I-Frame Delay (IFD) is a scheduling technique for adaptive streaming of MPEG video.
The idea behind it is that streaming scheduler drops video frames when the transmission buffer is full because of insufficient bandwidth, to reduce the transmitted bit-rate.
When a packet belonging to next frame is about to be written to the IFD buffer and the buffer is full, the IFD scheduler will drop a frame based on the priority assigned earlier.
When the network bandwidth is so low that also P-frames need to be dropped, then the GOP (Group of Pictures) is set to be "disturbed" and the rest of the GOP (which depends on the P-frame) is also dropped.
[1] For an IFD implementation with a buffer of the size of two frames the algorithm is shown in figure below.