Intelecommunications,round-trip delay(RTD) orround-trip time(RTT) is the amount of time it takes for a signal to be sentplusthe amount of time it takes for acknowledgement of that signal having been received. This time delay includespropagation timesfor the paths between the twocommunication endpoints.[1]In the context of computer networks, the signal is typically adata packet.RTT is commonly used interchangeably withping time,which can be determined with theping command.However, ping time may differ from experienced RTT with other protocols since the payload and priority associated withICMPmessages used by ping may differ from that of other traffic.
End-to-end delayis the length of time it takes for a signal to travel in one direction and is often approximated as half the RTT.
Protocol design
editRTT is a measure of the amount of time taken for an entire message to be sent to a destination and for a reply to be sent back to the sender. The time to send the message to the destination in its entirety is known as thenetwork latency,and thus RTT is twice the latency in the network plus a processing delay at the destination. The other sources of delay in a network that make up the network latency are processing delay in transmission, propagation time, transmission time and queueing time. Propagation time is dependent on distance. Transmission time for a message is proportional to the message size divided by the bandwidth. Thus higher bandwidth networks will have lower transmission time, but the propagation time will remain unchanged, and so RTT does fall with increased bandwidth, but the delay increasingly represents propagation time.[2]: 90, 91
Networks with both high bandwidth and a high RTT (and thus highbandwidth-delay product) can have large amounts ofdata in transitat any given time. Suchlong fat networksrequire a special protocol design.[3]One example is theTCP window scale option.
The RTT was originally estimated in TCP by:
whereis constant weighting factor ().[4]Choosing a value forclose 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 forclose to 0 makes the weighted average respond to changes in delay very quickly. This was improved by theJacobson/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
editAccurate round-trip time measurements overWi-FiusingIEEE 802.11mcare the basis for theWi-Fi positioning system.
See also
editReferences
edit- ^Round-trip delay time,Boulder, Colorado:National Telecommunications and Information Administration's Institute for Telecommunication Sciences, archived fromthe originalon 2021-10-17,retrieved2021-05-29
- ^Forouzan, Behrouz A.; Fegan, Sophia Chung (2007).Data communications and networking(4th ed.). Boston: McGraw-Hill Higher Education.ISBN9780072967753.
- ^Brian Heder (May 6, 2014),"Are your pipes too big?",Network World,archived fromthe originalon June 5, 2014,retrieved2016-01-09
- ^Douglas E. Comer(2000).Internetworking with TCP/IP - Principles, Protocols and Architecture(4th ed.). Prentice Hall. p. 226.ISBN978-0-13-018380-4.