White paper, SPDY: An experimental protocol for a faster web
14 Nov 2009
Executive summary
As part of the "Let's make the web faster" initiative, we are experimenting with alternative protocols to help reduce the latency of web pages. One of these experiments is SPDY (pronounced "SPeeDY"), an application-layer protocol for transporting content over the web, designed specifically for minimal latency. In addition to a specification of the protocol, we have developed a SPDY-enabled Google Chrome browser and open-source web server. In lab tests, we have compared the performance of these applications over HTTP and SPDY, and have observed up to 64% reductions in page load times in SPDY. We hope to engage the open source community to contribute ideas, feedback, code, and test results, to make SPDY the next-generation application protocol for a faster web.
Background: web protocols and web latency
Today, HTTP and TCP are the protocols of the web. TCP is the generic, reliable transport protocol, providing guaranteed delivery, duplicate suppression, in-order delivery, flow control, congestion avoidance and other transport features. HTTP is the application level protocol providing basic request/response semantics. While we believe that there may be opportunities to improve latency at the transport layer, our initial investigations have focussed on the application layer, HTTP.
Unfortunately, HTTP was not particularly designed for latency. Furthermore, the web pages transmitted today are significantly different from web pages 10 years ago such and demand improvements to HTTP that could not have been anticipated when HTTP was developed. The following are some of the features of HTTP that inhibit optimal performance:
- Single request per connection. Because HTTP can only fetch one resource at a time (HTTP pipelining helps, but still enforces only a FIFO queue), a server delay of 500 ms prevents reuse of the TCP channel for additional requests. Browsers work around this problem by using multiple connections. Since 2008, most browsers have finally moved from 2 connections per domain to 6.
- Exclusively client-initiated requests. In HTTP, only the client can initiate a request. Even if the server knows the client needs a resource, it has no mechanism to inform the client and must instead wait to receive a request for the resource from the client.
- Uncompressed request and response headers. Request headers today vary in size from ~200 bytes to over 2KB. As applications use more cookies and user agents expand features, typical header sizes of 700-800 bytes is common. For modems or ADSL connections, in which the uplink bandwidth is fairly low, this latency can be significant. Reducing the data in headers could directly improve the serialization latency to send requests.
- Redundant headers. In addition, several headers are repeatedly sent across requests on the same channel. However, headers such as the User-Agent, Host, and Accept* are generally static and do not need to be resent.
- Optional data compression. HTTP uses optional compression encodings for data. Content should always be sent in a compressed format.
Previous approaches
SPDY is not the only research to make HTTP faster. There have been other proposed solutions to web latency, mostly at the level of the transport or session layer:
- Stream Control Transmission Protocol (SCTP) -- a transport-layer protocol to replace TCP, which provides multiplexed streams and stream-aware congestion control.
- HTTP over SCTP -- a proposal for running HTTP over SCTP. Comparison of HTTP Over SCTP and TCP in High Delay Networks describes a research study comparing the performance over both transport protocols.
- Structured Stream Transport (SST) -- a protocol which invents "structured streams": lightweight, independent streams to be carried over a common transport. It replaces TCP or runs on top of UDP.
- MUX and SMUX -- intermediate-layer protocols (in between the transport and application layers) that provide multiplexing of streams. They were proposed years ago at the same time as HTTP/1.1.
These proposals offer solutions to some of the web's latency problems, but not all. The problems inherent in HTTP (compression, prioritization, etc.) should still be fixed, regardless of the underlying transport protocol. In any case, in practical terms, changing the transport is very difficult to deploy. Instead, we believe that there is much low-hanging fruit to be gotten by addressing the shortcomings at the application layer. Such an approach requires minimal changes to existing infrastructure, and (we think) can yield significant performance gains.
Goals for SPDY
The SPDY project defines and implements an application-layer protocol for the web which greatly reduces latency. The high-level goals for SPDY are:
- To target a 50% reduction in page load time. Our preliminary results have come close to this target (see below).
- To minimize deployment complexity. SPDY uses TCP as the underlying transport layer, so requires no changes to existing networking infrastructure.
- To avoid the need for any changes to content by website authors. The only changes required to support SPDY are in the client user agent and web server applications.
- To bring together like-minded parties interested in exploring protocols as a way of solving the latency problem. We hope to develop this new protocol in partnership with the open-source community and industry specialists.
