Priority qdisc
tc qdisc ... dev dev ( parent classid | root) [ handle major: ] prio [ bands bands ] [ priomap band band band... ] [ estimator interval timeconstant ]
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.
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.
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 socket(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.
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 0 Best Effort 1 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)
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.
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.
Alexey N. Kuznetsov, <[email protected]>, J Hadi Salim <[email protected]>. This manpage maintained by bert hubert <[email protected]>