|Internet protocol suite|
SPDY (pronounced speedy) is an open networking protocol developed primarily at Google for transporting web content. SPDY is similar to HTTP, with particular goals of reducing web page load latency and improving web security. SPDY achieves reduced latency through compression, multiplexing, and prioritization. The name "SPDY" is a trademark of Google and is not an acronym.
As of July 2012[update], the group developing SPDY has stated publicly that it is working toward standardisation (available as an Internet Draft). The first draft of HTTP 2.0 is using SPDY as the working base for its specification draft and editing.
The goal of SPDY is to reduce web page load time. This is achieved by prioritizing and multiplexing the transfer of web page subresources so that only one connection per client is required. TLS encryption is nearly ubiquitous in SPDY implementations, and transmission headers are gzip- or DEFLATE-compressed by design (in contrast to HTTP, where the headers are sent as human-readable text). Moreover, servers may hint or even push content instead of awaiting individual requests for each resource of a web page.
SPDY requires the use of SSL/TLS (with TLS extension NPN), and does not support operation over plain HTTP. The requirement for SSL is for security and to avoid incompatibility when communication is across a proxy.
Relation to HTTP
SPDY does not replace HTTP; it modifies the way HTTP requests and responses are sent over the wire. This means that all existing server-side applications can be used without modification if a SPDY-compatible translation layer is put in place. When sent over SPDY, HTTP requests are processed, tokenized, simplified and compressed. For example, each SPDY endpoint keeps track of which headers have been sent in past requests and can avoid resending the headers that have not changed; those that must be sent are compressed.
The server push mechanism pushes content regardless of existing cache which can result in waste of bandwidth. The workaround is to use the server hint mechanism.
SPDY is a versioned protocol. In its control frames there are 15 dedicated bits to indicate the version of the session protocol.
- Version 1: version 1 of the SPDY protocol is not used anymore.
- Version 2: soon to be discontinued. Nginx supports spdy/2. Firefox 28 and recent versions of Chrome drop support for SPDY version 2. OpenLiteSpeed 1.1 and up support SPDY/2.
- Version 3: SPDY v3 introduced support for flow control, updated the compression dictionary, and removed wasted space from certain frames, along with other minor bug fixes. Firefox supports SPDY v3 in Firefox 15. OpenLiteSpeed 1.1 and up support SPDY/3.
- Version 3.1: SPDY v3.1 introduced support for session-layer flow control, and removed the CREDENTIALS frame (and associated error codes). Firefox 27 has added SPDY 3.1 support. OpenLiteSpeed 1.2.7 introduces SPDY/3.1 support. Nginx 1.5.10 supports SPDY/3.1.
- Version 4.0: SPDY v4 alpha3 is more closely aligned with the HTTP 2.0 draft; it has a new stream flow control and error codes unified with the HTTP 2.0 draft.
Browser support and usage
- Google Chrome/Chromium. SPDY sessions in Chrome can be inspected via the URI:
chrome://net-internals/#events&q=type:SPDY_SESSION%20is:active. There is a command-line switch for Google Chrome (
--enable-websocket-over-spdy) which enables an early, experimental implementation of WebSocket over SPDY. SPDY protocol functionality can be (de)activated by toggling "Enable SPDY/4" setting on local
- Firefox supports SPDY 2 from version 11, and default-enabled since 13 and later. (Also SeaMonkey version 2.8+.) SPDY protocol functionality can be (de)activated by toggling the
about:config. Firefox 15 added support for SPDY 3. Firefox 27 has added SPDY 3.1 support. Firefox 28 has removed support of SPDY 2. about:networking shows if a websites uses SPDY.
- Internet Explorer 11 added support for SPDY version 3, but not for the Windows 7 version. A problem experienced by some users of Windows 8.1 and Internet Explorer 11 is that on initial loading, Google says "Page not found" but on reloading, it is fine. One fix for this is to disable SPDY/3 in Internet Options > Advanced. 
- Amazon's Silk browser for the Kindle Fire uses the SPDY protocol to communicate with their EC2 service for Web page rendering.
Server support and usage
As of April 2013[update], approximately 1% of all websites support SPDY, however there has been a decrease to about 0.6% As of January 2014[update]. Some Google services (e.g. Google search, Gmail, and other SSL-enabled services) use SPDY when available. Google's ads are also served from SPDY-enabled servers.
A brief history of SPDY support amongst major web players:
- In March 2012, Twitter enabled SPDY on its servers, at the time making it the second largest site known to deploy SPDY.
- In March 2012, the open source Jetty Web Server announced support for SPDY in version 7.6.2, while other open source projects were working on implementing support for SPDY, like node.js, Apache (mod_spdy), curl, and nginx.
- In April 2012 Google started providing SPDY packages for Apache servers which led some smaller websites to provide SPDY support.
- In May 2012 F5 Networks announced support for SPDY in its BIG-IP application delivery controllers.
- In June 2012 NGINX, Inc. announced support for SPDY in the open source web server Nginx.
- In July 2012 Facebook announced implementation plans for SPDY. By March 2013 SPDY was implemented by some of their public web servers.
- In August 2012 WordPress.com announced support for SPDY (using nginx) across all their hosted blogs.
- In June 2013, LiteSpeed Technologies announced support for SPDY/2 and SPDY/3 on OpenLiteSpeed, their open source HTTP server. Support for SPDY/3.1 was announced November 2013.
- In January 2014, Synology announced SPDY is included in the new DSM 5.0.
- In February 2014, CloudFlare announced automatic support for SPDY v3.1 for all customers with SSL/TLS certificates.
According to W3Techs most of SPDY-enabled websites use nginx.
- HTTP 2.0
- HTTP pipelining
- HTTP persistent connection
- Microsoft SM
- QUIC, another experimental Google protocol
- MarioNet split web browser, Optimized Protocol for Transport of Images to Client (OPTIC) protocol
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-  DSM 5.0 beta
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- Distribution of web servers among websites that use SPDY
- SPDY Documentation
- SPDY: Google wants to speed up the web by ditching HTTP
- Apache SPDY module
- SPDY Review and Analysis
- SPDY Protocol - RFC draft ietf httpbis http2-00
- Test if a website supports SPDY