The initial attempt for CAs to publish the revocation status of their issued certificates was through certificate revocation lists (CRLs). A CRL is simply a list of all the certificates the CA has ever revoked before their scheduled expiration. These are periodically updated by the CAs, and browsers were required to review them before each HTTPS connection. Over time, the CRLs grew in size, as did the task of each browser reviewing them. As the time required to download and parse a large (and growing) CRL grew, so did delays for the user. In an effort to mitigate these issues, browsers and CAs developed and implemented the Online Certificate Status Protocol (OCSP).