.

flowchart LR
    A[Heavy Traffic]

    B[TCP Slows Down]

    C[Stable Network]

    A --> B
    B --> C

This prevents network collapse.

Flow Control

A fast sender should not overwhelm a slow receiver.

flowchart LR
    A[Fast Sender]

    B[TCP Window]

    C[Slow Receiver]

    A --> B
    B --> C

TCP adjusts transmission speed automatically.

Common Port Numbers

Protocol Port
HTTP 80
HTTPS 443
FTP 21
SSH 22
SMTP 25
DNS 53
MySQL 3306
PostgreSQL 5432

TCP Use Cases

Use TCP when:

  • Banking Transactions
  • REST APIs
  • Spring Boot Applications
  • Database Communication
  • File Uploads
  • Email
  • E-commerce Orders
  • User Authentication

UDP Use Cases

Use UDP when:

  • Live Video Streaming
  • Voice Calls
  • Online Gaming
  • Live Sports Streaming
  • DNS Queries
  • IoT Sensor Data
  • Real-time Telemetry

Performance Comparison

flowchart LR
    A[Low Latency]

    B[UDP]

    C[TCP]

    D[High Reliability]

    A --> B
    B --> C
    C --> D

Generally:

  • UDP favors speed and low latency.
  • TCP favors reliability and correctness.

Monitoring

Monitor:

  • Packet Loss
  • Retransmissions
  • Round Trip Time (RTT)
  • Latency
  • Throughput
  • TCP Connection Count
  • Network Errors
  • Congestion Events

Tools

  • Wireshark
  • Prometheus
  • Grafana
  • Datadog
  • CloudWatch

Common Mistakes

❌ Using UDP for payment systems

❌ Using TCP for latency-sensitive gaming updates

❌ Ignoring packet loss

❌ Opening unnecessary ports

❌ Not monitoring network latency

Best Practices

  • Use TCP for critical business transactions.
  • Use HTTPS (HTTP over TCP) for web applications.
  • Use UDP for real-time communication where low latency is essential.
  • Encrypt sensitive data in transit.
  • Monitor packet loss and retransmissions.
  • Design applications to tolerate transient network failures.
  • Choose the protocol based on business requirements, not just speed.

Common Interview Questions

What is the main difference between TCP and UDP?

TCP is connection-oriented and guarantees reliable, ordered delivery. UDP is connectionless and prioritizes speed over reliability.

Why do banking systems use TCP?

Banking systems require guaranteed delivery, ordered packets, and data integrity. Losing or reordering packets could result in incorrect financial transactions.

Why do online games commonly use UDP?

Games prioritize low latency. Missing one position update is usually preferable to waiting for a retransmission, which could make gameplay feel unresponsive.

What is the TCP Three-Way Handshake?

It is the connection establishment process where the client and server exchange SYN, SYN-ACK, and ACK packets before data transfer begins.

Can HTTP use UDP?

Traditional HTTP/1.1 and HTTP/2 use TCP. HTTP/3 uses QUIC, which is built on UDP while providing reliability features at a higher layer.

Summary

TCP and UDP are the two primary transport protocols used in distributed systems.

In this article, we covered:

  • TCP fundamentals
  • UDP fundamentals
  • Three-Way Handshake
  • Reliability
  • Ordered delivery
  • Flow control
  • Congestion control
  • TCP vs UDP comparison
  • Banking, Amazon, Netflix, WhatsApp, Zoom, and gaming examples
  • Performance considerations
  • Best practices

Choosing between TCP and UDP depends on the application's priorities. If correctness and reliability are critical, TCP is the right choice. If low latency and speed are more important than guaranteed delivery, UDP is often the better option. Understanding these trade-offs is a fundamental skill for designing scalable, high-performance distributed systems.

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