Vyatta Network OS Documentation

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Path monitor policy compliance states and parameters

Policy compliance states and control of switching between the states.

A path monitor policy defines three compliance states:

  • Compliant
  • Marginally Compliant
  • Non-Compliant

To determine compliance, a path monitor probes its associated path monitor host, calculates the packet's statistics, and checks whether the measurements are within the values defined by the specified parameters.

The following sections use the RTT policy requirement to explain how policy compliance is determined. Other policy requirements (such as jitter) are processed in the same way with the corresponding metric.

Threshold

Defines the upper limit for RTT at or below which the RTT complies with a path monitor policy.

The threshold range is 5 to 10000 milliseconds (ms).

Tolerance

Tolerance is a dampening mechanism that is biases the system towards compliance.

The sum of threshold and tolerance defines the upper limit for RTT beyond which the RTT does not comply with a path monitor policy.

The tolerance range is 5 (default) to 1000 ms.

Robustness

Robustness is a dampening mechanism that aims to prevent excessive switching between policy states.

Policy compliance state transitions are determined by the number of consecutive path monitor samples that return the same result that differs from the current state.

For example, if the current state is Compliant and the robustness mechanism is triggered, then the compliance state changes to Marginally Compliant.

The robustness parameter allows a trade-off between stability in bursty traffic conditions and speed of detection.

The robustness range is 1 to 10. The default value is 1, which disables the robustness parameter.

Figure 1. Path monitor compliance states. The relationship between the path monitor compliance states and the threshold and tolerance parameters.
Table 1. Change of compliance states when robustness is disabledHow the path monitor policy engine determines the new compliance state of a path monitor when robustness has a value of 1 (disabled). Based on the current state of the path monitor and the RTT value.
Current state RTT valueNew state
Compliant, Marginally Compliant, or Non-Compliant
RTT <= Threshold
Compliant
Compliant or Marginally Compliant
Threshold < RTT <= (Threshold + Tolerance)
Marginally Compliant
Compliant or Marginally Compliant
RTT > (Threshold + Tolerance)
Non-Compliant
Non-Compliant
RTT > Threshold
Non-Compliant
Figure 2. Change of compliance states when robustness is disabled. How the path monitor policy engine determines the new compliance state of a path monitor when robustness has a value of 1 (disabled). Based on the current state of the path monitor and the RTT value.
Figure 3. Change of compliance states when robustness is enabled. How the path monitor policy engine determines the new compliance state of a path monitor when robustness has a value of 2. In this figure, the new states for the third and fourth samples are different, even though RTT is greater than the tolerable threshold in both samples. The reason for the different states is that it takes two consecutive Non-Compliant RTT values for the new state to change from Marginally Compliant to Non-Compliant.