own the so-called stack from layer to layer, additional information is added at each step of the way, as illustrated by the series of envelopes. In sum- mary then, the message to be transported is handed to layer seven, which performs Application Layer functions and then attaches a header to the beginning of the message that explains the functions performed by that layer SO that the receiver can interpret the message correctly. In our illustration, that header function is represented by information written on the envelope at each layer. When the receiving device is finally handed the message at the Physical Layer, each succeeding layer must open its own envelope until the kernel-the message-is exposed for the receiving application. Thus, OSI protocols really do work like a nested Russian doll. After peeling back layer after layer of the network onion, the core message is exposed. Let's now go back to our e-mail example, but this time we'll describe it within the detailed context of OSI's layered architecture. We begin with a lesson on linguistics. Esperanto There is an old and somewhat comforting cliché, which observes, "Wher- ever one goes, people speak English." In fact, less than ten percent of the world's population speaks English and, to their credit, many of them speak it as a second language. Many believe there is a real need for a 1There is an old joke among seasoned international travelers that goes likes this: "What do you call someone who speaks three languages?" Trilingual. "OK, what do you call someone who speaks two languages?" Bilingual. "OK, what do you call someone who speaks one lan- guage?" American. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols truly international language. In 1887, Polish physician Ludwig L. Zamenhof published a paper on the need for a universally spoken tongue. He believed that mos