SIP has gone through many changes since its humble beginnings. Still, SIP retains just enough of its simple former “endpoint-to-endpoint” characteristics to make it a bit difficult to pass across network borders and through firewalls.
If and when IM (Instant Messaging) and presence eventually become an IETF standard, the SIMPLE protocol will undoubtedly be the principal component, thanks in part to its ability to integrate IM/presence with voice, video, data-sharing, and other elements of conferencing and real-time collaboration.