Availability of an overlay network is a necessary condition for event delivery in event based systems. The availability of the overlay links depends on the underlying physical network. Overlay networks have to be underlay aware in order to provide assured levels of availability. We propose two models of availability for overlay networks, Manifest and Latent Availability. In the Manifest availability model, distinct paths at the overlay level are also node disjoint at the underlay and hence the alternate paths viewed by the overlay are independent in the underlay also. In the latent availability model, it is only guaranteed that any two overlay nodes have a guaranteed number of node disjoint paths between them in the underlay. We analyze both the models for complexity of formation and maintenance, and prove that in the general case, both are NP-complete. Then we identify a set of practical constraints applicable to large scale networks. We demonstrate that under these constraints, late...