An Investigation into Transport Protocols and Data Transport ...

5.3. Network Aid in Congestion Detection 99
of the TCP flow when a high fraction of losses is less likely.
5.3.2 Active Queue Management & Random Early Detection
The prevalence of drop-tail routers in the Internet can result in ‘lock-up’
[FJ92] and the unfair sharing of network resources. The former occurs when
a high throughput flow starves other competing (possibly lower throughput)
flows of any queue occupancy due to the fact that a higher throughput flow
is more likely to have a larger number of packets in an ingress queue. When
‘lock-up’ does not occur, higher throughput flows still occupy a majority of
the queue, hence being unfair to other flows sharing the bottleneck.
The Random Early Detection (RED) [FJ93] algorithm solves the problems
of over-buffering and fair sharing of buffer resources. [BCC98] recommends
that RED is used as the default queue management algorithm on
the Internet as there apparently no disadvantages and numerous advantages
[FJ93].
Active queue management attempts to prevent queue overflow by selectively
dropping packets in a queue. It also has the advantage that TCP flows
do not have to induce a buffer to overflow as an indication of the congestion.
RED is termed an Active Queue Management solution as a RED enabled
router detects the incipient stages of congestion by observing the exponentially
weighted moving average of the queue size.
Defined with minth and maxth, the two values define the thresholds from
which no packets should be dropped and where all packets should be dropped
respectively. Therefore, the average queue occupancy, q, should fall within
minth and maxth. This enables the unavoidable bursts of packets in the Inter-