Congestion avoidance prevents a network from being overloaded using a packet discarding policy. Congestion management ensures that high-priority services are preferentially processed based on the specified packet scheduling sequence.
On a traditional network, quality of service (QoS) is affected by network congestion. Congestion means the low data forwarding rate and delay resulting from insufficient network resources. Congestion results in delay of packet transmission, low throughput rate, and high resource consumption. Congestion frequently occurs in a complex networking environment where packet transmission and provision of various services are both required.
Congestion avoidance and congestion management are two flow control mechanisms for resolving congestion on a network.
Congestion Avoidance
Congestion avoidance is a flow control mechanism. A system configured with congestion avoidance monitors network resources such as queues and memory buffers. When congestion occurs or aggravates, the system discards packets.
The switch uses tail drop to implement congestion avoidance.
Tail drop is the traditional congestion avoidance mechanism that processes all packets equally without classifying the packets into different types. When congestion occurs, packets at the end of a queue are discarded until the congestion problem is solved.
Tail drop causes global TCP synchronization. In tail drop mechanism, all newly arrived packets are dropped when congestion occurs, causing all TCP sessions to simultaneously enter the slow start state and the packet transmission to slow down. Then all TCP sessions restart their transmission at roughly the same time and then congestion occurs again, causing another burst of packet drops, and all TCP sessions enter the slow start state again. The behavior cycles constantly, severely reducing the network resource usage.
By default, an interface uses tail drop.
Congestion Management
When a network is congested intermittently and delay-sensitive services require higher bandwidth than other services, congestion management adjusts the scheduling order of packets.
The device supports the following congestion management features:
PQ scheduling
Priority queuing (PQ) schedules packets in descending order of priority. Packets in queues with a low priority can be scheduled only after all packets in queues with a high priority have been scheduled.
By using PQ scheduling, the device puts packets of delay-sensitive services into queues with higher priorities and packets of other services into queues with lower priorities so that packets of delay-sensitive services are preferentially scheduled.
The disadvantage of PQ is that the packets in lower-priority queues are not processed until all the higher-priority queues are empty. As a result, a congested higher-priority queue causes all lower-priority queues to starve out.
In addition, WRR can dynamically change the time of scheduling packets in queues. For example, if a queue is empty, WRR ignores this queue and starts to schedule the next queue. This ensures efficient use of bandwidth.
WRR scheduling has two disadvantages:
WRR schedules packets based on the number of packets, whereas users concern the bandwidth. When the average packet length in each queue is the same or known, users can obtain the required bandwidth by setting WRR weight values. When the average packet length in each queue is variable, users cannot obtain the required bandwidth by setting WRR weight values.
Delay-sensitive services, such as voice services, cannot be scheduled in a timely manner.
WDRR scheduling
Weighted Deficit Round Robin (WDRR) implementation is similar to WRR implementation.
The difference between WDRR and WRR is as follows: WRR schedules packets based on the number of packets, whereas WDRR schedules packets based on the packet length. If the packet length is too long, WDRR allows the negative weight value so that long packets can be scheduled. In the next round, the queue with the negative weight value is not scheduled until its weight value becomes positive.
WDRR offsets the disadvantages of PQ scheduling and WRR scheduling. That is, in PQ scheduling, packets in queues with lower priorities cannot be scheduled for a long time if congestion occurs, in WRR scheduling, bandwidth is allocated improperly when the packet length of each queue is different or variable.
WDRR cannot schedule delay-sensitive services such as voice services in a timely manner.
When the all the queues participating in WDRR scheduling have the same weights, the result of WDRR scheduling is the same as that of DRR scheduling.
