@bob It's the implementation. The "protocol" has no current state. It is just PuSH (pubsubhubbub) delivery of atom feeds to subscribers. The implementation of the _hub_ chooses when and how to deliver and whether to try again upon various kinds of failures.

@maiyannah you might want to check out plugins/OStatus/lib/hubconfqueuehandler.php and plugins/OStatus/lib/huboutqueuehandler.php for a starter, also read plugins/OStatus/README:

$config['ostatus']['hub_retries']
(default 0)
Number of times to retry a PuSH send to consumers if using internal hub