IEEE 802.11p unicast considered harmful

We study the feasibility of IEEE 802.11p unicast communication in Vehicular Ad Hoc Networks (VANETs). In brief, we found unicast communication using MAC acknowledgement frames (ACKs) to be unsuitable for vehicular networks, because missing ACKs make protocol operations susceptible to pronounced head of line blocking effects. Worse, a transmit queue that is blocked by a missing ACK will delay messages for all protocols running on the same node that use the same access category. Other than in traditional networks, missing ACKs are especially prevalent in VANETs due to their high topology dynamics. Depending on the scenario, delays of messages could be shown to reach 200 ms and beyond - above the tolerable range of many VANET applications. Our findings are based on analytical calculations, measurements on hardware, as well as computer simulations; we conducted simulations both at small scale, for a baseline validation, and at a macroscopic level, to gauge the impact on a more complex protocol.