The standard protocol that AIM clients used to communicate is called Open System for CommunicAtion in Realtime (OSCAR). Most AOL-produced versions of AIM and popular third party AIM clients use this protocol. However, AOL also created a simpler protocol called TOC that lacks many of OSCAR's features, but was sometimes used for clients that only require basic chat functionality. The TOC/TOC2 protocol specifications were made available by AOL, while OSCAR is a closed protocol that third parties have had to reverse-engineer.
In January 2008, AOL introduced experimental Extensible Messaging and Presence Protocol (XMPP) support for AIM, allowing AIM users to communicate using the standardized, open-source XMPP. However, in March 2008, this service was discontinued. In May 2011, AOL started offering limited XMPP support. On March 1, 2017, AOL announced (via XMPP-login-time messages) that the AOL XMPP gateway would be desupported, effective March 28, 2017.