HTTP/3
HTTP/3 is the third major version of the Hypertext Transfer Protocol, built on top of QUIC instead of TCP to reduce connection latency and eliminate head-of-line blocking between multiplexed streams.
Definition
HTTP/3 is the third major version of the Hypertext Transfer Protocol, built on top of QUIC instead of TCP to reduce connection latency and eliminate head-of-line blocking between multiplexed streams.
Overview
HTTP/3 is the latest iteration of the protocol that underlies almost all web traffic. Where HTTP/2 improved on the original HTTP/1.1 by multiplexing multiple requests over a single TCP connection, it still suffered from head-of-line blocking at the transport layer: a single lost TCP packet could stall every stream sharing that connection. HTTP/3 solves this by replacing TCP with QUIC, a transport protocol built on UDP that gives each stream independent loss recovery, so one dropped packet only affects the stream it belongs to. Because QUIC integrates TLS 1.3 directly into the transport handshake, HTTP/3 connections are encrypted by default and can often be established in a single round trip (or zero round trips when resuming a session), which noticeably speeds up page loads on high-latency or lossy networks such as mobile connections. QUIC also supports connection migration, letting a session survive a client switching from Wi-Fi to cellular data without dropping the connection. Adoption has been driven largely by major content delivery networks and browsers rather than individual site operators configuring it from scratch — enabling it typically just means turning on a setting at the CDN or reverse-proxy layer. For teams building or debugging web APIs, understanding HTTP/3 alongside protocols like REST API design and CORS behavior rounds out a solid picture of how modern browsers talk to servers.
Specification
- Runs over QUIC instead of TCP, using UDP as the underlying transport
- Independent per-stream loss recovery eliminates transport-level head-of-line blocking
- TLS 1.3 encryption is built into the handshake, not layered on afterward
- Connection establishment can complete in 0-RTT or 1-RTT in most cases
- Connection IDs allow sessions to survive network changes (Wi-Fi to cellular)
- Multiplexes many independent streams over a single logical connection
- Backward-compatible negotiation via HTTP Alt-Svc headers or DNS records
- Widely supported by major browsers and CDNs as of the mid-2020s
Use Cases
History
HTTP/3 is the third major version of HTTP, published by the IETF as RFC 9114 in June 2022. Its defining change is the transport layer: where HTTP/1.1 and HTTP/2 run over TCP, HTTP/3 maps HTTP semantics onto QUIC, a transport built on UDP. QUIC provides stream multiplexing with per-stream flow control, low-latency (often 0-RTT or 1-RTT) connection establishment, built-in encryption, and connection migration across changing networks. Crucially, QUIC eliminates the head-of-line blocking that could stall all of HTTP/2's multiplexed streams when a single TCP packet was lost. HTTP/3 preserves the same request/response semantics and header model as HTTP/2 while delivering better performance on lossy or high-latency links such as mobile networks.
Sources
- IETF — RFC 9114: HTTP/3 · as of 2026-07-17
- IETF QUIC Working Group — quicwg.org · as of 2026-07-17