WEBVTT 00:05.460 --> 00:06.860 Welcome back, guys. 00:06.870 --> 00:11.400 So next, we will going to discuss a new design of a TCP server. 00:11.880 --> 00:16.500 And that design is called a TCP server with multiplexing. 00:16.770 --> 00:20.880 So such a TCP server is called multiplexed TCP server. 00:21.270 --> 00:28.560 So in the previous TCP server design, we discussed the limitation of the TCP server that a TCP server 00:28.590 --> 00:35.880 cannot entertain any other client as long as it is not finished with the current client it is communicating 00:35.880 --> 00:36.540 with. 00:36.720 --> 00:37.410 Right. 00:37.410 --> 00:42.360 But servers are not supposed to entertain only one client at a time. 00:42.360 --> 00:49.380 The usual property of the server is that that it should be able to entertain multiple clients at the 00:49.380 --> 00:50.400 same time. 00:51.810 --> 00:59.910 So therefore it is important to understand the implementation of TCP server with multiplexing that is, 00:59.910 --> 01:07.240 servers which could entertain and which could carry out communication with multiple clients at the same 01:07.240 --> 01:07.870 time. 01:08.200 --> 01:13.300 So you can see there are ten steps to implement the Dhcp server with multiplexing. 01:13.300 --> 01:20.560 If we compare these ten steps with the ten steps of the TCP server that we implemented and discussed 01:20.560 --> 01:28.570 previously, you can find that these ten steps are exactly same as those ten steps that we discussed, 01:28.870 --> 01:29.650 right? 01:30.940 --> 01:38.830 So we will see in what respect the multiplexed Dhcp server implementation is different from the previous 01:39.160 --> 01:41.320 TCP server implementation. 01:42.400 --> 01:48.760 So let me discuss the high level idea regarding implementing a TCP server with multiplexing. 01:48.760 --> 01:53.170 So you have a server, say S, and suppose you have a client. 01:53.170 --> 02:01.030 C1 Initially when the server boots up, it maintains only one file descriptor and that file descriptor 02:01.030 --> 02:03.220 is called master socket file descriptor. 02:03.220 --> 02:03.760 Right. 02:03.760 --> 02:07.240 We have discussed this master socket file descriptor enough now. 02:07.240 --> 02:12.640 So master socket file descriptor is used to detect new client connection requests. 02:12.670 --> 02:13.270 Right? 02:13.270 --> 02:19.750 So our TCP server is actually monitoring only the master socket file descriptor in the select system 02:19.750 --> 02:20.230 call. 02:20.260 --> 02:27.040 Now suppose the client C1 sends the connection initiation request to the server s, then the server 02:27.040 --> 02:34.000 s creates a new communication file descriptor using which it can carry out communication with the client. 02:34.030 --> 02:40.480 C1 So at this point of time the server s is maintaining two file descriptors the master socket file 02:40.480 --> 02:45.910 descriptor and the communication file descriptor of client C1 Right. 02:45.910 --> 02:54.880 Now suppose at some point of time that is later client C2 comes and it sends connection initiation request 02:54.880 --> 03:04.540 to the server s so server s will create a communication file descriptor for the client C2 and it will 03:04.540 --> 03:06.400 carry out communication with the client. 03:06.400 --> 03:09.190 C2 using that communication file descriptor. 03:09.940 --> 03:16.660 So you can see that every time a new client gets connected to the server, the server creates a communication 03:16.660 --> 03:22.540 file descriptor and keeps an array of those communication file descriptors. 03:24.010 --> 03:24.580 Right. 03:24.580 --> 03:31.900 So our server is able to carry out communication with multiple connected clients at the same time because 03:31.930 --> 03:37.690 our server is keeping track of the communication file descriptor of all the connected clients. 03:38.320 --> 03:43.510 So this data structure could be as simple as an array, right? 03:43.510 --> 03:49.600 So every time a new client gets connected to our TCP server, our server will create the communication 03:49.750 --> 03:57.100 file descriptor of that particular client and add that communication file descriptor in the array. 03:57.800 --> 03:58.520 Right. 04:00.900 --> 04:09.360 And every time a select system call is called by the TCP server, it will copy this entire array into 04:09.360 --> 04:11.160 read set. 04:13.370 --> 04:14.870 It will copy. 04:14.870 --> 04:20.180 And remember, the select system call is used to monitor all the file descriptors which are present 04:20.180 --> 04:21.590 in read set. 04:22.040 --> 04:22.700 Right. 04:22.700 --> 04:29.480 So using select system call or TCP server is actually monitoring all the file descriptors of all the 04:29.480 --> 04:30.800 connected clients. 04:32.700 --> 04:33.150 Right. 04:33.150 --> 04:39.330 And this is how our TCP server will be able to do multiplexing with multiple clients. 04:41.330 --> 04:48.410 So I am just describing you a high level design that how TCP server with multiplexing capability can 04:48.410 --> 04:50.360 be implemented in C. 04:52.240 --> 04:58.930 When we will have a code walk, you will have a clear idea that how this particular idea is implemented 04:58.930 --> 05:01.210 in network programming. 05:05.650 --> 05:12.040 Also note that suppose the client c to terminate its connection with the server s then what will server 05:12.070 --> 05:12.880 s will do? 05:12.910 --> 05:18.700 It will simply delete the communication file descriptor for the client C2 from the array. 05:19.900 --> 05:21.400 It will delete. 05:22.000 --> 05:22.690 Right. 05:22.690 --> 05:29.530 So in the read set, when this array will be copied in the read set, only the communication file descriptor 05:29.530 --> 05:31.750 of connected clients will be present. 05:32.110 --> 05:32.800 Right. 05:32.800 --> 05:38.560 So it means that the server s will not be communicating with the client C2 anymore. 05:39.250 --> 05:47.920 So let us discuss the code walk of TCP server with multiplexing and see how it differs from the previous 05:47.920 --> 05:50.230 TCP server that we discussed. 05:52.010 --> 05:59.120 So by now you have a fair idea that how TCP server with multiplexing capabilities actually works.