FA-7241 / Transport flow control / Open access
Duplicate ack counter resets on forward progress · case 01
Duplicate ack counter resets on forward progress.
ROOT CAUSE
The faulty implementation uses `duplicates+1` for the duplicate-ack-counter-resets-on-forward-progress decision.
VERIFIED REPAIR
Apply the explicit duplicate-ack-counter-resets-on-forward-progress contract, including the tested boundary and negative cases.
Unsuccessful approach: The attempted repair uses `duplicates if ack == previous else 0` and still violates a separate fixture.
Case contract
Bounded offline decision model: duplicate ack counter resets on forward progress. 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 duplicate-ack-counter-resets-on-forward-progress 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(previous, ack, duplicates):
return duplicates+1
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(10, 10, 2), 3)", "check('fixture 2', solve(10, 11, 2), 0)", "check('fixture 3', solve(0, 0, 0), 1)", "check('fixture 4', solve(10, 12, 0), 0)"]
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 | 3 | 0 | Failed |
| fixture 3 | 1 | 1 | Passed |
| fixture 4 | 1 | 0 | Failed |
| fixture 1 | 3 | 3 | Passed |
SHA-256 / 128543c9a71be0ca27998e95222b1be6c3d05202e47f9d046eb32ba8047800a8
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(previous, ack, duplicates):
return duplicates if ack == previous else 0
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(10, 10, 2), 3)", "check('fixture 2', solve(10, 11, 2), 0)", "check('fixture 3', solve(0, 0, 0), 1)", "check('fixture 4', solve(10, 12, 0), 0)"]
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 | 0 | 1 | Failed |
| fixture 4 | 0 | 0 | Passed |
| fixture 1 | 2 | 3 | Failed |
SHA-256 / 70aa470c2ca3e857b546c2c5e75fdc220abc29f3df8c193750d6c2b94b162ea4
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(previous, ack, duplicates):
return duplicates+1 if ack == previous else 0
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(10, 10, 2), 3)", "check('fixture 2', solve(10, 11, 2), 0)", "check('fixture 3', solve(0, 0, 0), 1)", "check('fixture 4', solve(10, 12, 0), 0)"]
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 | 1 | 1 | Passed |
| fixture 4 | 0 | 0 | Passed |
| fixture 1 | 3 | 3 | Passed |
SHA-256 / e3256659a1307500750b0987625557362ccbc4738e12efb324b37077d021ee45
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 2026-09-29T14:38:09.823996+00:00.
Case digest / 7d0b3ca4b0149617e0dd08f4a53c89fe70b60c79bc880716c40acd508ba36ca7