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 sendslope hicredit hicredit locredit locredit [ of-
fload 0|1 ]
DESCRIPTION
The CBS (Credit Based Shaper) qdisc implements the shaping algorithm de-
fined by the IEEE 802.1Q-2014 Section 8.6.8.2, which applies a well de-
fined 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
idleslope
Idleslope is the rate of credits that is accumulated (in kilobits
per second) when there is at least one packet waiting for trans-
mission. Packets are transmitted when the current value of cred-
its 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 pay-
load, and with no VLAN tags, you must add 14 bytes for the Ether-
net 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_trans-
mit_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 in-
terface so the CBS algorithm 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 transmission 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)
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