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  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

       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)