Hardware-assisted hierarchical QoS

Hardware-assisted HQoS is based on hardware aggregate shapers, which provide enforcement of aggregate rates for a group of queues in the hardware. In the FP4-based and later chipset, hardware aggregate shapers are available at egress only.

Hardware aggregate shapers are controlled by four parameters:

rate

sets the aggregate rate of the hardware shaper

burst-limit

sets the aggregate burst for the specific hardware aggregate shaper

adaptation-rule

sets how the administrative rates are translated into hardware operational values

queue-set size

sets the number of queues allocated as a block to the hardware aggregate shaper

Traffic is scheduled from the queues in a strict priority order based on the hardware scheduling priority assigned to each queue. This can be controlled using the sched-class command. There are 6 scheduling-classes available.

Hardware aggregate shapers do not need software-based HQoS to modify the oper-PIR of the queues to enforce an aggregate rate. This results in a significantly faster reaction time compared to traditional HQoS based on virtual schedulers.

The following examples show the configuration of a hardware aggregate shaper on a subscriber profile. The configuration is similar to traditional QoS configuration.

[ex:/configure subscriber-mgmt]
A:admin@node-2# info
    sub-profile "trialSub" {
        egress {
            qos {
                agg-rate {
                    rate 1500000
                    burst-limit auto
                }
            }
        }
    }
    sla-profile "trialSLA" {
        egress {
            qos {
                sap-egress {
                    policy-name "trial-egr"
                }
            }
        }
        ingress {
            qos {
                sap-ingress {
                    policy-name "trial-ingr"
                }
            }
        }

*A:node-2>config>subscr-mgmt# info
----------------------------------------------
        sla-profile "trialSLA" create
            ingress
                qos 2
                exit
            exit
            egress
                qos 2
                exit
            exit
        exit
        sub-profile "trialSub" create
            egress
                agg-rate-limit 1500000
            exit
        exit
----------------------------------------------

Hardware-assisted HQoS introduces a new way to implement queue-weight based bandwidth distribution.

The queue hardware-enforced FIR rate is used to provide weighted fair access between queues assigned to the same hardware scheduling priority (scheduling class) within a hardware aggregate shaper, by dynamically changing the queue's hardware scheduling priority as shown in Hardware FIR enforcement for dynamic fairness control.

Every queue in a hardware aggregate shaper is described by the following parameters:

scheduling class

sets assignment to hardware priorities

aggregate shaper weight

sets the queue fair-share of bandwidth at the specific scheduling class level

burst limit

sets the individual queue burst limit at PIR

FIR burst limit

sets the individual queue burst limit at FIR

The aggregate shaper weight algorithm operates similarly to the virtual-scheduler HQoS algorithm, meaning that the offered load of the queues at the same scheduling class is weighted against the aggregate shaper weight to determine the FIR, which then controls hardware-based scheduling.

A FIR-based algorithm is more efficient and more responsive to dynamic changes in offered load. The PIR is not adjusted, which prevents aggregate-rate underruns in case of a sudden change in an incoming load for some queues.

When hardware aggregate shapers are enabled, CIR as a queue parameter has no impact on the BW allocation algorithm, as in traditional HQoS. The CIR is still used for calculation of the default CBS. The queue-frame-based-accounting setting is also ignored when hardware aggregate shaping is configured.

The following examples show a SAP egress policy configuration. Weight-based bandwidth distribution in this example is only between queue 1 and queue 2. These queues are both assigned to the same scheduling class (sched-class 3), while queue 3 has a higher priority scheduling-class (sched-class 5) and is served with strict priority.

*[ex:/configure qos]
A:admin@node-2# info
    sap-egress "trial-egr" {
        queue 1 {
            agg-shaper-weight 5
            burst-limit 500
            mbs 102400
            sched-class 3
        }
        queue 2 {
            agg-shaper-weight 15
            burst-limit 500
            mbs 102400
            sched-class 3
        }
        queue 3 {
            burst-limit 500
            mbs 102400
            sched-class 5
        }
        fc be {
            queue 1
        }
        fc af {
            queue 2
        }
        fc ef {
            queue 3
        }

*A:node-2>config>qos>sap-egress# info
----------------------------------------------
            queue 1 create
                mbs 100 kilobytes
                burst-limit 500 bytes
                agg-shaper-weight 5
                sched-class 3
            exit
            queue 2 create
                mbs 100 kilobytes
                burst-limit 500 bytes
                agg-shaper-weight 15
                sched-class 3
            exit
            queue 3 create
                mbs 100 kilobytes
                burst-limit 500 bytes
                sched-class 5
            exit
            fc af create
                queue 2
            exit
            fc be create
                queue 1
            exit
            fc ef create
                queue 3
            exit
----------------------------------------------
*A:SR-1s-test>config>qos>sap-egress#

A hardware aggregate shaper scheduler policy is an alternative to a port scheduler. A hardware aggregate shaper policy can be assigned at the Vport level (for subscribers) or at the port level (for SAPs).

A virtual Vport scheduler is added to implement a bandwidth-distribution software algorithm to handle traffic oversubscription by subscriber hardware aggregate shapers at a vport.

The Vport represents a downstream bandwidth constraint and therefore the algorithm enforces a maximum rate, which is an Ethernet on-the-wire rate. This algorithm also implements the logic to distribute the available bandwidth at the Vport to its child hardware aggregate shapers.

Similarly, a port scheduler enforces bandwidth distribution between SAPs attached to the specific hardware aggregate shaper policy assigned to the specific port. For the bandwidth-distribution algorithm to operate effectively, all SAPs attached to it must have an explicit aggregate rate defined.

The algorithm performs a similar function to the existing HQoS port scheduler, however, instead of modifying the oper-PIRs of queues, policers, or schedulers, it modifies the oper-PIR of the hardware aggregate shapers connected to it, and consequently triggers recalculation of the individual queue’s FIR, if required.

Vport or port bandwidth distribution shows the functionality of a hardware aggregate shaper scheduler, including all parameters which the user can set. This architecture allows the user full flexibility to define SLA models.

The generic design in Vport or port bandwidth distribution can be mapped into two SLA models as shown in SLA models using subscriber management as an example. Similar examples can be constructed using SAPs

Both SLA models show two subscriber categories for simplicity, but this model can be extended to any number of subscriber categories.

The SLA model (a) in SLA models shows the overall Vport bandwidth distribution between gold and white categories in a ratio of 8:1 regardless of the number of subscribers in either category, and assuming that traffic passing through queue 7 for both subscriber categories is delay-critical but not significant in terms of volume.

The SLA model (b) in SLA models guarantees that the ratio between any gold and any white subscriber is 8:1. The overall bandwidth distribution between gold and white categories, however, it depends on the number of subscribers in each category.

SLA models are a design goal and must be tested to guarantee the expected behavior. SR OS configuration allows a user to construct other models by combining concepts and parameters in other ways. The resulting behavior is subject to a specific configuration and offered load.

When alternative SLA models are configured, the following considerations apply:

• If a group is used to enforce weighting between different scheduling classes, all queues attached to a specific hardware aggregate shaper must have the same scheduling class. This is because the software algorithm enforcing the hardware aggregate scheduler policy only controls the rate of the hardware aggregate shaper and not the rate of individual queues. If this recommendation is not followed, weights enforced at the group level may contradict weights enforced between queues within a single hardware aggregate shaper.

• Elevation-weight and groups for any set of queues are mutually exclusive.

The hardware aggregate scheduler policy implements a congestion threshold that is used to trigger software recalculation. If the overall offered load for a specific Vport is below this threshold (expressed as a percentage of the Vport aggregate rate), the software recalculation of all parameters is not performed to save CPU resources because it is assumed that all offered traffic passes through.

Similarly to a port scheduler policy, a hardware aggregate rate scheduler policy supports a monitor threshold that detects congestion. The configuration and operation are identical as described in Scheduler QoS policies.

The following example hardware aggregate shaper scheduler policy describes the SLA model from SLA models (a).

[ex:/configure qos hw-agg-shaper-scheduler-policy "trial"]
A:admin@node-2# info
    max-percent-rate 80.0
    group "g1" { }
    sched-class 3 {
        group "g1"
        weight 1
    }
    sched-class 4 {
        group "g1"
        weight 8
    }

[ex:/configure port 1/1/1/c1/1 ethernet access egress]
A:admin@ode-2# info
    virtual-port "dslam-a" {
        hw-agg-shaper-scheduler-policy "trial"
    }

*A:node-2>config>qos>hw-agg-shap-sched-plcy# info
----------------------------------------------
            max-rate percent 80.00
            group "g1" create
            exit
            sched-class 3 group "g1" weight 1
            sched-class 4 group "g1" weight 8
----------------------------------------------

*A:node-2# /configure port 1/1/1/c1/1 ethernet access egress
*A:SR-1s-test>config>port>ethernet>access>egress# info
----------------------------------------------
                    vport "dslam-a" create
                        hw-agg-shaper-scheduler-policy "trial"
                    exit
----------------------------------------------

The FP4-based and later chipsets support all existing QoS features including software-driven HQoS and hardware-assisted HQoS. The following sections describe how these features interact.