HTTP: The Definitive Guide

Distance Delays

Even if bandwidth isn’t a problem, distance might be. Every network router adds delays to Internet traffic. And even if there are not many routers between client and server, the speed of light alone can cause a significant delay.

The direct distance from Boston to San Francisco is about 2,700 miles. In the very best case, at the speed of light (186,000 miles/sec), a signal could travel from Boston to San Francisco in about 15 milliseconds and complete a round trip in 30 milliseconds.^[2]

Say a web page contains 20 small images, all located on a server in San Francisco. If a client in Boston opens four parallel connections to the server, and keeps the connections alive, the speed of light alone contributes almost 1/4 second (240 msec) to the download time (Figure 7-3). If the server is in Tokyo (6,700 miles from Boston), the delay grows to 600 msec. Moderately complicated web pages can incur several seconds of speed-of-light delays.

Figure 7-3. Speed of light can cause significant delays, even with parallel, keep-alive connections

Placing caches in nearby machine rooms can shrink document travel distance from thousands of miles to tens of yards.

^[2] In reality, signals travel at somewhat less than the speed of light, so distance delays are even worse.