tc-prio(8) -- Linux man page

NAME

SYNOPSIS

DESCRIPTION

Very useful for lowering latency when there is no need for slowing down traffic.

ALGORITHM

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

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 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

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 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:

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

BUGS

AUTHORS