\begin{thebibliography}{1}

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

\end{thebibliography}
