How Does QUIC Differ from TCP?

QUIC is a modern internet transport protocol designed to replace TCP. It eliminates TCP’s slow connection setup, reduces overhead, and improves performance, especially for unreliable networks. Unlike TCP, QUIC operates over UDP, encrypts everything by default, and handles packet loss more efficiently. 

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If you’re comparing QUIC vs TCP, think of it this way: TCP is reliable but slow due to its handshake and retransmission mechanisms, while QUIC is faster and more adaptive.

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Let’s break this down.

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The Fundamental Differences

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That’s the high-level view. Now, let’s break down each key difference.

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TCP Performance vs QUIC Performance

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TCP is reliable because it guarantees packet delivery—but at a cost. Every connection needs a 3-way handshake before sending data, and if a packet is lost, TCP stalls until it gets a new one.

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QUIC, on the other hand, is built for speed and resilience. It skips the handshake (or speeds it up significantly), encrypts everything by default, and doesn’t get stuck waiting for lost packets.

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TCP Overhead: Why It Slows Things Down

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TCP has a lot of baggage:

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• Three-Way Handshake: Before any actual data is sent, TCP requires a connection setup where the client and server exchange three messages (SYN, SYN-ACK, ACK).
• Separate Encryption (TLS): If encryption is required (which it usually is), TCP has to set up a TLS handshake before data transfer starts.
• Head-of-Line Blocking: If one packet is delayed or lost, all subsequent packets must wait, even if they were received correctly.
• Slow Recovery from Packet Loss: TCP retransmits entire packets instead of just the missing data, which adds more delay.

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All of this adds latency—especially on mobile networks or high-latency environments like satellite internet.

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How QUIC Solves These Issues

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• Zero Round Trip Time (0-RTT): QUIC can start sending data immediately if the client has connected before. Even for new connections, the handshake is much faster.
• No Separate TLS Setup: QUIC always encrypts traffic using TLS 1.3, so security is baked in without an extra handshake.
• Multiplexing Without Head-of-Line Blocking: QUIC allows multiple data streams inside a single connection. If one packet is lost, only that stream is affected, not the others.
• Efficient Packet Recovery: Instead of retransmitting entire packets, QUIC only resends the missing data, improving efficiency.

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QUIC Packet vs TCP Packet: What’s Inside?

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Every TCP packet carries:

1. Sequence numbers for ordering
2. Acknowledgments (ACKs) to confirm delivery
3. Flags for controlling connections (SYN, FIN, etc.)
4. Optional TLS payloads for encrypted connections

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A QUIC packet, on the other hand, looks completely different:

1. No sequence numbers at the transport layer – QUIC handles ordering internally.
2. Built-in encryption – No need for a separate TLS handshake.
3. No TCP flags – QUIC has its own connection management.
4. Smaller headers – Less overhead, leading to faster transmission.

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Why This Matters

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A smaller, simpler QUIC packet means:

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• Faster transmission (less waiting around for handshakes)
• Lower overhead (less wasted bandwidth)
• Built-in security (no plaintext packets or separate encryption layers)

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QUIC’s Multiplexing vs TCP’s Head-of-Line Blocking

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One of QUIC’s biggest advantages over TCP is how it handles multiple streams of data. 

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This is something you’ll notice in real-world performance, especially when loading websites or streaming media.

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What’s the Problem with TCP?

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TCP delivers data in order. That sounds great—until a single missing packet blocks everything behind it.

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Imagine a conveyor belt carrying packages. If one package gets lost, the whole line stops until the missing package is replaced. This is called head-of-line blocking (HOLB), and it’s a huge issue in TCP, especially for HTTP/2.

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Why? Because in HTTP/2 over TCP, all requests and responses within a connection share the same stream. If a packet carrying part of a response is delayed or lost, every other request using the same connection must wait until the missing packet is retransmitted.

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So if you're loading a webpage with multiple images, scripts, and stylesheets, a single lost packet can delay everything—even if the rest of the data was received correctly.

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How QUIC Fixes This

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QUIC eliminates head-of-line blocking by treating each stream independently, even within a single connection.

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With QUIC, if a packet containing part of one stream is lost, only that stream is affected—the others keep flowing smoothly.

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Let’s go back to the conveyor belt analogy. Instead of having a single belt where one lost package stops everything, QUIC runs multiple conveyor belts in parallel. If a package goes missing on one belt, the others keep moving.

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QUIC’s Smarter Congestion Control

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Congestion control is a critical part of how transport protocols manage network traffic. Both TCP and QUIC adjust their speed based on how much data can be sent without overwhelming the network.

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But TCP's congestion control is old and inefficient—it was designed in an era where networks were slow and unstable. QUIC improves on this by being faster and more adaptive.

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How TCP Handles Congestion (And Why It’s Outdated)

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TCP follows a conservative approach:

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1. Starts slow – TCP initially sends a small amount of data to test the network.
2. Gradually increases speed – If no packets are lost, TCP ramps up how much data it sends.
3. Slows down after packet loss – If a packet is dropped, TCP dramatically reduces speed and slowly increases again.

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This method works but has a big flaw: it reacts too aggressively to packet loss.

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• Even minor packet loss causes major slowdowns.
• It takes time for TCP to ramp up speed again.
• If the network fluctuates (like mobile data), TCP never reaches full speed.

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This is why your Wi-Fi might seem fast one moment and slow the next—TCP is constantly adjusting its speed based on congestion signals.

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How QUIC Handles Congestion Smarter

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QUIC learns from the network in real time and adapts much faster:

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• Better packet pacing – QUIC spaces out packet transmissions more efficiently, preventing sudden bursts of traffic that can cause congestion.
• Faster recovery from packet loss – QUIC doesn’t overreact to small losses. Instead of drastically slowing down, it adjusts its speed more smoothly.
• More accurate network measurements – QUIC monitors actual delivery times instead of just relying on packet loss signals.

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The Real-World Benefits of QUIC Over TCP

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If you’re just browsing the web, you might not see the difference between TCP and QUIC. But under the hood, QUIC makes everything faster, more reliable, and more secure.

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1. Web Browsing Feels Snappier

• Google’s tests showed 8% to 14% faster page load times with QUIC.
• On slow networks (like mobile data or weak Wi-Fi), QUIC performs even better.

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2. Video Streaming Becomes Smoother

• QUIC reduces buffering and interruptions, even with packet loss.
• YouTube and Netflix use QUIC to improve video playback.

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3. Online Gaming Gets Lower Latency

• Because QUIC handles packet loss better, game data is transmitted without delays.
• Less stuttering and lag in competitive games.

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4. Better Security with Always-On Encryption

• No unencrypted connections, unlike TCP.
• QUIC protects against man-in-the-middle attacks by default.

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Why Aren’t We Using QUIC Everywhere Yet?

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For all its advantages, QUIC isn’t universal yet. Here’s why:

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1. Not All Networks Support It – Some enterprise firewalls and legacy systems block QUIC since it runs over UDP.
2. TCP Is Deeply Embedded – Most infrastructure (like routers, proxies, and VPNs) is built around TCP.
3. QUIC Requires More CPU Power – Since it encrypts everything by default, it uses more processing power compared to TCP.

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But despite these challenges, QUIC is gaining traction. Google Chrome, YouTube, Facebook, and Cloudflare already use QUIC to improve performance. 

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Major browsers like Firefox, Edge, and Safari support it too.

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Why Big Tech Pushed QUIC Instead of TCP (Google’s Role in Its Development)

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QUIC isn’t just a random new protocol—it was born out of frustration with TCP’s limitations. And Google led the charge in developing it.

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Why Did Google Create QUIC?

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Google noticed that TCP was holding back performance on services like:

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• YouTube – TCP’s slow recovery from packet loss caused buffering.
• Google Search – TCP’s connection setup time slowed down page loads.
• Gmail & Drive – TCP’s head-of-line blocking made interactions feel sluggish.

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Since Google handles billions of requests per second, even a small improvement in speed means millions of saved hours for users.

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So instead of fixing TCP, Google decided to build a better transport protocol from scratch—QUIC.

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Google’s QUIC Experiments (Before It Became a Standard)

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• In 2013, Google enabled QUIC in Chrome as an experimental feature.
• By 2015, over 35% of Google’s traffic was running on QUIC.
• Google claimed QUIC reduced YouTube buffering by 30% and improved Google Search page load times by 8%.

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With results like that, Google pushed for QUIC to become an official standard, and in 2021, the IETF (Internet Engineering Task Force) officially standardized it as RFC 9000.

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Now, Cloudflare, Facebook, Apple, and Microsoft all support QUIC, and over 50% of internet traffic already runs on it.

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Why Didn’t Google Just Improve TCP?

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1. TCP is too deeply embedded – Changing TCP would require modifying every operating system, router, and firewall in the world.
2. QUIC doesn’t need OS-level support – Since QUIC runs over UDP, it can be implemented in apps and browsers without changing the OS.
3. Security was a priority – QUIC encrypts everything by default, while TCP’s encryption (TLS) is optional and adds extra overhead.

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