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Understanding Congestion Control in Transport Layer Protocols

Congestion control is essential for managing the data flow in network systems, preventing packet loss and delays arising from too many sources sending data too quickly. Unlike flow control, which manages data between sender and receiver, congestion control focuses on network capacity. This involves techniques such as TCP's Additive Increase/Multiplicative Decrease (AIMD) algorithm, which adjusts the transmission rate based on perceived network congestion. Through various scenarios, we explore the mechanisms involved in congestion management and the impact on throughput, delays, and overall network performance.

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Understanding Congestion Control in Transport Layer Protocols

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  1. Congestion Control Transport Layer

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

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

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

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

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

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

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

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

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

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

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

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

  14. TCP sender congestion control Transport Layer

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