Cisco Systems 8600 Series manual Traffic Policing Examples, Dual-Leaky Bucket An Analogy

Traffic Policing Examples

Traffic Policing Examples

Traffic Policing, also known as Usage Parameter Control (UPC), is implemented using either an

ATM Forum single or dual-leaky bucket algorithm. The buckets represent a GCRA (Generic Cell

Rate Algorithm) defined by two parameters:

•

•

Rate (where I, expected arrival interval is defined as 1/Rate)

Deviation (L)

If the cells are clumped too closely together, they are non-compliant and are tagged or discarded as applicable. If other cells arrive on time or after their expected arrival time, they are compliant, but three is no accrued credit.

Dual-Leaky Bucket (An Analogy)

A GCRA viewpoint is as follows:

•For a stream of cells in an ATM connection, the cell compliance is based on the theoretical arrival time (TAT).

•The next TAT should be the time of arrival of the last compliant cell plus the expected arrival interval (I) where I = 1/rate.

•If the next cell arrives before the new TAT, it must arrive no earlier than new TAT - CDVT to be compliant.

•If the next cell arrives after the new TAT, it is compliant, but there is no accrued credit.

CBR Traffic Policing Examples

CBR traffic is expected to be at a constant bit rate, have low jitter, and is configured for a constant rate equal to Peak Cell Rate (PCR). The connection is expected to be always at peak rate.

When a connection is added, a VPI.VCI address is assigned, and the UPC parameters are configured for the connection. For each cell in an ATM stream seeking admission to the network, the VPI.VCI addresses are verified and each cell is checked for compliance with the UPC parameters. The CBR cells are not enqueued, but are processed by the policing function and then sent to the network unless discarded.

For CBR, traffic policing is based on:

•Bucket 1

—PCR(0+1), Peak Cell Rate

—CDVT(0+1), Cell Delay Variation

The CBR connection may be configured with policing selected as either 4 or 5. With policing set to 5, there is no policing. With policing set to 4, there is single leaky bucket PCR policing as shown in Figure 7-11.The single leaky bucket polices the PCR compliance of all cells seeking admission to the network, both those with CLP = 0 and those with CLP =1. Cells seeking admission to the network with CLP set equal to 1 may have either encountered congestion along the user’s network or may have lower importance to the user and have been designated as eligible for discard in the case congestion is encountered. If the bucket depth CDVT (0+1) limit is exceeded, it discards all cells seeking admission. It does not tag cells. If leaky bucket 1 is not full, all cells (CLP =0 and CLP=1) are admitted to the network.

ATM Connections 7-23