Cell Loss Priority (CLP) is a flag bit in the ATM cell header that determines the probability of a cell being discarded if the network becomes congested. Cells where the CLP = 0 are insured traffic and unlikely to be dropped. Cells with CLP = 1 are best-effort traffic, which may be discarded in congested conditions in order to free up resources to handle insured traffic.
CLP is used as a control for a network traffic "policing mechanism". Policing is a process that determines if the cells meet pre-defined restrictions as they enter an ATM network. These restrictions include traffic rates and "burst sizes" that are agreed upon by the customer and the network provider.
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