This MIB module defines objects that describe the quality metrics of RTP streams, similar to those described by an RTCP Receiver Report packet [RFC3550]. GLOSSARY ============ Expected Packets - this value is formally defined as the extended last sequence number received less the initial sequence number received. An extended last sequence number is a 32-bit value, where the most significant 16-bit word indicates the number of sequence number cycles, and the least significant 16-bit word indicates the highest sequence number received. Flow Monitor - a hardware or software entity that classifies traffic flows, collects flow data, and periodically computes flow metrics. Flow Metric - a measurement that reflects the quality of a traffic flow. Inter-Arrival Jitter - an estimate of the statistical variance of the RTP data packet inter-arrival time. The inter-arrival jitter, J, is formally defined to be the mean deviation (smoothed absolute value) of the difference, D, in packet spacing at the flow monitor compared to the sender for a pair of packets. This is equivalent to the difference in the relative transit time for two packets; the relative transit time is the difference between a packet's RTP timestamp and the device's clock at the time of arrival (measured in the same units): D(i,j) = (Rj - Ri) - (Sj - Si) = (Rj - Sj) - (Ri - Si) where Si is the RTP timestamp from packet i, and Ri is the time of arrival in RTP timestamp units for packet i. The inter-arrival jitter SHOULD be calculated continuously for each RTP data packet received from source SSRCn, using this equation to compute difference for each packet and the previous packet (in order of arrival, not necessarily in sequence). |D(i-1,i)| - J(i-1) J(i) = J(i-1) + ------------------- 16 Measurement Interval - the length of time over which a flow monitor collects data related to a traffic flow, after which the flow monitor computes flow metrics using the collected data. Loss Distance - the difference between the sequence numbers delimiting the start of two consecutive loss intervals. Consider the following sequence of RTP data packets: 111111 111222 2233 33333 444 444 5 123456x890123xxxx8901xxx56789x123xx678x0 ^ ^ ^ ^ ^ ^ | | | | | | LI1 LI2 LI3 LI4 LI5 LI6 Loss Interval | Loss Distance ===============+=============== 1 | 2 | 7 3 | 8 4 | 8 5 | 4 6 | 5 Loss Fraction - the fraction of RTP data packets from source SSRCn lost during a measurement interval, expressed as a fixed-point number: Li Fi = ---- Ei where Fi is the loss fraction for measurement interval i, Li is the lost packets during measurement interval i, and Ei is the expected packets during measurement interval i. Observe that the number of packets lost includes packets that are late or duplicates, and hence this number can can have a theoretical theoretical value between negative infinity and one. The cumulative loss fraction is the fraction of RTP data packets from source SSRCn lost over the duration monitoring the flow: n sum [Li] i=1 Fn = ---------- n sum [Ei] i=1 where Fn is the cumulative loss fraction over n measurement intervals. Loss Interval - an interval in which consecutive packet losses were experienced. Consider the following sequence of RTP data packets: 111111 111222 2233 33333 444 444 5 123456x890123xxxx8901xxx56789x123xx678x0 ^ ^ ^ ^ ^ ^ | | | | | | LI1 LI2 LI3 LI4 LI5 LI6 LI1 through LI6 indicate the start of loss intervals observed in this sequence. Loss Interval Duration - the number of packets lost in a loss interval. Consider the following sequence of RTP data packets: 111111 111222 2233 33333 444 444 5 123456x890123xxxx8901xxx56789x123xx678x0 ^ ^ ^ ^ ^ ^ | | | | | | LI1 LI2 LI3 LI4 LI5 LI6 Loss Interval | Duration ===============+========== 1 | 1 2 | 4 3 | 3 4 | 1 5 | 2 6 | 1 Lost Packets - this value is formally defined as the number of packets expected less the number of packets actually received, where the number of packets received includes those which are late or duplicates. SSRCn - the SSRC identifier of the source. Traffic Flow - a unidirectional stream of packets conforming to a classifier. For example, packets having a particular source IP address, destination IP address, protocol type, source port number, and destination port number. Transit Time - the latency from the insertion into the network to the flow monitor. This value can be computed by taking the difference between a packet's RTP timestamp and the device's clock at the time of arrival (measured in the same units). REFERENCES ============== [RFC3550] H. Schlzrinne, S. Casner, R. Frederick, V. Jacobson, 'RTP: A Transport Protocol for Real-Time Applications', RFC-3550, July 2003.