Brocade Communications Systems 53-1001761-01 manual Congestion control and queuing

1 Layer 2 Ethernet overview

Congestion control and queuing

The Brocade FCoE hardware supports several congestion control and queuing strategies. As an output queue approaches congestion, Random Early Detection (RED) is used to selectively and proactively drop frames to maintain maximum link utilization. Incoming frames are classified into priority queues based on the Layer 2 CoS setting of the incoming frame, or the possible rewriting of the Layer 2 CoS field based on the settings of the CEE port or VLAN.

The Brocade FCoE hardware supports a combination of two scheduling strategies to queue frames to the egress ports; Priority queuing, which is also referred to as strict priority, and Deficit Weighted Round Robin (DWRR) queuing.

The scheduling algorithms work on the eight traffic classes as specified in 802.1Qaz Enhanced Transmission Selection (ETS).

Queuing features are described as follows:

•RED—RED increases link utilization. When multiple inbound TCP traffic streams are switched to the same outbound port, and some traffic streams send small frames while other traffic streams send large frames, link utilization will not be able to reach 100 percent. When RED is enabled, link utilization approaches 100 percent.

•Classification—Setting user priority.

-Inbound frames are tagged with the user priority set for the inbound port. The tag is visible when examining the frames on the outbound port. By default, all frames are tagged to priority zero.

-Externally tagged Layer 2 frames—When the port is set to accept externally tagged Layer 2 frames, the user priority is set to the Layer 2 CoS of the inbound frames.

•Queuing

-Input queuing—Input queuing optimizes the traffic flow in the following way. Suppose a CEE port has inbound traffic that is tagged with several priority values, and traffic from different priority settings is switched to different outbound ports. Some outbound ports are already congested with background traffic while others are uncongested. With input queuing, the traffic rate of the traffic streams switched to uncongested ports should remain high.

-Output queuing—Output queuing optimizes the traffic flow in the following way. Suppose that several ports carry inbound traffic with different priority settings. Traffic from all ports is switched to the same outbound port. If the inbound ports have different traffic rates, some outbound priority groups will be congested while others can remain uncongested. With output queuing, the traffic rate of the traffic streams that are uncongested should remain high.

-Multicast rate limit—A typical multicast rate limiting example is where several ports carry multicast inbound traffic that is tagged with several priority values. Traffic with different priority settings is switched to different outbound ports. The multicast rate limit is set so that the total multicast traffic rate on output ports is less than the specified set rate limit.

-Multicast input queuing—A typical multicast input queuing example is where several ports carry multicast inbound traffic that is tagged with several priority values. Traffic with different priority settings is switched to different outbound ports. Some outbound ports are already congested with background traffic while others are uncongested. The traffic rate of the traffic streams switched to the uncongested ports should remain high. All outbound ports should carry some multicast frames from all inbound ports. This enables multicast traffic distribution relative to the set threshold values.

6	Converged Enhanced Ethernet Administrator’s Guide
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