FA-7231 / Transport flow control / Open access

Sequence ordering handles modular wraparound · case 01

Sequence ordering handles modular wraparound.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The faulty implementation uses `a < b` for the sequence-ordering-handles-modular-wraparound decision.

VERIFIED REPAIR

Apply the explicit sequence-ordering-handles-modular-wraparound contract, including the tested boundary and negative cases.

Unsuccessful approach: The attempted repair uses `(b-a) % 256 <= 128` and still violates a separate fixture.

Case contract

Bounded offline decision model: sequence ordering handles modular wraparound. Inputs are already validated protocol fields; the explicit fixtures define the supported policy, not a complete protocol implementation.

Why this case matters

This deterministic transport flow control model isolates the sequence-ordering-handles-modular-wraparound policy at a network component boundary.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(a, b):
    return a < b
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(250, 3), True)", "check('fixture 2', solve(3, 250), False)", "check('fixture 3', solve(4, 4), False)", "check('fixture 4', solve(0, 128), False)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`True`	`False`	Failed
fixture 3	`False`	`False`	Passed
fixture 4	`True`	`False`	Failed
fixture 1	`False`	`True`	Failed

SHA-256 / 0eb3f8e8b5a592713ad4b73a23dc8dc2bb824b486f113318f5c9d63b7612efb9

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(a, b):
    return (b-a) % 256 <= 128
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(250, 3), True)", "check('fixture 2', solve(3, 250), False)", "check('fixture 3', solve(4, 4), False)", "check('fixture 4', solve(0, 128), False)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`False`	`False`	Passed
fixture 3	`True`	`False`	Failed
fixture 4	`True`	`False`	Failed
fixture 1	`True`	`True`	Passed

SHA-256 / 9464abf308caebdce9965e285e884e767bee3b83616bb27201cf81d17b4f0acb

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(a, b):
    return 0 < (b-a) % 256 < 128
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(250, 3), True)", "check('fixture 2', solve(3, 250), False)", "check('fixture 3', solve(4, 4), False)", "check('fixture 4', solve(0, 128), False)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`False`	`False`	Passed
fixture 3	`False`	`False`	Passed
fixture 4	`False`	`False`	Passed
fixture 1	`True`	`True`	Passed

SHA-256 / 97814155d2decb381e1d23ebc971ef91204494b50850ee0014cad64bbd17627a

Verification & scope

Bounded deterministic policy model over validated inputs; not a complete protocol stack or an interoperability claim. The five variants rotate the same explicit fixture set. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:38:09.673585+00:00.

Case digest / afd8ad147da5b1461c555cca8c728b9e452026a193f6c129f2e91b27360ec444