A reliability layer is a protocol component that makes control messages more dependable by tracking delivery, acknowledging packets, and retransmitting anything that appears lost. In a VPN such as OpenVPN, this sits on the control side of the connection, where the software negotiates sessions and keeps the tunnel state synchronized.
This matters because availability and session stability are security properties. If the reliability mechanism is disrupted, handshakes can fail, reconnections can loop, or the service can become unstable even though encryption is still enabled. Attackers may abuse this with packet loss, spoofed traffic, or malformed control messages to create a denial of service. Defenders care about it because robust retransmission logic, packet validation, and control-channel hardening help prevent remote-access outages and reduce the chance that small protocol errors turn into operational failures.