We study the performance of IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) under packet drop attacks. We consider an external jamming attacker who can selectively interfere with traffic around a targeted router. We show that the RPL implementation in Contiki OS allows such an attacker to continuously drop packets forwarded via the targeted router without triggering any rerouting, even when link-layer security mechanisms are in place. To counter such attacks, we design and analyze additional measures that can be built on RPL. In particular, we show that adding measures at the targeted router is more effective than doing so at affected children nodes. We also evaluate the performance of our enhanced RPL using Cooja, the Contiki network simulator. Our results show that our proposed measures have the potential to significantly reduce the fraction of packets being dropped without affecting RPL's performance when there is no attack.