QUIC Protocol

QUIC is a transport layer network protocol designed to make web communication faster and more reliable. Unlike older versions of HTTP that rely on TCP, QUIC works over UDP, combining speed and security in a single handshake.

In short:

HTTP/2 = TCP + TLS (multiple handshakes, higher latency)
HTTP/3 = QUIC (single handshake, low latency)

Why QUIC was Created

Before QUIC, web communication mostly used TCP (Transmission Control Protocol). TCP ensures reliable delivery of packets but at the cost of extra round trips during connection setup. When a client (like your browser) connects to a server:

1. It sends a SYN (synchronize) request.
2. The server replies with SYN-ACK.
3. The client responds with ACK.

That’s three round trips before any real data is transferred. If the connection is secure (HTTPS), another round trip is required for TLS/SSL certificate verification - adding even more delay. This means that even before your web page starts loading, your browser has already spent valuable time "saying hello" to the server.

Note: QUIC eliminates this handshake delay, dramatically reducing connection time.

How QUIC Works

QUIC operates in the transport layer, directly on top of UDP. It’s connectionless by nature, but QUIC adds reliability through:

• Packet sequencing
• Acknowledgments
• Loss recovery
• Encryption (built-in TLS 1.3)

Note: Unlike TCP, which separates the transport and encryption phases, QUIC merges them into one single handshake - meaning data can start flowing almost immediately after the first packet exchange.

QUIC - The Replacement for TCP + TLS

Traditionally, HTTP/2 uses TCP to handle data transmission and TLS to encrypt it. Both require multiple steps to establish a secure and reliable connection. With QUIC, both TCP and TLS functionalities are built right into the protocol. This means:

• Fewer round trips.
• Encrypted communication by default.
• Faster page load times.

Note: This combination gives QUIC both speed and security, making it ideal for modern web use.

UDP’s Role in QUIC

QUIC is built on top of UDP. UDP is connectionless, lightweight and extremely fast - but traditionally unreliable because it doesn’t guarantee packet delivery or order. QUIC extends UDP with reliability mechanisms like:

• Packet acknowledgment
• Retransmission of lost packets
• Stream multiplexing (multiple streams in a single connection)

Note: QUIC effectively makes UDP reliable, similar to TCP - but without TCP’s overhead. Because UDP is already widely supported, QUIC doesn’t need massive infrastructure changes. This makes it easier for browsers and servers to adopt it.

Where Is QUIC Used

QUIC powers HTTP/3, the latest version of the HTTP protocol. Major tech companies and browsers already use it:

• Google Chrome and YouTube
• Facebook
• Cloudflare
• Microsoft Edge
• Amazon and Akamai services

Note: If you’re using any Chromium-based browser today, chances are you’re already benefiting from QUIC - without even realizing it.

Enabling QUIC in Your Browser

Most modern browsers (like Chrome, Edge and Vivaldi) already have QUIC enabled by default. However, you can manually enable or disable it as follows:

Using Chrome Flags:

1. Open your browser.
2. Go to chrome://flags.
3. Search for "QUIC."
4. Set Experimental QUIC Protocol to Enabled or Disabled.

Using Command Line:

<browser.exe> --enable-quic --quic-version=h3-25
Example: chrome.exe --enable-quic --quic-version=h3-25

Using Enterprise Policy (Windows Registry):

Registry Key:
[HKLM/HKCU]/SOFTWARE/Policies/Google/Chrome
Value:
QuicAllowed (REG_DWORD): 1 (Allowed) or 0 (Blocked)

How to Check if a Site Uses QUIC (HTTP/3)

1. Open Developer Tools - Network Tab.
2. Load a website (like www.google.com).
3. Right-click the column headers and enable the Protocol column.
4. If you see h3 or http/3, the site is using QUIC.

View of Browser:

QUIC vs TCP - Latency Comparison

Experiment

Load a website (like Google) using both HTTP/2 (TCP) and HTTP/3 (QUIC) and note the latency.

QUIC's time

• Now, run in H3 Protocol and refer their latency. Then we can conclude the latency difference between H2 and H3.
• TCP time - We can see that time taken by H2 (TCP/IP) = 284 ms where as time taken by H3 (QUIC) is 261 ms.

Note: QUIC Took lesser time than TCP, to do this experiment, load the site www.google.com in H2 protocol & refer the latency time

Advantages of QUIC

• Low Latency: Uses UDP with a single handshake - faster startup.
• Built-in Security: Integrates TLS 1.3 for end-to-end encryption.
• Multiplexing Support: Multiple data streams in one connection (no TCP "head-of-line blocking").
• Easier Adoption: Runs on UDP, already supported by most networks.
• Connection Migration: Even if the IP changes (like switching from Wi-Fi to mobile), the session continues seamlessly.

Disadvantages of QUIC

• Limited Browser Support: Not all browsers or servers fully support QUIC yet.
• Debugging Challenges: Traditional TCP monitoring tools don’t work directly with QUIC.
• Complex Flow Control: Managing congestion and flow control over UDP is trickier than over TCP.
• Different Congestion Behavior: QUIC’s congestion control behaves differently than TCP, which can cause inconsistencies in some network conditions.