6.1 Introduction to the Link Layer — Transcript

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1. 0:00[Music]
2. 0:10well welcome to the link layer
3. 0:12we're now down at the lowest layer of
4. 0:14the protocol stack that we're going to
5. 0:15cover
6. 0:16in this class and we're going to see a
7. 0:18number of topics that we've encountered
8. 0:20already
9. 0:20such as reliable data transfer and flow
10. 0:23control
11. 0:24of course now between directly connected
12. 0:26nodes
13. 0:27but we've studied these topics before so
14. 0:29we've got a good feeling for them and we
15. 0:30won't need to cover them in depth here
16. 0:32but we're also going to cover a lot of
17. 0:34new and interesting topics
18. 0:36such as the multiple access problem
19. 0:39local area networks
20. 0:40addressing now with the link layer and
21. 0:42data center networks
22. 0:44of course we're going to cover the
23. 0:45instantiation the principles that we
24. 0:48learn
25. 0:48in practice protocols such as ethernet
26. 0:51that we've heard about
27. 0:52the address resolution protocol arp and
28. 0:54multi-protocol label switching
29. 0:56mpls so there's a lot for us to cover i
30. 0:59think you're going to find it
31. 1:00interesting so let's get going
32. 1:02well let's start off our study of the
33. 1:04link layer by laying out our goals
34. 1:06of course as always we're going to want
35. 1:08to look at both principles and practice
36. 1:10so let's start with principles well
37. 1:12we've already studied
38. 1:13error detection we looked at the
39. 1:15internet checksum earlier in the
40. 1:17transport layer
41. 1:18in the link layer we're going to study
42. 1:20much more powerful
43. 1:21error detection and correction
44. 1:23techniques as well
45. 1:25the heart of the principles that we're
46. 1:26going to take a look at coming up next
47. 1:28is the multiple access problem how do
48. 1:30multiple nodes
49. 1:32share a common communication channel
50. 1:34this to me is another
51. 1:36top 10 problem in networking and i think
52. 1:38you're going to find that really
53. 1:39interesting
54. 1:40and of course we want to look at the
55. 1:41issue of addressing now in the context
56. 1:43of the link layer
57. 1:44in terms of practice we're going to look
58. 1:46at some link layer protocols including
59. 1:48ethernet
60. 1:49virtual local area networks
61. 1:51multi-protocol label switching and then
62. 1:53finally data center networking as well
63. 1:56so that's what we're going to be
64. 1:57covering roughly in that order
65. 1:59so let's get started well let's start
66. 2:02off by defining some link layer
67. 2:04terminology depicted over here in the
68. 2:06diagram on the right
69. 2:08well of course we see the hosts and the
70. 2:10routers that we've seen throughout this
71. 2:11course and we'll refer to them simply as
72. 2:14nodes here in the context of the link
73. 2:16layer the role of the link layers to
74. 2:18serve as a communication channel
75. 2:20directly connecting to physically
76. 2:23adjacent nodes
77. 2:24and we'll want to parse that statement
78. 2:26quite carefully and discuss what we mean
79. 2:28by directly connecting and
80. 2:30physically adjacent two nodes might be
81. 2:33directly connected by a physical wire or
82. 2:35by a wireless link with one host
83. 2:37on each end that's simple or by a local
84. 2:40area network
85. 2:41the critical thing to remember here is
86. 2:43that the link layer connects
87. 2:45two nodes without any intervening
88. 2:48layer three that's to say network layer
89. 2:51router well maybe you just got a
90. 2:53flashback when i said
91. 2:54no intervening layer 3 router you may
92. 2:57recall when we were studying
93. 2:59subnets network layer addressing and
94. 3:01interfaces
95. 3:02back in section 4.3 let's flash back to
96. 3:05this slide here from section 4.3
97. 3:08we ask then how are interfaces connected
98. 3:11well we're going top down we're in
99. 3:12chapter 4
100. 3:14the network layer and we said well we'll
101. 3:15learn about that later in chapter six
102. 3:17well
103. 3:18here we are in chapter six and now we
104. 3:20can be more precise
105. 3:22now we can say that interfaces as shown
106. 3:25here
107. 3:26the diagram are connected by the link
108. 3:28layer
109. 3:29and that link layer can be a physically
110. 3:31connected wire
111. 3:32or wireless link but it can also be
112. 3:35a link layer with interconnected link
113. 3:37layer switches
114. 3:38which we'll refer to as a local area
115. 3:40network or a lan
116. 3:42or the link layer can consist of
117. 3:44wireless access points and
118. 3:46nodes as we'll see in chapter 7. but the
119. 3:49key thing here is that there are no
120. 3:51layer 3 that is network layer routers
121. 3:53between two nodes
122. 3:54that are connected at the link layer the
123. 3:57link layer consists of links and
124. 3:58switching possibly
125. 4:00that will switch layer 2 frames but a
126. 4:02link layer has no
127. 4:04layer 3 routers well let's return now to
128. 4:08our link layer vocabulary where the last
129. 4:10important definition is that of a frame
130. 4:12which is the layer 2 packet or protocol
131. 4:14data unit for the link layer
132. 4:16frame will typically encapsulate an ip
133. 4:19datagram
134. 4:20as its payload in an internet setting
135. 4:23well we can summarize by saying that the
136. 4:25link layer
137. 4:26has the responsibility of transferring a
138. 4:28datagram from one node to a physically
139. 4:30adjacent node
140. 4:32over a link and we should realize that
141. 4:34pretty much every word in that statement
142. 4:36is important
143. 4:38now let's put the link layer into the
144. 4:40larger end-to-end
145. 4:41context if we're to follow an ip
146. 4:43datagram from its initial source to its
147. 4:46final destination
148. 4:47we could see that along the way it could
149. 4:49be transferred
150. 4:50by different link protocols over
151. 4:52different links on its source to
152. 4:54destination journey
153. 4:56for example wi-fi on the first link
154. 4:58ethernet on the second link
155. 4:59and so on and different link layer
156. 5:02technologies provide different services
157. 5:04and have different characteristics
158. 5:06how noisy is the link does the link
159. 5:08provide link layer
160. 5:09error control or error detection and
161. 5:11correction if we take a traveler analogy
162. 5:14and say consider a trip from princeton
163. 5:16to lausanne and switzerland
164. 5:18we're going to take a car to get from
165. 5:19princeton to the jfk airport take a
166. 5:21plane
167. 5:22from the jfk airport to geneva and then
168. 5:24take a train from geneva to lausanne
169. 5:27in this analogy the tourist would be the
170. 5:29datagram who travels from source to
171. 5:30destination
172. 5:32an individual segment on the end-to-end
173. 5:35trip
174. 5:36the transport segment would be a
175. 5:37communication link each with different
176. 5:39services
177. 5:40hopefully with reliable transfer and the
178. 5:43transportation mode is the link layer
179. 5:45protocol here
180. 5:48let's next talk about the services that
181. 5:50are to be implemented by the link layer
182. 5:52like any layer the link layer implements
183. 5:54an encapsulation services
184. 5:56the case of the link layer link layer is
185. 5:58going to take a network layer datagram
186. 6:01at its own fields in a header wrapped
187. 6:03around the network layer datagram as
188. 6:05payload then pass the link layer frame
189. 6:08as we'll call it
190. 6:09down to the physical layer for bit level
191. 6:11transmission across the physical media
192. 6:13we're going to encounter a new service
193. 6:15at the link layer that we've not seen
194. 6:17before
195. 6:17media access when multiple nodes need to
196. 6:21share the same communication channel
197. 6:23their access to that channel needs to be
198. 6:26regulated it needs to be coordinated
199. 6:28of course there's a protocol for that
200. 6:30those protocols are known
201. 6:32as multiple access protocols also media
202. 6:35access protocols or macs so mac
203. 6:38protocols not like
204. 6:39mac computers and lastly we'll see that
205. 6:42at the link layer there's going to need
206. 6:44to be a new addressing scheme as well
207. 6:46sort of the bane of our existence as
208. 6:49networking students
209. 6:50we'll see that the link layer many link
210. 6:52layers implement a 48-bit
211. 6:54mac layer addressing scheme which is
212. 6:57distinct from the 32-bit or 128-bit
213. 7:00ip addressing scheme that we've studied
214. 7:02earlier mac addresses will
215. 7:05only be used in the context of an
216. 7:07individual link they won't be used
217. 7:09network
218. 7:10wide as in the case of the ip addresses
219. 7:12that we've studied
220. 7:14another service that can be provided by
221. 7:16the link layer is reliable delivery of
222. 7:18frames between adjacent nodes
223. 7:20from a principal standpoint we already
224. 7:22know how to do this
225. 7:23acts knacks error detection bits timeout
226. 7:27and retransmit
227. 7:28these are precisely the mechanisms that
228. 7:30are used at the link layer
229. 7:31now these techniques are seldom used on
230. 7:34low bit error links but they
231. 7:36are used in wireless links in a number
232. 7:38of cases
233. 7:40because wireless links are subject to
234. 7:42noise and interference and have high bid
235. 7:44error rates
236. 7:44and you might ask yourself well gee some
237. 7:47links provide error control
238. 7:49others don't provide error control we do
239. 7:51have error control
240. 7:52end to end why is it that we've got this
241. 7:55sort of mix of
242. 7:56local as well as end to end error
243. 7:58control
244. 8:00some of the other link layer services
245. 8:02that are offered by link layer protocols
246. 8:04include flow control
247. 8:06which we've learned is about speed
248. 8:07matching between the sending and
249. 8:09receiving sides of a link here
250. 8:11making sure that the link layer sender
251. 8:13doesn't overflow the link layer buffers
252. 8:15on the receiving side
253. 8:16there's error detection where more
254. 8:18powerful error detection and correction
255. 8:21codes
256. 8:21more powerful than the internet checksum
257. 8:23are used to detect and in some cases
258. 8:25correct
259. 8:26bit level errors in a frame we're going
260. 8:28to study error detection and correction
261. 8:30techniques in the following section
262. 8:32if errors are detected some link layer
263. 8:34protocols like bluetooth and 4g
264. 8:36lte cellular links use an acknack
265. 8:39re-transmission protocol
266. 8:41like we studied in chapter 3 to recover
267. 8:43from frame loss
268. 8:44and lastly we should note that some
269. 8:47links allow for data to be transferred
270. 8:49simultaneously in both directions
271. 8:51these are called full duplex links while
272. 8:54for other links data cannot be
273. 8:55transmitted and received at the same
274. 8:57time
275. 8:58these are called half duplex links
276. 9:02so each networked host has a link layer
277. 9:05the implementation of a host link layer
278. 9:07is particularly interesting because it's
279. 9:09here
280. 9:10where we're going to see the split
281. 9:11between hardware and software
282. 9:13part of the link layer is implemented in
283. 9:15hardware on chip
284. 9:16or in a network interface card which
285. 9:18attaches to the host
286. 9:20system bus the upper layer parts of the
287. 9:22link layer
288. 9:23demultiplexing up handling interrupts
289. 9:25for example
290. 9:26are handled in software in the host's
291. 9:28operating system
292. 9:30so as we see in the figure here the link
293. 9:32layers is where we see the split between
294. 9:34network functionality
295. 9:36implemented primarily in hardware and
296. 9:38the rest implemented in software
297. 9:40the lower parts of the link layer and
298. 9:42the physical layer are implemented in
299. 9:44hardware
300. 9:44the upper parts of the link layer and
301. 9:46the higher layers of the protocol stack
302. 9:48are implemented in software typically
303. 9:52and lastly here we can follow the data
304. 9:54on the sending side the interface is
305. 9:56going to encapsulate the ip datagram
306. 9:59in a link layer frame adding error
307. 10:01detection correction bits
308. 10:03link layers sending and receiving
309. 10:05addressing information
310. 10:06perhaps fields for sequence numbers and
311. 10:08acknowledgments flow control
312. 10:10and more all of these are the fields
313. 10:12that will allow the link layer to
314. 10:14implement its service
315. 10:15it then passes the frame to the physical
316. 10:17layer for bit level transmission
317. 10:19on the receiving side the link layer
318. 10:21receives the frame from the physical
319. 10:23layer
320. 10:24performs error checking flow control and
321. 10:26other services
322. 10:27then finally removes the payload
323. 10:29typically an ip datagram
324. 10:31and passes the datagram up to the
325. 10:33network layer
326. 10:35well that wraps up our introduction to
327. 10:37the link layer we've seen that there's
328. 10:39some ground
329. 10:40we've already covered flow control
330. 10:42reliable data transfer
331. 10:44addressing bit level error detection
332. 10:46with some new takes on these topics
333. 10:48but we've also seen that there are a
334. 10:50number of new interesting principles
335. 10:52and protocols that we'll want to take a
336. 10:54look at so let's get started
337. 11:12you