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Recap of Lecture 19

Recap of Lecture 19. If symptoms persist, please consult Dr Jacobson. Same caveat as the lecture …. “Everything […] is oversimplified” Read the RFCs or source code for details! (if you do, please tell us how TCP really works!) e.g.

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Recap of Lecture 19

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  1. Recap of Lecture 19 If symptoms persist, please consult Dr Jacobson

  2. Same caveat as the lecture … • “Everything […] is oversimplified” • Read the RFCs or source code for details! • (if you do, please tell us how TCP really works!) • e.g. When a TCP sender detects segment loss using the retransmission timer and the given segment has not yet been resent by way of the retransmission timer, the value of ssthresh MUST be set to no more than the value given in equation (4): ssthresh = max (FlightSize / 2, 2*SMSS) ... Implementation Note: An easy mistake to make is to simply use cwnd, rather than FlightSize, which in some implementations may incidentally increase well beyond rwnd. -- RFC 5681

  3. Congestion Control • What happens when two packets arrive at the same time? • Flow control vs. congestion control MaxWindow = min {congestion window, receiver window} • Who takes care of congestion?

  4. Utilization Average packet delay Average packet loss Load Load

  5. Detecting Congestion • ICMP Source Quench? • Could be lost • Adds to congestion • Packet delays? (knee of load-delay curve) • Noisy • Packet loss? • Fail-safe • Non-congestive loss

  6. Fast Retransmission • 3 duplicate ACKs • CWND ← CWND/2 • SSTHRESH ← CWND / 2 • CWND ← 1 • Retransmit first lost packet • Slow start until CWND > SSTHRESH, then AIMD

  7. Worksheet Q3 Q2 Q1

  8. CWND Flowchart Action when receive new ACK Slow Start CWND += MSS Start 3 duplicate ACKs timeout (CWND > SSTHRESH) CWND += CWND = max(, MSS) SSTHRESH = CWND = MSS 3 duplicate ACKs timeout Simple Fast Retransmission Loss detected by timeout Additive Increase Based on EE122 section slides, 10/31/11

  9. CWND: Advanced Fast Retransmit Action when receive new ACK Slow Start CWND += MSS Start SSTHRESH = CWND = timeout 3 duplicate ACKs (CWND > SSTHRESH) CWND += SSTHRESH = CWND = MSS CWND += MSS duplicate ACK timeout Loss detected by timeout CWND = SSTHRESH new ACK Additive Increase Based on EE122 section slides, 10/31/11

  10. What would go here? • Not all protocols TCP-friendly • High speed links • Question 1e • Bandwidth ∝ 1/RTT • Bandwidth ∝ # flows • Cheating • What is fairness?

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