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81 lines
3.1 KiB
Markdown
81 lines
3.1 KiB
Markdown
Quality of Service
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==================
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On occasion, most networks will experience congestion due to some
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extraordinary load being placed upon it. If the load is transient,
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switches and routers may be able to absorb such bursts of traffic by
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queuing packets in internal memories. However, if the load is
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sustained over long periods of time, queues will fill up and packets
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will start to be dropped. When such situations arise, it is the job of
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the network's Quality of Service (QoS) policy to define _which_
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packets to drop and which ones to prioritize, such that critical
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services remain operational.
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## Software Forwarded Traffic
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For packets which are processed by a CPU, i.e. typically routed
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traffic, and bridged traffic between interfaces that do not belong to
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the same hardware switching domain, an [nftables container][1] can be
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used to define a QoS policy.
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## Hardware Forwarded Traffic
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The default QoS policy for flows which are offloaded to a switching
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ASIC is defined by the hardware defaults of the device in question.
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### Marvell LinkStreet
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This family of devices, sometimes also referred to as _SOHO_, are
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managed by the `mv88e6xxx` driver in the Linux kernel. While older
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chips in this family where limited to 4 output queues per port, this
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documentation is _only_ valid for newer generations with 8 output
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queues per port.
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#### Default Policy
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##### Queueing
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Both layer 2 ([VLAN PCP][2]) and layer 3 ([IP DSCP][3]) priority marks
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are considered when selecting the output queue of an incoming
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frame. PCP to queue mapping is done 1:1. For IP packets, the 3 most
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significant bits of the DSCP is used to select the queue:
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| PCP | DSCP | ⇒ | Queue | Weight |
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|----:|------:|---|------:|-------:|
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| 0 | 0-7 | ⇒ | 0 | 1 |
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| 1 | 8-15 | ⇒ | 1 | 2 |
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| 2 | 16-23 | ⇒ | 2 | 3 |
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| 3 | 24-31 | ⇒ | 3 | 6 |
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| 4 | 32-39 | ⇒ | 4 | 12 |
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| 5 | 40-47 | ⇒ | 5 | 17 |
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| 6 | 48-55 | ⇒ | 6 | 25 |
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| 7 | 56-63 | ⇒ | 7 | 33 |
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For packets containing both a VLAN tag and an IP header, DSCP priority
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takes precedence over PCP priority. In cases where neither are
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available, packets are always assigned to queue 0.
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Each port's set of 8 egress queues operate on a Weighted Round Robin
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([WRR][4]) schedule, using the weights listed in the table above. The
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sum of all weights adds up to 99, meaning that the weight of any given
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queue is roughly equivalent to the percentage of the available
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bandwidth reserved for it.
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##### Marking
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Any priority marks available on ingress are left unmodified when the
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frame egresses an output port. In the case when an IP packet ingresses
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_without_ a VLAN tag, and is to egress _with_ a VLAN tag, its PCP is
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set to the 3 most significant bits of it. If no priority information
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is available in the frame on ingress (i.e. untagged non-IP), then
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packets will egress out of tagged ports with PCP set to 0.
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[1]: container.md#application-container-nftables
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[2]: https://en.wikipedia.org/wiki/IEEE_802.1Q
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[3]: https://en.wikipedia.org/wiki/Differentiated_services
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[4]: https://en.wikipedia.org/wiki/Weighted_round_robin
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