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 use-
       ful 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 sched-
	      uler moves to the next class. Defaults to the MTU of the	inter-
	      face. 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.



iproute2			 January 2010				 TC(8)