HTTP 3
Benefits of HTTP/3
HTTP/3, the latest version of the Hypertext Transfer Protocol, makes very deep changes to how HTTP connections are made in an effort to reduce latency, especially at the beginning of a connection.
Reliability and Encryption
The fundamental improvement of HTTP/3 is that, instead of using TCP for reliable transport and TLS (within TCP) for encryption, it uses QUIC, which combines both reliable transport and encryption in a single protocol. This reduces the number of round trips needed to establish a connection, which is especially important for mobile devices. Once the encryption/reliability layer is established, the HTTP/3 layer functions roughly the same as HTTP/2, with a few features that weren’t particularly useful left out.
Remaining (and new) challenges
Beyond the standard challenges of rolling out a new protocol worldwide (getting it into browsers, getting it into servers), there are some specific challenges to HTTP/3:
Connection Negotiation: H/3 is intended to be gradually rolled out,
rather than the impossible challenge of someone flipping a switch and suddenly
everyone using H/3 exclusively instead of H/1.1 or H/2. Thus, when a browser
connects to a server, it will always try to use the most widely supported
protocol first, which is H/1.1. If the server supports H/3, it can send an
Alt-Svc header indicating that H/3 is available, and the browser can try again
with H/3.
This is incredibly sub optimal though! The whole point of H/3 is to reduce latency, and this negotiation process adds even more latency to H/3 than even an HTTP/1.1 connection. This is why there are efforts to incorporate H/3 negotiation into the DNS response itself for a given domain, so that the browser can know how to connect to the server as part of looking up the IP address from the domain name.
Middleboxes are also a problem. Many networks have devices that inspect and modify traffic (firewalls, proxies, etc.). Much like with H/2, these devices may not understand QUIC, which could lead to connection failures or degraded performance. The problem is worse for HTTP/3 though, as many of these middleboxes are not equipped to handle UDP traffic, which QUIC uses.
The main benefit of UDP
Because QUIC and the HTTP/3 protocol are being used in tandem to deliver messages from different request and response streams, the “single in order byte stream” guarantee from TCP does not apply, and the HTTP/3 layer can continue processing incoming packets for other streams while waiting for lost messages to be resent. In this situation, only the specific stream that lost packets has to wait for the retransmission, and the others can still be processed.
Additional reading: