12.2 Requests within a Dialog

   Once a dialog has been established between two UAs, either of them
   MAY initiate new transactions as needed within the dialog.  The UA
   sending the request will take the UAC role for the transaction.  The
   UA receiving the request will take the UAS role.  Note that these may
   be different roles than the UAs held during the transaction that
   established the dialog.

   Requests within a dialog MAY contain Record-Route and Contact header
   fields.  However, these requests do not cause the dialog's route set
   to be modified, although they may modify the remote target URI.
   Specifically, requests that are not target refresh requests do not
   modify the dialog's remote target URI, and requests that are target
   refresh requests do.  For dialogs that have been established with an
   INVITE, the only target refresh request defined is re-INVITE (see
   Section 14).  Other extensions may define different target refresh
   requests for dialogs established in other ways.

      Note that an ACK is NOT a target refresh request.

   Target refresh requests only update the dialog's remote target URI,
   and not the route set formed from the Record-Route.  Updating the
   latter would introduce severe backwards compatibility problems with
   RFC 2543-compliant systems.

12.2.1 UAC Behavior

12.2.1.1 Generating the Request

   A request within a dialog is constructed by using many of the
   components of the state stored as part of the dialog.

   The URI in the To field of the request MUST be set to the remote URI
   from the dialog state.  The tag in the To header field of the request
   MUST be set to the remote tag of the dialog ID.  The From URI of the
   request MUST be set to the local URI from the dialog state.  The tag
   in the From header field of the request MUST be set to the local tag
   of the dialog ID.  If the value of the remote or local tags is null,
   the tag parameter MUST be omitted from the To or From header fields,
   respectively.

      Usage of the URI from the To and From fields in the original
      request within subsequent requests is done for backwards
      compatibility with RFC 2543, which used the URI for dialog
      identification.  In this specification, only the tags are used for
      dialog identification.  It is expected that mandatory reflection
      of the original To and From URI in mid-dialog requests will be
      deprecated in a subsequent revision of this specification.

   The Call-ID of the request MUST be set to the Call-ID of the dialog.
   Requests within a dialog MUST contain strictly monotonically
   increasing and contiguous CSeq sequence numbers (increasing-by-one)
   in each direction (excepting ACK and CANCEL of course, whose numbers
   equal the requests being acknowledged or cancelled).  Therefore, if
   the local sequence number is not empty, the value of the local
   sequence number MUST be incremented by one, and this value MUST be
   placed into the CSeq header field.  If the local sequence number is
   empty, an initial value MUST be chosen using the guidelines of
   Section 8.1.1.5.  The method field in the CSeq header field value
   MUST match the method of the request.


      With a length of 32 bits, a client could generate, within a single
      call, one request a second for about 136 years before needing to
      wrap around.  The initial value of the sequence number is chosen
      so that subsequent requests within the same call will not wrap
      around.  A non-zero initial value allows clients to use a time-
      based initial sequence number.  A client could, for example,
      choose the 31 most significant bits of a 32-bit second clock as an
      initial sequence number.

   The UAC uses the remote target and route set to build the Request-URI
   and Route header field of the request.

   If the route set is empty, the UAC MUST place the remote target URI
   into the Request-URI.  The UAC MUST NOT add a Route header field to
   the request.

   If the route set is not empty, and the first URI in the route set
   contains the lr parameter (see Section 19.1.1), the UAC MUST place
   the remote target URI into the Request-URI and MUST include a Route
   header field containing the route set values in order, including all
   parameters.

   If the route set is not empty, and its first URI does not contain the
   lr parameter, the UAC MUST place the first URI from the route set
   into the Request-URI, stripping any parameters that are not allowed
   in a Request-URI.  The UAC MUST add a Route header field containing
   the remainder of the route set values in order, including all
   parameters.  The UAC MUST then place the remote target URI into the
   Route header field as the last value.

   For example, if the remote target is sip:user@remoteua and the route
   set contains:

      <sip:proxy1>,<sip:proxy2>,<sip:proxy3;lr>,<sip:proxy4>

   The request will be formed with the following Request-URI and Route
   header field:

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