  Tomography is important for network design and routing optimization. Prior
approaches require either precise time synchronization or complex cooperation.
Furthermore, active tomography consumes explicit probeing resulting in limited
scalability. To address the first issue we propose a novel Delay Correlation
Estimation methodology named DCE with no need of synchronization and special
cooperation. For the second issue we develop a passive realization mechanism
merely using regular data flow without explicit bandwidth consumption.
Extensive simulations in OMNeT++ are made to evaluate its accuracy where we
show that DCE measured delay correlation is highly identical with the true
value. Also from test result we find that mechanism of passive realization is
able to achieve both regular data transmission and purpose of tomography with
excellent robustness versus different background traffic and package size.
