ALTQ (ALTernate Queueing) is the

network scheduler A network scheduler, also called packet scheduler, queueing discipline (qdisc) or queueing algorithm, is an arbiter on a node in a packet switching communication network. It manages the sequence of network packets in the transmit and receive q ...

for

Berkeley Software Distribution The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berk ...

. ALTQ provides queueing disciplines, and other components related to

quality of service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

(QoS), required to realize resource sharing. It is most commonly implemented on BSD-based routers. ALTQ is included in the base distribution of

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...

, and

DragonFly BSD DragonFly BSD is a free and open-source Unix-like operating system forked from FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and FreeBSD developer between 1994 and 2003, began working on DragonFly BSD in Ju ...

, and was integrated into the pf packet filter of

OpenBSD OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD 1.0. According to the website, the OpenBSD project em ...

but later replaced by a new queueing subsystem (it was deprecated with OpenBSD 5.5 release, and completely removed with 5.6 in 2014). With ALTQ, packets can be assigned to queues for the purpose of bandwidth control. The

scheduler A schedule or a timetable, as a basic time-management tool, consists of a list of times at which possible task (project management), tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order ...

defines the algorithm used to decide which packets get delayed, dropped or sent out immediately. There are five schedulers currently supported in the FreeBSD implementation of ALTQ: * — Class-based Queueing. Queues attached to an interface build a

tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...

, thus each queue can have further child queues. Each queue can have a priority and a bandwidth assigned. Priority mainly controls the time packets take to get sent out, while bandwidth has primarily effects on

throughput Network throughput (or just throughput, when in context) refers to the rate of message delivery over a communication channel, such as Ethernet or packet radio, in a communication network. The data that these messages contain may be delivered ov ...

. * — Controlled Delay. Attempts to combat bufferbloat. * — Fair Queuing. Attempts to fairly distribute bandwidth among all connections. * —

Hierarchical Fair Service Curve The hierarchical fair-service curve (HFSC) is a network scheduling algorithm for a network scheduler proposed by Ion Stoica, Hui Zhang and T. S. Eugene from Carnegie Mellon University at SIGCOMM 1997 It is based on a QoS and CBQ. An implementa ...

. Queues attached to an interface build a tree, thus each queue can have further child queues. Each queue can have a priority and a bandwidth assigned. Priority mainly controls the time packets take to get sent out, while bandwidth has primarily effects on throughput. * — Priority Queueing. Queues are flat attached to the

interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Inte ...

, thus, queues cannot have further child queues. Each queue has a unique priority assigned, ranging from 0 to 15. Packets in the queue with the highest priority are processed first.

References

External links

ALTQ home

in OpenBSD 5.4 and earlier
PF and ALTQ
documentation by the

project
pfSense Documentation
ALTQ Scheduler Types on

pfSense pfSense is a firewall/ router computer software distribution based on FreeBSD. The open source pfSense Community Edition (CE) and pfSense Plus is installed on a physical computer or a virtual machine to make a dedicated firewall/router for a ...

Firewall Firewall may refer to: * Firewall (computing), a technological barrier designed to prevent unauthorized or unwanted communications between computer networks or hosts * Firewall (construction), a barrier inside a building, designed to limit the spre ...

* * * * * * * * * {{DragonFly Network performance Network scheduling algorithms Quality of service Free software programmed in C Operating system technology I/O scheduling BSD software FreeBSD NetBSD DragonFly BSD

See also

References

External links