89
QCN algorithm
The QCN algorithm includes the CP algorithm and the RP algorithm.
CP algorithm
The CP measures the queue size by periodically sampling frames and computes the congestion
state based on the sampling result.
As shown in
, the CP algorithm includes the following parameters:
•
Q
—Indicates the instantaneous queue size at the sampling point.
•
Qeq
—Indicates the desired queue size.
•
Qold
—Indicates the queue size at the previous sampling point.
•
Fb
—Indicates the extent of congestion in the form of a quantized value.
The following formulas apply:
•
Qoff = Q – Qeq
•
Q
δ
= Q – Qold
•
Fb = – (Qoff + wQ
δ
)
where w is a constant to control the weight of Q
δ
in determining the value of Fb.
The CP determines whether to generate CNMs based on the Fb value.
•
When Fb
≥
0, no congestion occurs, and the CP does not generate a CNM.
•
When Fb < 0, congestion occurs, and the CP generates an CNM containing the QntzFb.
QntzFb is the quantized value of |Fb| and is calculated according to the following rules:
{
If Fb < – Qeq x (2 x w + 1), QntzFb takes the maximum value of 63.
{
Otherwise, QntzFb = – Fb x 63/(Qeq x (2 x w + 1)).
Figure 35 Congestion detection
RP algorithm
An RP decreases its transmission rate based on the value of |Fb| in the received CNM. The greater
the Fb value, the lower the RP reduces its transmission rate. After the RP reduces its transmission
rate, the RP gradually increases the transmission rate to the original level.
Summary of Contents for FlexFabric 5940 Series
Page 23: ...17 Figure 3 QoS processing flow ...
Page 84: ...78 Figure 26 MPLS label structure ...
Page 91: ...85 Switch burst mode enable ...
