Understanding the OSI Model - N10-008 CompTIA Network+ : 1.1 — Transcript

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1. 0:02if you work for any amount of time in
2. 0:04information technology you'll certainly
3. 0:06hear someone refer to a layer of the osi
4. 0:09model this stands for the open systems
5. 0:12interconnection reference model and it's
6. 0:14a way to describe the way that traffic
7. 0:17is moving from one part of the network
8. 0:19to another as the name implies the osi
9. 0:22model is a model it's a guideline it's a
10. 0:25very broad way to describe the way that
11. 0:28data moves across the network
12. 0:30and it allows us to communicate with
13. 0:32other it professionals in a way that we
14. 0:35both can understand exactly what we're
15. 0:37talking about if you do any type of
16. 0:40research into the osi model you may see
17. 0:42references to the osi protocol suite the
18. 0:46osi protocols themselves didn't catch on
19. 0:48and in fact today we use primarily tcpip
20. 0:52but the osi model which was built around
21. 0:55those osi protocols continues to be used
22. 0:58as a reference when we talk to other it
23. 1:01professionals the osi model consists of
24. 1:03these seven layers you can see them
25. 1:05represented here on the left side of the
26. 1:07screen and at each layer of the model is
27. 1:10a set of protocols and these protocols
28. 1:12may be very different depending on the
29. 1:14type of traffic going over your network
30. 1:17you'll use this osi model for the rest
31. 1:19of your career it's a common language
32. 1:21that we can use within it to help
33. 1:24describe how data is progressing from
34. 1:27the very beginning of the traffic flow
35. 1:29to the very end
36. 1:30one common way to memorize these
37. 1:33different layers of the osi model is to
38. 1:35start at the top with the application
39. 1:37layer through the presentation session
40. 1:40transport network data link and physical
41. 1:43layers one common way to remember the
42. 1:45different layers of the osi model is to
43. 1:48remember the mnemonic all people seem to
44. 1:51need data processing and those initials
45. 1:54for each one of those words correspond
46. 1:56to application presentation session
47. 1:59transport network data link and physical
48. 2:04let's start our talk about the osi model
49. 2:06at the very bottom this is layer one or
50. 2:09the physical layer of the osi model and
51. 2:12although we're talking about the
52. 2:14physical part of the network a lot of
53. 2:16what happens at this layer one is really
54. 2:19about the signaling or the signal that's
55. 2:22going over the network connections for
56. 2:24layer one we're talking about the cables
57. 2:26that you're using the fiber runs or just
58. 2:28the ability to get signal from one part
59. 2:31of the network to the other if someone
60. 2:33tells us that we're having a physical
61. 2:34layer problem then we'll want to look at
62. 2:36the cabling that we're using make sure
63. 2:38that all of our punch down connections
64. 2:40and connectors are working the way they
65. 2:42should we may want to run loopback tests
66. 2:44we may want to check or replace the
67. 2:46cables that we're using or perhaps even
68. 2:48swap the adapter cards to make sure that
69. 2:50we can resolve these layer 1 problems
70. 2:54the next layer of the osi model is layer
71. 2:562 or the data link layer this is the
72. 2:59layer that is a foundational layer for
73. 3:01the protocols that we will then begin to
74. 3:03stack on top of layer 2. you'll find
75. 3:06that layer 2 is sometimes referred to as
76. 3:08the data link control or dlc layer and
77. 3:11there will be a series of protocols that
78. 3:13run as dlc protocols a good example of
79. 3:17this is on ethernet networks where we
80. 3:19use the media access control addresses
81. 3:22or mac addresses we sometimes refer to
82. 3:25those as layer 2 addresses because they
83. 3:28correlate back to the data link control
84. 3:30protocols and since switches make their
85. 3:33forwarding decisions based on these mac
86. 3:35addresses we sometimes will refer to
87. 3:37layer 2 as the switching layer
88. 3:40this image describes communication at
89. 3:42layer 2. you can see the mac address of
90. 3:44this network interface card and the mac
91. 3:46address of this network interface card
92. 3:49and any communication between those two
93. 3:51that is using that mac address we can
94. 3:53refer to as a layer 2 communication
95. 3:57if we move up one layer in the osi model
96. 3:59where layer three or the network layer
97. 4:01sometimes you'll hear this referred to
98. 4:03as the routing layer because this is the
99. 4:05layer associated with ip addresses ip
100. 4:08addresses are obviously very common on
101. 4:10our networks and any device that is
102. 4:13making forwarding decisions based on
103. 4:15these ip addresses is communicating at
104. 4:17layer 3.
105. 4:19this is also the layer where frames will
106. 4:21be fragmented or broken into smaller
107. 4:23pieces in order to move those between
108. 4:26different types of networks so if you
109. 4:28move between an ethernet network off to
110. 4:30a wan network and then back to ethernet
111. 4:33you may find that the data is being
112. 4:34fragmented and that fragmentation is
113. 4:36occurring at layer 3.
114. 4:39layer 4 of the osi model is the
115. 4:41transport layer sometimes you'll hear
116. 4:43this referred to as the post office
117. 4:44layer because this is the layer that
118. 4:46describes how data is being delivered
119. 4:49and where it is being delivered into a
120. 4:51system the protocols used at layer 4 are
121. 4:54protocols such as tcp that is the
122. 4:56transmission control protocol and udp
123. 4:59which is the user datagram protocol this
124. 5:02layer is used when you're accessing a
125. 5:04web page but the web page itself is so
126. 5:06large that you can't send all of the
127. 5:08data across the network in one single
128. 5:10frame instead you have to split it up
129. 5:12into separate frames and send those
130. 5:15individual pieces across the network
131. 5:17where they are put back together on the
132. 5:19other side
133. 5:20layer 5 of the osi model or the session
134. 5:23layer is designed to start and stop
135. 5:26communication between one endpoint and
136. 5:29another this is where you would use
137. 5:30control protocols or tunneling protocols
138. 5:33in order to begin the communication of
139. 5:35data between one device and another for
140. 5:38example one device may ask in a browser
141. 5:40can we talk and it's communicating that
142. 5:42information to google which will then
143. 5:44set up a session and begin transferring
144. 5:46information
145. 5:48osi layer 6 is the presentation layer
146. 5:51and just before we're able to view
147. 5:52information on our screen the
148. 5:54presentation layer needs to take this
149. 5:56data and put it into a form that we can
150. 5:58understand anything dealing with the
151. 6:00encoding of characters or encryption of
152. 6:03application data will occur at layer 6.
153. 6:06this layer is often combined with the
154. 6:09application layer at layer 7 because the
155. 6:11functionality is so closely associated
156. 6:14with our ability to use these
157. 6:16applications
158. 6:17layer 7 of the osi model is the
159. 6:19application layer and this is the layer
160. 6:22that we get to see as humans using a
161. 6:25computer we go to a browser screen and
162. 6:27start a browser session and this
163. 6:29information that we're seeing on the
164. 6:31screen is the layer 7 application data
165. 6:34if you're transferring a file with ftp
166. 6:36or performing a name resolution with dns
167. 6:39or simply using a browser with http or
168. 6:42https you're using an application layer
169. 6:46protocol let's now take these very broad
170. 6:49concepts with the osi model and try to
171. 6:52associate them back to practical real
172. 6:54world examples let's look first at layer
173. 6:571 or the physical layer if you're
174. 6:59holding a cable or you're looking at an
175. 7:01analysis of signal going across the
176. 7:03network then you're working at the
177. 7:05physical layer of the osi model if
178. 7:07you're working with switch forwarding or
179. 7:09you're examining the mac address of a
180. 7:11device then you're working at layer 2 or
181. 7:14the data link layer moving up to layer 3
182. 7:17or the network layer we're now working
183. 7:19with ip addresses so if you're working
184. 7:21with routers that make forwarding
185. 7:22decisions based on ip address then
186. 7:24you're operating at layer 3.
187. 7:27layer 4 or the transport layer deals
188. 7:30with udp and tcp protocols and the port
189. 7:33numbers associated with tcp and udp
190. 7:36layer 5 or the session layer is our
191. 7:39control layer we're tunneling
192. 7:40information or setting up communication
193. 7:43between one device and another layer six
194. 7:46of the osi model or the presentation
195. 7:48layer is commonly associated with
196. 7:49encryption if we're communicating to a
197. 7:51web browser over an https or encrypted
198. 7:55communication then we're working at osi
199. 7:57layer 6. and lastly the web browser that
200. 8:00we're viewing and the information that's
201. 8:02on our screen is able to be shown to us
202. 8:05thanks to osi layer 7 or the application
203. 8:08layer
204. 8:10another practical view of the osi model
205. 8:12can be seen in a packet capture
206. 8:14application in this case i'm using
207. 8:17wireshark to capture data going across
208. 8:19the network and there are three windows
209. 8:21that you can see in this wireshark
210. 8:23communication the view at the top is the
211. 8:25summary view each line that's within
212. 8:27that summary view is showing us one
213. 8:29frame that's going across the network at
214. 8:32the bottom of the screen is a
215. 8:33hexadecimal and ascii representation of
216. 8:35that data but where we want to focus is
217. 8:38the middle window which is the detailed
218. 8:41window in this detail window we can see
219. 8:43each part of this communication broken
220. 8:46out into the separate osi layers for a
221. 8:49single frame for example we'll look at
222. 8:51this frame number 88 that's currently
223. 8:53selected frame 88 shows us that it's
224. 8:56using 2005 bytes on the wire and that is
225. 8:59describing the traffic that is being
226. 9:01received at layer 1 or the physical
227. 9:04layer just underneath that is a line of
228. 9:06information showing this is an ethernet
229. 9:082 frame it shows a source mac address
230. 9:11and a destination mac address which
231. 9:13means that it's referring to layer 2 or
232. 9:16the data link layer to see layer 3
233. 9:18information we go to the next line which
234. 9:20is showing us internet protocol or ip
235. 9:23and we know if there are ip addresses
236. 9:25such as the ones listed here then we
237. 9:26must be working at layer 3 or the
238. 9:29network layer
239. 9:30the next line underneath the layer 3
240. 9:32information is the layer 4 information
241. 9:35in this case it's the tcp protocol using
242. 9:37a port number of
243. 9:3918429 as the source and the destination
244. 9:42port is the https protocol using port
245. 9:46443 all of those tcp communications are
246. 9:49occurring at layer 4 or the transport
247. 9:52layer
248. 9:53not uncommonly you'll often see layers 5
249. 9:566 and 7 grouped together and in fact in
250. 9:58wireshark we see it grouped together
251. 10:00here as well you can see there is the
252. 10:02secure sockets layer which is the final
253. 10:05line in this detailed view and that
254. 10:07particular secure sockets layer details
255. 10:10the session presentation and application
256. 10:12layer traffic traversing the network
257. 10:16each time you use an application or send
258. 10:18traffic over the network you can
259. 10:20describe this communication in the
260. 10:22context of the osi model let's take for
261. 10:25example an application like google mail
262. 10:27we'll log into google mail at
263. 10:29mail.google.com
264. 10:30and as we are using that application on
265. 10:33our screen we're using osi layer 7 or
266. 10:36the application layer if we're sending
267. 10:39information back and forth to our
268. 10:40browser everything being communicated to
269. 10:43this mail server at google is being
270. 10:45encrypted
271. 10:46and it's being encrypted thanks to the
272. 10:48presentation layer at layer 6 which is
273. 10:50using ssl encryption
274. 10:52layer 5 is the session layer which is
275. 10:54linking together everything above at the
276. 10:57presentation and application layer to
277. 10:59all of the traffic below that will be
278. 11:01traversing the network and just below
279. 11:03that layer 5 is the layer 4 transport
280. 11:06information we know that this
281. 11:08communication is probably going to use
282. 11:10tcp 443 which means that we'll be using
283. 11:13the transport layer to send that tcp
284. 11:16data
285. 11:16there's of course ip addresses that are
286. 11:18used between your workstation and the
287. 11:20google email server and all of that
288. 11:23communication is occurring thanks to the
289. 11:25ip encapsulation associated with layer 3
290. 11:28or the network layer
291. 11:30all of that tcp and ip traffic is being
292. 11:33encapsulated within ethernet frames and
293. 11:36those ethernet frames are being sent
294. 11:37over the network thanks to layer 2 or
295. 11:40the data link layer and lastly the
296. 11:42physical signals to be able to send
297. 11:44these frames from one device to another
298. 11:47are occurring at layer 1 or the physical
299. 11:50layer
300. 11:51by separating the network communication
301. 11:53into this well-known series of layers
302. 11:55you're able to communicate with other it
303. 11:57professionals when you're
304. 11:59troubleshooting examining applications
305. 12:01or trying to get an understanding of how
306. 12:03applications may be working over your
307. 12:05network
308. 12:16you