A Half Century of Internet: How it works today

1. 00:00In our OpenHPI course, a half-century of internet, now we discuss
2. 00:05about the Client Server Principle. The client server principle is
3. 00:11the model for the interaction in the internet,
4. 00:17the interaction between the technical systems.
5. 00:21So common computer application works as follows, all the
6. 00:27computation is done on the same computer.
7. 00:31The inputs are sent by a keyboard or mouse to the CPU, and they are processed,
8. 00:39the results are shown on the desktop on the screen. So we have one system
9. 00:46here is the data processing and in the desktop, there is a presentation.
10. 00:53When we look at the internet, we have a different situation
11. 00:56we have distributed applications,
12. 00:59so the processing of the application is eventually
13. 01:05done on a different computer system, then the display
14. 01:13systems which contribute
15. 01:18to the processing to the input data to display, they all communicate
16. 01:25via the internet. So here we have an observer and we can connect
17. 01:32with different devices with smartphones, with a tablet,
18. 01:36with a computer and what's run and computed here
19. 01:41in the server and in the application behind the server that
20. 01:45can be displayed on different things. The action can be
21. 01:53put into the reality of when we speak about the internet of things in
22. 01:59in a distributed, way so the computation and the action or the display
23. 02:05they are distributed.
24. 02:09The question is how this interaction is organised,
25. 02:13of course, we need communication for such distributed applications
26. 02:21and to make this complex communication task as simple as possible.
27. 02:29There are the ideas modelling of such an interaction by the client-server paradigm,
28. 02:37so what is done on the internet, the communication of the distributed application
29. 02:45is organized, is implemented in a so-called
30. 02:51client-server paradigm.
31. 02:55So we have in this communication process, two parties, there is a client
32. 03:01and the other party is the server.
33. 03:05So the client is typically the active component,
34. 03:12the client sends a request to an application that's running on
35. 03:18another computer, so as the client is requesting
36. 03:23something from the other system and the server which
37. 03:29is the passive component is waiting for such kind of requests.
38. 03:34And then the server checks whether the requested resources are allowed
39. 03:40to transmit to the client and then if yes then the server sends in reply.
40. 03:48So complicated communication is
41. 03:53implemented in a very simple way, so one
42. 03:57requests something, asks system something and the system answers.
43. 04:03Then the next request can be sent, the next answer can come,
44. 04:08it's a different system, for example, says client system
45. 04:13can become a server system in communication with another
46. 04:19computer. So the client and server is not a characteristic
47. 04:25of a special computer.
48. 04:28It is a characteristic in one communication cycle which role is played
49. 04:35by the system.
50. 04:39This is a system which requests something
51. 04:42and this is a system which relies on. So the requesting party is a client
52. 04:48and the replying body is server.
53. 04:52So this client-server interaction, it's implemented in a way that
54. 04:59on each side, there is a so-called service access point as it's
55. 05:04needed for the communication. So such a service access point
56. 05:08is on the client-side, as well on the server-side.
57. 05:12We already spoke about this, the service access points
58. 05:17are called Sockets. So we have the client, we have the server,
59. 05:22they are connected over the internet. The client for a specific
60. 05:27communication with the server that means for using a special application
61. 05:34the server as there are sockets in place and these two sockets
62. 05:40are the communicating parties.
63. 05:43And remember as the sockets consist of the IP addresses and a port number,
64. 05:50and the IP addresses it characterizes the system.
65. 05:57it means the address of a
66. 05:58system on the internet and the port number that is the endpoint
67. 06:06for a certain application. So here with the transport layer with the port
68. 06:13an endpoint for the communication is provided with an input
69. 06:18buffer, with some other resources so that the communication can be organized
70. 06:23in such a client-server principle. So interaction is complicated
71. 06:29to implement this complicated communication infrastructure
72. 06:34one looks to such communication in a very
73. 06:38uniform way, one says in a specific a time of a communication
74. 06:45one system has the role of a client of
75. 06:50request something and the other is the role of
76. 06:54the server, it is able to answer such a request this is a client-server principle.
77. 06:59That is the basis for organizing the communication for
78. 07:05almost all applications in the internet.

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