WEBVTT 00:05.410 --> 00:14.500 So guys, let us discuss some observations regarding the TCP client server program that we have just 00:14.500 --> 00:15.550 discussed. 00:16.300 --> 00:23.620 So the TCP client server design that we just discussed, it had certain drawbacks. 00:23.710 --> 00:26.200 Let us discuss those drawbacks. 00:27.380 --> 00:34.280 So in this design we see that the server cannot accept other clients request while it is processing 00:34.280 --> 00:35.660 the current client. 00:36.110 --> 00:36.800 Right. 00:36.800 --> 00:38.480 So why it was happening? 00:38.480 --> 00:46.700 Because when the server had booted up, right as soon as the server boots up it creates a master socket 00:46.700 --> 00:54.290 file descriptor and after creating a master socket file descriptor, the server make a call to a select 00:54.290 --> 00:54.650 system. 00:54.650 --> 00:59.330 Call and select system call is a blocking system call. 00:59.780 --> 01:00.530 Right? 01:00.770 --> 01:07.550 So when the server boots up, it gets blocked immediately at a select system call after creating a master 01:07.550 --> 01:08.840 socket file descriptor. 01:09.560 --> 01:15.890 Now, at this point of time, if any client C sends a connection request to the server. 01:18.530 --> 01:26.510 Then the server gets unblocked from the select system call and immediately calls an accept system call. 01:27.110 --> 01:35.960 The Accept System call creates a communication file descriptor using which the server communicates with 01:35.960 --> 01:39.200 the client for the rest of the lifetime of the connection. 01:41.490 --> 01:49.110 So as soon as the communication file descriptor is created using accept system call, the server makes 01:49.110 --> 01:51.270 a call to the receive from system call. 01:51.270 --> 01:56.430 That is, the server waits for the data from the client. 01:57.090 --> 02:02.970 So the server gets blocked at the stage three. 02:04.170 --> 02:09.010 That is, the server is waiting for the data from the client. 02:09.030 --> 02:11.220 So let us call this client as C1. 02:11.910 --> 02:16.620 Now at this point of time, that is when the server is in stage three. 02:16.800 --> 02:22.710 Suppose there is another client, C2, which again sends a connection request to the server. 02:24.200 --> 02:32.510 Now because the server is blocked in the stage three The server do not entertain the connection request 02:32.510 --> 02:34.100 from the client see to. 02:35.820 --> 02:36.420 Right. 02:36.420 --> 02:43.470 So this was the drawback in our TCP client and server design that we had just discussed because the 02:43.470 --> 02:47.540 server is waiting for the data from the client. 02:47.560 --> 02:51.660 C1 That is, the server is blocked at the stage three. 02:51.690 --> 02:56.220 The server s cannot entertain any other client. 02:57.290 --> 02:58.010 Right. 02:58.160 --> 03:00.470 So because of this. 03:01.100 --> 03:08.570 This TCP server had a drawback that it can entertain only one client at a time. 03:09.880 --> 03:10.600 Right. 03:10.900 --> 03:15.160 So when the client C1 sends the data to the server. 03:15.520 --> 03:22.690 Now, note that it is not a connection request, but it is a data, so the server moves from the stage 03:22.690 --> 03:24.580 three to the stage four. 03:26.010 --> 03:35.280 Right and at the stage for the server can process the client data and reply back to the client even 03:36.090 --> 03:37.970 after replying back to the client. 03:38.130 --> 03:41.880 Even the server again moves back to the stage three. 03:43.140 --> 03:48.180 So you can see that the server is oscillating between the stage three and four. 03:49.960 --> 03:56.800 And while the server is oscillating between the stages three and four, the server is entertaining only 03:56.800 --> 03:58.030 the client C1. 03:58.840 --> 04:06.340 So the type of design that we had just discussed is a design in which the server can entertain only 04:06.340 --> 04:08.170 one client at a time. 04:09.180 --> 04:17.520 Right because the server can entertain another another client only when the server is blocked at the 04:17.520 --> 04:18.810 select system call. 04:19.260 --> 04:27.180 But in this design, once the server receives the connection request from the client C1 the server Unblocks 04:27.180 --> 04:34.500 from the select system call and get stuck between the stage three and four forever. 04:35.490 --> 04:36.150 Right. 04:36.150 --> 04:43.020 So this is the reason that in this design the server cannot entertain more than one client. 04:45.210 --> 04:45.690 Right. 04:45.690 --> 04:49.890 So you must be thinking then what is the use case of such a design? 04:50.400 --> 04:54.840 So it's not like that that this design is useless. 04:54.840 --> 05:03.960 This design is still useful in the scenario where there is only one client to be served by the server. 05:04.560 --> 05:12.810 So this kind of design is used where there is only one and only one dedicated client of a server. 05:12.840 --> 05:19.920 Such server processes are used in system softwares where there is only one client and one server. 05:21.090 --> 05:27.360 This server design cannot handle multiple clients connections that we have discussed. 05:27.390 --> 05:35.220 So remember, servers are just another process which accepts some data from another process. 05:35.220 --> 05:37.800 So nothing is special, right? 05:37.830 --> 05:44.100 Servers are just another processes which can accept the data from another processes which are called 05:44.200 --> 05:48.880 clients and can process that data and return back the result. 05:49.600 --> 05:57.130 So such a machine which accepts the data, process the data and return the result are usually called 05:57.430 --> 06:00.340 are usually termed as server machines. 06:01.060 --> 06:03.910 Server doesn't generate any data. 06:03.940 --> 06:08.080 That means server doesn't generate any data by itself. 06:08.110 --> 06:14.530 Server has the property that they just replies to the client after processing the client's data. 06:14.680 --> 06:21.620 Server by themselves never sends message to the client without client's initiation first. 06:21.640 --> 06:24.880 So this is the basic property of the server. 06:25.450 --> 06:30.970 So now we have discussed how to write a TCP server which can entertain only one client. 06:31.000 --> 06:37.540 Let us now discuss how to write a TCP client program. 06:38.080 --> 06:45.280 So writing a TCP client is fairly easy as compared to writing a TCP server, right? 06:45.400 --> 06:52.390 And after that we will going to discuss the design of a TCP server in which the server can actually 06:52.420 --> 06:55.420 handle multiple clients at the same time.