Frame Relay Traffic Policing and Shaping - Page 5
By Cisco Press
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Extended burst operates in the following fashion:
- If a packet arrives and needs to borrow n number of tokens because the token bucket contains fewer tokens than its packet size requires, then CAR compares the following two values:
- Extended burst parameter value
- Compounded debt, which is computed as the sum over all ai:
i indicates the packet that attempts to borrow tokens since the last time a packet was dropped.
- If the compounded debt is greater than the extended burst value, the exceed action of CAR takes effect. After a packet is dropped, the compounded debt is effectively set to 0. CAR computes a new compounded debt value equal to the actual debt for the next packet that needs to borrow tokens.
- If the actual debt is greater than the extended limit, all packets are dropped until the actual debt is reduced through accumulation of tokens in the token bucket.
NOTE: Although the entire compounded debt is forgiven when a packet is dropped, the actual debt is not forgiven, and the next packet to arrive to insufficient tokens is immediately assigned a new compounded debt value equal to the current actual debt. In this way, actual debt can continue to grow until it is so large that no compounding is needed to cause a packet to be dropped. In effect, the compounded debt is not really forgiven. This scenario leads to excessive drops on streams that continually exceed normal burst.
Cisco recommends the following values for the normal and extended burst parameters:
normal burst = configured rate 4 (1 byte)/(8 bits) 4 1.5 seconds extended burst = 2 4 normal burstNow look at an example that shows how the compounded debt is forgiven, but the actual debt accumulates.
In this example, the following parameters are assumed:
- Token rate is 1 data unit per time unit
- Normal burst size is 2 data units
- Extended burst size is 4 data units
- 2 data units arrive per time unit
Time DU arrivals Actual Debt Compounded Debt ------------------------------------------------------- 1 2 0 0 2 2 0 0 3 2 1 1 4 2 2 3 5 2 3 (temporary) 6 (temporary)The following actions occur at this time:
- A packet is dropped because the new compounded debt (6) would exceed the extended burst limit (4).
- When the packet is dropped, the compounded debt effectively becomes 0, and the actual debt is 2. (The values 3 and 6 were only temporary and do not remain valid if a packet is dropped.)
- The final values for time unit 5 follow. The stream begins borrowing again at time unit 6.
Time DU arrivals Actual Debt Compounded Debt ------------------------------------------------------- 5 2 2 0 6 2 3 3 7 2 4 (temporary) 7 (temporary) At time unit 6, another packet is dropped and the debt values are adjusted accordingly. Time DU arrivals Actual Debt Compounded Debt ------------------------------------------------------- 7 2 3 0
jfreund@internet.com
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