[Medium] Packet Loss During Rapid Topology Changes #21

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opened 2026-02-11 19:31:44 +00:00 by thabeta · 0 comments

thabeta commented

2026-02-11 19:31:44 +00:00

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Issue

The hello/IHU (I Heard You) sequence tracking logic is overly sensitive to out-of-order packets during rapid network topology changes, resulting in artificial packet loss metrics.

Location

mycelium/src/babel/hello.rs, mycelium/src/babel/ihu.rs

Problem Description

Babel uses sequence numbers to track link quality. If sequence numbers arrive out of order (due to multiple paths or network jitter), the current implementation treats the gap as loss rather than reordering. During rapid neighbor churn, this causes the link cost to spike artificially high, causing redundant routing recalculations (flapping).

Impact

Severity: MEDIUM (Performance/Stability)
Frequency: Common in dynamic wireless or mobile meshes
User Impact: Frequent routing route switching, lower throughput

Remediation

Implement a reordering window (sliding window) for sequence number tracking
Only count link loss if a sequence number is definitively missing after the window expires
Smooth the metric calculation with a low-pass filter (Exponential Moving Average)

Testing

Simulate 5% packet reordering with netem/tc
Verify metric stability compared to a baseline node

## Issue The hello/IHU (I Heard You) sequence tracking logic is overly sensitive to out-of-order packets during rapid network topology changes, resulting in artificial packet loss metrics. ## Location `mycelium/src/babel/hello.rs`, `mycelium/src/babel/ihu.rs` ## Problem Description Babel uses sequence numbers to track link quality. If sequence numbers arrive out of order (due to multiple paths or network jitter), the current implementation treats the gap as loss rather than reordering. During rapid neighbor churn, this causes the link cost to spike artificially high, causing redundant routing recalculations (flapping). ## Impact - **Severity**: MEDIUM (Performance/Stability) - **Frequency**: Common in dynamic wireless or mobile meshes - **User Impact**: Frequent routing route switching, lower throughput ## Remediation 1. Implement a reordering window (sliding window) for sequence number tracking 2. Only count link loss if a sequence number is definitively missing after the window expires 3. Smooth the metric calculation with a low-pass filter (Exponential Moving Average) ## Testing - Simulate 5% packet reordering with netem/tc - Verify metric stability compared to a baseline node

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Reference

geomind_code/mycelium_network#21

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