The network detects two degrees of congestion: minor and severe. Minor congestion is detected when the amount of data queued for a specific transmission priority to a particular transmission group exceeds a user-specified value. Severe congestion is detected when the aggregate amount of data for all transmission priorities exceeds the total user-specified value for a transmission group. At the first, less severe level, the number of PIUs allowed to be sent in a window is decreased by one until congestion clears. At the severe level, the window size is immediately decreased to its minimum value. These two levels can be specified with NCP transmission group flow control threshold parameters. These threshold parameters can be coded on the ER operand on the NCP PATH definition statement.

VTAM® detects only severe congestion. The reset window indicator is set in the PIU if VTAM cannot immediately get an IO buffer, either because of a shortage or because XPANPT is not high enough to allow expansion to complete before all the remaining buffers are allocated.