1 00:00:00,4 --> 00:00:03,74 Welcome, back. So, I hope you enjoyed that. 2 00:00:03,74 --> 00:00:06,914 Now, I want to make it real clear that, when I give you a 15 minute video of an 3 00:00:06,914 --> 00:00:11,202 amazing inventor and computer scientist, you don't have to remember every word 4 00:00:11,202 --> 00:00:14,824 that that person says. Okay? 5 00:00:14,824 --> 00:00:18,260 it's more important to get the gist of it. 6 00:00:18,260 --> 00:00:21,816 I try to cover the things I really want you to know in my slides. 7 00:00:21,816 --> 00:00:25,912 You might want to listen to it more than once, or listen to my slides and then go 8 00:00:25,912 --> 00:00:30,8 back, but you're not, I, I don't want you to memorize it, I, I wanted to give you 9 00:00:30,8 --> 00:00:34,232 smart people, I want you to hear from the smart people who did all this cool work, 10 00:00:34,232 --> 00:00:40,667 in their own words. But when we hear from their own words, 11 00:00:40,667 --> 00:00:44,486 they're sometimes pretty technical, so just relax and enjoy listening to these 12 00:00:44,486 --> 00:00:49,360 people and understand that you're not going to get everything. 13 00:00:49,360 --> 00:00:53,20 But hopefully you'll come back and you'll get more and more later. 14 00:00:53,20 --> 00:00:57,298 So the idea was that he really started with this idea of wireless which was, 15 00:00:57,298 --> 00:01:01,921 we're going to share one medium, and the wireless and the wire, the Ethernet wire 16 00:01:01,921 --> 00:01:06,475 that he built, the coaxial cable that he built, was like a giant radio, except it 17 00:01:06,475 --> 00:01:13,250 ran in the ceiling. And, so, the design ended up really 18 00:01:13,250 --> 00:01:17,730 simple and that's been good for Ethernet over the years, and later WiFi is a, a 19 00:01:17,730 --> 00:01:23,352 variation of Ethernet. and so it turns out to be sometimes it's 20 00:01:23,352 --> 00:01:27,192 best to build something simple but that just make it go really fast and make it 21 00:01:27,192 --> 00:01:31,909 really cheap. He also formed the company 3 Con which 22 00:01:31,909 --> 00:01:35,760 was one of the first manufacturers of PC cards. 23 00:01:35,760 --> 00:01:39,115 And so back in the day you would go buy a PC and you would go buy a 3COM card and 24 00:01:39,115 --> 00:01:42,770 you'd plug it into the back of your computer. 25 00:01:42,770 --> 00:01:47,630 And so, there was a time when San Francisco stadium Candlestick Park was 26 00:01:47,630 --> 00:01:54,902 named 3COM Stadium and our friend Bob Metcalfe was involved in all of that. 27 00:01:54,902 --> 00:01:58,542 So, he's done many things throughout his life, and we're honored to have met him. 28 00:01:58,542 --> 00:02:02,562 So, we started with this four layer architecture, that says we're going to 29 00:02:02,562 --> 00:02:06,917 break down a big problem of cooperating applications across various kinds of 30 00:02:06,917 --> 00:02:11,355 networks. We're going to break it down, and we just 31 00:02:11,355 --> 00:02:16,938 got them talking about the Link Layer. It's literally, 20,000, maybe 50,000 32 00:02:16,938 --> 00:02:21,452 engineers have spent the last 20 years figuring out how to make this work, 33 00:02:21,452 --> 00:02:26,262 because the layered architecture lets them think, although it was informed, 34 00:02:26,262 --> 00:02:31,72 they had 20,000, 50,000, I don't know, 20,000 engineers that think about that 35 00:02:31,72 --> 00:02:38,372 problem, and the, they ignore the rest of the problems. 36 00:02:38,372 --> 00:02:42,960 'Kay they ignore the rest of the problems. 37 00:02:42,960 --> 00:02:46,560 That's great, because they've gotten really good at this one problem. 38 00:02:46,560 --> 00:02:49,540 Now, we're going to go like, you know, I don't know how many engineers. 39 00:02:49,540 --> 00:02:53,560 Let's just make it up, 5k engineers think about this next problem. 40 00:02:53,560 --> 00:02:59,147 The next problem is called, oh my colors got changed, the internetwork layer. 41 00:03:00,230 --> 00:03:05,218 It is the notion of forwarding each of the post cards with a from and a to 42 00:03:05,218 --> 00:03:13,280 address, forwarding enough times to get them all the way across the network. 43 00:03:13,280 --> 00:03:16,430 That's the next problem we're going to solve, we're going to stop thinking about 44 00:03:16,430 --> 00:03:20,910 the link layer, we're just going to assume it works, it's just magic. 45 00:03:20,910 --> 00:03:23,100 That's what a layered architecture gives you. 46 00:03:23,100 --> 00:03:24,600 You don't worry about the stuff above you. 47 00:03:24,600 --> 00:03:26,790 You don't worry about the stuff below you. 48 00:03:26,790 --> 00:03:33,104 You focus like crazy on the stuff that you're focusing on. 49 00:03:33,104 --> 00:03:40,560 So, the link layer only works on one link, right? 50 00:03:40,560 --> 00:03:45,780 It, it worries about one link, and there might be 15 or so of these links. 51 00:03:45,780 --> 00:03:51,630 But the Internet layer worries about all the links, and the proper sequence of 52 00:03:51,630 --> 00:03:59,320 links to follow to get from to Stanford. That's a kind of complex problem. 53 00:03:59,320 --> 00:04:01,780 We're not even going to worry about reliability. 54 00:04:01,780 --> 00:04:05,501 We are just going to worry about if I had a pack-, if I had a postcard with a from 55 00:04:05,501 --> 00:04:09,620 and a to address, a packet, can I get it there? 56 00:04:09,620 --> 00:04:13,313 How will I forward it? I assume the link layer is perfect so the 57 00:04:13,313 --> 00:04:17,95 Internet layer is the first end to end, because if you recall this vertical box, 58 00:04:17,95 --> 00:04:21,60 this set of boxes is the host starting computer, and this vertical set of boxes 59 00:04:21,60 --> 00:04:28,120 is the destination computer. So in this little raspberry pie, all four 60 00:04:28,120 --> 00:04:33,547 of these things, application, transport, IP, and the link is all part of this one 61 00:04:33,547 --> 00:04:39,475 gadget, okay? So IP is best effort and it's okay to 62 00:04:39,475 --> 00:04:43,547 drop data if things go bad. And that's one of it's charming, most 63 00:04:43,547 --> 00:04:47,800 charming features. But what it had to introduce, in addition 64 00:04:47,800 --> 00:04:52,450 to the Ethernet addresses or the media access layer or Mac addresses, is an 65 00:04:52,450 --> 00:04:58,766 address of the destination. Now, if you remember, the Mac addresses, 66 00:04:58,766 --> 00:05:02,100 the addresses for the link layer, come from the manufacturer. 67 00:05:02,100 --> 00:05:05,628 The moment this equipment is manufactured, it is burned in a serial 68 00:05:05,628 --> 00:05:09,100 number. But these move all around the world. 69 00:05:09,100 --> 00:05:12,70 If this runs here at the University of Michigan, it needs one address to connect 70 00:05:12,70 --> 00:05:15,862 to the network. One IP address, if it runs at Stanford, 71 00:05:15,862 --> 00:05:19,622 it needs a different IP address. So we have to be able to change these, so 72 00:05:19,622 --> 00:05:24,222 they're assigned different. So it's the worldwide number that is your 73 00:05:24,222 --> 00:05:28,669 like postcard address. It's like a phone number, right, so 74 00:05:28,669 --> 00:05:33,213 wherever I'm at, you call my phone, and this gadget rings, so its address Well 75 00:05:33,213 --> 00:05:39,966 that's actually kind of a[SOUND] bad example cause I used a sync on it. 76 00:05:39,966 --> 00:05:41,680 Okay. Ignore that. 77 00:05:41,680 --> 00:05:44,351 Phones use magic, they use crazy magic. So, and computers they're not as cool as 78 00:05:44,351 --> 00:05:49,153 phones. You have to change the address everywhere 79 00:05:49,153 --> 00:05:52,4 you go. We'll talk about how you change the 80 00:05:52,4 --> 00:05:56,26 address everywhere you go. The IP addresses are based on where the 81 00:05:56,26 --> 00:06:00,422 station is connected. They do get reorganized once in a great 82 00:06:00,422 --> 00:06:04,232 while but not very often. And you can even go to various websites 83 00:06:04,232 --> 00:06:08,136 on the internet and say IP address look up is the most common search and it will 84 00:06:08,136 --> 00:06:12,704 tell you something about where your coming from. 85 00:06:12,704 --> 00:06:14,703 Right? You know, kind of be sometimes really 86 00:06:14,703 --> 00:06:18,442 weird, because you might be at Starbucks, and it might send it all to St. 87 00:06:18,442 --> 00:06:20,96 Louis, and it might say, oh you're coming from St. 88 00:06:20,96 --> 00:06:22,730 Louis, even if you're not. So you can look it up, and you can look 89 00:06:22,730 --> 00:06:26,16 up other addresses as well. You can say oh, here's an IP address, I'm 90 00:06:26,16 --> 00:06:30,52 going to look this one up. First it starts by looking yours, but you 91 00:06:30,52 --> 00:06:34,540 can put in different addresses, like in this particular one If I put a different 92 00:06:34,540 --> 00:06:40,150 address in here, it will actually go look that address up. 93 00:06:40,150 --> 00:06:45,10 So the IP address format, it's four numbers that are separated by dots. 94 00:06:45,10 --> 00:06:48,904 Each of the number can be between 1 and 2,000 and 255 and it suggests a 95 00:06:48,904 --> 00:06:53,62 representation of a 32 bit number, back in the day, it kept the numbers small 96 00:06:53,62 --> 00:06:58,804 because it didn't want to use all the computer memory. 97 00:06:58,804 --> 00:07:03,620 and so, this is an example of an IP address. 98 00:07:03,620 --> 00:07:09,338 Four numbers between zero and 255, separated by dots, okay? 99 00:07:09,338 --> 00:07:14,960 Now the concept, the address is broken into two parts. 100 00:07:14,960 --> 00:07:19,47 There is the network number part which is the prefix, and then there is the 101 00:07:19,47 --> 00:07:25,39 computer number within network, 'kay? And it's kind of like phone numbers were 102 00:07:25,39 --> 00:07:29,857 before cellphones where the area code was where the phone was, and then this was 103 00:07:29,857 --> 00:07:36,392 within that area code, where to find it. And even in the older days, these were 104 00:07:36,392 --> 00:07:40,104 actually geographic too, right, so these numbers would be neighborhoods or 105 00:07:40,104 --> 00:07:44,312 whatever. These days, it's all electronic, so the 106 00:07:44,312 --> 00:07:49,730 precise mapping of a phone number to a geography is less precise. 107 00:07:49,730 --> 00:07:52,475 And that's why you get a cell phone number and you move to a whole new state, 108 00:07:52,475 --> 00:07:56,600 you keep your old phone cell phone number because it's become electronic. 109 00:07:56,600 --> 00:08:00,696 But in the early days, when it was actual relays and switches making the phone 110 00:08:00,696 --> 00:08:04,918 numbers work, they actually had to do with where. 111 00:08:04,918 --> 00:08:07,980 It's actually kind of fascinating how phones work. 112 00:08:07,980 --> 00:08:11,576 I ought to, I ought to teach a class on like how phones switches worked in the 113 00:08:11,576 --> 00:08:16,164 1800s. Actually quite fascinating, because they 114 00:08:16,164 --> 00:08:20,495 were surprisingly simple. That's the interesting part is how simple 115 00:08:20,495 --> 00:08:24,942 turn of the century, well turn of last century, phone numbers were. 116 00:08:24,942 --> 00:08:30,540 So, lets get back to IP addresses. There's four numbers. 117 00:08:30,540 --> 00:08:33,450 Some part of those four numbers is a prefix. 118 00:08:33,450 --> 00:08:36,996 We call that the network number. And when the packet is in the middle of 119 00:08:36,996 --> 00:08:40,717 the network, it doesn't really look at these numbers, it only looks at the 120 00:08:40,717 --> 00:08:44,133 prefix, so it kind of thinks of all the packets going to a piece of the 121 00:08:44,133 --> 00:08:48,37 University of Michigan as 141.211.*.*, and then we, we assign these numbers 122 00:08:48,37 --> 00:08:52,185 within Michigan, and then this number is assigned to us by the internet authority 123 00:08:52,185 --> 00:08:59,306 that assigns numbers. So we say hey, we need some more and they 124 00:08:59,306 --> 00:09:03,262 give us a prefix. Then within that prefix we get to set up 125 00:09:03,262 --> 00:09:06,940 all the other things. So. 126 00:09:06,940 --> 00:09:10,660 This network number which is the prefix of the IP address is the way that a 127 00:09:10,660 --> 00:09:17,160 packet is routed through the internet as it progresses through the internet. 128 00:09:17,160 --> 00:09:21,696 And so it's sort of like if I start with, if I start with my computer here in U of 129 00:09:21,696 --> 00:09:26,770 M, and that's my number, that's my actual number. 130 00:09:26,770 --> 00:09:31,100 And here's a Stanford computer, and that's its number, alright? 131 00:09:31,100 --> 00:09:37,871 And I send a packet and I say, okay, I'm going to send it from 141 211 144 188 269 132 00:09:37,871 --> 00:09:43,960 67 149 yada yada. As soon as the packet enters the network, 133 00:09:43,960 --> 00:09:48,270 it doesn't throw the data away, but it stops thinking about it. 134 00:09:48,270 --> 00:09:52,486 It only looks at the prefix and so it's simplify, greatly simplify what the 135 00:09:52,486 --> 00:09:58,90 routers have to do as it go across the hops on the internet. 136 00:09:58,90 --> 00:10:01,30 It's greatly simplified because it only looks at the first part. 137 00:10:01,30 --> 00:10:02,592 It doesn't have to look at the whole thing. 138 00:10:02,592 --> 00:10:06,480 [COUGH] And then this starts making decisions as to how to get there. 139 00:10:06,480 --> 00:10:09,897 So it says okay, I'll hop it over here, again it's like in Kansas City and says, 140 00:10:09,897 --> 00:10:13,110 what am I going to do here and I'll pick one of those two links and I'll go over 141 00:10:13,110 --> 00:10:16,272 here and then, as it exits the network, it dumps it somehow onto Stanford's 142 00:10:16,272 --> 00:10:21,788 campus. And then, it finds its way. 143 00:10:21,788 --> 00:10:24,120 Oh, it didn't find it to this one;, it found its way to that one. 144 00:10:24,120 --> 00:10:27,49 There you go. Unpredictable computers. 145 00:10:27,49 --> 00:10:30,514 Now it, so within the Stanford campus, these numbers mean something and the 146 00:10:30,514 --> 00:10:35,660 Stanford campus uses those numbers to get the thing to the actual real computer. 147 00:10:35,660 --> 00:10:40,224 And so it sort of used this prefix. You know, it has a real number, but then 148 00:10:40,224 --> 00:10:45,326 it uses the prefix in the middle and then the real number reappears at the end. 149 00:10:45,326 --> 00:10:51,260 So that's called a network number. And it greatly simplifies. 150 00:10:51,260 --> 00:10:55,418 It's complex enough to be in the middle and have tot worry about all these things 151 00:10:55,418 --> 00:10:59,639 coming from Kansas City and Beijing and, like, where do these things go to have to 152 00:10:59,639 --> 00:11:05,510 look at each one and know where every computer is is crazy. 153 00:11:05,510 --> 00:11:09,454 So that's why they call it the Internetwork Protocol, because it says 154 00:11:09,454 --> 00:11:13,942 "there's one network, there's another network, and the only thing I care about 155 00:11:13,942 --> 00:11:19,608 is the network number, and I am the Internetwork Protocol. 156 00:11:19,608 --> 00:11:23,642 I" P, in a network protocol. I'm just getting these things from one 157 00:11:23,642 --> 00:11:26,678 network to another network, and then it's up to that network to figure out how to 158 00:11:26,678 --> 00:11:29,576 get the darn thing to the right computer within it, and it's up to Stanford to 159 00:11:29,576 --> 00:11:34,160 figure out how to get it to the right computer at Stanford. 160 00:11:34,160 --> 00:11:37,877 So these things are called the network number, it's the prefix of the IP 161 00:11:37,877 --> 00:11:40,928 address. Now the key thing that its really a 162 00:11:40,928 --> 00:11:44,648 beautiful, beautiful design. because the, because the center of the 163 00:11:44,648 --> 00:11:48,400 network is both exceedingly complex in one way. 164 00:11:48,400 --> 00:11:52,270 That it's so big and so fast. But exceedingly simple in the other. 165 00:11:52,270 --> 00:11:56,40 It just has to move the data from point A to point B. 166 00:11:56,40 --> 00:12:01,456 If you think about it, it's even simpler. If we look at it from the perspective of 167 00:12:01,456 --> 00:12:04,976 this router, the router here I've got circled. 168 00:12:04,976 --> 00:12:08,89 Right? It just received a packet, it looks at 169 00:12:08,89 --> 00:12:11,865 the network number, and it actually doesn't even care, to some degree, where 170 00:12:11,865 --> 00:12:16,760 it really belongs. It only has one of two choices. 171 00:12:16,760 --> 00:12:20,131 It's going to go this way, it's going to go that way, right? 172 00:12:20,131 --> 00:12:24,804 Which one's better? And it turns out, if you look at it, 173 00:12:24,804 --> 00:12:28,777 either one will work. It turns out this one is better but if I 174 00:12:28,777 --> 00:12:33,624 went this way, it would just take an extra tenth of a second or something. 175 00:12:33,624 --> 00:12:35,640 Right? So it turns out that decisions that are 176 00:12:35,640 --> 00:12:40,314 made don't even have to be perfect. You can make the wrong decision, and the 177 00:12:40,314 --> 00:12:44,950 network automatically corrects. That's part of the goal of the network. 178 00:12:44,950 --> 00:12:49,238 And so it really simplifies and limits the need for each router to understand 179 00:12:49,238 --> 00:12:53,258 the entire network, which makes these routers, I draw 'em small on these 180 00:12:53,258 --> 00:12:58,495 diagrams on purpose. Because we think of them as small and 181 00:12:58,495 --> 00:13:02,207 fast, and only solving a really tiny problem but doing it really super 182 00:13:02,207 --> 00:13:07,565 awesome. So routers maintain what we call router 183 00:13:07,565 --> 00:13:11,275 tables. And they maintain a list of network 184 00:13:11,275 --> 00:13:17,13 numbers and the best outbound route for each of the network numbers. 185 00:13:17,13 --> 00:13:22,620 Now, the other thing that they do is they pass routes back and forth. 186 00:13:22,620 --> 00:13:25,530 And this is how they adapt to errors, right? 187 00:13:25,530 --> 00:13:29,140 They get updated dynamically. They ask each other for the best place. 188 00:13:29,140 --> 00:13:33,26 If they see a network number they haven't seen before, they ask their neighbors, 189 00:13:33,26 --> 00:13:36,912 then their neighbors' neighbors, and they go, "Oh, God, OK; I got a good way to get 190 00:13:36,912 --> 00:13:40,624 to 16749 And so there's all kinds of communication, but it's relatively slow 191 00:13:40,624 --> 00:13:46,702 and it doesn't have to be perfect. And so it's router tables are what 192 00:13:46,702 --> 00:13:49,887 routers have but they're indexed by the network number of the packets, not the 193 00:13:49,887 --> 00:13:55,358 host of the destination. So that's an amazing, amazing improvement 194 00:13:55,358 --> 00:14:01,970 in the performance and efficiency of the Internet core, the IP core. 195 00:14:01,970 --> 00:14:07,228 So it's really quite simple, right? You basically have a local area network 196 00:14:07,228 --> 00:14:11,440 on your campus. This might actually be your house too. 197 00:14:11,440 --> 00:14:15,614 House is kind of like a campus. And you can do all the crazy things you 198 00:14:15,614 --> 00:14:19,124 want hundreds of computers thousands of hundreds of servers and thousands of 199 00:14:19,124 --> 00:14:23,922 laptops. And you get one address that is the 200 00:14:23,922 --> 00:14:29,276 address of your campus. It's the network number of your campus to 201 00:14:29,276 --> 00:14:33,316 the rest of the world. And then all over the world people can 202 00:14:33,316 --> 00:14:38,340 send to you and by simply looking at the prefix. 203 00:14:38,340 --> 00:14:43,26 Data makes it to you. Keeps this really simple and really fast, 204 00:14:43,26 --> 00:14:45,574 OK? So it's beautiful. 205 00:14:45,574 --> 00:14:50,562 That's with a network number, one area code, one network number. 206 00:14:50,562 --> 00:14:53,744 It's the only thing has to be kept track of. 207 00:14:53,744 --> 00:14:57,464 A whole campus can be characterized within the quarter of the network as 208 00:14:57,464 --> 00:15:02,544 basically a single network number. Now, the reality is, it's usually a few 209 00:15:02,544 --> 00:15:05,192 of these. because they give them to you in smaller 210 00:15:05,192 --> 00:15:08,342 chunks, so you end up with you know, 20 or 30 of these things for your campus if 211 00:15:08,342 --> 00:15:12,282 you're a medium to large campus. Okay. 212 00:15:12,282 --> 00:15:20,262 So now I want to talk a little bit about the problem of computers that move 213 00:15:20,262 --> 00:15:25,240 around. So you have a laptop, and you use it at 214 00:15:25,240 --> 00:15:29,84 your local coffee shop, and then you close the lid and you go home and open it 215 00:15:29,84 --> 00:15:33,114 and it works home, and then you close the lid and you go to school and you open it, 216 00:15:33,114 --> 00:15:38,828 and talk to wireless at school and it works. 217 00:15:38,828 --> 00:15:42,374 It's like, Hey Dr. Chuck you just told me that I have to 218 00:15:42,374 --> 00:15:46,680 have this address and I can't talk on the internet if don't have the right address. 219 00:15:46,680 --> 00:15:50,230 And the packets are routed based on the prefix of this address. 220 00:15:50,230 --> 00:15:54,500 Why is it that I can be in three places: coffee shop, home, and school and it 221 00:15:54,500 --> 00:16:00,700 seems like everything works? Well, That's because while your computer 222 00:16:00,700 --> 00:16:05,870 has an Ethernet address that's baked into the factory. 223 00:16:05,870 --> 00:16:09,582 Most computers are configured when they first open up and connect to a Wifi to 224 00:16:09,582 --> 00:16:14,300 ask instead of having an IP address configured in your computer. 225 00:16:14,300 --> 00:16:19,640 They send a request out to say, hey I'm new here. 226 00:16:19,640 --> 00:16:22,775 Is there anyone who will give me an IP address that I might use for this 227 00:16:22,775 --> 00:16:28,168 particular location? And, if there is an access point, say, at 228 00:16:28,168 --> 00:16:33,580 your Starbucks, it says sure, use this one. 229 00:16:33,580 --> 00:16:36,930 You're at Starbucks, that's a good number for Starbucks. 230 00:16:36,930 --> 00:16:40,950 Now, it turns out that the prefix of this is exactly the prefix that the, the world 231 00:16:40,950 --> 00:16:44,910 sees all this traffic to Starbucks, so there's actually kind of a real address, 232 00:16:44,910 --> 00:16:51,148 Okay, and it gives you an address. Okay, so you ask, when your thing comes 233 00:16:51,148 --> 00:16:56,910 up, it asks, what can I get, and then it's told what address to use. 234 00:16:56,910 --> 00:17:00,558 So using one address while you're at Starbucks, a different address at home, 235 00:17:00,558 --> 00:17:03,910 and a different address yet again at school. 236 00:17:03,910 --> 00:17:09,88 So that's called the dynamic host configuration protocol. 237 00:17:09,88 --> 00:17:18,200 Now it turns out, with schools and homes and, there's just too many computers Give 238 00:17:18,200 --> 00:17:26,745 every computer a real address. And so we have these special addresses, 239 00:17:26,745 --> 00:17:30,446 they're called the non-routable addresses. 240 00:17:30,446 --> 00:17:34,352 So you'll probably notice if you go to one person's house, you will have a IP 241 00:17:34,352 --> 00:17:39,290 address of 192 dot 168 dot something dot something. 242 00:17:39,290 --> 00:17:41,690 Go to a different person's house it'll also be 192.168. 243 00:17:41,690 --> 00:17:45,3 something dot something. And you go to your house and it's 244 00:17:45,3 --> 00:17:47,656 192.168. something dot something. 245 00:17:47,656 --> 00:17:51,972 And you're like, how can that work? I seem to have the same address. 246 00:17:51,972 --> 00:17:56,732 Well, that's because of a technology called network address translation, where 247 00:17:56,732 --> 00:18:01,352 each of the home routers actually has a unique and distinct address, but you're 248 00:18:01,352 --> 00:18:05,916 not seeing it. It's giving you a temporary address, an 249 00:18:05,916 --> 00:18:09,244 address that really can't run at all on the Internet.It only lives within the 250 00:18:09,244 --> 00:18:12,676 house, but then as your packets leave the house or leave the Starbucks, the real 251 00:18:12,676 --> 00:18:16,4 address is put in, and then when it comes back the real address is taken out and 252 00:18:16,4 --> 00:18:22,22 your local address is put in. So they're called non-routable addresses 253 00:18:22,22 --> 00:18:24,710 because if they ever escaped into the real internet, they'd be like, oh those 254 00:18:24,710 --> 00:18:27,398 are for your house, they're only for, they're not supposed to go very far and I 255 00:18:27,398 --> 00:18:32,247 don't even know where they go. They go nowhere in the core of the 256 00:18:32,247 --> 00:18:36,320 internet but they go properly inside your house. 257 00:18:36,320 --> 00:18:42,365 So the way this ends up working Is if you are at your coffee shop, the coffee shop 258 00:18:42,365 --> 00:18:48,558 has an address. You associate with their base access 259 00:18:48,558 --> 00:18:51,712 point. It gives you a non-routable address to 260 00:18:51,712 --> 00:18:55,79 use locally, and then as your traffic gets sent. 261 00:18:56,450 --> 00:18:59,540 This address is changed to the address, that address. 262 00:18:59,540 --> 00:19:02,544 You never see this happening. It's done as the packet goes through the 263 00:19:02,544 --> 00:19:05,856 base station. And then when the packet comes back, it 264 00:19:05,856 --> 00:19:09,974 comes back with this address. But then as it goes to the base station 265 00:19:09,974 --> 00:19:13,943 it switches back to this address. So even though this address is not the 266 00:19:13,943 --> 00:19:18,118 real address the network sees, they see you having this address here. 267 00:19:18,118 --> 00:19:21,835 So if you would do an IP lookup from a coffee shop, the coffee shop will be 268 00:19:21,835 --> 00:19:26,739 identified. You'll see the IP address here, and if 269 00:19:26,739 --> 00:19:33,372 you looked at your laptop, your laptop would not have that same address, because 270 00:19:33,372 --> 00:19:39,836 it's being translated in the access point. 271 00:19:39,836 --> 00:19:47,228 So then, you go home, and at home, your access point has a different address, but 272 00:19:47,228 --> 00:19:54,956 your computer asks your as, ask, address, asks your access point foreign address, 273 00:19:54,956 --> 00:20:04,190 and it gets a local address that's generated locally. 274 00:20:04,190 --> 00:20:08,923 And again, there is a mapping. It's called NAT, NAT. 275 00:20:08,923 --> 00:20:13,890 Oh I'll change the color. NAT, Network Address Translation. 276 00:20:13,890 --> 00:20:17,250 So as the packet goes through, it takes out the one address and puts in the 277 00:20:17,250 --> 00:20:19,620 second. Dave comes back. 278 00:20:19,620 --> 00:20:22,156 He takes the address out and puts the local one in. 279 00:20:22,156 --> 00:20:25,641 And the same thing happens at school. Alright, so, you're at school finally and 280 00:20:25,641 --> 00:20:28,611 you get a different address. They're not the same, but you look and 281 00:20:28,611 --> 00:20:31,549 the prefixes are the same. It's like they should be the same, but 282 00:20:31,549 --> 00:20:34,783 they're completely different because your school has a different address and 283 00:20:34,783 --> 00:20:40,40 there's a translation that goes back and forth as the data goes round and round. 284 00:20:40,40 --> 00:20:44,566 So just for the distance of the local Wi-Fi, or whatever, use these 192.168 285 00:20:44,566 --> 00:20:49,457 numbers, and then they're translated to the real numbers by your network access 286 00:20:49,457 --> 00:20:55,755 points. So these are illegal inside this network, 287 00:20:55,755 --> 00:21:03,420 so if it ever saw a 192.168 It would just throw that packet away. 288 00:21:03,420 --> 00:21:12,657 They're only for very local connections. So with that, I'm wondering if by now, 289 00:21:12,657 --> 00:21:26,84 you know why this is funny. I'll give you a minute. 290 00:21:26,84 --> 00:21:36,387 OK. So the reason this is funny Is she traced 291 00:21:36,387 --> 00:21:42,723 the killer's IP address and it had a prefix of 192.168 which means that killer 292 00:21:42,723 --> 00:21:48,960 had to be within a short radius using likely the same WiFi access point which 293 00:21:48,960 --> 00:22:00,10 means it was close. Very scary. 294 00:22:00,10 --> 00:22:05,242 Hope you think it's funny. 'Course, XKCD is never exactly funny, but 295 00:22:05,242 --> 00:22:11,29 it hopefully makes you smile a little bit. 296 00:22:11,29 --> 00:22:15,973 So up till now,[COUGH] I've been talking about. 297 00:22:15,973 --> 00:22:25,330 [SOUND] So, up to now, I've been talking about this cloud network that's magic. 298 00:22:25,330 --> 00:22:29,120 And the packets take different routes and we don't know, but they show up. 299 00:22:29,120 --> 00:22:33,947 Remember the mail box, like, they just show up or don't show up, right? 300 00:22:33,947 --> 00:22:37,758 So, it turns out. That sometimes, we as engineers, want to 301 00:22:37,758 --> 00:22:41,920 take a look at what's going on inside the internet. 302 00:22:41,920 --> 00:22:45,616 And it turned out that there was a feature they added early on to help 303 00:22:45,616 --> 00:22:50,550 diagnose problems in the internet that we still use today. 304 00:22:50,550 --> 00:22:53,718 And it's so convenient that it's built right into your operating system. 305 00:22:53,718 --> 00:22:57,480 Y'know, in Macintosh or Linux, Traceroute is built in, and if you have Windows, 306 00:22:57,480 --> 00:23:01,632 you've got to install Traceroute. So just say Windows and Install 307 00:23:01,632 --> 00:23:06,460 Traceroute, and you'll find something. But there was a problem. 308 00:23:06,460 --> 00:23:12,155 So if you recall, I said that each router sees the world very narrowly, and simply 309 00:23:12,155 --> 00:23:18,130 sees a packet and makes a decision, one of one place. 310 00:23:18,130 --> 00:23:23,186 So let's say that here comes a packet on the way to Stanford, and these routers 311 00:23:23,186 --> 00:23:28,794 are sort of strangely configured. This router thinks it should go there, 312 00:23:28,794 --> 00:23:33,920 this router thinks it should go there, and this router thinks it shoud go there. 313 00:23:33,920 --> 00:23:36,376 Well it comes in. It goes like, oh, well I know where that 314 00:23:36,376 --> 00:23:40,680 one goes, there. So if you end up with this misconfigured 315 00:23:40,680 --> 00:23:48,400 router situation, you end up with your data going round and round in circles. 316 00:23:48,400 --> 00:23:51,969 It would actually, like, crush the network because it's like a whirlpool. 317 00:23:53,410 --> 00:23:57,863 You can't even notice that it's happening, because there might acutally 318 00:23:57,863 --> 00:24:01,449 be, you know, ten. You know there might be a bunch of them, 319 00:24:01,449 --> 00:24:04,250 and then it comes back and you send it around again. 320 00:24:04,250 --> 00:24:07,870 So you're filling up all your bandwidth. It's never going to get there, unless 321 00:24:07,870 --> 00:24:11,390 something changes, unless something crashes. 322 00:24:11,390 --> 00:24:13,870 It's not going to change because these routers think that's the best thing to 323 00:24:13,870 --> 00:24:15,740 do. They're mistaken. 324 00:24:15,740 --> 00:24:19,898 But they are, they can be mistaken because they're, they're operating with 325 00:24:19,898 --> 00:24:24,800 imperfect information. So how would you solve this problem? 326 00:24:24,800 --> 00:24:29,816 The routers are imperfect but they solve the problem with a thing called Time to 327 00:24:29,816 --> 00:24:34,376 Live field. So much like the network entrance 328 00:24:34,376 --> 00:24:38,406 translation tweaks the addresses on the way in and the way out, Time to Live is a 329 00:24:38,406 --> 00:24:43,648 field that routers change. Every time a packet goes through a router 330 00:24:43,648 --> 00:24:48,200 it subtracts one from this field. And it starts with a number between 25, 331 00:24:48,200 --> 00:24:51,648 it can be as high as 255 but it usually like 25. 332 00:24:51,648 --> 00:24:57,360 And what happens is, is every time the packet goes through a router, the number 333 00:24:57,360 --> 00:25:02,614 goes down by one. So, if it was, it would be come in here 334 00:25:02,614 --> 00:25:07,310 as 255 and it will go through here 254, and 253. 335 00:25:07,310 --> 00:25:11,862 It will come back it will be 252 and what would happen is eventually. 336 00:25:11,862 --> 00:25:17,217 We get to one of these guys and it hit 0 and they would decide okay, you've been 337 00:25:17,217 --> 00:25:22,726 running around to long we will throw you away. 338 00:25:22,726 --> 00:25:27,57 So the number goes down, and it always goes down but then when it hits 0 they 339 00:25:27,57 --> 00:25:33,600 throw the packet away. They say you have been through 255 hops. 340 00:25:33,600 --> 00:25:37,260 Chances are good you're never going to get to your destination. 341 00:25:37,260 --> 00:25:44,586 So the Traceroute command sort of cheats. Normal packets are sent with a TTL, or 342 00:25:44,586 --> 00:25:49,190 time to live, of like 30, 30 hops or 40 hops. 343 00:25:49,190 --> 00:25:54,455 But what Traceroute does is send broken packets It turns out that when a router 344 00:25:54,455 --> 00:25:59,477 throws away your packet most of the time it is courteous, and it sends back a 345 00:25:59,477 --> 00:26:04,661 notification and says hi I got your packet, I decremented it, subtracted one 346 00:26:04,661 --> 00:26:13,540 and it got to zero and I threw it away. Sorry about that. 347 00:26:13,540 --> 00:26:16,110 Here's here's who I am. I mean I really feel bad about throwing 348 00:26:16,110 --> 00:26:18,349 your packet away. Maybe you want to figure something else 349 00:26:18,349 --> 00:26:19,30 out. I don't know. 350 00:26:19,30 --> 00:26:21,630 Something must be messed up. It can't be my fault, but I threw it 351 00:26:21,630 --> 00:26:24,6 away. So what it does is, it first sends a 352 00:26:24,6 --> 00:26:28,456 packet with a time to live of one. So the first router goes like hello, this 353 00:26:28,456 --> 00:26:31,720 has been around a long time. Sets to zero, throws it away. 354 00:26:31,720 --> 00:26:36,520 Then sends a note back Then Traceroute sends a packet of two across. 355 00:26:36,520 --> 00:26:40,470 It goes hop, hop, and then it gets thrown away, and a little note comes back. 356 00:26:40,470 --> 00:26:45,244 So you can kind of build a map by sending enough packets and getting kind of a 357 00:26:45,244 --> 00:26:51,340 return rejection from one of the routers. It would have got there. 358 00:26:51,340 --> 00:26:54,939 So if, for example, I do a Traceroute from the University of Michigan to 359 00:26:54,939 --> 00:27:00,72 Stanford, you'd, if I did it now, it'd be a different set of things. 360 00:27:00,72 --> 00:27:05,700 [COUGH] I get this output, and if I take a look at this, this is the hop. 361 00:27:05,700 --> 00:27:10,227 So there's the first one, two, three, four through 14, 14 hops. 362 00:27:10,227 --> 00:27:13,444 So it takes 14 hops. Now interestingly, again I don't remember 363 00:27:13,444 --> 00:27:17,191 what the hop count was in the store in former days but is quite a few. 364 00:27:17,191 --> 00:27:21,525 And that's because it's optimizing geography. 365 00:27:21,525 --> 00:27:26,772 and so you can see the first hop is on my campus. 366 00:27:26,772 --> 00:27:32,530 Then the second and third hop are bouncing around my campus some more. 367 00:27:32,530 --> 00:27:34,730 Now we don't quite know where this one is. 368 00:27:34,730 --> 00:27:38,415 But now it's on an, on a national network called internet 2, internet 2, internet 369 00:27:38,415 --> 00:27:41,424 2. This is probably going across the 370 00:27:41,424 --> 00:27:44,280 country. And then it ends up on scenic, which I 371 00:27:44,280 --> 00:27:48,351 think is sort of California's network, it bounces through California's network a 372 00:27:48,351 --> 00:27:55,96 couple of times. Let's see hprlax, that's Los Angeles. 373 00:27:55,96 --> 00:28:00,352 And this is, I don't know where that is. Los Angeles, that looks like it's making 374 00:28:00,352 --> 00:28:04,820 Oakland, I don't know, there's probably some meaning to these things. 375 00:28:04,820 --> 00:28:09,760 So it's making it through Oakland, then it's going to Stanford from something 376 00:28:09,760 --> 00:28:12,860 Okland. I don't know. 377 00:28:12,860 --> 00:28:16,565 But now it's on Stanford campus, and now it has three more hops to get across the 378 00:28:16,565 --> 00:28:20,530 Stanford campus To the Stanford campus's web server. 379 00:28:20,530 --> 00:28:25,890 Now what's also going on here, is it's keeping track of how long it's taking. 380 00:28:25,890 --> 00:28:29,823 It sends each one a couple of times, and so these are milliseconds, so 381 00:28:29,823 --> 00:28:36,475 milliseconds are thousandths of a second, so 534 milliseconds is a half a second. 382 00:28:36,475 --> 00:28:40,917 490 is a half a second. So these are, like, half a second, half a 383 00:28:40,917 --> 00:28:45,471 second, oh, wait, no, no, sorry, not half a second, that'd be 4,000, well, 1,000 384 00:28:45,471 --> 00:28:51,136 milliseconds would be one second so I got that all wrong. 385 00:28:51,136 --> 00:28:56,265 So, 0.49 milliseconds is like half of a thousandth of a second. 386 00:28:56,265 --> 00:29:01,568 So that's fast, fast. 6 milliseconds is 6 1/1000ths of a 387 00:29:01,568 --> 00:29:07,410 second. 76 milliseconds, which is 9 hops away, 388 00:29:07,410 --> 00:29:15,311 that's 76,000 or 7, 100s. So it takes about 7, 100s. 389 00:29:15,311 --> 00:29:19,147 Yeah, so like 77 over 1000. So, no 7 100's. 390 00:29:19,147 --> 00:29:26,161 Sorry, 7 100's of a second. It's less than a 10th of a second to get 391 00:29:26,161 --> 00:29:39,114 through 14 routers from Michigan to Stanford Less than a tenth of a second. 392 00:29:39,114 --> 00:29:44,247 Pretty impressive. Now, if I do a trace route from Ann 393 00:29:44,247 --> 00:29:49,288 Arbor, Michigan to East Lansing Michigan, Michigan State University, we have a very 394 00:29:49,288 --> 00:29:54,817 close connection. I mentioned the Merit Network, where 395 00:29:54,817 --> 00:29:59,397 we've have a close connection for a long time. 396 00:29:59,397 --> 00:30:06,68 So not only is it fewer hops, it's only eight hops to get to Michigan State. 397 00:30:06,68 --> 00:30:11,519 if you look at the, if you look at the hops here I'm bouncing around the campus 398 00:30:11,519 --> 00:30:16,654 for three, I'm bouncing through the state for two and bouncing on the Michigan 399 00:30:16,654 --> 00:30:23,204 State campus for three. So it's total of eight, two hops to get, 400 00:30:23,204 --> 00:30:26,986 two, three hops to get across my campus, two hops to get across the State of 401 00:30:26,986 --> 00:30:33,178 Michigan and three hops to get on campus. And it's really fast. 402 00:30:33,178 --> 00:30:40,350 It's 9 1000ths of a second which is less than a hundredth of a second. 403 00:30:40,350 --> 00:30:44,246 So really fast, you can kind of see it. Now if you ran this tracer out more than 404 00:30:44,246 --> 00:30:48,556 once, this might change. It doesn't change too fast, but legally 405 00:30:48,556 --> 00:30:51,880 it could change. I mean it, there's no guarantee it's 406 00:30:51,880 --> 00:30:54,876 going to be the same. It's highly likely it's going to be the 407 00:30:54,876 --> 00:30:58,600 same because the most efficient way is not going to change within a few seconds. 408 00:30:58,600 --> 00:31:01,101 But if you start in the middle of the day, and you do it the next day, it might 409 00:31:01,101 --> 00:31:05,840 change quite a bit. So it'd be something to play with. 410 00:31:05,840 --> 00:31:08,82 Run it. Print it out. 411 00:31:08,82 --> 00:31:11,787 And run it again at midnight, run it at noon, see if your Traceroute is 412 00:31:11,787 --> 00:31:16,248 different. Right, interesting. 413 00:31:16,248 --> 00:31:21,84 So here's an example of a Traceroute From University of Michigan to Peking 414 00:31:21,84 --> 00:31:26,894 University in China and and so it's again, you know it's. 415 00:31:26,894 --> 00:31:32,542 {COUGH} it bounces thru the state of my campus for a couple of hops. 416 00:31:32,542 --> 00:31:37,794 Alright my campus for a couple of hops. It bounces around the United States for a 417 00:31:37,794 --> 00:31:42,924 couple of hops. and then it starts crossing the Pacific 418 00:31:42,924 --> 00:31:46,456 Ocean. It goes this, there's traffic actually 419 00:31:46,456 --> 00:31:50,476 went through Seoul, Korea. And then it ended up in Beijing. 420 00:31:50,476 --> 00:31:55,156 Now the interesting thing is you can see that it looks like it's taking about 61 421 00:31:55,156 --> 00:31:59,764 thousandths or six one hundredths, just over a, just almost, well, a half of a 422 00:31:59,764 --> 00:32:05,340 tenth of a second to get across the country. 423 00:32:05,340 --> 00:32:09,159 And then it starts taking longer. And then by the time it's going all the 424 00:32:09,159 --> 00:32:13,575 way to China and back, it's doing 256 milliseconds which is about a quarter of 425 00:32:13,575 --> 00:32:19,9 a second. Now the big different here is likely not 426 00:32:19,9 --> 00:32:23,920 traffic It is likely the speed of the light. 427 00:32:23,920 --> 00:32:29,407 It is how fast it takes light to get across the Pacific Ocean, so it takes 428 00:32:29,407 --> 00:32:35,378 awhile. the reason I think it's most 429 00:32:35,378 --> 00:32:42,578 likely[INAUDIBLE] numbers[INAUDIBLE][INAUDIBLE] load 430 00:32:42,578 --> 00:32:49,530 traffic because these are very consistent. 431 00:32:49,530 --> 00:32:56,721 [INAUDIBLE] pretty much all the time. So that suggests that we're not waiting 432 00:32:56,721 --> 00:32:59,87 for any traffic. We're getting through as fast as we 433 00:32:59,87 --> 00:33:05,72 physically can. Which is some combination of the speed of 434 00:33:05,72 --> 00:33:13,498 the link and the speed of light. So we just got done talking about how we 435 00:33:13,498 --> 00:33:19,615 add to every packet this global number, the IP address. 436 00:33:19,615 --> 00:33:23,575 It has the prefix of the network number in the internet part, the inter network, 437 00:33:23,575 --> 00:33:27,595 where it's really moving data from one network to another network and leaving it 438 00:33:27,595 --> 00:33:32,706 up to those destination networks how to, how to move it. 439 00:33:32,706 --> 00:33:37,442 we end up really simplifying the postcard, and the postcard ends up being 440 00:33:37,442 --> 00:33:41,982 a really apt example. But the key thing, especially when 441 00:33:41,982 --> 00:33:45,573 thinking about the four layer architecture that, the reason that, I 442 00:33:45,573 --> 00:33:49,794 think, TCP/IP succeeded was in this real complex problem of moving data between 443 00:33:49,794 --> 00:33:55,940 billions of computers, it kept the part in the middle real simple. 444 00:33:55,940 --> 00:33:58,995 It doesn't try to be perfect. It doesn't try to retransmit data. 445 00:33:58,995 --> 00:34:02,375 Doesn't try to store it, it doesn't try to keep it in the right order, it doesn't 446 00:34:02,375 --> 00:34:05,651 try to say that "if this packet went here I'm going to make sure the next one goes 447 00:34:05,651 --> 00:34:11,30 there." If the two packets, one gets ahead, we don't really care. 448 00:34:11,30 --> 00:34:15,119 It means it really fast and really scalable. 449 00:34:15,119 --> 00:34:20,219 And by keeping it simple and really fast, It solves really an amazing problem, but 450 00:34:20,219 --> 00:34:27,231 we yet have other problems to solve. So I want to close this lecture by 451 00:34:27,231 --> 00:34:34,71 introducing you to another person. So, Vint Cerf was a graduate student as 452 00:34:34,71 --> 00:34:38,828 the whole notion of packet-switching was being sort of examined and, both in the 453 00:34:38,828 --> 00:34:43,940 Federal Government at the Defense Advance Research Project Agency, DARPA, and in 454 00:34:43,940 --> 00:34:50,568 higher education. Both in the United States and Europe UK 455 00:34:50,568 --> 00:34:54,38 and elsewhere. So, Vint Cerf was kind of at the right 456 00:34:54,38 --> 00:34:58,570 place at the right time and we can be very thankful for that. 457 00:34:58,570 --> 00:35:02,540 He's credited as being one of the father's of the ARPANET. 458 00:35:02,540 --> 00:35:09,93 Which of course begat the internet. And so, Vint is going to talk to us, in a 459 00:35:09,93 --> 00:35:14,405 sense, going back even pre-ARPANET and bring us up packets and what packet's 460 00:35:14,405 --> 00:35:19,800 mean and then how that all flowed into the ARPANET And how the ARPANET evolved, 461 00:35:19,800 --> 00:35:25,677 to become the internet, Enjoy.