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And in this section we gonna look at TCP/IP transport layer or layer 4 of the OSI model. 2

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We are specifically gonna concentrate on the TCP and UDP protocol that reside at this layer. 3

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I’d like to start up with the comparison of UDP and TCP 4

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and show you the differences between those 2 protocols 5

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we need to have a discussion of port numbers and explain how ports numbers are used. 6

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We look at the details of UDP and the details of TCP 6

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and we look at some of the mechanisms that TCP uses including the TCP 3 way handshake. 7

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We gonna look at windowing and we're gonna look at sequence numbers. 8

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So there's quite a bit to cover on these 2 protocols. 9

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Before discussing TCP and UDP I'd like to remind you 10

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that IP or internet protocol is connectionless. 11

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Every packet it’s treated individually and separately by routers or routers in the network. 12

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So this PC on the left hand side send is sending traffic to this server on the right hand side. 13

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Individual packets from the PC could take the upper path 14

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or they could traverse the lower link. 15

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The internet consist of many, many devices and multiple parts to destinations 16

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and it’s important to realize that traffic from one host to another host 17

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could take different paths even though that traffic 18

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is part of the same conversation or same session. 19

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So if the PC is communicating to the server across the internet 20

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traffic from that PC could use multiple parts 21

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depending on how the traffic is routed through the internet. 22

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That means the traffic could arrive out of order. 23

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So for example if the PC sends packet 1 24

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Packet 2 could arrive before packet 1 25

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and does the packet arrive at the server in this order 26

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packet 2, packet 1, and packet 3 27

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that server requires a mechanism to reorder the packets into the original sequence. 28

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IP also does not guarantee the delivery of packets 30

so there’s no guarantee that the packets actually arrived at the server 29

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or they arrived in the correct order or free from errors. 30

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Higher layer protocols need to ensure the reordering of packets 31

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and checking that the packets actually arrived successfully. 32

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The 2 protocols that we're gonna concentrate on this section, as mentioned are TCP and UDP. 33

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TCP and UDP reside at layer 4 the OSI model, in other words the transport layer. 34

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Just to recap at the physical layer we'll have Ethernet 37

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or another type of physical presentation. 35

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At layer 2 on Ethernet, we use MAC addresses 36

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on WAN interfaces, we would use another type of encapsulation such as frame relay or PPP 37

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At layer 3 were using IP and devices are represented by IP addresses 38

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and at layer 4 we have TCP and UDP. 39

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Here’s a quick analogy explaining the difference between 40

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UDP or User Datagram Protocol and TCP or Transmission Control Protocol. 41

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UDP like IP is connectionless, it does not guarantee the delivery of packets. 42

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It requires high layer protocols for instance 43

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at the application layer to ensure the successful delivery of packets 44

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as an analogy UDP services are similar to using regular mail offered by the Post Office 48

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you would write your letter and enclose it in an envelope 45

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by the same token, you would take your data and encapsulate it in UDP 46

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you would then post your letter and hopefully the postal service 47

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will deliver the letter to the recipient 48

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there’s is however no guarantee that the letter 49

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will be collected, transported, delivered 50

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and opened by the recipient as this is regular mail. 51

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You as the sender do not get an acknowledgement 52

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or proof of delivery of that letter. 57

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The postal service once again, does not guarantee delivery and is not responsible 53

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for letting you know that the delivery was successful or unsuccessful. 54

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but has the inherent disadvantage of unreliability. 55

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In a UDP environment, UDP at layer 4 does not guarantee delivery 62

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higher layer protocols at the application layer 56

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will need to ensure reliability and delivery acknowledgement if required. 57

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TCP on the other hand, does provide delivery acknowledgement and reliability 58

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but with the disadvantage of the additional overhead 59

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as an analogy, a TCP session could be seen as the telephone call 60

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TCP is connection orientated in the same way that a telephone call is. 61

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The person on the left makes a call to the person on the right. 62

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The person on the right is notified of the incoming connection 63

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or incoming call by the phone ringing. 71

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When the person on the right hand side answers the call 64

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they will acknowledge that by saying like 65

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"hello, it's David speaking" 66

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the person on the left hand side or the caller is does notify 67

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that the telephone call has been answered. 68

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The caller can then say 69

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"hello David, it's Peter speaking". 70

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So the called party or the person on the right hand side, knows you've made the call. 71

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So there sort of a three-way handshake that takes place here 72

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with the person on the right hand side initiates the call 73

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the person on the right hand side acknowledges the call. 74

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and the person on the left hand side then indicates who's speaking 75

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Once the three-way handshake has taking place, and please note once again 84

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that this is just an analogy and in a telephone environment 76

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there are many more steps that take place 77

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but once as an analogy that has taken place, there’s communication between the 2 parties. 78

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If the party on the left hand side, wanted to provide some information 79

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to the party on the right hand side, let’s say for instance a telephone number. 80

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The party on the left hand side would read out the number 81

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and in this example I’m just keeping it simple 82

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and imagine for the moment that this is very long and complicated number 83

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the party on the left hand side would say the number is 555 84

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and then the party on the right hand side would acknowledge receipt of that number 85

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by reading back the number received which is 555. 86

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The party on the left hand side, can check that the number received is the correct number 87

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and then can continue with the subsequent digits like 1234. 88

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The party on the right hand side would read those number back 89

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to ensure correct receipt of the number. 90

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Now in TCP environment the information is not echoed 91

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but there is the use of sequence numbers to ensure successful delivery 92

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or successful receive of information 93

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Before continuing I'd like to explain what a socket is 94

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as this term is often used in networking and I'm going to mention it in the upcoming slides 95

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A socket is the combination of the IP Address of the host 96

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in other words the location of a computer for example and the port number used. 97

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Port numbers are used to identify applications 98

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for instance port 80, identified http and the transport protocol used 99

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in other words TCP or UDP, these 3 are then combined into a single entity 100

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in the same way like a telephone connection is the combination of the phone number and a particular extension 101

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We'll be discussing TCP and UDP protocols in more detail in a moment 102

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We'll also be looking at various port numbers and how they are used to identify applications 103

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and in the previous section we've discussed IP addressing 104

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A socket is a combination of these 3 things 105

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used to identify connection between hosts 106

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TCP and UDP allow for session multiplexing 116

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which is when a single computer or host with the single IP address is able to communicate 117

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with multiple servers or multiple devices 107

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and have multiple sessions occurs simultaneously. 119

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The session is created when a source host needs to send data or information to a destination host. 108

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Replies are often received but aren't mandatory 121

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the session is created and controlled within the network application 109

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which contains the functionality of OSI layers 5 through 7. 123

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In a best effort environment the settings are very simple 110

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sessions parameters are sent to UDP, which as we know now as a best effort protocol. 111

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Information is just sent to a destination IP address and destination port number. 126

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It’s important to remember that each transmission is a totally separate event 112

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with no memory or association between various transmissions retain. 113

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When using a reliable service like TCP, as discussed a connection must first be established 114

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between the sender and receiver before any data can be transmitted. 115

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TCP will open a connection and negotiate various connection parameters 116

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which I'll discuss in more detail in a moment before actually transmitting any data. 117

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Sharing data flow, TCP will maintain reliable delivery of the data 133

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and will close the connection once complete. 118

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An example of this, is that on my machine, I can’t open up multiple connections 119

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to different servers at the same time. 120

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So I could open a connection to Gmail, I could open up connection to yahoo 121

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I could open up a connection to Google and so forth and so on. 122

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On this individual machine that has 1 IP address 123

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I’m connecting to multiple servers simultaneously, therefore using session multiplexing.