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In its simplest form, FIFO queuing involves storing packets when the network is congested and forwarding them in order of arrival when the network is no longer congested.
In some instances, FIFO is the default queuing algorithm and
requires no router configuration. However, FIFO has several
shortcomings. Most importantly, FIFO queuing makes no decision about packet priority; the order of arrival determines bandwidth, promptness, and buffer allocation.
Nor does it provide protection against ill-behaved applications
(sources). Bursty sources can cause long delays in delivering time-sensitive application traffic, and potentially to network control and signaling messages.
These large file transfers and other high-volume network
applications often generate series of packets of associated data. These related packets are known as
packet trains.
As shown in the Figure,
packet trains are groups of packets that tend to move together
through the network. These packet trains can consume all available bandwidth and starve out other traffic.
FIFO queuing was a necessary first step in controlling network traffic, but today's intelligent networks need more sophisticated algorithms.
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