Round-trip delay time

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One problem that must be resolved when using a Transmission Control Protocol (TCP) is how to deal with timeouts and retransmissions. The round-trip delay time (RTD) or round-trip time (RTT) is a big factor in helping to decide what to do in each case. RTT may also be used to find the best possible route.

In telecommunications, the RTT is the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgment of that signal to be received.

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

In space technology, the round-trip delay time or Round Trip Light Time is the time light (and hence any signals) takes to go to the spacecraft and to return.

Network links with both a high bandwidth and a high RTT can have a very large amount of data (the bandwidth-delay product) "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: RTT = (α * Old_RTT) + ((1-α) * New_Round_Trip_Sample)[1] This was improved by the Jacobson/Karel 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.

See also

  • ping, a network utility for measuring round-trip delay time

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).


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