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.