Abstract: A probe virtual network function (VNF) is deployed in a software defined network (SDN), where the probe VNF computes delays and determines operation status of other VNFs as ‘available’ or ‘unavailable’ based on whether the computed delays are bounded or unbounded (or if a packet fails to arrive at a given VNF). The computed delay and the determined operation-status are then reported to a control function. The availability of such delay measurements using the probe VNF makes the routing algorithm within the controller more intelligent by incorporating the delay sensitivity of various service function chains.