What you describe is known as IP tunneling, where a gateway/router connected to both an AppleTalk network and an IP network wraps a DDP packet inside a UDP packet. It then transmits that packet across the IP network to another gateway/router that is capable of de-encapsulating the packet.
The IP Extension to the Apple Internet Rounter (AIR) does just that. When talking to another AIR running the IP Extension, you can tunnel AppleTalk through an IP network, including the Internet.