CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies. Ben Piper

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CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper


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Presentation

       5. Session

       4. Transport

       3. Network

       2. Data Link

       1. Physical

      The seven layers are taught zealously in most introductory networking courses. You may have had them permanently drilled into your head with the help of one or two fun little mnemonics! (My favorite is “All people seem to need data processing.”) As we discuss the functions of the different layers, keep in mind that the layers of the OSI model are arbitrary. They're not written on stone tablets, nor are they the result of a rigorous scientific process that conclusively proved that the perfect network has these seven layers. The ISO arrived at each layer by attempting to group similar network functions together in a layer and then organizing the layers in a hierarchical fashion so that each layer of functions is dependent on the one below it. This led to impressive results in layers 1–4 (the lower layers) and utter confusion in layers 5–7 (the upper layers).

Layer Name Example protocols
1 Physical Thicknet (10BASE5)
Thinnet (10BASE2)
1000BASE-T
T1/E1
2 Data Link IEEE 802.3/Ethernet II (DIX)
Point-to-Point Protocol (PPP)
High-Level Data Link Control (HDLC)
3 Network IPv4
IPv6
4 Transport TCP
UDP

      One thing that has always been clear about the OSI model is that the Application layer includes application data and application protocols. The Hypertext Transfer Protocol (HTTP) is an application protocol that a web browser uses for communicating with web servers. Application data would be an HTTP GET request that the browser sends to a web server. Likewise, the web page that the server sends in response would also be application data. In short, application data is whatever the application sends or receives over the network.

      Incidentally, an application can use more than one protocol. For example, when a web browser uses the Hypertext Transfer Protocol Secure (HTTPS) protocol to send a request to a web server, it's making use of two protocols: HTTP and Transport Layer Security (TLS). Despite the latter's confusing name, both are application protocols.

      For all practical purposes, the upper layers (Session, Presentation, and Application) are one layer: the Application layer. The actual functions of the Session and Presentation layers—things like authentication and negotiating an application protocol—occur in the application anyway. They don't include any network functions and are concerned only with application data and application protocols.

      Making Sense of Layers

      The ISO never clearly defined what a layer is. The closest they came was a circular definition. But we can infer from the OSI reference model what they had in mind.

      

For the curious, the ISO defined a layer as a “subdivision of the OSI architecture, constituted by subsystems of the same rank” (ISO/IEC 7498-1). While it's tautological that “subsystems of the same rank” are conceptually in the same layer, it still doesn't tell us what a layer is.

      The concept of layering comes straight from software development (many of the OSI folks were operating system developers). The idea was that applications would treat the network as a software abstraction, somewhat like a filesystem. A filesystem acts as a layer that sits between the application and physical storage (e.g., disks). When the application needs to store some data, it just sends that data to the filesystem layer, which in turn takes care of the specifics of writing it to disk.

      The OSI folks thought that in the same way that an application can store data on a filesystem without having to know anything about the underlying disks, so could it also send data over a network without requiring any network-specific coding or knowing anything about the network's infrastructure. Each layer would consist of a set of network-related functions implemented by the operating system or some middleware that would sit between the application and the host's physical network interface. Collectively, these layers would handle all the mechanisms of getting the application data onto the network and giving the network enough information to make sure the data got to its destination.

      

You may see some striking similarities between the layers in Figure 1.1 and the so-called TCP/IP or Internet protocol suite model. It and the OSI model are often juxtaposed as competing models. The fact is that the TCP/IP model is just a specific implementation of the OSI model based on the TCP/IP protocol suite.


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