Let's have a look at debugging tools then so what tools could you use to debug your network at layer one so layer one is the physical layer that means you're looking at things like cables and fibers and radio spectrum so the kind of things you could use might be a cable tester to check out your copper cables for fibre cables you can get a very cheap device which is just a red led laser that you can shine down the fiber and at the other end you can see if the light comes out the other end and you can see if you've got a if the fiber takes the path you expect and if there's a big break in it or not more expensive tools very special tools called an OTDR an optical time delay reflectometer will actually send test pulses of light down the fiber and look at the reflections coming back and time how long they take to come back and by doing that they can tell you exactly where there are faults in your fiber finder breaks badly made joints bad connectors and so on expensive piece of equipment but very very useful one for debugging fibre at layer 1. radio networks at layer 1 you can get things like spectrum analyzers that will tell you how much noise there is on a particular radio channel and help you to track down where those sources of noise are at layer 2 well this is a bit more difficult there aren't very many tools you can use that are layer two um testing tools you can you can send a ping of course you can ping a device that's on the same layer two network as you and check that you get a response and that is obviously testing the network at layer 2 but it doesn't really tell you much else so in particular there's no equivalent to trace route at layer 2. so you can't for example tell whether that packet went through one switch or it went through 10 switches you can't see which path it took so it's actually very hard to debug networks at layer two and this is another good reason for keeping your layer two domains as small as you can because it makes it easier to see what's going on at layer three test tools i'm sure you're familiar with uh ping we'll do uh send an ICMP echo request packet and when it reaches the destination that device should send back an icmp echo reply and then you receive that and you can see how long it took to get there whether it successfully got there or not if it didn't come back successfully it could be because the packet didn't arrive or it could be because the packet coming back was lost but at least you can see whether it got there or not trace route we've already talked about it shows you the sequence of routers that a packet goes through on its way to reach a particular destination there's another tool called mtr which is like a trace route that keeps running continuously so you can see a trace route for multiple passes across the network and it will keep track of the lowest and highest and average round-trip times that are seen and so on so that's useful for monitoring a network on the ongoing to see particularly if it's varying higher layers so debugging your networks at layer 4 and above you're probably going to be looking at things like application log files firewall logs to be able to see what's going on to try and get your applications to report what problems it sees and to see whether you can determine the problem you may be able to do tests at these layers so for example for a web server you could connect to port 80 using just telnet on port 80 and you can send an http request by hand and get a response and also you can do the same with an email you can tell that to port 25 and send smtp by hand and get a response so to do those kind of tests you need to understand how the protocols actually work and how to format a message in http and how to format a message with smtp they're very useful low-level tools and on top of all of that across all of these layers tools that decode packets on the wire like wireshark or t-sharp is the text version of that or even tcp dump it's not as good at decoding packets as wireshark and t-shark but they can show you all the headers all the different layers that are going on so you can see the layer two headers the layer three layer four and work out where the traffic is going if it's not getting somewhere by monitoring the traffic at different points sometimes you can find out what's going on as well so that's in summary so we have the OSI Model of networking layers one to seven these are all real physical things that you will see on your network there are different devices that work at these different levels the different parts of each packet that you see on the network which are related to these levels and the levels work together so that we have the ability to do end-to-end applications across the internet.

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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