It was decided in the Flow Bit Rate Properties ADR to follow option 3b.
Option 3b defines the max_bit_rate and avg_bit_rate Flow properties to be segment bit rates (rather than essence bit rates).
Option 3b adds a segment_duration Flow property that is the target duration for the segments.
The segment_duration and max_bit_rate can be used for calculating a minimum receiver buffer size.
This AppNote defines how clients can calculate the bit rates if they are not known from the process that is creating them (e.g. an encoder).
Further background on the definition and calculations can be found in the Flow Bit Rate Properties ADR.
The avg_bit_rate is calculated as int(total_segment_bit_size / (total_segment_duration_sec * 1000)) (kbit/sec) for the segments of the Flow.
The avg_bit_rate could be calculated from the segments that are present in the TAMS.
The value could be an expectation of what could be present in the TAMS (e.g. derived from an encoder / muxer setting for a live ingest).
The value could be an approximation.
The value could change over time as segments becomes available or are removed from TAMS.
The max_bit_rate is calculated as int(peak_segment_bit_rate / 1000) (kbit/sec) for the segments of the Flow.
The peak_segment_bit_rate corresponds to the definition in HLS (HTTP Live Streaming), where it is defined as the maximum bit rate for a contiguous sequence of segments that has a duration between 0.5 and 1.5 the target segment duration.
The HLS definition is extended to include single segments above 1.5 the target segment duration to handle Flows that don't follow the HLS retrictions for segment durations, e.g. segmentation optimised for best quality, with variable and potentially large GOP sizes etc..
The max_bit_rate could be calculated from the segments that are present in the TAMS.
The value could be an expectation of what could be present in the TAMS (e.g. derived from an encoder / muxer setting for a live ingest).
The value could be an approximation, ideally an upper bound (e.g. the bit rate from the H.264 hrd_parameters plus some container overhead approximation).
The value could change over time as segments become available or are removed from TAMS.
The segment_duration is the media segment duration that the media segmenter (e.g. encoder) is targetting.
The segment durations may vary, e.g. to ensure that video segments with variables size GOPs always start with a key frame.
The description in SCTE 214-1 (Society of Cable Telecommunications Engineers, MPEG DASH for IP-Based Cable Services, Part 1: MPD Constraints and Extensions), section 10.3 includes a formula for calculating the buffer size required for continuous playback.
The buffer size is defined as 1.1 * MSR[R] * SDmax.
MSR[R] in the TAMS context is max_bit_rate, SDmax is the maximum segment duration and 1.1 adds a 10% overhead for additional event data.
The duration of segments is assumed to vary between 0.5 and 1.5 times the segment_duration.
The SDmax is therefore approximately segment_duration * 1.5 and the minimum buffer size in bytes is then approximately max_bit_rate * 1000 * SDmax * 8.