User guide

QoS implementation for DIFFSERV QOS
7-11
iMG/RG Software Reference Manual (Quality of Service)
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.