\section{{\OPTIONS}}
\label{sec:OPTIONS}

The {\OPTIONS} method is used to query a server as to its capabilities. 
A server that believes it can contact the user, such as a user agent
where the user is logged in and has been recently active, {\MAY} respond
to this request with a capability set.  A called user agent {\MAY}
return a status reflecting how it would have responded to an invitation,
e.g., 600 (Busy).  A server {\SHOULD} return \header{Allow},
\header{Accept}, \header{Accept-Encoding}, \header{Accept-Language} and
\header{Supported} header fields.  The response {\MAY} contain a message
body indicating the capabilities of the end system (rather than
properties of any existing call).

The use of the \header{Call-ID} header field is discussed in
Section~\ref{sec:Call-ID}.  An {\OPTIONS} requests for an existing
call-id has no impact on that call.

This method {\MUST} be supported by SIP user agents and registrars.
