Riverstone Networks WICT1-12 - Manual (Page 570)

26-8 Riverstone Networks RS Switch Router User Guide Release 8.0
Weighted Random Early Detection (WRED) QoS Configuration
26.6.1 Allocating Bandwidth for a Weighted-Fair Queuing Policy
If you enable the weighted-fair queuing policy on the RS, you can allocate bandwidth for the queues on the RS. To
allocate bandwidth for each queue, enter the following command in the Configure mode:
26.7 WEIGHTED RANDOM EARLY DETECTION
(WRED)
WRED is a dynamic process for controlling congestion on RS ports and the segments of the network associated with
the WRED enabled ports. The WRED process consists of setting a minimum queue threshold (min-threshold) and a
maximum queue threshold (max-threshold) on any of the four queues (low, medium, high, and control) belonging to a
port. Associated with these thresholds is an average queue size, which is dynamically calculated as the instantaneous
average buffer depth. If the average queue size is below the min-threshold, no packets are discarded. However, if the
average queue size rises above the min-threshold, WRED uses a packet-marking probability algorithm to randomly
mark packets for discard. As the average queue size increases toward the max-threshold, the probability of packet drop
also increases. Eventually, if the average queue size exceeds the max-threshold, the probability of packet drop becomes
1 (100% of packets are dropped). This increase in the probability of packet drop increases in a linear fashion from 0
to 1 (0% dropped to 100% dropped). Notice that the probability of packet drop roughly depends on bandwidth, i.e.;
the more packets sent by a particular connection, the greater the probability that packets will be dropped from that
connection.
26.7.1 WRED’s Effect on the Network
WRED’s full capabilities to reduce congestion are best used with TCP (and other connection-oriented protocols). As
TCP traffic increases on a WRED port, and the average queue size rises above the min-threshold, some TCP packets
begin to drop. Each TCP source interprets dropped packets as an indication of congestion. As a result, each TCP source
that has experienced a dropped packet reduces its window, the average queue size decreases, and congestion is
alleviated.
Although connection-less protocols do not have the response capability of TCP to sense congestion. Nevertheless,
WRED’s technique of dropping packets based on rising probability assures that those connections that are sending the
most packets or using the most bandwidth will be more likely to have their packets dropped than lower bandwidth
connections. This provides at least some assurance of equality of throughput on a WRED port for connection-less
protocols.
26.7.2 Weighting Algorithms in WRED
WRED provides for the “fine-tuning” of both the average queue size algorithm and the packet-marking probability
algorithm. Control over the average queue size algorithm is provided by the
exponential-weighting-constant
parameter, while control over the packet-marking probability algorithm is provided by the
mark-prob-denominator
parameter.
Allocate bandwidth for a
weighted-fair queuing policy.
qos set weighted-fair control
<percentage>
high
<percentage>
medium
<percentage>
low
<percentage>
port
<port list>
| all-ports