OK, so Representational State Transfer
was initially described in the context of HTTP, but is not limited to that protocol.
REST - to contrast it with RPC - follows a slightly different concept than "traditional" RPC mechanisms. Some people think that the way communication is handled with REST is "better" than with RPC.
Now, what interests me, is whether there are actually any working frameworks that are RESTful, but do not rely on HTTP as the underlying transfer mechanism.
Restlet is a Java framework that provides a uniform interface (as far as possible) for a number of protocols. The model of the API is based on HTTP, but it can be used with other protocols. However, some features might not be directly available, depending on the protocol used.
There are also libraries that use XMPP.
However, for the REST principles to work, it's useful to stick with the same protocol, otherwise, the two "worlds" won't be able to have a uniform interface (in terms of protocols, not API).
[...] but do not rely on HTTP as the underlying transport.
I should add that HTTP is not a transport protocol, but a transfer protocol: "Hypertext Transfer Protocol". It's a fundamental difference when it comes to REST. (It can be used for transport by other protocols, e.g. SOAP, but that's effectively a misuse.)
