What is AWDL (Apple Wireless Direct Link) and how does it work?

I'd like to provide a more precise answer as to how the protocol works internally. I quote part of the abstract of this paper.

In short, each AWDL node announces a sequence of Availability Windows (AWs) indicating its readiness to communicate with other AWDL nodes. An elected master node synchronizes these sequences. Outside the AWs, nodes can tune their Wi-Fi radio to a different channel to communicate with an access point, or could turn it off to save energy.

From a user perspective, AWDL allows a device remain connected to an infrastructure-based Wi-Fi network and communicate with AWDL peers "at the same time" by quickly hopping between the channels of the two networks (AWDL uses fixed social channels 6, 44, and 149). In contrast to the previous answer, we found that current versions of AWDL work fairly well and channel hopping only induces a small overhead.

Disclaimer: I'm co-author of this paper and we retrieved this information by means of reverse engineering. If you are interested in the details, please read the paper and have a look at the Wireshark dissector (published soon).

AWDL recently caught a lot of attention when it caused Wi-Fi issues in iOS 8 and OS X Yosemite devices.

What is AWDL? AWDL (Apple Wireless Direct Link) is a low latency/high speed WiFi peer-to peer-connection Apple uses for everywhere you’d expect: AirDrop, GameKit (which also uses Bluetooth), AirPlay, and perhaps elsewhere. It works using its own dedicated network interface, typically “awdl0".

While some services, like Instant HotSpot, Bluetooth Tethering (of course), and GameKit advertise their services over Bluetooth SDP, Apple decided to advertise AirDrop over WiFi and inadvertently destroyed WiFi performance for millions of Yosemite and iOS 8 users.

How does AWDL work? Since the iPhone 4, the iOS kernels have had multiple WiFi interfaces to 1 WiFi Broadcom hardware chip.

• en0 — primary WiFi interface
• ap1 — access point interface used for WiFi tethering
• awdl0 — Apple Wireless Direct Link interface (since iOS 7?)

By having multiple interfaces, Apple is able to have your standard WiFi connection on en0, while still broadcasting, browsing, and resolving peer to peer connections on awdl0 (just not well).

You can find more info here and here.