Incast
Beginner
Many senders all sending to the same receiver at the same moment, overflowing its connection.
Novice
A many-to-one traffic pattern in which several senders transmit to one receiver at once. Their combined rate exceeds the receiver’s link, so the last switch’s buffer fills and packets are delayed, dropped or paused.
Expert
Synchronized fan-in that oversubscribes the last-hop link. Common in collectives and storage; switch buffers are small on-chip memories shared across all ports, so line-rate incast from even a handful of senders fills them in microseconds.
Explained in Scale-out networking (Systems).
See also: ECN (explicit congestion notification).