The Online Certificate Status Protocol (OCSP) is an Internet protocol used for obtaining the revocation status of an X.509 digital certificate. It is used by https clients (browsers) to confirm that the certificate sent by the server they have connected to is a valid one. Every time the client connects to a server it needs to contact the server's CA OCSP responder and check the validity of its certificate. An OCSP responder (a server typically run by the certificate issuer) may return a signed response signifying that the certificate specified in the request is 'good', 'revoked', or 'unknown'. If it cannot process the request, it may return an error code. It has several benefits over Certificate Revocation List (CRL) since an OCSP response contains less information and does not need to be parsed.

Nginx by default contains the core modules needed which makes it light and lean web server. Any additional stuff needed have to be recompiled and added as modules since Nginx doesn't have a dynamic (plug-able) module infrastructure like Apache for example.