1. The Speed Limit Issue Of QOS CAR
1.1 a brief overview of equipment to achieve CAR mechanism.
CAR mechanism is achieved by Token Bucket. When the equipment receive any packets first it will get the proper token use for packets transit. Hence the packets transit required sufficient token in token bucket. If the number of token is less than the packets required, then it will be discard by router.
The command of configuring CAR is “qos car cir XXX pir XXX cbs XXX pbs XXX”. The size of Token Bucket is the CBS value. The filling speed of Token Bucket is the CIR value. Every packet will use several token. The system of equipment will calculate the time interval(△T) of last received packet and first received packet, and complete token filling. The amount of filling packet will equal to CIR*△T.
As per above analysis, system need to record the time of first received packet(t1) and last received packet(t2), in order to calculate △T. △T= t2 – t1. But the buffer used to record the time is limited, which means it will be reset by system periodically. Suppose this period is Nms. If the traffic continued time exceed Nms, after the buffer reseted, the △T calculation will be not correct. The calculated △T will be less than the expecting value, and the filling token also will be less than the expecting value, which will cause some packets discard by equipment, as there is no sufficient token in token bucket.
This is the route reason of why packets discarded by equipment while the traffic have not reach the speed limit.