A protocol? The whole point of REST is that you just ask a resource for a representation that happens to contain links to other resources. Pretty much any resource can tell you about other resources that happen to be relevant. There isn't a protocol specifically for discovery precisely because discovery is built into the whole architecture.
Some RESTful services publish service descriptions in a format like WADL, which could allow a client to predict the URIs for the some of the sorts of resources that the service provides. But that's not a substitute for actually getting links from other resources published by the service, as those links are concrete whereas WADL talks about URI templates.