\documentclass[11pt]{article}
\usepackage{i-d}

\newcommand{\INVITE}{\textsf{INVITE}}
\newcommand{\BYE}{\textsf{BYE}}
\newcommand{\ACK}{\textsf{ACK}}
\newcommand{\CANCEL}{\textsf{CANCEL}}
\newcommand{\OPTIONS}{\textsf{OPTIONS}}
\newcommand{\REGISTER}{\textsf{REGISTER}}
\newcommand{\header}[1]{\textsf{#1}}
\newcommand{\host}[1]{\texttt{#1}}

\pubdate{\today}
\expires{May 2002}
\filename{draft-schulzrinne-sip-reason-01}
\title{The Reason Header Field for the Session Initiation Protocol}
\author{Schulzrinne/Oran/Camarillo}
\address{Columbia U./Cisco/Ericsson}
\shorttitle{SIP}
\wg{SIP WG}
\begin{document}
\maketitle

\status{}

\begin{abstract}
For creating services, it is often useful to know why a SIP request was
issued. This document defines an optional informational header field,
\header{Reason}, that provides this information.
\end{abstract}

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

The same SIP \cite{rfc2543} request can be issued for a variety of
reasons.  For example, a SIP {\CANCEL} request can be issued if the call
has completed on another branch or was abandoned before answer.  While
the protocol and system behavior is the same in both cases, namely,
alerting will cease, the user interface may well differ.  In the second
case, the call may be logged as a missed call, while this would not be
appropriate if the call was picked up elsewhere.

Third party call controllers sometimes generate a SIP request upon
reception of a SIP response from another dialog.  Gateways generate SIP
requests after receiving messages from a different protocol than SIP. 
In both cases the client may be interested in knowing what triggered the
SIP request.

SIP responses already have a means of informing the user of why a
request failed.  The simple mechanism in this draft accomplishes
something roughly similar for requests.

Initially, the request header field defined here appears to be most
useful for {\BYE} and {\CANCEL} methods, but it is defined that it can
appear in any method.

Clients are free to ignore this header. It has no impact on protocol
processing.

\subsection{Terminology}
\label{sec:terminology}

In this document, the key words ``{\MUST}'', ``{\MUSTNOT}'',
``{\REQUIRED}'', ``{\SHALL}'', ``{\SHALLNOT}'', ``{\SHOULD}'',
``{\SHOULDNOT}'', ``{\RECOMMENDED}'', ``{\MAY}'', and ``{\OPTIONAL}''
are to be interpreted as described in RFC 2119 \cite{rfc2119} and
indicate requirement levels for compliant SIP implementations.

\section{The \header{Reason} Request Header Field}

The \header{Reason} request header field can appear in any method. 
\begin{syntax}
Reason            & = & "Reason" HCOLON reason-code *(SEMI reason-params)\\
reason-code       & = & 1*DIGIT\\
reason-params     & = & triggered-by $|$ protocol-cause $|$ reason-text\\
                  &   & $|$ reason-extension\\
triggered-by      & = & "triggered" EQUAL protocol\\
protocol          & = & quoted-string\\
protocol-cause    & = & "cause" EQUAL cause\\
cause             & = & 1*DIGIT\\
reason-text       & = & "text" EQUAL text\\
text              & = & *$<$TEXT-UTF8, excluding CR, CF$>$\\
reason-extension  & = & generic-param\\
\end{syntax}

Examples are:
\begin{verbatim}
  Reason: 1;text="Call completed elsewhere"
  Reason: 4;triggered=Q.850; cause=16; text="Terminated"
  Reason: 7;triggered=SIP/2.0; cause=600; text="Busy Everywhere"
\end{verbatim}
The syntax of the header field follows the standard SIP parameter syntax.

The following reason codes and text phrases have been defined:

\begin{enumerate}
\item (Call completed elsewhere): {\CANCEL} request; another
branch completed the call.

\item (Abandoned): {\CANCEL} request; the call attempt was
abandoned.

\item (Timed out): {\CANCEL} request; the call attempt timed out.

\item (Terminated): {\BYE} request; the caller or callee
terminated the call.

\item (Transfer completed): {\BYE} request; the call
transfer was completed.

\item (No media):  {\BYE} request; the call was terminated since the
other side did not receive media for an extended period of time. 

\item (Interworking):  {\BYE} and {\CANCEL} requests; the call was
terminated because a gateway or a third party call controller received
an error or a release message from the other side.  This reason code
{\SHOULD} only be used if none of the other reason codes is applicable.
\end{enumerate}

The text phrase {\MAY} be modified, e.g., translated into different
languages or enhanced with additional call-specific information, for
example,
\begin{verbatim}
  Reason: 2;text="Anrufer gibt nach 30 Sekunden auf"
\end{verbatim}

The following values for the triggered parameter have been defined:
\begin{enumerate}
\item SIP/2.0: The cause parameter contains a SIP status code.

\item Q.850:  It refers to the ITU-T Q.850 recommendation.  The cause
parameter contains a Q.850 cause value (decimal representation).
\end{enumerate}

Proxies generating a {\CANCEL} request upon reception of a {\CANCEL}
from the previous hop that contains a \header{Reason} header field
{\SHOULD} copy it into the new {\CANCEL} request.

\section{IANA Considerations}

IANA registers new reason codes.  All new reason codes {\MUST} be
defined in an RFC.

IANA registers new values for the triggered parameter. 

\section{Security Considerations}

While spoofing or removing the \header{Reason} header field has no
impact on protocol operation, the user interface may change and end
systems may provide services based on this header field.  Thus, it is
{\RECOMMENDED} that this field is protected by a suitable integrity
mechanism.

\section{Acknowledgments}

We wish to thank Jonathan Rosenberg for his comments and suggestions.

\section{Authors' Addresses}

\begin{flushleft}
Henning Schulzrinne\\
Dept. of Computer Science\\
Columbia University\\
1214 Amsterdam Avenue\\
New York, NY 10027\\
USA\\
electronic mail: {\sf schulzrinne@cs.columbia.edu}
\end{flushleft}

\begin{flushleft}
Dave Oran\\
Cisco\\
\end{flushleft}

\begin{flushleft}
Gonzalo Camarillo\\
Ericsson\\
Advanced Signalling Research Lab.\\
FIN-02420 Jorvas\\
Finland\\
electronic mail:  {\sf Gonzalo.Camarillo@ericsson.com}
\end{flushleft}

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\input{../copyright}

\end{document}