1. Congestion Avoidance and Control CSCI 780, Fall 2005

2. Contributions Packet conservation principle Self-clocking How to reach equilibrium Slow-start (ssthresh) How to set RTO (retransmission timer) Consider variance estimation Exponential back-off Adjust window size on congestion (AIMD) Additive increase if no congestion Multiplicative decrease if congested

3. Conservation principle A new should not put into the network until an old packet leaves Transmit a new data packet after receiving an ACK Pre-condition: the connection is “ in equilibrium ” How to reach equilibrium (slow-start) How to maintain at equilibrium (congestion- avoidance)

4. Slow-start NOT slow, the window size exponentially increases per RTT cwnd + 1 per ACK cwnd doubles per RTT Quickly reach the equilibrium (fill the empty pipe) Slow-start threshold (ssthresh) value is crucial

5. Exponential growth of window size

6. Retransmission timer Detection of packet loss, implying congestion occurs It is not easy to set RTO properly Old RTO ignore the RTT variance New RTO setup considers RTT variance estimation Exponential back-off (double RTO) for each retranmission

7. Congestion-avoidance Slow-start — > Congestion-avoidance cwnd reaches ssthresh Window size linearly increases cwnd + 1 per RTT Congestion is detected via RTO or fast retransmit Multiplicatively reduce window size Switch back to slow-start

8. Karn ’ s algorithm Purpose: avoid retransmission ambiguity problem Cannot update RTT estimation when retransmission occurs Reuse the backed off RTO, update RTO until an ACK received for an un-retransmitted packet