CRLs are published periodically. CRLs are issued according to a time-based schedule. CRLs can be issued every single time a certificate is revoked, at a specific time of day, or once every so-many minutes.

If the interval is set to 3, for example, then the first CRL publishing is both a full and delta CRL, then the next two publishing updates are only delta CRLs, and then the fourth interval is both a full and delta CRL again. In other words, every third publishing interval has both a full CRL and a delta CRL.

Interval   1, 2, 3, 4, 5, 6, 7 ...
Full CRL   1        4        7 ...
Delta CRL  1, 2, 3, 4, 5, 6, 7 ...

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