Specifies the return SDP-ID to be used by the far-end node for
its message reply. This parameter is optional and is valid only if
tmnxOamPingCtlTestMode has a value of 'sdpPing'.
If this SDP-ID does not exist on the far-end, terminates on another
node different than the originating node, or some other issue
prevents the far-end from using the specified SDP-ID, the OAM Echo
message reply is sent using generic IP/GRE OAM encapsulation. The
received forwarding class (as mapped on the ingress network interface
for the far-end) defines the forwarding class encapsulation for the
reply message.