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. 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

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?

Utilization Average packet delay Load Average packet loss Load

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

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

Worksheet Q2 Q1 Q3

CWND Flowchart Based on EE122 section slides, 10/31/11 Start CWND += MSS timeout3 duplicate ACKs Action when receive new ACK (CWND > SSTHRESH) timeout3 duplicate ACKs Simple Fast Retransmission Additive Increase Loss detected by timeout Slow Start

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

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