14 · Parent improvement, handoff, abort, rollback and drain

Starting state

Healthy old parent exists; a better eligible candidate remains preferred through hysteresis and spare capacity permits overlap.

Ordered events

1. Child → Candidate

   Establish standby and receive signed revision-bound path_offer

2. Child → Candidate

   handoff_prepare reserves generation and bounded lease

3. Candidate → Child

   Both directions exchange authenticated readiness probes; send ready

4. Child → Candidate

   Atomic local role swap; handoff_activate

5. Candidate → Child

   Activate child role and return exact handoff_ack

6. Child → OldParent

   Publish new path; handoff_retire then scoped drain

Fields and messages

attempt/generation/revision bind all coordination; probe nonce/sequence binds bidirectional UDP; active/standby/draining are distinct from advertised parent.

Authorization and invariants

At most one active parent locally; revoke old forwarding generation at swap; standby/draining never accept/flood source media. Revalidate path/lease before commit.

Timers

Probe 1 s, three sends 250 ms apart; preparation lease 2 s; ack 1 s; drain 500 ms.

Resource lifetime

Source keys, SRTP index/replay and unrelated branches persist. Rollback uses new attempt and newer parent generation, never resurrects an old authorization.

What the rider sees

Sanitized handoff phase/reason and parent-presence flags expose transaction progress without private topology IDs.

Failure branches

Probe/setup failure aborts candidate; lost ack rolls back only to healthy usable retained parent; old loss can preserve candidate; candidate loss preserves old; stale tuple ignored; crossed preparations use peer-ID ordering and leases.

Resulting state

Successful exact-tuple activation replaces only the active parent, then publishes path and drains old request; failed candidate retains or restores old usable parent.

Evidence boundary

Production handoff interleavings, known-cycle changes, lost ack, capacity failure and standby replay isolation tests; endurance includes successful improvement.