# tc-cbs(8) - man - phpMan

[CBS(8)](https://www.chedong.com/phpMan.php/man/CBS/8/markdown)                                          Linux                                         [CBS(8)](https://www.chedong.com/phpMan.php/man/CBS/8/markdown)



## NAME
       CBS - Credit Based Shaper (CBS) Qdisc

## SYNOPSIS
       **tc** **qdisc** **...** **dev** dev **parent** classid **[** **handle** major: **]** **cbs** **idleslope** idleslope **sendslope** send‐
       slope **hicredit** hicredit **locredit** locredit **[** **offload** 0|1 **]**


## DESCRIPTION
       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 traf‐
       fic.

       This queueing discipline is intended to be used by TSN (Time Sensitive  Networking)  applica‐
       tions,  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)](https://www.chedong.com/phpMan.php/man/mqprio/8/markdown).**


## PARAMETERS
       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 cal‐
              culating  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  se‐
              quence  (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  fol‐
              lows, (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. Hi‐
              credit 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)](https://www.chedong.com/phpMan.php/man/cbs/8/markdown)** will try to configure the network interface so the CBS algo‐
              rithm runs in the controller. The default is 0.


## EXAMPLES
       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)](https://www.chedong.com/phpMan.php/man/mqprio/8/markdown),** 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 transmis‐
       sion rate is 1Gbit/s and the maximum interfering frame size is 1500 bytes.


## AUTHORS
       Vinicius Costa Gomes <<vinicius.gomes@intel.com>>



iproute2                                    18 Sept 2017                                      [CBS(8)](https://www.chedong.com/phpMan.php/man/CBS/8/markdown)