\begin{thebibliography}{10}

\bibitem{rfc2119}
S.~Bradner, \htmladdnormallink{``Key words for use in {RFCs} to indicate
  requirement levels,''}{http://www.rfc-editor.org/rfc/rfc2119.txt} RFC 2119,
  Internet Engineering Task Force, Mar. 1997.

\bibitem{rfc3261}
J.~Rosenberg, H.~Schulzrinne, G.~Camarillo, A.~R. Johnston, J.~Peterson,
  R.~Sparks, M.~Handley, and E.~Schooler, \htmladdnormallink{``{SIP:} session
  initiation protocol,''}{http://www.rfc-editor.org/rfc/rfc3261.txt} RFC 3261,
  Internet Engineering Task Force, June 2002.

\bibitem{Q.735.3}
{International Telecommunication Union}, \htmladdnormallink{``Stage 3
  description for community of interest supplementary services using signalling
  system no. 7: Multi-level precedence and preemption,''}{http://www.itu.int}
  Recommendation Q.735.3, Telecommunication Standardization Sector of ITU,
  Geneva, Switzerland, Mar. 1993.

\bibitem{rfc3428}
B.~Campbell, J.~Rosenberg, and H.~Schulzrinne, eds.,
  \htmladdnormallink{``Session initiation protocol {(SIP)} extension for
  instant messaging,''}{http://www.rfc-editor.org/rfc/rfc3428.txt} RFC 3428,
  Internet Engineering Task Force, Dec. 2002.

\bibitem{rfc3311}
J.~Rosenberg, \htmladdnormallink{``The session initiation protocol {(SIP)}
  {UPDATE} method,''}{http://www.rfc-editor.org/rfc/rfc3311.txt} RFC 3311,
  Internet Engineering Task Force, Oct. 2002.

\bibitem{rfc3265}
A.~B. Roach, \htmladdnormallink{``Session initiation protocol (sip)-specific
  event notification,''}{http://www.rfc-editor.org/rfc/rfc3265.txt} RFC 3265,
  Internet Engineering Task Force, June 2002.

\bibitem{draft-ietf-sip-identity}
J.~L. Peterson, \htmladdnormallink{``Enhancements for authenticated identity
  management in the session initiation protocol
  {(SIP)},''}{http://www.ietf.org/internet-drafts/draft-ietf-sip-identity-01.t%
xt} internet draft, Internet Engineering Task Force, Mar. 2003.
\newblock Work in progress.

\bibitem{draft-ietf-sipping-basic-call-flows}
A.~R. Johnston, \htmladdnormallink{``Session initiation protocol basic call
  flow
  examples,''}{http://www.ietf.org/internet-drafts/draft-ietf-sipping-basic-ca%
ll-flows-02.txt} internet draft, Internet Engineering Task Force, Apr. 2003.
\newblock Work in progress.

\bibitem{rfc3420}
R.~Sparks, \htmladdnormallink{``{Internet} media type
  message/sipfrag,''}{http://www.rfc-editor.org/rfc/rfc3420.txt} RFC 3420,
  Internet Engineering Task Force, Nov. 2002.

\bibitem{rfc791}
J.~B. Postel, \htmladdnormallink{``{Internet}
  protocol,''}{http://www.rfc-editor.org/rfc/rfc791.txt} RFC 791, Internet
  Engineering Task Force, Sept. 1981.

\end{thebibliography}
