Question

computer network

(a) In the TCP mechanism for congestion control, generally the techniques falls into two categories; retransmission timer management and window management. Describe with suitable examples these two techniques. 6 marks)

Answer 1

Congestion control refers to some techniques that can either prevent congestion before it happens or remove congestion if it is already happens.

1) Retransmission timer management

TCP use retransmission timer to retransmit a lost segment.

Timer start when TCP send a segment and timer stop when TCP received acknowledgement.

If timer expired before receiving acknowledgement in this case segment retransmission occurred.

For example if sender has send 1st packet it start its timer, if acknowledgement receive before timer get expired then stop the timer and assume packet is received at receiver side without any packet lost

But if sender does not receive any acknowledgement and timer get expired then sender assume packet loss occure and retransmission of packet occurred.

2) window management

TCP use congestion window that avoid congestion.

If network can not delivered data as fast created by sender then it tells the sender to slow down.

Congestion policy :

1) slow start

In slow start phase after every round trip time

congestion window size increased exponentially.

2) congestion avoidance

After reaching threshold it increase additive.

3) congestion detection

If congestion occurred window size decreased.

Sender can guess that congestion occurred if retransmission of segment is needed.

Retransmission occure in two cases if timer expires and if 3 duplicates acknowledgement are received.