Reducing HTTP latency with SPDY

Google unveiled an experimental open source project in early November aimed at reducing web site load times. SPDY, as it is called, is a modification to HTTP designed to target specific, real-world latency issues without altering GET, POST, or any other request semantics, and without requiring changes to page content or network infrastructure. It does this by implementing request prioritization, stream multiplexing, and header compression. Results from tests on a SPDY-enabled Chrome and a SPDY web server show a reduction in load times of up to 60%.

SPDY is part of Google's "Let's make the web faster" initiative that also includes projects targeting JavaScript speed, performance benchmarking, and analysis tools. Mike Belshe and Roberto Peon announced SPDY on November 11 on both the Chromium and Google Research blogs, noting that "HTTP is an elegantly simple protocol that emerged as a web standard in 1996 after a series of experiments. HTTP has served the web incredibly well. We want to continue building on the web's tradition of experimentation and optimization, to further support the evolution of websites and browsers."

Finding the latency in HTTP

The SPDY white paper details the group's analysis of web latency, beginning with the observation that although page requests and responses rely on both HTTP as the application-layer protocol and TCP as the transport-layer protocol, it would be infeasible to implement changes to TCP. Experimenting on HTTP, on the other hand, requires only a compliant browser and server and can be tested on real network conditions.

The group found four factors to be HTTP's biggest sources of latency. First, relying on a single request per HTTP connection makes inefficient use of the TCP channel and forces browsers to open multiple HTTP connections to send requests, adding overhead. Second, the size of uncompressed HTTP headers, which comprise a significant portion of HTTP traffic because of the large number of HTTP requests in a single page. Third, the sending of redundant headers — such as User-Agent and Host — that remain the same for a session. Finally, the exclusive reliance on the client to initiate all HTTP requests, when there are cases where the server knows that related content will be requested, but cannot push it to the client.

SPDY tackles these weaknesses by multiplexing an unlimited number of concurrent streams over a single TCP connection, by allowing the client to assign priorities to HTTP requests in order to avert channel congestion, and by compacting HTTP request and response headers with gzip compression and omitting the redundant transmission of headers. The SPDY draft specification also includes options for servers to initiate content delivery. The available methods are "server push," in which the server initiates transmission of a resource via an X-Associated-Content header, and "server hint," in which the server only suggests related resources to the client with X-Subresources.

In addition, SPDY is designed to run on top of SSL, because the team decided it was wiser to build security into its implementation now than to add it later. Also, because SPDY requires agents to support gzip compression for headers, it compresses the HTTP data with gzip too.

The important thing to note is that SPDY's changes affect only the manner in which data is sent over the wire between the client and the server; there are no changes to the existing HTTP protocol that a web page owner would notice. Thus, SPDY is not a replacement for HTTP so much as a set of possible enhancements to it.

Comments on the blog posts indicate that although most readers see the value in header compression and request prioritization, some are skeptical of the need to multiplex HTTP requests over a single TCP connection. Other alternatives have been tried in the past, notably HTTP pipelining and the Stream Control Transmission Protocol (SCTP).

The white paper addresses both. SCTP, it says, is a transport-layer protocol designed to replace TCP, and although it may offer some improvements, it would not fix the problems with HTTP itself, which SPDY attempts to do. Implementing SCTP would also require large changes to client and server networking stacks and web infrastructure. The latter is also true for similar transport-layer solutions like Structured Stream Transport (SST), intermediate-layer solutions like MUX, and HTTP-replacements like Blocks Extensible Exchange Protocol (BEEP).

The problem with pipelining, it says, is that even when multiple requests are pipelined into one HTTP connection, the entire connection remains first-in-first-out, so a lost packet or delay in processing one request results in the delay of every subsequent request in the pipeline. On top of that, HTTP pipelining is difficult for web proxies to implement, and remains disabled by default in most browsers. The fully multiplexed approach taken by SPDY, however, allows multiple HTTP requests and responses to be interleaved in any order, more efficiently filling the TCP channel. A lost packet would still be retransmitted, but other requests could continue to be filled without pausing to wait for it. A request that requires server-side processing would form a bottleneck in an HTTP pipeline, but SPDY can continue to answer requests for static data over the channel while the server works on the slower request.

Implementation and test results

The development team wrote a SPDY web server and added client support in a branch of the Chrome browser, then ran tests serving up "top 100" web site content over simulated DSL and cable home Internet connections. The test included SSL and non-SSL runs, single-domain and multiple-domain runs, and server push and server hint runs. The resulting page load times were smaller in every case, ranging from 27.93% to 63.53% lower.

The team's stated goal is a 50% reduction in load time; the average of the published tests in all of their variations is 48.76%. Though it calls the initial results promising, the team also lists several problems — starting with the lack of well-understood models for real world packet loss behavior.

SPDY remains an experiment, however, and the team solicits input on a number of open questions, including dealing with the latency introduced by SSL handshakes, recovering from a lost TCP connection, and how best to implement the server-side logic to truly take advantage of server push and server hint. Interested people are encouraged to join the mailing list and download the code.

So far, only the modified Chrome client code is available, and that from the public Subversion repository, not binary downloads. Peon said that the server release is coming soon, and the project page says that the test suite and benchmarking code used in Google's test will be released under an open source license as well.

A 50% reduction in page load times is nothing to sneer at, particularly when all of the gains come from tweaking HTTP's connection and data transfer behavior. Header compression alone gives noticeable savings; the white paper states that it resulted in an "~88% reduction in the size of request headers and an ~85% reduction in the size of response headers." The future of the web may indeed include new protocols like SCTP and BEEP, but SPDY is already demonstrating that there is plenty of room for improvement without drastically altering the protocol stack.