TC(8)								       Linux								     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 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 classi-
       fied, 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 suc-
       ceeds. 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.

iproute2							   January 2010 							     TC(8)