H3C Technologies H3C S6300 Series Switches - Manual (Page 63)

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ECN
By dropping packets, WRED alleviates the influence of congestion on the network. However, the network
resources for transmitting packets from the sender to the device which drops the packets are wasted.
When congestion occurs, it is a better idea to inform the sender of the congestion status and have the
sender proactively slow down the packet sending rate or decrease the window size of packets. This better
utilizes the network resources.
RFC 2482 defined an end-to-end congestion notification mechanism named Explicit Congestion
Notification (ECN). ECN uses the DS field in the IP header to mark the congestion status along the packet
transmission path. A ECN-capable terminal can determine whether congestion occurs on the
transmission path according to the packet contents, and then adjusts the packet sending speed to avoid
deteriorating congestion. ECN defines the last two bits (ECN field) in the DS field of the IP header as
follows:
• Bit 6 indicates whether the sending terminal device supports ECN, and is called the "ECN-Capable
Transport (ECT)" bit.
• Bit 7 indicates whether the packet has experienced congestion along the transmission path, and is
called the "Congestion Experienced (CE)" bit.
For more information about the DS field, see " Appendixes ."
In actual applications, the packets with ECT set to 1 and CE set to 0 and the packets with ECT set to 0
and CE set to 1 are considered as packets that an ECN-capable endpoint transmits.
After you enable ECN on a device, congestion management processes packets as follows:
• When the average queue size is below the lower threshold, no packet is dropped, and the ECN
fields of packets are not identified or marked.
• When the average queue size exceeds the lower threshold and is below the upper threshold, before
the device drops a packet which should be dropped according to the drop probability, the device
examines the ECN field of the packet.
{
If the ECN field shows that the packet is sent out of ECN-capable terminal, the device sets both
the ECT bit and the CE bit to 1 and forwards the packet.
{
If the ECN field shows that the packet has experienced congestion along the transmission path
(both the ECT bit and the CE bit are 1), the device forwards the packet without modifying the
ECN field.
{
If both the ECT bit and the CE bit are 0s, the device drops the packet.
• When the average queue size exceeds the upper threshold, the device drops the packet, no matter
whether the packet is sent out of an ECN-capable terminal.
ECN is enabled on a per-queue basis. You can configure the switch to identify and mark the ECN fields
of packets for a specific queue.
Configuring and applying a WRED table
The switch supports the queue-based WRED table. You can configure separate drop parameters for
different queues. When congestion occurs, packets of a queue are randomly dropped based on drop
parameters of the queue.
Determine the following parameters before configuring WRED: