3.1 KiB
Quality of Service
On occasion, most networks will experience congestion due to some extraordinary load being placed upon it. If the load is transient, switches and routers may be able to absorb such bursts of traffic by queuing packets in internal memories. However, if the load is sustained over long periods of time, queues will fill up and packets will start to be dropped. When such situations arise, it is the job of the network's Quality of Service (QoS) policy to define which packets to drop and which ones to prioritize, such that critical services remain operational.
Software Forwarded Traffic
For packets which are processed by a CPU, i.e. typically routed traffic, and bridged traffic between interfaces that do not belong to the same hardware switching domain, an nftables container can be used to define a QoS policy.
Hardware Forwarded Traffic
The default QoS policy for flows which are offloaded to a switching ASIC is defined by the hardware defaults of the device in question.
Marvell LinkStreet
This family of devices, sometimes also referred to as SOHO, are
managed by the mv88e6xxx driver in the Linux kernel. While older
chips in this family where limited to 4 output queues per port, this
documentation is only valid for newer generations with 8 output
queues per port.
Default Policy
Queueing
Both layer 2 (VLAN PCP) and layer 3 (IP DSCP) priority marks are considered when selecting the output queue of an incoming frame. PCP to queue mapping is done 1:1. For IP packets, the 3 most significant bits of the DSCP is used to select the queue:
| PCP | DSCP | ⇒ | Queue | Weight |
|---|---|---|---|---|
| 0 | 0-7 | ⇒ | 0 | 1 |
| 1 | 8-15 | ⇒ | 1 | 2 |
| 2 | 16-23 | ⇒ | 2 | 3 |
| 3 | 24-31 | ⇒ | 3 | 6 |
| 4 | 32-39 | ⇒ | 4 | 12 |
| 5 | 40-47 | ⇒ | 5 | 17 |
| 6 | 48-55 | ⇒ | 6 | 25 |
| 7 | 56-63 | ⇒ | 7 | 33 |
For packets containing both a VLAN tag and an IP header, DSCP priority takes precedence over PCP priority. In cases where neither are available, packets are always assigned to queue 0.
Each port's set of 8 egress queues operate on a Weighted Round Robin (WRR) schedule, using the weights listed in the table above. The sum of all weights adds up to 99, meaning that the weight of any given queue is roughly equivalent to the percentage of the available bandwidth reserved for it.
Marking
Any priority marks available on ingress are left unmodified when the frame egresses an output port. In the case when an IP packet ingresses without a VLAN tag, and is to egress with a VLAN tag, its PCP is set to the 3 most significant bits of it. If no priority information is available in the frame on ingress (i.e. untagged non-IP), then packets will egress out of tagged ports with PCP set to 0.