HTTP: The Definitive Guide
by David Gourley, Brian Totty, Marjorie Sayer, Anshu Aggarwal, Sailu Reddy
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.
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Read now
Unlock full access