\documentclass[11pt]{article}
\usepackage{times,graphics,ifthen,i-d,html,txtpsfig,tight}
\usepackage[dvips]{changebar}

\input{../defs}

\title{SIP extension for tracking locations attempted}
\author{Oran/Schulzrinne}
\shorttitle{SIP}
\filename{oran-sip-visited-00}
\wg{SIP WG}
\expires{December 2000}
\address{Cisco/Columbia U.}
\pubdate{\today}

\begin{document}
\maketitle
\begin{abstract}
Proxying hides the destinations tried by the proxy. Since this
information is sometimes useful to the requestor, this draft proposes a
new optional SIP request header called \header{Contacts-Tried} listing the
locations tried unsuccessfully during a search.
\end{abstract}

\section{Introduction}

It may be useful for the initiator of a request to track which locations
have been tried while forking calls, particularly if recursive
indirection is involved.  For example, this allows a caller to avoid
manually calling a person that has already been tried as part of proxy
forking. To provide this functionality, a proxy adds a
\header{Contacts-Tried} header to the provisional or final responses
returned to the upstream client. Also, a UAC or forking proxy
operating in sequential search mode add {\MAY} add this header field to
requests to indicate locations that have been tried unsuccessfully.
The interpretation of the information in the header field is left to the
receiver. Receivers {\MAY}, for example, use this information to avoid
revisiting applications tried unsuccessfully before.

NEED EXAMPLE USES

\section{Syntax}

The \header{Contacts-Tried} general header has the following syntax:

\begin{syntax}
Contacts-Tried & = & "Contacts-Tried" ":" # ( name-addr $|$ addr-spec \\
               &   & *(";" ct-params )\\
ct-params      & = & "reason" = quoted-string\\
               &$|$& "status" = Status-Code\\
\end{syntax}

The ``reason'' parameter contains the reason phrase from the response,
while the ``status'' parameter identifies the numeric status code.  The
``reason'' parameter {\MAY} be present if and only if the ``status''
parameter is present.

The order of elements has no significance.  If added by a proxy to a
response, each proxy adds to the list in responses returned from
downstream servers.

Example:
\begin{verbatim}
  Contacts-Tried: "A. G. Bell" <a.g.bell@bell-tel.com>
    ;reason="Busy" ;status=600,
    "T. Bell" <t.bell@bell-tel.com>
    ;reason="I don't talk to telemarketers" ;status=603
\end{verbatim}

\section{Security Considerations}

Users may expect proxying and forwarding information to remain private. 
Thus, proxies {\MUSTNOT} reveal this information by default and {\MUST}
only reveal this information only if the party designated in the
\header{To} header field has agreed to have this information revealed.

\section{Authors' Addresses}

\begin{flushleft}
David R. Oran\\
Cisco Systems\\
7 Ladyslipper Lane\\
Acton, MA 01720\\
USA\\
Email: {\sf oran@cisco.com}
\end{flushleft}

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

\end{document}
