figure 1. sPDY gateway translates between sPDY-capable clients and conventional httP servers.

situated on high-speed
internet backbone
httP
nytimes.com
sPd Y gateway
httP
facebook.com
sPdY
httP
netflix.com

figure 2. timeline of sPDY-related development milestones.

SPDY
implemented
in Google Chrome
browser
Google begins
serving Search,
Gmail, Maps,
and other major
services over
SPDY to
compatible
Amazon
announces
Kindle Fire tablet
using SPDY
between client
devices and
Amazon servers
to accelerate
browsing

Open-source iOS
SPDY client library
commissioned by
SPDY author
Mike Belshe

worldwide to deploy SPDY gateways on the high-speed Internet. Content-delivery networks have begun offering SPDY gateway services to Web site owners as a means of accelerating the performance (as experienced by users) of their HTTP Web sites. 1, 26 Vendors of mobile devices might deploy SPDY gateways to accelerate the notoriously slow high-latency mobile browsing experience, a marketable feature. Even more intriguing are the incentives for large Web companies to deploy open (publicly available) SPDY gateways to collect and mine rich information about users’ Web-browsing behavior, a lucrative commodity in the business of online advertising. Interestingly, the SPDY gateway’s ability to aggregate certain critical resources may provide benefits above and beyond regular SPDY, as described later.

SPDY implemented
in Google Android
Honeycomb browser
SPDY
announced as
“early-stage
research
project”
SPDY
implemented in
Mozilla Firefox
browser

Nov.2009

Sept.2010

Feb. 2011

Feb. 2012

Sept. 2011

M ar. 2012

November 2009 post6 on the Google
Chromium blog ( http://blog.chro-
mium.org) describing the protocol as
“an early-stage research project,” part
of Google’s effort to “make the Web
faster.” By September 2010, SPDY
had been implemented in the stable
version of the Google Chrome brows-
er, 14 and by February 2011, Google
quietly flipped the server-side switch
on SPDY, using the protocol to serve
major services (such as Gmail, Maps,
and search) to Chrome and other
SPDY-enabled clients. 4 In February
2011, the Android Honeycomb mo-
bile Web browser received client-side
SPDY support, also with little public-
ity. 13 In September 2011, Amazon an-
nounced its Kindle Fire tablet, along
with the Silk Web browser that speaks
SPDY to Amazon cloud servers. 3 In
January 2012, Mike Belshe, SPDY co-
author, commissioned development
of an open-source Apple iOS SPDY cli-
ent library. 20 In March 2012, Firefox
11 implemented the SPDY protocol, 19
which, by June 2012, was enabled by
default in Firefox 13, 22 bringing com-
bined client-side support (Chrome +
Firefox) to approximately 50% of the
desktop browser market25 (see Figure
3). SPDY is currently an Internet En-
gineering Task Force (IETF) Internet
draft in its third revision. 7

sPDY Protocol

SPDY is designed primarily to address performance inhibitors inherent in HTTP, including HTTP’s poor support for pipelining and prioritization, the inability to send compressed headers, and lack of resource push capabilities. SPDY’s hallmark features—request multiplexing/prioritization, header compression, and server push—are described in the following sections:

Request multiplexing and prioritization. SPD Y multiplexes requests and responses over a single TCP connection in independent streams, with request multiplexing inspired by HTTP pipelining while removing several limitations. HTTP pipelining allows multiple HTTP requests to be sent over a TCP connection without waiting for corresponding responses (see Figure 4). Though pipelining has been specified since the 1999 HTTP 1. 1 RFC, 11 Opera is the only browser that both implements the feature and enables it by default. Other major browsers either do not implement pipelining or disable pipelining by default, as compatibility with older Web proxies and servers is problematic. Besides a lack of widespread adoption, HTTP pipelining also suffers from head-of-line blocking, as the specification mandates resources be returned in the order they are requested, meaning a large resource, or one associated with a time-consuming