Tuning the SBAU calculation

The router allows for tuning the calculation of the SBAU (as used by the RED slopes to calculate a packet’s drop probability) after assigning buffers for a packet entering a queue. The router implements a time average factor (TAF) parameter in the buffer policy that determines the contribution of the historical SBU and the instantaneous SBU in calculating the SBAU.

The TAF defines a weighting exponent used to determine the portion of the shared buffer instantaneous utilization and the previous SBAU used to calculate the new SBAU. To derive the new SBAU, the buffer pool takes a portion of the previous SBAU and adds it to the inverse portion of the instantaneous SBU. The formula used to calculate the SBAU is:

where:

SBAUn = SBAU for event n

SBAUn - 1 = SBAU for event (n - 1)

SBU = Instantaneous SBU

TAF = Time average factor

Table: TAF impact on shared buffer average utilization calculation describes the effect that the allowed values of TAF have on the relative weighting of the instantaneous SBU and the previous SBAU (SBAUn-1), when calculating the current SBAU (SBAUn).

The value specified for the TAF affects the speed at which the shared buffer average utilization tracks the instantaneous shared buffer utilization. A low value weights the new shared buffer average utilization calculation more to the shared buffer instantaneous utilization. When TAF is zero, the shared buffer average utilization is equal to the instantaneous shared buffer utilization. A high value weights the new shared buffer average utilization calculation more to the previous shared buffer average utilization value. The TAF value applies to all high- and low-priority RED slopes for ingress and egress buffer pools controlled by the buffer policy.