175 lines
5.3 KiB
HTML
175 lines
5.3 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of CBS</TITLE>
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</HEAD><BODY>
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<H1>CBS</H1>
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Section: Linux (8)<BR>Updated: 18 Sept 2017<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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CBS - Credit Based Shaper (CBS) Qdisc
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>tc qdisc ... dev</B>
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dev
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<B>parent</B>
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classid
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<B>[ handle</B>
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major:
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<B>] cbs idleslope</B>
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idleslope
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<B>sendslope</B>
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sendslope
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<B>hicredit</B>
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hicredit
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<B>locredit</B>
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locredit
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<B>[ offload</B>
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0|1
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<B>]</B>
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<P>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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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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<P>
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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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<P>
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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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<B><A HREF="/cgi-bin/man/man2html?8+mqprio">mqprio</A>(8).</B>
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<P>
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<A NAME="lbAE"> </A>
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<H2>PARAMETERS</H2>
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<DL COMPACT>
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<DT id="1">idleslope<DD>
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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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<P>
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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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<P>
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<DT id="2">sendslope<DD>
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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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<P>
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sendslope = idleslope - port_transmit_rate
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<P>
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<DT id="3">hicredit<DD>
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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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<P>
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hicredit = max_interference_size * (idleslope / port_transmit_rate)
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<P>
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<DT id="4">locredit<DD>
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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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<P>
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locredit = max_frame_size * (sendslope / port_transmit_rate)
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<P>
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<DT id="5">offload<DD>
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When
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<B>offload</B>
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is 1,
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<B><A HREF="/cgi-bin/man/man2html?8+cbs">cbs</A>(8)</B>
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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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<P>
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</DL>
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<A NAME="lbAF"> </A>
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<H2>EXAMPLES</H2>
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<P>
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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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<B><A HREF="/cgi-bin/man/man2html?8+mqprio">mqprio</A>(8),</B>
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and configure it like this:
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<P>
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# tc qdisc add dev eth0 handle 100: parent root mqprio num_tc 3 \
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<TT> </TT>map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \<BR>
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<TT> </TT>queues 1@0 1@1 2@2 \<BR>
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<TT> </TT>hw 0<BR>
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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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# tc qdisc replace dev eth0 parent 100:4 cbs \
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<TT> </TT>locredit -1470 hicredit 30 sendslope -980000 idleslope 20000<BR>
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<P>
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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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<P>
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<A NAME="lbAG"> </A>
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<H2>AUTHORS</H2>
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Vinicius Costa Gomes <<A HREF="mailto:vinicius.gomes@intel.com">vinicius.gomes@intel.com</A>>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="6"><A HREF="#lbAB">NAME</A><DD>
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<DT id="7"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="8"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="9"><A HREF="#lbAE">PARAMETERS</A><DD>
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<DT id="10"><A HREF="#lbAF">EXAMPLES</A><DD>
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<DT id="11"><A HREF="#lbAG">AUTHORS</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:06:17 GMT, March 31, 2021
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</BODY>
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</HTML>
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