1 00:00:00,025 --> 00:00:10,700 Oh, oh hi, sorry. I was in, I was in the forums. 2 00:00:10,700 --> 00:00:13,830 [SOUND] I guess it's time for this weeks video. 3 00:00:14,890 --> 00:00:19,310 So this week we're moving from history to technology. 4 00:00:19,310 --> 00:00:23,510 And in a sense were going to tell the same story again. 5 00:00:23,510 --> 00:00:26,348 you know, we'll sort of start from the beginning, but we're going to start from 6 00:00:26,348 --> 00:00:29,143 the bottom and move upwards so it sounds as though we're kind of taking about the 7 00:00:29,143 --> 00:00:33,000 same things, but just from a more technical perspective. 8 00:00:33,000 --> 00:00:36,100 Don't worry, you'll be fine. we're not going to over, there's no math, 9 00:00:36,100 --> 00:00:39,860 I promise, there's no programming. Don't worry about that. 10 00:00:39,860 --> 00:00:43,746 So of course, I appreciate IEEE Computer Magazine, who I write for to let me use 11 00:00:43,746 --> 00:00:47,458 the articles that are associated with some of the videos, as well as the folks 12 00:00:47,458 --> 00:00:51,940 from up in Michigan who have helped me, so. 13 00:00:51,940 --> 00:00:55,150 Hang on a sec. Now hang on, I've got a phone call. 14 00:00:55,150 --> 00:00:59,050 I'll be right back. Okay, well that's been sent. 15 00:00:59,050 --> 00:01:02,408 Sorry for that interruption. again I thank everybody for the use of 16 00:01:02,408 --> 00:01:06,862 the copyright materials. part of my hidden agenda in this class is 17 00:01:06,862 --> 00:01:11,182 that you will be able to look at this XKCD comic and you'll understand the 18 00:01:11,182 --> 00:01:14,560 humor. So, I'm going to stop for a moment and 19 00:01:14,560 --> 00:01:16,905 let you look at it and see if you get the humor. 20 00:01:16,905 --> 00:01:28,545 Okay so maybe you did, maybe you didn't. But hopefully by the time we're done, 21 00:01:28,545 --> 00:01:31,380 you'll have a better chance of getting here. 22 00:01:31,380 --> 00:01:36,750 So if you recall, the most academics throughout the 60s 70s and 80s. 23 00:01:36,750 --> 00:01:39,974 The best they had access to was this store and forward networking where you 24 00:01:39,974 --> 00:01:43,198 would send a message and it would go into a computer and it might sit for some 25 00:01:43,198 --> 00:01:46,784 time. Then it would find its way across a 26 00:01:46,784 --> 00:01:50,160 network and it would sit for a little while longer. 27 00:01:50,160 --> 00:01:52,960 Find, and, and hop, and there are these multiple hops. 28 00:01:52,960 --> 00:01:57,240 We call these hops, and each of these would be a couple hundred miles perhaps. 29 00:01:57,240 --> 00:02:03,430 And to go from say Michigan to Stanford, there might be fifteen or twenty hops. 30 00:02:03,430 --> 00:02:07,918 Hops are as much, defined by geography where we're all trying to optimized the 31 00:02:07,918 --> 00:02:13,419 cost of these long distance connections. So the thing that really characterized 32 00:02:13,419 --> 00:02:17,810 store and forward networking was that there wasn't a lot of sharing. 33 00:02:17,810 --> 00:02:21,202 One message was being sent across one of these links at a time and all the other 34 00:02:21,202 --> 00:02:24,570 messages just kind of waited in time, in line. 35 00:02:24,570 --> 00:02:29,193 Now, that the, the, the innovation that happened in the research networks in the 36 00:02:29,193 --> 00:02:33,747 60's through the 80's ultimately unbodied in the ARPAnet was the fact that they 37 00:02:33,747 --> 00:02:40,928 were going to be packet networks. First, how do we share one link, so that 38 00:02:40,928 --> 00:02:45,862 a long message doesn't clog it up? And how do we deal with outages more 39 00:02:45,862 --> 00:02:48,960 dynamically? With the idea that, eventually, you could 40 00:02:48,960 --> 00:02:53,140 send a packet across the country. And back into a computer and back out in 41 00:02:53,140 --> 00:02:56,494 maybe a half of a second. And that was seen as something to do 42 00:02:56,494 --> 00:03:00,245 rather than. Somewhere between ten minutes and a day 43 00:03:00,245 --> 00:03:03,950 or two. So how to do this efficiently. 44 00:03:03,950 --> 00:03:07,930 It was a research project. And it was a research project that lasted 45 00:03:07,930 --> 00:03:13,500 almost 20 year-, well, 20 years. And, 20 or more years. 46 00:03:13,500 --> 00:03:16,650 And, and it, one of the neatest things about this is they were able to throw it 47 00:03:16,650 --> 00:03:21,594 away and rewrite it a bunch of times. So that they could put something in 48 00:03:21,594 --> 00:03:25,752 production, see its flaws, see what was good about it, and they had the money 49 00:03:25,752 --> 00:03:30,310 from Darpa to throw it away and rewrite it. 50 00:03:30,310 --> 00:03:34,750 Now by the late 1970s it had 100ish computers on it, here's a picture, and 51 00:03:34,750 --> 00:03:40,595 these are not the schools, these are the computers, these are. 52 00:03:40,595 --> 00:03:43,270 You know, listing all of the computers that are there. 53 00:03:45,650 --> 00:03:50,402 And so the engine, the, the innovation that they spent 20 years perfecting, was 54 00:03:50,402 --> 00:03:55,252 the notion of packet-switching. So how can we simply move data 55 00:03:55,252 --> 00:04:01,130 simultaneously across the connection? And the essential brilliant idea Is break 56 00:04:01,130 --> 00:04:06,548 the message into packets. And again to sort of review, here I have 57 00:04:06,548 --> 00:04:11,460 a message and I have post cards that only can handle 10 characters. 58 00:04:11,460 --> 00:04:15,300 And i'm going to send this "Hello there have a nice day" message to Daphne in 59 00:04:15,300 --> 00:04:20,690 California. And so I have 3 10 character post cards. 60 00:04:20,690 --> 00:04:24,466 I basically put a from address and a to address on each postcard and a sequence 61 00:04:24,466 --> 00:04:28,242 number on each postcard, 1, 2 and 3, and I put ten characters on each and I stick 62 00:04:28,242 --> 00:04:32,950 them in my mailbox. I just put them in. 63 00:04:32,950 --> 00:04:38,394 Put the sign up and then I wait. Now let's just say you know, that the 64 00:04:38,394 --> 00:04:43,464 post office person comes and picks this first one up and it goes to Chicago from 65 00:04:43,464 --> 00:04:48,378 Michigan than it goes to Omaha then it goes to Denver then the second one kind 66 00:04:48,378 --> 00:04:57,730 of falls on the floor and it ends up going to Charlotte North Carolina. 67 00:04:57,730 --> 00:05:00,691 And then they're like, Why is this here? And then they send it to Atlanta to get 68 00:05:00,691 --> 00:05:02,610 checked. And then they make sure. 69 00:05:02,610 --> 00:05:06,040 And, and this one goes and. And Number 3 ends up going to St. 70 00:05:06,040 --> 00:05:08,220 Louis. And then Tulsa. 71 00:05:08,220 --> 00:05:10,638 And then Colorado. And this one gets routed back to 72 00:05:10,638 --> 00:05:15,370 Charlotte because they weren't sure why they sent it to them in the first place. 73 00:05:15,370 --> 00:05:18,379 And then they send it to Memphis, Tennessee and then this one makes it to 74 00:05:18,379 --> 00:05:21,556 California. And this other one sort of goes to 75 00:05:21,556 --> 00:05:26,290 Phoenix and this one finally gets to Dallas, Texas. 76 00:05:26,290 --> 00:05:30,160 And this other one goes finally. And the third one goes to Tucson, 77 00:05:30,160 --> 00:05:34,500 Arizona, then Las Vegas Nevada and finally he makes it. 78 00:05:34,500 --> 00:05:39,310 So, these poor little postcards I put in They have each a different journey. 79 00:05:39,310 --> 00:05:41,344 Right? It's a little adventure for each of the 80 00:05:41,344 --> 00:05:46,550 postcards that they go through. And, so, we don't, I don't see how this 81 00:05:46,550 --> 00:05:49,780 works on my end. I put in three postcards. 82 00:05:49,780 --> 00:05:53,410 I numbered them, one, two, three. But on the far end, in California, Daphne 83 00:05:53,410 --> 00:05:57,460 opens her mailbox. And has scribbling all over her mailbox. 84 00:05:57,460 --> 00:05:59,953 Oh, the shame! No, actually we'll get rid of the 85 00:05:59,953 --> 00:06:02,776 scribbling. So Daphne opens up her mailbox, and she 86 00:06:02,776 --> 00:06:06,327 doesn't know how these packets got there, I mean, postcards, how they got there, 87 00:06:06,327 --> 00:06:09,850 out comes the first one. She goes, oh looks like I just got a 88 00:06:09,850 --> 00:06:12,310 message from Chuck, but I only got part of it. 89 00:06:12,310 --> 00:06:14,440 Then out comes the second one shortly thereafter. 90 00:06:14,440 --> 00:06:17,307 Then after a long period of time, finally the third one comes out, but it's 91 00:06:17,307 --> 00:06:21,010 actually the second one. And now she understands that she's got 92 00:06:21,010 --> 00:06:24,010 the entire message, and she reassembles it, and away we go. 93 00:06:24,010 --> 00:06:28,244 And so this is the basic notion of how all those postcards can share the 94 00:06:28,244 --> 00:06:31,666 infrastructure. All, they can all be invited at the same 95 00:06:31,666 --> 00:06:34,010 time. They can take different paths. 96 00:06:34,010 --> 00:06:37,210 You don't have to connect them all together like trains, basically. 97 00:06:37,210 --> 00:06:43,300 and, and this led to a shared network infrastructure. 98 00:06:43,300 --> 00:06:47,186 And so the computers that were in a store and forward network, went from, sort of, 99 00:06:47,186 --> 00:06:51,014 big powerful computers with disk drives to really tiny computers with a single 100 00:06:51,014 --> 00:06:56,200 purpose of forwarding packets. Rather than longs term storage of 101 00:06:56,200 --> 00:06:59,424 message. And so, so the, the store in forward had, 102 00:06:59,424 --> 00:07:03,800 longs term storage of messages, in the routers. 103 00:07:03,800 --> 00:07:07,300 So the, but, I mean, not in the routers. In long, in the store in forward, the old 104 00:07:07,300 --> 00:07:10,200 one, long term storage was somewhere in the network. 105 00:07:10,200 --> 00:07:14,130 But this is only short term, right, short-term storage, short-term storage. 106 00:07:14,130 --> 00:07:18,482 And so when a packet comes out, it simply has to find its way through a series of 107 00:07:18,482 --> 00:07:22,620 hops. It still hops and it's still connections, 108 00:07:22,620 --> 00:07:26,750 but basically it has to find its way across the Internet. 109 00:07:26,750 --> 00:07:30,284 And then we would take whole campuses Like University of Michigan, say, or 110 00:07:30,284 --> 00:07:34,306 Stanford for example. And, then we'd have sort of various kinds 111 00:07:34,306 --> 00:07:37,254 of local networks on those campuses, and computers on the campuses, and servers 112 00:07:37,254 --> 00:07:40,909 and various things. And they would route all their packets, 113 00:07:40,909 --> 00:07:43,618 all their data out to the Internet, and then the packets would find their way 114 00:07:43,618 --> 00:07:46,456 across the Internet, and then we might have a home connection as well that might 115 00:07:46,456 --> 00:07:52,164 connect. And so, the shared, shared Network 116 00:07:52,164 --> 00:08:01,365 infrastructure focuses only on packets, not reliability or anything else. 117 00:08:01,365 --> 00:08:03,440 [COUGH] So, this notion of hops didn't go away. 118 00:08:03,440 --> 00:08:07,040 As a matter of fact, I don't know how many hops it took to get from Ann Arbor 119 00:08:07,040 --> 00:08:12,423 to Palo Alto in using BITNET. I'm going to guess it might have been 16 120 00:08:12,423 --> 00:08:16,460 or 20, in these days it takes 16 or 20 hops. 121 00:08:16,460 --> 00:08:19,852 The difference is in the internet, the 16 or 20 hops happens in a hundredth of a 122 00:08:19,852 --> 00:08:24,170 second, or a tenth of a second. So the ho-, notion of hops, and the 123 00:08:24,170 --> 00:08:28,250 notion of intermediate computers is still present in the internet, and the TCPIP 124 00:08:28,250 --> 00:08:33,610 networks that we use today. And so your, your, your message sort of 125 00:08:33,610 --> 00:08:37,390 leaves you the host that you're in, hops to the first router, and then hops to 126 00:08:37,390 --> 00:08:41,494 however many routers. Dot dot dot, this is more of a dot dot 127 00:08:41,494 --> 00:08:44,232 dot thing. And then finally hops its way out to the 128 00:08:44,232 --> 00:08:46,993 far host. And so this is sort of either your 129 00:08:46,993 --> 00:08:50,276 computer, maybe Stanford's web server, and then there's this series of routers 130 00:08:50,276 --> 00:08:55,030 in the middle that it hops through. Okay? 131 00:08:55,030 --> 00:09:01,204 And so, the problem of what data goes between here and here is you gotta solve 132 00:09:01,204 --> 00:09:07,264 a lot of problems, right? How to, all, all kinds of problems, and 133 00:09:07,264 --> 00:09:11,294 so, in order to simplify the solution or break the solution into simpler, more 134 00:09:11,294 --> 00:09:16,467 manageable parts. They came up with a layered network 135 00:09:16,467 --> 00:09:20,004 model. Now this is a cartoon and computer people 136 00:09:20,004 --> 00:09:25,090 love drawing cartoons and saying this is our architecture. 137 00:09:25,090 --> 00:09:27,750 This is our frame work. This is our approach. 138 00:09:27,750 --> 00:09:30,220 Sometimes they're helpful. Sometimes they're not helpful. 139 00:09:30,220 --> 00:09:32,350 Sometimes the cartoon is just kind of a cartoon. 140 00:09:33,910 --> 00:09:37,550 But what's usually being communicated in these kinds of pictures is they're taking 141 00:09:37,550 --> 00:09:40,618 a big problem and breaking it down into some subset, some set of smaller 142 00:09:40,618 --> 00:09:45,150 problems. So, the whole problem they've gotta solve 143 00:09:45,150 --> 00:09:48,790 of getting data reliability reliably across the whole country is that big and 144 00:09:48,790 --> 00:09:55,296 if we can break into four pieces. And work separately and come up with ways 145 00:09:55,296 --> 00:09:59,436 to let these pieces interact, work separately with each onem then maybe 146 00:09:59,436 --> 00:10:05,134 we'll have a better solution, okay? So take a problem that's so large and so 147 00:10:05,134 --> 00:10:08,611 complex that we might not be able to solve it and break it into four smaller 148 00:10:08,611 --> 00:10:13,710 problems Gives us a better chance of solving the four problems. 149 00:10:13,710 --> 00:10:18,340 Now, there's a certain art to picking how you break the problem up. 150 00:10:18,340 --> 00:10:21,950 And there's more than one network model. The one that we use in the Internet is 151 00:10:21,950 --> 00:10:25,214 the TCP/IP or the Internet Protocol Suite model, and we'll meet some of the people 152 00:10:25,214 --> 00:10:29,280 who designed that. there was also a model called the 7 layer 153 00:10:29,280 --> 00:10:32,395 OSI model, Open System Interconnection model. 154 00:10:32,395 --> 00:10:38,790 So there's a model out there that has 1, 2, 3, 4, 5, 6, 7 layers. 155 00:10:38,790 --> 00:10:40,614 And they all have names, and they all have purposes, and they all have 156 00:10:40,614 --> 00:10:43,745 definitions. this was not quite as po-, this was not 157 00:10:43,745 --> 00:10:48,699 very popular. And I doubt that it's many places. 158 00:10:48,699 --> 00:10:51,615 because the TCPI model is the one that kind of won. 159 00:10:51,615 --> 00:10:54,440 Um, [COUGH] and so that's the one that we're going to study. 160 00:10:54,440 --> 00:10:56,480 But that doesn't mean that, that has to be the only one. 161 00:10:56,480 --> 00:10:59,790 But it is the one that has become popular on the internet. 162 00:11:01,050 --> 00:11:04,350 So, once you break the problem down from a big problem into four small problems, 163 00:11:04,350 --> 00:11:08,570 you have to develop documents about how these layers work together. 164 00:11:08,570 --> 00:11:12,370 How various computers work together. How routers work. 165 00:11:12,370 --> 00:11:16,594 All these things and so early in the process of the development of the Arpanet 166 00:11:16,594 --> 00:11:22,190 they created an open process to build these specifications. 167 00:11:22,190 --> 00:11:25,490 Where they would vite, invite engineers from all kinds of companies and 168 00:11:25,490 --> 00:11:30,420 universities and who knows what any expertise to show up at a meeting. 169 00:11:30,420 --> 00:11:33,670 Several times a year. I mention the IETF meeting when I was 170 00:11:33,670 --> 00:11:37,422 talking about the history of the Internet and Tim Berners-Lee boff, that only 15 171 00:11:37,422 --> 00:11:41,798 people showed up to. That's this meeting, the Internet 172 00:11:41,798 --> 00:11:45,230 Engineering Task Force, which is just a bunch of engineers that get into rooms, 173 00:11:45,230 --> 00:11:48,480 many rooms, and solve a bunch of problems. 174 00:11:48,480 --> 00:11:51,270 And so the standards that come out of this, you can go read them, they're all 175 00:11:51,270 --> 00:11:54,830 open documents. They're very throwback documents. 176 00:11:54,830 --> 00:11:58,673 They've got this text thing that's dial this like, beautiful upper case, it 177 00:11:58,673 --> 00:12:03,570 reminds me of back when printers didn't have a lower case. 178 00:12:03,570 --> 00:12:07,503 and these are documents that describe how most components of the internet work 179 00:12:07,503 --> 00:12:16,260 together. So, the layered architecture that I just 180 00:12:16,260 --> 00:12:23,000 described where you have a computer and then it sends data out to back of it. 181 00:12:23,000 --> 00:12:26,082 And then hop, hop, hop, hop, hop through routers, and then the data arrives at the 182 00:12:26,082 --> 00:12:29,072 destination computer, And then it, the destination computer sends it back it 183 00:12:29,072 --> 00:12:33,520 hop, hop, hop, hop, hop coming back and it comes to you. 184 00:12:33,520 --> 00:12:35,120 We're going to expand on this a little bit. 185 00:12:35,120 --> 00:12:42,050 So each of these hosts on the two ends is going to expand to this much right here. 186 00:12:42,050 --> 00:12:44,930 So that's one host. That's another host. 187 00:12:44,930 --> 00:12:47,700 And each of these routers is going to expand into this. 188 00:12:48,870 --> 00:12:54,160 So this bottom picture is just an expanded version of this upper picture. 189 00:12:54,160 --> 00:12:57,590 So that we can see how the Internet layers work together. 190 00:12:59,060 --> 00:13:04,082 So when you send a message in on your computer, it's -- this is all software, 191 00:13:04,082 --> 00:13:09,910 application, transport, Internet link. So this is all software, and then it all 192 00:13:09,910 --> 00:13:12,855 comes out the little plug on the back of your computer. 193 00:13:12,855 --> 00:13:17,494 Right. [SOUND] So, maybe your computer has a 194 00:13:17,494 --> 00:13:19,960 little plug like this on the back of it. Right? 195 00:13:19,960 --> 00:13:23,184 That's an ethernet plug and here's an ethernet wire and I plug my local area 196 00:13:23,184 --> 00:13:25,280 network in. Right? 197 00:13:25,280 --> 00:13:27,780 And so, this right here is that right there. 198 00:13:29,520 --> 00:13:32,747 That's kind of the plug. It might be WiFi which means it's kind of 199 00:13:32,747 --> 00:13:35,736 like air but If you have a wire like this, then it's a little more tangible, 200 00:13:35,736 --> 00:13:39,520 and there's something on the other end of that wire. 201 00:13:39,520 --> 00:13:42,710 This is a very short wire, but there would be a router on the other end of 202 00:13:42,710 --> 00:13:46,514 that wire. [COUGH] The router takes the data off the 203 00:13:46,514 --> 00:13:50,672 wire and then forwards it just like a post office, intermediate post office 204 00:13:50,672 --> 00:13:55,740 would do, onto the next link. On to the next one. 205 00:13:55,740 --> 00:13:58,780 And this would be multiple hops in here, they're only showing 2 hops. 206 00:13:58,780 --> 00:14:03,690 And finally it's on the last link going to that computer at Stanford. 207 00:14:03,690 --> 00:14:07,286 And then it goes up through these 4 layers of software and then goes and does 208 00:14:07,286 --> 00:14:11,176 whatever it's going to do. Now when they send you the response back 209 00:14:11,176 --> 00:14:16,300 it kind of goes the other way. And back up and back to you. 210 00:14:16,300 --> 00:14:20,615 So if this was you in a web browser, you know and then here's me. 211 00:14:20,615 --> 00:14:24,780 And here's a slide I've got, right. This web browser and you moving your 212 00:14:24,780 --> 00:14:28,135 cursor in the web browser is sending stuff back and forth to the [UNKNOWN] 213 00:14:28,135 --> 00:14:31,920 servers right here. And that's what's happening, alright. 214 00:14:31,920 --> 00:14:37,590 So If as you look at this picture over here, this left colmn is the host you're 215 00:14:37,590 --> 00:14:41,998 coming from. The right column is the host you're going 216 00:14:41,998 --> 00:14:45,425 to, and the riders are these things in the middle, okay? 217 00:14:45,425 --> 00:14:53,482 Okay. So let's start looking at all four 218 00:14:53,482 --> 00:14:56,022 layers. We are going to start at the bottom, at 219 00:14:56,022 --> 00:14:58,670 the link layer, then we're going to move up. 220 00:15:00,130 --> 00:15:04,550 So if you recall when I talked about the, the, the basic reason why we even have a 221 00:15:04,550 --> 00:15:10,520 layered architecture at all, is to simplify the problem. 222 00:15:10,520 --> 00:15:14,260 To simplify the problem. So the idea of the link layer. 223 00:15:14,260 --> 00:15:19,151 Is, the link layer is a, it, it on-, only worries about getting the data across one 224 00:15:19,151 --> 00:15:23,495 hump, right? Only worries about getting the data from, 225 00:15:23,495 --> 00:15:27,940 across one piece of wire. White like voltage goes on these little 226 00:15:27,940 --> 00:15:32,170 wires, and. How we send the stuff and if more than 227 00:15:32,170 --> 00:15:35,370 one computer's using the same wire, how do we share? 228 00:15:36,700 --> 00:15:40,400 It's a complex enough problem but, we don't worry about the whole world. 229 00:15:40,400 --> 00:15:44,420 We worry about this one link layer and so the link layer is sort of like the 230 00:15:44,420 --> 00:15:50,040 connection out of one computer and into another computer. 231 00:15:50,040 --> 00:15:54,330 And, and this might be fiber optic, might be, you know, 40 miles or it might be 40 232 00:15:54,330 --> 00:15:58,900 feet for all we know. But it's one link, okay? 233 00:15:58,900 --> 00:16:03,452 And then, a router pulls it off that link and then forwards it onto another link. 234 00:16:03,452 --> 00:16:07,027 If you go to the post office, you could think of the, the person who picks your 235 00:16:07,027 --> 00:16:10,547 mail up from your house, with the thing on their shoulder, that's one link And 236 00:16:10,547 --> 00:16:17,310 they put it in a truck which takes it to a place that puts it in a semi truck. 237 00:16:17,310 --> 00:16:20,340 And then it puts it to another place that puts it on a train. 238 00:16:20,340 --> 00:16:24,884 And the semi truck and the train and the postman or postwoman are these little 239 00:16:24,884 --> 00:16:29,272 link layers. Each person or semi truck is not taking 240 00:16:29,272 --> 00:16:33,100 it all the way, they're just getting a little farther. 241 00:16:33,100 --> 00:16:36,820 So that's the link layer So the link layer doesn't worry about the rest of 242 00:16:36,820 --> 00:16:39,900 this stuff. It really, the stuff that is defined on 243 00:16:39,900 --> 00:16:43,625 this wire, it doesn't even care if there's a world wide web or anything. 244 00:16:43,625 --> 00:16:48,740 Its job is to get data across one foot. That's what its job is. 245 00:16:48,740 --> 00:16:51,600 Is it up, is it down, how do we share? It doesn't care. 246 00:16:51,600 --> 00:16:54,210 It's got a very narrow view. So. 247 00:16:54,210 --> 00:16:57,000 What it means is we can zoom in on this problem, right. 248 00:16:57,000 --> 00:17:01,300 We can zoom in and focus and ignore everything else. 249 00:17:02,740 --> 00:17:06,100 So the link layer basically asks questions like, you know, I've got some 250 00:17:06,100 --> 00:17:10,660 data inside the computer and I want to send it out. 251 00:17:10,660 --> 00:17:13,870 How do I encapsulate it? What if this is shared? 252 00:17:13,870 --> 00:17:17,958 How do I deal with that? And common Link technologies that we see 253 00:17:17,958 --> 00:17:23,780 are like Ethernet or WiFi or cable modem, DSL, satellite or optical. 254 00:17:23,780 --> 00:17:28,320 These are all links layers of one form or another. 255 00:17:28,320 --> 00:17:31,335 So if you're going back to this Ethernet link layer, which is one of our favorites 256 00:17:31,335 --> 00:17:35,696 cause it's kind of ubiquitous. Other than wireless it is probably 257 00:17:35,696 --> 00:17:37,880 Ethernet is the most ubiquitous link layer. 258 00:17:41,240 --> 00:17:46,000 [SOUND] So, when the manufacturer builds an ethernet or a wireless adapter, they 259 00:17:46,000 --> 00:17:50,700 actually build into it, which is probably right here. 260 00:17:50,700 --> 00:17:55,620 No, it could be right here, they build into it a serial number. 261 00:17:55,620 --> 00:17:58,660 So, they, they mark this as having a serial number. 262 00:17:58,660 --> 00:18:01,884 And you can actually whether you're on Windows or Mac, you can work your way 263 00:18:01,884 --> 00:18:07,470 down to find the serial number for your actual manufacturer piece of equipment. 264 00:18:07,470 --> 00:18:09,830 So there's mine on the Mac. That's an example one on the Mac. 265 00:18:09,830 --> 00:18:14,100 And that's an example one on the PC. They tend to be six two digit numbers 266 00:18:14,100 --> 00:18:18,650 concatonated together often with colons or in this case with Windows it shows 267 00:18:18,650 --> 00:18:23,630 dashes. And so this is a raspberry pie and all 268 00:18:23,630 --> 00:18:27,726 the raspberry pies are going to actually have similar prefixes and then serial 269 00:18:27,726 --> 00:18:32,820 numbers within that prefix. And so the raspberry pies get 270 00:18:32,820 --> 00:18:35,920 manufactured, they come out of the manufacturing line with this sequence 271 00:18:35,920 --> 00:18:39,672 number just kind of going chug chug chug, up up up up up. 272 00:18:39,672 --> 00:18:43,020 And, then they get shipped to all corners of the world. 273 00:18:44,440 --> 00:18:49,197 And so, these numbers are not the numbers for this to get across the world, all the 274 00:18:49,197 --> 00:18:53,883 number, all these serial numbers at the link layer are good for are to get across 275 00:18:53,883 --> 00:18:59,975 one connection, in case that connection is shared. 276 00:18:59,975 --> 00:19:02,945 And if you were to plug this into a hub, and you're plugging other computers into 277 00:19:02,945 --> 00:19:06,080 that same hub. Well then you're sharing that connection. 278 00:19:06,080 --> 00:19:10,110 And this computer might see the traffic for that other computer. 279 00:19:10,110 --> 00:19:14,142 And so we use the physical addresses so that this computer knows which of the 280 00:19:14,142 --> 00:19:17,800 packets belong to it on the piece of wire. 281 00:19:17,800 --> 00:19:21,050 So, wired ethernet or wifi certainly if you're sitting in a room with a bunch of 282 00:19:21,050 --> 00:19:24,270 wifi computers. You're sharing the air. 283 00:19:24,270 --> 00:19:28,113 So let's come up with rules how to share this, how to share nicely and how to 284 00:19:28,113 --> 00:19:32,840 behave. So the way it works, ccc, is you 285 00:19:32,840 --> 00:19:39,450 could...the ethernet could easily have a bunch of computers hooked to it right. 286 00:19:39,450 --> 00:19:42,245 Cause its really shared, and so they're all sitting here, they're all talking 287 00:19:42,245 --> 00:19:45,040 simultaneously. No. 288 00:19:45,040 --> 00:19:47,820 But, we have a com, we have a pair of computers that want to talk. 289 00:19:47,820 --> 00:19:50,900 So, it's not just one. It's got a bunch of folks sitting here. 290 00:19:50,900 --> 00:19:53,728 [SOUND] Just draw a bunch of em. Bunches of folks. 291 00:19:53,728 --> 00:19:57,622 They're all talking or all potentially talking [SOUND] and we gotta figure out 292 00:19:57,622 --> 00:20:01,348 how in the shared medium. Like this is, think of this as a hub and 293 00:20:01,348 --> 00:20:05,285 they're all connected into this hub including the two we want to talk. 294 00:20:05,285 --> 00:20:09,800 They're all connected in the hub, including the two that we want to talk. 295 00:20:09,800 --> 00:20:13,076 And how can these two make sure that the data goes, is, is the data that's sent 296 00:20:13,076 --> 00:20:17,580 with the intention of going to this computer, this router. 297 00:20:17,580 --> 00:20:22,214 How can it make sure that it gets there? Well, if it knows the address of the 298 00:20:22,214 --> 00:20:27,690 sending and receiving unit It just encodes that, in the packet. 299 00:20:29,360 --> 00:20:32,165 And that way, it sort of goes by all these folks. 300 00:20:32,165 --> 00:20:36,143 Let me change colors here. The packet we send goes by all these 301 00:20:36,143 --> 00:20:39,198 folks, and even if they all heard every one of the packets, they all know that it 302 00:20:39,198 --> 00:20:42,253 doesn't belong to them because they all have a different number and this is the 303 00:20:42,253 --> 00:20:48,495 only one that responds to it. So that allows them to share the wireless 304 00:20:48,495 --> 00:20:53,935 or the wire, and, share the wireless to the wire and have it all work out for 305 00:20:53,935 --> 00:20:58,726 them. So, [COUGH] so that's the idea, the link 306 00:20:58,726 --> 00:21:01,913 layer. Now, again, remember this might only be 307 00:21:01,913 --> 00:21:05,560 fifty feet, or fifty yards, or fifty meters. 308 00:21:05,560 --> 00:21:08,228 And they can be meters. It's only 50 meters, but you still have 309 00:21:08,228 --> 00:21:12,765 to share the network in that 50 meters. And then your router takes it off of that 310 00:21:12,765 --> 00:21:17,890 and forwards it on a different link. And that's how we get across the country. 311 00:21:17,890 --> 00:21:20,040 But that's the next layer up, and let's not worry about that for now. 312 00:21:20,040 --> 00:21:24,493 So, the idea is think about one link, whether it's fiber optic or cable, or 313 00:21:24,493 --> 00:21:28,566 wireless, or wifi. And so what are the kind of problems that 314 00:21:28,566 --> 00:21:33,425 you have to solve on this link layer? Well, so one of the things that's cool 315 00:21:33,425 --> 00:21:37,110 about many link layers is, like wireless and ethernet, is they can be shared, and 316 00:21:37,110 --> 00:21:41,080 it makes it real easy just plug new computers in. 317 00:21:41,080 --> 00:21:43,663 So you just sort of like, here's a hub, and you just plug another computer in, 318 00:21:43,663 --> 00:21:47,446 and it's on the network. But they have to come up with a way to 319 00:21:47,446 --> 00:21:52,400 avoid the chaos when they're sharing. And so, the way Ethernet does this is 320 00:21:52,400 --> 00:22:00,485 with a technique called, Carrier Sense Media Access with Collision Detection. 321 00:22:00,485 --> 00:22:04,985 And as you'll see, a lot of things we do on the internet have a lot to do with 322 00:22:04,985 --> 00:22:10,360 courtesy. Meaning that we're just nice. 323 00:22:10,360 --> 00:22:11,935 And if everybody's nice, it all works out. 324 00:22:11,935 --> 00:22:15,781 kind of like driving in a car. Just everyone can't run through the stop 325 00:22:15,781 --> 00:22:18,770 sign at the same time. Some of you gotta stop, got a green 326 00:22:18,770 --> 00:22:23,972 light, your turn, my turn, whatever. This probably wouldn't work so well with 327 00:22:23,972 --> 00:22:27,160 cars, because sometimes you do have a collision of packets. 328 00:22:27,160 --> 00:22:30,980 You just have to wait. Packet collisions don't crush your car. 329 00:22:30,980 --> 00:22:34,470 They just slow your data down. So, here we go. 330 00:22:34,470 --> 00:22:39,170 This is basically carrier sense media access with collision detection. 331 00:22:39,170 --> 00:22:42,020 The first thing you do if you want to send some data, say we got some data 332 00:22:42,020 --> 00:22:45,370 inside our little computer and we want to send it out, knowing that there might be 333 00:22:45,370 --> 00:22:49,660 other data going by The first thing we do is listen. 334 00:22:51,600 --> 00:22:54,920 We listen to what's on there. If it's not silent we just wait until it 335 00:22:54,920 --> 00:22:59,622 goes silent and then we start sending. So if someone's already using it, why 336 00:22:59,622 --> 00:23:02,742 crash into their data and crush their data? 337 00:23:02,742 --> 00:23:05,060 Because there's, it's shared, there's only one. 338 00:23:05,060 --> 00:23:07,649 You can't both be sending at the same time. 339 00:23:08,660 --> 00:23:12,776 So first you listen. Once it's silent you begin transmitting 340 00:23:12,776 --> 00:23:16,000 data. [COUGH] And then what you do is you also 341 00:23:16,000 --> 00:23:21,120 listen to your own data and if your own data is coming back, then it's sounding 342 00:23:21,120 --> 00:23:29,680 pretty good. and And then if, if you. 343 00:23:29,680 --> 00:23:32,956 If there is a collision, because there's a chance that two computers will want to 344 00:23:32,956 --> 00:23:36,022 send at the same time and they'll collide. 345 00:23:36,022 --> 00:23:39,850 Then what they have to do is back off.. And they have a sophisticated random 346 00:23:39,850 --> 00:23:44,380 number calculation, so that they back off not always the same amount. 347 00:23:44,380 --> 00:23:47,380 Each computer backs off a different amount, and they make it so that it's 348 00:23:47,380 --> 00:23:50,255 fair. So that one computer's not always backing 349 00:23:50,255 --> 00:23:52,100 the most off. So it's very fair. 350 00:23:52,100 --> 00:23:59,150 So it's, when you detect a collision, you retransmit after a random backup. 351 00:23:59,150 --> 00:24:03,395 First, you avoid collisions. But in the, in the rare case that you 352 00:24:03,395 --> 00:24:07,994 have a collision, then you do this. And so, 353 00:24:10,390 --> 00:24:18,312 [NOISE] I want to introduce you to the person who invented Ethernet, Ethernet. 354 00:24:18,312 --> 00:24:23,520 Hmm, This is Robert Melcalf and he was working at Xerox, Palo Alto research 355 00:24:23,520 --> 00:24:29,855 center, PARC, everyone calls it PARC. And they had, they were building what is 356 00:24:29,855 --> 00:24:34,320 most considered the first computer, the Alto computers. 357 00:24:34,320 --> 00:24:38,290 They were connecting line printers, or, fast printers to these. 358 00:24:38,290 --> 00:24:46,050 Pate, Xerox was building the first, laser printers. 359 00:24:46,050 --> 00:24:49,018 And they needed something faster. And so they just started building, and 360 00:24:49,018 --> 00:24:52,580 they built this thing called Ethernet. Now it's a little different than the 361 00:24:52,580 --> 00:24:54,956 Ethernet that we see, the first Ethernet that they built is a little different, 362 00:24:54,956 --> 00:24:58,679 you can see a picture of it down here. If you ever go to Xerox Park and get in, 363 00:24:58,679 --> 00:25:01,195 you'll go see this little museum they have. 364 00:25:01,195 --> 00:25:06,025 it used a single piece of cable that ran along, they just ran it down the hallway, 365 00:25:06,025 --> 00:25:12,850 and they would connect a tap in And then they would use that to send the data. 366 00:25:12,850 --> 00:25:16,630 So it would go down the hallway and a tap come out at each office, and they were 367 00:25:16,630 --> 00:25:20,540 truly sharing the media. This comment is not shared, except when 368 00:25:20,540 --> 00:25:23,540 you plug it into a hub. That's what makes the shared. 369 00:25:23,540 --> 00:25:28,160 it was inspired by an earlier wireless network where they were doing not 370 00:25:28,160 --> 00:25:33,060 collision detection, but retransmission called Aloha, and a lot of early network, 371 00:25:33,060 --> 00:25:38,380 packet network inspiration comes from the Alohanet in Uni-, comes from University 372 00:25:38,380 --> 00:25:44,038 of Hawaii. And because they didn't have anything 373 00:25:44,038 --> 00:25:47,027 except wireless, but they did have wireless, they did a lot of really cool 374 00:25:47,027 --> 00:25:52,149 early research on wireless. And so let's go ahead and meet Bob 375 00:25:52,149 --> 00:25:56,580 Metcalfe the inventor, one of the inventors of ethernet.