FA-7316 / Transport flow control / Open access
Path mtu payload budget subtracts all protocol overheads · case 01
Path mtu payload budget subtracts all protocol overheads.
ROOT CAUSE
The faulty implementation uses `mtu-ip` for the path-mtu-payload-budget-subtracts-all-protocol-overheads decision.
VERIFIED REPAIR
Apply the explicit path-mtu-payload-budget-subtracts-all-protocol-overheads contract, including the tested boundary and negative cases.
Unsuccessful approach: The attempted repair uses `max(0,mtu-max(ip,transport))` and still violates a separate fixture.
Case contract
Bounded offline decision model: path mtu payload budget subtracts all protocol overheads. 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 path-mtu-payload-budget-subtracts-all-protocol-overheads 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(mtu, ip, transport):
return mtu-ip
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(1500, 20, 20), 1460)", "check('fixture 2', solve(30, 20, 20), 0)", "check('fixture 3', solve(100, 0, 0), 100)", "check('fixture 4', solve(1280, 40, 8), 1232)"]
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 | 10 | 0 | Failed |
| fixture 3 | 100 | 100 | Passed |
| fixture 4 | 1240 | 1232 | Failed |
| fixture 1 | 1480 | 1460 | Failed |
SHA-256 / dd05307b040411c223190ad9ed2aa99220e53e1b8049d51d0c2095d20a2c2751
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(mtu, ip, transport):
return max(0,mtu-max(ip,transport))
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(1500, 20, 20), 1460)", "check('fixture 2', solve(30, 20, 20), 0)", "check('fixture 3', solve(100, 0, 0), 100)", "check('fixture 4', solve(1280, 40, 8), 1232)"]
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 | 10 | 0 | Failed |
| fixture 3 | 100 | 100 | Passed |
| fixture 4 | 1240 | 1232 | Failed |
| fixture 1 | 1480 | 1460 | Failed |
SHA-256 / fc339315f18104cb06da64c702562b367fb7960a2c89bed3a83e8708fe70130a
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(mtu, ip, transport):
return max(0,mtu-ip-transport)
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(1500, 20, 20), 1460)", "check('fixture 2', solve(30, 20, 20), 0)", "check('fixture 3', solve(100, 0, 0), 100)", "check('fixture 4', solve(1280, 40, 8), 1232)"]
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 | 0 | 0 | Passed |
| fixture 3 | 100 | 100 | Passed |
| fixture 4 | 1232 | 1232 | Passed |
| fixture 1 | 1460 | 1460 | Passed |
SHA-256 / 9adb6345c5f43286f3acd1630d60ec7fa48fb1fdb3b2670a79954d2a8eaa1df0
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 2026-09-29T14:38:10.607379+00:00.
Case digest / fc632d4cf5abaf6c40fc4345ab58a1dfaa7ad13c65dbecaed2d8b8d6b50f0889