Detect blogs that are no longer receiving updates
It's possible for a blog subscriber to be cut off from the blog's author due to upstream subscribers unsubscribing or leaving the network. We could use some combination of keepalive messages from the author and an adaptive timeout at the subscriber to detect this and mark the blog as inactive or unreachable.
If we use keepalives, the interval between keepalives should adapt to the average interval between posts. If we use a timeout at the subscriber based on the arrival times of updates, TCP's running estimates of round-trip time and round-trip time variance might be a good place to start.
Depending on the mechanism used, this might also be applicable to other kinds of group.