- TCP sessions slow down to the approximate rate of output-link bandwidth.
- Average queue size is small (much less than the maximum queue size).
- TCP sessions are desynchronized by random drops.
RED is a dropping mechanism that randomly drops packets before a queue is full. The dropping strategy is based primarily on the average queue length—that is, when the average size of the queue increases, RED will be more likely to drop an incoming packet than when the average queue length is shorter.
Because RED drops packets randomly, it has no per-flow intelligence. The rationale is that an aggressive flow will represent most of the arriving traffic and therefore it is likely that RED will drop a packet of an aggressive session. RED therefore punishes more aggressive sessions with higher statistical probability and is, therefore, able to somewhat selectively slow down the most significant cause of congestion. Directing one TCP session at a time to slow down allows for full utilization of the bandwidth, rather than utilization that manifests itself as crests and troughs of traffic.
As a result of implementing RED, the problem of TCP global synchronization is much less likely to occur, and TCP can utilize link bandwidth more efficiently. In RED implementations, the average queue size also decreases significantly, as the possibility of the queue filling up is reduced. This is because of very aggressive dropping in the event of traffic bursts, when the queue is already quite full.
RED distributes losses over time and normally maintains a low queue depth while absorbing traffic spikes. RED can also utilize IP precedence or differentiated services code points (DSCPs) to establish different drop profiles for different classes of traffic, in which case it is referred to as weighted random early detect (WRED) or DSCP-based WRED, respectively.
This topic describes the elements of a RED traffic profile used to implement the RED packet-dropping strategy.
A RED traffic profile is used to determine the packet-dropping strategy and is based on the average queue length. The probability of a packet being dropped is based on three configurable parameters contained within the RED profile:
■ Minimum threshold: When the average queue length is equal or above the minimum threshold, RED starts dropping packets. The rate of packet drop increases linearly as the average queue size increases, until the average queue size reaches the maximum threshold.
■ Maximum threshold: When the average queue size is above the maximum threshold, all packets are dropped.
■ Mark probability denominator: This is the fraction of packets that are dropped when the average queue depth is at the maximum threshold. For example, if the denominator is 512, one out of every 512 packets is dropped when the average queue is at the maximum threshold. The linear increase of packet drops from the minimum threshold (0 drops) to the maximum threshold is based on this parameter and the queue size between the minimum and maximum thresholds.
The minimum threshold value should be set high enough to maximize the link utilization. If the minimum threshold is too low, packets may be dropped unnecessarily, and the transmission link will not be fully used.
The difference between the maximum threshold and the minimum threshold should be large enough to avoid global synchronization. If the difference is too small, many packets may be dropped at once, resulting in global synchronization.
The mark probability has the effect of controlling the number of packets that are dropped when the average queue length reaches the maximum threshold. If the value is set too low, it will result in too many dropped packets. If the value is set too large, RED dropping can be rendered ineffective.
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