In this paper, we propose a new centralized routing protocol named WRP that aims at maximizing the network lifetime. In WRP, the sink node is assumed to be with more capabilities and basically collects each node's estimated wakeup time. This enables the sink node to estimate the wakeup times of all sensor nodes even if some of them are multi-hops away from the sink node. The estimated information is used to find a new route without flooding when some links are broken. Owing to these features, WRP solves the problem of high energy consumption mainly caused by the clock drift in ultra low duty cycled environments. It achieves longer network lifetime independently of the node density because the amount of control traffic does not increase with the node density. We investigate the performance of WRP through extensive simulations and show that WRP increases the network lifetime by more than 10 times against existing routing protocols. WRP can be a very promising routing protocol applicable to the monitoring case like AMR (Automatic Metering Reading) and AMI (Advanced Metering Infrastructure).