Network scheduler

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For the scheduling of processes, see process scheduler.
Packets queuing in a FIFO (first in, first out) data structure.

On a node in packet switching communication network, a network scheduler, also called packet scheduler, is an arbiter program that manages the sequence of network packets in the transmit and receive queues of the network interface controller, which is a circular data buffer. There are several network schedulers available for the different operating system kernels, that implement many of the existing network scheduling algorithms.

The network scheduler logic decides, in a way similar to statistical multiplexers,[citation needed] which network packet to forward next from the buffer. The buffer works as a queuing system, storing the network packets temporarily until they are transmitted. The buffer space may be divided into different queues, with each of them holding the packets of one flow according to configured packet classification rules; for example, packets can be divided into flows by their source and destination IP addresses. Network scheduling algorithms and their associated settings determine how the network scheduler manages the buffer.

Also, network schedulers are enabling accomplishment of the active queue management and traffic shaping.

Network scheduling algorithms[edit]

In the course of time several network scheduling algorithms (also called queueing discipline, qdisc or queueing algorithm) have been developed. Each of the scheduling algorithms used internally for these queuing disciplines provides specific reordering or dropping of network packets inside various transmit or receive buffers.[1][2] Queuing disciplines are commonly used as attempts to compensate for various networking conditions, like reducing the latency for certain classes of network packets, and are generally used as part of the quality of service (QoS) measures.[3][4][5]

Examples of algorithms suitable for managing network traffic include:

Several of the above have been implemented as Linux kernel modules[16] and are freely available.

Bufferbloat[edit]

Main article: Bufferbloat

Bufferbloat is a phenomenon in packet-switched networks generally, in which excess buffering of packets causes high latency and packet delay variation (also known as jitter), as well as reducing the overall network throughput. When a router device is configured to use excessively large buffers, even very high-speed networks can become practically unusable for many interactive applications like voice calls, chat, and even web surfing.

The CoDel algorithm attempts to reduce this problem by improving upon the RED algorithm.[citation needed] CoDel is less prone to the effects of bufferbloat than the common tail drop disciplines.[citation needed]

Terminology[edit]

  • Active queue management (AQM) – the employment of an arbiter program
  • Network traffic control – an umbrella term for all measures aimed at the control of traffic
  • Traffic shaping – a form of AQM, where the available bandwidth is being limited to match certain use cases
  • Link sharing – this term is used when more individuals or customers share the same Internet connection
  • Traffic prioritizing – a form of AQM, that selectively prioritizes certain network packets (e.g. VoIP-packets)
  • TCP Turbo – by prioritizing ACK-packets on the upload, a slow-down of the download rate of a TCP connection is prevented
  • Bandwidth management – the management of the available bandwidth

Implementations[edit]

Linux kernel[edit]

As the default queuing discipline, Linux kernel uses a FIFO implementation called pfifo_fast.[17]

The currently available network scheduler for the Linux kernel is an integral part of the Linux network stack; it arbiters the ring buffers of all NICs, hence it works on layer 2 of the OSI model and handles Ethernet frames. It manages the transmit and receive buffers of all present NICs.

The user space programs ifconfig and ip enable the system administrator to regulate the buffer sizes: txqueuelen /rxqueuelen; the unit being number of Ethernet frames (regardless of their size) for each device separately. The Linux network stack contains several other buffers, which are not managed by the network scheduler. The overall size of all buffers has been the point of critique by the Bufferbloat project. The project provided a partial solution with CoDel, which was primarily tested in OpenWrt.

Another network scheduler is being developed as part of Netfilter and nftables.[clarification needed]

BSD[edit]

ALTQ is the implementation of a network scheduler for BSDs.

See also[edit]

References[edit]

  1. ^ "Traffic Control HOWTO". tldp.org. Retrieved November 24, 2013.  |chapter= ignored (help)
  2. ^ Saravanan Radhakrishnan (September 30, 1999). "QoS Support in Linux". qos.ittc.ku.edu. Retrieved March 18, 2014.  |chapter= ignored (help)
  3. ^ "Traffic Control HOWTO". tldp.org. Retrieved November 24, 2013.  |chapter= ignored (help)
  4. ^ "Traffic Control HOWTO". tldp.org. Retrieved November 24, 2013.  |chapter= ignored (help)
  5. ^ "Queuing Disciplines: Order of Packet Transmission and Dropping" (PDF). tau.ac.il. October 25, 2006. Retrieved March 18, 2014. 
  6. ^ http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.108.4477&rep=rep1&type=pdf
  7. ^ "DRR Linux kernel network scheduler module". kernel.org. Retrieved 2013-09-07. 
  8. ^ "Heavy-Hitter Filter qdisc". kernel.org. 
  9. ^ "HTB Linux kernel network scheduler module". kernel.org. Retrieved 2013-09-07. 
  10. ^ "QFQ Linux kernel network scheduler module". kernel.org. Retrieved 2013-09-07. 
  11. ^ "Fair Queue packet scheduler committed to Linux kernel 3.12". 
  12. ^ "Network emulator Linux kernel network scheduler module". kernel.org. Retrieved 2013-09-07. 
  13. ^ "Proportional Integral controller Enhanced (PIE)". kernel.org. 
  14. ^ "SFQ Linux kernel network scheduler module". kernel.org. Retrieved 2013-09-07. 
  15. ^ "TBF Linux kernel network scheduler module". kernel.org. Retrieved 2013-09-07. 
  16. ^ "The Linux kernel network scheduler". kernel.org. 2012-12-26. Retrieved 2013-09-07. 
  17. ^ "Linux Advanced Routing and Traffic Control HOWTO". lartc.org. 2012-05-19. Retrieved 2014-09-19.  |chapter= ignored (help)