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Parameter priomap use blank instead of comma to separate bands, update manpage to confirms to this.
188 lines
6.2 KiB
Groff
188 lines
6.2 KiB
Groff
.TH PRIO 8 "16 December 2001" "iproute2" "Linux"
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.SH NAME
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PRIO \- Priority 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 | root) [ handle
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major:
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.B ] prio [ bands
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bands
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.B ] [ priomap
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band band band...
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.B ] [ estimator
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interval timeconstant
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.B ]
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.SH DESCRIPTION
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The PRIO qdisc is a simple classful queueing discipline that contains
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an arbitrary number of classes of differing priority. The classes are
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dequeued in numerical descending order of priority. PRIO is a scheduler
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and never delays packets - it is a work-conserving qdisc, though the qdiscs
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contained in the classes may not be.
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Very useful for lowering latency when there is no need for slowing down
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traffic.
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.SH ALGORITHM
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On creation with 'tc qdisc add', a fixed number of bands is created. Each
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band is a class, although is not possible to add classes with 'tc qdisc
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add', the number of bands to be created must instead be specified on the
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command line attaching PRIO to its root.
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When dequeueing, band 0 is tried first and only if it did not deliver a
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packet does PRIO try band 1, and so onwards. Maximum reliability packets
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should therefore go to band 0, minimum delay to band 1 and the rest to band
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2.
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As the PRIO qdisc itself will have minor number 0, band 0 is actually
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major:1, band 1 is major:2, etc. For major, substitute the major number
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assigned to the qdisc on 'tc qdisc add' with the
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.B handle
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parameter.
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.SH CLASSIFICATION
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Three methods are available to PRIO to determine in which band a packet will
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be enqueued.
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.TP
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From userspace
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A process with sufficient privileges can encode the destination class
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directly with SO_PRIORITY, see
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.BR socket(7).
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.TP
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with a tc filter
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A tc filter attached to the root qdisc can point traffic directly to a class
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.TP
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with the priomap
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Based on the packet priority, which in turn is derived from the Type of
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Service assigned to the packet.
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.P
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Only the priomap is specific to this qdisc.
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.SH QDISC PARAMETERS
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.TP
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bands
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Number of bands. If changed from the default of 3,
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.B priomap
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must be updated as well.
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.TP
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priomap
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The priomap maps the priority of
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a packet to a class. The priority can either be set directly from userspace,
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or be derived from the Type of Service of the packet.
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Determines how packet priorities, as assigned by the kernel, map to
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bands. Mapping occurs based on the TOS octet of the packet, which looks like
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this:
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.nf
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0 1 2 3 4 5 6 7
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+---+---+---+---+---+---+---+---+
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|PRECEDENCE | TOS |MBZ|
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+---+---+---+---+---+---+---+---+
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.fi
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The four TOS bits (the 'TOS field') are defined as:
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.nf
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Binary Decimal Meaning
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-----------------------------------------
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1000 8 Minimize delay (md)
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0100 4 Maximize throughput (mt)
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0010 2 Maximize reliability (mr)
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0001 1 Minimize monetary cost (mmc)
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0000 0 Normal Service
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.fi
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As there is 1 bit to the right of these four bits, the actual value of the
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TOS field is double the value of the TOS bits. Tcpdump -v -v shows you the
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value of the entire TOS field, not just the four bits. It is the value you
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see in the first column of this table:
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.nf
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TOS Bits Means Linux Priority Band
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------------------------------------------------------------
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0x0 0 Normal Service 0 Best Effort 1
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0x2 1 Minimize Monetary Cost 1 Filler 2
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0x4 2 Maximize Reliability 0 Best Effort 1
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0x6 3 mmc+mr 0 Best Effort 1
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0x8 4 Maximize Throughput 2 Bulk 2
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0xa 5 mmc+mt 2 Bulk 2
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0xc 6 mr+mt 2 Bulk 2
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0xe 7 mmc+mr+mt 2 Bulk 2
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0x10 8 Minimize Delay 6 Interactive 0
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0x12 9 mmc+md 6 Interactive 0
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0x14 10 mr+md 6 Interactive 0
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0x16 11 mmc+mr+md 6 Interactive 0
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0x18 12 mt+md 4 Int. Bulk 1
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0x1a 13 mmc+mt+md 4 Int. Bulk 1
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0x1c 14 mr+mt+md 4 Int. Bulk 1
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0x1e 15 mmc+mr+mt+md 4 Int. Bulk 1
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.fi
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The second column contains the value of the relevant
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four TOS bits, followed by their translated meaning. For example, 15 stands
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for a packet wanting Minimal Monetary Cost, Maximum Reliability, Maximum
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Throughput AND Minimum Delay.
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The fourth column lists the way the Linux kernel interprets the TOS bits, by
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showing to which Priority they are mapped.
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The last column shows the result of the default priomap. On the command line,
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the default priomap looks like this:
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1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
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This means that priority 4, for example, gets mapped to band number 1.
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The priomap also allows you to list higher priorities (> 7) which do not
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correspond to TOS mappings, but which are set by other means.
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This table from RFC 1349 (read it for more details) explains how
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applications might very well set their TOS bits:
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.nf
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TELNET 1000 (minimize delay)
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FTP
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Control 1000 (minimize delay)
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Data 0100 (maximize throughput)
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TFTP 1000 (minimize delay)
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SMTP
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Command phase 1000 (minimize delay)
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DATA phase 0100 (maximize throughput)
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Domain Name Service
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UDP Query 1000 (minimize delay)
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TCP Query 0000
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Zone Transfer 0100 (maximize throughput)
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NNTP 0001 (minimize monetary cost)
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ICMP
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Errors 0000
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Requests 0000 (mostly)
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Responses <same as request> (mostly)
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.fi
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.SH CLASSES
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PRIO classes cannot be configured further - they are automatically created
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when the PRIO qdisc is attached. Each class however can contain yet a
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further qdisc.
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.SH BUGS
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Large amounts of traffic in the lower bands can cause starvation of higher
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bands. Can be prevented by attaching a shaper (for example,
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.BR tc-tbf(8)
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to these bands to make sure they cannot dominate the link.
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.SH AUTHORS
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Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>, J Hadi Salim
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<hadi@cyberus.ca>. This manpage maintained by bert hubert <ahu@ds9a.nl>
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