Two IP communication standards offer the promise of interoperability among different Instant Messaging (IM) products. In 1999, the Internet Engineering Task Force (IETF) released the Session Initiation Protocol (SIP) to support interactive connections between different types of clients. IM clients can use SIP to establish public or invitation-only private chat sessions. But SIP wasn't developed specifically for IM—it handles only session initiation and disconnects. After the session is established, SIP hands off the chat to a higher-level protocol, such as HTTP or XML.
Key IM vendors such as Microsoft, IBM, and AOL have created the SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE—http://www.ietf.org/html.charters/simple-charter.html) IETF standard as a way to customize SIP for IM. SIMPLE is still in final development, but it's starting to become widely used. Although many corporate clients that claim to have SIP or SIMPLE support don't yet connect to other such clients because both standards are still being fleshed out, vendors that claim their clients support SIP or SIMPLE will probably adopt the final standard when it's available and are more likely to achieve interoperability than is a client that doesn't yet support the standards. If interoperability is important to your company, ask your IM vendor for details about its support for SIP and SIMPLE.