\section{Introduction}
\label{sec-intro}

Fax is a popular means for transferring documents between locations. 
Traditionally, this has been done over the telephone network, as defined
in the ITU specification T.30 \cite{T.30}.  Some of the reasons for the
populatity of fax is that the sender of a fax gets a notification that
the fax has been successfully sent, and that the receiver gets
information on the senders telephone number and the time the fax was
received.  Another reason is that fax transmission over a telephone
connection can not easily be eavesdropped.  When introducing fax over
IP, these benefits should be preserved.  The ITU standard T.38
\cite{T.38} defines how to transport the fax signals over IP.  It is
expected that the fax channel is set up by some other means, e.g.,
through H.323 or SIP.  Annex D of H.323 \cite{H.323} describes how H.323
supports fax over IP.  In this document we will describe how SIP and SDP
can do the same.  It would probably be possible to even support the T.38
scope with SIP and SDP, but that is for future work.
