Promoting the use of end to end congestion control
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Promoting the Use of End-to-End Congestion Control . Tapan Karwa CS590F. What is Congestion Control. Its a control problem. Its concerned with allocation of resources. The problem – limited resources. Will infinite BW, buffer space and super fast processors help?

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What is congestion control l.jpg
What is Congestion Control

  • Its a control problem.

  • Its concerned with allocation of resources.

  • The problem – limited resources.

  • Will infinite BW, buffer space and super fast processors help?

  • CC is not a resource shortage problem.

  • Congestion will occur. It needs to be handled. Dynamically.

Classical congestion collapse l.jpg
Classical Congestion Collapse

  • Congestion collapse occurs when the network is increasingly busy, but little useful work is getting done.

  • Problem : paths clogged with unnecessarily retransmitted packets.

  • Reasons : heterogeneity, dynamic network conditions, bad timers with Go-back-N retransmissions.

  • Fix : Modern TCP retransmit timer and congestion control algorithms. Based on the assumption that sources will cooperate.

Congestion collapse from undelivered packets l.jpg
Congestion Collapse from undelivered packets

  • Problem : Paths clogged with packets that are discarded before they reach the receiver.

  • Reasons : open-loop apps not performing end-to-end CC, best-effort apps increasing their sending rate on packet loss.

  • Unresponsive flows. No cooperation. Router mechanisms (scheduling mechanisms, ECN) don’t help either.

  • Fix : Either end-to-end CC, along with mechanisms to detect unresponsive flows, or virtual-circuit style of guarantee packet delivery.

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Why do we need CC

  • So that the application can better achieve its goals of minimizing loss and delay, maximizing throughput.

  • Fairness. Especially with unresponsive flows around.

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What is the fairness goal?

  • No connection /session/end-node should hog the network resources.

    • TCP is the dominant transport protocol in the Internet (90-95% of the bytes/packets).

    • Routers are likely to use FIFO scheduling.

    • New forms of traffic/new applications that compete with TCP as best-effort traffic in FIFO queues should not be significantly more (or less) aggressive than TCP.

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Mechanisms within the network

  • Mechanisms within the network infrastructure to restrict unresponsiveness during times of congestion.

  • Max sending rate :

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Identifying TCP-unfriendly flows

  • Given R and B, if the steady-state packet drop rate is ‘x’, then the arrival rate of the flow should be at most ‘y’.

  • R is set to twice the propagation delay of the attached link.

  • Limitations :

    • Difficult to determine B, R.

    • Applies only to non-bursty packet drop behavior. During severe congestion, multiple packet drops are very likely.

    • Measurements should be taken over large intervals in comparison to the RTT of the connection.

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Identifying unresponsive flows

  • To test if a high BW connection is responsive, check if, when the steady-state drop rate increases by factor x, does the arrival rate decrease by a factor close to squareroot(x).

  • Can be applied to aggregated traffic too.

  • Limitations :

    • Requires estimates of a flows arrival rate and packet drop rate over long time intervals.

    • Flows could start with high initial BW and then reduce it. This means that just testing for unresponsiveness might not be enough.

Identifying flows with disproportionate bw l.jpg
Identifying flows with disproportionate BW

  • Applied when a flow is consuming a larger share than other flows who could use more BW.

  • Two components :

    • Check if flow is using more than its share.

    • Check if flow has a high arrival rate, relative to the level of congestion, as reflected by the drop rate.

  • Limitations :

    • Difficult to access how unsatisfied a flow is.

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Other approaches and Conclusion

  • Instead of using the mechanisms proposed, we could use per-flow scheduling.

  • Limitations : difficult to implement, FCFS is more optimal.

  • End-to-end congestion control is required for reasonable usage of the Internet.

  • Mechanisms to punish non-responsive flows are also required to prevent pathological situations.