HTTP/2 vs HTTP/3: Multiplexing Over TCP vs QUIC

Sat, 01 Aug 2026 20:02:00 +0900

Overview

HTTP/2 and HTTP/3 both let a browser send many requests over one logical connection, but they diverge at the transport layer. HTTP/2 rides on TCP, inheriting its single ordered byte stream and its packet-loss stalls; HTTP/3 replaces that with QUIC over UDP, giving each stream independent loss recovery and letting connections survive network changes.

Comparison Diagram

Comparison Table

Aspect	HTTP/2	HTTP/3
Transport protocol	Runs over TCP	Runs over QUIC (built on UDP)
Connection handshake	Separate TCP handshake, then TLS handshake (1-2 RTT)	TLS 1.3 is integrated into the QUIC handshake, often 1-RTT or 0-RTT on resumption
Stream multiplexing	Multiple streams share one ordered TCP byte stream	Each stream is a distinct, independently-sequenced QUIC stream
Head-of-line blocking	A single lost TCP segment stalls delivery of every stream until it’s retransmitted	Loss on one stream only stalls that stream; others keep flowing
Header compression	HPACK	QPACK
Connection identity and migration	Bound to the source/destination IP and port 4-tuple; changing networks breaks it	Bound to a connection ID; survives IP or network changes, e.g. Wi-Fi to cellular
Congestion control and loss recovery	Implemented in the OS kernel’s TCP stack	Implemented in user-space by the QUIC library, easier to iterate on
Network and middlebox support	Ubiquitous; TCP port 443 is rarely blocked	UDP is sometimes blocked or throttled by firewalls, requiring a TCP fallback

Key Differences

HTTP/2 multiplexes streams inside a single TCP connection, while HTTP/3 gives each stream its own loss-recovery in QUIC
TCP’s in-order delivery means one dropped packet causes head-of-line blocking across all HTTP/2 streams; HTTP/3 avoids this by design
HTTP/3 folds the transport and TLS handshakes together for faster 0-RTT setup on reconnection
QUIC’s connection ID lets HTTP/3 sessions survive a client’s IP or network change without reconnecting
HTTP/3 depends on UDP reaching the server, so restrictive middleboxes can force a fallback to HTTP/2

When to Use Each

HTTP/2

Http2 on IT Comparison

HTTP/2 vs HTTP/3: Multiplexing Over TCP vs QUIC

Overview

Comparison Diagram

Comparison Table

Key Differences

When to Use Each