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HTTP/3

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In brief · quick answer

HTTP/3 is the latest major revision of HTTP that replaces TCP with QUIC, a transport protocol built on UDP, to eliminate head-of-line blocking and reduce connection latency.

§ 1 Definition

HTTP/3 is the third major version of HTTP. Its defining characteristic is replacing TCP with QUIC (a transport protocol developed by Google and standardized by the IETF). QUIC runs over UDP instead of TCP. This fundamental change solves the TCP-level head-of-line blocking problem that even HTTP/2 could not fix. In HTTP/2, a single lost TCP packet blocks all multiplexed streams until the packet is retransmitted. QUIC handles lost packets independently per stream. QUIC also combines the TLS handshake with the transport handshake, reducing connection establishment to a single round trip (zero round trips for returning connections with session resumption). HTTP/3 is the fastest, most resilient version of HTTP. Adoption has grown rapidly. Most major CDNs, browsers, and server platforms now support it.

§ 2 How it works

HTTP/3 uses QUIC as its transport layer. QUIC is built on UDP but provides reliability, congestion control, multiplexing, and encryption. Each stream in QUIC is independent: packet loss on one stream does not affect others. Connection migration is built in: if your device switches from WiFi to cellular, the QUIC connection continues with a new connection ID, without re-establishing. The TLS 1.3 handshake is integrated into QUIC, providing encryption from the first packet.

§ 3 Performance benefits

Zero-round-trip connection resumption for returning users. No head-of-line blocking across streams. Better performance on lossy networks (mobile, satellite). Connection migration for network changes. These benefits are particularly pronounced on mobile networks where packet loss is higher and connections are less stable.

§ 4 Common misconception

HTTP/3 does not obsolete HTTP/2. HTTP/2 is still excellent for most use cases. HTTP/3's advantages are most visible on networks with high packet loss or latency. On a stable, low-latency connection, the difference may be imperceptible. Migrating to HTTP/3 requires server and CDN support but no application code changes.

§ 5 Note

HTTP/3 adoption is increasing but not universal. Ensure your CDN and server software support it. Fallback to HTTP/2 (and HTTP/1.1) is transparent to users. Check support via DevTools Network panel (protocol column) or online tools.

§ 6 Common questions

Q. Is HTTP/3 the same as QUIC?: A. No. QUIC is the transport protocol. HTTP/3 is HTTP semantics mapped onto QUIC.
Q. Does HTTP/3 work over UDP?: A. Yes. QUIC runs over UDP instead of TCP. Some corporate firewalls block UDP, so HTTP/3 includes fallback to HTTP/2.

Key takeaways

HTTP/3 replaces TCP with QUIC over UDP, eliminating transport-level head-of-line blocking.
QUIC combines transport and TLS handshake for faster connection establishment.
HTTP/3 benefits are most visible on lossy mobile networks.

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# HTTP/3

HTTP/3 is the latest major revision of HTTP that replaces TCP with QUIC, a transport protocol built on UDP, to eliminate head-of-line blocking and reduce connection latency.

Category: Web Development (also: Performance, Infrastructure)

Author: Atomic Glue Team

## Definition

HTTP/3 is the third major version of HTTP. Its defining characteristic is replacing TCP with QUIC (a transport protocol developed by Google and standardized by the IETF). QUIC runs over UDP instead of TCP. This fundamental change solves the TCP-level head-of-line blocking problem that even [HTTP/2](/glossary/http2) could not fix. In HTTP/2, a single lost TCP packet blocks all multiplexed streams until the packet is retransmitted. QUIC handles lost packets independently per stream. QUIC also combines the TLS handshake with the transport handshake, reducing connection establishment to a single round trip (zero round trips for returning connections with session resumption). HTTP/3 is the fastest, most resilient version of HTTP. Adoption has grown rapidly. Most major CDNs, browsers, and server platforms now support it.

## How it works

## Performance benefits

## Common misconception

## Note

## Common questions

Q: Is HTTP/3 the same as QUIC?

A: No. QUIC is the transport protocol. HTTP/3 is HTTP semantics mapped onto QUIC.

Q: Does HTTP/3 work over UDP?

A: Yes. QUIC runs over UDP instead of TCP. Some corporate firewalls block UDP, so HTTP/3 includes fallback to HTTP/2.

## Key takeaways

HTTP/3 replaces TCP with QUIC over UDP, eliminating transport-level head-of-line blocking.
QUIC combines transport and TLS handshake for faster connection establishment.
HTTP/3 benefits are most visible on lossy mobile networks.

## Related entries

Last updated June 2026. Permalink: atomicglue.co/glossary/http3