In this paper, we investigate issues of synchronization in highly
aggregated flows such as would be found in the Internet backbone.
Understanding this phenomenon is important since it leads to reduced
network utilization.  Our hypothesis  is that regularly spaced loss events
lead to window synchronization in long lived flows.  We argue that window
synchronization is likely to be more common in the Internet than previously
reported.  We support our argument with evidence of the existence and
evaluation of the characteristics of periodic discrete congestion events
using active probe data gathered in the Surveyor infrastructure.  When
connections experience loss events which are periodic, the aggregate
offered load to neighboring links rises and falls in cadence with the loss
events. Connections whose cWnd values grow from W/2 to W at approximately
the same rate as the loss event period soon synchronize their cWnd additive
increases  and multiplicative decreases.  We find that this window
synchronization can  scale to large numbers of connections depending on the
diversity of  roundtrip times of individual flows.  Through simulation we
investigate  conditions under which window synchronization occurs.  A model
is presented  that predicts important characteristics of the loss events in
window  synchronized flows including the quantity, intensity, and duration.
The  model effectively explains the prevalence of discrete loss events in
fast  links with high multiplexing factors as  well as the queue buildup
and  queue draining phases of congestion.  We then show how the model
scales well for common traffic engineering tasks at the multi-hop level.