It is not clear how long PubSub security keys can be used if it is not possible to get new keys from the SKS (e.g. SKS down, no network connection possible).

8.4.1 SecurityGroupType definition
The Property KeyLifetime defines the lifetime of a key in milliseconds.

We have definitions for update behavior and update cycles e.g. check for next key in a cycle that is half the key lifetime.
But we have no definition what happens if the update fails.

In Client/Server we define that update should start 1/3 before lifetime expires and keys are valid for 1/3 after lifetime expires.
A similar definition for expiry would make sense.