0RTT

0-RTT, or zero round-trip time, is a technique used in TLS 1.3 and QUIC to reduce connection latency by allowing the client to send application data before the handshake completes. This is possible when the client has a previously established session with the same server and presents a pre-shared key or session resumption ticket. The server, using this material, can decrypt and process the early data without waiting for a full handshake to finish.

In TLS 1.3, 0-RTT data is optional and subject to security considerations. Early data is protected with keys derived from the PSK and does not provide forward secrecy for that data. It is vulnerable to replay attacks, and servers may reject, drop, or selectively allow 0-RTT data. Clients are generally advised to limit 0-RTT use to idempotent or safe operations.

In QUIC, 0-RTT is integrated into the protocol so a client can send application data as part of the initial packets. This further reduces latency for HTTP/3 and other protocols layered on QUIC. However, the same replay and anti-replay concerns apply, and servers may apply anti-replay checks or drop 0-RTT data.

Operational notes: Because of security risks, 0-RTT may be disabled by default on many servers or libraries; its use is typically tempered by policy and application requirements. Standards include TLS 1.3 (RFC 8446) and QUIC (RFC 9000 and 9001).