Data Communication - N10-008 CompTIA Network+ : 1.1 — Transcript

Full transcript

1. 0:02Getting data moved from one part of the network
2. 0:04to the other relies on something called
3. 0:07a PDU, or a protocol data unit.
4. 0:09Sometimes you'll hear these referred to
5. 0:11as transmission units, because we're
6. 0:13taking a little bit of data and transferring it
7. 0:16across the network as a single unit.
8. 0:18For example, if we're running Ethernet,
9. 0:20we know that Ethernet is going to send everything
10. 0:23within an Ethernet frame from one device or MAC address
11. 0:27on the network to another MAC address that's on the network.
12. 0:30Ethernet, though, doesn't care what's
13. 0:32on the inside of this particular frame.
14. 0:35It's simply encapsulating all of the data that
15. 0:37may be underneath it and sending that across the network.
16. 0:41A similar thing happens at the next layer up with layer 3,
17. 0:44or the IP layer, where everything within the IP layer
18. 0:48is being sent across the network from one IP address
19. 0:51to the other.
20. 0:52Inside of that IP packet is UDP data, TCP data,
21. 0:56or some other type of data, but IP doesn't
22. 0:59care what's on the inside.
23. 1:01It simply knows that it's moving data
24. 1:03across the network from one IP address to another.
25. 1:06And where there is a TCP header or UDP header,
26. 1:09there's probably going to be a TCP segment, or what
27. 1:12we call a UDP datagram within that particular part
28. 1:16of the packet.
29. 1:18Here's an illustrated view of how
30. 1:20this operates at each layer of the OSI model,
31. 1:23and how data is encapsulated between each one
32. 1:26of those parts.
33. 1:27For example, let's start with using an application.
34. 1:30We're sending data within our application,
35. 1:32such as a web browser, and we need
36. 1:34to get that data sent from the web server to the web client.
37. 1:38That application data exists at OSI layers
38. 1:415, 6, and 7, or what we will generically
39. 1:43call the application layers.
40. 1:45To be able to send that application data,
41. 1:47we need to transport it across the network using the TCP
42. 1:50protocol, so we'll put a TCP header right
43. 1:53in front of that application data
44. 1:55and send it across using that OSI layer 4,
45. 1:58or transport layer.
46. 2:00We have to have an IP address that
47. 2:02is able to send this TCP data and the application data that
48. 2:06might be inside of it, so we need
49. 2:08to add an IP header so that we can send information
50. 2:11from one IP address to another IP address.
51. 2:14And lastly, over Ethernet, we need
52. 2:16to have a DLC, or data link control, layer 2 frame header.
53. 2:21And in many cases, there's also a frame trailer
54. 2:23to indicate where the end of this frame might be.
55. 2:27The layer 2 frame header encapsulates
56. 2:29all of the information within.
57. 2:31So inside of this frame will be an IP header, a TCP header,
58. 2:35and ultimately, the application data
59. 2:37that we would like to send to the other side of the network.
60. 2:41Let's expand this view to show both the sending and receiving
61. 2:45device, and view the way the network might
62. 2:48interoperate with all of this.
63. 2:49So there's our application data at OSI layers 5, 6, and 7.
64. 2:53On the left side is the source address
65. 2:55that's sending this data, and we somehow
66. 2:57want to get that application data
67. 2:59to this destination device on the other side of the network.
68. 3:03We'll start by encapsulating this data by
69. 3:05including a TCP header, an IP header, and ultimately,
70. 3:09the frame header and frame trailer.
71. 3:11That information is going to be sent across the network, where
72. 3:15it will then be received by the destination device.
73. 3:18The destination device, though, needs the application data
74. 3:21that's inside--
75. 3:22it doesn't need all of this other information.
76. 3:24So it will begin decapsulating this information-- removing
77. 3:28the frame headers, removing the IP header,
78. 3:30and ultimately removing the TCP header
79. 3:33so that it can then access the application data.
80. 3:36And finally, we've been able to transfer this data
81. 3:38from the source device through the encapsulation
82. 3:41process, the decapsulation process,
83. 3:44and finally be able to receive the application
84. 3:47data at the destination.
85. 3:49Here's another way to visualize how
86. 3:51this data is being encapsulated and decapsulated on both sides.
87. 3:56We can start with our layer 5 through 7 application, data
88. 3:59such as HTTPS, IMAP email information, or SSH terminal
89. 4:04screens, where we're then going to take all of that
90. 4:07and encapsulate it within some type of layer for protocol.
91. 4:11This is commonly TCP or UDP.
92. 4:13We will then encapsulate the layer 4 traffic
93. 4:16within layer 3--
94. 4:17which, these days, is commonly IP traffic--
95. 4:20And then finally, layer 2 information on Ethernet.
96. 4:23That would be a media access control address or MAC address,
97. 4:27and that encapsulates all of this data within it.
98. 4:31In previous views of this data, we
99. 4:33mentioned that there's header information that
100. 4:35precedes the data associated with these layers.
101. 4:38And the header information is important
102. 4:40because it tells the rest of the data how
103. 4:43it should be processed.
104. 4:44For example, on layer 4, we have TCP data.
105. 4:48And within the TCP header, such as we have here,
106. 4:51there's information called a TCP flag.
107. 4:55This helps us understand how we can process this data as it's
108. 4:58going through the network.
109. 4:59This control information is setting
110. 5:01bits that are within the header of this packet,
111. 5:04and each one of those bits has a particular definition.
112. 5:08This means that the device that's receiving this data
113. 5:11can interpret those bits and understand how
114. 5:13to process the data properly.
115. 5:15We call these bits control flags,
116. 5:18and we can identify whether a flag has
117. 5:20been turned on or turned off, and then
118. 5:23decide how these particular flags can affect the data flow.
119. 5:26For example, we can look at the flags
120. 5:28in this particular protocol decode
121. 5:30and we can see that one flag has been set to 1.
122. 5:33That means the data that's contained within this TCP
123. 5:36part of the packet is acknowledgment data that
124. 5:39has been set.
125. 5:40You can see a number of different flags.
126. 5:42For example, if the SYN flag is set,
127. 5:45that means that there's a synchronization of sequence
128. 5:47numbers that's occurring.
129. 5:49If the push flag, or PSH flag, has been set,
130. 5:52it pushes the data to the application
131. 5:54without buffering anything else that might be incoming.
132. 5:57There's also a reset flag, or RST,
133. 5:59that resets the connection, and a FIN flag
134. 6:02that designates that this is the last packet that
135. 6:04will be sent by the sender.
136. 6:06By turning on or off different flags,
137. 6:08we can change how a device may interpret
138. 6:11the rest of the data that is being
139. 6:13sent using that TCP header.
140. 6:16Not only are there flags within the TCP or UDP header--
141. 6:20there are also flags within the IP header.
142. 6:23You can see an example of those flags right here.
143. 6:26Most of the flags being used at the IP header deal
144. 6:29with the fragmentation of data.
145. 6:31There may be times when you want to send traffic
146. 6:33across the network, but because of the architecture
147. 6:36or design of the network, you're,
148. 6:38not able to send packets that are very large.
149. 6:40In those cases, you may need to fragment
150. 6:42the data to be able to get through the smaller size
151. 6:45networks.
152. 6:46We determine the maximum size that you're
153. 6:48able to send using something called a maximum transmission
154. 6:52unit, or an MTU.
155. 6:54This designates the size of the data
156. 6:56that we're able to send through the network
157. 6:58without having to fragment any of that information further.
158. 7:02The reason we don't want to fragment
159. 7:04is that it commonly slows down the overall flow of traffic,
160. 7:07and if you can optimize your network communication
161. 7:10so that you're not fragmenting, you'll
162. 7:12have a much higher throughput of traffic.
163. 7:15This also eliminates any overhead
164. 7:17of having to chop the data into smaller pieces,
165. 7:20send those individual pieces across the network,
166. 7:23and then rebuild those pieces when
167. 7:24they get to the other side.
168. 7:26That's why it's important to know the MTU value that
169. 7:29would be used all the way through the network
170. 7:31from the beginning of the communication all
171. 7:34the way to the very end.
172. 7:35But understanding what the MTU might be
173. 7:38could sometimes be a very complicated process.
174. 7:41There may be many different hops that
175. 7:43are used to be able to communicate from point A
176. 7:45to point B, and each one of those hops
177. 7:48may be using a different MTU.
178. 7:50There is an automated process that your system will
179. 7:53use to attempt to determine what the MTU is when communicating
180. 7:57to that other device, but unfortunately,
181. 7:59if ICMP is filtered in that communication,
182. 8:02there's no way to automate that process, which
183. 8:05means you'll have to manually set the MTU on your side.
184. 8:09Let's take a look at what this fragmentation really means.
185. 8:12We've seen before, where we've taken some TCP, data
186. 8:15we put a TCP header in front of it, an IP header
187. 8:18in front of that, and finally, a DLC
188. 8:21header on the outside to send it across our Ethernet network.
189. 8:25The data that's on the inside from the IP header
190. 8:27all the way through to the data that's being transmitted
191. 8:30is our IP packet, and the maximum size
192. 8:33of an IP packet on an Ethernet network is 1,500 bytes.
193. 8:38If there's no fragmentation that's occurring anywhere
194. 8:41on the network, you'll be able to send all 1,500
195. 8:44bytes through the network without having any of that data
196. 8:47fragmented along the way.
197. 8:49Let's take an example, where we can only
198. 8:51send a very small amount of data through the network.
199. 8:54In this example, 16 bytes of data
200. 8:57is the maximum transmission unit that's
201. 8:59supported on this particular network.
202. 9:02That means we might have 44 bytes of data
203. 9:05that needs to be fragmented, and as we're sending it
204. 9:07through the network, we're going to fragment the first section
205. 9:10of it, or the first 16 bytes.
206. 9:12We'll then send another frame of data that has another 16 bytes,
207. 9:16and the last frame is going to send the remaining
208. 9:19amount of data, up to 16 bytes.
209. 9:22So if we do need to send data across the network that's
210. 9:2444 bytes in length, but the MTU of this network
211. 9:28is only 16 bytes, we're going to end up taking a single frame
212. 9:32and splitting it up into three separate frames.
213. 9:36Fortunately, when you're designing and creating
214. 9:38a network, the MTU is usually set during that creation
215. 9:42process, so once the network is built,
216. 9:44it's very unusual for that MTU to change.
217. 9:47We also know that there are going
218. 9:49to be MTU changes if we have to tunnel any information,
219. 9:53so if you're using a VPN of any type,
220. 9:55you're probably going to need to set some MTU
221. 9:58or ensure that the MTU is going to be properly set
222. 10:01automatically.
223. 10:02There is a flag within the IP part of the header that
224. 10:06will specify that the information you're sending
225. 10:08should not be fragmented as it goes through the network.
226. 10:12And if you send data that's too large with a Don't Fragment bit
227. 10:15set, that data may not be able to make it all
228. 10:18the way through the network.
229. 10:19And very commonly, you'll receive an ICMP message
230. 10:22saying that this data is too large
231. 10:25to send through this network.
232. 10:27If you'd like to test your network between one
233. 10:30device and another to see exactly what the MTU might be,
234. 10:33you can test by using the ping utility.
235. 10:36You can specify in ping to set that Don't Fragment bit,
236. 10:40and then you can force a particular size of data
237. 10:42to be sent through the network.
238. 10:44In the case of Ethernet, the maximum size
239. 10:46would be 1,472 bytes.
240. 10:50Now, normally, that would be 1,500 bytes,
241. 10:52but the ping command is specifying
242. 10:54that 1,500 bytes minus the ICMP header of 8 bytes,
243. 10:58and minus the 20-byte header of the IP
244. 11:01address itself, leaving us with 1,472 as the maximum MTU that
245. 11:08would be used for the ping command on an Ethernet network.
246. 11:11In Windows, you can test this by using the ping command
247. 11:15with the -f, which tells us to ping with the Don't Fragment
248. 11:19bit set, and then with -l, which specifies the length of data
249. 11:24you would like to use in bytes.
250. 11:25And in this case, it's 1,472 bytes.
251. 11:29And if you'd like to try this against Google's DNS server,
252. 11:32you can then use the IP address of 8.8.8.8.
253. 11:36Let's try this ping with the -f to don't fragment.
254. 11:40We'll do -l of 1,472 bytes, and let's
255. 11:43specify the DNS for Google.
256. 11:46And you can see that we are able to send traffic with 1,472-byte
257. 11:52ping frames from our device all the way through
258. 11:55to Google's DNS, and then we do receive a response back from
259. 11:58Google saying that that information was received.
260. 12:01If we tried to do a packet that was larger than 1,472--
261. 12:05let's try, for example, 1,482--
262. 12:08we'll send that information, and it
263. 12:09says this information needs to be fragmented,
264. 12:12but you've set the Don't Fragment bit, which
265. 12:15means it's not going to be able to traverse
266. 12:17this particular network, and you're going
267. 12:19to need to use a lower MTU.
268. 12:22If this was going over VPN, we would simply
269. 12:24keep moving that number lower and lower
270. 12:27until finally it was able to send through the network,
271. 12:30and we would be able to receive replies to our pings.