3.5.1.1 Indicating Support for BENOTIFY

The client signals support for the BENOTIFY mechanism by inserting two headers in the SUBSCRIBE request.

 Supported: ms-benotify
 Proxy-Require: ms-benotify

The Proxy-Require header ensures that any intermediate SIP proxy that does not understand this extension and that expects a SIP response to every SIP request to maintain proper SIP transaction state will reject the initial subscription at which time the client can resend the SUBSCRIBE minus these headers and disable the BENOTIFY mechanism for this subscription.