10.1. Hardware Aggregate Shapers

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 chipset, hardware aggregate shapers are available at egress only. The queues are allocated in a block of eight to each hardware aggregate shaper.

Hardware aggregate shapers are controlled by three parameters:

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.

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10.2. Fair Bandwidth-distribution Algorithm

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 Figure 35.

Figure 35:  Hardware FIR Enforcement for Dynamic Fairness Control 

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

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 is ignored.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.

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10.3. Hardware Aggregate Shaper Scheduler Policy

A hardware aggregate shaper scheduler policy is an alternative to a port scheduler. A hardware aggregate shaper policy can only be assigned at the vport level.

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.

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.

Figure 36 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.

Figure 36:  Vport Bandwidth Distribution 

The generic design in Figure 36 can be mapped into two SLA models as shown in Figure 37. Both SLA models show two subscriber categories for simplicity, but this model can be extended to any number of subscriber categories.

Figure 37:  SLA Models  

The SLA model (a) in Figure 37 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 Figure 37 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.

The hardware aggregate scheduler policy implements a congestion threshold that is used to trigger software recalculation. If the overall offered load for a given 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 Figure 37 (a).

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10.4. Interactions with Other QoS Features

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