WEBVTT 00:00:00.000 --> 00:00:03.680 Welcome back to CSE 316 — Data Communication and Networking. 00:00:03.730 --> 00:00:09.400 This is the detailed video version of Session 3, and the rest of the course depends on it. 00:00:09.450 --> 00:00:18.400 It is dense by design, and it is meant to be watched the way you would use a textbook chapter — in pieces, with the pause button, and again before an exam. 00:00:19.500 --> 00:00:25.420 There is a checkpoint at the end of each section so you can tell whether to go on or go back. 00:00:25.470 --> 00:00:28.340 Here is the shape of it. 00:00:28.390 --> 00:00:31.630 First, why layers exist at all. 00:00:31.680 --> 00:00:36.760 Second, the five layers of the TCP/IP suite, one floor at a time. 00:00:36.810 --> 00:00:39.110 Third, encapsulation. 00:00:39.160 --> 00:00:44.640 Fourth, the OSI model— the seven-layer rival of the TCP/IP suite that lost. 00:00:44.690 --> 00:00:53.640 And fifth, the answer to the question I am about to ask you, counted out hop by hop. 00:00:54.226 --> 00:00:58.616 Picture something you did today, probably within the last hour. 00:00:58.666 --> 00:01:01.376 Suppose you load a web page. 00:01:01.426 --> 00:01:08.866 Your browser produces one single HTTP request (Hypertext Transfer Protocol) — one message, asking one server for one page. 00:01:08.916 --> 00:01:17.866 Here is the question: how many separate times is that request wrapped, readdressed and unwrapped before the server actually reads it? 00:01:20.236 --> 00:01:29.186 Pause the video and commit to a number — a plain count of wrap-and-unwrap events — and write it down somewhere. 00:01:30.976 --> 00:01:39.926 Section five counts them hop by hop so you can check your answer. The shape of it: three wraps at your laptop, two events at every router on the path, three strips at the server. 00:01:42.916 --> 00:01:51.716 In Session 1 we established that a protocol is an agreement — what to say, when to say it, what to do when something breaks. 00:01:51.766 --> 00:02:00.716 Today's question is quantity: how many agreements does one web page need, and why does stacking them — rather than writing one enormous agreement — turn out to be the design idea that made the Internet possible? 00:02:04.756 --> 00:02:06.616 That is this session's job. 00:02:06.666 --> 00:02:11.430 Everything else is detail. 00:02:11.480 --> 00:02:12.760 Section one. 00:02:12.810 --> 00:02:14.160 Why layers at all. 00:02:14.210 --> 00:02:17.300 One rule-book covers a simple conversation. 00:02:17.350 --> 00:02:26.300 A real system needs a stack of them, and this section shows why engineers chose it on purpose. 00:02:26.444 --> 00:02:29.614 Let us start exactly where Session 1 left off. 00:02:29.664 --> 00:02:33.784 A protocol is the agreement that makes communication possible. 00:02:33.834 --> 00:02:40.564 Chapter 2 of your textbook asks the next question: how many agreements does a real system need? 00:02:40.614 --> 00:02:44.354 For simple communication, the answer is one. 00:02:44.404 --> 00:02:51.204 Two people talking face to face need one simple protocol: greet, take turns, say goodbye. 00:02:51.254 --> 00:02:57.104 One rule-book covers the whole exchange, and nobody has ever needed to write it down. 00:02:57.154 --> 00:03:00.244 Complex communication is different. 00:03:00.294 --> 00:03:09.024 When the task is large, we divide it between layers — each layer takes one slice of the job — and then we need a protocol at EACH layer. 00:03:09.074 --> 00:03:15.024 That is protocol layering, and it is the design idea the rest of this course is built on. 00:03:15.074 --> 00:03:17.784 And what does the stack buy us? 00:03:17.834 --> 00:03:20.064 Three things, in one breath. 00:03:20.114 --> 00:03:24.184 Division of labor — each layer solves one problem well. 00:03:24.234 --> 00:03:31.534 Independence between floors — you can rebuild one layer without touching the others, and slide 7 demonstrates it. 00:03:31.584 --> 00:03:37.764 And the third one, the one with money attached: cheap middle boxes that only need SOME of the layers. 00:03:37.814 --> 00:03:43.794 Without that, every box in the middle of the Internet would have to be as complex as your laptop. 00:03:43.844 --> 00:03:49.794 Slide 17 draws that cheap middle box. 00:03:49.844 --> 00:03:56.024 Before any diagram from the textbook, here is the analogy the whole session lives inside. 00:03:56.074 --> 00:03:59.644 Picture the journey of one letter, and who reads what. 00:03:59.694 --> 00:04:01.104 The letter itself. 00:04:01.154 --> 00:04:03.404 What the sender actually wrote. 00:04:03.454 --> 00:04:08.884 It is read by the recipient — and by no one else, anywhere on the way. 00:04:08.934 --> 00:04:13.414 The letter goes into an envelope, with a name written on the front. 00:04:13.464 --> 00:04:20.774 The envelope is read at the very last step, to hand the letter to the right person in the right house. 00:04:20.824 --> 00:04:25.954 The envelope goes into a courier bag, with a city code on the tag. 00:04:26.004 --> 00:04:33.024 The bag's tag is read at every sorting depot along the way, to choose the bag's next journey. 00:04:33.074 --> 00:04:38.394 And the bag goes into a shipping container, with a routing label on the outside. 00:04:38.444 --> 00:04:47.394 The container label is read by the driver hauling it — and this word matters — the bag is RE-PACKED into a different container at every depot. 00:04:48.234 --> 00:04:50.124 So here is the question. 00:04:50.174 --> 00:04:55.194 The driver hauling that container — which address does he read? 00:04:55.244 --> 00:04:59.754 You said it already: the container label, and only that. 00:04:59.804 --> 00:05:08.164 He does not read the city code, he does not read the name on the envelope, and he certainly does not read the letter. 00:05:08.214 --> 00:05:16.161 That answer is layering, encapsulation and addressing at once, and slide 35 spells it out. 00:05:16.211 --> 00:05:20.061 Now the same idea, in the form your textbook gives it. 00:05:20.111 --> 00:05:26.791 Forouzan's scenario: Maria and Ann are friends who exchange confidential ideas by post. 00:05:26.841 --> 00:05:28.811 Three layers appear. 00:05:28.861 --> 00:05:32.941 Layer three, at the top — the thinking layer. 00:05:32.991 --> 00:05:38.801 Maria talks, meaning she produces the letter; Ann listens, meaning she reads it. 00:05:38.851 --> 00:05:46.861 And notice the dashed line between them: under layer three, both sides hold the identical object — the plaintext letter. 00:05:46.911 --> 00:05:50.331 What Maria wrote is exactly what Ann reads. 00:05:50.381 --> 00:05:55.941 Layer two — the secrecy layer, because the ideas are confidential. 00:05:55.991 --> 00:05:59.351 Maria's side encrypts; Ann's side decrypts. 00:05:59.401 --> 00:06:02.951 And again the identical object: the ciphertext. 00:06:03.001 --> 00:06:07.341 What leaves Maria's layer two is exactly what arrives at Ann's. 00:06:07.391 --> 00:06:09.861 Layer one — logistics. 00:06:09.911 --> 00:06:18.641 Put the ciphertext in an envelope, take it to the post office; on the other side, receive the mail and take the letter out. 00:06:18.691 --> 00:06:21.401 Identical object: a piece of mail. 00:06:21.451 --> 00:06:23.431 Now observe two things. 00:06:23.481 --> 00:06:30.371 First, each layer performs two tasks, and the two tasks are opposites — one for each direction. 00:06:30.421 --> 00:06:31.711 Talk and listen. 00:06:31.761 --> 00:06:33.691 Encrypt and decrypt. 00:06:33.741 --> 00:06:35.451 Send and receive. 00:06:35.501 --> 00:06:40.971 Each layer undoes, on the way in, exactly what its twin did on the way out. 00:06:41.021 --> 00:06:43.171 That is the first principle. 00:06:43.221 --> 00:06:47.101 And second — the post office path at the bottom. 00:06:47.151 --> 00:06:50.541 This is the only real journey in the whole picture. 00:06:50.591 --> 00:06:59.541 Everything above it is a logical connection: Maria's layer three believes it talks straight to Ann's layer three, but nothing actually travels there. 00:07:00.141 --> 00:07:05.761 The dashed lines are imaginary — a logical connection, not a physical one. 00:07:05.811 --> 00:07:09.011 The post office signs only the bottom agreement. 00:07:09.061 --> 00:07:10.351 It moves mail. 00:07:10.401 --> 00:07:14.988 It cannot read letters. 00:07:15.038 --> 00:07:21.718 Now do something rather than watch something. This is where layering stops being a diagram and becomes a reason. 00:07:21.768 --> 00:07:28.478 Suppose Maria and Ann decide their cipher is too weak, and they switch to a much stronger one. 00:07:28.528 --> 00:07:34.298 Layer two — the secrecy layer — is torn out and replaced entirely. 00:07:34.348 --> 00:07:38.988 New algorithm, new keys, new everything inside that floor. 00:07:39.038 --> 00:07:46.748 Pause the video and list everything else in the system that has to change. 00:07:46.798 --> 00:07:50.838 Look up one floor, and down one floor, and write your list. 00:07:50.888 --> 00:07:52.968 Here is the answer: nothing. 00:07:53.018 --> 00:07:54.928 Nothing else changes. 00:07:54.978 --> 00:08:02.818 Layer three still hands down a plaintext letter and still gets one back — it cannot even tell the cipher changed. 00:08:02.868 --> 00:08:09.518 Layer one still posts whatever envelope it is given — it never knew there was a cipher in the first place. 00:08:09.568 --> 00:08:14.398 The renovation happened on one floor, and the other floors did not notice. 00:08:14.448 --> 00:08:21.078 That is why engineers use this design: you can renovate one floor without evacuating the building. 00:08:21.128 --> 00:08:30.078 Here is the precise reason it works, and the vocabulary you need: each layer offers a fixed service to the layer above it, and demands a fixed service from the layer below it. 00:08:32.728 --> 00:08:36.318 Those meeting points are called the interfaces between layers. 00:08:36.368 --> 00:08:42.468 As long as the interfaces hold, what happens inside a floor is that floor's private business. 00:08:42.518 --> 00:08:51.468 This is separation of concerns — and it is why your browser did not need an update when your building's Wi-Fi was upgraded, and why the Wi-Fi did not care when your browser was. 00:08:55.859 --> 00:08:59.879 One more principle before the checkpoint, stated carefully. 00:08:59.929 --> 00:09:04.139 Maria's layer three believes it talks straight to Ann's layer three. 00:09:04.189 --> 00:09:05.869 It never does. 00:09:05.919 --> 00:09:12.579 Each layer behaves as if it were in direct conversation with the same layer on the other side. 00:09:12.629 --> 00:09:19.949 We call these peer processes — same-level layers on the two sides, speaking their own layer's protocol to each other. 00:09:19.999 --> 00:09:27.399 Encrypt on one side, decrypt on the other: that pair is having a conversation, in a very real sense. 00:09:27.449 --> 00:09:29.779 The agreement between them is real. 00:09:29.829 --> 00:09:31.849 But the journey is not. 00:09:31.899 --> 00:09:40.849 What actually happens is this, every single time: data flows DOWN the sender's stack, ACROSS the one physical path at the bottom, and UP the receiver's stack. 00:09:42.959 --> 00:09:44.499 There is no other road. 00:09:44.549 --> 00:09:51.039 The dashed lines between peers carry nothing; the peer connection is logical, not physical. 00:09:51.089 --> 00:09:53.469 So why keep the hallucination? 00:09:53.519 --> 00:09:57.309 Because it is what makes each agreement writable. 00:09:57.359 --> 00:10:06.309 It lets transport-to-transport rules be written without ever mentioning Wi-Fi, and link-to-link rules be written without ever mentioning the browser. 00:10:06.699 --> 00:10:10.779 Every layer gets a small, simple world to reason about. 00:10:10.829 --> 00:10:16.419 Peer-to-peer communication is logical; the physical flow is down, across, and up. 00:10:16.469 --> 00:10:25.196 Write that sentence down — it is worth marks, and it is worth understanding. 00:10:25.436 --> 00:10:26.756 First checkpoint. 00:10:26.806 --> 00:10:31.516 Pause the video and answer these three on paper, without scrolling back. 00:10:31.566 --> 00:10:32.776 One. 00:10:32.826 --> 00:10:36.546 Maria and Ann switch to a stronger cipher. 00:10:36.596 --> 00:10:39.516 What else in the three-layer system has to change? 00:10:39.566 --> 00:10:40.726 Two. 00:10:40.776 --> 00:10:43.986 Each layer performs two tasks. 00:10:44.036 --> 00:10:46.126 How are the two tasks related? 00:10:46.176 --> 00:10:47.346 Three. 00:10:47.396 --> 00:10:51.706 Maria's layer three 'talks to' Ann's layer three. 00:10:51.756 --> 00:10:55.416 What actually travels between them? 00:10:55.466 --> 00:11:00.286 Pause now. 00:11:00.336 --> 00:11:01.036 Answers. 00:11:01.086 --> 00:11:02.176 The first: nothing. 00:11:02.226 --> 00:11:07.326 Layer three still hands down plaintext; layer one still posts an envelope. 00:11:07.376 --> 00:11:14.116 Only the rebuilt floor changes, and that independence is the entire point of layering. 00:11:14.166 --> 00:11:17.986 The second: they are opposites, one per direction. 00:11:18.036 --> 00:11:21.536 Talk and listen, encrypt and decrypt, send and receive. 00:11:21.586 --> 00:11:25.606 Each layer undoes on the way in what its twin does on the way out. 00:11:25.656 --> 00:11:27.896 And the third: nothing at all. 00:11:27.946 --> 00:11:30.206 The peer connection is logical. 00:11:30.256 --> 00:11:39.075 Real data flows only down the stack, across the physical path, and up the other side. 00:11:39.125 --> 00:11:40.415 Section two. 00:11:40.465 --> 00:11:41.835 The TCP/IP model. 00:11:41.885 --> 00:11:46.315 Maria and Ann needed three layers for letters. 00:11:46.365 --> 00:11:52.185 The Internet needs five for bits — the same logic, with a much bigger post office. 00:11:52.235 --> 00:11:58.375 In this section we take the tower one floor at a time. 00:11:58.425 --> 00:12:04.665 Here is the whole suite on one slide, and I am going to build it floor by floor, from the top. 00:12:04.715 --> 00:12:13.665 For each floor, three things: the job, the parcel it makes — its unit — and, later, the address written on that parcel. 00:12:14.065 --> 00:12:17.075 Layer five, the application layer. 00:12:17.125 --> 00:12:22.405 Two running programs exchange messages — process-to-process. 00:12:22.455 --> 00:12:25.175 Its unit is the message. 00:12:25.225 --> 00:12:27.965 Layer four, the transport layer. 00:12:28.015 --> 00:12:35.635 Carry the message from the program on this host to the MATCHING program on the other host — end-to-end. 00:12:35.685 --> 00:12:44.355 Its unit is the segment, or the user datagram, depending which protocol you hire; slide 13 comes back to that. 00:12:44.405 --> 00:12:47.165 Layer three, the network layer. 00:12:47.215 --> 00:12:53.855 Host-to-host, across many networks, with routers choosing the route in between. 00:12:53.905 --> 00:12:56.755 Its unit is the datagram. 00:12:56.805 --> 00:12:59.615 Layer two, the data link layer. 00:12:59.665 --> 00:13:07.245 Move the datagram across ONE link — this LAN, this WAN, this single hop. 00:13:07.295 --> 00:13:09.895 Its unit is the frame. 00:13:09.945 --> 00:13:12.795 And layer one, the physical layer. 00:13:12.845 --> 00:13:17.595 Carry the individual bits as signals on the medium. 00:13:17.645 --> 00:13:20.595 Its unit is the bit itself. 00:13:20.645 --> 00:13:27.275 Say the five units back to yourself before you go on: message, segment, datagram, frame, bit. 00:13:27.325 --> 00:13:31.955 Each of those words means its own layer and no other. 00:13:32.005 --> 00:13:38.255 And the tower is hierarchical: every upper layer is served by the ones below it. 00:13:38.305 --> 00:13:46.035 The application does not carry bits; it is CARRIED, by four floors of machinery it never has to think about. 00:13:46.085 --> 00:13:51.899 The next five slides take one floor each. 00:13:51.949 --> 00:13:55.259 The top floor first: the application layer. 00:13:55.309 --> 00:13:58.229 The job is process-to-process communication. 00:13:58.279 --> 00:14:03.719 A process is simply a running program, and being exact about that matters. 00:14:03.769 --> 00:14:06.919 A browser talks to a web server. 00:14:06.969 --> 00:14:09.679 A mail program talks to a mail server. 00:14:09.729 --> 00:14:13.979 The conversation is between programs, not between machines. 00:14:14.029 --> 00:14:21.369 Your laptop is not browsing the web; a particular program on your laptop is. 00:14:21.419 --> 00:14:23.329 Who lives on this floor? 00:14:23.379 --> 00:14:27.239 Every protocol you know from daily life. 00:14:27.289 --> 00:14:28.289 HTTP for the web. 00:14:28.339 --> 00:14:32.359 SMTP (Simple Mail Transfer Protocol) for mail. 00:14:32.409 --> 00:14:36.359 FTP (File Transfer Protocol) for moving files. 00:14:36.409 --> 00:14:40.419 DNS (Domain Name System) for turning names into addresses. 00:14:40.469 --> 00:14:46.019 If you have heard of a protocol before taking this course, it almost certainly lives here. 00:14:46.069 --> 00:14:54.169 Its parcel is the message, and its addresses are names — someorg.com, somebody@coldmail.com. 00:14:54.219 --> 00:14:57.469 Human-shaped addresses, because humans type them. 00:14:57.519 --> 00:15:00.069 This design runs on trust. 00:15:00.119 --> 00:15:06.829 The application layer does not know, and does not ask, how its message will cross the world. 00:15:06.879 --> 00:15:15.829 It hands the message down and trusts the floors below — and every layer in this tower extends exactly that kind of trust to the one beneath it. 00:15:18.828 --> 00:15:21.778 Layer four: the transport layer. 00:15:21.828 --> 00:15:29.468 The job: carry the message from one program to the matching program on the other host — end-to-end. 00:15:29.518 --> 00:15:38.248 And 'end-to-end' is doing real work in that sentence: no machine in the middle of the network takes any part in this layer's job. 00:15:38.298 --> 00:15:47.248 As far as transport is concerned, the two hosts are connected by one direct logical pipe, however many routers actually sit between them. 00:15:47.448 --> 00:15:52.458 The parcel depends on which courier you hire, because this floor employs two. 00:15:52.508 --> 00:16:01.458 Hire TCP — the Transmission Control Protocol — and the message rides in a segment, with all the safety equipment: ordering, error control, congestion control. 00:16:05.058 --> 00:16:13.248 Hire UDP (User Datagram Protocol) and it rides in a user datagram, with none of it — and it is therefore light and fast. 00:16:13.298 --> 00:16:21.698 Same floor, two very different couriers, and the application chooses based on what it can afford to lose. 00:16:21.748 --> 00:16:29.968 The address is the port number, and be precise about it: ports identify the two PROGRAMS at each end. 00:16:30.018 --> 00:16:32.188 Not the machines. 00:16:32.238 --> 00:16:40.048 Your browser and your mail program share one laptop and one network connection; port numbers are what keep their traffic apart. 00:16:40.098 --> 00:16:49.048 When data arrives at your laptop, the port number is the answer to the question 'which program gets this?' 00:16:49.728 --> 00:16:52.288 Layer three: the network layer. 00:16:52.338 --> 00:16:57.958 This is the layer the middle of the Internet runs on, so give it your attention. 00:16:58.008 --> 00:17:01.808 The job: host-to-host delivery, across many networks. 00:17:01.858 --> 00:17:10.808 From this machine to that machine — across your LAN, across your ISP, across an ocean, into somebody else's data center. 00:17:11.608 --> 00:17:16.598 At every router along the way, one decision is made: which way next? 00:17:16.648 --> 00:17:20.348 Choosing routes is this layer's whole occupation. 00:17:20.398 --> 00:17:26.708 The parcel is the datagram — the transport segment, wrapped in a network header. 00:17:26.758 --> 00:17:35.358 Some books say packet; Forouzan says datagram; at this layer they name the same parcel, and I will use Forouzan's word. 00:17:35.408 --> 00:17:41.798 The address is the IP address, and its character is logical and hierarchical. 00:17:41.848 --> 00:17:49.078 Like a postal address, it encodes WHERE in the world you are — this network, then this host on it. 00:17:49.128 --> 00:17:54.808 It is globally meaningful, and it stays the same from end to end of the journey. 00:17:54.858 --> 00:17:59.738 IP — the Internet Protocol — is deliberately bare. 00:17:59.788 --> 00:18:01.238 No flow control. 00:18:01.288 --> 00:18:02.828 No error control. 00:18:02.878 --> 00:18:04.678 No congestion control. 00:18:04.728 --> 00:18:08.478 Bare delivery, done fast, with no promises. 00:18:08.528 --> 00:18:11.678 That is not an oversight; it is the design. 00:18:11.728 --> 00:18:20.678 If an application needs those guarantees, the transport layer must supply them — and that, precisely, is why TCP exists. 00:18:21.008 --> 00:18:29.958 Keep this pairing in mind: a bare network layer underneath, and a careful transport layer on top when you need one. 00:18:31.328 --> 00:18:33.928 Layer two: the data link layer. 00:18:33.978 --> 00:18:40.018 The job: take the datagram across THIS one link — this LAN, this WAN, this single hop — and no further. 00:18:40.068 --> 00:18:42.568 Its whole world is one link. 00:18:42.618 --> 00:18:49.348 It delivers to the next stop only: the router down the corridor, not the server across the ocean. 00:18:49.398 --> 00:18:54.628 Somebody else — the network layer — worries about the ocean. 00:18:54.678 --> 00:19:03.548 The parcel is the frame, and the frame is the eccentric of the family: it is the only parcel with a header AND a trailer. 00:19:03.598 --> 00:19:11.308 Link addresses go in the header at the front; error-check bits go in the trailer at the tail. 00:19:11.358 --> 00:19:13.808 No other layer adds a trailer. 00:19:13.858 --> 00:19:22.088 The address is the MAC address (Media Access Control): forty-eight bits, flat, with no structure at all. 00:19:22.138 --> 00:19:23.938 Contrast it with IP. 00:19:23.988 --> 00:19:29.678 An IP address is like a postal address — it tells you where in the world the machine is. 00:19:29.728 --> 00:19:35.268 A MAC address is like a serial number — it tells you which machine, and nothing about where. 00:19:35.318 --> 00:19:43.098 It is meaningful on this one network only — local currency — and the link addresses on a parcel change at every hop. 00:19:43.148 --> 00:19:51.028 One more remarkable fact about this floor: TCP/IP does not even define a protocol here. 00:19:51.078 --> 00:19:58.848 Any link technology that can carry a datagram is welcome — Ethernet, Wi-Fi, fiber links, cellular. 00:19:58.898 --> 00:20:07.848 That open door is why the same Internet runs over copper, glass and air without ever noticing the difference. 00:20:08.708 --> 00:20:12.538 And layer one: the physical layer, the ground floor. 00:20:12.588 --> 00:20:21.538 The job: take each bit of the frame and turn it into something physical — a voltage on copper, a pulse of light in glass, a radio wave in the air. 00:20:23.318 --> 00:20:26.648 One bit at a time, onto the medium. 00:20:26.698 --> 00:20:30.758 Its unit is the bit, the smallest parcel in the whole tower. 00:20:30.808 --> 00:20:33.418 This layer does not wrap anything. 00:20:33.468 --> 00:20:40.128 Every other working layer adds something around its cargo; the physical layer adds nothing. 00:20:40.178 --> 00:20:44.068 It shatters the frame into individual bits and signals them, one by one. 00:20:44.118 --> 00:20:52.338 Keep that in mind for the next section — when we count wrappers, the physical layer will not be on the list. 00:20:52.388 --> 00:21:01.338 And it has no address, for a reason you can reconstruct yourself in an exam: its unit is a single bit, and a single bit cannot carry an address. 00:21:03.198 --> 00:21:05.848 There is nowhere to write one. 00:21:05.898 --> 00:21:09.048 So: five layers, but only four address pairs. 00:21:09.098 --> 00:21:13.068 And you now know the reason for that asymmetry, not just the fact. 00:21:13.118 --> 00:21:15.578 One final line for this floor. 00:21:15.628 --> 00:21:22.138 The medium itself — the actual fiber in the wall — sits BELOW even this layer. 00:21:22.188 --> 00:21:27.388 The medium carries signals, not bits, and nothing about it has an address. 00:21:27.438 --> 00:21:36.388 The next session of this course lives exactly there: how a bit becomes a wave, and what that costs. 00:21:36.737 --> 00:21:40.907 Now the question with money in it: who needs how many floors? 00:21:40.957 --> 00:21:48.687 Not every machine carries the whole tower, and the ones that do not are the reason the Internet was affordable to build. 00:21:48.737 --> 00:21:52.617 Your laptop — a host — needs all five. 00:21:52.667 --> 00:21:58.457 It runs the programs, so it needs the application layer, and therefore everything beneath it. 00:21:58.507 --> 00:22:04.477 Both ends of any communication are hosts, and hosts implement the full stack. 00:22:04.527 --> 00:22:07.617 A router needs the bottom three. 00:22:07.667 --> 00:22:15.077 It must reach the network layer, because choosing routes IS the network layer's job — that is what a router is for. 00:22:15.127 --> 00:22:21.447 But it has no business in your transport data or your messages, so those floors simply are not built. 00:22:21.497 --> 00:22:25.577 A router routes datagrams; it never opens your mail. 00:22:25.627 --> 00:22:29.617 And a link-layer switch needs only the bottom two. 00:22:29.667 --> 00:22:35.447 It forwards frames inside one network, and it never even learns that IP exists. 00:22:35.497 --> 00:22:39.457 The switch in the GPL lab is exactly this machine. 00:22:39.507 --> 00:22:45.667 Now step back and look at what this slide is really saying, because it is the third advantage from section one, drawn. 00:22:45.717 --> 00:22:51.617 The middle of the network is dumb on purpose, and cheap on purpose. 00:22:51.667 --> 00:22:57.857 An access switch implements two layers and costs an order of magnitude less than a host. 00:22:57.907 --> 00:23:02.717 A router implements three, and costs less than the servers it serves. 00:23:02.767 --> 00:23:11.717 Cheap, ignorant middle boxes are why the Internet could grow without anyone's permission — nobody had to make the middle smart, because the design never asked it to be. 00:23:13.177 --> 00:23:15.057 And look at the router's stack. 00:23:15.107 --> 00:23:18.647 Bottom floors only, moving parcels it cannot read. 00:23:18.697 --> 00:23:21.737 This is the post office from the Maria-and-Ann slide. 00:23:21.787 --> 00:23:26.946 Same character, bigger network. 00:23:26.996 --> 00:23:28.556 Second checkpoint. 00:23:28.606 --> 00:23:32.786 Same rule — pause, paper, no scrolling back. 00:23:32.836 --> 00:23:34.046 One. 00:23:34.096 --> 00:23:41.056 Name the five TCP/IP layers bottom-up, with the unit each one exchanges. 00:23:41.106 --> 00:23:42.276 Two. 00:23:42.326 --> 00:23:47.436 Why must a router reach the network layer — and why must it go no higher? 00:23:47.486 --> 00:23:48.666 Three. 00:23:48.716 --> 00:23:54.116 Which is the only layer that adds a trailer as well as a header? 00:23:54.166 --> 00:23:58.596 Pause now. 00:23:58.646 --> 00:23:59.536 Answers. 00:23:59.586 --> 00:24:08.536 The first: physical with bits, data link with frames, network with datagrams, transport with segments or user datagrams, and application with messages. 00:24:10.476 --> 00:24:17.936 If you recited them top-down, that is acceptable — but practice bottom-up, because that is how Forouzan numbers them. 00:24:17.986 --> 00:24:24.586 The second: choosing routes is the network layer's own job, so a router must climb exactly that high. 00:24:24.636 --> 00:24:33.076 Transport is end-to-end, between the two hosts only — so a router has no business there, and the floor is simply not built. 00:24:33.126 --> 00:24:42.076 And the third: the data link layer — link addresses in the header, error-check bits in the trailer. 00:24:42.890 --> 00:24:44.360 Section three. 00:24:44.410 --> 00:24:45.810 Encapsulation. 00:24:45.860 --> 00:24:47.570 You now own the tower. 00:24:47.620 --> 00:24:56.570 For the next stretch of this session we put one message in the lift, ride it down, across a router, and back up — and we read what is written on every wrapper. 00:24:57.660 --> 00:25:04.073 This is the densest part of the session; take it slowly. 00:25:04.123 --> 00:25:07.943 We start at the source host, at the top of the tower. 00:25:07.993 --> 00:25:13.113 The application produces the message — say, your HTTP request — and hands it down. 00:25:13.163 --> 00:25:18.583 Now watch the wrapping, one floor at a time. 00:25:18.633 --> 00:25:23.713 Transport wraps the message in TH — the transport header. 00:25:23.763 --> 00:25:29.333 Inside TH live the port numbers: the addresses of the two PROGRAMS. 00:25:29.383 --> 00:25:32.093 The parcel is now a segment. 00:25:32.143 --> 00:25:40.533 The word is mechanical: to encapsulate is to put in a capsule — the payload goes inside, the header goes around it. 00:25:40.583 --> 00:25:48.043 The network layer wraps NH around the segment — source and destination IP addresses. 00:25:48.093 --> 00:25:51.013 The parcel is now a datagram. 00:25:51.063 --> 00:25:53.873 And transport's header just became cargo. 00:25:53.923 --> 00:26:00.903 The network layer does not read TH, does not understand TH, does not care that TH exists. 00:26:00.953 --> 00:26:05.723 It treats everything handed down from above as one opaque payload. 00:26:05.773 --> 00:26:10.583 Every layer does this, and it is the reason the whole scheme works. 00:26:10.633 --> 00:26:14.633 The data link layer adds a header AND a trailer. 00:26:14.683 --> 00:26:21.553 DH carries the link addresses for this one hop; DT carries the error-check bits at the tail. 00:26:21.603 --> 00:26:27.053 The parcel is now a frame — ready for this link, and only this link. 00:26:27.103 --> 00:26:30.253 And the physical layer does not wrap anything. 00:26:30.303 --> 00:26:36.253 It shatters the frame into individual bits and signals them onto the medium. 00:26:36.303 --> 00:26:38.513 Three wraps, then bits. 00:26:38.563 --> 00:26:42.903 Now look back up the slide at the grey block that appears in every row. 00:26:42.953 --> 00:26:49.693 Through all of this — three wrappers, three sets of addresses — the message itself was never touched. 00:26:49.743 --> 00:26:53.153 Not compressed, not edited, not even read. 00:26:53.203 --> 00:26:57.239 Wrapped, only. 00:26:57.289 --> 00:26:59.159 Now the interesting machine. 00:26:59.209 --> 00:27:07.499 The bits cross the first link and arrive at a router — and remember from section two: a router owns the bottom three floors only. 00:27:07.549 --> 00:27:09.569 The frame arrives on link one. 00:27:09.619 --> 00:27:18.209 The data link layer checks the frame is addressed to it — and at the link level it IS; that is exactly why the frame came here. 00:27:18.259 --> 00:27:26.129 So data link strips the link-one envelope: DH-one and DT-one are removed and discarded. 00:27:26.179 --> 00:27:28.719 Link one's frame is now garbage. 00:27:28.769 --> 00:27:30.729 Its whole life was one hop. 00:27:30.779 --> 00:27:36.729 What remains is the datagram: NH, TH, message, intact. 00:27:36.779 --> 00:27:39.979 The datagram goes up one floor. 00:27:40.029 --> 00:27:43.599 The network layer reads NH — and ONLY NH. 00:27:43.649 --> 00:27:51.539 Destination IP address, into the forwarding table, out comes a decision: the next hop is link two. 00:27:51.589 --> 00:27:53.829 That is the router's entire job. 00:27:53.879 --> 00:28:02.829 TH and the message stay sealed — the router does not know whether it is carrying your exam results or a video, and that ignorance is a feature, not a failing. 00:28:05.959 --> 00:28:14.909 Decision made, the datagram goes back down, and data link builds a brand-new frame for link two — new link addresses in DH-two, new trailer in DT-two. 00:28:17.119 --> 00:28:23.539 The parcel has been readdressed at the link level, while the IP addresses inside NH were not touched. 00:28:23.589 --> 00:28:25.519 It leaves on link two. 00:28:25.569 --> 00:28:28.289 Write these two lines down. 00:28:28.339 --> 00:28:30.579 Two links — two frames. 00:28:30.629 --> 00:28:32.959 One datagram, end to end. 00:28:33.009 --> 00:28:41.959 That second line is the identical-objects principle from Maria and Ann: the datagram is the identical object that both hosts' network layers hold, no matter how many frames were born and died carrying it. 00:28:49.455 --> 00:28:51.505 And the destination host. 00:28:51.555 --> 00:29:00.505 The ascent mirrors the descent, so this is brisk. Each floor asks one question and strips one wrapper, and here are all four questions. 00:29:01.265 --> 00:29:04.695 Data link asks: is this frame for me? 00:29:04.745 --> 00:29:06.665 The link address says yes. 00:29:06.715 --> 00:29:10.895 It strips DH and DT, checking the error bits on the way. 00:29:10.945 --> 00:29:13.095 The frame has done its job. 00:29:13.145 --> 00:29:17.095 The network layer asks: is this IP address me? 00:29:17.145 --> 00:29:22.275 Yes — which means the journey is over; there is no next hop to compute. 00:29:22.325 --> 00:29:24.205 It strips NH. 00:29:24.255 --> 00:29:28.825 Transport asks: which program does this belong to? 00:29:28.875 --> 00:29:34.125 The port number in TH answers, and the segment is handed to the right program. 00:29:34.175 --> 00:29:36.155 It strips TH. 00:29:36.205 --> 00:29:39.635 And the application receives the message. 00:29:39.685 --> 00:29:43.685 Ann reads exactly — bit for bit — what Maria wrote. 00:29:43.735 --> 00:29:47.875 Three wrappers on, three wrappers off, nothing else touched. 00:29:47.925 --> 00:29:53.475 The order of the ascent is not a convention someone chose; it is a necessity. 00:29:53.525 --> 00:30:02.475 Each layer removes exactly the header its twin added, in reverse order — because you can only ever read the OUTERMOST label. 00:30:02.705 --> 00:30:07.475 It is the same reason you cannot read a letter without opening the envelope first. 00:30:07.525 --> 00:30:14.072 Decapsulation is forced to mirror encapsulation. 00:30:14.122 --> 00:30:19.982 You have now seen the descent, the router, and the ascent as three separate slides. 00:30:20.032 --> 00:30:26.782 Here is the demonstration — all of it in motion, thirty-eight seconds. Watch the colors. 00:30:26.832 --> 00:30:30.992 Watch it once, clean, without pausing. 00:30:31.042 --> 00:30:38.532 Follow one thing all the way through: the message block, and what happens around it. 00:30:38.582 --> 00:30:42.162 Now replay it, and pause at each wrap. 00:30:42.212 --> 00:30:50.372 At the source: message, then segment, then datagram, then frame, then bits — say the unit names as they appear. 00:30:50.422 --> 00:30:59.372 At the router: watch the frame die and a new one get born, while the datagram inside rides straight through. 00:31:01.722 --> 00:31:09.972 And at the destination: the wrappers come off in exactly the reverse order they went on. 00:31:10.022 --> 00:31:17.932 Before you go on, answer one question from the freeze frame: which machines touched the blue transport header? 00:31:17.982 --> 00:31:23.657 The next slide checks your answer. 00:31:23.707 --> 00:31:26.207 Two questions about what you just watched. 00:31:26.257 --> 00:31:33.727 Answer each one before you let its answer appear — this pair of questions is the entire section, compressed. 00:31:33.777 --> 00:31:35.267 Question one. 00:31:35.317 --> 00:31:44.047 Across the whole journey — source, routers, destination — which machines touched TH, the transport header? 00:31:44.097 --> 00:31:46.057 Only the two hosts. 00:31:46.107 --> 00:31:51.377 Every router carried TH as sealed cargo inside the datagram, never opened, never read. 00:31:51.427 --> 00:31:59.677 Transport is end-to-end — nothing in the middle of the path even has the floor that could read it. 00:31:59.727 --> 00:32:01.247 Question two. 00:32:01.297 --> 00:32:05.197 And the frame headers — DH and DT? 00:32:05.247 --> 00:32:07.997 Different ones on each link. 00:32:08.047 --> 00:32:13.067 Born at one end of a link, dead at the other, replaced at every router. 00:32:13.117 --> 00:32:15.307 Two links, two frames. 00:32:15.357 --> 00:32:19.207 The wrapper is per-hop; the datagram is per-journey. 00:32:19.257 --> 00:32:23.407 If both answers came back right, encapsulation has landed. 00:32:23.457 --> 00:32:27.697 To drive this yourself, open the layer stepper on the course page. 00:32:27.747 --> 00:32:36.697 Step a message down and up the stack one layer at a time, first on the preset 'Host to host, same LAN', then on 'Across one router'. 00:32:37.607 --> 00:32:43.027 Ask yourself at every step: what unit am I holding, and who can read which header? 00:32:43.077 --> 00:32:50.487 Watch particularly how high the switch climbs — layer two and no further — and what the router does to the frame. 00:32:50.537 --> 00:32:59.487 And take the closing sentence with you: each layer reads only its own header, and treats everything inside as opaque payload. 00:32:59.947 --> 00:33:08.221 That is the driver reading the container label — drawn in full, with the machinery showing. 00:33:08.271 --> 00:33:13.011 So far the wrappers were anonymous — time to read what is actually written on them. 00:33:13.061 --> 00:33:19.041 Any communication between two parties needs two addresses: source and destination. 00:33:19.091 --> 00:33:21.731 Five layers, but only four pairs. 00:33:21.781 --> 00:33:23.871 Let us go down the tower. 00:33:23.921 --> 00:33:30.871 The application layer: names. someorg.com; somebody@coldmail.com. 00:33:30.921 --> 00:33:33.891 Human-shaped, because humans type them. 00:33:33.941 --> 00:33:36.901 Transport: port numbers. 00:33:36.951 --> 00:33:41.421 And be precise about what they identify — the PROGRAMS at each end. 00:33:41.471 --> 00:33:42.751 Not machines. 00:33:42.801 --> 00:33:44.291 Programs. 00:33:44.341 --> 00:33:47.421 Network: the IP address. 00:33:47.471 --> 00:33:56.041 Logical, hierarchical, globally meaningful — the whole Internet is its scope, and it is unchanged from end to end. 00:33:56.091 --> 00:33:59.231 Data link: the MAC address. 00:33:59.281 --> 00:34:08.231 Forty-eight bits, flat, no structure — local currency, THIS network only, and as you watched three times today, new at every hop. 00:34:09.301 --> 00:34:11.721 And the physical layer: nothing. 00:34:11.771 --> 00:34:16.631 Its unit is one bit, and a bit cannot carry an address. 00:34:16.681 --> 00:34:18.791 Five layers, four pairs. 00:34:18.841 --> 00:34:21.381 One footnote, one sentence only. 00:34:21.431 --> 00:34:30.381 The headers do one more job besides addressing: each carries a field naming which upper protocol the payload belongs to, so that transport knows whether to hand up to the browser or the mail program. 00:34:33.981 --> 00:34:42.931 That mechanism is called multiplexing and demultiplexing. Section four of this session is where it is taught; no later session returns to it. 00:34:46.784 --> 00:34:51.824 You have watched the descent, the ascent, and the router in between. 00:34:51.874 --> 00:34:57.494 Now here is the same journey with a step button, so you can stop it wherever you are unsure. 00:34:57.544 --> 00:35:06.064 Maria's five floors on the left, Ann's five floors on the right, and the scenario at the top: same LAN, through a switch. 00:35:06.114 --> 00:35:11.504 Every click is exactly one layer's work — no more, no less. 00:35:11.554 --> 00:35:16.004 Step, and the application layer hands down a message. 00:35:16.054 --> 00:35:22.074 Step again, and transport adds its header — TH — and the message becomes a segment. 00:35:22.124 --> 00:35:26.574 Notice the unit name changes at the same instant the header goes on. 00:35:26.624 --> 00:35:30.224 That is not a coincidence; it is the definition. 00:35:30.274 --> 00:35:38.734 Network adds NH and it becomes a datagram; data link adds a header and a trailer and it becomes a frame. 00:35:38.784 --> 00:35:47.734 And at the bottom, the wrapping stops and the frame becomes bits — individual signals on the medium, with no blocks left to draw. 00:35:48.444 --> 00:35:51.184 Then the same thing backwards. 00:35:51.234 --> 00:35:56.554 Every strip on Ann's side is performed by the layer that matches the one that wrapped it. 00:35:56.604 --> 00:35:59.174 Layer two opens what layer two wrote. 00:35:59.224 --> 00:36:02.164 Layer three opens what layer three wrote. 00:36:02.214 --> 00:36:06.154 Nobody peeks upward, and nobody peeks downward. 00:36:06.204 --> 00:36:09.544 Now the scenario across one router. 00:36:09.594 --> 00:36:16.104 Watch the middle column — the router has three floors, not five, and the message stops at layer three. 00:36:16.154 --> 00:36:22.264 It reads the network header, decides where to send it, and then builds a NEW frame for link two. 00:36:22.314 --> 00:36:27.264 Different frame, different data-link header, same datagram inside. 00:36:27.314 --> 00:36:36.264 And if you only care about one floor, put the lens on it — only layer three — and the tool shows you nothing but that layer's work, at both hosts and at the router. 00:36:38.524 --> 00:36:41.834 Thirteen steps in total on this one-router path. 00:36:41.884 --> 00:36:50.587 Step through it once yourself before you answer the session's question, which uses a ten-router path. 00:36:50.827 --> 00:36:57.157 Third checkpoint — and this one is timed in the classroom version, so give yourself sixty seconds. 00:36:57.207 --> 00:36:59.017 Pause and answer on paper. 00:36:59.067 --> 00:37:00.277 One. 00:37:00.327 --> 00:37:05.027 A link-layer switch, forwarding — which address does it read? 00:37:05.077 --> 00:37:06.227 Two. 00:37:06.277 --> 00:37:11.927 A router in Singapore, forwarding your datagram — which address does it read? 00:37:11.977 --> 00:37:13.157 Three. 00:37:13.207 --> 00:37:17.527 The fiber in the wall — how many layers does it hold? 00:37:17.577 --> 00:37:21.617 Pause now. 00:37:21.667 --> 00:37:23.097 Answers. 00:37:23.147 --> 00:37:27.107 One — the link address, the MAC address. 00:37:27.157 --> 00:37:31.877 Layer two is all a switch has; IP is invisible to it. 00:37:31.927 --> 00:37:37.517 Two — the destination IP address, in the network header, and nothing above it. 00:37:37.567 --> 00:37:40.177 The ports and the message stay sealed. 00:37:40.227 --> 00:37:41.447 Three — zero. 00:37:41.497 --> 00:37:44.367 The medium sits BELOW the physical layer. 00:37:44.417 --> 00:37:49.137 It carries signals, not bits, and nothing about it has an address. 00:37:49.187 --> 00:37:58.137 And a warning: if you swapped answers one and two, your scope column is inverted — you have link addresses global and IP local, which is exactly backwards. 00:37:59.447 --> 00:38:02.367 Link is local; IP is global. 00:38:02.417 --> 00:38:10.207 Fix it tonight; the addressing sessions from Session 9 onward depend on it. 00:38:10.257 --> 00:38:11.647 Section four. 00:38:11.697 --> 00:38:14.417 Before we answer the driver — the rival. 00:38:14.467 --> 00:38:18.877 There is a seven-layer model in every textbook, including yours. 00:38:18.927 --> 00:38:22.537 On paper it is more complete than TCP/IP. 00:38:22.587 --> 00:38:29.841 The Internet does not run it, and the reasons why are worth five minutes. 00:38:29.891 --> 00:38:38.841 In the late nineteen-seventies, the ISO — the International Organization for Standardization, a multinational standards body — set out to design the model for connecting all systems: the Open Systems Interconnection model, OSI. 00:38:41.731 --> 00:38:46.831 Seven layers, covering all aspects of network communication. 00:38:46.881 --> 00:38:55.831 From the bottom: physical, data link, network, transport — so far, familiar — and then three more: session, presentation, and application at the top. 00:38:56.981 --> 00:39:04.211 It was thorough, it was elegant, and it was intended to be THE standard, for everyone. 00:39:04.261 --> 00:39:07.631 Now put the five-layer TCP/IP suite beside it. 00:39:07.681 --> 00:39:14.331 Physical, data link, network, transport line up exactly, floor for floor. 00:39:14.381 --> 00:39:16.891 And then look at the top. 00:39:16.941 --> 00:39:23.161 Two floors are missing from TCP/IP: session and presentation. 00:39:23.211 --> 00:39:32.161 TCP/IP's application box swallows all three top floors of OSI — one layer doing, when it needs to, what OSI assigned to three. 00:39:36.471 --> 00:39:45.421 The next slide asks what those two extra floors were supposed to do, and where their jobs went. 00:39:47.366 --> 00:39:51.246 So what did OSI put between transport and application? 00:39:51.296 --> 00:39:55.496 Layer five, the session layer: manage the dialog. 00:39:55.546 --> 00:40:01.946 Open a conversation between the two sides, keep it healthy, close it down properly. 00:40:01.996 --> 00:40:03.756 Decide who speaks when. 00:40:03.806 --> 00:40:12.756 And place checkpoints along a long exchange — so that a two-hour transfer that breaks in the second hour can resume from the last checkpoint instead of restarting from zero. 00:40:14.466 --> 00:40:19.346 Layer six, the presentation layer: translate the data. 00:40:19.396 --> 00:40:28.346 Two different machines may represent information differently — different character codes, different number formats — so somebody must agree on a common representation at the boundary. 00:40:30.046 --> 00:40:38.996 OSI also placed encryption and compression here: transforming the data's form, on the way through, without changing its meaning. 00:40:39.246 --> 00:40:44.826 Those jobs are not fake, and this is the point to be precise about. 00:40:44.876 --> 00:40:48.766 The jobs are real; the FLOORS were not needed. 00:40:48.816 --> 00:40:57.766 TCP/IP never added them, because transport already covers some of their duties, and applications build the rest themselves whenever they actually need it. 00:40:58.756 --> 00:41:07.706 Your browser still encrypts, still compresses, still resumes broken downloads — all of it living inside the application layer, built per-application instead of imposed as a mandatory floor for everyone. 00:41:12.816 --> 00:41:20.706 That is a genuine design disagreement, not an accident: OSI said 'everyone gets these services, as layers'. 00:41:20.756 --> 00:41:24.216 TCP/IP said 'take them if you need them'. 00:41:24.266 --> 00:41:30.116 Keep that disagreement in mind for the next slide. 00:41:30.166 --> 00:41:32.916 So why does the Internet not run OSI? 00:41:32.966 --> 00:41:35.686 Three reasons, straight from your textbook. 00:41:35.736 --> 00:41:38.256 Reason one: too late. 00:41:38.306 --> 00:41:47.256 By the time OSI was ready to be adopted, TCP/IP was fully in place and running, with serious time and serious money already sunk into it. 00:41:49.576 --> 00:41:53.846 Nobody rebuilds a working network to honor a diagram. 00:41:53.896 --> 00:41:56.846 Reason two: never finished. 00:41:56.896 --> 00:42:05.666 Session and presentation had their services listed — but the protocols to deliver those services were never fully defined or built. 00:42:05.716 --> 00:42:08.106 A specification is not a product. 00:42:08.156 --> 00:42:11.226 You cannot deploy a table of contents. 00:42:11.276 --> 00:42:14.606 Reason three: never faster. 00:42:14.656 --> 00:42:21.986 Where OSI implementations were actually built and deployed, they never performed well enough to tempt anyone across. 00:42:22.036 --> 00:42:26.866 Switching has a cost, and there was no prize on the other side. 00:42:26.916 --> 00:42:34.336 The moral, and it is worth writing down because it recurs in engineering forever: deployed-and-working beats elegant-and-promised. 00:42:34.386 --> 00:42:43.336 Remember that sentence in Session 19, when we meet IPv6 — a better design, moving slowly for exactly these reasons. 00:42:44.166 --> 00:42:47.396 And yet — OSI is not gone. 00:42:47.446 --> 00:42:53.096 It survives as vocabulary, and its words are the industry's everyday shorthand. 00:42:53.146 --> 00:42:54.896 A 'layer 2 switch'. 00:42:54.946 --> 00:42:56.956 A 'layer 3 device'. 00:42:57.006 --> 00:42:59.016 A 'layer 7 firewall'. 00:42:59.066 --> 00:43:05.616 The whole industry numbers its boxes with OSI's floor plan, even though the building was never built. 00:43:05.666 --> 00:43:14.616 So learn the seven names: not because the Internet runs them, but because everyone speaks them. 00:43:14.756 --> 00:43:16.166 Fourth checkpoint. 00:43:16.216 --> 00:43:18.036 Pause and answer on paper. 00:43:18.086 --> 00:43:19.296 One. 00:43:19.346 --> 00:43:26.746 Which two layers does OSI have that TCP/IP does not — and where do they sit in the tower? 00:43:26.796 --> 00:43:27.946 Two. 00:43:27.996 --> 00:43:31.636 Give the three reasons OSI lost. 00:43:31.686 --> 00:43:32.836 Three. 00:43:32.886 --> 00:43:36.626 A vendor sells you a 'layer 7 firewall'. 00:43:36.676 --> 00:43:41.086 Which model is that number from — and what does the Internet actually run? 00:43:41.136 --> 00:43:45.186 Pause now. 00:43:45.236 --> 00:43:46.666 Answers. 00:43:46.716 --> 00:43:53.596 One — session and presentation, sitting between transport and application. 00:43:53.646 --> 00:43:59.086 Two — too late: TCP/IP was already deployed, with money sunk in. 00:43:59.136 --> 00:44:05.496 Never finished: the extra layers' protocols were listed but never fully defined or built. 00:44:05.546 --> 00:44:11.396 Never faster: existing implementations gave nobody a reason to switch. 00:44:11.446 --> 00:44:16.746 Three — the number is OSI's; layer seven is OSI's application layer. 00:44:16.796 --> 00:44:20.146 The Internet itself runs the five-layer TCP/IP suite. 00:44:20.196 --> 00:44:29.146 OSI is not deployed; its layer numbering is the industry's standard vocabulary. 00:44:30.715 --> 00:44:32.465 Section five. 00:44:32.515 --> 00:44:39.115 Back to the question from minute one — and this time we count the wraps, hop by hop. 00:44:39.165 --> 00:44:44.231 Get out the number you wrote down. 00:44:44.281 --> 00:44:53.231 Here was the question: one HTTP request, from your browser to the server — how many separate times is it wrapped, readdressed and unwrapped before the server reads it? 00:44:55.861 --> 00:45:04.811 To count, we need a concrete path, so take a typical one: your laptop, the campus switch, ten routers across the Internet, and the server. 00:45:05.761 --> 00:45:08.821 Now count with me, machine by machine. 00:45:08.871 --> 00:45:11.991 Your laptop wraps it three times. 00:45:12.041 --> 00:45:15.041 Transport wraps TH around the request. 00:45:15.091 --> 00:45:17.721 Network wraps NH around the segment. 00:45:17.771 --> 00:45:21.821 Data link wraps DH and DT around the datagram. 00:45:21.871 --> 00:45:29.241 Three wrap events — and then the physical layer signals the bits, which, as you now know, is not a wrap. 00:45:29.291 --> 00:45:31.441 The switch: zero. 00:45:31.491 --> 00:45:36.751 It reads the frame's link address and forwards the frame untouched. 00:45:36.801 --> 00:45:43.231 No wrapper added, no wrapper removed — which is exactly what 'bottom two floors' buys. 00:45:43.281 --> 00:45:46.871 Each router: one strip and one wrap. 00:45:46.921 --> 00:45:55.871 The old frame dies; the network layer reads the IP address and chooses the next link; a new frame is born, with new link addresses. 00:45:56.501 --> 00:46:02.341 Two events per router — and the datagram is readdressed at the link level every single time. 00:46:02.391 --> 00:46:05.941 Ten routers on our path: twenty events. 00:46:05.991 --> 00:46:14.941 And the server strips three times: off comes the frame, off comes the datagram's network header, off comes the transport header — and at last a web server program reads your HTTP request, exactly as your browser wrote it. 00:46:22.131 --> 00:46:24.311 Add it up. 00:46:24.361 --> 00:46:33.311 Three plus twenty plus three: twenty-six separate wrap-and-unwrap events — thirteen wraps and thirteen strips — for one request, on one perfectly ordinary path. 00:46:34.991 --> 00:46:38.731 Compare that with the number you wrote down. 00:46:38.781 --> 00:46:45.321 And the count does not include the request itself: it was never touched, not once, by anything. 00:46:45.371 --> 00:46:54.321 If your path has more routers, the total grows by two per router — the structure, three plus two-per-router plus three, is the real answer; the twenty-six is just our path's arithmetic. 00:47:00.401 --> 00:47:04.111 And now the driver from section one, paid off in full. 00:47:04.161 --> 00:47:07.541 Line the two worlds up, wrapper by wrapper. 00:47:07.591 --> 00:47:10.181 The letter is the message. 00:47:10.231 --> 00:47:16.671 Read by Ann alone — by the application at the far end, and by nothing else in the world. 00:47:16.721 --> 00:47:20.871 The envelope, with a name on it, is the segment. 00:47:20.921 --> 00:47:27.511 Opened at the very last step by Ann's transport layer, and the name on it is the port — which program. 00:47:27.561 --> 00:47:32.261 The courier bag, with its city code, is the datagram. 00:47:32.311 --> 00:47:37.381 Every router on the path reads the bag's IP label — and only that. 00:47:37.431 --> 00:47:40.191 And the container is the frame. 00:47:40.241 --> 00:47:49.191 Read by THIS hop's driver, and re-packed at every depot — exactly like the frame, born and dying on every link. 00:47:49.651 --> 00:47:53.231 So: which address does the driver read? 00:47:53.281 --> 00:47:54.781 The outermost one. 00:47:54.831 --> 00:47:55.841 His own. 00:47:55.891 --> 00:47:59.841 He does not know there is a letter inside, and he does not need to. 00:47:59.891 --> 00:48:04.461 One agreement per job, and each layer reads only its own header. 00:48:04.511 --> 00:48:13.461 That single sentence is layering, encapsulation and addressing at once — and it is why the driver is cheap, the post office is dumb, the letter is private, and the middle of the Internet could scale. 00:48:17.111 --> 00:48:26.061 If your answer at the start was 'the container label, obviously', your instinct was right. This session supplied the vocabulary for it. 00:48:28.769 --> 00:48:32.849 Before the recap, the five mistakes I see every year. 00:48:32.899 --> 00:48:37.839 For each one, predict the correction before I give it. 00:48:37.889 --> 00:48:44.109 Mistake one: 'a router rebuilds all five layers.' No. Bottom three only. 00:48:44.159 --> 00:48:50.599 It reads the network header, swaps the frame, and never opens the transport header or the message. 00:48:50.649 --> 00:48:59.079 If an exam asks what a router does with a segment, the answer is: nothing — it never sees one. 00:48:59.129 --> 00:49:08.079 Mistake two: 'every hop gives the datagram a new IP header.' No. The FRAME is new on every link; the datagram normally survives end to end, identical at both hosts. 00:49:12.309 --> 00:49:18.099 Carry this error into the addressing sessions and subnetting stops making sense. 00:49:18.149 --> 00:49:26.699 Mistake three: 'a link-layer switch reads IP addresses.' No. A switch lives at layers one and two. 00:49:26.749 --> 00:49:30.829 It forwards frames; IP is invisible to it. 00:49:30.879 --> 00:49:37.819 Mistake four: 'a port number identifies a computer.' No. Ports pick the PROGRAM. 00:49:37.869 --> 00:49:39.529 IP picks the host. 00:49:39.579 --> 00:49:42.879 Link addresses pick the stop on this one network. 00:49:42.929 --> 00:49:48.409 Three address types, three different questions being answered. 00:49:48.459 --> 00:49:57.409 And mistake five: 'OSI replaced TCP/IP', or its cousin, 'TCP/IP has seven layers.' No. OSI lost — too late, never finished, never faster — and the Internet runs the five-layer suite. 00:50:01.939 --> 00:50:10.889 The seven-layer picture is a model you must know, not a network that exists. 00:50:12.654 --> 00:50:17.534 Let me close the content by naming exactly what you should be able to do now. 00:50:17.584 --> 00:50:26.534 Explain why complex communication needs a stack of protocols rather than one, and what layering buys — division of labor, independence between floors, and the cheap middle box. 00:50:30.644 --> 00:50:39.554 Recite the five TCP/IP layers bottom-up, with each layer's job and its unit: bits, frames, datagrams, segments, messages. 00:50:39.604 --> 00:50:48.554 Trace one message down the stack, across a router, and back up — naming every header added and removed, in order, and saying who reads what. 00:50:53.894 --> 00:51:02.844 Match the four address pairs to their layers — names, ports, IP, MAC — and say which layer has none, and why. 00:51:03.574 --> 00:51:12.524 And compare OSI with TCP/IP: the two extra layers and what they were for, the three reasons OSI lost, and where its vocabulary survives. 00:51:15.244 --> 00:51:19.464 If any of those feel shaky, the section that covers it is still there. 00:51:19.514 --> 00:51:24.828 This is a video — use it as one. 00:51:24.878 --> 00:51:26.618 Five last questions. 00:51:26.668 --> 00:51:28.638 Pause and answer them on paper. 00:51:28.688 --> 00:51:29.888 One. 00:51:29.938 --> 00:51:32.598 A router receives a frame. 00:51:32.648 --> 00:51:36.038 Which headers does it read — and which does it never open? 00:51:36.088 --> 00:51:37.248 Two. 00:51:37.298 --> 00:51:40.348 Name the unit exchanged at each of the five layers, bottom-up. 00:51:40.398 --> 00:51:42.228 Three. 00:51:42.278 --> 00:51:46.838 Which is the only layer that adds a trailer? 00:51:46.888 --> 00:51:48.108 Four. 00:51:48.158 --> 00:51:51.198 A path crosses two routers. 00:51:51.248 --> 00:51:55.688 How many different frames carry the message — and how many datagrams? 00:51:55.738 --> 00:51:57.008 Five. 00:51:57.058 --> 00:52:02.498 Name the two OSI layers that TCP/IP does not have. 00:52:02.548 --> 00:52:06.598 Pause now. 00:52:06.648 --> 00:52:09.338 Take your time on these. 00:52:09.388 --> 00:52:17.918 One — the link header, to check the frame is for it, and the network header, to choose the next hop. 00:52:17.968 --> 00:52:21.618 It never opens the transport header or the message. 00:52:21.668 --> 00:52:26.278 Two — bits, frames, datagrams, segments or user datagrams, messages. 00:52:26.328 --> 00:52:29.188 Three — the data link layer. 00:52:29.238 --> 00:52:35.508 Four — two routers means three links, so three frames — and one datagram, end to end. 00:52:35.558 --> 00:52:38.288 Five — session and presentation. 00:52:38.338 --> 00:52:46.877 If you got all five, you own Chapter 2 — and Chapter 2 is most of the course's map. 00:52:46.927 --> 00:52:48.427 So take this with you. 00:52:48.477 --> 00:52:54.647 Wrap on the way down, strip on the way up — and read only your own label. 00:52:54.697 --> 00:53:01.757 That is layering, that is encapsulation, that is addressing — and it is most of the map of this course. 00:53:01.807 --> 00:53:09.267 Before the next session, read Forouzan sections three point one to three point three — signals, where bits become electricity. 00:53:09.317 --> 00:53:17.097 And watch the sine-wave clip on the course page; it is thirty-five seconds, and the next session opens assuming you have seen it. 00:53:17.147 --> 00:53:23.247 From memory, before you arrive: the five layers with their units, and the four address pairs. 00:53:23.297 --> 00:53:27.857 Bring a calculator from the next session onward — Chapter 3 computes. 00:53:27.907 --> 00:53:29.777 And two notes for your diary. 00:53:29.827 --> 00:53:38.637 Weekly Online Quiz A1 is live on the course website now — twenty minutes, one attempt, and it closes on Saturday at midnight. 00:53:38.687 --> 00:53:47.637 And Classroom Quiz 1 comes in Week 5, in the first thirty minutes of Session 9 — thirty minutes, twenty marks, in class and on paper. 00:53:48.547 --> 00:53:55.387 Several versions of that paper circulate in the room, so the only working strategy is to do the examples. 00:53:55.437 --> 00:53:59.977 See you in Session 4.