TC(8)                               Linux                              TC(8)

NAME         top

       drr - deficit round robin scheduler

SYNOPSIS         top

       tc qdisc ... add drr [ quantum bytes ]

DESCRIPTION         top

       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         top

       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         top

              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         top

       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

       tc filter add .. protocol ip .. $handle flow hash keys
       src,dst,proto,proto-src,proto-dst divisor 1024 perturb 10

SOURCE         top

       o      M. Shreedhar and George Varghese "Efficient Fair Queuing using
              Deficit Round Robin", Proc. SIGCOMM 95.

NOTES         top

       This implementation does not drop packets from the longest queue on
       overrun, as limits are handled by the individual child qdiscs.

SEE ALSO         top

       tc(8), tc-htb(8), tc-sfq(8)

AUTHOR         top

       sched_drr was written by Patrick McHardy.

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
       networking and traffic) project.  Information about the project can
       be found at 
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iproute2                        January 2010                           TC(8)