| idleslope | 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 = idleslope - port_transmit_rate |
| hicredit | hicredit = max_interference_size * (idleslope / port_transmit_rate) |
| locredit | 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. |
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 [ 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 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
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 = idleslope - port_transmit_rate
- hicredit
hicredit = max_interference_size * (idleslope / port_transmit_rate)
- locredit
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.
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 0To 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 20000These 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 <[email protected]>