[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort) — run with the Makefile flags on l2tpv2.py (mypy clean, isort clean, pylint only the pre-existing W0622 on type)
• make test passes, and a test case covers the change — per-module runs listed below, not the full make test
• Added a changelog entry — N/A, added centrally after the wave

What is the purpose of your pull request?

• fix — corrects a defect

---

Closes #1172

Cause. _make_data returned the parsed Length as length, which ProtocolBase.__init__ takes as the parse length, so make() never saw it and cleared L. make() now takes total_length (as #1166 did for IPv4/UDP). tests/protocols/link/test_link_unit.py called make(length=20) directly and is updated to the new keyword.

Probe. L2TPv2.from_data(L2TPv2(bytes.fromhex('c802000c0001000000010002')).info).data.hex(): before 88020001000000010002, after c802000c0001000000010002. Over every T/L/S/O combination, with and without a 3-octet zero pad: 12/24 round-trip before, 24/24 after. No capture in examples/captures/ carries L2TP (0 frames out of 103 UDP).

Tests. New tests/protocols/link/test_l2tpv2_length_roundtrip_unit.py: 4 passed, 24 subtests passed; with the fix reverted, 15 failed, 1 passed, 12 subtests passed. test_link_unit 24 passed, test_l2tp_version_unit 7 passed, test_make_data_keys_unit 2 passed, test_construction_keyword_check_unit 21 passed, test_option_roundtrip_unit 6 passed, tests/project 389 passed, 1 skipped. util/run_unittest_leg.py protocols --exclude protocols/internet: 831 tests, 0 failures, 0 errors before; 835 tests, 0 failures, 0 errors after.

[github-actions]

Coverage: 88.89% (unit tier, Python 3.14, 77800615b, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	4	20	4	93.55%
pcapkit/const	18757	1034	2342	845	90.24%
pcapkit/corekit	1878	91	580	22	94.34%
pcapkit/dumpkit	142	0	42	0	100.00%
pcapkit/foundation	2442	102	852	39	94.38%
pcapkit/interface	112	7	40	5	92.11%
pcapkit/protocols	15707	188	3974	163	98.19%
pcapkit/toolkit	487	65	144	1	85.42%
pcapkit/utilities	429	4	122	4	98.55%
pcapkit/vendor	4409	2359	1006	158	42.84%

Per-file detail: the coverage-html artifact of this run.

[JarryShaw]

Cross-review verdict on 77800615b: GOOD TO GO (ran on Sonnet; author Opus). Labelled breaking.

• Root cause confirmed: _make_data returned the Length under length, which ProtocolBase.__init__ takes as the parse length. The repro now rebuilds c802000c0001000000010002; main gives 88020001000000010002.
• Flag matrix is clean: 48 datagrams covering T/L/S/O, zero offset pad, with and without payload, from bytes and from BytesIO, gave 0 mismatches. On main, every L=1 case fails.
• Interaction with fix(protocols): a payload layer parsed from a short stream rebuilds default-filled octets #1171's guard, checked on a merge with main: a 12-octet ZLB with L=1 and a 10-octet one with L=0 both still parse as L2TPv2, including through UDP 1701. The protocols (rest) ordering leg runs 842 tests with 0 failures and 0 errors.
• Fails without the fix: 15 tests fail.
• Why breaking: make(length=) was renamed to make(total_length=). Nothing in the repo uses the old name, and the constructor never let it through. A direct make(length=...) call now silently drops the L flag instead of raising.
• Follow-ups, not blocking, filed separately:
  • The L2TP Type enum is inverted against RFC 2661 §3.1.
  • make() emits L=0 for control messages; §3.1 says L MUST be 1.
  • total_length is written without checking it.
  • A non-zero offset pad is zeroed on rebuild.