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205 lines
7.0 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of CoDel</TITLE>
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</HEAD><BODY>
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<H1>CoDel</H1>
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Section: Linux (8)<BR>Updated: 23 May 2012<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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CoDel - Controlled-Delay Active Queue Management algorithm
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>tc qdisc ... codel</B>
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[
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<B>limit</B>
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PACKETS ] [
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<B>target</B>
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TIME ] [
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<B>interval</B>
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TIME ] [
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<B>ecn</B>
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<B>noecn</B>
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] [
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<B>ce_threshold</B>
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TIME ]
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<P>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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CoDel (pronounced "coddle") is an adaptive "no-knobs" active queue management
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algorithm (AQM) scheme that was developed to address the shortcomings of
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RED and its variants. It was developed with the following goals
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in mind:
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<BR> o It should be parameterless.
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<BR> o It should keep delays low while permitting bursts of traffic.
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<BR> o It should control delay.
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<BR> o It should adapt dynamically to changing link rates with no impact on
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utilization.
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<BR> o It should be simple and efficient and should scale from simple to
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complex routers.
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<P>
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<A NAME="lbAE"> </A>
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<H2>ALGORITHM</H2>
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CoDel comes with three major innovations. Instead of using queue size or queue
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average, it uses the local minimum queue as a measure of the standing/persistent queue.
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Second, it uses a single state-tracking variable of the minimum delay to see where it
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is relative to the standing queue delay. Third, instead of measuring queue size
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in bytes or packets, it is measured in packet-sojourn time in the queue.
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<P>
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CoDel measures the minimum local queue delay (i.e. standing queue delay) and
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compares it to the value of the given acceptable queue delay
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<B>target.</B>
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As long as the minimum queue delay is less than
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<B>target</B>
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or the buffer contains fewer than MTU worth of bytes, packets are not dropped.
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Codel enters a dropping mode when the minimum queue delay has exceeded
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<B>target</B>
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for a time greater than
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<B>interval.</B>
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In this mode, packets are dropped at different drop times which is set by a
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control law. The control law ensures that the packet drops cause a linear change
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in the throughput. Once the minimum delay goes below
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<B>target,</B>
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packets are no longer dropped.
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<P>
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Additional details can be found in the paper cited below.
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<P>
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<A NAME="lbAF"> </A>
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<H2>PARAMETERS</H2>
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<A NAME="lbAG"> </A>
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<H3>limit</H3>
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hard limit on the real queue size. When this limit is reached, incoming packets
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are dropped. If the value is lowered, packets are dropped so that the new limit is
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met. Default is 1000 packets.
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<P>
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<A NAME="lbAH"> </A>
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<H3>target</H3>
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is the acceptable minimum standing/persistent queue delay. This minimum delay
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is identified by tracking the local minimum queue delay that packets experience.
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Default and recommended value is 5ms.
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<P>
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<A NAME="lbAI"> </A>
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<H3>interval</H3>
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is used to ensure that the measured minimum delay does not become too stale. The
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minimum delay must be experienced in the last epoch of length
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<B>interval.</B>
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It should be set on the order of the worst-case RTT through the bottleneck to
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give endpoints sufficient time to react. Default value is 100ms.
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<P>
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<A NAME="lbAJ"> </A>
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<H3>ecn | noecn</H3>
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can be used to mark packets instead of dropping them. If
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<B>ecn</B>
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has been enabled,
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<B>noecn</B>
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can be used to turn it off and vice-a-versa. By default,
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<B>ecn</B>
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is turned off.
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<P>
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<A NAME="lbAK"> </A>
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<H3>ce_threshold</H3>
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sets a threshold above which all packets are marked with ECN Congestion
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Experienced. This is useful for DCTCP-style congestion control algorithms that
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require marking at very shallow queueing thresholds.
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<P>
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<P>
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<A NAME="lbAL"> </A>
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<H2>EXAMPLES</H2>
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<BR> # tc qdisc add dev eth0 root codel
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<BR> # tc -s qdisc show
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<BR> qdisc codel 801b: dev eth0 root refcnt 2 limit 1000p target 5.0ms
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interval 100.0ms
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<BR> Sent 245801662 bytes 275853 pkt (dropped 0, overlimits 0 requeues 24)
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<BR> backlog 0b 0p requeues 24
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<BR> count 0 lastcount 0 ldelay 2us drop_next 0us
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<BR> maxpacket 7306 ecn_mark 0 drop_overlimit 0
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<P>
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<BR> # tc qdisc add dev eth0 root codel limit 100 target 4ms interval 30ms ecn
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<BR> # tc -s qdisc show
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<BR> qdisc codel 801c: dev eth0 root refcnt 2 limit 100p target 4.0ms
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interval 30.0ms ecn
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<BR> Sent 237573074 bytes 268561 pkt (dropped 0, overlimits 0 requeues 5)
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<BR> backlog 0b 0p requeues 5
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<BR> count 0 lastcount 0 ldelay 76us drop_next 0us
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<BR> maxpacket 2962 ecn_mark 0 drop_overlimit 0
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<P>
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<P>
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<A NAME="lbAM"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?8+tc">tc</A></B>(8),
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<B><A HREF="/cgi-bin/man/man2html?8+tc-red">tc-red</A></B>(8)
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<P>
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<A NAME="lbAN"> </A>
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<H2>SOURCES</H2>
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o Kathleen Nichols and Van Jacobson, "Controlling Queue Delay", ACM Queue,
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<A HREF="http://queue.acm.org/detail.cfm?id=2209336">http://queue.acm.org/detail.cfm?id=2209336</A>
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<P>
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<A NAME="lbAO"> </A>
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<H2>AUTHORS</H2>
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CoDel was implemented by Eric Dumazet and David Taht. This manpage was written
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by Vijay Subramanian. Please reports corrections to the Linux Networking
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mailing list <<A HREF="mailto:netdev@vger.kernel.org">netdev@vger.kernel.org</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="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">ALGORITHM</A><DD>
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<DT id="5"><A HREF="#lbAF">PARAMETERS</A><DD>
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<DL>
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<DT id="6"><A HREF="#lbAG">limit</A><DD>
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<DT id="7"><A HREF="#lbAH">target</A><DD>
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<DT id="8"><A HREF="#lbAI">interval</A><DD>
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<DT id="9"><A HREF="#lbAJ">ecn | noecn</A><DD>
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<DT id="10"><A HREF="#lbAK">ce_threshold</A><DD>
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</DL>
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<DT id="11"><A HREF="#lbAL">EXAMPLES</A><DD>
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<DT id="12"><A HREF="#lbAM">SEE ALSO</A><DD>
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<DT id="13"><A HREF="#lbAN">SOURCES</A><DD>
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<DT id="14"><A HREF="#lbAO">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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