Glossary
OCSP (Online Certificate Status Protocol)
The Online Certificate Status Protocol lets a verifier ask, in real time, whether one specific certificate has been revoked, rather than downloading and searching through an entire revocation list to answer the exact same question. This trades one problem for another: it avoids the cost and staleness of a periodically-downloaded list, at the cost of a live, per-check network dependency — the responder answering the query has to actually be reachable, which introduces exactly the kind of availability dependency a locally cached list doesn't have.
Neither approach is strictly better in every case; each accepts a different tradeoff between freshness and reliability. A revocation-checking design has to choose deliberately between them — or combine both, falling back to a cached list when a live check isn't reachable — rather than assuming one approach is simply correct and the other obsolete.