Dell 53-1002116-01 manual Congestion control, Tail drop

10 Congestion control

Congestion control

Queues can begin filling up due to a number of reasons, such as over subscription of a link or backpressure from a downstream device. Sustained, large queue buildups generally indicate congestion in the network and can affect application performance through increased queueing delays and frame loss.

Congestion control covers features that define how the system responds when congestion occurs or active measures taken to prevent the network from entering a congested state.

Tail drop

Tail drop queueing is the most basic form of congestion control. Frames are queued in FIFO order and queue buildup can continue until all buffer memory is exhausted. This is the default behavior when no additional QoS has been configured.

The basic tail drop algorithm does not have any knowledge of multiple priorities and per traffic class drop thresholds can be associated with a queue to address this. When the queue depth breaches a threshold, then any frame arriving with the associated priority value will be dropped. Figure 6 describes how you can utilize this feature to ensure that lower priority traffic cannot totally consume the full buffer memory. Thresholds can also be used to bound the maximum queueing delay for each traffic class. Additionally if the sum of the thresholds for a port is set below 100 percent of the buffer memory, then you can also ensure that a single port does not monopolize the entire shared memory pool.

FIGURE 6 Queue depth

The tail drop algorithm can be extended to support per priority drop thresholds. When the ingress port CoS queue depth breaches a threshold, then any frame arriving with the associated priority value will be dropped. Figure 6 describes how you can utilize this feature to ensure lower priority traffic cannot totally consume the full buffer memory. Thresholds can also be used to bound the maximum queueing delay for each traffic class. Additionally if the sum of the thresholds for a port is set below 100 percent of the buffer memory then you can also ensure that a single CoS does not monopolize the entire shared memory pool allocated to the port.

100	Dell Converged Enhanced Ethernet Administrator’s Guide
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