Question

Describe the various mechanisms for avoiding and controlling congestion employed by TCP, namely: slow start, congestion avoidance phase and reaction to congestion. Further, assume that a TCP connection at steady state with AIMD parameters alpha = 1 Byte and beta = 0.5 is currently sending 107B of data (i.e., congestion_window = 107B). Assuming no congestion drops occur in the next three rounds of transmission, what is the congestion-window in the next three rounds? If there is a congestion drop after the third round, what is the new congestion_window? (Hint: use the AIMD algorithms with given values of alpha and beta to arrive at these answers)

Answer 1

Various mechanism for avoiding and controlling congestion employed by TCP are as follows

Congestion policy in TCP –

1. Slow Start Phase: starts slowly increment is exponential to threshold
2. Congestion Avoidance Phase: After reaching the threshold increment is by 1
3. Congestion Detection Phase: Sender goes back to Slow start phase or Congestion avoidance phase.

Slow Start Phase : exponential increment – In this phase after every RTT the congestion window size increments exponentially.

Initially cwnd = 1
After 1 RTT, cwnd = 2^(1) = 2
2 RTT, cwnd = 2^(2) = 4
3 RTT, cwnd = 2^(3) = 8

Congestion Avoidance Phase : additive increment – This phase starts after the threshold value also denoted as ssthresh. The size of cwnd(congestion window) increases additive. After each RTT cwnd = cwnd + 1.

Initially cwnd = i
After 1 RTT, cwnd = i+1
2 RTT, cwnd = i+2
3 RTT, cwnd = i+3

Congestion Detection Phase : multiplicative decrement – If congestion occurs, the congestion window size is decreased. The only way a sender can guess that congestion has occurred is the need to retransmit a segment. Retransmission is needed to recover a missing packet which is assumed to have been dropped by a router due to congestion. Retransmission can occur in one of two cases: when the RTO timer times out or when three duplicate ACKs are received.

• Case 1 : Retransmission due to Timeout – In this case congestion possibility is high.

  (a) ssthresh is reduced to half of the current window size.
  (b) set cwnd = 1
  (c) start with slow start phase again.

• Case 2 : Retransmission due to 3 Acknowledgement Duplicates – In this case congestion possibility is less.

  (a) ssthresh value reduces to half of the current window size.
  (b) set cwnd= ssthresh
  (c) start with congestion avoidance phase

AIMD algorithm

W(t+1) =w(t) +a, if Congestion not detected

=w(t) *b, if congestion detected

If there is no congestion drop in first three round of transmission then window size increase linearly..

Window size after first round=107+1=108 bytes

Window size after second round=108+1=109 bytes

Window size after third round=109+1=110 bytes

If there is congestion drop after that.. Then the new window size will be 110*1/2= 55 bytes.