\section{Backwards Compatibility}
\label{sec:backwards}

The resource priority mechanism described in this document is fully
backwards compatible with SIP systems following RFC 3261
\citenorm{rfc3261}.  Naturally, systems not understand the mechanism can
only deliver standard, not elevated, service priority.  User agent
servers and proxies can ignore any \header{Resource-Priority} header
field just like any other unknown header field and then treat the
request like any other request.  Naturally, the request may still
succeed.

Introducing \header{Require} or \header{Proxy-Require} would not help,
as systems that do not support the mechanism will not improve by
rejecting the request due to feature failure.  Since the intent of
resource priority indications is to increase the probability of call
completion, adding failure modes appears counterproductive.
