HTTP/3 is the third major version of the Hypertext Transfer Protocol (HTTP), built on QUIC, a modern transport protocol that operates over UDP.
QUIC provides secure and reliable data transport, while HTTP/3 defines how web requests and responses are exchanged over QUIC.
Together, they address some of the transport limitations of HTTP/2, enabling more efficient connections and improved resilience to packet loss.
How Does HTTP/3 Use QUIC?
Unlike HTTP/2, which runs over TCP, HTTP/3 uses QUIC as its underlying transport protocol.
The protocol stack is:
HTTP/3 → QUIC → UDP → IP
Each protocol serves a different purpose:
- HTTP/3: Handles HTTP requests, responses, and message framing, using QPACK for header compression.
- QUIC: Manages secure connection establishment, stream multiplexing, loss recovery, and congestion control.
- UDP: Carries QUIC packets across the network.
QUIC integrates TLS 1.3 into its connection handshake, allowing HTTP/3 to establish secure connections with fewer round trips in many scenarios.
While HTTP/3 depends on QUIC, QUIC can also support applications beyond HTTP.
Why Does HTTP/3 Use QUIC Instead of TCP?
HTTP/2 supports multiple concurrent streams, but all streams share an ordered TCP byte stream. When a TCP packet is lost, other HTTP/2 streams can be delayed until the missing data is recovered.
QUIC avoids this cross-stream head-of-line blocking by managing streams independently.
QUIC also introduces two important capabilities:
- Faster Connection Establishment: Integrated transport and TLS handshakes can reduce connection setup latency, with 0-RTT early data available for eligible resumed connections.
- Connection Migration: QUIC connection identifiers can help maintain connections when devices switch networks, such as from Wi-Fi to cellular data.
These capabilities make HTTP/3 particularly relevant to mobile users and networks experiencing latency or packet loss.
For a deeper explanation, see our guide to the QUIC protocol.
What Do QUIC and HTTP/3 Mean for CDN Performance?
For content delivery networks (CDNs), HTTP/3 over QUIC can improve communication between compatible clients and edge servers, particularly under challenging network conditions.
Potential benefits include reduced connection setup delays, less cross-stream blocking during packet loss, and better connection continuity when network paths change.
However, HTTP/3 does not guarantee faster performance in every environment. Results depend on network quality, server implementation, and application behavior.
HTTP/3 commonly uses UDP port 443. When QUIC connections are unavailable, clients can generally fall back to supported TCP-based HTTP versions.
Understanding the HTTP/2 protocol helps explain why HTTP/3 adopted QUIC and how the two HTTP versions differ.
