Doi: 10.1145/1538788.1538806
Article development led by queue.acm.org
These concepts are not generally understood, however, and even when they are, they’re sometimes forgotten.
So, what are the basic principles related to speed of light and network hops that all software developers should be acquainted with? This article answers that question by first working out some quantitative preliminaries with an example, then moving on to the networking implications, and then covering applications. Finally, it provides some rules of thumb to keep in mind as applications and architectures evolve in reaction to new network capabilities and unchanging physics.
(SaaS) has become an attractive option for companies looking to save money and simplify their computing infrastructures. SaaS is an interesting group of techniques for moving computing from the desktop to the cloud. however, as it grows in popularity, engineers should be aware of some of the fundamental limitations they face when developing these kinds of distributed applications—in particular, the finite speed of light.
Consider a company that wants to build a distributed application that does interprocess communication (IPC) over the long haul. The obvious advice is “just say no”—don’t do it. If you’re going far outside your local networking environment, the physics of distance and the speed of light, combined with the delays that come from the Internet’s routing infrastructure, tell us that it will be much too slow.
The speed of light in a vacuum is exactly 299,792,458m/sec. 2 This is as fast as you can move a bit of data, and according to our current understanding of physics, it is a fundamental constraint of the universe in which we live. In fiber, the speed of light is 2. 14 × 108 m/ sec or about 70% of the speed of light in a vacuum. If a fiber were stretched in a straight line from New York to San Francisco, it would be about 4,125km long, and it would take about 19ms ( 4,125 ÷ 214) for light to make the one-way trip. Assuming an 8,250km length of fiber was used, you can just double this time to get an estimate for minimum round-trip time.
At first glance, 19ms might seem like a short time, certainly on a human scale. As computer scientists, however, we are usually concerned with a different time scale, that of the computer. Here we can calculate the 19ms in terms of instructions, the fundamental units of work for computers. As an example, we can use a 2003-vintage single-core machine: the Intel Pentium 4 Extreme Edition, which at a 3.2GHz clock rate was rated at 9,726MIPS: 9,726 × 0.019 is 184 million instructions—sufficient, for example, to search through or sort millions of names.
It is always important to keep in mind the purpose of computer networking is to interconnect computers, and that computers operate on very
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