Editing Latency (engineering) (section)

==={{Anchor|PACKET-SWITCHED}}Packet-switched networks===
{{See also|Bandwidth-delay product|Network delay}}

'''Network latency''' in a [[packet-switched]] network is measured as either [[One-way delay|one-way]] (the time from the source sending a packet to the destination receiving it), or [[round-trip delay time]] (the one-way latency from the source to the destination plus the one-way latency from the destination back to the source). Round-trip latency is more often quoted, because it can be measured from a single point. Many software platforms provide a service called [[Ping (networking utility)|ping]] that can be used to measure round-trip latency. Ping uses the [[Internet Control Message Protocol]] (ICMP) ''echo request'' which causes the recipient to send the received packet as an immediate response, thus it provides a rough way of measuring round-trip delay time. Ping cannot perform accurate measurements,<ref>{{cite web|url=http://www.c0t0d0s0.org/archives/7479-Dont-misuse-ping!.html|title=Don't misuse ping!|access-date=29 April 2015|archive-date=12 October 2017|archive-url=https://web.archive.org/web/20171012190836/http://www.c0t0d0s0.org/archives/7479-Dont-misuse-ping!.html|url-status=dead}}</ref> principally because ICMP is intended only for diagnostic or control purposes, and differs from real communication protocols such as [[Transmission Control Protocol|TCP]]. Furthermore, [[Router (computing)|routers]] and [[internet service provider]]s might apply different [[traffic shaping]] policies to different protocols.<ref>{{cite web
 | url = http://www.knowplace.org/pages/howtos/traffic_shaping_with_linux/network_protocols_discussion_traffic_shaping_strategies.php
 | archive-url = https://web.archive.org/web/20070109033459/http://www.knowplace.org/pages/howtos/traffic_shaping_with_linux/network_protocols_discussion_traffic_shaping_strategies.php
 | title = Network Protocols Discussion / Traffic Shaping Strategies
 | archive-date = 2007-01-09 | year = 2005
 | author = Shane Chen | website = knowplace.org
}}</ref><ref>{{cite web |url=http://aitaseller.wordpress.com/2012/09/19/basic-qos-part-1-traffic-policing-and-shaping-on-cisco-ios-router/ |title=Basic QoS part 1 – Traffic Policing and Shaping on Cisco IOS Router|work=The CCIE R&S |date=19 September 2012|access-date=29 April 2015}}</ref> For more accurate measurements it is better to use specific software, for example: [[hping]], [[Netperf]] or [[Iperf]].

However, in a non-trivial network, a typical packet will be forwarded over multiple links and gateways, each of which will not begin to forward the packet until it has been completely received.  In such a network, the minimal latency is the sum of the transmission delay of each link, plus the forwarding latency of each gateway. In practice, minimal latency also includes queuing and processing delays. [[Queuing delay]] occurs when a gateway receives multiple packets from different sources heading toward the same destination. Since typically only one packet can be transmitted at a time, some of the packets must queue for transmission, incurring additional delay. [[Processing delay]]s are incurred while a gateway determines what to do with a newly received packet. [[Bufferbloat]] can also cause increased latency that is an order of magnitude or more. The combination of propagation, serialization, queuing, and processing delays often produces a complex and variable network latency profile.

Latency limits total [[throughput]] in reliable two-way communication systems as described by the [[bandwidth-delay product]].