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Deficit Round Robin

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• Generalizing the Round Robin Scheduling to achieve Weight Max-Min fairness
  • The round robin scheduling can provide Max-Min fairness when:
    1. all flows have equal weights (i.e., unweighted)

    2. all packets have the same size

  • Sheerhar and Varghese presented in "Efficient Fair Queueing using Deficit Round Robin" ( click here ) a very nice adaptation of the round robin scheduler that can provide a very good approximate Max-Min fairness when:
    1. flows have different weights

    2. packets can have different sizes

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• The Deficit Round Robin Scheduler
  • The router maintains a deficit counter for each active flow

  • The deficit counter is the number of bytes that a flow is allowed to transmit when it is its turn.

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  • A DRR router uses a "Quantum Size" parameter that is pre-defined.

  • Packets from flows are transmitted in a round robin manner

  • A packet from a flow is only transmitted if the deficit counter of that flow is greater than or equal to the packet length

    When a packet is transmitted, the value of the packet length is subtracted from the deficit counter

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  • The deficit counter of a flow is initially set to zero

  • The Quantum is added to the deficit counter of a flow before servicing a flow.

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• Example of an unweighted Deficit Round Robin Scheduler
  • Consider the following situation in a DRR scheduler

    There are 3 flows with different packet sizes in the corresponding queues:

    Deficit counter +-----+ +------------+ +---------------------------+ flow 1 | 0 | | 200 bytes | | 400 bytes | +-----+ +------------+ +---------------------------+ +-----+ +-----+ +------------+ +-------------------+ flow 2 | 0 | | 100 | | 200 | | 300 bytes | +-----+ +-----+ +------------+ +-------------------+ +-----+ +------------+ +------------+ +------------+ flow 3 | 0 | | 200 bytes | | 200 | | 200 | +-----+ +------------+ +------------- +------------+ Quantum = 500

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  • Flow 1 is serviced:

    Deficit counter +-----+ +------------+ +---------------------------+ flow 1 | 500 | | 200 bytes | | 400 bytes | +-----+ +------------+ +---------------------------+ +-----+ +-----+ +------------+ +-------------------+ flow 2 | 0 | | 100 | | 200 | | 300 bytes | +-----+ +-----+ +------------+ +-------------------+ +-----+ +------------+ +------------+ +------------+ flow 3 | 0 | | 200 bytes | | 200 | | 200 | +-----+ +------------+ +------------- +------------+ Quantum = 500

    Flow 1 has enough credit to transmit the first packet of 200 bytes:

    Deficit counter +-----+ +---------------------------+ flow 1 | 300 | | 400 bytes | +-----+ +---------------------------+ +-----+ +-----+ +------------+ +-------------------+ flow 2 | 0 | | 100 | | 200 | | 300 bytes | +-----+ +-----+ +------------+ +-------------------+ +-----+ +------------+ +------------+ +------------+ flow 3 | 0 | | 200 bytes | | 200 | | 200 | +-----+ +------------+ +------------- +------------+ Quantum = 500

    Now Flow 1 does not have enough credits and the turn goes to Flow 2.

    Flow 1 will carry its residual transmission credit over to the next service round

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  • Flow 2 is being serviced:

    Deficit counter +-----+ +---------------------------+ flow 1 | 300 | | 400 bytes | +-----+ +---------------------------+ +-----+ +-----+ +------------+ +-------------------+ flow 2 | 500 | | 100 | | 200 | | 300 bytes | +-----+ +-----+ +------------+ +-------------------+ +-----+ +------------+ +------------+ +------------+ flow 3 | 0 | | 200 bytes | | 200 | | 200 | +-----+ +------------+ +------------- +------------+ Quantum = 500

    Flow 2 has enough credit to transmit its first packet:

    Deficit counter +-----+ +---------------------------+ flow 1 | 300 | | 400 bytes | +-----+ +---------------------------+ +-----+ +------------+ +-------------------+ flow 2 | 400 | | 200 | | 300 bytes | +-----+ +------------+ +-------------------+ +-----+ +------------+ +------------+ +------------+ flow 3 | 0 | | 200 bytes | | 200 | | 200 | +-----+ +------------+ +------------- +------------+ Quantum = 500

    Flow 2 has enough credit to transmit its second packet:

    Deficit counter +-----+ +---------------------------+ flow 1 | 300 | | 400 bytes | +-----+ +---------------------------+ +-----+ +-------------------+ flow 2 | 200 | | 300 bytes | +-----+ +-------------------+ +-----+ +------------+ +------------+ +------------+ flow 3 | 0 | | 200 bytes | | 200 | | 200 | +-----+ +------------+ +------------- +------------+ Quantum = 500

    Now Flow 2 does not have enough credits and the turn goes to Flow 3.

    Flow 2 will carry its residual transmission credit over to the next service round

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  • Flow 3 is being serviced:

    Deficit counter +-----+ +---------------------------+ flow 1 | 300 | | 400 bytes | +-----+ +---------------------------+ +-----+ +-------------------+ flow 2 | 200 | | 300 bytes | +-----+ +-------------------+ +-----+ +------------+ +------------+ +------------+ flow 3 | 500 | | 200 bytes | | 200 | | 200 | +-----+ +------------+ +------------- +------------+ Quantum = 500

    Flow 3 has enough credit to transmit its first packet:

    Deficit counter +-----+ +---------------------------+ flow 1 | 300 | | 400 bytes | +-----+ +---------------------------+ +-----+ +-------------------+ flow 2 | 200 | | 300 bytes | +-----+ +-------------------+ +-----+ +------------+ +------------+ flow 3 | 300 | | 200 | | 200 | +-----+ +------------- +------------+ Quantum = 500

    Flow 3 has enough credit to transmit its second packet:

    Deficit counter +-----+ +---------------------------+ flow 1 | 300 | | 400 bytes | +-----+ +---------------------------+ +-----+ +-------------------+ flow 2 | 200 | | 300 bytes | +-----+ +-------------------+ +-----+ +------------+ flow 3 | 100 | | 200 | +-----+ +------------+ Quantum = 500

    Now Flow 3 does not have enough credits and the turn goes BACK to Flow 1.

    Flow 1 will get another quantum added to its residual transmission credit

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  • Flow 1 is being serviced:

    Deficit counter +-----+ +---------------------------+ flow 1 | 800 | | 400 bytes | +-----+ +---------------------------+ +-----+ +-------------------+ flow 2 | 200 | | 300 bytes | +-----+ +-------------------+ +-----+ +------------+ flow 3 | 100 | | 200 | +-----+ +------------+ Quantum = 500

  • And so on....

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  • From the operation, it should be clear that:
    • Flow will share the bandwidth fairly if they are unsaturated

    • The saturated flows (flows that transmit at a lower rate than the fair share) will receive its complete service rate.

      The left over transmission rate will be divided among the unsaturated flows

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• Weighted Round Robin Scheduling
  • It should be easy to see that by assigning a different quantum to different flows, you can obtain a weighted version of DRR

  • Example:

    Flow Deficit Quantum counter +-----+ +-----+ +------------+ +---------------------------+ flow 1 | 800 | | 0 | | 200 bytes | | 400 bytes | +-----+ +-----+ +------------+ +---------------------------+ +-----+ +-----+ +-----+ +------------+ +-------------------+ flow 2 | 400 | | 0 | | 100 | | 200 | | 300 bytes | +-----+ +-----+ +-----+ +------------+ +-------------------+ +-----+ +-----+ +------------+ +------------+ +------------+ flow 3 | 600 | | 0 | | 200 bytes | | 200 | | 200 | +-----+ +-----+ +------------+ +------------- +------------+

  • At the start of servicing a flow, the flow quantum is added to the deficit counter