Brocade Converged Enhanced Ethernet Administrator's Guide v6.1.2_cee (53-1001258-01, June 2009)

104 Converged Enhanced Ethernet Administrator’s Guide
53-1001258-01
Rewriting
8
• Congestion control
When queues begin filling up and all buffering is exhausted, packets are dropped. This has a
detrimental effect on application throughput. Congestion control techniques are used to
reduce the risk of queue overruns without adversely affecting network throughput. Congestion
control features include IEEE 802.3x Ethernet Pause, Tail Drop, and Brocade proprietary
Ethernet Per Priority Pause (PPP).
• Multicast rate limiting
Many multicast applications cannot be adapted for congestion control techniques and the
replication of packets by switching devices can exacerbate this problem. Multicast rate limiting
controls packet replication to minimize the impact of multicast traffic.
• Scheduling
When multiple queues are active and contending for output on a common physical port the
scheduling algorithm selects the order the queues are serviced. Scheduling algorithms include
Strict Priority (SP), Weighted Round Robin (WRR), and Deficit Weighted Round Robin (DWRR)
queueing. The scheduler supports a hybrid policy combining SP and WRR/DWRR servicing.
Under a hybrid scheduler configuration, the highest priority queues are serviced by SP while
lower priority queues share the remaining bandwidth using the WRR service.
• Converged Enhanced Ethernet
CEE describes an enhanced Ethernet that will enable convergence of various applications in
data centers (LAN, SAN, and IPC) onto a single interconnect technology.
Rewriting
Rewriting a packet header field is typically performed by an edge device. Rewriting occurs on
packets as they enter or exit a network because the neighboring device is untrusted, unable to
mark the packet, or is using a different QoS mapping.
The packet rewriting rules set the Ethernet CoS and VLAN ID fields. Egress Ethernet CoS rewriting is
based on the user-priority mapping derived for each packet as described later in the queueing
section.
Queueing
In this section:
•User-priority mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
•Traffic class mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Queue selection begins by mapping an incoming packet to a configured user priority, then each
user-priority mapping is assigned to one of the switches eight unicast traffic class queues or one of
the four multicast traffic class queues.