Traffic policing parameters

Most modern switches and network interface devices (NID) can set different traffic policing parameters. When activated, a policer will monitor the incoming frames and determine their color mode (CM). If the CM is set to color-aware, the policer will then monitor incoming frames and assign them the relative color of green or yellow based on the frame header matching the policer setting and current information rate (IR).

Once confirmed to be color-aware, the frames are passed to the first of two token buckets. Frame headers are then checked to confirm the color they are tagged, commonly via the DSCP or priority code point (PCP) bits within the header. An incoming green frame IR is compared to the committed burst size (CBS) tokens in the bucket. If tokens remain in the bucket, the frame will consume a token and be passed to the egress of the network element. When the IR is lower than the CBS token replenishment rate, the bucket will be classified as full.

Frames tagged as yellow and frames passed from the CBS bucket will be checked against the excess burst signal (EBS) bucket tokens. If the IR is lower than the number of tokens, they will be allowed to pass. Any frames above this rate will be tagged as red and discarded.

With networks evolving, a new standard – ITU-T Y.1564 – was developed. It addresses these different network configurations and ensures quality is maintained across networks with multiple streams with different policing parameters. The new standard also enables the engineer to input the service acceptance criteria (SAC) information that is normally based on a subset of the user’s SLA. By inputting the SAC information before the test is conducted, it’s possible to set basic pass/fail criteria to simplify the results and locate possible points of concern for the engineer. To make the tests more efficient and address all areas, the standard has been written around two core components -- Service Configuration Test and Service Performance Test.

October 13, 2011

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