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Since I've mentioned Wireshark several times in this course already, I've decided to create a bonus

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lesson to show you what the

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communication between our server and our client looks like in Wireshark.

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I'm going to run the first program we made - simple_tcp, and there's only one change we need to make, and

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it's going to be on the client side. Instead of connecting to the loopback address,

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We're going to connect to the network address of this host. To see what that address is,

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I'm going to use comment prompt. I'm gonna say "ipconfig" - and here we have the address.

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It's 10.0.2.15. Let's type that in.

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Save the code and then open Wireshark.

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I'm going to choose this adapter, and then I'm going to go back to Eclipse, run the server, and then run

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the client.

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When we open Wireshark, we can see our results. And I'm going to stop the capture. In the first few packets

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we can see the typical TCP connection being established.

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So this is called the "three way handshake".

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We have a SYN packet, and then SYN acknowledgement, and then another acknowledgement.

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The first packet that is actually a part of our protocol is this packet, and we can recognize it because

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it has this push flag turned on. When we look at what's in the packet,

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we can see this "Welcome!" message that we can recognize.

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And then when we look at the length of that message, it says 10 bytes.

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Let me show you what that 10 bytes actually is.

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It's actually "Welcome!" and then new line - or it's like this.

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One - two - three - four - five - six - seven - eight - nine - ten.

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So that's the ten bytes we're transmitting in this particular packet.

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Now regarding the push flag, when you go to the TCP part of the packet, we're going to look at the flags.

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And it tells us right away which flags are on.

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So it's the acknowledgement flag and the push flag.

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All the other packets don't have the push flag, they are anyway just SYN, acknowledgement and FIN packets,

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which are a regular part of any TCP connection.

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Let's find our reply from the client.

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It's "Thanks!"

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And that's the entire communication.

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So the session was opened, we sent "Welcome!" and then we sent "Thanks!" back and then we closed the socket,

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which initiated the termination of the TCP connection.