Instant messaging/presence is at an inflection point today. The lack of standards-based interoperable IM/presence systems makes it difficult for IT to control and monitor deployment of this popular business tool. Proprietary networks and protocols also impede IM users from communicating with others outside their organizations.
What's needed is a unified protocol that enables IM/presence, just as Simple Mail Transfer Protocol, HTTP and Real-Time Protocol (RTP) do for e-mail, Web and voice traffic. The solution is Session Initiation Protocol for IM and Presence Leveraging Extensions (SIMPLE).
The Internet Engineering Task Force's SIMPLE working group, in the applications area, is chartered to specify a set of profiles and extensions to SIP in order to enable IM/presence. The group has produced an extensive set of Internet drafts and is expected to publish a proposed standard this year. The IM and Presence Protocol working group of the IETF has published the general requirements for and model of IM/presence as RFCs 2778 and 2779. The basics of the proposed protocol have been widely implemented.
The internals of SIMPLE are the same as those of SIP: Instead of data-retrieval methods such as GET and POST, SIP has signaling methods such as INVITE and BYE to start and end a call or session.
SIMPLE adds a new request method called MESSAGE to send a one-shot IM, called pager-mode IM. SUBSCRIBE is used to request presence information to be sent to the requester, while NOTIFY is used to transport the presence information.
Longer IM sessions, in which participants exchange multiple messages over time, are signaled using INVITE and a transport protocol called Message Session Relay Protocol (MSRP). With SIMPLE, MSRP is used to transport the text of IMs, just as with SIP RTP is used to transport voice packets in an IP phone call.
Much of the IM/presence infrastructure reuses SIP without change. For example, an IM client sends a REGISTER message to a SIP registrar server to signal that it is available to receive IMs. Just as in an ordinary SIP system, registrar servers process logons from endpoints.
Spreading the message
IM clients send the actual IM traffic and presence updates to each other directly or via SIP proxy servers and SIP redirect servers. SIP proxy servers relay SIP requests between SIP system elements such as SIP phones, while redirect servers are used to tell clients about participants that have moved.
An IM client uses MIME to send multimedia requests. Multiparty IM and chat rooms already are supported, as SIP was designed to route signaling to a group of endpoints as easily as to one endpoint.
IM/presence's relationship with SIP is analogous to SMS' relationship with the mobile phone signaling system. SMS piggybacks text messages on the mobile phone network. IM/presence piggybacks on SIP, which is the Internet's form of telephone signaling.
By using SIMPLE, IM/presence automatically gains the benefits of SIP, which combines multimedia, multiparty and group-aware features, as well as the ability to support this same functionality for mobile users.
IM/presence is likely to go the way of e-mail and telephony, and become an IETF standard. SIMPLE is ideally suited for integrating IM/presence with voice, data-sharing, video and other real-time collaboration features. Nearly all leading manufacturers of IP and telecom equipment and every major IM service provider have announced support for SIMPLE, and there are enough SIMPLE-based offerings that there is no longer a question of whether SIMPLE will gain widespread deployment.