[e2e] Re: crippled Internet

k claffy kc at ipn.caida.org
Fri Apr 27 16:28:25 PDT 2001

  Looking at the NLANR data (http://watt.nlanr.net/active/) we see
  measurements such as "average RTT of 60 ms plus standard deviation of 5
  ms", one may be tempted to conclude that life is good -- average one way
  delay plus two std sums to 40 ms, much less than 150. However, there are
  two caveats. First, a significant portion of the 150 ms budget must be
  reserved for the processing function -- maybe half of it in the current
  state of the art; this leaves a target of 75 ms for the network delay.
  Second, measuring the std is a very poor approximation of the behavior
  of jitter compensation algorithms. We need a much finer grain
  understanding of the structure of the delay.
  Stanislav mentions that "Surveyor shows 0th, 50th, and 90th percentile
  graphs by default." It is a good start, but there are two issues. First,
  surveyor uses the IPPM defined methodology, sending probes at
  pseudo-random intervals; this is not a good modeling of the VoIP
  traffic, which mostly consists of "talk spurts", trains of packets sent
  at a 20 ms or 30 ms interval; the interesting measurement is the maximum
  cumulative jitter within a packet train. Second, even if we accepted
  random sampling, we would have to check for the 99th percentile, rather
  than the 90th percentile. We could only derive this percentile from the
  surveyor data if we new the structure of the delay distribution: if the
  distribution is Poisson, then the 99th percentile will be about 2.4 time
  farther from the mean than the 90th percentile; if it is a power law
  with a 1.5 exponent, it will be 6.5 times farther.
  Can we get better data?

can you (or someone) be more specific?


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