This object specifies the value of the
'a=direction:<role>' SDP attribute supported by
the user agent. The direction attribute is used
to describe the role of the user agent (as an
endpoint for a connection-oriented media stream)
in the connection setup procedure.
none : No role is specified.
passive : The user agent will advertise itself
as a 'passive' entity (inside the NAT)
in the SDP.
active : The user agent will advertise itself
as a 'active' entity (outside the NAT)
in the SDP.