OCSP (Online Certificate Status Protocol) is one of two common schemes for maintaining the security of a server and other network resources. The other, older method, which OCSP has superseded in some scenarios, is known as Certificate Revocation List (CRL).
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OCSP overcomes the chief limitation of CRL: the fact that updates must be frequently downloaded to keep the list current at the client end. When a user attempts to access a server, OCSP sends a request for certificate status information. The server sends back a response of "current", "expired," or "unknown." The protocol specifies the syntax for communication between the server (which contains the certificate status) and the client application (which is informed of that status). OCSP allows users with expired certificates a grace period, so they can access servers for a limited time before renewing.