\section{Extensions to SDP}
\label{sec-sdpext}

The Session Description Protocol (SDP) (RFC 2327 \cite{rfc2327})
provides mechanisms for describing sessions.  The information that needs
to be represented in SDP for T.38 is

\begin{itemize}
\item the fact that T.38 is to be used,
\item whether to use TCP or UDP for transport, and
\item which type of error control to be used by T.38.
\end{itemize}

Thus, the SDP message could be the following:

\begin{verbatim}
  v=0
  o=faxgw1 2890844526 2890842807 IN IP4 128.59.19.68
  s=FAX message
  e=faxsupport@company.com
  t=2873397496 0
  c=IN IP4 128.59.19.68
  m=application 49170 udp t38
  a=t38errctl:parFEC
\end{verbatim}

In order to do this, ``application/t38'' needs to be registered as a
MIME type according to the recommendations in \cite{rfc2327}.  The
choice of TCP or UDP can already be represented in SDP, and the error
control scheme should be represented as an attribute.
