| 3.1 | Network Services | ||
| 3.1.4 | Connection services and the OSI Model |
| After you have identified and corrected any physical or data link
layer troubleshooting targets, you can proceed to troubleshoot at
the higher layers.
Failures at the lower layers of the OSI model tend to propagate to the higher layers. For example, a bad Ethernet data link affects the connected router Ethernet interface. This, in turn, affects the TCP/IP routing tables, in turn affecting the connection made by an application. Sometimes there can be multiple potential causes for a network problem, especially where there are multiple technologies, protocols, and data-link hops that the application uses. At the network layer are two types of protocols. Routed protocols send user data between hosts. Routing protocols communicate information between routers about the network paths to use. The connection sequences that protocol suites use involve both of these types of protocols as well as higher-layer protocols within the protocol suites. In the next section, we examine the TCP/IP connection sequence.
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