Mac68k Forums

I did some checking on AppleTalk over Wi-Fi a while ago. I'm struggling to completely remember it, but I know I had it figured out at one point. Here is at least one reason that a lot of Wi-Fi access points have trouble with AppleTalk packets (I think).

AppleTalk uses IEEE 802.2 SNAP packets. For some weird reason, AARP (AppleTalk Address Resolution Protocol) packets have a 0 for the OUI (Organizationally Unique Identifier) code instead of Apple's assigned OUI code. DDP packets, on the other hand, do use Apple's correct OUI. The problem is that a value of 0 is a special value that means the packet is an encapsulated Ethernet II frame. Wi-Fi also encapsulates packets as SNAP frames. The problem (as I'm recalling) is that access points supporting AppleTalk need to know that they're supposed to keep the AARP packets in SNAP format when they spit them out the Ethernet side, or something like that, and they end up getting sent out as Ethernet II frames instead because of the OUI of 0. The Ethernet II frames are not recognized by the Mac's AppleTalk stack, so it doesn't work.

I was able to observe this incorrect behavior in action on at least one wireless access point I have at home. Good APs that work with AppleTalk will recognize that packets with an OUI of 0, that are also AARP packets, should remain as SNAP frames instead of being translated to Ethernet II.

I also believe it would be technically possible to make a pass-through device you could attach to your access point that would translate all AARP frames it generated from Ethernet II into SNAP form. What I don't know, though, is if there are other similar compatibility problems.

Edit: I did some more research to confirm what I was remembering. If you download the 802.11-2012 spec from IEEE, Annex P (page 2744 in the PDF) specifically talks about compatibility with AppleTalk. It has examples of what exactly you're supposed to do when converting to/from Ethernet in order to remain compatible with AppleTalk and IPX. I think the correct behavior is actually a bit more complicated than I originally said, because EtherTalk Phase I and II are slightly different. The IEEE spec shows exactly what to do in all cases.

Message was edited by: dougg3

Yeah, a lot of their standards are inaccessible, but 802.11 is actually freely available. You do have to say what type of user you are and enter your email address though:

http://standards.ieee.org/getieee802/download/802.11-2012.pdf