All language subtitles for 5. TCP 3 Way Handshake

When using TCP, devices must first establish a connection with a peer system
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before data transmission can take place
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so connection orientated session will be established between host A and host B
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One machine will initiate the connection which must then the accepted by the other machine.
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Protocols software modules in the operating systems of the host devices
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communicate with each other by sending messages across the network
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to verify that the transfer is authorized
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and that both sides are ready for the transmission of data.
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For this to take place a three-way handshake occurs between the host devices using TCP.
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So host initiating the session will set the SYN flag or SYN bit
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in the TCP header of the initial segment send to host B.
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Host A will also choose an initial sequence number
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which in this example let's say 100.
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So the control flag SYN is set on
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and the sequence number is set to an initial value of 100.
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This is then used to start the handshake process.
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This synchronization segment also specifies the number of the port
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to which the sender wants to connect for instance port 80 or HTTP.
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The host on the right hand side will be waiting for a connection request
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from the remote client, in this case host A.
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when the SYN is received and accepted
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host b will send back a TCP segment with both the SYN and ACK flags set.
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So the control flags SYN and ACK are set on
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and use to negotiate the connection
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and acknowledge receipt of the initials synchronization segment of the sender.
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B also set an initial sequence number
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to indicate the next sequence number of the next byte of data
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it expects to receive from host A.
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host B also sets the acknowledgement flag to, in this case 101
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an ACK flag indicates, the next portion of data, the host expects to receive.
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So host A initially sent a sequence number of a 100
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and host B, in this case were assuming a window size of 1
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sends back an acknowledgement of a 101.
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The third step in the three-way handshake is where the initiating host
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in this case host A has received the SYN from host B and sends back a TCP segment
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with the control field set to ACK, in other words acknowledgement.
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Host A is therefore acknowledging the next segment it expect to receive from host B
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in this case 301, initially host B sent a sequence number of 300.
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So host A is expecting the next segment 301.
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Host A sets it's sequence number to 101.
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The initial segment sent was 100 and the next one in this case is 101
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because the SYN bit or SYN flag is unset
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this confirms that three-way handshake has completed successfully.
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Just to reiterate, the control bits or flags initially A
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sends a segment with the SYN bit or SYN flag set on.
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So the control flag SYN is set to 1.
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Host B in the second step of the three-way handshake
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sets its control flags or bits to SYN ACK.
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In other words the SYN bit is set to 1
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and the ACK bit is set to 1.
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In the last step of the three-way handshake
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Host A sets the ACK bit to 1 or the ACK flag is set on.
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The SYN bit or SYN flag is set to 0 to indicate
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that the three-way handshake has completed successfully.
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Now sequence numbers and acknowledgements can cause a lot confusion
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So I'm going to explain them in more detail now.
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We're assuming in this example, that a window size of 1 is used.
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Now if you remember the window size is the maximum amount of data
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that the receiver can receive from a sender and process correctly.
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So we're going to assume that only 1 segment can be transmitted
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at any given time before an acknowledgement is received
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to acknowledge receipt of that segment.
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So let’s assume A starts with an initial sequence number of 5
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because of a window size of 1, only 1 segment can be sent from A to B.
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B successfully receives the segment from A
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and acknowledges the next segment that it wants to receive.
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so rather than acknowledging receipt of sequence number of 5
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it acknowledges for sequence number 6
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which implies that all previous sequence numbers will receive correctly.
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So B acknowledges in this case for sequence number 6
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but B may start with an initial sequence number of 10.
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so in the TCP header B tells A that its initial sequence number is 10
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and that its successfully received sequence number 5 from A
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and is expecting sequence number 6 from A in the next packet.
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Let’s assume successfully receives that segment
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so A will now send segment 6 to B
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in other words the next sequence number.
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A also acknowledges receipt of segment 10 from host B.
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So A has successfully received the segment with sequence number 10.
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Please note once again that the host
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can randomly choose the initial sequence numbers
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and thus in the initial three-way handshake
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that information needs to be communicated between the 2 hosts
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so that they know what the initial sequence numbers are.
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So once again A sends sequence number 6 to B and acknowledges sequence 11.
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Let’s assume B successfully receives that segment
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and therefore will acknowledge for segment 7.
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The next segment it expects to receive
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once again 7 in the acknowledgement indicates that previous segment was successfully received.
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So host B by setting the sequence number to 7 is telling host A
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that sequence number 6 was successfully received.
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Host B sends sequence number 11
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because that’s the next sequence number that A expects to receive.
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The thing to note once again, is that initial sequence numbers
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that the host expects to receive and the sequence of number of for instance 11
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implies that sequence number 10 and previous sequence number was successfully received.
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Now once again flow control prevents an issue
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where the sender is sending so much data
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that the buffers of the receiver are overflowed.
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If this is a very powerful machine and this is an older machine
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that is not as powerful, it’s possible that A can overrun
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the buffers of B because it’s sending so much data.
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So B needs a mechanism to tell A to slowdown
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so that D can successfully process the traffic that it’s receiving.
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So as an example let’s assume
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that the window size in this example is 3 rather than 1.
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So A can send 3 segments of data before getting an acknowledgement.
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The advantage of increasing the window size is that throughput
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can increase dramatically because a host can send more data
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with fewer acknowledgements and therefore the round trip timers decrease dramatically.
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So in this example A sends 3 segments to B.
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Let’s assume that the received buffer of B is full
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and it can't handle that amount of data
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B will send a not ready indicator to A
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and it does this by setting the windows size to 0.
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This tells the sender to stop sending data
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and wait for ready indicator from the receiver.
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Assuming that host B has now been able to process the data
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in it’s receive buffer and can now receive more data.
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It can send a ready indicator to A, to tell it to resume sending of datagrams.
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so A resumes the transmission by sending for instance 3 segments to B.
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Because the window size is 3.
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So please note in the background the TCP host involve in the conversation
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can negotiate various parameters and one of them is flow control
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where a receiver, can tell a sender to slowdown or to stop sending data
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until the receiver has buffer space available to receive transmitted segments.
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This is allows for communication between a very powerful
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or fast machine and a slower or less powerful machine
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where they can negotiate the rate of transmission.