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PRIO


Section: Linux (8)
Updated: 16 December 2001
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NAME

PRIO – Priority qdisc
 

SYNOPSIS

tc qdisc … dev

dev
( parent

classid
| root) [ handle

major:
] prio [ bands

bands
] [ priomap

band,band,band...
] [ estimator

interval timeconstant
]


 

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.


 

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
command line 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
handle

parameter.


 

CLASSIFICATION

Three methods are available to PRIO to determine in which band a packet will
be enqueued.


From userspace

A process with sufficient privileges can encode the destination class
directly with SO_PRIORITY, see
tc(7).

with a tc filter

A tc filter attached to the root qdisc can point traffic directly to a class
with the priomap

Based on the packet priority, which in turn is derived from the Type of
Service assigned to the packet.

Only the priomap is specific to this qdisc.
 

QDISC PARAMETERS


bands

Number of bands. If changed from the default of 3,
priomap

must be updated as well.

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:



0 1 2 3 4 5 6 7
+—+—+—+—+—+—+—+—+
| | | |
|PRECEDENCE | TOS |MBZ|
| | | |
+—+—+—+—+—+—+—+—+


The four TOS bits (the ‘TOS field’) are defined as:



Binary Decimal 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


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:



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


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 Monetary 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 command line,
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:



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)




 

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.


 

BUGS

Large amounts of traffic in the lower bands can cause starvation of higher
bands. Can be prevented by attaching a shaper (for example,
tc-tbf(8)

to these bands to make sure they cannot dominate the link.


 

AUTHORS

Alexey N. Kuznetsov, <[email protected]>, J Hadi Salim
<[email protected]>. This manpage maintained by bert hubert <[email protected]>





 

Index



NAME

SYNOPSIS

DESCRIPTION

ALGORITHM

CLASSIFICATION

QDISC PARAMETERS

CLASSES

BUGS

AUTHORS



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