1. Apparatus comprising: a first device generating a first knowledge digest encoding first configuration information, the first configuration information associated with communications to a second device, the first device receiving messages from the second device, the messages including second knowledge digests encoding second configuration information, the first device comparing the first knowledge digest to the second knowledge digests, and producing an unavailability indication during a predetermined time interval following generating of the first knowledge digest encoding the first configuration information only if the second configuration information encoded in a second knowledge digest received during the predetermined time interval is not a subset of the first configuration information encoded in the first knowledge digest, and producing the unavailability indication after the predetermined time interval if the second configuration information encoded in a second knowledge digest received after the predetermined time interval does not match the first configuration information encoded in the first knowledge digest, the first device indicating the availability of communications between the first device and the second device as long as second knowledge digests continue to be received from the second device and no unavailability indication occurs, and for indicating the unavailability of communications between the first device and the second device if the unavailability indication occurs.