Hierarchical Quality of Service (HQoS) Overview

A profile configuration defines the QoS profile that is attached to either a Subscriber interface or an L3 interface.

Profile maps the following QoS constructs to a Subscriber or an L3 interface:

Classifiers assign the class to which a packet belongs. BA classification is performed on the ingress and maps incoming packet codepoint to a predefined class. BA Classification relies upon markings (that is, codepoint) placed in the headers of incoming packets:

Classifier configuration has the following guidelines and limitations:

Multifield (MF) classifiers assign the class to which a packet belongs based on multiple fields. Unlike the BA classifier, where only CoS fields are used for classification, the MF classifier additionally uses the following fields:

The actions supported by a Multifield Classifier are:

The Multifield Classifiers can be bound globally (global.qos.global.config) or via QoS profile (global.qos.profile.config). The global Multifield Classifier applies to all traffic from any instance or interface. The Multifield Classifier assigned via the QoS profile applies only to ingress traffic received on the interface where the profile is bound to it.

The Multifield Classifier is processed after BA classification which allows it to match on selected class from BA classification or to change the assigned class by more granular match conditions. Both classification stages (BA and MF) are optional, they can be combined or used alone, controlled by configuration.

Multifield Classifiers cannot be bound to MPLS core interfaces. Therefore, the downstream traffic (from core to subscriber) should be classified via global Multifield Classifier, while upstream traffic (from subscriber to core) can be classified via Multifield Classifier from the QoS profile, which is instantiated per subscriber with an implicit match on ingress logical interface (InLIF).

Policer defines the rate at which certain applications can access the hardware resource. So as to rate-limit the traffic from an application, the policer hard-drops the unwanted packets on the ingress side.

In RBFS, policers support the “two-rate, three-color” type in a 4-level cascaded mode. This means that each policer level has two rates (CIR and PIR) and three colors (green, yellow and red) with two token buckets as shown below.

This means that traffic below CIR is marked green. Traffic above CIR but below PIR is yellow and above PIR is red. Traffic marked red will be dropped. Traffic marked yellow can be demoted by changing ToS, TC, or EXP using remark map.

In 4-level cascade mode, unused tokens can be passed from higher priority levels to lower priorities where level 1 has the highest and level 4 has the lowest priority as shown in the figure below.

Therefore a lower level configured with CIR 0 can still serve traffic if higher priority levels are not consuming all available tokens.

The available tokens per level are calculated by remaining CIR credits from upper levels and additional credits based on configured CIR per level. Per default, the resulting tokens are not limited. The optional max CIR rate attribute limits tokens from CIR and upper levels. Let us assume levels 1 and 2 are both configured with a CIR of 2Mbps. Without max CIR or max PIR (default behaviour) level 2 can reach up to 4Mbps (level 1 CIR/PIR plus level 2 CIR/PIR). This can be limited by max CIR (for example, 3Mbps). If both level 1 and level 2 have a committed information rate (CIR) of 2Mbps, then level 2 can reach a maximum of 4Mbps (which is the sum of level 1 CIR and level 2 CIR) without any consideration for the maximum CIR. However, the maximum CIR is not relevant for level 1.

In columns 2 through 4 of the preceding example table, L1 consumes only 1Mbps of the available 4Mbps and passes the remaining 3Mbps to L2 which adds 6m based on their own configured CIR resulting in 9m.

In columns 5 through 7 of the preceding example table, L1 consumes only 1Mbps of the available 4Mbps and passes the remaining 3m to L2 which adds 6Mbps based on their own configured CIR resulting in 9Mbps. But because of the CIR limit set to 8Mbps, only 8Mbps of 9Mbps can be used at this level. The remaining 1Mbps is now passed to L3 which does not add additional CIR-based credits. In both examples, L4 would be able to reach up to 10Mbps if upper levels are not consuming credits.

The RBFS RADIUS services support dynamic policer rate updates. The dynamic policer rate when configured affects only the QoS instance of the corresponding subscriber but not other subscribers.

Queuing helps to drop unwanted traffic in advance at the ingress side in case of congestion. This is to ensure bandwidth for qualified services.

RBFS supports the following queueing techniques:

A Queue Group defines the Queue bundle. A Queue Group contains bundle of either 1 or 4 or 8 queues.

A scheduler configuration defines scheduler parameters such as type and shaping rate. The shaping rate defined for a scheduler applies to queue(s) associated with it. NOTE: For 1 Queue in Queue-Group, scheduler is not applicable.

The following scheduler types are supported:

The following SP scheduler types are supported:

Hierarchical QOS (HQoS) on RBFS is implemented by connecting or chaining queues to scheduler elements (Q — > SE), scheduler elements to each other (SE — > SE), and scheduler elements to ports (SE — > PORT). Each scheduler element can have different child connection points based on types described in the section Scheduler.

This means that sched_0 in the example below is not scheduling between the attached queues, but between the different child connection points SP0 to SP3. The scheduler element sched_0 cannot differentiate between Q1 and Q2 in this example because both are connected to SP2.

Without priority propagation, each scheduler element can have multiple child connection points but just one parent connection point. Therefore traffic leaving a scheduler element cannot be differentiated by the parent scheduling element. The parent scheduler element sched_1 receives the traffic from sched_0 on the selected child connection point. As already mentioned scheduling within a scheduler element happens between child connection points. Second, a scheduler element has only one parent connection point which can be connected to a child connection point of another scheduler element (output of sched_0 → input of sched_1). This results in the situation that all traffic from this SE is handled equally regardless of the queue. This may lead to dropped priority traffic like voice or control traffic in case of congestion in parent elements. For example, if sched_1 has a shaping rate lower than the one of sched_0, it will drop traffic unaware of its original priority.

This problem is addressed with priority propagation which is enabled by default.

With priority propagation, the scheduler elements operate in a dual-flow mode with high and low-priority flows. The credits generated from the physical interface will be consumed by all attached high-priority flows first and only the remaining credits will be available for low-priority flows. In this mode, an implicit FQ element is created for each scheduler element. All queues assigned to low-priority flow will be attached to this element.

An additional composite option of the scheduler element allows also the differentiation between multiple low-priority queues if required. This composite type is created implicitly and does not need to be configured.

Without priority propagation enabled, each scheduler element consumes only one scheduler resource compared to two elements if enabled. The composite type consumes three scheduler elements.

With priority propagation disabled, all traffic is considered a high-priority flow.

Now for each queue, we can select if connected to high-priority or low-priority flow where high-priority flow is selected per default if not explicitly mentioned.

Assuming the example as before but with priority propagation and Q0 assigned to low-priority flow and Q1 - Q3 assigned to high-priority flow.

The figure below shows a typical multi-level QoS configuration without priority propagation on the left and with priority propagation on the right side.

The credits generated from the physical interface will be consumed by high-priority flow first and the remaining credits will be available for low-priority flow. The high-flow traffic at any one element is scheduled based on the type and connection point. Between schedulers, it depends on how they are connected to the parent scheduling element. Per default all levels there is FQ for low and FQ for high-priority flows. The port scheduler is also FQ.

In this mode, each shaper supports two different rates for low and high-priority where the actual shaper rate is the sum of low and high-priority rates. If the low-priority rate is zero, this flow is only served if the high-priority flow is not consuming all credits. An example might be a high rate of 9Mbps and a low rate of 1Mbps which results in 10Mbps for low-priority flow if high-priority flow is not consuming any packets but at least 1Mbps is ensured.

The following example shows a typical access service provider configuration with priority propagation enabled with and without composite type.

Shaper is used to rate-limit the traffic at the egress. In RBFS, shapers can be attached to both Queue and Scheduler.

A shaper configuration defines the shaping rate in Kilo-bits-per-seconds (Kbps).

Scheduler Map defines the set of relationships between parents and children in egress scheduling hierarchy. A child in a Scheduler Map configuration can be either Queue or Scheduler. A parent in a Scheduler Map configuration can be either a Port or Scheduler.

Child-queue or child-scheduler in a scheduler map configuration is connected to the parent-scheduler at “connection point (CP)“. Connection point configuration also has “weight” associated with it if the parent has a WFQ scheduler corresponding to that connection point. The valid connection point value for a child to connect to parent WFQ/FQ scheduler is no_priority and to connect to parent SP/Hybrid scheduler is between strict_priority_0 to strict_priority_3 (based on a number of Strict Priority points in parent scheduler).

Connection Types

There are five connection types in a scheduler map entry:

The packet markers set the codepoint in a packet to a particular value, adding the marked packet to a particular behavior aggregate. When the marker changes the codepoint in a packet, it "remarks" the packet. The codepoint in a packet can be IPv4-ToS, IPv6-TC, MPLS-EXP, or IEEE 802.1p field.

The following remarking options are supported in RBFS:

IPv4/v6 and IEEE 802.1p remark-map are applied on an interface - subscriber-ifl or l3ifl using Profile Name.

MPLS-IPv4/v6 remark-map is applied either globally or per instance (to support multiple VPN marking schemes) using Remark-Map Name.

In RBFS, remarking can be performed at the ingress or egress:

Here Color is used to set different remark-codepoint for same match-codepoint based on color marked by the Policer (i.e. green or yellow). Color is a mandatory field in remark-map. To set the same remark-codepoint for a match-codepoint irrespective of color, we have to set color as “all”.

IPv4-ToS, IPv6-TC, or MPLS-EXP remarking:

IEEE 802.1p VLAN remarking:

L2BSA L2X VLAN Priority Remarking:

The L2BSA L2X VLAN priority remarking support for the platforms is as follows: Only A10NSP switches support VLAN operations, access-leaf is transparent.

For more information about L2BSA configurations, see the L2BSA User Guide.

The Layer 2 Tunneling Protocol (L2TP) QoS for upstream is similar to any other locally terminated subscriber. The QoS Profile is mapped dynamically via RADIUS for the L2TP subscribers.

The L2TP QoS for Downstream requires IPv4-TOS based BA Classifier which is mapped to L2TP Tunnel. The same can be achieved by attaching l2tp-classifier-name in global QoS configuration.

For Downstream queueing, there is no change. Queueing is applied using QoS Profile similar to locally terminated Subscribers.

The following features are supported for L2TP QoS.

A sample 5-level requirement looks like this.

The figure below shows the diagram along with QoS representing Level-1 to Level-5 Hierarchical scheduling.

The levels 4 and 5 are configured per logical interface (i.e. subscriber-ifl or l3-ifl). Separate scheduler-map representing levels 1 to 3 connectivity shall be statically configured and mapped to the corresponding physical interface (IFP).

The child scheduler in a subscriber scheduler map is connected to the parent scheduler in a physical interface scheduler map using the following way:

The figure below shows the same details as the preceding figure before with the different levels but from the DPU-PON-IFP scheduler-map point of view.

The MPLS HQoS has both UNIFORM and PIPE modes. These modes provide the following functionality:

For the Uniform MPLS mode mapping between IPv4-ToS or IPv6-TC to MPLS-EXP see the table below:

Not all features are necessarily supported on each hardware platform. Refer to the Platform Guide for the features and the sub-features that are or are not supported by each platform.