Module path_sensor 

Path sensing from the peer’s TTL / ECN observations.

The receiver reads the IP TTL and ECN bits off every datagram (a passive cmsg, no protocol cost) and echoes them back in a PathFrame. The sender feeds that stream here to derive two feed-forward signals the adaptive controller fuses alongside loss and delay-trend:

• Path shift: a change in hop count means a router-level path change (a re-route, a link failover). It often precedes a throughput change, so it pre-arms protection before loss materializes. The signal spikes to 1.0 on the change and decays.
• ECN-CE rate: an AQM router marks Congestion-Experienced before it tail-drops. A rising CE rate is a direct “queue is building” signal that, like a rising delay trend, calls for protection ahead of loss.

The estimator holds no clock and does no I/O - the caller supplies each (hop_count, ecn) observation - so it is deterministic and exhaustively testable with synthetic traces.

Structs

PathSensor

Rolling estimator over the peer’s path observations.

Constants

ECN_CE

The two-bit ECN codepoint marking Congestion Experienced (RFC 3168). An AQM router sets this on a packet it would otherwise have to drop.

Functions

hop_count_from_ttl

Derive a hop count from an observed TTL: pick the smallest standard initial TTL not below the observed value, and subtract.