Okay. So now we are talking about the last thing, which is, as I mentioned to you guys, TCP is, TCP is the? Which layer? Transport layer and what are the most important under TCP? We have reliability, that means we keep track of the data, right? To keep track of data, we need to, we call it as establish a connection between sender and the receiver. That's very important so that we can keep track of the data that we send and then whether the receiver received the data we are sending. Now, keep track of the data. Okay, let's talk about one last story, then when we talk about the one last story, we are done the session today. Alright. So remember my last thing happens? I sent a letter to my girlfriend. What happened? My wife changed it, right? And then she left me, life was very boring. Nothing exciting. I come to my office, work hard, understand, go back to home. Nothing exciting, it's very, very boring. So you know what I have done? I went out to, I went out one Sunday, I just went off Central, I just walking around, then I saw very beautiful girl. So as soon as I see her, what I feel like to? I feel like, when you see a beautiful girl, how do you feel? I just go, "Oh no! Beautiful girl. Close eyes, no, no, I don't want to look at it," Are you going to do like that? Of course not, I want to what? To talk to her. Talk to her, that means to establish a? Connection. Connection, yes. I want have a, establish a connection with her. So I want to establish a connection, I saw her and I come back. Does it work? No, so what I'm going do? I go and introduce myself, does it work? May not be the best way to do it. So she was shopping and I was just following her, she entered to a shop, I was trying to pretend that I also want to buy, she went to a shop that had ladies' clothes. I was telling the shopkeeper, I'm not saying that I'm buying to my wife or somebody, I'm buying to my sister. I'm looking for the buying some clothes to my sister and then she was looking at the aisle one side, I was looking, she was looking at this side, so we were like looking at the clothes. So when we were looking at the clothes, you know what I did? Then of course we happened to see each other. Then I come home, we saw each other, we come home, can I establish a connection? No. So what I should do? Her address, phone number, I'm a stranger, will she give it to me directly? No. No, this is not a good strategy. So I have better strategies than that. So you know what I do? "You looks very familiar to me, are you from UST? Are you from UST?" Of course she is not from UST, right? So I ask, "You looks very familiar, are you from UST?" She just, "Yes, I'm not, sorry, no, I'm not from UST." Okay, then we start talking. Then I come home, does it work? No, so what I need to do? Ask her phone number? She may not give it, I take out my wallet, I take out my name card, Professor HKUST sounds good? Sounds good? Yes, she took it, sounds good and then what happened? Once I gave her my name card to her, what she is going to do? She gave her name card back to me, can I come home now? Yes, can we establish a connection? Yes. That is, that is how I established a connection. This is Percy, this is Percy. This is girlfriend, so what we do is I exchange our name cards so we call we exchange our initial sequence numbers. So initial, why is it initial? Is it the only thing we are going to exchange? No. This is the very beginning, right? Don't ask me what I exchanged after that, we exchanged many things, right? Correct? This is the first beginning of the exchange. So initial sequence number, that's what we exchange. So intial sequence number could be a large random number. My phone number and her phone number, they are not same, right? Correct. So this is my initial sequence number is 10 and her initial sequence number is 20, is it possible? Yes. So I'm sending my sequence number, sequence number, 10, to her, right? Understand? And then next thing is that, I want to make sure that, that assume that we did online. So I want to make sure that she'll receive it. How can I make sure? I sent sequence number 10 to her. She acknowledgement, right? So she is going to acknowledge, acknowledge 10, right? Correct? What is next? I sent my sequence number, then what is next? She going to send her sequence number to me which is sequence number 20, right? And then acknowledgment 20, alright. So we have how many messages? Four messages. Can be combine some of them? Which one we can combine together? Can we combine some of them? Which one? Is that right? She send the sequence, acknowledgment and then her sequence number, right? Correct? So that means we call it as, we call it as this Three-Way Handshake. We call it as Three-Way Handshake. We exchange three messages, correct? Now, acknowledgment. I send, I tell her my sequence number is 10. She says thank you very much for 10, correct? But our acknowledgment is different. Yeah. Okay, let's change the subject. I send a beautiful flowers to her, if she likes it, how she's going to acknowledge? She always say thank you, right? Is it something indicating that she likes it very much? Not necessarily, right? So if she likes it very much, what would her acknowledgment would be? Sorry? Send hearts? Yes, but still not sure. Okay, we all adults. I give her a kiss, if she likes it, what would she tell me? Yes, have you ever given a kiss to a girl? I'm not going to ask you, sorry. So when you do a kiss, you do a kiss, if she likes it very much, what's her acknowledgment? Is she going to say thank you? Your techniques are very bad, right? So if you, if you give her a kiss and if she says thank you, the techniques are bad, so then come to me, I can coach you guys, okay? Look, what would be her acknowledgment if you really likes it? Kiss you back. Kiss you back, always she does it. If she likes it very much, you know what? When I give a kiss to my girlfriend, you know what she does? Percy, can I have one more? Does it mean that she really likes it, yes or not? Yes or no? Yes, she really likes it and she wants one more. So this acknowledgment, we call it, she's expecting more, correct? She's looking forward for more, that's what we call expectational acknowledgements and we call it as looking forward for more. She call it as forward acknowledgment, understand? Give me one more, correct? So now the acknowledgment is for her is what? She's not saying "thank you very much for 10." She's telling me "give me one more", correct? So she acknowledged 10 plus 1, one more. Is that right? Understand? What is my acknowledgment? Of course, when she gives me a kiss, I like it very much, I ask her, one more, understand? So this is what we call connection establishment. Okay, that is what we call connection establishment and this is what we have here. I think that you can understand. The connection establishment between A and B computers. How many messages altogether? Three, correct. So what is the initial sequence number of A? X. X is sent to the receiver, correct? What is the receiver going to do? Thank you very much for x? No. What is this? I need one more that means acknowledgment, X plus one, correct? And then at the same time, send a sequence number Y and then acknowledgment Y plus one. So this is what we call the connection establishment of TCP. After connection establishment, we can keep track, correct? So that means we always keep waiting for the acknowledgment. And then during the connection establishment, we don't exchange anything, then after connections have been established, we start sending and receiving data then after the data collection is finished, we are going to terminate the session. So that's how TCP works.
