The MDI subset (RFC-4445) is a useful quality indicator for constant bit rate compressed video but is not defined for variable bit rate stream (VBR) networks. MDI was always touted as a relatively simple measurement to make and scale to hundreds of channels. The MDI RFC only specifies how to measure MDI for constant bitrate streams (CBR). The complexity of the measurement in VBR systems goes up dramatically however--as IneoQuest (RFC sponsor) has acknowledged in their formal publications.
The MDI consists of two components: the Delay Factor (DF) and the Media Loss Rate (MLR). The Delay Factor gives a hint of the minimum size of the buffer required at the next downstream node. Instead of relying upon MDI Delay Factor (and it’s inherent problems with VBR streams) to estimate buffer requirements, FaultLine directly measures network induced jitter using the highly accurate 27MHz system time clock embedded in the MPEG2 transport stream. This measurement, called packet inter-arrival jitter, is computed identically for both CBR and VBR streams and can be reliably used to calculate buffer requirements, the same purpose as the MDI Delay Factor.