|International standard||RFC 7540|
|Introduced||May 14, 2015|
HTTP/2 (originally named HTTP/2.0) is a major revision of the HTTP network protocol used by the World Wide Web. It was derived from the earlier experimental SPDY protocol, originally developed by Google. HTTP/2 was developed by the HTTP Working Group (also called httpbis, where "bis" means "twice") of the Internet Engineering Task Force (IETF). HTTP/2 is the first new version of HTTP since HTTP/1.1, which was standardized in RFC 2068 in 1997. The Working Group presented HTTP/2 to the Internet Engineering Steering Group (IESG) for consideration as a Proposed Standard in December 2014, and IESG approved it to publish as Proposed Standard on February 17, 2015 (and was updated in February 2020 in regard to TLS 1.3). The HTTP/2 specification was published as RFC 7540 on May 14, 2015.
The standardization effort was supported by Chrome, Opera, Firefox, Internet Explorer 11, Safari, Amazon Silk, and Edge browsers. Most major browsers had added HTTP/2 support by the end of 2015. About 97% of web browsers used have the capability. As of October 2021[update], 47% (after topping out at just over 50%) of the top 10 million websites supported HTTP/2.
Its successor is HTTP/3, a major revision that builds on the concepts established by HTTP/2.
The working group charter mentions several goals and issues of concern:
The proposed changes do not require any changes to how existing web applications work, but new applications can take advantage of new features for increased speed. HTTP/2 leaves all of HTTP/1.1's high-level semantics, such as methods, status codes, header fields, and URIs, the same. What is new is how the data is framed and transported between the client and the server.
Websites that are efficient minimize the number of requests required to render an entire page by minifying (reducing the amount of code and packing smaller pieces of code into bundles, without reducing its ability to function) resources such as images and scripts. However, minification is not necessarily convenient nor efficient and may still require separate HTTP connections to get the page and the minified resources. HTTP/2 allows the server to "push" content, that is, to respond with data for more queries than the client requested. This allows the server to supply data it knows a web browser will need to render a web page, without waiting for the browser to examine the first response, and without the overhead of an additional request cycle.
Additional performance improvements in the first draft of HTTP/2 (which was a copy of SPDY) come from multiplexing of requests and responses to avoid some of the head-of-line blocking problem in HTTP 1 (even when HTTP pipelining is used), header compression, and prioritization of requests. However, as HTTP/2 runs on top of a single TCP connection there is still potential for head-of-line blocking to occur if TCP packets are lost or delayed in transmission. HTTP/2 no longer supports HTTP/1.1's chunked transfer encoding mechanism, as it provides its own, more efficient, mechanisms for data streaming.
SPDY (pronounced like "speedy") was a previous HTTP-replacement protocol developed by a research project spearheaded by Google. Primarily focused on reducing latency, SPDY uses the same TCP pipe but different protocols to accomplish this reduction. The basic changes made to HTTP/1.1 to create SPDY included: "true request pipelining without FIFO restrictions, message framing mechanism to simplify client and server development, mandatory compression (including headers), priority scheduling, and even bi-directional communication".
The HTTP Working Group considered Google's SPDY protocol, Microsoft's HTTP Speed+Mobility proposal (SPDY based), and Network-Friendly HTTP Upgrade. In July 2012, Facebook provided feedback on each of the proposals and recommended HTTP/2 be based on SPDY. The initial draft of HTTP/2 was published in November 2012 and was based on a straight copy of SPDY.
The biggest difference between HTTP/1.1 and SPDY was that each user action in SPDY is given a "stream ID", meaning there is a single TCP channel connecting the user to the server. SPDY split requests into either control or data, using a "simple to parse binary protocol with two types of frames". SPDY showed evident improvement over HTTP, with a new page load speedup ranging from 11% to 47%.
The development of HTTP/2 used SPDY as a jumping-off point. Among the many detailed differences between the protocols, the most notable is that HTTP/2 uses a fixed Huffman code-based header compression algorithm, instead of SPDY's dynamic stream-based compression. This helps to reduce the potential for compression oracle attacks on the protocol, such as the CRIME attack.
On February 9, 2015, Google announced plans to remove support for SPDY in Chrome in favor of support for HTTP/2. That took effect, starting with Chrome 51.
|December 20, 2007||First HTTP/1.1 Revision Internet Draft|
|January 23, 2008||First HTTP Security Properties Internet Draft|
|Early 2012||Call for Proposals for HTTP 2.0|
|October 14 – November 25, 2012||Working Group Last Call for HTTP/1.1 Revision|
|November 28, 2012||First WG draft of HTTP 2.0, based upon draft-mbelshe-httpbis-spdy-00|
|Held/Eliminated||Working Group Last Call for HTTP Security Properties|
|September 2013||Submit HTTP/1.1 Revision to IESG for consideration as a Proposed Standard|
|February 12, 2014||IESG approved HTTP/1.1 Revision to publish as a Proposed Standard|
|June 6, 2014||Publish HTTP/1.1 Revision as RFC 7230, 7231, 7232, 7233, 7234, 7235|
|August 1, 2014 – September 1, 2014||Working Group Last call for HTTP/2|
|December 16, 2014||Submit HTTP/2 to IESG for consideration as a Proposed Standard|
|December 31, 2014 – January 14, 2015||IETF Last Call for HTTP/2|
|January 22, 2015||IESG telechat to review HTTP/2 as Proposed Standard|
|February 17, 2015||IESG approved HTTP/2 to publish as Proposed Standard|
|May 14, 2015||Publish HTTP/2 as RFC 7540|
|February 2020||RFC 8740: HTTP/2 with TLS 1.3|
HTTP/2 is defined both for HTTP URIs (i.e. without TLS encryption, a configuration which is abbreviated in h2c) and for HTTPS URIs (over TLS using ALPN extension where TLS 1.2 or newer is required, a configuration which is abbreviated in h2).
Although the standard itself does not require usage of encryption, all major client implementations (Firefox, Chrome, Safari, Opera, IE, Edge) have stated that they will only support HTTP/2 over TLS, which makes encryption de facto mandatory.
The FreeBSD and Varnish developer Poul-Henning Kamp asserts that the standard was prepared on an unrealistically short schedule, ruling out any basis for the new HTTP/2 other than the SPDY protocol and resulting in other missed opportunities for improvement. Kamp criticizes the protocol itself for being inconsistent and having needless, overwhelming complexity. He also states that the protocol violates the protocol layering principle, for example by duplicating flow control that belongs in the transport layer (TCP). He also suggested that the new protocol should have removed HTTP Cookies, introducing a breaking change.
Initially, some members[who?] of the Working Group tried to introduce an encryption requirement in the protocol. This faced criticism.
Critics stated that encryption has non-negligible computing costs and that many HTTP applications have actually no need for encryption and their providers have no desire to spend additional resources on it. Encryption proponents have stated that this encryption overhead is negligible in practice. Poul-Henning Kamp has criticized the IETF for hastily standardizing Google's SPDY prototype as HTTP/2 due to political considerations. The criticism of the agenda of mandatory encryption within the existing certificate framework is not new, nor is it unique to members of the open-source community – a Cisco employee stated in 2013 that the present certificate model is not compatible with small devices like routers, because the present model requires not only annual enrollment and remission of non-trivial fees for each certificate, but must be continually repeated on an annual basis. Working Group finally did not reach consensus over the mandatory encryption, although most client implementations require it, which makes encryption a de facto requirement.
The HTTP/2 protocol also faced criticism for not supporting opportunistic encryption, a measure against passive monitoring similar to the STARTTLS mechanism that has long been available in other Internet protocols like SMTP. Critics have stated that the HTTP/2 proposal goes in violation of IETF's own RFC 7258 "Pervasive Monitoring Is an Attack", which also has a status of Best Current Practice 188. RFC7258/BCP188 mandates that passive monitoring be considered as an attack, and protocols designed by IETF should take steps to protect against passive monitoring (for example, through the use of opportunistic encryption). A number of specifications for opportunistic encryption of HTTP/2 have been provided, of which draft-nottingham-http2-encryption was adopted as an official work item of the working group, leading to the publication of RFC 8164 in May 2017.
Although the design of HTTP/2 effectively addresses the HTTP-transaction-level head-of-line blocking problem by allowing multiple concurrent HTTP transactions, all those transactions are multiplexed over a single TCP connection, meaning that any packet-level head-of-line blocking of the TCP stream simultaneously blocks all transactions being accessed via that connection. This head-of-line blocking in HTTP/2 is now widely regarded as a design flaw, and much of the effort behind QUIC and HTTP/3 has been devoted to reduce head-of-line blocking issues.
Main article: Comparison of web server software
2014-12-16 IESG state changed to Publication Requested
HTTP/2 does not modify the application semantics of HTTP in any way
HTTP/2 uses DATA frames to carry message payloads. The "chunked" transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be used in HTTP/2
Update: To better align with Chrome's release cycle, SPDY and NPN support will be removed with the release of Chrome 51.
TL;DR: Support for HTTP/2 is widespread enough that SPDY/3.1 support can be dropped.
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