Parse & encode the IPv6 fragment header in network byte order

[deusmatrix]

The 13-bit fragment offset and the "more fragments" (M) flag were encoded/decoded with the wrong bit positions: the offset's low bits were packed into byte 3 bits 4..0 and M was read from byte 3 bit 7. Per RFC 8200 §4.5, bytes 2..3 form a big-endian 16-bit word where the offset is the top 13 bits (word >> 3) and M is the least significant bit.

The encode and decode bugs were symmetric, so to_bytes() round-trips parsed back cleanly and every existing test passed — but the bytes on the wire did not match the RFC or any other stack. A hand-crafted byte-level test (network_byte_order) is added to break that symmetry and lock the layout.

Summary by CodeRabbit

• Bug Fixes

  • Enhanced IPv6 fragment header parsing with improved safety mechanisms and RFC 8200 compliance.
  • Refined fragment offset and more-fragments bit decoding logic for increased accuracy.

• Tests

  • Added comprehensive test coverage for IPv6 fragment header network byte order serialization and deserialization.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 040b690c-9dbf-4d06-9a7f-68082fc925be

📥 Commits

Reviewing files that changed from the base of the PR and between 7cb4b6f and 6889de9.

📒 Files selected for processing (2)

• etherparse/src/net/ipv6_fragment_header.rs
• etherparse/src/net/ipv6_fragment_header_slice.rs

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📝 Walkthrough

Walkthrough

This PR refactors IPv6 fragment header bit-parsing to centralize wire-format decoding in slice accessors, updates header serialization to match the new bit layout, consolidates deserialization to use slice-based parsing, and adds comprehensive RFC 8200 compliance testing through round-trip verification.

Changes

IPv6 Fragment Header Bit-Layout Refactoring

Layer / File(s)	Summary
Slice bit-decoding refactoring etherparse/src/net/ipv6_fragment_header_slice.rs	fragment_offset() now derives the 13-bit offset by reading two bytes as a big-endian u16 and right-shifting by 3. more_fragments() tests the M flag using a simplified bitmask. is_fragmenting_payload() is rewritten to use accessor methods instead of unsafe byte-level checks. Documentation example updated for the new M-bit value.
Header serialization bit computation etherparse/src/net/ipv6_fragment_header.rs (lines 151–161, 379–390)	to_bytes() serialization refactored to compute fragment-offset + M-flag bits via left-shift of fragment_offset.value() combined with conditional M-flag insertion. Test expectations adjusted to match the new computation.
Header deserialization consolidation etherparse/src/net/ipv6_fragment_header.rs (lines 60–62, 82–84)	read() and read_limited() consolidated to call Ipv6FragmentHeaderSlice::from_slice_unchecked(&buffer).to_header(), eliminating manual byte-to-field parsing and adding safety comments.
RFC 8200 compliance testing and documentation etherparse/src/net/ipv6_fragment_header.rs (lines 15, 400–435)	Documentation typo corrected ("Identifcation" → "Identification"). New network_byte_order test verifies exact RFC 8200 §4.5 wire layout through round-trip to_bytes() and from_slice() operations for maximum offset, offset with/without M flag, and edge cases.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~50 minutes

Suggested reviewers

• JulianSchmid

Poem

🐰 Bits align in perfect rows,
Fragment headers now pristine and clear,
Where slices parse what RFC shows,
Round-tripping checks without a fear! 🎯

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Parse & encode the IPv6 fragment header in network byte order' directly and clearly summarizes the main change: fixing the IPv6 fragment header to use proper network byte order per RFC 8200. This is the core objective of the PR.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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