OSI Model: A Practical Perspective - Part 2 - Networking Fundamentals - Lesson 2 — Transcript

Full transcript

1. 0:01hello welcome to another video from the
2. 0:02first module of my new course on
3. 0:04networking fundamentals
4. 0:05this module will teach you everything
5. 0:07you need to know to understand how
6. 0:09packets move through the internet
7. 0:11in this video we'll continue our
8. 0:12discussion of the osi model from a
9. 0:14practical perspective
10. 0:16in the first part of this lesson we
11. 0:17unpacked how layer 1 layer 2 and layer 3
12. 0:20contribute to the overall goal of
13. 0:21networking
14. 0:23we discussed that layer 1 the physical
15. 0:24layer was responsible for transporting
16. 0:26ones and zeros across the wire
17. 0:29we then illustrated how if this host has
18. 0:31some data needs to send to this host
19. 0:33it needs to start by adding a layer 3
20. 0:35header which will facilitate carrying
21. 0:37the data from one end to the other
22. 0:39then a new layer 2 header will be added
23. 0:41to carry the data from one hop to the
24. 0:43next
25. 0:44this will continue across the entire
26. 0:46path between these two hosts
27. 0:48if you haven't watched that video go
28. 0:50ahead and pause this video and watch
29. 0:52that one first
30. 0:53there'll be a link in the description
31. 0:54otherwise we'll continue right where we
32. 0:56left off by discussing layer 4 of the
33. 0:58osi model
34. 0:59the transport layer the overall goal of
35. 1:03the transport layer of the osi model
36. 1:05is what i'm going to call service to
37. 1:07service delivery
38. 1:08let me explain what i mean by that here
39. 1:10is a computer
40. 1:12this computer has an ip address and has
41. 1:14a mac address
42. 1:16in a pretty common scenario whoever is
43. 1:18using this computer has a web browser
44. 1:20open and is browsing the web
45. 1:22but maybe at the same time that user is
46. 1:24also running some sort of chat program
47. 1:26something like slack or discord or irc
48. 1:29and at the very same time that user
49. 1:30might
50. 1:31also be running some sort of online game
51. 1:33well
52. 1:34each of those programs is meant to send
53. 1:36and receive data on the wire
54. 1:38all that data will be destined to this
55. 1:40layer 3 header to accomplish end to end
56. 1:42delivery
57. 1:43and this layer 2 headers to accomplish
58. 1:45hop to hop delivery
59. 1:47the question then is how do we make sure
60. 1:50the right program
61. 1:51receives the right packets well that's
62. 1:53where layer 4 comes into play
63. 1:55layer 4 is there to distinguish data
64. 1:57streams it's going to take all the
65. 1:59incoming data
66. 2:00and make sure that the right program
67. 2:02receives the right data
68. 2:05just like layer 3 had an addressing
69. 2:07scheme to do
70. 2:08end-to-end delivery and layer 2 had an
71. 2:10addressing scheme to do
72. 2:12hopped hop delivery layer 4 is also
73. 2:14going to be using its own addressing
74. 2:16scheme to accomplish its goal of service
75. 2:18to service delivery
76. 2:20layer 4's addressing scheme involves
77. 2:22ports
78. 2:23now there are two sets of ports there's
79. 2:25zero to sixty five thousand for tcp
80. 2:28and zero to sixty five thousand for udp
81. 2:31tcp and udp are two different strategies
82. 2:34for distinguishing data streams
83. 2:36tcp favors reliability and udp favors
84. 2:39efficiency
85. 2:41both tcp and udp are simply different
86. 2:43strategies for accomplishing the goal of
87. 2:45layer 4.
88. 2:46their inner workings are both
89. 2:48significant and fascinating but
90. 2:49unfortunately they are outside the scope
91. 2:51of this lesson
92. 2:52they will both be covered in detail in a
93. 2:54later lesson in this course
94. 2:56but the way they work is that every
95. 2:58single program that's expected to
96. 2:59receive
97. 3:00or send data on the wire is going to be
98. 3:02associated with a particular port number
99. 3:04then when data arrives on the wire it's
100. 3:07going to include
101. 3:07a layer 4 header in addition to the
102. 3:10layer 3 and the layer 2 header that
103. 3:11we've already discussed
104. 3:13that layer 4 header will indicate which
105. 3:14particular program should be receiving
106. 3:16that data
107. 3:17and that's what layer four is going to
108. 3:18use to make sure the right program
109. 3:20receives the right
110. 3:21ones and zeros so that's a high level
111. 3:24overview of how layer four distinguishes
112. 3:26data streams
113. 3:28but let's pick it apart in more detail
114. 3:32here is a client and three servers as we
115. 3:34discussed in the first lesson
116. 3:36a server is nothing more than a computer
117. 3:38which is running software which knows
118. 3:40how to respond to specific requests
119. 3:44each of these pieces of software is
120. 3:46assigned a predefined
121. 3:47well-known port number which correlates
122. 3:49to the underlying network
123. 3:51application let's say bank.com is
124. 3:54listening for secure web request
125. 3:56using https which is assigned tcp port
126. 3:59443
127. 4:00and the server for site.com is
128. 4:02responding to general web requests using
129. 4:04http
130. 4:06which by default listens on tcp port 80.
131. 4:09and this chat server is running irc
132. 4:12which stands for internet relay chat
133. 4:14which is an online chat application
134. 4:15which runs on udp port 6667
135. 4:19when the client is making a request to
136. 4:21these servers it's not only making a
137. 4:22request to the ip address it's also
138. 4:25going to make a request to the specific
139. 4:26port in question
140. 4:28then for each request made by the client
141. 4:30the client is going to choose a random
142. 4:32port number to use as the source port
143. 4:34for the connection
144. 4:36so a connection from our client to
145. 4:38site.com would look like this
146. 4:42includes a source port of 9999 that's
147. 4:45the port that the client randomly
148. 4:46selected for this particular connection
149. 4:48the destination of this packet is
150. 4:50destined to the ip address of site.com
151. 4:53going to tcp port 80 which is the http
152. 4:56application
153. 4:57keep in mind that the source and
154. 4:59destination ip addresses are going to
155. 5:01exist in the layer 3 header of the
156. 5:02packet
157. 5:03and the source and destination port are
158. 5:05going to exist in the layer 4 header of
159. 5:07the packet
160. 5:09now this randomly selected source port
161. 5:11is actually very important
162. 5:13it is actually the port the client will
163. 5:15listen to for the response to the
164. 5:17original request
165. 5:19meaning when this server responds to
166. 5:21this web request
167. 5:22that packet will look like this notice
168. 5:25the destination port
169. 5:27is 9999 that's the same port that was
170. 5:30initially selected by the client
171. 5:31randomly on the outbound initial packet
172. 5:35so for all connections there is always a
173. 5:37source port and a destination port that
174. 5:39are involved
175. 5:40the destination port is typically
176. 5:42governed by the application in use
177. 5:44and the source port is randomly selected
178. 5:46by the client
179. 5:47so our connection from the client to
180. 5:49site.com has the following attributes
181. 5:52it is a tcp connection from this ip
182. 5:54address and port
183. 5:55to this ip address and port notice that
184. 5:58in the initial packet
185. 6:00this was the source and this was the
186. 6:02destination
187. 6:03and in the response packet it's the
188. 6:04exact opposite this was the source
189. 6:07and this was the destination
190. 6:10this process occurs for each connection
191. 6:12made by the client
192. 6:13in each case the client is selecting a
193. 6:15new random source port
194. 6:18this makes it so that when all these
195. 6:19servers respond whatever comes back
196. 6:21on port 8888 will be given to the web
197. 6:24browser
198. 6:25and whatever comes back on port 7777
199. 6:28will be given to the irc client
200. 6:30in this way the data streams are kept
201. 6:32isolated from each other
202. 6:34these ports will ensure that the right
203. 6:36application gets the right data
204. 6:40this process also allows the client to
205. 6:42make multiple connections to the same
206. 6:43server
207. 6:44consider when you are browsing the
208. 6:46internet i imagine you've often had
209. 6:48multiple tabs open to the same website
210. 6:50well the reason each of those browser
211. 6:51tabs don't accidentally display the data
212. 6:53from another tab
213. 6:55is that each time you open a new tab
214. 6:57your client meaning your web browser
215. 6:59generates a new random source port
216. 7:03so that is how data streams are
217. 7:04distinguished from one another
218. 7:06it is a function of udp or tcp which are
219. 7:09both layer 4 protocols
220. 7:11and again layer 4's ultimate goal is
221. 7:12what i'm calling service to service
222. 7:14delivery of data
223. 7:18which finally brings us to the last
224. 7:20three layers of the osi model
225. 7:22back when they first created the osi
226. 7:24model each of these layers had a
227. 7:26distinct function
228. 7:27independent from the rest however
229. 7:29currently
230. 7:30the distinction between these layers is
231. 7:32somewhat vague
232. 7:33every application is in fact free to
233. 7:35implement the functions of layer 5 6 and
234. 7:377 as they choose
235. 7:38therefore often these three layers are
236. 7:41simply considered as a single
237. 7:43universal application layer in fact
238. 7:46the other popular internet communication
239. 7:48model actually does
240. 7:49exactly that the tcp model combines all
241. 7:52the functions of the osi layers 5
242. 7:546 and 7 into a single layer
243. 7:58now the goal of this module is to teach
244. 8:00you how data flows
245. 8:01through the internet and the most
246. 8:03critical layers to understand for that
247. 8:04is layers one through four
248. 8:06therefore we aren't going to pick apart
249. 8:08the original intended distinction
250. 8:10between layers 5 6 and 7. if you're
251. 8:13curious however
252. 8:14go ahead and let me know in the comments
253. 8:16below and i can cover it in another
254. 8:17video
255. 8:18as for us we're going to follow the tcp
256. 8:20model's lead and simply consider
257. 8:22all three of these layers as the
258. 8:24application
259. 8:29so now that we've talked through the
260. 8:31entire osi model i want to show you
261. 8:33what's actually happening when hosts are
262. 8:35communicating to each other using this
263. 8:36networking stack
264. 8:38this host has an application that's
265. 8:40going to generate data that is meant to
266. 8:41be sent to the other side
267. 8:43what that host is going to go through is
268. 8:45what's known as the encapsulation
269. 8:46process
270. 8:47that data will be first sent to layer 4.
271. 8:50layer 4 is going to add a header to that
272. 8:52data which can facilitate the goal of
273. 8:54layer 4 which is service to service
274. 8:56delivery
275. 8:57in this case it is a tcp header which is
276. 8:59going to include
277. 9:00the source port and destination port for
278. 9:02this particular data
279. 9:04the construct of a layer 4 header plus
280. 9:06data is known as a segment
281. 9:08that segment is going to be passed down
282. 9:10the osi stack
283. 9:12to the next layer in this case layer 3
284. 9:14the network layer
285. 9:15and the network layer is going to add
286. 9:17another header to this data
287. 9:19this header is going to facilitate the
288. 9:21goal of layer 3 which is end to end
289. 9:23delivery meaning in this header you'll
290. 9:25have something like a source ip address
291. 9:27and a destination iputrus
292. 9:31the construct of a layer 3 header and
293. 9:33its data is known as a packet
294. 9:36now notice that inside the data of the
295. 9:38packet is the layer 4 header that was
296. 9:40above it
297. 9:41but from layer 3's perspective it
298. 9:43doesn't know or doesn't care what's
299. 9:44inside that data
300. 9:45it's simply a bunch of ones and zeros
301. 9:47that need to be delivered to the other
302. 9:49end
303. 9:50either way layer 3 will then take that
304. 9:52packet and hand it off to layer 2
305. 9:54and layer 2 will once again add another
306. 9:56header to that data
307. 9:58to accomplish layer 2's goal of hop to
308. 10:00hop delivery
309. 10:01meaning this header will include
310. 10:03something like a source mac address and
311. 10:04a destination mac address
312. 10:06the construct of a layer 2 header and
313. 10:08its ensuing data is known as a frame
314. 10:11and that frame gets converted into ones
315. 10:13and zeros and finally put
316. 10:15on the wire on the other side the
317. 10:17receiving host is going to do the
318. 10:19opposite process it's going to do d
319. 10:22encapsulation
320. 10:23meaning it's going to take those ones
321. 10:25and zeros off the wire to convert them
322. 10:26back into their frame
323. 10:28layer two is going to look at the layer
324. 10:29two header to make sure that it is
325. 10:31indeed addressed to this
326. 10:32host's nic if it is it's going to
327. 10:35discard that header and pass that up the
328. 10:37stack
329. 10:38then layer 3 is going to look at the ip
330. 10:40header to confirm it is indeed addressed
331. 10:42to this host's ip address if it is it's
332. 10:45going to discard that layer 3 header and
333. 10:47pass it up the stack
334. 10:48to the transport layer layer 4 will then
335. 10:51take a look at the layer 4 header
336. 10:52to identify the port that this data is
337. 10:54destined to and will deliver the data to
338. 10:57the appropriate application
339. 10:58the application can then finally process
340. 11:00the data
341. 11:02that is the process of encapsulation and
342. 11:04de-encapsulation
343. 11:06notice each layer is going to add
344. 11:07information to the data
345. 11:09in order to accomplish its goal
346. 11:13and this brings us to the final idea i
347. 11:14want to communicate to you about the osi
348. 11:16model
349. 11:17throughout this lesson we've mentioned
350. 11:19that networking devices
351. 11:21operate at specific layers for example
352. 11:23switches and routers operate at layer 2
353. 11:25and layer 3 respectively what that means
354. 11:28is that they only look into the datagram
355. 11:30up to their respective layer for example
356. 11:33switches which are layer 2 devices
357. 11:35are only looking at the frame meaning
358. 11:37they're only looking at the layer 2
359. 11:39header to make a decision
360. 11:40they're not looking inside the data
361. 11:42payload into the layer 3 and layer 4
362. 11:44headers which we know exists there
363. 11:46they're only looking up to the layer 2
364. 11:48header
365. 11:49moreover we've discussed that there are
366. 11:51various protocols which operate at
367. 11:52specific layers
368. 11:54for instance the ip protocol is a layer
369. 11:563 protocol
370. 11:57tcp and udp are layer 4 protocols
371. 12:01that said neither of these are strict
372. 12:04rules
373. 12:05there are exceptions to each of these
374. 12:06that exist
375. 12:08for example if you take a router which
376. 12:10typically operates at layer 3
377. 12:12but configure access list on that router
378. 12:15now that router is looking into the
379. 12:16layer 4 header to make a decision
380. 12:18so the router isn't purely a layer 3
381. 12:20device anymore
382. 12:22moreover earlier in this lesson we
383. 12:24discussed the arp protocol and we
384. 12:25discussed how arp
385. 12:26links a layer 2 address to a layer 3
386. 12:28address links an ip address to a mac
387. 12:30address
388. 12:31which means arp doesn't purely fit in
389. 12:33layer 3 nor does it purely fit in layer
390. 12:352.
391. 12:36the point i'm trying to make is that the
392. 12:38osi model is simply a model
393. 12:40it's not rigid rules that everything in
394. 12:42networking adheres to
395. 12:44it's simply a way to conceptualize what
396. 12:46is required for data to flow
397. 12:48through the internet and that wraps up
398. 12:50our lesson on the osi model
399. 12:53the main takeaway for this lesson is
400. 12:55understanding that each layer of the osi
401. 12:57model
402. 12:57has a specific function that contributes
403. 12:59the overall goal
404. 13:01of allowing two hosts to share data with
405. 13:03one another
406. 13:04each layer uses its own addressing
407. 13:06scheme to accomplish their various goals
408. 13:08and finally there are various devices
409. 13:10and protocols which operate at specific
410. 13:12layers of the osi model
411. 13:14which serve in accomplishing each
412. 13:16layer's goal but do keep in mind
413. 13:18exceptions do exist in the next lesson
414. 13:21we'll be looking at everything a host
415. 13:23does to put data on the wire
416. 13:24but that's it for this lesson i hope you
417. 13:27enjoyed this video
418. 13:28i want to thank you for watching and
419. 13:30i'll see you in the next lesson when we
420. 13:31discuss
421. 13:32everything a host does when
422. 13:33communicating on the internet
423. 13:35hey youtube i hope you enjoyed that free
424. 13:37lesson for my new course on networking
425. 13:39fundamentals
426. 13:39i'll be releasing the entire first
427. 13:41module for free here on youtube
428. 13:43i want this course to be the ultimate
429. 13:45networking fundamentals course and since
430. 13:47i'm still scoping out the outline you
431. 13:48could have a say in what topics will be
432. 13:49covered
433. 13:50let me know in the comments below what
434. 13:52subjects you want included in this
435. 13:53course
436. 13:54otherwise remember to like and subscribe
437. 13:56and of course if you learned something
438. 13:57from this video the best way to thank me
439. 13:59is to share this video
440. 14:00it's a small act of gratitude but one i
441. 14:02appreciate greatly
442. 14:04i hope you enjoyed this lesson i want to
443. 14:06thank you for watching and we'll see you
444. 14:07in the next one