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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 CNP

Fast CNP

NOTE:

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

Context

The following describes the process of traditional Congestion Notification Packet (CNP), according to the steps illustrated in Figure 2-6:
  1. DeviceA finds that the port is congested and then tags the packet with the ECN field (the ECN field is set to 11), indicating that there is congestion on the network.
  2. After receiving the packet, DeviceB finds that the destination address of the packet is not the local device and does not process it. It forwards the packet to the destination server.
  3. After receiving the packet, the destination server finds that the packet is tagged with the ECN field (the ECN field is 11) and sends a CNP to instruct the source server to reduce the traffic rate.

As illustrated in the figure, the congestion point is on DeviceA, but the CNP is sent by the destination server. If the path where the CNP is transmitted is too long, there is a delay in reducing the rate of traffic on the source server. As a result, the congestion of the forwarding device may be further aggravated. In extreme cases, the entire network stops sending packets due to PFC.

Figure 2-6 Traditional CNP

Fast CNP can solve the problem. After fast CNP is enabled on the forwarding device, the forwarding device records packet information in the flow table entry when forwarding the packet. It sends the CNP to the source server based on the learned flow entry information when receiving a packet tagged with the ECN field. The path along which the CNP is transmitted is shortened. In this way, the traffic rate of the source server can be adjusted with little delay, which relieves the congestion in the buffer of the forwarding device.

Implementation

The following describes the process of fast CNP, according to the steps illustrated in Figure 2-7:
  1. DeviceA finds that the port is congested and then tags the packet with the ECN field (the ECN field is set to 11), indicating that there is congestion on the network.
  2. After receiving the packet, DeviceB finds that the packet is tagged with the ECN field (the ECN field is 11) and sends a CNP to instruct the source server to reduce the traffic rate.
  3. DeviceB finds that the destination address of the packet is not the local device and forwards the packet to the destination server.
Figure 2-7 Fast CNP
After receiving the packet tagged with the ECN field, the destination server sends the CNP to the source server. For the same packet tagged with the ECN field sent by DeviceA, if both DeviceB and the destination server send a response and a CNP to the source server, the traffic rate of the source server will be reduced excessively. You can perform the following operations to solve this problem:
  1. Disable the destination server from sending the CNP to the source server in response to receiving the packet tagged with the ECN field.
  2. On DeviceB, set the aggregation time for sending the CNP. After receiving the CNP from the downstream device (destination server), DeviceB checks the time that has passed since it sent the last CNP packet. If the time is shorter than the aggregation time, the CNP received from the downstream device is discarded.
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Updated: 2019-04-20

Document ID: EDOC1100040243

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