he protocol. An Internet-Draft containing bug fixes and clarifications to SIP was submitted beginning in July 2000, referred to as RFC 2543 "bis". This docu- ment was eventually published as RFC 3261 [4], which obsoletes (or replaces) the original RFC 2543 specification. In addition, several SIP extension RFC documents have been published. The popularity of SIP in the IETF has lead to the formation of other SIP- related working groups. The Session Initiation Protocol investigation (SIPPING) working group was formed to investigate applications of SIP, develop requirements for SIP extensions, and publish best current practice (BCP) documents about the use of SIP. This working group also publishes SIP event package RFCs. The SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE) working group was formed to standardize related proto- cols for presence and instant messaging applications. Other working groups that make use of SIP include PSTN and Internet Internetworking (PINT) working group and Service in the PSTN/IN requesting Internet Services (SPIRITS) working group. To advance from Proposed Standard to Draft Standard, a protocol must have multiple independent interworking implementations and limited opera- tional experience. Since the early days of RFC 2543, SIP interoperability test events, called SIPit (formerly called "bakeoffs"), have been held a few times per year. For the latest information about SIPit, visit the SIP Forum Web site: http://www.sipforum.org. (Note that the SIP Forum is a marketing/promotion organization for SIP and does not have any standardization function.) The final level, Standard, is achieved after operational success has been demonstrated [5]. With the documented interoperability from SIPit, SIP will move to Draft Stan- dard status in the future. SIP incorporates elements of two widely used Internet protocols: Hyper Text Transport Protocol (HTTP) used for Web browsing and Simple Mail Transport Protocol (SMTP) used for e-mail. From HTTP, SIP borrowed a SIP: Understanding t