FA-7246 / Transport flow control / Open access

Fast retransmit fires on threshold crossing once · case 01

Fast retransmit fires on threshold crossing once.

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

ROOT CAUSE

The faulty implementation uses `new >= threshold` for the fast-retransmit-fires-on-threshold-crossing-once decision.

VERIFIED REPAIR

Apply the explicit fast-retransmit-fires-on-threshold-crossing-once contract, including the tested boundary and negative cases.

Unsuccessful approach: The attempted repair uses `new > threshold` and still violates a separate fixture.

Case contract

Bounded offline decision model: fast retransmit fires on threshold crossing once. 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 fast-retransmit-fires-on-threshold-crossing-once 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(old, new, threshold):
    return new >= threshold
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(2, 3, 3), True)", "check('fixture 2', solve(3, 4, 3), False)", "check('fixture 3', solve(0, 2, 3), False)", "check('fixture 4', solve(1, 4, 3), True)"]
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`	`True`	Passed
fixture 1	`True`	`True`	Passed

SHA-256 / 74df6f9d438f31449cb7fd561e4f215c314c1bf95a918292baff5750d0bcf6a7

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(old, new, threshold):
    return new > threshold
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(2, 3, 3), True)", "check('fixture 2', solve(3, 4, 3), False)", "check('fixture 3', solve(0, 2, 3), False)", "check('fixture 4', solve(1, 4, 3), True)"]
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`	`True`	Passed
fixture 1	`False`	`True`	Failed

SHA-256 / e22f4b7881aa8e7845afa9773613a1d98291af3cb32c3290c30994f666fb0eaf

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(old, new, threshold):
    return old < threshold <= new
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(2, 3, 3), True)", "check('fixture 2', solve(3, 4, 3), False)", "check('fixture 3', solve(0, 2, 3), False)", "check('fixture 4', solve(1, 4, 3), True)"]
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	`True`	`True`	Passed
fixture 1	`True`	`True`	Passed

SHA-256 / 22259f0d6a0a92b875b3e130f02a937645511546f65b38ee6af4aa2efd2bb54d

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 2026-09-29T14:38:09.831510+00:00.

Case digest / 6026f5650bfd26d18c4a70829636e592d3d998bfde1d7db275a244a21e2925b9