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NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | EXAMPLES | AUTHORS | COLOPHON |
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CBS(8) Linux CBS(8)
CBS - Credit Based Shaper (CBS) Qdisc
tc qdisc ... dev dev parent classid [ handle major: ] cbs idleslope
idleslope sendslope sendslope hicredit hicredit locredit locredit [
offload 0|1 ]
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
Sepcification. 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).
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 ocurring. 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.
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 queue‐
ing 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 interfer‐
ing frame size is 1500 bytes.
Vinicius Costa Gomes <vinicius.gomes@intel.com>
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
netdev@vger.kernel.org, shemminger@osdl.org. This page was obtained
from the project's upstream Git repository
⟨git://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git⟩
on 2018-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2018-01-29.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
iproute2 18 Sept 2017 CBS(8)
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