SPDY (pronounced speedy) is an open networking protocol developed primarily at Google for transporting web content. As of July 2012[update], the group developing SPDY has stated publicly that it is working toward standardization (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. Implementations of SPDY exist in Chromium, Mozilla Firefox, Opera, Amazon Silk, and Internet Explorer. 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.
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.
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: As of April 2012[update], Version 2 is current, with version 3 being tested in Chrome version 19 and higher, with the aim of replacing Version 2 in Chrome 22.
- 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.
- Version 3.1: SPDY v3.1 introduced support for session-layer flow control, and removed the CREDENTIALS frame (and associated error codes).  Firefox 27 will add SPDY 3.1 support, and retire SPDY/2 support. 
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 included in 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 version 3 of this protocol.
- Internet Explorer 11 added support for SPDY version 3, but not for the Windows 7 version.
- 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. 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.
- Cloudflare is also providing a beta of SPDY on their servers (using Nginx) from June 2012, though users who would like to use/test it must be paying customers as SPDY is built on top of TLS, and only paying customers can use SSL/TLS Certificates.
- In June 2012, LiteSpeed Technologies announced support for SPDY on OpenLiteSpeed, their open source HTTP server.
- 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.
- 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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- Chromium: SPDY proxy examples.
- List of Chromium Command Line Switches.
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- Opera 12.10 Changelog
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- Usage of SPDY for websites.
- spdy-dev mailing list: SPDY on Google servers?.
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- 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