4.7.2. Burst Metrics

4.7.2. Burst Metrics

A burst is a period during which a high proportion of packets are either lost or discarded due to late arrival. A burst is defined, in terms of a value Gmin, as the longest sequence that (a) starts with a lost or discarded packet, (b) does not contain any occurrences of Gmin or more consecutively received (and not discarded) packets, and (c) ends with a lost or discarded packet.

A gap, informally, is a period of low packet losses and/or discards. Formally, a gap is defined as any of the following: (a) the period from the start of an RTP session to the receipt time of the last received packet before the first burst, (b) the period from the end of the last burst to either the time of the report or the end of the RTP session, whichever comes first, or (c) the period of time between two bursts.

For the purpose of determining if a lost or discarded packet near the start or end of an RTP session is within a gap or a burst, it is assumed that the RTP session is preceded and followed by at least Gmin received packets, and that the time of the report is followed by at least Gmin received packets.

A gap has the property that any lost or discarded packets within the gap must be preceded and followed by at least Gmin packets that were received and not discarded. This gives a maximum loss/discard rate within a gap of: 1 / (Gmin + 1).

A Gmin value of 16 is RECOMMENDED, as it results in gap characteristics that correspond to good quality (i.e., low packet loss rate, a minimum distance of 16 received packets between lost packets), and hence differentiates nicely between good and poor quality periods.

For example, a 1 denotes a received packet, 0 a lost packet, and X a discarded packet in the following pattern covering 64 packets:

11110111111111111111111X111X1011110111111111111111111X111111111
|---------gap----------|--burst---|------------gap------------|

The burst consists of the twelve packets indicated above, starting at a discarded packet and ending at a lost packet. The first gap starts at the beginning of the session and the second gap ends at the time of the report.

If the packet spacing is 10 ms and the Gmin value is the recommended value of 16, the burst duration is 120 ms, the burst density 0.33, the gap duration 230 ms + 290 ms = 520 ms, and the gap density 0.04.

This would result in reported values as follows (see field descriptions for semantics and details on how these are calculated):

loss rate             12, which corresponds to 5%
discard rate          12, which corresponds to 5%
burst density         84, which corresponds to 33%
gap density           10, which corresponds to 4%
burst duration       120, value in milliseconds
gap duration         520, value in milliseconds

burst density: 8 bits The fraction of RTP data packets within burst periods since the beginning of reception that were either lost or discarded. This value is expressed as a fixed point number with the binary point at the left edge of the field. It is calculated by dividing the total number of packets lost or discarded (excluding duplicate packet discards) within burst periods by the total number of packets expected within the burst periods, multiplying the result of the division by 256, limiting the maximum value to 255 (to avoid overflow), and taking the integer part. This field MUST be populated and MUST be set to zero if no packets have been received.

gap density: 8 bits The fraction of RTP data packets within inter-burst gaps since the beginning of reception that were either lost or discarded. The value is expressed as a fixed point number with the binary point at the left edge of the field. It is calculated by dividing the total number of packets lost or discarded (excluding duplicate packet discards) within gap periods by the total number of packets expected within the gap periods, multiplying the result of the division by 256, limiting the maximum value to 255 (to avoid overflow), and taking the integer part. This field MUST be populated and MUST be set to zero if no packets have been received.

burst duration: 16 bits The mean duration, expressed in milliseconds, of the burst periods that have occurred since the beginning of reception. The duration of each period is calculated based upon the packets that mark the beginning and end of that period. It is equal to the timestamp of the end packet, plus the duration of the end packet, minus the timestamp of the beginning packet. If the actual values are not available, estimated values MUST be used. If there have been no burst periods, the burst duration value MUST be zero.

gap duration: 16 bits The mean duration, expressed in milliseconds, of the gap periods that have occurred since the beginning of reception. The duration of each period is calculated based upon the packet that marks the end of the prior burst and the packet that marks the beginning of the subsequent burst. It is equal to the timestamp of the subsequent burst packet, minus the timestamp of the prior burst packet, plus the duration of the prior burst packet. If the actual values are not available, estimated values MUST be used. In the case of a gap that occurs at the beginning of reception, the sum of the timestamp of the prior burst packet and the duration of the prior burst packet are replaced by the reception start time. In the case of a gap that occurs at the end of reception, the timestamp of the subsequent burst packet is replaced by the reception end time. If there have been no gap periods, the gap duration value MUST be zero.