CBS(8)                              Linux                             CBS(8)

NAME         top

       CBS - Credit Based Shaper (CBS) Qdisc

SYNOPSIS         top

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

DESCRIPTION         top

       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).

PARAMETERS         top

              Idleslope is the rate of credits that is accumulated (in
              kilobits 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 bandwidth that will be consumed.  Note that
              when calculating idleslope, the entire packet size must be
              considered, including headers 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
              payload, and with no VLAN tags, you must add 14 bytes for the
              Ethernet 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 /

              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.

EXAMPLES         top

       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 queue‐
       ing 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 interfer‐
       ing frame size is 1500 bytes.

AUTHORS         top

       Vinicius Costa Gomes <>

COLOPHON         top

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iproute2                        18 Sept 2017                          CBS(8)