53
•
Drop probability.
Relationship between WRED and queuing mechanisms
Figure 17 Relationship between WRED and queuing mechanisms
Through combining WRED with WFQ, the flow-based WRED can be realized. Each flow has its own
queue after classification.
•
A flow with a smaller queue size has a lower packet drop probability.
•
A flow with a larger queue size has a higher packet drop probability.
In this way, the benefits of the flow with a smaller queue size are protected.
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 perform the following actions:
•
Inform the sender of the congestion status.
•
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. Then, it 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 "
Packets to be sent
through this interface
Packets sent
Sending queue
Interface
Classify
Schedule
……
Queue 1 weight 1
Queue 2 weight 2
Queue N-1 weight N-1
Queue N weight N
……
WRED drop
Packets
dropped
Summary of Contents for FlexFabric 5940 Series
Page 23: ...17 Figure 3 QoS processing flow ...
Page 84: ...78 Figure 26 MPLS label structure ...
Page 91: ...85 Switch burst mode enable ...
