Round-trip delay

In telecommunications, the round-trip delay (RTD) or round-trip time (RTT) is the amount of time it takes for a signal to be sent plus the amount of time it takes for an acknowledgement of that signal to be received. This time delay includes the propagation times for the paths between the two communication endpoints.

In the context of computer networks, the signal is generally a data packet, and the RTT is also known as the ping time. An internet user can determine the RTT by using the ping command.

End-to-end delay is the length of time it takes for a signal to travel in one direction and is often approximated by half the RTT.

Protocol design

Network links with both a high bandwidth and a high RTT (and thus high bandwidth-delay product) can have a very large amount of data "in flight" at any given time. Such "long fat pipes" require a special protocol design. One example is the TCP window scale option.

The RTT was originally estimated in TCP by:

$\mathrm {RTT} =\alpha \cdot \mathrm {old\_RTT} +(1-\alpha )\cdot \mathrm {new\_round\_trip\_sample}$

where α is constant weighting factor ($0\leq \alpha <1$). Choosing a value for α close to 1 makes the weighted average immune to changes that last a short time (e.g., a single segment that encounters long delay). Choosing a value for α close to 0 makes the weighted average respond to changes in delay very quickly. This was improved by the Jacobson/Karels algorithm, which takes standard deviation into account as well. Once a new RTT is calculated, it is entered into the equation above to obtain an average RTT for that connection, and the procedure continues for every new calculation.

Wi-Fi

Accurate round-trip time measurements over Wi-Fi using IEEE 802.11mc is the basis for the Wi-Fi positioning system.

See also

• End-to-end delay
• Lag
• Latency (engineering)
• Minimum-Pairs Protocol
• Network delay
• Time of flight