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TCP

TCP. EE122 Discussion 10/18/13. TCP Sequence Numbers. Initial sequence number for the connection is not zero Sequence number refers to the first byte in the TCP packet payload. Example (from class). Packet arrives: – Seq : 2323 – Ack : 4001 – W=3000 – [no payload]

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TCP

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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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