The current status of the keep-alive protocol.
The value 'alive' indicates that the far-end
ESR is replying the SDP Echo Requests messages
sent by this device indicating no error condition.
The value 'noResponse' indicates that the number
of consecutive SDP Echo Request messages unack-
nowledged by the far-end ESR exceeded the limit
defined by sdpKeepAliveMaxDropCount. The values
'senderIdInvalid' and 'responderIdError' are
two error conditions detected by the far-end ESR.
The value 'disabled' indicates that the keep-alive
protocol is not enabled.