| 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. |
