# tc-drr

## 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.
    


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