Allied-Telesis AT-iMG624A Router Manual PDF (Setup & Configuration Guide)

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•

Priority Queuing.

•

WFQ2+ Queuing.

•

Priority Queuing and Shaping.

•

WFQ2+ Queuing and Shaping.

•

Shaping and Algorithmic Dropper.

•

Priority Queuing and Algorithmic Dropper.

•

WFQ2+ Queuing and Algorithmic Dropper.

•

Priority Queuing and Shaping and Algorithmic Dropper.

•

WFQ2+ Queuing and Shaping and Algorithmic Dropper.

The task of qualifying packets arriving on the incoming interfaces to different priority streams is handled by the

Classifier device. The Classifier and Meter devices associate a

scheduling priority

and a

drop priority

value respec-

tively, with the incoming buffer. The service provided by the

Scheduler

to a packet is determined by these values.

•

Scheduling Priority

: This value is used by the Scheduler to identify different priority streams to provide appro-

priate scheduling behavior. The Classifier sets the configured scheduling priority for a traffic stream match-

ing a classification rule. Multiple traffic streams may map to the same scheduling priority. At present 8 levels

(values 0 to 7) of scheduling priority are defined. The value 7 is the highest priority and 0 is the lowest pri-

ority.

•

Drop Priority

: The Scheduler implements Algorithmic Dropper functionality, which when configured drops

packets selectively during congestion conditions in Scheduler queues.

The packets with higher drop priority have a higher probability of getting dropped than lower priority ones.

The Drop priority value is

set by the Meter

. The Classifier is used to set the Drop priority if the Meter has not

been configured.

The following diagram provides an overview of the functionality provided by the Scheduler:

FIGURE 7-3

Overview of Scheduler Functionality

Packet

Packet Queue of Size 32

Action in Terms of Service Discipline

Service Discipline of

Priority Queuing

WF2Q+

ALD

Packet Desqueue

Shaping

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7.1.3.4.1 Service discipline

The Scheduler Service discipline determines the service provided to packets belonging to distinct scheduling

priority streams. The incoming packets are buffered in multiple queues based on the Scheduling Priority value

(contained in the buffer) associated with the respective packet buffers. These packets are then selected for

transmission, based on the configured Service Discipline (the packet selection algorithm).

There are two types of service disciplines that can be used for transmission.

•

Priority Queuing

Priority queuing aims at providing better treatment to higher priority traffic as compared to lower priority traf-

fic. The Scheduler supports 8 queues, corresponding to the scheduling priority values (0-7), with 7 being the

highest priority and 0 the lowest.

A lower priority queue is only served if the queues having higher priority are not backlogged. This scheme may

cause starvation to low priority traffic packets if there is a continuous flow of high priority packets.

•

WF2Q+ (WF2Qplus)

WF2Q+ is a version of WF2Q (Worst case weighted fair queuing) algorithm. The WF2Q+ algorithm requires

participating queues to have an associated weight value. It distributes the link bandwidth among these queues in

the ratio of their respective configured weights.

A single default queue with a weight of 100 percent is created by default. Packets with scheduling priority 0 are

always enqueued in the default queue. Weight values may be configured for the other scheduling priority levels

1-7.

As weight values are configured for a scheduling priority level, the weight allocation for the default queue is dec-

remented by the same amount. That is, the total weight allocation for all priority levels is not allowed to exceed

100.

Packets belonging to the scheduling priority level for which a weight was configured, are enqueued in a separate

queue and the bandwidth is allocated for all participating queues as per the configured weights.

Packets with scheduling priority for which no weight allocation is configured are enqueued in the default queue.

7.1.3.4.2 Shaping

The Scheduler device may also be configured in a

Shaping mode

to limit the aggregate outgoing traffic to a cer-

tain rate using a Token Bucket. Rate limiting for individual traffic streams is not supported.

Note:

The Scheduler considers the complete length of a packet for shaping calculations. However, the extra

bytes added by lower layer devices than the Scheduler may not be considered. For example, if the

Scheduler is applied over an Ethernet interface, the padding and FCS are NOT considered. Also, if the

Scheduler is configured over an ATM interface (e.g. PPPoE over RFC1483 over Utopia) the LLC header

and the AAL-5 header and trailer are not considered.

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7.1.3.4.3 Algorithmic dropper

The Scheduler also supports an active queuing mechanism called the

Algorithmic Dropper (ALD)

. ALD uses RED

(Random Early Detection) and WRED (Weighted RED) algorithms for congestion avoidance.

The user can configure the Algorithmic Dropper on any created queue by setting their relevant attributes. This

will lead to the association of an Algorithmic Dropper with that particular queue.

The Dropper will decide at the time of packet enqueuing whether the packet should be enqueued in the queue

or dropped. When there is no congestion, all packets are queued for transmission on the outgoing interface.

When there is congestion or when congestion is about to occur the ALD is used to determine which packets

to drop.

The RED algorithm uses a single set of parameters (ALD profiles) to all packets being enqueued, to decide

whether to drop a packet.

The WRED algorithm uses three sets of parameters (ALD Profiles). A Drop Priority value associated with each

packet is used to determine the ALD profile to use. Based on the profile parameters it is decided whether or

not to drop the packet.

The drop priority value is set by the Meter or the Classifier device (if configured).

7.1.3.4.4 Scheduler Profiles

There are two types of profiles used by the Scheduler:

1.

Scheduler profiles

2.

Algorithmic Dropper Profiles

Scheduler profiles are used to define the following parameters:

•

Type of Service Discipline (WF2Q+ or Priority Queuing)

•

Parameters for configuring the Scheduler for Shaping

•

Values of queue parameters (Algorithmic Dropper profile and queue weights) for WF2Q+ algorithm.

It is possible to create and store multiple profiles. Any one of these profiles can then be used to create multiple

Schedulers, each of which can be applied on a different channel.

•

Algorithmic Dropper configurations are stored as Algorithmic Dropper Profiles. It is possible to create and

store multiple profiles. These profiles can then be associated with a configured queue in a Scheduler profile.

To configure the RED algorithm on a created Scheduler queue, only one ALD profile needs to be set for that

queue; the ALD Green profile attribute needs to be set to an existing ALD profile.

To configure WRED on a created Scheduler queue, three ALD profiles need to be created; one profile needs to

be set for each drop priority corresponding to the Green, Yellow and Red drop priorities.

For the WRED algorithm, ALD profiles should be configured and applied to Scheduler queues in such a way

that packets with a higher drop priority are more likely to be dropped than packets with lower drop priority

value.

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7.1.3.4.5 Scheduling packets

Packets forwarded to the Scheduler are buffered internally in multiple queues based on the scheduling priority

value associated with each packet. If the Algorithmic Dropper is associated with that particular queue then the

Dropper will be invoked, at the time of enqueuing, to decide whether the packet should be enqueued in the

queue or dropped.

The flow chart below summarises the scheduling process for packet enqueuing.

FIGURE 7-4

Scheduling Process for Packet Enqueuing

Enqueue

Get scheduling priority associated with the packet.

Get the queue corresponding with the scheduling priority

Enqueue packet

in the queue

Invoke ALD

Drop the Packet

ALD configured for queue

for given scheduling priority?

Should the packet

be dropped?

Is the queue full

STOP

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7.1.3.4.6 Packet Dequeue

The packets present in various queues are selected based on the configured service discipline and transmitted

to the driver below. As an optimization measure, the Scheduler tries to utilize a low level driver queue fully by

pushing as many packets as possible (limited by the driver queue size) to the driver. The flow chart below sum-

marises the scheduling process for packet dequeuing.

FIGURE 7-5

Scheduling Process for Packet Dequeuing

7.1.3.4.7 Configuring the Scheduler

This section explains the basic steps to follow in order to configure the Scheduler.

To configure the Scheduler, use the following CLI commands:

1.

Create a Scheduler profile using the command scheduler add profile {wf2qplus | priority}

Dequeue

Select a packet according to the configured service discipline,

for transmission to the driver

Register for a timer for duration

in which enough tokens can be

accumulated to transmit the packet

Transmit packet

to the driver

Consume number

of tokens according

to the packet size

STOP

Packet found

Perform

shaping?

Enough

tokens?

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