What is HTTP/2?

HTTP/2 is the first major HTTP protocol update since 1997 when HTTP/1.1 was first published by the IETF. The new HTTP protocol is needed to keep up with the exponential growth of the web. The successor of HTTP/1.1 brings significant improvement in efficiency, speed and security and is supported by most modern web browsers. A list of browsers that support HTTP/2 can be found on caniuse.com.

• HTTP/2 is binary, instead of textual.
• It is fully multiplexed, sending multiple requests in parallel over a single TCP connection.
• It uses header compression HPACK to reduce overhead.
• It allows servers to “push” responses proactively into client caches instead of waiting for a new request for each resource
• It uses the new ALPN extension which allows for faster-encrypted connections since the application protocol is determined during the initial connection.
• It reduces additional round trip times (RTT), making your website load faster without any optimization.
• Domain sharding and asset concatenation is no longer needed with HTTP/2.

HTTP/2 introduces other improvements, more details: HTTP/2 RFC7540

This test can check HTTP and HTTPS but most browsers only support HTTP/2 for HTTPS, which means you must migrate to HTTPS if you want to take advantage of the performance benefits.

Does this test also verify SPDY?

Not directly, but it tests which TLS extension is supported to negotiate the protocols. The tested TLS extensions are either NPN or ALPN. Next Protocol Negotiation (NPN) appeared as part of the SPDY protocol but it has been deprecated. Application Layer Protocol Negotiation (ALPN) is the successor of NPN and is approved by the IETF (RFC7301). However, NPN advertises the supported protocols from the server to the client and this test will show the advertised protocols if HTTP/2 is not supported. The protocol advertising has been reversed in ALPN (client to server). The first phase of this HTTP/2 check runs the ALPN test with only H2 in the protocol list.

How to test HTTP/2 via command line?

Get the latest curl release and use this command: curl --http2.