Network Layer Message Delivery Exercise — Transcript

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1. 0:02if
2. 0:03machine a at ip address 6.6.6.2
3. 0:08wanted to communicate with machine c
4. 0:11at ib address 8.8.8.4
5. 0:14over the ipv4 protocol the following
6. 0:17process would occur
7. 0:19first machine a would create a new ip
8. 0:24packet with the fields
9. 0:28source address
10. 0:32set to be 6.6.6.2
11. 0:36and destination address
12. 0:428.8.
13. 0:48i would also have a ttl
14. 0:52of 64. and many other fields but
15. 0:56those are less interesting to us
16. 1:02okay so it would send this off to its
17. 1:05default gateway
18. 1:06and since there's only one router on
19. 1:08this network the default gateway will be
20. 1:10this endpoint over here
21. 1:14and we'll assume that all these devices
22. 1:16are connected with
23. 1:18ethernet okay so it's going to
24. 1:22wrap this segment
25. 1:26up with a ethernet header
26. 1:30where the source mac address is going to
27. 1:33be
28. 1:33addressed with all a's so i'll just
29. 1:36shorten that to
30. 1:38a a and i'll put two colons up here
31. 1:42for the rest the destination address
32. 1:46will be our first router okay and that's
33. 1:49because
34. 1:49again the ethernet only provides a
35. 1:52single hop
36. 1:54and we have to chain these together to
37. 1:56get to our final destination
38. 1:57so destination in this case is going to
39. 1:59be the mac address
40. 2:06okay so just abbreviated as 1 1.
41. 2:10okay once it reaches the router
42. 2:14the ethernet header is torn off we don't
43. 2:17need this value anymore
44. 2:19the ttl is decremented by one
45. 2:22to be 63. if this ever hits zero we
46. 2:25throw it out
47. 2:27because there's likely some type of loop
48. 2:29okay
49. 2:31so but these this source and destination
50. 2:34address are preserved so we don't
51. 2:36really change these values okay because
52. 2:38they sort of help us
53. 2:39guide us to our final destination in
54. 2:41case that there was
55. 2:43um in a more typical example it wouldn't
56. 2:46be sort of three networks it would be
57. 2:47maybe
58. 2:4810 networks or 20 networks in between we
59. 2:50don't have any control of how many
60. 2:52networks are
61. 2:52our intermediaries okay
62. 2:56so the source and destination are sort
63. 2:57of guiding us to
64. 2:59um to our final destination and
65. 3:02we'll talk about routing protocols in
66. 3:03the next lecture but again for
67. 3:05for for now just assume that we have
68. 3:07some oracle device that this router
69. 3:10knows that if i want to go to
70. 3:13if i'm trying to send the message to
71. 3:158.8.8.4
72. 3:17i know that i have to go through here
73. 3:19and that going through
74. 3:21here is is not the right answer okay
75. 3:24so we'll just assume that for now and in
76. 3:26the next lecture we'll we'll go into the
77. 3:28details of how it decides
78. 3:30between two paths okay so i'm just gonna
79. 3:33quickly rewrite this 6.6.6.2
80. 3:37and 8.8.8.4
81. 3:40okay so that's our source and
82. 3:42destination
83. 3:43ip address okay so this router
84. 3:47it has two mac addresses one for each
85. 3:49network okay
86. 3:52it's going to add a new
87. 3:55data link layer header okay so it's
88. 3:57source mac address
89. 3:59is now two
90. 4:00[Music]
91. 4:03the address of all twos and the
92. 4:04destination mac address is the one with
93. 4:06all fours okay and the ttl
94. 4:11for this one is 63.
95. 4:15okay so that's how we get from um
96. 4:19this router to the router on the
97. 4:22correct network okay so at this point
98. 4:24we're actually
99. 4:25not finished yet okay we're going to go
100. 4:28with the worst case scenario in the best
101. 4:30case scenario that this router
102. 4:32inside of its arp table it already has
103. 4:36machine c so it knows about machine c
104. 4:39but it's also possible that
105. 4:41machine c when it connected it got its
106. 4:45ip address allocated by one of these
107. 4:47other two routers okay so it did dhcp
108. 4:50and because it was so close to
109. 4:53this router okay
110. 4:56this router gave machine c a i p address
111. 5:00and not this one okay
112. 5:03um the consequence of that is that this
113. 5:06router
114. 5:06may not know the ip address
115. 5:10of machine c okay so assume that
116. 5:13this router does not have inside of its
117. 5:15arp table
118. 5:16it does not have an entry for 8.8.8.4
119. 5:20okay but it knows that it's part of this
120. 5:22network because it fits the
121. 5:24decider address okay so what it's going
122. 5:26to do
123. 5:27is it's going to broadcast an arp
124. 5:29request
125. 5:30okay so a arp request is a broadcast
126. 5:33message that goes
127. 5:34out to every device on the network
128. 5:35saying something like
129. 5:37hey i'm looking for the address 8.8.8.4
130. 5:43okay so we have other machines on these
131. 5:45networks because they don't own netip
132. 5:47address they'll just ignore that request
133. 5:49but c because it knows it owns this ip
134. 5:51address it will
135. 5:53respond back to our router okay and then
136. 5:56our
137. 5:57router will update its arp table with
138. 6:00the correct address
139. 6:01and then finally it will make the the
140. 6:03final
141. 6:05the the final message which is going to
142. 6:08be
143. 6:09let's clear this up a bit
144. 6:14okay so
145. 6:17again this stays the same
146. 6:24then ttl decreases again to 62
147. 6:28and this time wrapper from our source
148. 6:32mac address is
149. 6:36by the address of all fives
150. 6:40and our destination is
151. 6:43the address of all c's okay and that's
152. 6:45how
153. 6:47our machine a can deliver a message
154. 6:50to machine c by chaining together
155. 6:55multiple data link layer hops