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