Here you go: link
I believe that federation of cloud services is the next important step for truly secure and flexible file sync and share cloud software. Because of that we are working for a while on the needed technologies and now have the first draft of an open specification ready.
The goal of the Federated Cloud Sharing API is to be a common language for sharing files across different file sharing server implementations. That only works if a wide audience provides input, which is why we started the Open Cloud Mesh initiative and have already been working with and talking about earlier drafts to partners and other open source projects.
The document describes an API which consists of two parts, WebDAV for file transfer and a simple REST based API to initiate sharing and exchange metadata. It is a very simple and pragmatic model, re-using as much existing technology as possible to ease implementation and migration. Rather than re-thinking the entire infrastructure of the web to enable federation, it combines the existing model of email (username @ server) and file transport protocols.
For more information and background on the architecture, read my earlier blog post.
For more general information on Federation, see my preceding post about the Next Generation File Sync and Share technology
This draft is more than theory as it is already implemented in ownCloud, being introduced as ‘server to server sharing’ with ownCloud 7 and matured to its current state which you can try out in ownCloud 8.1. See the video below for Federated Cloud Sharing in action.