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Understanding TCP Sequence Numbers and Congestion Control Mechanisms

This discussion explores the fundamentals of TCP (Transmission Control Protocol) sequence numbers and congestion control strategies. It elaborates on the significance of the initial sequence number in establishing a TCP connection, and how sequence numbers are essential for data integrity. Additionally, it delves into congestion control mechanisms such as slow-start and AIMD (Additive Increase Multiplicative Decrease), emphasizing the importance of the congestion window (cwnd) and how TCP mitigates network congestion while ensuring data is transmitted effectively.

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Understanding TCP Sequence Numbers and Congestion Control Mechanisms

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  1. TCP EE122 Discussion 10/18/13

  2. TCP Sequence Numbers • Initial sequence number for the connection is not zero • Sequence number refers to the first byte in the TCP packet payload

  3. Example (from class) • Packet arrives: – Seq: 2323 – Ack: 4001 – W=3000 – [no payload] • Appropriate response? Seq: 4001, payload: 4001-8000 Seq: 2001, payload: 2001-5000 Seq: 4001, payload: 4001-5000 Seq: 5001, payload: 5001-6001 Seq: 8001, payload: 8001-9000

  4. 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

  5. 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

  6. ‘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!

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