How to Eat a T-Rex, OSI Style
| The Tyrannosaurus Rex was one of the largest dinosaurs. One website I found re-created a T-rex, at 26 meters long! (See http://www.dinosaurvalley.com/drcdt/wld/.) So, what do dinosaurs have to do with networking? Well, in the same general timeframe that TCP/IP was evolving into a legitimate networking model in the business world, a competing public networking model, called OSI, was being developed by an organization called the International Organization for Standardization (ISO). If TCP/IP is like an elephant, the OSI networking model is more like a T-Rex. In a word, it is huge! It is also complex. In its defense, the ISO wanted to build the end-all networking modela model that, once implemented by all computers on the planet, would allow pervasive communications among all computers from all vendors in all countries everywhere! The short version of the history is this: Most computers in the world use the TCP/IP networking model, and almost none use the OSI model. Although the OSI model might have been the bigger networking model and might have had some great features, it was developed much more slowly than TCP/IP. So, TCP/IP took over the marketplace before OSI could be finished. (For you dinosaur fans, yes, I know the T-Rex was faster than elephants, so the analogy fails here.) Also, you should be aware that ISO still plays an active role in standards development today, working with the IETF, ITU, and other standards bodies. So, although the OSI model lost out to TCP/IP, the organization that created it thrives today. T-Rex Versus the ElephantWhy bother mentioning the OSI model at all? As it turns out, some of the terminology used by the OSI model ends up being used a lot today. In particular, the names of the layers of the OSI model are used to describe other networking models, TCP/IP included. To talk about networking, you need to know the names of the OSI model's seven layers. OSI uses a seven-layer model, instead of the four (or five) layers in the TCP/IP model. Figure 3-10 depicts how the layers in the two models match up. For instance, OSI defines that error recovery between end user computers should be implemented by transport layer protocols. So, the TCP/IP transport layer, which also defines how end user computers do error recovery with TCP, equates to OSI Layer 4. Figure 3-10. OSI Model Versus TCP/IP Model (a.k.a. T-Rex Versus the Elephant)
Knowing the OSI model and the names of the layers is useful because most networking jargon refers to protocols and standards relative to the OSI model. For instance, IP sits at the internetwork layer of TCP/IP. That layer is most closely related to the OSI network layer, or OSI Layer 3. So, using popular networking jargon today, IP would be a network layer protocol. Notice that this term uses OSI's name for the equivalent layer and does not use internetworkthe name of TCP/IP's equivalent layer. Here are some more examples of phrases you might use today, relating to OSI terminology:
If you are new to networking, you will likely never use a network that implements the OSI model. I've been in networking more than 20 yearsI even worked at IBM when they thought that OSI would take over the world of networkingand I've never used a computer with a full implementation of OSI. But, some of the jargon stuck around. So, if you haven't already, you should memorize the layers of the OSI model and how they relate to the TCP/IP model. If you want some tricks to help you remember the first letters of the names of the seven layers, you might benefit from the following list of mnemonic phrases. The first letters in each word of these phrases match the first letters of the OSI layer names:
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EAN: 2147483647
Pages: 173