[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort) — ran mypy and pylint directly on the three SCTP files: they report nothing new on changed lines. I did not run the make targets.
• make test passes, and a test case covers the change — only the modules listed below
• Added a changelog entry under docs/source/changelog/ and regenerated CHANGELOG.md, if the change is user-visible — N/A, added centrally

What is the purpose of your pull request?

• fix — corrects a defect
• feat — adds a feature
• perf — changes performance, not behaviour
• refactor — changes neither behaviour nor performance
• test — tests only
• docs — documentation only
• ci — workflows or build tooling
• release — bumps the version or rolls up a distribution
• chore — anything else

---

Description of your pull request and other information

Closes #1221
Closes #1222

• fix(sctp): chunk flags and reserved fields are written back as zero #1221. Ten chunk data models gain a raw flags field, and their makers gain a flags keyword: INIT, INIT ACK, SACK, HEARTBEAT, HEARTBEAT ACK, SHUTDOWN, SHUTDOWN ACK, ERROR, COOKIE ECHO and COOKIE ACK. DATAChunkFlags and TBitFlags gain a reserved subfield. InvalidStreamIdentifierCause gains reserved.
• fix(sctp): chunk length always counts the last parameter's padding #1222. Six chunks carry a nested list: INIT, INIT ACK, HEARTBEAT, HEARTBEAT ACK, ABORT and ERROR. Their length now excludes the last item's padding, as RFC 9260 §3.2 requires. A parsed length that counts the padding, which the RFC lets receivers accept, is kept as received.
• Breaking change. These are new required data-class fields. make() output changes for an unpadded final parameter: for example, an ABORT with bye is now 11 bytes, not 12. test_sctp_unit.py pinned the old lengths (INIT ACK 32, ABORT 12) and is updated.

Probe. Each input is rebuilt with SCTP.from_data(SCTP(raw).info).

• Before: INIT 01a50014 → 01000014; 0100001b → 0100001c.
• After: both rebuild byte-exactly, as do the DATA f3, ABORT fe and cause beef cases.
• All 34 SCTP frames in examples/captures/* round-trip.

Tests. The new test_sctp_roundtrip_unit.py fails without the fix (20 failed, 5 passed) and passes with it (7 passed, 18 subtests). Results of the other runs:

• test_sctp_unit 37 passed
• test_transport_unit 11 passed
• test_ngap_unit 23 passed
• test_option_roundtrip_unit 6 passed
• tests/project 389 passed, 1 skipped
• run_unittest_leg.py protocols --exclude protocols/internet: 857 tests, 0 failures

[github-actions]

Coverage: 88.89% (unit tier, Python 3.14, 3351bd690, 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	15821	200	4064	175	98.08%
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 3351bd690: GOOD TO GO (Sonnet reviewed; the author ran on Opus).

• Repros: the INIT flags a5 and length 1b repros now rebuild exactly; I re-ran both myself. DATA f3, ABORT fe, SHUTDOWN COMPLETE ff and the beef cause also rebuild exactly. All of these fail on main.
• RFC 9260 §3.2: the Chunk Length counts the padding of non-final parameters but not the final one, and receivers should accept either form. The helper implements both rules for every chunk type that carries a nested list (INIT, INIT ACK, HEARTBEAT, HEARTBEAT ACK, ABORT, ERROR). COOKIE ECHO is opaque bytes and needs no change.
• Pinned lengths: the two edited pins in test_sctp_unit.py (INIT ACK 29, ABORT 11) are the RFC-canonical lengths; the old pins (32 and 12) encoded padded output.
• Fixtures: all 34 SCTP frames rebuild byte-exactly. NGAP and the generators build only DATA chunks, whose length is unchanged.
• Tests: the new module fails 20 cases on main. run_unittest_leg.py protocols --exclude protocols/internet: 857 tests, 0 failures.
• Breaking change: the new required flags and reserved fields break any code that constructs these data classes directly.
• Nit: flags is bytes on ten chunk types, while reserved is an int on DATA, ABORT and SHUTDOWN COMPLETE.