  SDN-enabled Integrated satellite-terrestrial networks (ISTNs), can provide
several advantages including global seamless coverage, high reliability, low
latency, etc. and can be a key enabler towards next generation networks. To
deal with the complexity of the control and management of the integrated
network, leveraging the concept of software-defined networking (SDN) will be
helpful. In this regard, the SDN controller placement problem in SDN-enabled
ISTNs becomes of paramount importance. In this paper, we formulate an
optimization problem for the SDN controller placement with the objective of
minimizing the average failure probability of SDN control paths to ensure the
SDN switches receive the instructions in the most reliable fashion.
Simultaneously, we aim at deploying the SDN controllers close to the satellite
gateways to ensure the connection between the two layers occurs with the lowest
latency. We first model the problem as a mixed integer linear program (MILP).
To reduce the time complexity of the MILP model, we use submodular optimization
techniques to generate near-optimal solutions in a time-efficient manner.
Finally, we verify the effectiveness of our approach by means of simulation,
showing that the approximation method results in a reasonable optimality gap
with respect to the exact MILP solution.
