In telecommunications, the term round-trip delay time or round-trip time (RTT) has the following meanings:

1. The elapsed time for transit of a signal over a closed circuit, or time elapsed for a message to a remote place and back again.
2. In primary or secondary radar systems, the time required for a transmitted pulse to reach a target and for the echo or transponder reply to return to the receiver.

Round-trip delay time is significant in systems that require two-way "interactive" communication, such as voice telephony, or ACK/NAK data systems where the round-trip time directly affects the throughput rate, such as the Transmission Control Protocol (TCP). It may range from a very few microseconds for a short line-of-sight (LOS) radio system to many seconds for a multiple-link circuit with one or more satellite links involved. This includes the node delays as well as the media transit time.

In regards to TCP communication the RTT time is calculated from the 3-way handshake by measuring the time between segment transmission and ACK receipt.

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