The Online Certificate Status Protocol (OCSP) is an Internet protocol used for obtaining the revocation status of an X.509 digital certificate.
It was created as an alternative to certificate revocation lists (CRL).
Messages communicated via OCSP are encoded in ASN.1 and are usually communicated over HTTP. The “request/response” nature of these messages leads to OCSP servers being termed OCSP responders.
Using OCSP, a client can obtain revocation information regarding a certificate. OCSP responders accept requests that contain a serial and reply with a signed statement about that single certificate. This avoids the potentially large overhead of downloading a CRL and ostensibly means you can get “fresher” revocation information about a certificate.