SIP is the widely deployed Internet Standard Session Initiation Protocol (RFC 3621). XMPP is the Internet Standard eXtensible Messaging and Presence Protocol. There are substantial architectural similarities between services of these types, and advanced clients need presence, messaging, and session initiation capabilities. Integrating these services makes some sense, and two key attempts have been made.
SIMPLE is a family of protocols that builds presence and messaging over SIP. This layering has resulted in a specification of substantial complexity and rather poor performance, which suggests that this layering is not a good choice. These technical issues are a key factor as to why XMPP is winning over SIMPLE as the standard of choice for presence and instant messaging.
JINGLE is a session initiation protocol built over XMPP. This layering works better, and it is clear that JINGLE will have an important role in providing session initiation between clients that support XMPP. It is a clean and effective approach to help XMPP clients support voice and video within a protocol and security framework that is already implemented.
However, JINGLE does not provide a complete replacement for SIP.
A SIP client implementation is a very straightforward mechanism to set up a phone call, and is supported on a large number of phones. If call connect is all that is needed, SIP is ideal. JINGLE only makes sense if XMPP is needed for other reasons. VOIP phones will continue to be SIP based, and VOIP deployments will be based around PABX systems that include SIP servers.
Another reason for this is that JINGLE does not provide a framework for phone type services such as call forwarding, putting calls on hold and voicemail integration. The underlying reason is that JINGLE is based on communication starting out with (XMPP) presence, which is a model of user to user communication, as distinct from the more traditional model underlying SIP of device to device communication.
JINGLE use will grow, based on voice and video services in the context of XMPP clients. SIP use will continue to grow in the context of VOIP phone deployment. There will be co-existence, and gateways between SIP and XMPP/JINGLE.