\begin{thebibliography}{1}

\bibitem{rfc2119}
S.~Bradner, ``Key words for use in {RFCs} to indicate requirement levels,''
  Request for Comments 2119, Internet Engineering Task Force, Mar. 1997.

\bibitem{rfc2778}
M.~Day, J.~Rosenberg, and H.~Sugano, ``A model for presence and instant
  messaging,'' Request for Comments 2778, Internet Engineering Task Force, Feb.
  2000.

\bibitem{rfc2543}
M.~Handley, H.~Schulzrinne, E.~Schooler, and J.~Rosenberg, ``{SIP:} session
  initiation protocol,'' Request for Comments 2543, Internet Engineering Task
  Force, Mar. 1999.

\bibitem{rfc2142}
D.~Crocker, ``Mailbox names for common services, roles and functions,'' Request
  for Comments 2142, Internet Engineering Task Force, May 1997.

\bibitem{draft-schulzrinne-sip-911}
H.~Schulzrinne, ``Providing emergency call services for {SIP}-based internet
  telephony,'' {Internet Draft}, Internet Engineering Task Force, Mar. 2001.
\newblock Work in progress.

\bibitem{rfc3087}
B.~Campbell and R.~Sparks, ``Control of service context using {SIP}
  {Request-URI},'' Request for Comments 3087, Internet Engineering Task Force,
  Apr. 2001.

\end{thebibliography}
