Congestion Control in TCP
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1 Congestion Control in CP Antonio Carzaniga Faculty of Informatics University of Lugano Apr 7, Antonio Carzaniga 1 Intro to congestion control Outline Input rate vs. output throughput Congestion window Congestion avoidance Slow start Fast recovery Antonio Carzaniga 2
2 Understanding Congestion A router behaves a lot like a kitchen sink λ 3 = /2 λ 1 = /2 λ 2 = /2 max throughput throughput= / Antonio Carzaniga 3 Queuing Delay otal latency is the sum of link latency, processing time, and the time that a packet spends in the input queue L= X + CPU + q where q = q / Ideal case: constant input data rate λ in < In this case the q = 0, because q =0 Extreme case: constant input data rate λ in > In this case q =(λ in )t and therefore q = λ in Antonio Carzaniga 4 t
3 Steady-state queuing delay 0 λ in < q = λ in t λ in > Queuing Delay q q λ in ideal input flow λ in constant λ in realistic input flow λ in variable Antonio Carzaniga 5 Queuing Delay Conclusion: as the input rateλ in approaches the maximum throughput, packets will experience very long delays More realistic assumptions and models finite queue length (buffers) in routers effects of retransmission overhead λ out congestion λ in Antonio Carzaniga 6
4 What to Do? What to do when the network is congested and queues are full? λ 3 = /2 λ 1 = /2 λ 2 = /2 max throughput throughput= Antonio Carzaniga 7 Congestion Control (in CP) Approach: the sender limits its output rate according to the status of the network the sender output rate becomes (part of) the input rate for the network (λ in ) Issues how does the sender measure the status of the network? i.e., how does the sender detect congestion? how does the sender effectively limit its output rate? how should the sender modulate its output rate? i.e., what algorithm should the sender use to decrease or increase its output rate? Antonio Carzaniga 8
5 Detecting Congestion If all traffic is correctly acknowledged, then the sender assumes (quite correctly) that there is no congestion Congestion means that the queue of one or more routers between the sender and the receiver overflow the visible effect is that some segments are dropped herefore the server assumes that the network is congested when it detects a segment loss time out (i.e., no ACK) multiple acknowledgements (i.e., NACK) Antonio Carzaniga 9 he sender maintains a congestion window W Congestion Window he congestion window limits the amount of bytes that the sender pushes into the network before blocking waiting for acknowledgments where LastByteSent LastByteAcked W W = min(congestionwindow, ReceiverWindow) he resulting maximum output rate is roughly λ= W 2L Antonio Carzaniga 10
6 How does CP modulate its output rate? Congestion Control Additive-increase and multiplicative-decrease Slow start Reaction to timeout events Antonio Carzaniga 11 Additive-Increase/Multiplicative-Decrease How W is reduced: at every loss event, CP halves the congestion window e.g., suppose the window size W is currently 20Kb, and a loss is detected CP reduces W to 10Kb How W is increased: at every (good) acknowledgment, CP increments W by 1MSS/W, so as to increase W by MSS every round-trip time 2L. his process is called congestion avoidance e.g., suppose W = and MSS= 1460, then the sender increases W to after 10 acknowledgments acknowledgments Antonio Carzaniga 12
7 Additive-Increase/Multiplicative-Decrease Window size W over time W ime Antonio Carzaniga 13 What is the initial value of W? Slow Start he initial value of W is MSS, which is quite low for modern networks In order to get quickly to a good throughput level, CP increases its sending rate exponentially for its first growth phase After experiencing the first loss, CP cuts W in half and proceeds with its linear push his process is called slow start, because of the small initial value of W Antonio Carzaniga 14
8 imeouts vs. NACKs As we know, three duplicate ACKs are interpreted as a NACK Both timeouts and NACKs signal a loss, but they say different things about the status of the network A timeout indicates congestion hree (duplicate) ACKs suggest that the network is still able to deliver segments along that path So, CP reacts differently to a timeout and to a triple duplicate ACKs Antonio Carzaniga 15 Assuming the current window size is W = W imeout go back to W= MSS run slow start until W reaches W/2 then proceed with congestion avoidance imeouts vs. NACKs NACK cut W in half: W = W/2 run congestion avoidance, ramping up W linearly his is called fast recovery Antonio Carzaniga 16
9 Sender Behavior SS=slow start CA=congestion avoidance SS CA SS CA CA CA NACK W NACK timeout NACK MSS ime Antonio Carzaniga 17
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