FA-7321 / Transport flow control / Open access

Dont fragment policy rejects packets exceeding path mtu · case 01

Dont fragment policy rejects packets exceeding path mtu.

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

ROOT CAUSE

The faulty implementation uses `size <= mtu` for the dont-fragment-policy-rejects-packets-exceeding-path-mtu decision.

VERIFIED REPAIR

Apply the explicit dont-fragment-policy-rejects-packets-exceeding-path-mtu contract, including the tested boundary and negative cases.

Unsuccessful approach: The attempted repair uses `not df or size < mtu` and still violates a separate fixture.

Case contract

Bounded offline decision model: dont fragment policy rejects packets exceeding path mtu. 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 dont-fragment-policy-rejects-packets-exceeding-path-mtu 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(size, mtu, df):
    return size <= mtu
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(1500, 1500, True), True)", "check('fixture 2', solve(1501, 1500, True), False)", "check('fixture 3', solve(1501, 1500, False), True)", "check('fixture 4', solve(0, 0, True), 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`	`True`	Failed
fixture 4	`True`	`True`	Passed
fixture 1	`True`	`True`	Passed

SHA-256 / b6a6719730c1a1e9e7fd43d3a1d4093ae0477b5672cb1341ff355d28b44b6d0b

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(size, mtu, df):
    return not df or size < mtu
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(1500, 1500, True), True)", "check('fixture 2', solve(1501, 1500, True), False)", "check('fixture 3', solve(1501, 1500, False), True)", "check('fixture 4', solve(0, 0, True), 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	`True`	`True`	Passed
fixture 4	`False`	`True`	Failed
fixture 1	`False`	`True`	Failed

SHA-256 / bfdf287295ac9be4a0b86a50060939473cf8850b00c1ba2f4aed5bf8f425a13d

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(size, mtu, df):
    return not df or size <= mtu
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(1500, 1500, True), True)", "check('fixture 2', solve(1501, 1500, True), False)", "check('fixture 3', solve(1501, 1500, False), True)", "check('fixture 4', solve(0, 0, True), 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	`True`	`True`	Passed
fixture 4	`True`	`True`	Passed
fixture 1	`True`	`True`	Passed

SHA-256 / 6f520cf0a5a075ff37eb19678413fa4563c74b33a209fcea528badb7fc40132a

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 2026-09-29T14:38:10.614865+00:00.

Case digest / 4ed207273cf6b582fdf6c2f5c9e2b918801653ad3ea785c17291f044f9e4f710