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Fix 2 typos on the man page of the CBS qdisc. Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com> Reviewed-by: Simon Horman <simon.horman@netronome.com> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
125 lines
3.8 KiB
Groff
125 lines
3.8 KiB
Groff
.TH CBS 8 "18 Sept 2017" "iproute2" "Linux"
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.SH NAME
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CBS \- Credit Based Shaper (CBS) Qdisc
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.SH SYNOPSIS
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.B tc qdisc ... dev
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dev
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.B parent
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classid
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.B [ handle
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major:
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.B ] cbs idleslope
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idleslope
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.B sendslope
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sendslope
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.B hicredit
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hicredit
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.B locredit
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locredit
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.B [ offload
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0|1
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.B ]
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.SH DESCRIPTION
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The CBS (Credit Based Shaper) qdisc implements the shaping algorithm
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defined by the IEEE 802.1Q-2014 Section 8.6.8.2, which applies a well
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defined rate limiting method to the traffic.
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This queueing discipline is intended to be used by TSN (Time Sensitive
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Networking) applications, the CBS parameters are derived directly by
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what is described by the Annex L of the IEEE 802.1Q-2014
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Specification. The algorithm and how it affects the latency are
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detailed there.
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CBS is meant to be installed under another qdisc that maps packet
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flows to traffic classes, one example is
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.BR mqprio(8).
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.SH PARAMETERS
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.TP
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idleslope
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Idleslope is the rate of credits that is accumulated (in kilobits per
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second) when there is at least one packet waiting for transmission.
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Packets are transmitted when the current value of credits is equal or
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greater than zero. When there is no packet to be transmitted the
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amount of credits is set to zero. This is the main tunable of the CBS
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algorithm and represents the bandwidth that will be consumed.
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Note that when calculating idleslope, the entire packet size must be
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considered, including headers from all layers (i.e. MAC framing and any
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overhead from the physical layer), as described by IEEE 802.1Q-2014
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section 34.4.
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As an example, for an ethernet frame carrying 284 bytes of payload,
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and with no VLAN tags, you must add 14 bytes for the Ethernet headers,
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4 bytes for the Frame check sequence (CRC), and 20 bytes for the L1
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overhead: 12 bytes of interpacket gap, 7 bytes of preamble and 1 byte
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of start of frame delimiter. That results in 322 bytes for the total
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packet size, which is then used for calculating the idleslope.
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.TP
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sendslope
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Sendslope is the rate of credits that is depleted (it should be a
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negative number of kilobits per second) when a transmission is
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occurring. It can be calculated as follows, (IEEE 802.1Q-2014 Section
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8.6.8.2 item g):
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sendslope = idleslope - port_transmit_rate
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.TP
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hicredit
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Hicredit defines the maximum amount of credits (in bytes) that can be
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accumulated. Hicredit depends on the characteristics of interfering
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traffic, 'max_interference_size' is the maximum size of any burst of
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traffic that can delay the transmission of a frame that is available
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for transmission for this traffic class, (IEEE 802.1Q-2014 Annex L,
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Equation L-3):
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hicredit = max_interference_size * (idleslope / port_transmit_rate)
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.TP
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locredit
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Locredit is the minimum amount of credits that can be reached. It is a
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function of the traffic flowing through this qdisc (IEEE 802.1Q-2014
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Annex L, Equation L-2):
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locredit = max_frame_size * (sendslope / port_transmit_rate)
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.TP
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offload
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When
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.B offload
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is 1,
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.BR cbs(8)
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will try to configure the network interface so the CBS algorithm runs
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in the controller. The default is 0.
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.SH EXAMPLES
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CBS is used to enforce a Quality of Service by limiting the data rate
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of a traffic class, to separate packets into traffic classes the user
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may choose
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.BR mqprio(8),
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and configure it like this:
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.EX
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# tc qdisc add dev eth0 handle 100: parent root mqprio num_tc 3 \\
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map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \\
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queues 1@0 1@1 2@2 \\
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hw 0
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.EE
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.P
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To replace the current queuing disciple by CBS in the current queueing
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discipline connected to traffic class number 0, issue:
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.P
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.EX
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# tc qdisc replace dev eth0 parent 100:4 cbs \\
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locredit -1470 hicredit 30 sendslope -980000 idleslope 20000
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.EE
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These values are obtained from the following parameters, idleslope is
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20mbit/s, the transmission rate is 1Gbit/s and the maximum interfering
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frame size is 1500 bytes.
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.SH AUTHORS
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Vinicius Costa Gomes <vinicius.gomes@intel.com>
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