\section{Changes from RFC 2543}

In addition to editorial clarifications, this document changes or adds
the following features to SIP as specified in RFC 2543:

\begin{itemize}
\item Extensions developed by the IETF no longer use the
\header{org.ietf} prefix.

\item Tag syntax was generalized.

\item \header{Via} header branch parameters were extended to allow
``spirals'', where two requests that differ only in the request URI are
not treated as copies.

\item New optional header fields, \header{Alert-Info},
\header{Call-Info}, \header{In-Reply-To}.

\end{itemize}
