1RTT

1RTT, or one round-trip time, is a term used in networking to describe the minimum latency required for a complete two-way exchange between a client and a server, often in the context of establishing a secure connection. It contrasts with zero- and multi-RTT handshakes and is commonly used to describe the latency of secure handshake protocols.

In the context of TLS 1.3, a 1RTT handshake refers to the typical handshake flow in which the client and server exchange messages over a single round trip to establish keys and begin data transfer. After this exchange, application data can be transmitted with encryption already in place. This design reduces handshake latency compared with earlier TLS versions that could require more RTTs.

A typical TLS 1.3 1RTT handshake involves the client sending a ClientHello, the server replying with a ServerHello and subsequent handshake messages, and the client completing the handshake with its final messages. Once the Finished messages are exchanged, the connection is ready for application data. The overall handshake latency is therefore roughly one RTT plus processing time on both sides. Some protocols also support 0RTT data, which allows data to be sent before the full handshake completes if a session has been resumed, though this comes with security tradeoffs such as replay risks and reduced forward secrecy.

In practice, the measured 1RTT latency depends on network RTT and server processing time. It is a key concept in performance discussions for latency-sensitive applications and services, informing decisions about protocol choice and infrastructure. See also: round-trip time, TLS 1.3, 0RTT, QUIC.