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7 layer OSI Model

The Transport Layer

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I know this is a bit complex to understand but I hope you got the picture. In a nutshell every time a congestion is detected, meaning an ack is not received for the window sent, the window size is reduced to half and resent, this process is called slow start, and if there is a successful transmission during the slow start phase then the system would go into the fast start state when it starts multiplying the window size by 2 until it reaches the original window size. Dividing the window size by 2 when congestion is detected makes logical sense because when there is a congestion you don't want to make the situation worse by sending more data. Wait till the congestion ceases and start sending again at full capacity.

Now arises another logical question; what if the ACK packet is lost during transmission? In other words let's say the sender sends 8000 bytes and the receiver receives these 8000 bytes and sends an ACK for this window to the sender but along the path the ACK is dropped so that the sender doesn't receive it.

So the sender would wait till the retransmission timer expires and then it would transmit 4000 bytes (slow start) of the initial 8000 bytes. So the receiver would now receive duplicate packets instead of receiving the new window. This has also been taken care of. The receiver keeps tab of all the sequence number which have already been received during the previous window transmission and it would compare the sequence numbers received with the sequence numbers received previously, if it finds that there has been a duplication then it would send an ACK for the received packets but would drop all the packets (4000 bytes) because it has no need for them as they are duplicates, makes logical sense. Now the receiver gets the ACK and proceeds to send a window of 8000 bytes again (fast start).

I hope we are clear about how the congestion control mechanism works for TCP. Now lets move on to the concept of session termination.

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