CBS(8)                               Linux                              CBS(8)

       CBS - Credit Based Shaper (CBS) Qdisc

       tc  qdisc  ...  dev  dev parent classid [ handle major: ] cbs idleslope
       idleslope sendslope sendslope hicredit  hicredit  locredit  locredit  [
       offload 0|1 ]

       The  CBS  (Credit  Based Shaper) qdisc implements the shaping algorithm
       defined by the IEEE 802.1Q-2014 Section, which applies  a  well
       defined rate limiting method to the traffic.

       This  queueing discipline is intended to be used by TSN (Time Sensitive
       Networking) applications, the CBS parameters are  derived  directly  by
       what is described by the Annex L of the IEEE 802.1Q-2014 Sepcification.
       The algorithm and how it affects the latency are detailed there.

       CBS is meant to be installed under another qdisc that maps packet flows
       to traffic classes, one example is mqprio(8).

              Idleslope  is  the rate of credits that is accumulated (in kilo-
              bits per second) when there is at least one packet  waiting  for
              transmission.  Packets are transmitted when the current value of
              credits is equal or greater than zero. When there is  no  packet
              to  be transmitted the amount of credits is set to zero. This is
              the main tunable of the CBS algorithm and represents  the  band-
              width  that will be consumed.  Note that when calculating idles-
              lope, the entire packet size must be considered, including head-
              ers  from all layers (i.e. MAC framing and any overhead from the
              physical layer), as described by IEEE 802.1Q-2014 section 34.4.

              As an example, for an ethernet frame carrying 284 bytes of  pay-
              load,  and with no VLAN tags, you must add 14 bytes for the Eth-
              ernet headers, 4 bytes for the Frame check sequence  (CRC),  and
              20  bytes  for  the  L1 overhead: 12 bytes of interpacket gap, 7
              bytes of preamble and 1 byte of start of frame  delimiter.  That
              results  in  322  bytes for the total packet size, which is then
              used for calculating the idleslope.

              Sendslope is the rate of credits that is depleted (it should  be
              a negative number of kilobits per second) when a transmission is
              ocurring. It can be calculated  as  follows,  (IEEE  802.1Q-2014
              Section item g):

              sendslope = idleslope - port_transmit_rate

              Hicredit  defines  the maximum amount of credits (in bytes) that
              can be accumulated. Hicredit depends on the  characteristics  of
              interfering traffic, 'max_interference_size' is the maximum size
              of any burst of traffic that can delay  the  transmission  of  a
              frame that is available for transmission for this traffic class,
              (IEEE 802.1Q-2014 Annex L, Equation L-3):

              hicredit =  max_interference_size  *  (idleslope  /  port_trans-

              Locredit  is  the minimum amount of credits that can be reached.
              It is a function of the traffic flowing through this qdisc (IEEE
              802.1Q-2014 Annex L, Equation L-2):

              locredit = max_frame_size * (sendslope / port_transmit_rate)

              When  offload  is  1,  cbs(8)  will try to configure the network
              interface so the CBS  algorithm  runs  in  the  controller.  The
              default is 0.

       CBS  is  used to enforce a Quality of Service by limiting the data rate
       of a traffic class, to separate packets into traffic classes  the  user
       may choose mqprio(8), and configure it like this:

       # tc qdisc add dev eth0 handle 100: parent root mqprio num_tc 3 \
            map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
            queues 1@0 1@1 2@2 \
            hw 0

       To  replace the current queuing disciple by CBS in the current queueing
       discipline connected to traffic class number 0, issue:

       # tc qdisc replace dev eth0 parent 100:4 cbs \
            locredit -1470 hicredit 30 sendslope -980000 idleslope 20000

       These values are obtained from the following parameters,  idleslope  is
       20mbit/s,  the transmission rate is 1Gbit/s and the maximum interfering
       frame size is 1500 bytes.

       Vinicius Costa Gomes <>

iproute2                         18 Sept 2017                           CBS(8)
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