RANDOM EARLY DETECTION (RED)
A second
mechanism, called random early detection
(RED), is similar to the DECbit scheme in that each router is programmed to
monitor its own queue length, and when it detects that congestion is imminent,
to notify the source to adjust its congestion window. RED, invented by Sally
Floyd and Van Jacobson in the early 1990s, differs from the DECbit scheme in
two major ways.
The first
is that rather than explicitly sending a congestion notification message to the
source, RED is most commonly implemented such that it implicitly notifies the source of congestion by
dropping
one of its packets. The source is, therefore, effectively notified by the
subsequent timeout or duplicate ACK. In case you haven’t already guessed, RED
is designed to be used in
conjunction
with TCP, which currently detects congestion by means of timeouts (or some
other means of detecting packet loss such asduplicate ACKs). As the “early”
part of the RED acronym
suggests,
the gateway drops the packet earlier than it would have to, so as to notify the
source that it should decrease its congestion window sooner than it would
normally have.
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