Round-trip delay time
Encyclopedia
In telecommunication
s, the round-trip delay time (RTD) or round-trip time (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. This time delay therefore consists of the transmission times between the two points of a signal.
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:
Where α is constant weighting factor(0 ≤ α < 1). Choosing a value 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.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, the round-trip delay time (RTD) or round-trip time (RTT) is the length of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
it takes for a signal to be sent plus the length of time it takes for an acknowledgment of that signal to be received. This time delay therefore consists of the transmission times between the two points of a signal.
- In the context of computer networks, the signal is generally a data packet, and the RTT is also known as the pingPingPing is a computer network administration utility used to test the reachability of a host on an Internet Protocol network and to measure the round-trip time for messages sent from the originating host to a destination computer...
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
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
and a high RTT can have a very large amount of data (the bandwidth-delay product
Bandwidth-delay product
In data communications, bandwidth-delay product refers to the product of a data link's capacity and its end-to-end delay . The result, an amount of data measured in bits , is equivalent to the maximum amount of data on the network circuit at any given time, i.e. data that has been transmitted but...
) "in flight" at any given time. Such "long fat pipes" require a special protocol design. One example is the TCP window scale option
TCP window scale option
The TCP window scale option is an option to increase the TCP receive window size above its maximum value of 65,535 bytes.This TCP option, along with several others, is defined in IETF RFC 1323 which deals with Long-Fat Networks, or LFN....
.
The RTT was originally estimated in TCP by:
- RTT = (α · Old_RTT) + ((1 − α) · New_Round_Trip_Sample)
Where α is constant weighting factor(0 ≤ α < 1). Choosing a value 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.