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I read the aforementioned draft with interest, as I believe
that it is important to have a way of quantifying the burstiness of packet loss. I must admit to having read it too quickly, as I am
traveling and wanted to get some comments in before the upcoming meeting. While I believe that the metrics defined are a good first
step, I must admit to feeling unsatisfied with them. I simply am not sure that the concept of burstiness has been
captured sufficiently well. Yes, the duration of a packet burst and the frequency of
bursts are nice to have, but first we need to quantify how bursty the loss is –
if it is not bursty then these two metrics are meaningless. For this purpose the draft proposes loss-pair-counts and
bi-packet-loss-ratio, but loss-pair-counts are too raw and bi-packet-loss-ratio
doesn't seem to describe the right thing. Another minor gripe I have with the methodology is the
introduction of time. I think of burstiness in terms of packet (transmit) sequence
number. Is it really necessary to introduce a time scale ? It seems
to complicate things. What did I expect as a metric ? Well, I have become used to
the use of Gilbert-Elliott models, where there are two states – loss and non-loss, with
easily measured probabilities of transitions between the states, and probabilities of loss or not in
either state. These probabilities seem to capture well the subjective idea of burstiness, but it takes
a while to get used to them. Intuitively, after defining the probability of loss, we need
to describe the higher moments or cumulants. So a simple derived metric would be the probability of
consecutive packets being lost divided by the PLR^2, the probability of three consecutive packets lost divided by
PLR^3, etc. As a complement one could do the probability of two randomly
chosen distant packets both being lost divided by PLR^2, etc. Can someone map the metrics described in the draft to these
ideas ? Y(J)S |