As a protocol feature baked into TLS 1.3, 0-RTT (zero round-trip time) helps clients more quickly establish secure connections via key exchanges with the server. The data that traverses this connection is cryptographically protected, and there's no need to share key exchange protocol messages before transmission.
0-RTT provides perfect forward secrecy since encryption and decryption keys are changed automatically and often. This constant refresh cycle strengthens security and minimizes secure data leakage during a hack.
How does 0-RTT work?
The 0-RTT process works in three steps:
Client "Hello" – The client packages together 0-RTT data (occasionally from a previous session, if possible) with a "client hello" message to the server. Any sent data is encrypted and requires a shared secret for decryption.
Server "Hello" – Upon receiving the client's initial message, the server will verify the client's identity and then decrypt any received data. This requires the server to recognize the client and the encrypted data.
Server response – The server sends a response to the client.
Since the encrypted data arrives alongside the handshake, a network roundtrip is saved. However, organizations must still watch out for targeted replay attacks, in which attackers intercept original client messages and attempt to read them. While no security solution is perfect, cryptography helps maintain data integrity and privacy.
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Does HAProxy support 0-RTT?
Yes! HAProxy has supported 0-RTT since version 1.8, and it remains a key SSL/TLS feature in HAProxy Enterprise. When a client uses 0-RTT, HAProxy can connect to backend servers the same way and enable 0-RTT with a single line in your bind or server configuration.
Explore our SSL/TLS termination solution to learn more.