PFC Priority Flow Control
Pausing traffic on an Ethernet link for one priority class at a time rather than all of it. The prerequisite for running RDMA over Ethernet.
The old Ethernet pause mechanism stopped everything on the link, which makes it useless when the same cable carries both storage traffic that tolerates no loss and ordinary traffic that may happily drop a packet. PFC splits it into eight classes so only the sensitive one is halted. Two well-known problems come with it. Pause frames propagate backwards hop by hop, so a single congested port can throttle traffic far away in the fabric — congestion spreading. And in unfortunate topologies a cyclic buffer dependency can arise where the fabric locks up permanently, a PFC deadlock. That is why end-to-end congestion control is layered on top and PFC is regarded as a last resort rather than the primary mechanism.