TC(8)                    (January 2010)                     TC(8)

     NAME
          drr - deficit round robin scheduler

     SYNOPSIS
          tc qdisc ... add drr [ quantum bytes ]

     DESCRIPTION
          The Deficit Round Robin Scheduler is a classful queuing dis-
          cipline as a more flexible replacement for Stochastic Fair-
          ness Queuing.

          Unlike SFQ, there are no built-in queues -- you need to add
          classes and then set up filters to classify packets accord-
          ingly.  This can be useful e.g. for using RED qdiscs with
          different settings for particular traffic. There is no
          default class -- if a packet cannot be classified, it is
          dropped.

     ALGORITHM
          Each class is assigned a deficit counter, initialized to
          quantum.

          DRR maintains an (internal) ''active'' list of classes whose
          qdiscs are non-empty. This list is used for dequeuing. A
          packet is dequeued from the class at the head of the list if
          the packet size is smaller or equal to the deficit counter.
          If the counter is too small, it is increased by quantum and
          the scheduler moves on to the next class in the active list.

     PARAMETERS
          quantum
               Amount of bytes a flow is allowed to dequeue before the
               scheduler moves to the next class. Defaults to the MTU
               of the interface. The minimum value is 1.

     EXAMPLE & USAGE
          To attach to device eth0, using the interface MTU as its
          quantum:

          # tc qdisc add dev eth0 handle 1 root drr

          Adding two classes:

          # tc class add dev eth0 parent 1: classid 1:1 drr
          # tc class add dev eth0 parent 1: classid 1:2 drr

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     TC(8)                    (January 2010)                     TC(8)

          You also need to add at least one filter to classify pack-
          ets.

          # tc filter add dev eth0 protocol .. classid 1:1

          Like SFQ, DRR is only useful when it owns the queue -- it is
          a pure scheduler and does not delay packets. Attaching non-
          work-conserving qdiscs like tbf to it does not make sense --
          other qdiscs in the active list will also become inactive
          until the dequeue operation succeeds. Embed DRR within
          another qdisc like HTB or HFSC to ensure it owns the queue.

          You can mimic SFQ behavior by assigning packets to the
          attached classes using the flow filter:

          tc qdisc add dev .. drr

          for i in .. 1024;do
               tc class add dev .. classid $handle:$(print %x $i)
               tc qdisc add dev .. fifo limit 16
          done

          tc filter add .. protocol ip

     SOURCE
          o    M. Shreedhar and George Varghese "Efficient Fair Queu-
               ing using Deficit Round Robin", Proc. SIGCOMM 95.

     NOTES
          This implementation does not drop packets from the longest
          queue on overrun, as limits are handled by the individual
          child qdiscs.

     SEE ALSO
          tc(8), tc-htb(8), tc-sfq(8)

     AUTHOR
          sched_drr was written by Patrick McHardy.

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