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ALTQ(4) FreeBSD Kernel Interfaces Manual ALTQ(4)

alternate queuing of network packets

options ALTQ

options ALTQ_CBQ
options ALTQ_CODEL
options ALTQ_RED
options ALTQ_RIO
options ALTQ_HFSC
options ALTQ_CDNR
options ALTQ_PRIQ
options ALTQ_FAIRQ

The ALTQ system is a framework which provides several disciplines for queuing outgoing network packets. This is done by modifications to the interface packet queues. See altq(9) for details.

The user interface for ALTQ is implemented by the pfctl(8) utility, so please refer to the pfctl(8) and the pf.conf(5) man pages for a complete description of the ALTQ capabilities and how to use it.

The following options in the kernel configuration file are related to ALTQ operation:

Enable ALTQ.
Build the “Class Based Queuing” discipline.
Build the “Controlled Delay” discipline.
Build the “Random Early Detection” extension.
Build “Random Early Drop” for input and output.
Build the “Hierarchical Packet Scheduler” discipline.
Build the traffic conditioner. This option is meaningless at the moment as the conditioner is not used by any of the available disciplines or consumers.
Build the “Priority Queuing” discipline.
Build the “Fair Queuing” discipline.
Required if the TSC is unusable.
Enable additional debugging facilities.

Note that ALTQ-disciplines cannot be loaded as kernel modules. In order to use a certain discipline you have to build it into a custom kernel. The pf(4) interface, that is required for the configuration process of ALTQ can be loaded as a module.

The driver modifications described in altq(9) are required to use a certain network card with ALTQ. They have been applied to the following hardware drivers: ae(4), age(4), alc(4), ale(4), an(4), aue(4), axe(4), bce(4), bfe(4), bge(4), bxe(4), cas(4), cxgbe(4), dc(4), em(4), epair(4), et(4), fxp(4), gem(4), igb(4), ixgbe(4), jme(4), le(4), liquidio(4), msk(4), mxge(4), my(4), nfe(4), nge(4), npe(4), qlxgb(4), re(4), rl(4), sge(4), sis(4), sk(4), ste(4), stge(4), ti(4), udav(4), vge(4), vr(4), vte(4), and xl(4).

The ndis(4) framework also has support for ALTQ and thus all encapsulated drivers.

The tun(4) and ng_iface(4) pseudo drivers also do support ALTQ.

The vlan(4) driver does not directly support ALTQ, but as packets (mbufs) are passed to the underlying interface, a queue can be defined for the underlying interface, and any packets directed to the queue will be processed at the interface level. An example:

altq on igb0 cbq queue { def aq }
queue def bandwidth 90% cbq (default borrow)
queue aq bandwidth 10Mb cbq

pass in on igb0.10 proto udp all queue aq keep state

pf(4), pf.conf(5), ipfw(8), pfctl(8), altq(9)

The ALTQ system first appeared in March 1997 and found home in the KAME project ( It was imported to FreeBSD in 5.3 .
December 26, 2020 FreeBSD 13.1-RELEASE

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