A domain consists of many servers and clients. Servers across domains speak with essentially the same protocol. Client opens a long-lived TCP connection to server, and authenticate only once at the beginning of the session. Client may send many commands asynchronously, without waiting for responses or errors. Servers may open many connections with each other.
Servers connecting across domains authenticate to each other.
Generally, the protocol used is similar to the client-server protocol. It is possible to see how the additional requirements in the AOL-IMX document can be implemented by a suitable development of these ideas. The same notification mechanism is used for IM sends, and Presence status updates. HTTP Digest authentication is used when authentication is needed. XML data is used in the body of the HTTP message.