vide both instant messages and presence information. The service in question provides a user with information about the sta- tus of his or her friends and enables the user to send them instant mes- sages. SIP is another way to establish multimedia sessions by using the same network's infrastructure of SIP servers. The user interface of such an application is usually a buddy list dis- played on the screen. Each person in the list has a label indicating his or her current status. Because Laura has included Bob in her buddy list, his name will appear on the list. If Bob is busy on his workstation, his name appears with the label available. When Bob goes for lunch, his label changes to unavailable. This way, Laura knows if she can send him a short message at any given moment. Now Laura clicks on Bob's name, selects send instant message, and dashes a message off. If, after exchanging some messages, Laura wants to make a voice call to Bob to clarify something, she just has to click again on Bob's name and select establishing a voice call instead. SIMPLE Working Group The Instant Messaging and Presence Protocol (IMPP) working group was chartered to build a protocol that enables applications combining instant messages and presence information. However, after working on the require- ments for such a protocol [RFC 2779], the working group could not come to a consensus. Different groups of people within the IMPP working group had completely different proposals. All of the different proposals had pros and cons. After merging some proposals, the working group ended up with three main approaches to provide instant messaging and presence service: APpli- cation EXchange (APEX), Presence and Instant Messaging (PRIM), and SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE). Chapter 7 We will focus on the third approach, SIMPLE, because it is based on SIP. The SIMPLE working group met for the first time at the 50th Internet Engi- neering Task Force (IETF) meeting in Minneapolis (March 2001). Its task was to define