output packet queueing?

Where do network drops typically happen in darwin? Eg, I'm seeing packets get dropped somewhere between udp_output() and my driver. (comparing counters in my driver and in udp_output()). The culprit is returning ENOBUFS. My driver has control of its own if_output (eg, its not going through IOKit). Since the BSD per-interface ifp->if_snd output queuing seems to be gone (or replaced somehow), I'm not sure where output queuing is happening. Can somebody shed some light on this, please? Thanks, Drew