1231 Sending the Request

Local policy MAY specify an alternate set of destinations to attempt.

   If the Request-URI contains a SIPS URI, any alternate destinations

   MUST be contacted with TLS.  Beyond that, there are no restrictions

   on the alternate destinations if the request contains no Route header

   field.  This provides a simple alternative to a pre-existing route

   set as a way to specify an outbound proxy.  However, that approach

   for configuring an outbound proxy is NOT RECOMMENDED; a pre-existing

   route set with a single URI SHOULD be used instead.  If the request

   contains a Route header field, the request SHOULD be sent to the

   locations derived from its topmost value, but MAY be sent to any

   server that the UA is certain will honor the Route and Request-URI

   policies specified in this document (as opposed to those in RFC

   2543).  In particular, a UAC configured with an outbound proxy SHOULD

attempt to send the request to the location indicated in the first

   Route header field value instead of adopting the policy of sending

   all messages to the outbound proxy.

 

      This ensures that outbound proxies that do not add Record-Route

      header field values will drop out of the path of subsequent

      requests.  It allows endpoints that cannot resolve the first Route

      URI to delegate that task to an outbound proxy.

 

   The UAC SHOULD follow the procedures defined in [4] for stateful

   elements, trying each address until a server is contacted.  Each try

   constitutes a new transaction, and therefore each carries a different

   topmost Via header field value with a new branch parameter.

   Furthermore, the transport value in the Via header field is set to

   whatever transport was determined for the target server.