Traceroute, RSSI, SNR, and per-hop link budget

LowMesh includes mesh traceroute: an originator sends TRACEROUTE_REQ (payloadType = 0x0D); relays toward targetId record per-hop RF metrics; the destination (or last reachable hop) returns TRACEROUTE_RSP (0x0E) with a fixed-size hop report list — same types referenced in Payloads and IoT.

Diagnostics frames let operators and firmware measure multi-hop paths and RF quality toward each relay.

Metrics definitions (per reception)

Metric	Typical source	Unit
RSSI	LoRa modem / RSSI after packet	dBm (often reported as negative integer)
SNR	LoRa packet SNR	dB (can be negative)
Link margin	Implementation-defined estimate	dB — headroom above required SNR for current SF

Per-hop entry records the receiver’s measurement of the inbound transmission from the previous hop (not the forward TX).

TRACEROUTE_REQ (0x0D)

Fixed body only — no mutable traces[] accumulated in the request. Relays forward the same inner body toward targetId; hop data is collected only in the response.

Field	Size	Description
traceId	2	LE — originator picks; echoed in TRACEROUTE_RSP
targetId	4	Destination nodeId32 (LE uint32)
maxHops	1	Upper bound on hop reports (1–63)
requestFlags	1	Reserved 0 on TX unless defined later

innerBodyLen MUST be 8.

Forwarding: Unicast or flood toward targetId. Each relay decrements ttlRaw per Routing. When the probe reaches targetId, that node (or policy-designated responder) builds TRACEROUTE_RSP using the same traceId and measured path. Long paths MAY use fragmentation for the response body only.

TRACEROUTE_RSP (0x0E)

Returned toward the originator (unicast when addressed; otherwise flood toward the request’s srcId with bounded ttlRaw per policy).

Response body (after mesh header + decryption)

Field	Size	Description
traceId	2	LE — MUST match the request
hopCount	1	Number of HopReport records following (0–63; 0 = error / no data)
hopReports	hopCount × 16	Fixed 16-byte records — see below

If the path is incomplete (target unreachable), implementations MAY return a partial list with hopCount reflecting how far the probe got, or hopCount = 0 with a product-defined error extension.

Hop record (HopReport, 16 bytes fixed)

Field	Size	Description
nodeId	4	Relay nodeId32 that received from previous hop (LE uint32)
rssiDbm	1	int8 measured at this hop’s RX; 0x80 = unknown
rxSnrQ4	1	int8 SNR in quarter-dB units (decodedSnrDb = rxSnrQ4 / 4.0); 0x80 = unknown (same encoding as ACK_BASIC)
rxMarginDb	2	int16 LE link margin estimate
txPowerDbm	1	int8 forward TX power for outbound leg (if known); 0x80 = unknown
radioProfileId	1	uint8
relayDelayMs	2	uint16 LE queue/airtime delay at this hop; 0xFFFF = unknown
reserved	4	MUST be 0 on TX

Origin is hop 0 (source). Maximum records = path length, capped by maxHops in the request.

Link budget summary

Implementations SHOULD compute a scalar score for UI:

hopQualityDb = snrDb - snrRequiredForSf(sf)   // simplified
pathScore = min(hopQualityDb across hops)         // bottleneck hop

Display per-hop RSSI/SNR in a table; optional sparkline for path.

Example: 3-hop trace (conceptual)

Hop	nodeId	RSSI	SNR	margin
1	R1	-95	+4	6
2	R2	-102	+2	3
3	G	-108	-1	1

The bottleneck is hop 3 — user may relocate R2 or switch profile.

Neighbor table (runtime)

Even without traceroute, every node stores last RSSI/SNR per neighbor from any decoded frame:

Field	Description
neighborId	nodeId32 (4 bytes LE)
lastRssi	int16
lastSnr	int16
lastSeenMs	monotonic clock
ewmaSnr	smoothed

This feeds path-quality estimates for policy (gateway pick, TTL tuning) in Routing — not mesh-wide predictive tables.

C++ API: a documented RAM row shape is lowmesh::runtime::Neighbor in Embedded storage (nodeId, lastRssiDbm, lastSnrDb, lastSeenMs, ewmaSnrDb, …).

Security

Traceroute reveals topology. On the default community channel (channelHash == 0x0000), ciphertext hides inner payloads, but participation patterns may still leak. On other channels, only members who hold the key can decrypt responses — still operator-sensitive.