Fix interface error/discard percentage calculation exceeding 100%

[c-kr]

Problem Description

My customer encountered this issue on several firewall switches with WAN interfaces where the plugin reported error values exceeding 4000%. This mathematical impossibility indicated a bug in the percentage calculation logic.

Root Cause Analysis

The current calculation uses only successful packets as the denominator:

100 * $self->{delta_ifInErrors} / $self->{delta_ifInPkts}

However, according to RFC 2863, the ifInPkts/ifOutPkts counters represent only successfully received packets, excluding errored/discarded packets. This means:

• Total packets on the wire = Successful packets + Error packets
• Current formula: Errors / Successful_packets * 100
• Correct formula: Errors / (Successful_packets + Errors) * 100

Note: This analysis was assisted by AI and should be verified against the full RFC specification.

Solution

This PR corrects the mathematical formula for both error and discard percentage calculations by including the errors/discards in the denominator.

Related Issue

This fix addresses issue #307 opened by @andytlc68, who independently identified the same problem and proposed an identical solution. The issue already has community support with additional users confirming the problem, indicating this affects at least 3+ users in different environments.

Testing

• ✅ Successfully tested in customer environment
• ✅ Error rates > 100% eliminated
• ✅ Mathematical results now bounded to 0-100% range

Disclaimer and Considerations

⚠️ Please review carefully before merging:

This fix modifies a core component of the plugin. While I've implemented this to the best of my knowledge and successfully tested it in our customer environment, I cannot provide absolute guarantee of correctness due to several considerations:

1. Potential Side Effects: This change affects core interface calculations and may have implications for other parts of the plugin that weren't immediately apparent.

2. Scope Limitation: I've only been able to verify this issue with error rates, but applied the same fix to discard calculations based on logical consistency.

3. Calculation Precision: The new calculation may still have edge cases or imprecision issues (e.g., lack of distinction between unicast/broadcast/multicast traffic).

4. Vendor Implementation: It's possible that only certain vendors incorrectly implement the SNMP specification, and this change might affect others differently.

Recommendation: Please conduct thorough testing across different network equipment vendors and gather community feedback before merging this PR.

Fixes #307

[lausser]

sounds correct. i am on holiday right now, will check and test next week.

[lausser]

It's even
100 * $self->{delta_ifInErrors} / ($self->{delta_ifInPkts} + $self->{delta_ifInErrors} + $self->{delta_ifInDiscards})
unfortunately i am waiting for a customer feedback who seems to be on vacation too, i need it before i can release.

[lausser]

Unfortunately it takes time...still waiting for a customer who is testing the current work-in-progress

[c-kr]

Sure, no hurry. We also need to upgrade to mod-gearman Go worker first before we can use the latest nwc-health with ePN support