Converged networks with Fibre Channel over Ethernet and Data Center Bridging

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Figure 6 illustrates the same scenario up to the point where the receiving node needs to send a pause
frame. A PPP frame dictates pausing the red TC. The pause takes advantage of the class of service fields to
restrict the pause to only classes of traffic that have nearly full queues. The transmitting station stops sending
red traffic; the latency-sensitive green traffic continues to flow properly.
Figure 6. PFC-based flow control
Receive queues in a DCB Ethernet device will have high and low watermarks. If the queues fill up to the
high watermark, the device generates a PPP frame. If the level of the queue drops below the low
watermark, the device will send a PPP frame specifying a zero time to indicate that the link partner may
send traffic for the affected TCs immediately. This allows an XON/XOFF-type operation on a per
priority/TC. PPP frames allow a single frame to specify XON/XOFF behavior independently for any of up
to eight priorities/TCs. This reduces the control frame overhead if devices support PFC on multiple TCs.
The FCoE protocol requires DCB-enabled Ethernet devices to support only one PFC-enabled priority/TC. Not
all eight priorities/TCs must support PFC, and not all priorities/TCs have to support PFC. Many devices on
the market today support only one PFC-enabled priority/TC. In the future, devices should support a greater
number of PFC-enabled priorities/TCs, but that is not required for basic FCoE transport over DCB-enabled
Ethernet links.
Enhanced Transmission Selection
Legacy Ethernet supports multiple traffic management elements called traffic classes (TCs). IEEE 802.1Q
(VLAN) tags with a class of service (CoS) field assign a transmission priority to each TC. You can implement
up to eight TCs (TC0 through TC7) in an Ethernet device. Current standards and product implementations
focus on transmitting the traffic classes in strict priority order. For applications operating completely at layer
2, the MAC layer, strict priority does not allow for fair, deterministic bandwidth control typically preferred