It is often desirable for a Jabber/XMPP application (commonly but not necessarily a client) to take different actions depending on the capabilities of another application from which it receives presence information. Examples include:
- Showing a different set of icons depending on the capabilities of other clients.
- Not sending XHTML-IM content to plaintext clients such as cell phones.
- Allowing the initiation of Voice over IP (VoIP) sessions only to clients that support VoIP.
- Not showing a "Send a File" button if another user's client does not support File Transfer.
Recently, some existing Jabber clients have begun sending Software Version requests to each entity from which they receive presence. That solution is impractical on a larger scale, particularly for users or applications with large rosters. This document proposes a more robust and scalable solution: namely, a presence-based mechanism for exchanging information about entity capabilities.