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.

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.

The picture illustrates packets having their priority determined at ingress, here interface e1 and e3. In this example, both packets are forwarded to the same outgoing interface (e2), subject to output queueing. The sections below provides more information on these topics.

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, PCP priority takes precedence over DSCP 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.

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.

For VLAN interfaces, Infix provides support for mapping the Priority Code Point (PCP) to internal priority on ingress, and the reverse on egress.

These ingress-qos and egress-qos settings are done per VLAN, both defaulting to '0'. The example below shows how to keep the PCP priority for packets being routed between two VLAN interfaces.

admin@example:/config/> edit interface e1.10
admin@example:/config/interface/e1.10/> set vlan ingress-qos priority from-pcp 
admin@example:/config/interface/e1.10/> up
admin@example:/config/> edit interface e1.20
admin@example:/config/interface/e1.20/> set vlan egress-qos pcp from-priority 
admin@example:/config/interface/e1.20/> leave
admin@example:/> 

A complex example

The picture below shows a packet flow being subject both to software forwarding and hardware offloading.