Chapter 12 offline policy model, 2026-09-20
Run: python3 policy.py
Python standard library only; result.json is deterministic normal stdout.

A=64512, B=64513, C=64514. C originates the sole modeled prefix
203.0.113.0/24. Export whitelist: C->A, C->B, B->A. Import whitelist:
C->A preference100, C->B preference100, B->A preference200 by default.
Missing directional policy rejects. Prefix is fixed for every announcement;
this is not a multi-prefix parser or general routing policy language.

C exports an initially empty originated path as [C]. B imports and selects
that candidate, then prepends B when exporting to A, yielding [B,C].
Local preference is assigned by the receiving AS, not carried in the
Advertisement dataclass across eBGP. A selects [B,C] at preference200 ahead
of [C] at preference100. Removing B's candidate leaves C's intact. Removing
C's candidate afterward leaves no route.

Controls run on every invocation: absent B->A export yields no advertisement
and A uses directC; absent A import policy for B rejects B's advertisement;
AS_SEQUENCE [B,A,C] is rejected by A because its own ASN occurs in the path.
The teaching selection order is higher local preference, shorter AS_SEQUENCE,
then lower neighbor ASN as a deterministic model tie-break. The final tie-break
is not claimed to be the complete BGP decision process.

CLI checks: default and preference201 exit0; --help exit0;
preference100, negative, noninteger and unknown option exit2.
Repeated normal stdout is byte-identical to result.json.
Python no-excuse checker: zero violations.

All next hops are assumed reachable inputs. No TCP, BGP wire encoding,
session FSM, keepalive/hold timers, import convergence, AS_SET/confederation,
recursive next-hop resolution, multi-prefix selection, route reflector,
policy interactions beyond this topology, RPKI or data-plane verification.
Withdrawal events mutate only the receiver's modeled per-neighbor candidates;
no network withdrawal message or convergence duration is observed.