Priority-based flow control frames are a special type of Ethernet frame defined in IEEE 802.1Qbb. Control frames are identified from other frame types by a defined value placed into the length/type field (the MAC Control Type code). The priority-based flow control frame format is shown in the following figure.
A PFC frame is a special type of control frame identified by a defined value placed into the OPCODE field. This is shown in the previous figure.
The 16-bit priority enable vector contains
eight priority enable bits with all other bits set to zero. The priority vector field
contains eight 16-bit quanta values, one for each priority, with priority 0 being the first.
This defines the number of pause_quantum (512-bit times of the particular implementation)
for the priority pause duration request. For 10 Gb Ethernet, a single pause_quantum corresponds to 51.2 ns.