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Understanding TCP Flow and Congestion Control Mechanisms

This discussion focuses on TCP flow control, aiming to prevent a sender from overwhelming a receiver. It explains the use of the advertised window (Adv_Win) to manage data transmission and its relationship with packet size (MSS). The text delves into TCP congestion control, detailing mechanisms such as the congestion window (cwnd) and the slow-start phase, where the cwnd increases based on acknowledgments (ACKs). It also covers the dynamics of timeout scenarios and fast retransmission techniques to address packet loss effectively.

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Understanding TCP Flow and Congestion Control Mechanisms

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  1. TCP EE122 Discussion 10/31/11

  2. TCP Flow Control • Keep sender from overwhelming receiver • Data not necessarily pushed to app layer S R Adv_Win: 300 PKT_MAX = 200 Adv_Win: 300 Adv_Win: 100 Adv_Win: 100 ACK ACK ACK MSS = min(PKT_MAX, Adv_Win) Push data to application MSS = min(PKT_MAX, Adv_Win)

  3. Congestion Control [1] • Avoid overwhelming the network • cwnd – congestion window • SSTHRESH –threshold until which exponential slow-start happens • TCP is either in slow-start mode or AIMD mode

  4. Congestion Control [2] • TCP is either in slow-start mode or AIMD mode • Slow-start: for every ACK, increase cwnd by MSS • “AI”: for every window of packets being ACK-ed, increase cwnd by MSS

  5. ‘Vanilla’ TCP • Exponential slow-start till SSTHRESH • Set timer, which expires when packets are lost • At each new data ACK the timeout is RESET • On time-out, set SSTHRESH to (cwnd/2), and cwnd itself to 1 • Timeout = receiving nothing for an entire RTO • Usually a disaster!

  6. ‘Vanilla’ TCP Set SSTHRESH = 5; Enter Slow Start SSTHRESH = 5 crossed. Enter AIMD Set SSTHRESH = 1; Enter directly into AIMD since SSTHRESH is already crossed.

  7. TCP Retransmission + Fast Recovery Set SSTHRESH = 5; However, enter AIMD directly! Set SSthresh = 1; Enter directly into AIMD since SSthresh is already crossed.

  8. TCP Congestion Control: CWND Flowchart Slow Start Start 3 duplicate ACKs timeout (CWND > SSTHRESH) Additive Increase Simple Fast Retransmission Loss detected by timeout 3 duplicate ACKs timeout

  9. TCP Congestion Control: CWND Flowchart Action when receive ACK CWND+= MSS Start 3 duplicate ACKs timeout (CWND > SSTHRESH) CWND += CWND = max(, MSS) SSTHRESH = CWND = MSS 3 duplicate ACKs timeout

  10. Problem 1

  11. Problem 2

  12. Problem 3

  13. Advanced Fast Retransmit Action when receive ACK CWND+= MSS Start SSTHRESH = CWND = timeout 3 duplicate ACKs (CWND > SSTHRESH) CWND += SSTHRESH = CWND = MSS CWND += MSS duplicate ACK timeout CWND = SSTHRESH new ACK

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