0-RTT (TLS Early Data)

0-RTT, or early data, is a TLS 1.3 feature that lets a client resuming a previous session send encrypted application data along with its very first message, without waiting for the handshake to finish. It removes one network round trip for repeat connections.

Why it matters

The saved round trip helps latency on slow mobile or long-distance links. The cost is that early data is not protected against replay: an attacker who captures the first flight can resend it, and the server may process it twice. For that reason servers and CDNs that enable 0-RTT should only let it carry safe, idempotent requests such as a GET for a static page, never a payment or a state change. RFC 8470 defines the 425 Too Early status code a server can return to tell the client to retry after the full handshake.

Example

nginx can accept early data and pass a hint to the application:

ssl_early_data on;
proxy_set_header Early-Data $ssl_early_data;

The backend sees Early-Data: 1 and can answer 425 for any unsafe request.

Common mistakes

• Enabling it globally without checking that POST handlers reject early data.
• Believing replay is a TLS flaw. It is a documented tradeoff of the feature.
• Forgetting that forward secrecy is weaker for early data than for the rest of the connection.

Related

• TLS 1.3
• TLS session resumption
• TLS handshake
• Forward secrecy

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