Traffic policing (communications)

In communications, traffic policing is the process of monitoring network traffic for compliance with a traffic contract and taking steps to enforce that contract. Traffic sources which are aware of a traffic contract may apply traffic shaping to ensure their output stays within the contract and is thus not discarded. Traffic exceeding a traffic contract may be discarded immediately, marked as non-compliant, or left as-is, depending on administrative policy and the characteristics of the excess traffic.

The recipient of traffic that has been policed will observe packet loss distributed throughout periods when incoming traffic exceeded the contract. If the source does not limit its sending rate (for example, through a feedback mechanism), this will continue, and may appear to the recipient as if link errors or some other disruption is causing random packet loss. The received traffic, which has experienced policing en route, will typically comply with the contract, although jitter may be introduced by elements in the network downstream of the policer.

With reliable protocols, such as as opposed to , the dropped packets will not be acknowledged by the receiver, and therefore will be resent by the emitter, thus generating more traffic.

Sources with feedback-based congestion control mechanisms (for example ) typically adapt rapidly to static policing, converging on a rate just below the policed sustained rate.

Co-operative policing mechanisms, such as packet-based discard facilitate more rapid convergence, higher stability and more efficient resource sharing. As a result, it may be hard for endpoints to distinguish TCP traffic that has been merely policed from TCP traffic that has been shaped.

Where cell-level dropping is enforced (as opposed to that achieved through packet-based policing) the impact is particularly severe on longer packets. Since cells are typically much shorter than the maximum packet size, conventional policers discard cells which do not respect packet boundaries, and hence the total amount of traffic dropped will typically be distributed throughout a number of packets. Almost all known packet reassembly mechanisms will respond to a missing cell by dropping the packet entirely, and consequently a very large number of packet losses can result from moderately exceeding the policed contract.

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