What is HTTP/2?
HTTP/2 is the new version of HTTP (Hypertext Transfer Protocol), superseding HTTP/1.1 with several optimizations designed to improve the speed of web communications. It is fully backwards-compatible with HTTP/1.1 and retains the core HTTP semantics including methods, status codes, URIs, and header fields. HTTP/2 is based on an earlier experimental protocol called SPDY which was designed and tested by Google.
Major Changes in HTTP/2
Here is a summary of the major changes in HTTP/2:
Frames, Atreams, and Multiplexing
Rather than transporting data in plaintext format, data is encoded as binary and encapsulated inside frames which can be multiplexed (or interleaved) along bidirectional channels known as streams – all over a single TCP connection. This allows for many parallel requests/responses to take place concurrently with no head-of-line blocking since the frames can be received out of order and automatically reassembled client-side. Multiplexing also eliminates the need for hacky HTTP/1.1 workarounds such as domain sharding, image sprites, and file concatenation.
Stream Prioritization and Dependency
Priority levels can be assigned to any stream, allowing the server to allocate more resources to high-priority request/response types. In addition, streams can be considered as parents, siblings, or children of other streams, permitting fine-grained control over the order in which data should be received.
Reduced Header Size
HPACK compression is used to reduce request/response header overhead. Compressed headers are also indexed for quick re-use of previously-seen header fields and values.
Flow Control
HTTP/2 offers a flow control mechanism to limit and regulate the resource use of any particular stream, or the connection as a whole.
Server Push
A completely new feature in which the server can proactively send multiple responses to the client in reply to a single request. Server Push helps reduce page load time by sending all the extra resources needed to render a page (e.g. CSS / JS files) along with the initial response. These resources can then be cached and re-used across different pages on the same domain.
HTTP/2 is supported by all major web browsers, although it should be noted that most browsers do not support unencrypted (plaintext) HTTP/2 traffic by default.