Probe-based (active) measurements of packet loss have formed the basis for
much of our empirical understanding of loss behavior in wide area networks.
Router-based (passive) loss measurements via SNMP, while not widely
available, offer the potential for a more detailed perspective.  In this
paper we present a case study that quantitatively assesses and  compares
the  viewpoints provided by both of these methods for measuring packet
loss.  Our hypothesis was that probe-based and router-based loss
measurements should correlate.  We investigated this by first comparing
SNMP loss  measurements on router interfaces to those extracted from packet
traces in  a series of laboratory experiments.  We found these two passive
measures of  loss to be highly correlated.  Next, we evaluated packet loss
data gathered over three weeks in a widely deployed infrastructure, using
both backbone router interfaces for passive measurements and
co-located hosts that sent active probes in a full mesh.  We found 
little correlation between time series of passive, router-based
measures of loss and active loss probes for all of the paths in our
measurement infrastructure.  We also compared the distributional
characteristics of these loss measurements including lengths of loss free
periods, loss rates during lossy periods, and measures of loss constancy.
We found the level of agreement between passive measures and active
measures for each of these  characteristics to be quite low.  Deeper
evaluation of our data indicated  that current methods for active probing
for packet loss suffer from high  variance inherent in standard sampling
techniques and from effects of  end-host interface loss, which we also
characterize in this work.