Several readers are reporting UDP/3478 (STUN) traffic to Amazon AWS address 54.84.9.242. If you got packets or know what it is, please share below.
Update Apr 29 19:30 UTC:
Thanks everyone for pitching in and providing packets and logs! With your help, we were able to link these requests to a project conducted by Dan Kaminsky at White Ops.">At White Ops, weve been exploring some of the more interesting corners of web browsers, and what they expose via JavaScript, to detect the presence of some of the more interesting abusers of web browsers, i.e. bots.">STUN, as part of the new WebRTC protocol stack, actually exposes certain classes of bot behavior. People are welcome to contact me privately if they if it was dangerous to users, we">In general, network administrators should expect a significant increase in STUN (and UDP) traffic over the next year that will ultimately be traced to web browsers. TCP has been a fantastic workhorse but between the rise of videoconferencing (which requires entirely different network topologies in order to provide reasonable latencies and echo cancellation) and the constant push of the web away from request/response and towards push semantics for web apps, WebRTC will likely take a position alongside other normal protocols like HTTP, DNS, etc.
There is also a discussion thread on this in the SANS ISC Forum at https://isc.sans.edu/forums/STUN+traffic/745/ and https://isc.sans.edu/forums/STUN+traffic/745/2/