.TH CBS 8 "18 Sept 2017" "iproute2" "Linux"

.SH NAME

CBS \- Credit Based Shaper (CBS) Qdisc

.SH SYNOPSIS

.B tc qdisc ... dev

dev

.B parent

classid

.B [ handle

major:

.B ] cbs idleslope

idleslope

.B sendslope

sendslope

.B hicredit

hicredit

.B locredit

locredit

.B [ offload

0|1

.B ]

.SH 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 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

.BR mqprio(8).

.SH PARAMETERS

.TP

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.

.TP

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

.TP

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)

.TP

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)

.TP

offload

When

.B offload

is 1,

.BR cbs(8)

will try to configure the network interface so the CBS algorithm runs

in the controller. The default is 0.

.SH 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

.BR mqprio(8),

and configure it like this:

.EX

# 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

.EE

.P

To replace the current queuing disciple by CBS in the current queueing

discipline connected to traffic class number 0, issue:

.P

.EX

# tc qdisc replace dev eth0 parent 100:4 cbs \\

	locredit -1470 hicredit 30 sendslope -980000 idleslope 20000

.EE

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.

.SH AUTHORS

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