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. HTTP redirection is used as needed.
Message is responded to.
Basically, each interaction must be self-contained. Some of the issues in III apply. Note that SIP users may be mobile, i.e. move from terminal to terminal. Supporting such features is not a requirement for IMPP, and unlikely to become so. While it is great to have such a significant extension worked out, it would be even better to see a protocol framework where the complexity of handling mobility need not be paid for if it is not used.