On 7210 SAS platforms RED slopes support is as follows:
On 7210 SAS-T (both network mode and access-uplink mode), each queue, supports a high-priority RED slope and a low-priority RED slope.
On 7210 SAS-Mxp, 7210 SAS-R6, and 7210 SAS-R12, each queue supports a high-priority RED slope and a low-priority RED slope, which are configurable using the queue-management policies.
On 7210 SAS-Sx/S 1/10GE and 7210 SAS-Sx 10/100GE, each queue supports a high-priority RED slope and a low-priority RED slope.
The high-priority RED slope manages access to the shared portion of the buffer pool for high-priority or in-profile packets. The low-priority RED slope manages access to the shared portion of the buffer pool for low-priority or out-of-profile packets.
By default, all slopes are disabled.
The WRED uses average queue lengths, queue thresholds provisioned, and drop probability to calculate the packet’s eligibility to be enqueued. The committed portion of the buffer pool is exclusively used by a queue to enqueue traffic within committed rate.
For the queues within a buffer pool, packets are either queued using committed burst size (CBS) buffers or shared buffers. The CBS buffers are simply buffer memory that has been allocated to the queue while the queue depth is at or below its CBS threshold.
When a queue depth exceeds the queue’s CBS, packets received on that queue must contend with other queues exceeding their CBS for shared buffers. To resolve this contention, the buffer pool uses two RED slopes to determine buffer availability on a packet by packet basis. A packet that was either classified as high priority or considered in-profile is handled by the high-priority RED slope. This slope should be configured with RED parameters that prioritize buffer availability over packets associated with the low-priority RED slope. Packets that had been classified as low priority or out-of-profile are handled by this low-priority RED slope.
The following is a simplified overview of how a RED slope determines shared buffer availability on a packet basis:
The RED function keeps track of shared buffer utilization and shared buffer average utilization.
At initialization, the utilization is 0 (zero) and the average utilization is 0 (zero).
When each packet is received, the current average utilization is plotted on the slope to determine the packet’s discard probability.
A random number is generated associated with the packet and is compared to the discard probability.
The lower the discard probability, the lower the chances are that the random number is within the discard range.
If the random number is within the range, the packet is discarded which results in no change to the utilization or average utilization of the shared buffers.
A packet is discarded if the utilization variable is equal to the shared buffer size or if the used CBS (actually in use by queues, not just defined by the CBS) is oversubscribed and has stolen buffers from the shared size, lowering the effective shared buffer size equal to the shared buffer utilization size.
If the packet is queued, a new shared buffer average utilization is calculated using the time-average-factor (TAF) for the buffer pool. The TAF describes the weighting between the previous shared buffer average utilization result and the new shared buffer utilization in determining the new shared buffer average utilization. (See Tuning the shared buffer utilization calculation.)
The new shared buffer average utilization is used as the shared buffer average utilization next time a packet’s probability is plotted on the RED slope.
When a packet is removed from a queue (if the buffers returned to the buffer pool are from the shared buffers), the shared buffer utilization is reduced by the amount of buffers returned. If the buffers are from the CBS portion of the queue, the returned buffers do not result in a change in the shared buffer utilization.
A RED slope itself is a graph with an X (horizontal) and Y (vertical) axis. The X-axis plots the percentage of shared buffer average utilization, going from 0 to 100 %. The Y-axis plots the probability of packet discard marked as 0 to 1. The actual slope can be defined as four sections in (X, Y) points (as shown in the following figure):
Figure: RED slope characteristics
Section A is (0, 0) to (start-avg, 0). This is the part of the slope that the packet discard value is always zero, preventing the RED function from discarding packets when the shared buffer average utilization falls between 0 and start-avg.
Section B is (start-avg, 0) to (max-avg, max-prob). This part of the slope describes a linear slope where packet discard probability increases from zero to max-prob.
Section C is (max-avg, max-prob) to (max-avg, 1). This part of the slope describes the instantaneous increase of packet discard probability from max-prob to one. A packet discard probability of 1 results in an automatic discard of the packet.
Section D is (max-avg, 1) to (100%, 1). On this part of the slope, the shared buffer average utilization value of max-avg to 100% results in a packet discard probability of 1.
Plotting any value of shared buffer average utilization will result in a value for packet discard probability from 0 to 1. Changing the values for start-avg, max-avg and max-prob allows the adaptation of the RED slope to the needs of the access or network queues using the shared portion of the buffer pool, including disabling the RED slope.