PFC (priority flow control)
Beginner
A signal that tells the switch sending data to pause for a moment, because the next switch is nearly full.
Novice
Priority-based flow control: when a switch’s buffer for one traffic class passes a threshold, it sends a pause frame to the device feeding it, which stops sending that class until told to resume. It keeps packets from being dropped, at the cost of pausing traffic that may not be causing the problem.
Expert
Per-priority XOFF/XON pause (IEEE 802.1Q). Needs per-port headroom proportional to cable length × rate; pauses propagate hop by hop and cause head-of-line blocking, congestion spreading, pause storms and, with cyclic buffer dependencies, deadlock.
Explained in Scale-out networking (Systems).
See also: Head-of-line blocking, Credit-based flow control.