HTTP/3 is the third major version of the Hypertext Transfer Protocol used to exchange information on the World Wide Web, complementing the widely-deployed HTTP/1.1 and HTTP/2.
Unlike previous versions which relied on the well-established TCP (published in 1974), HTTP/3 uses QUIC, a multiplexed transport protocol built on 59.32 UDP.On 6 June 2022, IETF published HTTP/3 as a Proposed Standard in RFC 9114.HTTP/3 uses similar semantics compared to earlier revisions of the protocol, including the same request methods, status codes, and message fields, but encodes them and maintains session state differently. However, partially due to the protocol's adoption of QUIC, HTTP/3 has lower latency and loads more quickly in real-world usage when compared with previous versions: in some cases over four times as fast than with HTTP/1.1 (which, for many websites, is the only HTTP version deployed).
HTTP/3 is (at least partially) supported by 97% of tracked web browser installations (thereof of 98% of "tracked mobile" web browsers), and 29% of the top 10 million websites. It has been supported by Chromium (and derived projects including Google Chrome, Microsoft Edge, Samsung Internet, and Opera) since April 2020 and by Mozilla Firefox since May 2021. Safari 14 implemented the protocol but it remains disabled by default.
In HTTP/2, we saw the TCP head-of-line blocking problem, i.e., 59.29 TCP segments must be delivered in order.
- HTTP/3 uses QUIC protocol.
- Like HTTP/2, QUIC has streams.
- But QUIC uses 59.32 UDP instead.
- The application decides the boundary.
Therefore, the TCP segments have been replaced with UDP/QUIC datagrams. Here, the streams are independent, and the server does not care about the order.
If a datagram is lost, then except that stream rest, all will continue as it is.
QUIC is a transport-layer protocol that provides several features to improve performance and security, while HTTP/3 is the application-layer protocol that benefits from these features to make server connections faster, more secure, and more reliable.
Pros
- QUIC has many other benefits
- Merges Connection setup + TLS in one handshake
- Has congestion control at the stream level
- Connection migration -> Even if the IP address changes, the connection is not dropped.
Cons
- The header compression algorithm (HPAC) requires ordered header entries, which is not possible in QUIC. That's why QPAC was created.
- It takes a lot of CPU (parsing logic)
- UDP could be blocked
- IP fragmentation is the enemy.
