(SIP for Instant Messaging and Presence Leveraging Extensions) Enhancements that add instant messaging and presence to the SIP protocol. Developed by the SIMPLE working group of the IETF, SIMPLE adds buddy list subscriptions and notifications and instant messaging commands.

SIMPLE has two modes. Session mode, which is the more efficient mode, sets up the call and provides a raft of features including conferencing. Page mode uses no call setup and is more like a short messaging service for delivering one-shot messages and announcements.

The IETF's Instant Messaging and Presence Protocol (IMPP) working group, from which the SIMPLE group was split off, focuses on helping people build gateways to SIMPLE from other IM systems. In 2001, AOL said it would support SIMPLE, but subsequently abandoned its plans. See SIP.