HTTP/2 vs HTTP/3

HTTP/2 and HTTP/3 are both modern HTTP versions designed to improve web performance, but they differ fundamentally in their transport layer. HTTP/2 runs over TCP. HTTP/3 runs over QUIC, a UDP-based transport protocol.

The Head-of-Line Blocking Problem

HTTP/2 solved HTTP-level head-of-line blocking by multiplexing multiple streams over a single TCP connection. But TCP itself has head-of-line blocking — if a single TCP packet is lost, all streams are blocked until it is retransmitted.

HTTP/3 solves this completely. QUIC streams are independent at the transport level. A lost packet in one stream does not affect other streams. This is especially impactful on lossy mobile networks.

Connection Setup Speed

HTTP/2 over TLS 1.3 requires 2 round trips: one for TCP, one for TLS. HTTP/3 combines transport and encryption in a single QUIC handshake — 1 round trip for new connections, 0 round trips for repeat connections (0-RTT).

On a 200ms latency connection, this saves 200-400ms on every new connection.

Connection Migration

When a mobile device switches from WiFi to cellular, its IP address changes. TCP connections break because they are identified by the (source IP, source port, dest IP, dest port) tuple. QUIC connections use a connection ID that survives IP changes, enabling seamless migration.

For more on networking protocols, see our concepts library and system design interview guide. Explore pricing for practice.

The Bottom Line

HTTP/3 is the future of web transport. Its advantages — no head-of-line blocking, faster connections, connection migration — are most impactful on mobile and high-latency networks. HTTP/2 remains the safe default where UDP is blocked or server support is limited.