On waveprotocol.org, there is a white paper about wave client-server protocol. http://www.waveprotocol.org/whitepapers/internal-client-server-protocol
However, the white paper is a high level view of what the protocol should look like, it does not yet specify what the protocol should look like.
I think client-server protocol may be important to help people who build other server and client to maintain compatibility. This is especially important for desktop client. If email protocol was not standardized, Outlook and Thunderbird email client may only talk to only certain email servers. Hopefully, a client-server protocol standard may emerge.