  A source submits status updates to a network for delivery to a destination
monitor. Updates follow a route through a series of network nodes. Each node is
a last-come-first-served queue supporting preemption in service. We
characterize the average age of information at the input and output of each
node in the route induced by the updates passing through. For Poisson arrivals
to a line network of preemptive memoryless servers, we show that average age
accumulates through successive network nodes.
