How to identify GOOSE communications failure?

It is of the interest how GOOSE communication failure signal is generated and how this signal can be used in configurable logic of an IED. As far as I understand GOOSE-messages are sent periodically even if there are no status changes. And in case if status change occurs, generation of GOOSE-messages occurs with shorter time intervals, increasing up to constant value T0 which is retransmission time for steady state. Here the following questions arise:
  1. What happens if timeAllowedtoLive time interval has elapsed?
  2. Does the subscriber set all the values to default when timeAllowedtoLive time interval has elapsed?
  3. How GOOSE-communications failure is detected? Which data object/attribute or service facilitates in detection of the failure?
  4. Is it reasonable to implement the following logic: Besides the required data, include constant value (BOOLEAN type) in a transmitted dataset. Assign True value for this constant. Detect the value at the subscriber’s side. If the value goes to FALSE (reset to default value on GOOSE-communication failure) on subscriber’s side, can this be used to identify GOOSE-communications failure?
Please, let me know if there are any misunderstandings in my view. Thanks in advance.

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