How the TCP/IP Model Actually Works | CCNA Day 3 — Transcript

Full transcript

1. 0:06Hi from Tokyo, Japan, and welcome to Jeremy's IT  Lab. This video is about the Internet Protocol
2. 0:11Suite, often called TCP/IP, the family of  protocols that makes communication over the
3. 0:17Internet and other networks possible. You've  probably heard of some of these protocols:
4. 0:22IP, TCP, UDP, HTTP and so on. If you  haven't heard of them, no problem:
5. 0:29we'll cover many of these  core protocols in this course.
6. 0:32From this suite of protocols, we can build  what people usually call the TCP/IP model:
7. 0:38a way of grouping protocols into layers based  on the job they do. In this video we’re going
8. 0:43to build that model step by step and use it as  a simple map of “who does what” in a network.
9. 0:48This is going to be one of the longest videos  in the course, but there actually isn’t a lot
10. 0:52you have to memorize: we’re covering a big-picture  framework about how networks work. Later, when you
11. 0:58learn about things like IP addressing, routing and  switching, TCP and UDP, and so on, you’ll be able
12. 1:04to drop each new topic into this framework and see  where it fits and how it works with the others.
13. 1:10There’s also a lot of history - and a bit  of argument - around different models,
14. 1:14like the OSI model and various versions of the  TCP/IP model. In this lesson I’ll give you a
15. 1:20practical way to think about layers that will  help you understand how networks actually operate.
16. 1:25As you continue studying, you can add nuance  and explore the debates if you’re interested,
17. 1:30but you don’t need to solve those issues today.
18. 1:32So, relax, don’t worry about memorizing  every concept, and just focus on the big
19. 1:37ideas and relationships between the  layers. We’ll revisit these concepts
20. 1:41many times in later videos. Alright,  enough talking. let’s get started.
21. 1:46Let’s begin with the importance  of protocols and standards.
22. 1:50A protocol is a set of rules defining how data  should be communicated between devices over
23. 1:55a network. Protocols are the languages that  computers use to communicate. Just as someone
24. 2:00who only speaks Chinese can’t talk with someone  who only speaks English, computers that “speak”
25. 2:06different protocols can’t exchange data. Since the  early days of computer networking, there have been
26. 2:12several attempts to define the functions needed  for computers to communicate with each other.
27. 2:17These protocols were often developed by a specific  vendor, such as IBM, to be used with their own
28. 2:23products. With this proprietary approach, enabling  communications between different vendors’ products
29. 2:28was difficult, if not impossible. For example,  if we have an Apple iMac and an IBM server,
30. 2:35each using its own proprietary protocols,  the IBM server wouldn’t be able to understand
31. 2:40a request from the iMac. I’m just using  modern devices here as a visual example,
32. 2:46but this kind of problem was common in the  early days of networking. To solve this issue,
33. 2:51today’s networks and the devices connected to  them use standard, vendor-neutral protocols and
34. 2:56technologies. A standard is an agreed-upon  specification that describes how a protocol
35. 3:02or technology should work. When devices follow  vendor-neutral standards, devices of all types can
36. 3:09communicate with each other. For example, an Apple  MacBook can access a website hosted on a Linux web
37. 3:15server. And a PC running Windows can send an  email that can be read on a smartphone running
38. 3:20Android. As long as the devices follow the same  standards, they can work together on the network.
39. 3:28How did TCP/IP come about? Let’s cover a bit of  history. Early work on the computer networks that
40. 3:34would evolve into today’s Internet began in the  1960s, mainly in the US. The US Department of
41. 3:40Defense’s ARPA, Advanced Research Projects Agency,  funded ARPANET. It came online in 1969 to connect
42. 3:48mainframes at universities and labs around the  country. Here’s a map showing ARPANET in the
43. 3:531970s, connecting locations mostly around the east  and west coasts of the country. ARPANET originally
44. 4:00used a protocol called NCP, the Network Control  Program. There was no TCP/IP in the early days.
45. 4:08In 1974, Vint Cerf and Bob Kahn, two of  the fathers of the modern Internet, began
46. 4:14developing TCP, Transmission Control Program,  as an internetworking protocol. TCP went through
47. 4:21a few revisions, and was later divided into two  protocols that are still used today: Transmission
48. 4:26Control Protocol, TCP. Notice the name change from  “Program” to “Protocol”. And Internet Protocol,
49. 4:33IP. Together, these two protocols form the  foundation of the protocol suite widely known
50. 4:39as TCP/IP today. And the ARPANET fully switched to  use TCP/IP on January 1st, 1983. Over time, TCP/IP
51. 4:50became dominant over other vendor-proprietary  solutions at the time because it was published
52. 4:55as a set of open standards that any vendor could  implement, and it could run over many different
53. 5:00types of networks. And here we are several  decades later, still learning about TCP/IP.
54. 5:08That’s enough history. Now who actually defines  these standards? Most networking standards are
55. 5:13created by independent standards organizations,  not by a single vendor. Engineers from many
56. 5:19different companies participate in the process.  When it comes to networking, there are two
57. 5:24organizations you should be aware of. The first  one is the IEEE, the Institute of Electrical and
58. 5:30Electronics Engineers. I mentioned the IEEE in  the previous video when talking about Ethernet
59. 5:35cables. The IEEE develops many of the technologies  we use on local area networks, such as Ethernet,
60. 5:43which is defined in the 802.3 set of standards.  The IEEE also develops Wi-Fi, which is defined
61. 5:50in the 802.11 standards. These standards include  physical specifications like Ethernet cable types,
62. 5:57Wi-Fi radio frequency, and how to transmit signals  over the physical medium, whether a cable or a
63. 6:03radio wave. But they also specify how to format  messages to send them to another device over the
64. 6:09medium. Both are important topics that we’ll cover  in this course. The second standards group is the
65. 6:16IETF, the Internet Engineering Task Force. The  IETF is an open community that defines many of
66. 6:23the protocols used on the Internet. A few examples  are TCP, IP, UDP, HTTP, and DNS. If you don’t know
67. 6:33these protocols yet, don’t worry: they’re all in  this course. The IETF publishes its standards in
68. 6:39documents called RFCs, Requests for Comments, all  of which are freely available on the Internet.
69. 6:46The IEEE and IETF create the standards, and then  vendors like Cisco implement them so that devices
70. 6:53from different companies can work together. But  there are many different standards and protocols,
71. 6:58each solving a different part of the overall  communication problem. To make sense of them,
72. 7:03it helps to group their jobs into  layers, making a layered model.
73. 7:09So let’s look at that layered model. Networks  do a lot of different jobs when they move data
74. 7:14from one computer to another. Things like the  physical transmission of signals, local delivery
75. 7:19of messages on a LAN, routing traffic between  networks, maintaining end-to-end conversations,
76. 7:25and the applications themselves. A model lets us  group related jobs into layers. Each layer has a
77. 7:33specific role, and each layer uses the services  of the layer below and provides services to the
78. 7:38layer above. We’ll come back to this idea  later. Protocols live mostly at one layer.
79. 7:45I say “mostly” because sometimes the lines are  blurred and it’s hard to categorize a protocol.
80. 7:50Some examples of protocols that we’ll look at  later are IP, TCP, and HTTP. Together they form
81. 7:58a “stack” of protocols that work as a team, often  called the network stack. Here’s an example of a
82. 8:04protocol stack from RFC 791, which defines IP, the  Internet Protocol. It was published in 1981, but
83. 8:12this is still the current standard we use today.  If you’re curious, do a Google search for RFC 791:
84. 8:19as I said in the previous slide, RFCs are freely  available online, although they can be quite
85. 8:24dense reading. You don’t need to read them for the  CCNA. We can divide this stack into a few layers
86. 8:31like this. At the top, we have the application  layer, with protocols like Telnet, FTP, and TFTP.
87. 8:39I won’t define these protocols now, but they’re  all covered in this course. Then below it, we
88. 8:45have the Transport layer, with protocols like TCP  and UDP. Below that is the Internet layer. Here,
89. 8:53“Internet” doesn’t just mean THE Internet, the  public Internet you’re probably using to watch
90. 8:57this video. It refers to an internetwork,  multiple networks connected together. That’s
91. 9:02the real job of this layer. This is where we  find IP, which has two versions in use today:
92. 9:08IP version 4 and IP version 6, both important  CCNA topics. Finally, we have the Link layer,
93. 9:16including local network protocols like  Ethernet and Wi-Fi. Each of these layers,
94. 9:21and the protocols in them, plays a different  role in the network. This is the TCP/IP model,
95. 9:27at least one version of it. The model is a  description, not a law. For example, different
96. 9:33textbooks and courses use slightly different  models, some with 4 layers, some with 5 layers,
97. 9:38and some with more. In this course I’ll use  a five-layer model that builds on this one.
98. 9:46Before talking about networks specifically,  let’s use an analogy for this layered approach:
99. 9:51sending a letter to a friend via the post. I  don’t use analogies like this often because
100. 9:56they can cause misunderstandings if you take them  too seriously, but I think this one is helpful.
101. 10:01On the left, you’re in your house. And on the  right, Bob and his wife are in their house,
102. 10:06with a couple of post offices between, connected  by roads. You write a letter, which is addressed
103. 10:12to Bob. You then put the letter in an envelope  addressed to Bob’s house. Bob’s house is a bit
104. 10:18far from where you live, so instead of driving  directly to his house, you deliver the envelope
105. 10:22in your car to post office A, like this. Notice  the three different destinations. Your car is
106. 10:29going to post office A. The envelope inside your  car is going to Bob’s house. And the letter inside
107. 10:35the envelope is going to Bob himself. Post office  A then moves the envelope to a truck and delivers
108. 10:42it to post office B, like this. The truck’s  destination is post office B, but the envelope and
109. 10:48letter inside keep their original destinations.  Then, post office B moves the envelope to a new
110. 10:55truck and delivers it to Bob’s house, like this.  Once again, this truck has a different destination
111. 11:00from the previous truck: Bob’s house. The envelope  and letter inside still have their original
112. 11:06destinations. The envelope is now delivered to  Bob’s house, opened, and the letter addressed to
113. 11:12Bob is read by him. So in this simple mail system,  different parts care about different things: the
114. 11:18message itself, who should read it, which house  it goes to, and how it moves along the route.
115. 11:25We can turn those roles into a layered model,  and then compare that model to how TCP/IP works.
116. 11:32So, let’s build that model. Don’t worry about  remembering this model or the names of the layers:
117. 11:37it’s just an analogy for now. At the top we  have a content layer, the text of the letter:
118. 11:42what you actually want to say. I’ll call the next  layer the recipient layer: the “To: Bob” part,
119. 11:49indicating who inside the house should read the  letter. Below that is the address layer, which
120. 11:54is the destination address of the house where  that person lives. Next is the local delivery
121. 12:00layer, which handles moving the envelope to the  next stop on the path using cars and trucks:
122. 12:05from your house to post office A, then post  office B, then Bob’s house. With these four
123. 12:11roles we already have a simple 4-layer model,  similar to the TCP/IP stack I showed before.
124. 12:17But we can add one more layer at the  bottom. I’ll call it the infrastructure
125. 12:22layer: the roads and other paths that  the vehicles travel on. But really,
126. 12:26these bottom two layers are related and always  work together. If traveling over ground,
127. 12:32local delivery uses a car or truck. If  traveling in the air, local delivery uses
128. 12:37an airplane. If traveling by water, local  delivery uses a ship. And often the letter
129. 12:43will travel over multiple kinds of paths in its  journey: ground, then air, then ground again,
130. 12:49for example. You can either think of these  bottom two layers as one combined delivery layer,
131. 12:54or split them into two layers. And that’s why some  networking models use 4 layers and some use 5.
132. 13:03One of the benefits of a layered system like  this is how the layers work together, but
133. 13:08remain separate. Each layer has its own job. They  work together to deliver the message, but each
134. 13:14one focuses on its own task. The Content layer  focuses on the actual contents of the letter,
135. 13:21and that doesn’t change throughout the journey.  The Recipient layer focuses on the individual
136. 13:26person who should receive the letter, and also  remains the same from start to finish. Likewise,
137. 13:32the Address layer focuses on the address of the  house or building to which the letter should be
138. 13:36delivered, and that stays the same as well. The  Local Delivery layer focuses on getting the letter
139. 13:43to the next stop in the route: post office A,  then post office B, and finally Bob’s house. The
140. 13:50Infrastructure layer is the system of roads and  mail trucks that the delivery process relies on.
141. 13:56What happens inside one layer doesn’t change the  job of the other layers. For example, changing the
142. 14:02content of the letter doesn’t change the delivery  steps. And changing the delivery path doesn’t
143. 14:07affect the letter itself. When you’re writing the  letter to Bob, you don’t have to think about how
144. 14:12the postal service will go about delivering it. Of  course, that doesn’t mean the layers are entirely
145. 14:18independent. For example, if you address the  letter to somewhere overseas, that will influence
146. 14:23how it’s delivered. But that’s not your concern  as you write the letter. This separation of layers
147. 14:30is true in our mail system model example, and  it’s true of networks as well. Now let’s replace
148. 14:35letters with data and roads with networks, and  build the 5-layer TCP/IP model we’ll actually use.
149. 14:44Here are the five layers of our mail system.  Let’s map them to the TCP/IP model. The Content
150. 14:50layer is equivalent to the TCP/IP Application  layer. The Recipient layer maps to the Transport
151. 14:56layer. The Address layer matches the Internet  layer. These are the same names I showed before
152. 15:02for the TCP/IP model. But I’m going to use  a different name next. The Local Delivery
153. 15:08layer matches what I’ll call the Local Network  layer. This is the layer used for delivering
154. 15:13messages within a LAN, a local area network.  There are a few different names for this layer,
155. 15:18but I’ll go with “Local Network”. And finally  the Infrastructure layer is equivalent to the
156. 15:23Physical layer. At this point, these  names probably don’t mean much to you,
157. 15:29so next we’ll see how they actually work in  a real network between a client and a server.
158. 15:36Here is a simple network I’ll use to demonstrate.  You’ve seen these icons before. On the left,
159. 15:41we have PC1, a client that will send a request  to SRV1, the server on the right. Between them
160. 15:48are two routers, R1 and R2, and two switches: SW1  between PC1 and R1, and SW2 between R2 and SRV1.
161. 15:58PC1 is running a web client application; in  other words, a web browser such as Chrome.
162. 16:04SRV1 is running a couple of processes: a  web server and a file server. PC1’s user
163. 16:11wants to access a web page hosted on SRV1, so  the web client on PC1, let’s say it’s Chrome,
164. 16:17needs to send a request to the web server  on SRV1. That’s the role of the application
165. 16:23layer: it includes protocols for communication  between application processes and is responsible
166. 16:28for creating and interpreting the data. But  there’s a problem: there are multiple processes
167. 16:34running on SRV1: a web server process and a  file server process, and possibly more. So,
168. 16:41how can PC1 ensure that its request reaches  the correct process on SRV1? That’s the role
169. 16:47of the Transport layer. Each process on SRV1  has an associated transport “port number”:
170. 16:5480 for the web server and 21 for the file server.  Don’t worry about the exact numbers for now;
171. 17:00that’s for later in the course. The important  point is that, to send the message to the web
172. 17:05server, PC1 addresses it to port 80. Don’t  be confused by the name “port”; these aren’t
173. 17:11physical ports like we looked at in the previous  lecture. Here, a port is just a number used to
174. 17:17identify a process running on a host. The role  of the Transport layer is to provide end-to-end
175. 17:23communication between application processes, such  as the web browser on PC1 and the web server on
176. 17:29SRV1, using port numbers. But we still have a  problem: even if the message is addressed to
177. 17:35the correct port on SRV1, we still have to make  sure the message reaches SRV1 in the first place.
178. 17:42SRV1 has an IP address for that purpose. For  example, let’s say its IP address is 10.1.1.1.
179. 17:50Again, don’t worry about the format of the address  itself yet; that’s a topic for another video. By
180. 17:57addressing its message to SRV1’s IP address,  PC1 tells the routers in the path which host
181. 18:02the message should be delivered to. That’s the  role of the Internet layer: it provides end-to-end
182. 18:08communication between hosts, from the source host  to the destination host, using IP addresses and
183. 18:14routers. When you think of the Internet layer,  think “IP addresses and routers.” And “host”,
184. 18:20by the way, just means a device connected to  the network that can send and receive data,
185. 18:25such as PC1 and SRV1. So, we’re getting close  to achieving our goal of getting PC1’s request
186. 18:32to SRV1, but we’re not quite there yet. There are  several devices between PC1 and SRV1, and we need
187. 18:39to make sure the message is properly passed along  between them. That’s the role of the Local Network
188. 18:45layer. Using protocols at this layer, such as  Ethernet, each device sends the message to the
189. 18:50next device on the local network: PC1 to R1 via  SW1,R1 forwards the message to R2. And finally,
190. 19:00R2 forwards the message to SRV1 via SW2. The Local  Network layer provides hop-to-hop delivery within
191. 19:07a local network using MAC addresses and switches.  I’ll explain the term “hop-to-hop” more in a bit,
192. 19:14and MAC addresses are a topic for another video.  Looking at this diagram, you might wonder about
193. 19:19the role of the switches; they connect devices in  a LAN and pass messages between them. We’ll look
194. 19:26at how that works in detail in future  videos. Okay, and last but not least,
195. 19:31we have the Physical layer: all of these cables  connecting the devices, and the transceivers on
196. 19:36the devices that transmit and receive signals. The  Physical layer sends bits as electrical, optical,
197. 19:43or radio signals over the physical medium.  Electrical signals over copper UTP cables,
198. 19:49optical signals over fiber-optic cables, and  radio signals over wireless Wi-Fi connections.
199. 19:57We’ve covered the role of each layer.  But now let’s go from the bottom up,
200. 20:01from the Physical layer to the Application layer,  and look at a few more details. The Physical
201. 20:07layer, also called “Layer 1”, is responsible  for sending and receiving bits as electrical,
202. 20:12optical, or radio signals over the medium.  It defines things like cables, connectors,
203. 20:18signal levels, and link speeds: all of the  physical aspects of communication. Examples
204. 20:24include copper UTP and fiber-optic cables, Wi-Fi  radios and antennas, and network interface cards,
205. 20:31also called NICs. Here are a few examples: UTP  ports and a cable on the left, fiber-optic in the
206. 20:38middle, and an image of a NIC on the right. Each  network interface has a NIC like this inside of
207. 20:44the device. The physical aspects of transmitting  data are very complex, involving a lot of
208. 20:50electrical or optical engineering. Fortunately,  network engineers typically don’t have to know
209. 20:56the low-level details of how it all works. So,  that’s the Physical layer: Layer 1 of our TCP/IP
210. 21:02model. In some models this layer is combined  with the next layer, but I think it’s useful
211. 21:07to differentiate between the physical concerns  like these and the logical aspects of Layer 2.
212. 21:14Next, we have the Local Network layer, Layer  2. This layer provides hop-to-hop delivery of
213. 21:20messages on a local network. So, what exactly  is a “hop”? A hop is one step along the path
214. 21:27between two devices: from one router or host,  to the next router or host in the path. When PC1
215. 21:34sends a message to SRV1, how many hops are there?  The first hop is from PC1 to R1. The second hop
216. 21:42is from R1 to R2. And the third hop is from R2 to  SRV1, so three hops in total. Switches don’t count
217. 21:50as hops: a switch just extends the local network,  allowing multiple devices to connect. To keep the
218. 21:56diagram simple I only show one host connected to  each of the switches, but there could be many more
219. 22:02connected, like this. The switch allows them all  to connect to the same local area network. We’ll
220. 22:08look at switches and LANs in detail in later  videos, so don’t worry about them for now.
221. 22:13Layer 2 uses MAC, Media Access Control, addresses  to identify interfaces. Each device connected to
222. 22:21a LAN has a unique MAC address for that  specific interface. Since R1 and R2 have
223. 22:27multiple interfaces connected to the network,  let’s add some simple interface labels. G1, for
224. 22:34GigabitEthernet1, and G2, for GigabitEthernet2,  meaning these interfaces operate at a speed of 1
225. 22:40gigabit per second. PC1 sends the message  to the MAC address of R1’s G1 interface,
226. 22:47its NIC. That’s the interface that will receive  PC1’s message. R1 sends the message to the MAC
227. 22:54address of R2’s G1 interface. And R2 sends  the message to the MAC address of SRV1’s
228. 23:00interface. Once again, we’ll cover MAC addresses  in another video when we learn about switches.
229. 23:06The key protocols at this layer that you should  know for the CCNA are Ethernet and Wi-Fi. Others
230. 23:13exist, of course, but these are by far the  most commonly used Layer-2 protocols today.
231. 23:20Next up we have the Internet layer, which  is Layer 3 of our model. This layer provides
232. 23:25end-to-end delivery between hosts across multiple  networks. We call it “end-to-end” because it
233. 23:31focuses on getting the message from the source  host all the way to the final destination host,
234. 23:37instead of worrying about each individual  hop in the middle. Remember, “Internet”
235. 23:42means “internetwork”, between networks. It uses  IP addresses to identify hosts in the network,
236. 23:48kind of like a home address. In our example, SRV1  has the IP address 10.1.1.1. So when PC1 sends
237. 23:57a message to SRV1, it addresses the message to  SRV1’s IP address. Routers operate mainly at this
238. 24:05layer, using the message’s destination address to  forward the message toward its final destination
239. 24:10host. There are a few protocols at this layer  that you’ll learn about for the CCNA: IP,
240. 24:16both version 4 and version 6, and ICMP,  the Internet Control Message Protocol.
241. 24:24Next is Layer 4, the Transport layer. This  layer provides end-to-end communication between
242. 24:30application processes. And why is that needed?  Well, in our example, our server, SRV1, provides
243. 24:37multiple services. It’s running a web server and  a file server, and perhaps other services. If SRV1
244. 24:44receives a message, it needs a way to know which  of these services should receive the message.
245. 24:49This can also be called “process-to-process”  or “service-to-service” communication. This
246. 24:55layer uses port numbers to identify the processes  on each host, such as port 80 for the web server
247. 25:01and port 21 for the file server. So, when the web  client on PC1 wants to send a request to the web
248. 25:08server running on SRV1, it addresses the message  to port 80. And if PC1 wanted to access the file
249. 25:15server instead, it would address its messages  to port 21. The Transport layer allows hosts
250. 25:21to differentiate between these different streams  of data. Layer 4 runs mainly on the communicating
251. 25:27hosts, PC1 and SRV1. Routers normally operate  based on IP, Layer 3, not on Transport-layer
252. 25:35information. There are exceptions of course,  but that’s a topic for later in the course.
253. 25:41Layer 4 is primarily a conversation between the  two communicating hosts. There are a few protocols
254. 25:48used at this layer. The two most prominent are  UDP, User Datagram Protocol, and TCP, Transmission
255. 25:55Control Protocol. They both offer some different  features, and we’ll cover them in this course.
256. 26:03Finally, we have layer 5: the Application  layer. This is where network communications
257. 26:09meet applications. Small side note:  this layer is usually called “Layer
258. 26:147”. I’ll explain why later when we talk about  another model, the OSI model. The Application
259. 26:20layer defines how application processes  format, send, and interpret data. So,
260. 26:26when Chrome on PC1 sends a request  to the web server process on SRV1,
261. 26:31it uses an Application-layer protocol such  as HTTP to format and send the message,
262. 26:37and that same protocol tells the web server how  to interpret the message it receives. Protocols
263. 26:43at this layer define message formats and rules  for specific tasks. Here are a few examples. For
264. 26:50browsing web pages, we use HTTP, or more often  its secure version, HTTPS. For file transfers,
265. 26:58we can use FTP, TFTP, or another similar  protocol. And email uses its own protocols, too.
266. 27:06Don’t worry about the specific protocols or their  names at this point, right now we’re just talking
267. 27:11big concepts. Network infrastructure devices  like routers and switches typically don’t care
268. 27:18about Application-layer details. This is too  high-level for them. They just move messages
269. 27:23across the network. Only the communicating  hosts, PC1 and SRV1 in this case, actually
270. 27:30look at and interpret the Application-layer  data. Now that we’ve seen what each layer does,
271. 27:36how does a single message actually include all of  this information at once? Let’s see how the layers
272. 27:42combine into a stack and how data moves through  them using encapsulation and decapsulation.
273. 27:49To show how encapsulation and decapsulation  work, let’s simplify the network, making it a
274. 27:54direct connection between PC1 and SRV1. First,  the Application layer prepares the data to be
275. 28:01sent over the network, for example an HTTP request  that Chrome on PC1 sends to a web server running
276. 28:08on SRV1. As the message moves down the stack, each  layer encapsulates the data with a header that
277. 28:16contains the information needed by that layer.  For example, the source and destination addresses:
278. 28:22Layer 4 port numbers, layer 3 IP addresses, and  Layer 2 MAC addresses, among other information.
279. 28:30So first, the Transport layer encapsulates  the data with its header, the Layer 4 header,
280. 28:35with source and destination port numbers  and other information. Then the Internet
281. 28:40layer adds its header with source and destination  IP addresses. Then, Layer 2 encapsulates the data
282. 28:47with both a header and a trailer. The trailer  is used by the receiving device to check for
283. 28:52transmission errors. We’ll look at this in  more detail in a later video about Ethernet.
284. 28:58Finally, the Physical layer transmits the  bits as signals over the physical medium,
285. 29:03which is an Ethernet cable in this case. I’m  showing the message in order from left to
286. 29:08right here. So the Layer 2 header is transmitted  first, and Layer 2 trailer is transmitted last.
287. 29:17So that was the encapsulation process, now let’s  look at decapsulation. The receiving device
288. 29:23receives the message as a stream of bits at layer  1. Layer 1 simply passes those bits up to the next
289. 29:29layer. Now it’s up to the other layers to  interpret those bits. The device examines the
290. 29:35information in the Layer 2 header and trailer, and  then removes them. This is called “decapsulation”,
291. 29:41or sometimes “de-encapsulation”. The decapsulation  process continues up the stack. So, Layer
292. 29:483 examines and removes the Layer 3 header. Then  Layer 4 examines and removes the Layer 4 header,
293. 29:55and the data is delivered to the Application  layer. Now the application processes the data and,
294. 30:01if needed, generates a response that goes back  down the stack, and is then transmitted back to
295. 30:06PC1. So, that’s the decapsulation process, which  is basically the encapsulation process in reverse.
296. 30:15Each device has its own network stack.  A message from PC1 goes down its stack,
297. 30:21crosses the wire, and then goes up SRV1’s  stack where the Application layer processes
298. 30:25it. The response does the same thing in the  opposite direction. It goes down SRV1’s stack,
299. 30:31is sent over the wire to PC1, and then goes  up PC1’s stack where the data is interpreted
300. 30:36by the application. In a real network there are  switches, routers, and other devices between the
301. 30:42hosts. That adds more steps to the delivery  process, just like the mail system example,
302. 30:47where the letter is moved between cars and trucks.  But the overall flow is still the same: each
303. 30:53host sends the message down its stack, across the  network, and up the other host’s stack. We’ll look
304. 31:00at how those intermediate devices - the routers  and switches - handle messages in later videos.
305. 31:07Now that we’ve seen how encapsulation and  decapsulation work, let’s look at what to call
306. 31:12these messages. At each stage in the encapsulation  and decapsulation process, there is a name given
307. 31:18to the message. The combination of data and a  Layer 4 header is called a segment when using TCP,
308. 31:25or a datagram when using UDP. Remember that point:  TCP creates segments, and UDP creates datagrams.
309. 31:33This difference has to do with how TCP and UDP  treat the data, but we don’t need those details
310. 31:38yet. The combination of a segment or datagram and  a Layer 3 header is called a packet. “Packet” is
311. 31:46the most common term we use when talking about  messages being sent over a network, but strictly
312. 31:51speaking it refers to the message at this stage in  the process. Finally, the combination of a packet
313. 31:57and a Layer 2 header and trailer is called  a frame. This is what is actually sent over
314. 32:02the wire. You’ll never see a packet, segment,  or datagram travelling over the wire itself;
315. 32:08they are always sent inside a frame. We can also  use alternative names to describe the message at
316. 32:14each stage, using the term protocol data unit,  or PDU. A segment or datagram is a Layer 4 PDU,
317. 32:21or L4PDU. A packet is a Layer 3 PDU, or L3PDU. And  a frame is a Layer 2 PDU, or L2PDU. Both names are
318. 32:33common, so I recommend knowing both. And there’s  one more important term to learn. The contents
319. 32:39of each PDU, that is everything encapsulated by  the layer’s header and trailer, are called the
320. 32:44payload. So, at Layer 4 a segment or datagram’s  payload is the application data itself. At Layer
321. 32:513, a packet’s payload is a segment or datagram,  including the Layer 4 header and the data. And at
322. 32:58Layer 2, a frame’s payload is a packet, including  the Layer 3 and Layer 4 headers and the data. Just
323. 33:05remember that the payload is what’s inside the  PDU, not including that layer’s header or trailer.
324. 33:13Finally, let’s talk about how these layers  interact with each other. I’ve replaced the
325. 33:17generic layer names with examples of common  protocols. Also note the distinction between
326. 33:23Layer 2 Ethernet, which defines MAC addressing  and frames, and Ethernet at the physical layer,
327. 33:29which defines the signaling and physical  media. Each layer in the stack provides a
328. 33:34service to the layer above it, and is serviced by  the layer below it. This is called adjacent-layer
329. 33:40interaction. For example, Layer 4 provides a  service to Layer 5 by delivering data to the
330. 33:47correct application using port numbers. Layer  3 provides a service to Layer 4 by delivering
331. 33:53segments and datagrams to the correct destination  host using IP addresses. Layer 2 provides a
332. 34:00service to Layer 3 by delivering packets to  the next hop using MAC addresses. And Layer
333. 34:061 provides a service to Layer 2 by sending and  receiving the frame’s bits as electrical, optical,
334. 34:11or radio signals over the physical medium. Each  layer relies on the layer below it to do its job.
335. 34:20There’s also a related concept. Each layer  communicates with the same layer on other devices,
336. 34:26and this is called same-layer interaction.  For example, the Application layer on one host
337. 34:31sends data to the Application layer on the other  host. A segment or datagram is addressed to the
338. 34:37Layer 4 port number of the correct application  on the destination host. A packet is addressed
339. 34:43to the Layer 3 IP address of the destination  host. A frame is addressed to the Layer 2 MAC
340. 34:49address of the next hop. And signals sent  out of a physical port are received by a
341. 34:54physical port on the connected device.  This layered cooperation - within each
342. 34:59device and between devices - is what  makes network communication possible.
343. 35:05Just like in our mail system example, the  separation of layers is key to how networks
344. 35:10work. Each layer has its own job and provides a  specific service to the layers above. Because the
345. 35:17layers are modular, we can swap protocols at one  layer without changing the others. For example,
346. 35:23instead of a web page exchange using HTTP and  TCP, maybe it’s a file exchange using TFTP,
347. 35:30Trivial File Transfer Protocol, over UDP.  The lower layers in the stack can use IP
348. 35:36and Ethernet without worrying about  the details of what the upper layers
349. 35:40are doing. And if instead of a wired Ethernet  connection it’s a wireless Wi-Fi connection,
350. 35:46Layers 1 and 2 can change to Wi-Fi without  affecting the upper layers. As long as each
351. 35:51layer keeps its “contract” with the layers above  and below, we can improve or replace protocols at
352. 35:57different layers without redesigning everything.  That flexibility is one of the main benefits of a
353. 36:02layered model. Now that we’ve built  this 5-layer TCP/IP model, let’s see
354. 36:08how it compares to other models you’ll see in  other courses and books, like the OSI model.
355. 36:15You might have heard of the OSI model  before. If you’ve studied IT before,
356. 36:19you almost certainly have. As I said at the  beginning of this video, TCP/IP development
357. 36:24started in the 1970s, with work on ARPANET and  the early TCP/IP specifications. But in the late
358. 36:311970s and 1980s, the International Organization  for Standardization, the ISO, designed a 7-layer
359. 36:39model called OSI, Open Systems Interconnection,  and a matching protocol suite. The goal was to
360. 36:45create international, vendor-neutral networking  standards that could unify existing proprietary
361. 36:50stacks and potentially replace TCP/IP as the  vendor-neutral choice. Here is that model,
362. 36:56with seven layers from top to bottom: Application,  Presentation, Session, Transport, Network, Data
363. 37:04Link, and Physical. Governments, including the  US, promoted OSI as the preferred or recommended
364. 37:10stack for new deployments. And for a while many  people thought that OSI was the future. But OSI
365. 37:17protocols ended up being developed too late and  were too complex, and they never gained the same
366. 37:22level of deployment as TCP/IP. I wasn’t around  at the time, but my understanding is that this
367. 37:28was largely due to the bureaucratic nature of  the approach to developing OSI and its protocol
368. 37:33stack. The process was very top-down: committees  designed the protocols in detail, and vendors were
369. 37:40expected to implement them exactly as specified.  TCP/IP used a more bottom-up approach. In the end,
370. 37:47TCP/IP won in the real world, although some  OSI technologies are still used. Today,
371. 37:54almost all networks use the TCP/IP protocol stack,  but the 7-layer OSI model survives as a reference
372. 38:00and teaching model, and as a common way to talk  about layers. Actually, most networking resources
373. 38:06use a 5-layer model like the one I covered in this  video, but with names borrowed from the OSI model.
374. 38:12Here is that 5-layer model. I foreshadowed this  earlier in the video, but this is why the TCP/IP
375. 38:19Application layer is often called Layer 7: because  the Application layer is Layer 7 of the OSI model.
376. 38:26I’ve also highlighted the two layer names that  are different from the model I introduced:
377. 38:30Layer 3 is called the Network layer, and  Layer 2 is called the Data Link layer.
378. 38:37And to make things even more confusing, here’s  a selection from Wikipedia of different models
379. 38:42proposed by various authors. Here we have the  four-layer TCP/IP model I showed at the beginning
380. 38:48of this video. Here is the seven-layer OSI model.  And the adapted five-layer model I just showed in
381. 38:55the previous slide. When you see all of these  variations, you might wonder which model and
382. 39:01which names you should actually learn. Of course,  I recommend the five-layer model I presented in
383. 39:06this video, because I think its names best match  the job of each layer. But this five-layer model
384. 39:12shown here is probably the most common in books  and other resources. The only differences from my
385. 39:18model are that Layer 3 is called the Network  layer and Layer 2 is called the Data Link
386. 39:23layer. In the end, it’s not that important:  in practice people usually just refer to
387. 39:28“Layer 2” and “Layer 3”, and you won’t be  quizzed about layer names on the CCNA exam.
388. 39:35Before wrapping up, let’s review the key  concepts from this video. I won’t explain
389. 39:40everything again here; I’ll just point out what  you should focus on as you review. The first is,
390. 39:45of course, the TCP/IP model itself. You should  know the general purpose of each of these
391. 39:50layers. I also recommend knowing these more  commonly used names borrowed from the OSI model.
392. 39:57It’s also important to understand the  encapsulation and decapsulation processes.
393. 40:02In encapsulation, the sending  host adds headers, and a trailer,
394. 40:05to the data to prepare it for transmission  over the physical medium. In decapsulation,
395. 40:10the receiving host removes the headers and trailer  layer by layer until it gets to the data inside.
396. 40:17You should also know the names of the different  PDUs, protocol data units. The Layer 4 PDU is a
397. 40:23segment when using TCP and a datagram when using  UDP. The Layer 3 PDU is called a packet. And the
398. 40:30Layer 2 PDU is called a frame; the frame is what  is actually transmitted over the physical medium.
399. 40:36And finally, you should understand adjacent-layer  interaction and same-layer interaction. These
400. 40:42concepts describe how the different layers  work together within each host and with
401. 40:45their counterparts on other devices to achieve  communication between applications over a network.
402. 40:52That’s all for this video on the TCP/IP model. By  now you should have a mental picture of the five
403. 40:58layers, what each one is responsible for, and  how they work together using encapsulation and
404. 41:03decapsulation. You don’t need to remember every  example or every historical detail; the important
405. 41:08thing is that you know the layers, their general  roles, and the idea of PDUs and layer interaction.
406. 41:14We’ll keep coming back to this model throughout  the course. You’ll hear the terms “Layer 1”,
407. 41:19“Layer 2”, “Layer 3”, and so on many times.  As we dive into topics like Ethernet,
408. 41:25IP addressing, TCP, and routing, you’ll be  able to place each new concept at the right
409. 41:30layer and see how it fits into  the bigger picture. Just remember,
410. 41:33this is a model, not a law. Real protocols  don’t always fit neatly into a single layer,
411. 41:38and that’s okay. The model is just a tool to help  you think about what’s happening in the network.
412. 41:43If you’d like a structured companion to  these videos, I’ve also written two books:
413. 41:47Acing the CCNA Exam Volume 1 and Volume 2. They  go into more detail than I can fit in the videos,
414. 41:53and include hundreds of diagrams and practice  questions. I honestly think they’re my best
415. 41:58work when it comes to teaching the CCNA. If  you’re interested, you can find them on Amazon,
416. 42:03or use the links in the description to get them  directly from the publisher. Thanks for watching.
417. 42:08Before we wrap up, I want to say a quick thank  you to the channel members whose names you see
418. 42:12on screen. Your monthly support helps me keep  making these videos and keep this content free
419. 42:17for everyone on YouTube. I really appreciate  it: thank you for your support. If the video
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