tc-cbs(8) — Linux manual page

NAME \| SYNOPSIS \| DESCRIPTION \| PARAMETERS \| EXAMPLES \| AUTHORS \| COLOPHON

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 8.6.8.2, 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
              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
              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 8.6.8.2 item g):

              sendslope = idleslope - port_transmit_rate

       hicredit
              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_transmit_rate)

       locredit
              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)

       offload
              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 <[email protected]>

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 
       ⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
       If you have a bug report for this manual page, send it to
       [email protected], [email protected].  This page was
       obtained from the project's upstream Git repository
       ⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
       2025-08-11.  (At that time, the date of the most recent commit
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       COLOPHON (which is not part of the original manual page), send a
       mail to [email protected]

iproute2                       18 Sept 2017                        CBS(8)