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Adding Acknowledgement Congestion Control to TCP

Adding Acknowledgement Congestion Control to TCP. S. Floyd, A. Arcia, D. Ros, and J. Iyengar draft-floyd-tcpm-ackcc-02.txt TCPM December 2007. How would TCP’s ACK Congestion Control work?. Negotiation between sender and receiver: (Ack-Congestion-Control-Permitted option).

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Adding Acknowledgement Congestion Control to TCP

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  1. Adding Acknowledgement Congestion Control to TCP S. Floyd, A. Arcia, D. Ros, and J. Iyengar draft-floyd-tcpm-ackcc-02.txt TCPM December 2007

  2. How would TCP’s ACK Congestion Control work? • Negotiation between sender and receiver: • (Ack-Congestion-Control-Permitted option). • Start with an Ack Ratio of 2. • The sender detects lost Ack packets: • And tells the receiver the new Ack Ratio. • The sender uses Appropriate Byte Counting and rate-based pacing (in response to Acks acking more than two packets).

  3. Changes from last time: • Added a section on "Keep-alive Packets". Feedback from Anantha Ramaiah. • Added a section on "Possible Complication: TCP Implementations that Skip ACK Packets". Motivated by reports at IETF that many high-bandwidth TCPs don't follow the MUST of sending an ACK for every other packet, if they don't have time. • Added that receivers might have buffer limitations that require that they ack at least every K packets, for some K. Feedback from Sara Landstrom. • Added to the discussion of "Possible Complication: Two-Way Traffic". Feedback from Sara Landstrom.

  4. More changes from last time: • Added a section on "Possible Complication: Router or Middlebox-based ACK Mechanisms". Feedback from Sara Landstrom. • Added that SACK is required with ACK congestion control. Feedback from Sara Landstrom. • Added a discussion of "Reducing the TCP Acknowledgment Frequency" to the related work section. • Added an appendix on "Design Considerations", with a subsection on "The TCP ACK Ratio Option, or an AckNow bit in data packets?". • General editing from feedback from Alfred Hoenes.

  5. Changes indraft-floyd-tcpm-ackcc-03b.txt: • General editing. Feedback from Alfred Hoenes. • Added more about keep-alive packets and window update packets. Feedback from Anantha Ramaiah.

  6. Possible Complication: TCP Implementations that Skip ACK Packets • “One possibility for addressing this problem would be for TCP receivers using ACK congestion control to be required to send an acknowledgement for each R packets, for ACK Ratio R.” • “A second possibility would be to define a TCP option or flag that the TCP receiver could use, when sending an ACK packet, to inform the sender that the TCP receiver `skipped' some ACK packets, so that the sender should not infer ACK loss if some ACK packets seem to be missing.”

  7. Future work: • Simulations and other evaluation of proposed mechanism. • Ready to be a working group document, targeted as Experimental?

  8. Slides from last time:

  9. Possible Complications: • Delayed acknowledgements. • Duplicate acknowledgements. • Two-way traffic. • Reordering of Ack packets. • Abrupt changes in the Ack path. • …

  10. Congestion on the reverse path: • Does pure Ack traffic really contribute to congestion? • Yes, somewhat, if the queue is in units of packets. • Measurement studies of congested links? • How might ackcc be useful to the connection? • ECN-capable ACK packets. • Possibly reducing the ACK drop rate even without ECN. • How might ackcc be harmful to the connection? • Costs of a larger Ack Ratio.

  11. Security Considerations: • Cheating with ECN-capable ACK packets? • If the receiver cheats, the sender could detect it. • If the sender cheats, the receiver can’t easily detect it. • Middleboxes probably could detect it.

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