WEBVTT 00:00:00.940 --> 00:00:05.570 Network communications have required a lot of thought and engineering, 00:00:05.740 --> 00:00:10.680 and how can we send data between two different systems and ensure that 00:00:10.680 --> 00:00:16.680 it's properly understood? When we talk as people we use analog types of 00:00:16.680 --> 00:00:22.740 communications. Our voice varies by frequency and amplitude, frequency 00:00:22.740 --> 00:00:27.500 being then the number of cycles per second, and amplitude being the 00:00:27.500 --> 00:00:28.890 strength of the signal. 00:00:29.480 --> 00:00:34.590 But when we talk in a computer, we're using digital signals, 00:00:34.600 --> 00:00:39.340 which means there's only two values, 1 or 0, for the most part. 00:00:40.000 --> 00:00:44.340 So, if I'm going to send a million 1s and 0s to a 00:00:44.340 --> 00:00:47.240 destination device in the next few seconds, 00:00:48.130 --> 00:00:51.910 that destination device needs to know, what does that 1 00:00:51.910 --> 00:00:57.500 mean in this place? So that is why they developed models 00:00:57.500 --> 00:00:59.860 and protocols for communication. 00:01:00.910 --> 00:01:05.140 The first of these that was really developed as the international standard 00:01:05.150 --> 00:01:10.830 was the ISO 7498, the Open Systems Interconnect Model. 00:01:11.810 --> 00:01:17.240 It also had a ‑2, or a second part, to the standard, which talked 00:01:17.240 --> 00:01:21.170 about how to put security into the OSI Model. 00:01:21.940 --> 00:01:27.130 We don't actually really use the OSI standard for communication. 00:01:27.140 --> 00:01:32.580 Instead, we use it as a reference to describe how communications work. 00:01:32.960 --> 00:01:34.540 So let's take a look at that. 00:01:35.510 --> 00:01:40.040 There are seven layers to the OSI stack, as we call it. Starting 00:01:40.040 --> 00:01:43.390 at the top with the Application, Presentation, 00:01:43.390 --> 00:01:48.210 Session, Transport, Network, Data Link, and Physical. 00:01:49.010 --> 00:01:55.470 The application layer is used to interface between the user's 00:01:55.470 --> 00:02:00.680 application and the communication system. Let's say a user writes an 00:02:00.690 --> 00:02:04.640 email using some type of email application. 00:02:05.020 --> 00:02:08.979 So that user application now has the content of that 00:02:08.979 --> 00:02:14.380 email, and the user hits Send. They want to send this data 00:02:14.390 --> 00:02:17.130 across to a destination device. 00:02:18.170 --> 00:02:21.890 The data itself contains the content of the email. 00:02:23.260 --> 00:02:25.900 When this Send button is pressed, 00:02:26.520 --> 00:02:32.560 this piece of data passes down to the application layer of the OSI stack. 00:02:33.160 --> 00:02:34.930 The application layer says, 00:02:34.930 --> 00:02:41.870 well, eventually this email is going to get to a destination device, 00:02:42.310 --> 00:02:46.380 and that destination device will need to know what to do with it, so 00:02:46.380 --> 00:02:48.610 I'll put a header on there, then I'll say, 00:02:48.610 --> 00:02:50.160 hey, this is an email. 00:02:50.490 --> 00:02:54.300 So when it's received by the application layer at the far end, 00:02:54.400 --> 00:03:00.170 it knows to pass it to the destination email's application, 00:03:00.280 --> 00:03:05.700 not to their web browser or their FTP server, or some other communication. 00:03:06.560 --> 00:03:11.010 So this header that is put on by the application layer is 00:03:11.010 --> 00:03:14.750 intended for the application layer at the far end. 00:03:15.810 --> 00:03:20.990 It's kind of an interesting thing to point out here, that the application 00:03:20.990 --> 00:03:24.470 layer of the OSI stack is not really the application, 00:03:24.510 --> 00:03:29.360 it's the interface to the application that the user is using. 00:03:30.430 --> 00:03:32.900 So the purpose of this header, as we said, 00:03:33.080 --> 00:03:38.650 is so that when the sender uses their user application to send traffic 00:03:38.660 --> 00:03:43.220 down through the application, presentation, and so on, down finally to 00:03:43.220 --> 00:03:49.360 the physical layer, it traverses across some type of physical media, 00:03:49.480 --> 00:03:54.380 and that data now reaches the receiver's machine. The receiver's 00:03:54.380 --> 00:03:59.960 machine then passes the data back up those seven layers of the OSI 00:03:59.960 --> 00:04:03.040 stack, and of course, as we said, 00:04:03.040 --> 00:04:07.220 in order for the application layer on the receiver's machine to know 00:04:07.220 --> 00:04:13.360 what to do with it, it then reads the header that was provided by its 00:04:13.360 --> 00:04:18.600 corresponding or peer level on the sender's machine, and it knows to 00:04:18.600 --> 00:04:21.550 send it to the correct user application. 00:04:22.280 --> 00:04:25.680 If you looked at the traffic going over the physical media, 00:04:25.710 --> 00:04:31.400 you would see the application header that indicates it's an email, and the data, 00:04:31.400 --> 00:04:33.820 the content, of the email itself. 00:04:34.790 --> 00:04:39.710 We can actually encrypt emails, which would mean we'd encrypt that data 00:04:39.710 --> 00:04:44.430 that's being sent, maybe using products like PGP or S/MIME, 00:04:44.430 --> 00:04:49.150 for example. When we go down to the presentation layer, 00:04:49.360 --> 00:04:52.830 this is where we deal with things like formatting and compression. 00:04:53.220 --> 00:04:57.580 If I'm going to compress some type of traffic to save space, 00:04:58.330 --> 00:05:03.240 then, I will use the presentation layer to do that. I'll put a 00:05:03.240 --> 00:05:07.430 header on there, the presentation header, that will tell the 00:05:07.430 --> 00:05:10.290 presentation layer at the far end, oh, by the way, 00:05:10.490 --> 00:05:14.730 this is compressed so you need to expand it back before you give it to the 00:05:14.730 --> 00:05:19.910 application layer or it won't know what to do with it. At the session 00:05:19.910 --> 00:05:23.160 layer, this is where we log in to a remote device. 00:05:23.860 --> 00:05:26.000 This is where we do session management. 00:05:26.150 --> 00:05:28.890 So if you're doing something like online banking, 00:05:28.930 --> 00:05:33.670 we put in here some type of indication which session this 00:05:33.670 --> 00:05:37.260 communication belongs to, so your traffic goes to your 00:05:37.260 --> 00:05:39.240 session and not to somebody else's. 00:05:39.760 --> 00:05:43.970 And here we see a number of different protocols used, things like 00:05:43.980 --> 00:05:48.550 Layer 2 Tunneling Protocol, or Point‑to‑Point Tunneling Protocol, 00:05:48.800 --> 00:05:50.900 that help us manage the session. 00:05:51.910 --> 00:05:57.280 We can also encrypt at this layer using things like Secure Shell.