 an OSP Authoriza- tion Token," IETF Internet-Draft, Work in Progress, February 2003. [16] European Telecommunications Standards Institute, "Telecommunications and Internet Protocol Harmonization Over Networks (TIPHON); Open Settlement Protocol (OSP) for Inter-domain Pricing, Authorization, and Usage Exchange," Technical Specification 101 321, Version 2.1.0. [17] Jennings, C., J. Peterson, and M. Watson, "Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks," RFC 3325, 2002. [18] Schulzrinne, H., D. Oran, and G. Camarillo, "The Reason Header Field for the Session Initiation Protocol (SIP)," RFC 3326, 2002. [19] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method," RFC 3515, 2003. Sparks, R., "The SIP Referred-By Mechanism," IETF Internet-Draft, Work in Progress, February 2003. Donovan, S., and Rosenberg, J., "Session Timers in the Session Initiation Protocol (SIP)," IETF Internet-Draft, Work in Progress, November 2002. Handley, M., and V. Jacobson, "SDP: Session Description Protocol," RFC 2327, 1998. [23] Sparks, R., "Internet Media Type message/sipfrag," RFC 3420, 2003. Rosenberg, J., and H. Schulzrinne, "An INVITE Initiated Dialog Event Package for the Session Initiation Protocol (SIP)," IETF Internet-Draft, Work in Progress, March 2003. Rosenberg, J., and H. Schulzrinne, "A Session Initiation Protocol (SIP) Event Package for Conference State," IETF Internet-Draft, Work in Progress, June 2002. SIP: Understanding the Session Initiation Protocol [26] Sugano, H., et al., "Common Presence and Instant Messaging (CPIM) Presence Informa- tion Data Format," IETF Internet-Draft, Work in Progress, 2002 Zimmerer, E., et al., "MIME Media Types for ISUP and QSIG Objects," RFC 3204, December 2001. [28] Olson, S., "A Mechanism for Content Indirection in Session Initiation Protocol (SIP) Messages," IETF Internet-Draft, Work in Progress, February 2003. [29] Freed, M., and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME). Part One: Format of Internet Me