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.