Definition
Validation Service
A validation service is a component of a PKI that offers real-time queries for the revocation status of a certificate.
Implementation
OCSP
Implemented with standard protocols like OCSP (Online Certificate Status Protocol).
Connection Latencies
Introduces connection latencies when certificates are used to set up a TLS connection.
Privacy Concerns
Possible privacy concerns, since the usage of a particular certificate is revealed to another party (albeit trusted).