1 Computer Networks Congestion Avoidance

2 Recall TCP Sliding Window Operation

3 Important Terms Maximum Segment Size (MSS) Window Retransmission Timer Delayed ACK Duplicate ACK

4 Congestion Window Sender based flow control Sender uses min(cwnd,advertised window) Tranmission window Detect network condition and adjust Use timers, ACKs and network feedback

5 Retransmitting Segments TCP starts timer after sending a segment If ACK returns, reset timer If no ACK, retransmit and increase timer (2x) after resetting

6 Round Trip Time (RTT) TCP measures RTT and uses RTT to calculate timer values If ACKs returning quickly, timer values are kept low Retransmit quickly when throughput is high Allow time for ACK to return when throughput is low

7 Slow Start Initialize cwnd to 1 MSS Increase cwnd by 1 MSS for every ACK Not very slow!

8 Congestion Avoidance When packet loss occurs, TCP must slow its flow of traffic When retranmission timer expires, save value of transmission window * ½ (ssthresh) If congestion occurred due to a timeout, set cwnd to 1 segment When data is ACK'd, increase cwnd via slow start up to ssthresh Thereafter increase cwnd by 1/cwnd for each ACK

9 Congestion Avoidance Illustrated

10 Fast Retransmit If > 2 duplicate ACKs are received, a segment was probably lost Sender assumes enough time has elapsed for out of order segments to have arrived and assembled Retransmit segment without waiting for a retransmission timer to expire Enter Fast Recovery

11 Fast Recovery Sender has to assume data is still flowing due to the reception of duplicate ACKs Loss was probably a rare or temporary event Do not go into slow start, just go into congestion avoidance mode

12 Other mechanisms Selective Acknowledgements (SACK) TCP Pacing Packeteer RED ECN Network pricing