EE 122: Congestion Control The Sequel October 1, 2003
Quick Review § Slow-Start: cwnd++ upon every new ACK § Congestion avoidance: AIMD if cwnd > ssthresh - ACK: cwnd = cwnd + 1/cwnd - Drop: ssthresh =cwnd/2 and cwnd=1 § Fast Recovery: - duplicate ACKS: cwnd=cwnd/2 - Timeout: cwnd=1 2
TCP Flavors § TCP-Tahoe - cwnd =1 whenever drop is detected § TCP-Reno - cwnd =1 on timeout - cwnd = cwnd/2 on dupack § TCP-new. Reno - TCP-Reno + improved fast recovery § TCP-Vegas, TCP-SACK 3
TCP Vegas § Improved timeout mechanism § Decrease cwnd only for losses sent at current rate - avoids reducing rate twice § Congestion avoidance phase: - compare Actual rate (A) to Expected rate (E) if E-A > , decrease cwnd linearly if E-A < , increase cwnd linearly rate measurements ~ delay measurements see textbook for details! 4
TCP-SACK § SACK = Selective Acknowledgements § ACK packets identify exactly which packets have arrived § Makes recovery from multiple losses much easier 5
Standards? § How can all these algorithms coexist? § Don’t we need a single, uniform standard? § What happens if I’m using Reno and you are using Tahoe, and we try to communicate? 6
Equation-Based CC § Simple scenario - assume a drop every k’th RTT (for some large k) - w, w+1, w+2, . . . w+k-1 DROP (w+k-1)/2, (w+k-1)/2+1, . . . § Observations: - § In steady state: w= (w+k-1)/2 so w=k-1 Average window: 1. 5(k-1) Total packets between drops: 1. 5 k(k-1) Drop probability: p = 1/[1. 5 k(k-1)] Throughput: T ~ (1/RTT)*sqrt(3/2 p) 7
Equation-Based CC § Idea: - Forget complicated increase/decrease algorithms - Use this equation T(p) directly! § Approach: - measure drop rate (don’t need ACKs for this) - send drop rate p to source - source sends at rate T(p) § Good for streaming audio/video that can’t tolerate the high variability of TCP’s sending rate 8
Question! § Why use the TCP equation? § Why not use any equation for T(p)? 9
Cheating § Three main ways to cheat: - increasing cwnd faster than 1 per RTT - using large initial cwnd - Opening many connections 10
Increasing cwnd Faster A C y D x B y E x increases by 2 per RTT y increases by 1 per RTT Limit rates: x = 2 y x 11
Increasing cwnd Faster A D x y B E 12
Larger Initial cwnd A D x y B E x starts SS with cwnd = 4 y starts SS with cwnd = 1 13
Open Many Connections A D x y B E Assume • A starts 10 connections to B • D starts 1 connection to E • Each connection gets about the same throughput Then A gets 10 times more throughput than D 14
Cheating and Game Theory A D A x B y E D Increases by 1 Increases by 5 22, 22 10, 35 35, 10 15, 15 Individual incentives: cheating pays Social incentives: better off without cheating Classic PD: resolution depends on accountability (x, y) Too aggressive Losses Throughput falls 15
Lossy Links § TCP assumes that all losses are due to congestion § What happens when the link is lossy? § Recall that Tput ~ 1/sqrt(p) where p is loss prob. § This applies even for non-congestion losses 16
Example p=0 p = 1% p = 10% 17
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