H3C Technologies H3C S3100 Series Switches - Manual (Page 399)

Operation Manual – QoS
H3C S3100-52P Ethernet Switch Chapter 1 QoS Configuration
1-6
1.1.7 Queue Scheduling
When the network is congested, the problem that many packets compete for resources
must be solved, usually through queue scheduling.
In the following section, strict priority (SP) queueing and weighted round robin (WRR)
queueing are introduced.
1) SP
queueing
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queue Dequeue
queue 7
queue 6
queue 5
queue 4
queue 3
queue 2
queue 1
queue 0
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queue Dequeue
queue 7
Packets sent via this
interface Packets sent
Sending queue Dequeue
queue 7
queue 6
queue 5
queue 4
queue 3
queue 2
queue 1
queue 0
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queue Dequeue
queue 7
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queue Dequeue
queue 7
queue 6
queue 5
queue 4
queue 3
queue 2
queue 1
queue 0
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queue Dequeue
queue 7
Packets sent via this
interface Packets sent
Sending queue Dequeue
queue 7
queue 6
queue 5
queue 4
queue 3
queue 2
queue 1
queue 0
Figure 1-4
Diagram for SP queueing
SP queue-scheduling algorithm is specially designed for critical service applications.
An important feature of critical services is that they demand preferential service in
congestion in order to reduce the response delay. Assume that there are eight output
queues on the port and the preferential queue classifies the eight output queues on the
port into eight classes, which are queue7, queue6, queue5, queue4, queue3, queue2,
queue1, and queue0. Their priorities decrease in order.
In queue scheduling, SP sends packets in the queue with higher priority strictly
following the priority order from high to low. When the queue with higher priority is
empty, packets in the queue with lower priority are sent. You can put critical service
packets into the queues with higher priority and put non-critical service (such as e-mail)
packets into the queues with lower priority. In this case, critical service packets are sent
preferentially and non-critical service packets are sent when critical service groups are
not sent.
The disadvantage of SP queue is that: if there are packets in the queues with higher
priority for a long time in congestion, the packets in the queues with lower priority will be
“starved” because they are not served.
2) WRR
queueing