04 Autonomous Routing and Policy Engine

04 — Autonomous Routing & Policy Engine

Corresponding Specifications: sys-arch/03-transport-routing-policy-engine-architecture.md, sys-arch/12-multipath-networking-architecture.md
Key Crates: crates/siar-routing-policy, crates/siar-routing, crates/siar-connectivity

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1. Multi-Metric Path Scoring Model

Unlike traditional IP routers that route solely on minimum hop counts or static OSPF costs, SIAR evaluates paths dynamically across six real-time physical dimensions:

$$\text{Score}(P) = w_l \cdot L(P) + w_b \cdot \frac{1}{B(P)} + w_e \cdot E(P) + w_p \cdot \text{PDR}(P) + w_c \cdot C(P) + w_s \cdot S(P)$$

Metric Dimension	Notation	Weight ($w$)	Description
Latency ($L$)	Milliseconds ($0–5000\text{ms}$)	Medium	Round-trip time (RTT) measured via micro-probes.
Bandwidth ($B$)	Megabits/sec ($0.01–1000\text{Mbps}$)	Medium	Available link throughput capacity.
Energy Cost ($E$)	Milliwatts ($10–2500\text{mW}$)	High (on battery)	Energy drain of radio interface (BLE vs Wi-Fi vs Cellular).
Packet Delivery Ratio ($\text{PDR}$)	Fraction ($0.0–1.0$)	High	Historical delivery success over last 60 seconds.
Financial Cost ($C$)	Score ($0–100$)	High	Metered cellular data ($>0$) vs unmetered Wi-Fi/mesh ($0$).
Link Stability ($S$)	Contact Duration ($s$)	Medium	Expected time before peer moves out of physical radio range.

graph TD
    InboundFrame[Inbound Outbox Message] --> Classify{Traffic Class}
    Classify -->|Emergency SOS| ForceBroadcast[Low-Power Multi-Radio Flood]
    Classify -->|Realtime Voice/Call| LowLatency[Opt for LAN / Wi-Fi Aware / Lowest RTT]
    Classify -->|Large File Blob| HighBandwidth[Opt for Unmetered Wi-Fi / LAN / Batch DTN]
    Classify -->|Standard Text Chat| LowEnergy[Opt for BLE Mesh / Opportunistic Mule]
    
    LowLatency --> PathScorer[Dynamic Path Scorer]
    HighBandwidth --> PathScorer
    LowEnergy --> PathScorer
    PathScorer --> Dispatch[Radio Link Dispatcher]

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2. Dynamic Link Health Monitoring & Probing

The LinkHealthMonitor in siar-routing-policy maintains active telemetry for each peer link:

pub struct LinkMetrics {
    pub rtt_ms: u32,
    pub smoothed_rtt_ms: u32,
    pub packet_loss_rate: f32,
    pub tx_bytes_per_sec: u64,
    pub rx_bytes_per_sec: u64,
    pub last_seen: Instant,
    pub is_metered: bool,
    pub signal_rssi: Option<i8>,
}

Probing Strategy

1. Active Probes: Lightweight 32-byte keepalive pings sent on high-bandwidth links every 5–15 seconds.
2. Passive Probes: Piggyback telemetry on routine message ACKs and DTN custody signals to conserve radio airtime.
3. Degradation Detection: If packet loss exceeds 25% or RSSI drops below -85 dBm, the link is flagged as Degraded and the scheduler switches traffic to a standby warm path in $< 50\text{ms}$.

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3. Multipath Scheduler & Warm Failover

SIAR's multipath engine avoids single-point-of-failure routing by maintaining primary, secondary, and fallback routes simultaneously:

[Outbox Dispatcher]
        |
        +---> [Active Path: Wi-Fi Direct] (Primary: High Throughput)
        |
        +---> [Standby Path: BLE GATT]   (Warm Backup: Zero-Setup Failover)
        |
        +---> [Cold Path: DTN Relay]     (Asynchronous Storage Queue)

• Warm Failover: If the Wi-Fi Direct socket disconnects due to physical distance, outgoing frames instantly divert to the active BLE connection without application-layer timeouts.
• Deduplication Barrier: Monotonic sequence IDs and message hashes ensure that if duplicate frames arrive over multiple paths, the destination storage layer idempotently acknowledges and ignores copies.

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4. Hysteresis, Retry Policies & Priority Route Dispatch

To prevent route flapping and handle intermittent links gracefully:

• Stickiness & Hysteresis (HysteresisPolicy): Requires candidate routes to exceed the current active route's score by a minimum threshold margin (e.g. 15%) before triggering a switch, avoiding wasteful connection thrashing.
• Exponential Retry Backoff (RetryPolicy): Failed transmission attempts incur randomized exponential backoff with jitter to protect congested mesh airwaves.
• Priority-Fair Dispatch Queue (RouteDispatchQueue): Combines traffic classification (SOS > Voice > Messages > Blobs) with fair queue scheduling (FairScheduler from siar-protocol-ext), ensuring critical signaling always preempts bulk transfers.
• Pooled Socket Multiplexing: siar-transport manages pooled peer connections so that high-volume message exchanges reuse existing multiplexed streams instead of incurring round-trip handshake penalties on each packet.