1. Autumn 2000John Kristoff1 Computer Networks Congestion Avoidance

2. Autumn 2000John Kristoff2 Recall TCP Sliding Window Operation

3. Autumn 2000John Kristoff3 Important Terms ÑMaximum Segment Size (MSS) ÑWindow ÑRetransmission ÑTimer ÑDelayed ACK ÑDuplicate ACK

4. Autumn 2000John Kristoff4 Congestion Window ÑSender based flow control ÑSender uses min(cwnd,advertised window) ÑTranmission window ÑDetect network condition and adjust ÑUse timers, ACKs and network feedback

5. Autumn 2000John Kristoff5 Retransmitting Segments ÑTCP starts timer after sending a segment ÑIf ACK returns, reset timer ÑIf no ACK, retransmit and increase timer (2x) after resetting

6. Autumn 2000John Kristoff6 Round Trip Time (RTT) ÑTCP measures RTT and uses RTT to calculate timer values ÑIf ACKs returning quickly, timer values are kept low ÑRetransmit quickly when throughput is high ÑAllow time for ACK to return when throughput is low

7. Autumn 2000John Kristoff7 Slow Start ÑInitialize cwnd to 1 MSS ÑIncrease cwnd by 1 MSS for every ACK ÑNot very slow!

8. Autumn 2000John Kristoff8 Congestion Avoidance Ñ When packet loss occurs, TCP must slow its flow of traffic Ñ When retranmission timer expires, save value of transmission window * ½ (ssthresh) Ñ If congestion occurred due to a timeout, set cwnd to 1 segment Ñ When data is ACK'd, increase cwnd via slow start up to ssthresh Ñ Thereafter increase cwnd by 1/cwnd for each ACK

9. Autumn 2000John Kristoff9 Congestion Avoidance Illustrated

10. Autumn 2000John Kristoff10 Fast Retransmit ÑIf > 2 duplicate ACKs are received, a segment was probably lost ÑSender assumes enough time has elapsed for out of order segments to have arrived and assembled ÑRetransmit segment without waiting for a retransmission timer to expire ÑEnter Fast Recovery

11. Autumn 2000John Kristoff11 Fast Recovery ÑSender has to assume data is still flowing due to the reception of duplicate ACKs ÑLoss was probably a rare or temporary event ÑDo not go into slow start, just go into congestion avoidance mode

12. Autumn 2000John Kristoff12 Other mechanisms ÑSelective Acknowledgements (SACK) ÑTCP Pacing ÑPacketeer ÑRED ÑECN ÑNetwork pricing