NAME

       drr - deficit round robin scheduler

SYNOPSIS

       tc qdisc ... add drr [ quantum bytes ]

DESCRIPTION

       The  Deficit  Round  Robin  Scheduler  is a classful queuing discipline as a more flexible
       replacement for Stochastic Fairness Queuing.

       Unlike SFQ, there are no built-in queues -- you need  to  add  classes  and  then  set  up
       filters  to  classify  packets  accordingly.  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

       You also need to add at least one filter to classify packets.

       # 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 .. $handle flow hash  keys  src,dst,proto,proto-src,proto-dst
       divisor 1024 perturb 10

SOURCE

       o      M.  Shreedhar  and  George  Varghese  "Efficient  Fair  Queuing 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.