Frame Relay Traffic Policing and Shaping - Page 5

 By Cisco Press | Posted Feb 11, 2002
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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:
    a indicates the actual debt value of the flow after packet i is sent. Actual debt is simply a count of how many tokens the flow has currently borrowed.
    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.
Dropped packets do not count against a rate or burst limit. In other words, when a packet is dropped, no tokens are removed from 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 burst
Now 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
After two time units, the stream has used up its normal burst and must begin borrowing one data unit per time unit, beginning at time unit 3:
 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

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