Credit-based flow control
Beginner
A rule that a sender may only send as much as the receiver has said it has room for, like tickets for parking spaces.
Novice
The receiving end of each link tells the sender how much buffer space it has (credits). The sender spends credits as it sends and stops when they run out, so nothing is ever dropped for lack of space.
Expert
Hop-by-hop, per-virtual-channel buffer accounting. Lossless like PFC but insensitive to cable delay misestimates (a long cable only lowers throughput) and it gives the sender per-VC occupancy for scheduling. Native to InfiniBand; Ultra Ethernet defines an Ethernet version (CBFC).
Explained in Scale-out networking (Systems).
See also: PFC (priority flow control), InfiniBand.