Original subtitles

TCP or Transmission Control Protocol is a transport layer protocol 2

residing at layer 4 in OSI model and is connection orientated 3

it once again allows high layer protocols access to the network layer or IP layer 4

but in this case providing reliability. 5

It is connection orientated before transmission 6

a session is establish between 2 devices. 7

TCP generally implements a full duplex mode of operation. 8

There are some exceptions but we won’t go into them here 9

In other words a TCP connection is a pair of virtual circuits 10

one in each direction which operate in full duplex mode. 11

The transmitter can receive data at the same time its transmitting. 12

Both host in a conversation can transmit and receive at the same time. 13

there is error checking in TCP 14

because there is a checksum in the datagram to verify that there’s no corruption. 15

TCP segments are also numbered in sequence 15

so that the destination can re-order segments and determine if data is missing. 16

There is also acknowledgement of receipt of data, so all data is acknowledge by the receiver. 17

the transmitter or sender can re-transmit the segment or terminate the connection 18

if it determines that the receiver is no longer involve in the conversation. 19

TCP implements data recovery features 20

in other words there can be re-transmission of lost data. 21

So if there’s no acknowledgement of a segment, the segment will be re-transmitted. 22

TCP segment are sent using IP packets. 23

The TCP header will follow the IP header supplying information specific to the TCP protocol. 24

As you can see here the TCP header has many more options than the UDP header. 25

So firstly you have a 16 bit source port number which identifies the sending port. 26

We have a 16 bit destination port, which identifies the receiving port. 27

There’s a 32 bit sequence number, if the SYN bit is set 28

then this is the initial sequence number. 29

The sequence number of the actual first data byte are then this sequence number plus 1. 30

If the SYN bit is not set then the sequence number, is the accumulated sequence number 31

of the first data byte of this packet for the current session. 32

It then has a 32 bit acknowledgement number. 33

If the ACK flag is set or bit is set 34

then the value of the acknowledgement number 35

is the next sequence number that the receiver is expecting to receive. 36

This field acknowledges receipt of all prior bytes. 37

The fist ACK or acknowledgement sent by each end 38

acknowledges the other ends initial sequence number but no data. 39

The header length or data offset specifies the size of the TCP header in 32 bit words. 40

The minimum size of the header is 5 words and the maximum is 15 words. 41

The minimum size of the header is 20 bytes 42

and the maximum size of the header is 60 bytes in IPv4 43

which allows for up to 40 bytes of options in the header. 44

The reserve fill the set to 0 and is reserve for future use. 45

Now there are lot of flags or control bits available in the TCP header 46

and we won’t go through all of them. 47

The Congestion Window Reduced Flag is part of a congestion notification mechanism 48

used in conjunction with the ECE bit or flag 49

or echo congestion notification echo field or flag. 50

Once again used in congestion notification. 51

This can be used in quality of service where the network 52

and the host communicate to indicate congestion 53

therefore letting the transmitter know that it needs to slowdown. 54

The URG flag can indicate that this segment is urgent 55

and should be process as soon as possible. 56

The ACK flag as mentioned is used for acknowledgement of data. 57

PSH is the flag set by TCP sender 58

to cause the TCP receiver to immediately pass 60

that segment's data to the receivers application socket 59

along with all other in order data that the receiver is yet to give to that application. 60

Reset; resets the connection in other words, the connection is turned down. 61

SYN is used to synchronize sequence numbers. 62

Only the first packet sent from each end will have this flag set. 63

FIN means that there is no more data from the sender. 64

The window size which is 16 bits in length 65

specifies the size of the received window 66

which is the number of bytes that the receiver is currently willing to receive 67

we'll be talking more about flow control and window sizes in a moment. 68

TCP also includes a 16 bits TCP checksum 69

which is used for error checking of the header and the data. 70

The 16 bit urgent pointer is used with the URG flag 71

which when set on means that the 16 bit urgent pointer is used. 72

This indicates an offset from the sequence number indicating the last urgent data byte. 73

There are also various options available in the TCP 74

but this are out of the scope of this course. 75

And lastly we have the data, which is the data from higher layer protocols 76

encapsulated within the TCP header. 77

There are some examples of applications that rely on either TCP or UDP. 78

Examples are file transfer protocols include; FTP or File Transfer Protocol 79

TFTP or Trivial File Transfer Protocol, NFS or Network File System. 80

In email we tend to use POP3 or Post Office Protocol to receive mail 81

Simple Mail Transfer Protocol or SMTP to send mail 82

or IMAP or Internet Message Access Protocol 83

which is another protocol used for email retrieval 86

For remote log in to devices, we could use telnet 84

which sends traffic in clear text 85

and is therefore in secure or secure SHELL or SSH 86

which allows for a secure connection to remote devices. 87

For network management we can use Simple Network Management Protocol or SNMP 88

and for name management we can use Domain Name System 89

which allows for the use of names rather than IP addresses 90

and translates those meaningful domain names, into IP addresses 91

so for instance cisco.com will be converted to an IP address 92

when a user browses the internet. 93

Before continuing I wanna mention again how the mappings work 94

between the different layers of the OSI model. 95

At layer 2 in an Ethernet 2 frame, there’s a field called the type number 99

which allows a host to differentiate between multiple layer 3 protocols 100

at layer 3, remember you could be using a protocol like IPv4 or IPv6 101

or in the old days IPX or apple talk. 96

So at layer 2 the NEC needs to know which layer 3 protocol to send this traffic to 97

and the type number is used to differentiate the different layer 3 protocols. 98

At layer 3 a protocol number is used to differentiate 105

the different protocols running at layer 4, so in an IP header 99

the protocol field will denote where the TCP or UDP is being used at layer 4 100

At layer 4 a port number is used to differentiate multiple applications being use at layer 7. 108

So it’s important to note that at layer 4 the way TCP or UDP 101

know which application this traffic is destined to is by the port number.