Communications - December 2012

back-end process, delays all other resources (see Figure 5).

SPDY implements pipelining without HTTP’s head-of-line blocking limitation. Resources transferred through a SPDY connection are carried in annotated “streams,” or independent sequences of bidirectional data divided into frames; annotated streams allow SPDY to not only return resources in any order but interleave resources over a single TCP connection7 (see Figure 6).

SPDY also includes request prioritization, allowing the client to specify that certain resources be returned with a higher priority than others. Unlike many quality-of-service mechanisms that work on prioritizing packets in queues at lower layers in the network stack, SPDY prioritization works at the application layer, designed to allow the client to specify what is important. One use of prioritization is to request that resources that block progressive page rendering (such as cascading style sheets and JavaScript) be returned with higher priority. Another use of prioritization is to increase the priority of resources being downloaded for the currently visible browser tab while decreasing priority of resources belonging to a currently loading but hidden tab. A further implication of implementing priority at the application layer is that a server can, at least theoretically, prioritize not only the order in which resources are transmitted over the wire but also the order in which resources are generated on the back-end if the task is time intensive.

Header compression. HTTP requests
and responses all include a set of HTTP
headers that provide additional infor-
mation about the request or response
(see Figure 7). There is significant re-
dundancy in these headers across re-
quests and responses; for example,
the “User-Agent” header describing
the user’s browser (such as Mozilla/5.0
compatible, MSIE 9.0, Windows NT
6. 1, WOW64, and Trident/5.0 for In-
ternet Explorer 9) is sent to the server
many times over. Likewise, cookie
headers, which describe state infor-
mation about the client, are repeated
many times over across requests. Such
redundancy means HT TP headers tend
to compress relatively effectively. To
further improve compression, SPDY
seeds an out-of-band compression dic-
tionary based on a priori knowledge of
common HTTP headers. 29

figure 3. Global browser usage share, as recorded by statcounter ( http://gs.statcounter. com/#browser-ww-monthly-201103-201202); in february 2012, chrome had 29.84% and firefox 24.88%.