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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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Fast ECN

Fast ECN

NOTE:

Only the CE8860EI, CE8850EI, CE6860EI, CE6865EI, CE6870EI, and CE6875EI support the function.

Context

When the number of packets in the forwarding queue exceeds the ECN threshold, the device sends a packet carrying the ECN field to the destination server to notify it of the congestion on the network. After receiving the packet carrying the ECN field, the destination server sends a CNP to instruct the source server to reduce the traffic rate.

In Figure 2-4, when a packet enters a queue, traditional Explicit Congestion Notification (ECN) allows the device to determine whether the buffer used by the queue exceeds the ECN threshold. If so, the device tags the packet with the ECN field of 11. The time for the destination server to receive the packet tagged with the ECN field is the packet forwarding time in the device queue (the time taken from the device adding the ECN field to the packet to forwarding the tagged packet to the device) plus the tagged packet forwarding time in the network. If there is severe network congestion, the traditional mode may exacerbate queue congestion. In extreme cases, the entire network may stop sending packets due to PFC.

Figure 2-4 Traditional ECN

Fast ECN can solve the problem. It allows the device to tag the packet when the packet goes out of the queue, instead of when entering the queue. This shortens the time taken from the device adding the ECN to the packet to forwarding the tagged packet to the device.

Implementation

Fast ECN allows the device to tag the packet when the packet goes out of the queue. When a packet goes out of a queue, the device determines whether the buffer used by the queue exceeds the ECN threshold. If so, the packet is tagged with the ECN field (the ECN field in the packet is set to 11). The device then tags the packet with the ECN field and forwards the tagged packet. As the packet has already left the queue, the time for forwarding tagged packets in the queue is shortened. The destination server can receive the tagged packet with the least delay possible.

Figure 2-5 Fast ECN
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Updated: 2019-04-20

Document ID: EDOC1100040243

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