tc-cbs(8) — Linux manual page


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 Specification. 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

              Sendslope is the rate of credits that is depleted (it
              should be a negative number of kilobits per second) when a
              transmission is occurring. 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 /

              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
       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.

AUTHORS         top

       Vinicius Costa Gomes <>

COLOPHON         top

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iproute2                      18 Sept 2017                        CBS(8)