man > tc-cbs(8)

CBS(8)                                          Linux                                         CBS(8)

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).


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) 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), 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 AT intel.com>

iproute2                                    18 Sept 2017                                      CBS(8)