This package contains definitions for the protocol messages used by the Magicicada file storage/sharing service (open source fork of Ubuntu One).
The protocol message definitions here are provided as Google Protocol Buffer (.proto) files (see http://code.google.com/apis/protocolbuffers/docs/overview.html). These are compiled to Python using Google's "protoc" tool, which is available on Ubuntu via the protobuf-compiler package, and use the python-protobuf bindings. The package may be built and installed in the usual fashion using the python setup tools (python setup.py build && sudo python setup.py install).
However, note that unless you are very comfortable with what you are doing, if you are installing on an Ubuntu system it is probably better to build and install a Debian package. Recent versions of Ubuntu do not load python modules from /usr/local by default, and you are likely to already have an installed magicicadaprotocol package in any case.
Protocol Overview:
Since it is not well-documented elsewhere, I'll also give a very brief outline of the protocol interactions here.
Most client/server communication is in the form of client-initiated requests. Each message from the client initiating a request is given a request ID which is intended to be unique for the lifetime of the connection.
The actual method for generating request IDs doesn't matter as long as they are even numbers (e.g. 0, 2, 4, ...) and aren't re-used by different requests on the same connection (but note that some requests may involve multiple messages). Server responses to a client request will use the client-supplied request ID, but server-originated messages will have a server-assigned ID which is odd (e.g. 1, 3, 5, ...). In effect, when the low bit of the request ID is set, it indicates a server-initiated request.
The protocol is asynchronous in that multiple requests may be "in flight" at once, their messages (if there are multiple messages in the request) arbitrarily interleaved.
The main exception to the rule that communication is client-initiated are node state change notifications. A client will receive unsolicited node state messages from the server whenever a node (a file or directory) in the user's storage (or in any shares which the user has accepted) changes.
At present, node state extends to the hash (currently a sha1) of a node's content, but may later extend to metadata like executable flags and so on. In addition to these "push" notifications, node state messages may be sent in response to explicit requests by the client.
Every node in the storage system is identified by a UUID, and every node (whether a file or a directory) has content associated with it. For directories, that content will be a sorted enumeration of the directory's contents, serialized using the structures defined in dircontent.proto.
When downloading the content of a node, the client and server have a brief exchange followed by a series of messages bearing data, all with the same request id. Uploads work similarly -- in both cases, the request ID identifies a particular in-progress upload or download.