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Configuration Guide - Low Latency Network

CloudEngine 8800, 7800, 6800, and 5800 V200R005C00

This document describes the configurations of Buffer optimization of lossless queues, Fast ECN, Fast CNP, Dynamic ECN threshold of lossless queues, and Dynamic load balancing.
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Dynamic ECN Threshold of Lossless Queues

Dynamic ECN Threshold of Lossless Queues

NOTE:

Only the CE6865EI and CE8850-64CQ-EI support the function.

Context

The device measures the buffer usage of queues based on the buffer threshold. To control traffic of lossless queues and reduce congestion in the buffer for lossless queues, you can configure ECN and PFC thresholds for lossless queues. Figure 2-8 illustrates the functions of ECN and PFC thresholds.

Figure 2-8 Functions of ECN and PFC thresholds
  1. When congestion occurs in a lossless queue (Queue-n) and the buffer used by the queue exceeds the ECN threshold, the device adds the ECN field of 11 to packets to be forwarded.
  2. After receiving the packet tagged with the ECN field, the destination server sends a Congestion Notification Packet (CNP) to the source server. After the source server receives the CNP, it reduces the packet sending rate.
  3. When congestion of Queue-n is aggravated and the buffer used by the queue exceeds the PFC threshold, the device sends a PFC notification packet to the source server. After receiving the PFC notification packet, the source server stops sending packets from the queue with a specified priority.
  4. When congestion of Queue-n is relieved and the buffer used by the queue is lower than the PFC threshold, the device sends a PFC pause packet to the source server. After receiving the PFC pause packet, the source server continues to send packets from the queue with a specified priority.

According to the preceding process, there is a period of time from ECN field marking to reduction of the packet sending rate. During this period of time, the source server still sends traffic to the device at the original packet sending rate. As a result, congestion in the queue buffer on the device is aggravated continuously and the device stops sending packets due to PFC. In this case, the ECN threshold needs to be set properly. By doing this, the buffer space between the ECN threshold and PFC threshold can accommodate traffic during the period from ECN field marking to reduction of the packet sending rate on the source server, providing as much assurance as possible that PFC is not triggered.

Traditionally, the ECN threshold is configured manually. However, manually configuring a fixed value cannot meet requirements when both delay-sensitive mice flows and throughput-sensitive elephant flows exist on a network. The following describes the effect of manually configuring the ECN threshold:
  • When a low ECN threshold is used, ECN field marking is triggered at the earliest possible time. The source server is then requested to reduce the packet sending rate. This ensures the low buffer depth (low queue latency), which is better for delay-sensitive mice flows. However, a low ECN threshold affects throughput-sensitive elephant flows by limiting their bandwidth.
  • When a high ECN threshold is used, ECN field marking can be delayed. This ensures the capability to cope with burst traffic in queues and bandwidth of throughput-sensitive elephant flows. When congestion occurs in a queue, there is a long queue delay. In addition, a long ECN threshold is invalid for delay-sensitive mice flows.

The dynamic ECN threshold for lossless queues can address the preceding issue. The device analyzes forwarded network traffic to dynamically adjust the ECN threshold for lossless queues. This function prevents PFC from being triggered and meets bandwidth requirements of delay-sensitive mice flows and throughput-sensitive elephant flows as much as possible.

Implementation

In the many-to-one incast model, to provide as much assurance as possible for the lossless forwarding of lossless services, the device dynamically adjusts the ECN threshold for lossless queues based on the Incast value (N:1) and percentage of elephant and mice flows. A larger value of N indicates a large Incast value and higher burst pressure of the buffer.
  • When a high Incast value is used, a low ECN threshold needs to be set to ensure low latency of queues, increase the buffer space between the ECN threshold and PFC threshold, and reduce the frequency of PFC being triggered. When a low Incast value is used, a high ECN threshold needs to be set to reduce the buffer space between the ECN threshold and PFC threshold and ensure high throughput of queues.
  • When the percentage of mice flows is high, a low ECN threshold needs to be set to ensure low latency of most mice flows. When the percentage of elephant flows is high, a high ECN threshold needs to be set to ensure high throughput of most elephant flows.
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Updated: 2019-04-20

Document ID: EDOC1100040243

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