ay be created for a single INVITE request. As a result, Contact and Record- Route header fields must be returned in responses that create early dialogs. Appendix A: Changes in the SIP Specification from RFC 2543 to RFC 3261 RFC 3261 also included new header fields such as Alert-Info, Call-Info, Reply-To, In-Reply-To, and Error-Info, - which add new and useful functionality to SIP. About the Author Alan B. Johnston is a distinguished technical member with MCI in their engi- neering department, and is also an adjunct assistant professor of electrical and systems engineering at Washington University in St. Louis, Missouri. He is cur- rently working with the SIP protocol in their MCI Advantage product. Prior to MCI, he worked at a competitive local exchange carrier (CLEC), Brooks Fiber Properties, Telcordia (formerly Bellcore), and SBC Technology Resources. Dr. Johnston is a coauthor of the new SIP specification RFC 3261 and the cochair of the new XCON Centralized Conferencing IETF Working Group. He has a Ph.D. from Lehigh University, Bethlehem, Pennsylvania, in electrical engineer- ing and a B.S. in engineering (first class honors) from the University of Melbourne, Melbourne, Australia, in electrical engineering. Born in Melbourne, Australia, Dr. Johnston currently resides in a national historic district in St. Louis, Missouri, with his wife Lisa, son Aidan, and daughter Nora. Index 3GPP architecture, 201-2 402 Payment Required response, illustrated, 201 IMS elements, 201 403 Forbidden response, 115 IPv6 addresses, 202 404 Not Found response, 115 Mobile IP reliance, 202 405 Method Not Allowed response, signaling compression, 202 3GPP header fields, 203-4 407 Proxy Authentication 3GPP wireless call flow, 235-54 Required response, 115-16 defined, 235-36 408 Request Timeout response, 116 illustrated, 236 410 Gone response, 116 100 Trying response, 109 411 Length Required response, 116 180 Ringing response, 109 413 Request Entity Too Large 181 Call Is Being Forwarded response, 117 response, 109 414 Request-URI - Too Lon