AirTunes + Multicast(Bonjour) = ‘It Just Works’
About a year ago I watched a video on Bonjour, which is Apple’s Zero Configuration Networking. It seemed like netbios and apple talk all over again, and I thought no more about it.
I’ve now experienced Bonjour in action, and believe it’s strengthened the Apple meme/culture of ‘it just works’. For me this was getting an Airport Express and playing with AirTunes, which works perfectly, out of the box for any local machine. I was amazed at how this was possible, thinking broadcasts must be going every which way on my network to get this working.
Firing up Wireshark, I was determined to figure out how it was working. The secret of the ‘it just works’ is Bonjour.
When firing up iTunes, a Multicast DNS Query (MDNS) is sent out to 224.0.0.251 looking for _appletv._tcp.local as a PTR Record. AirTunes replies, via Multicast, and the iTunes PC then does an ARP/Broadcast to resolve the IP.
This process enables the remote speaker to be seen.
When playing a music over AirTunes/Remote Speaker, another multicast packet is sent out to stream the music. Again this is over 224.0.0.251, now with a SRV record, Sending type of TXT looking for iTunes_Ctrl
At this point, PDU’s start flying and the speakers come alive.
I could have just read the data sheet, for the one line answer, but I’d have no greater understanding, and where’s the fun in that? 
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