Congestion Control Transport Layer 1 Principles of Congestion

Congestion Control Transport Layer 1

Principles of Congestion Control Congestion: r informally: “too many sources sending too much data too fast for network to handle” r different from flow control! r manifestations: m lost packets (buffer overflow at routers) m long delays (queueing in router buffers) r a top-10 problem! Transport Layer 2

Scenario 1: Queuing Delays Host A r two senders, two receivers r one router, infinite buffers r no retransmission Host B lout lin : original data unlimited shared output link buffers r large delays when congested r maximum achievable throughput Transport Layer 3

Scenario 2: Retransmits r one router, finite buffers r sender retransmission of lost packet Host A lin : original data lout l'in : original data, plus retransmitted data Host B finite shared output link buffers Transport Layer 4

Scenario 3: Congestion Near Receiver r four senders Q: what happens as l in and l increase ? r multihop paths in r timeout/retransmit Host A lin : original data lout l'in : original data, plus retransmitted data finite shared output link buffers Host B Transport Layer 5

Approaches towards congestion control Two broad approaches towards congestion control: End-end congestion control: r no explicit feedback from network r congestion inferred from end-system observed loss, delay r approach taken by TCP Network-assisted congestion control: r routers provide feedback to end systems m single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) m explicit rate sender should send at Transport Layer 6

TCP Congestion Control r end-end control (no network assistance) r sender limits transmission: Last. Byte. Sent-Last. Byte. Acked Cong. Win r Roughly, rate = Cong. Win Bytes/sec RTT r Cong. Win is dynamic, function of perceived network congestion How does sender perceive congestion? r loss event = timeout or 3 duplicate acks r TCP sender reduces rate (Cong. Win) after loss event three mechanisms: m m m AIMD slow start conservative after timeout events Transport Layer 7

TCP AIMD multiplicative decrease: cut Cong. Win in half after loss event additive increase: increase Cong. Win by 1 MSS every RTT in the absence of loss events: probing Long-lived TCP connection Transport Layer 8

TCP Slow Start r When connection begins, Cong. Win = 1 MSS m m Example: MSS = 500 bytes & RTT = 200 msec initial rate = 20 kbps r When connection begins, increase rate exponentially fast until first loss event r available bandwidth may be >> MSS/RTT m desirable to quickly ramp up to respectable rate Transport Layer 9

TCP Slow Start (more) r When connection m m double Cong. Win every RTT done by incrementing Cong. Win for every ACK received RTT begins, increase rate exponentially until first loss event: Host A Host B one segme nt two segme nts four segme nts r Summary: initial rate is slow but ramps up exponentially fast time Transport Layer 10

Refinement Philosophy: r After 3 dup ACKs: m Cong. Win m window is cut in half then grows linearly r But after timeout event: m Cong. Win instead set to 1 MSS; m window then grows exponentially m to a threshold, then grows linearly • 3 dup ACKs indicates network capable of delivering some segments • timeout before 3 dup ACKs is “more alarming” Transport Layer 11

Refinement (more) Q: When should the exponential increase switch to linear? A: When Cong. Win gets to 1/2 of its value before timeout. Implementation: r Variable Threshold r At loss event, Threshold is set to 1/2 of Cong. Win just before loss event Transport Layer 12

Summary: TCP Congestion Control r When Cong. Win is below Threshold, sender in slow-start phase, window grows exponentially. r When Cong. Win is above Threshold, sender is in congestion-avoidance phase, window grows linearly. r When a triple duplicate ACK occurs, Threshold set to Cong. Win/2 and Cong. Win set to Threshold. r When timeout occurs, Threshold set to Cong. Win/2 and Cong. Win is set to 1 MSS. Transport Layer 13

TCP sender congestion control Event State TCP Sender Action Commentary ACK receipt Slow Start for previously (SS) unacked data Cong. Win = Cong. Win + MSS, If (Cong. Win > Threshold) set state to “Congestion Avoidance” Resulting in a doubling of Cong. Win every RTT ACK receipt Congestion for previously Avoidance unacked data (CA) Cong. Win = Cong. Win+MSS * (MSS/Cong. Win) Additive increase, resulting in increase of Cong. Win by 1 MSS every RTT Loss event detected by triple duplicate ACK SS or CA Threshold = Cong. Win/2, Cong. Win = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. Cong. Win will not drop below 1 MSS. Timeout SS or CA Threshold = Cong. Win/2, Cong. Win = 1 MSS, Set state to “Slow Start” Enter slow start Duplicate ACK SS or CA Increment duplicate ACK count for segment being acked Cong. Win and Threshold not changed Transport Layer 14