Activation Under Apache 2

On Apache 2, HTTP/2 is not enabled by default; you have to do this manually by installing http2_module and enabling it,

LoadModule http2_module modules/mod_http2.so

in the configuration file. The second directive to be entered is,

Protocols h2 h2c HTTP/1.1

which makes HTTP/2 the preferred protocol if the client also supports that version; otherwise, the fallback is to HTTP/1.1.

Downward Compatibility

According to statistics from w3techs.com [4], a good year after the HTTP/2 specification was published, less than 10 percent of the websites at the time were using the new protocol. More importantly, however, these sites handled about two thirds of all page views. This rapid acceptance has been helped because HTTP/2 is fully downward compatible, so a change in technology does not initially require any adjustments to websites and applications.

Only if a company fully wants to leverage HTTP/2 with its website or application do the web developers have to step in and break down the individual files to the extent possible – as described earlier – and define the server push rules.

From TCP and TLS to QUIC

As mentioned earlier, the main focus of the change from HTTP/1.1 to HTTP/2 was multiplexing and the resulting superior bandwidth utilization. However, if a packet is lost during transmission, the entire TCP connection stops until the packet has reached the receiver again from the sender. As a result, depending on the loss rate, HTTP/2 can have a lower speed than HTTP/1.1, because all data is sent over a single TCP connection; therefore, all transmissions are affected by the packet loss.

To solve this problem, a working group at Google standardized the Quick UDP Internet Connection (QUIC) network protocol in July 2016, which removes the restrictions on the use of TCP and is based on UDP. One requirement is that data is always encrypted with TLS 1.3, which increases security. A split header creates further advantages. The first part is only used to establish the encrypted connection and does not contain any other data. Only when the connection is established does the system transmit the header to transfer the data. When a new connection is established to the same host, QUIC does not have to renegotiate the encryption but can start transmission directly over the secured connection. Another advantage is that QUIC is no longer limited to HTTP transmissions.

Because it is difficult to establish completely new protocols on the Internet, Google presented QUIC to the IETF. The latter has adopted QUIC as the basis for HTTP/3 and has been working on standardization since the beginning of 2017. The latest draft is from December 14, 2020 [5]. HTTP/3 completely breaks with TCP and will use QUIC in the future. For HTTP/3 to replace HTTP/2 in the future, web servers will need to support QUIC. Apache, still the most widely used HTTP daemon, has not yet made an announcement in this direction. Nginx unveiled a first version in June 2020 that is based on the IETF QUIC document. LiteSpeed is currently the only server that has fully implemented QUIC and, as a consequence, HTTP/3.