User guide

QOS QoS implementation for DIFFSERV
iMG/RG Software Reference Manual (Quality of Service)
7-12
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.