protocols: size MH's MN-ID option from its subtype, not identifier's Python type

docs/source/pep.rst

@@ -223,9 +223,6 @@ What is left, and why:
   so putting it in that chain would make ``layer=`` and ``protocol=`` limits
   behave wrongly. Changing the parsed shape from :obj:`bytes` to ``Raw`` also
   changes what existing captures dump to, so it is its own change.
-* **The MN-ID option's constructor mis-sizes a non-address identifier**, tracked
-  as `#448 <https://github.com/JarryShaw/PyPCAPKit/issues/448>`__. Pre-existing
-  and outside the registry-completion work, so it is filed rather than fixed here.
 
 Two wire-format traps are worth knowing before touching this code, since both
 look like ordinary fields and are not:

examples/generators/options.py

@@ -810,10 +810,6 @@ def _mh_option_overrides() -> 'dict[Any, dict[str, Any]]':
         Enum_Option.Authorization_Data: {'data': b'\xa5' * 8},
         Enum_Option.Mobility_Header_Link_Layer_Address_option: {
             'address': b'\x00\x11\x22\x33\x44\x55'},
-        # The default identifier is the *string* ``'::'``, whose ``len()`` is 2
-        # rather than the 16 octets the field packs -- so the declared length
-        # is 3 going out and 17 coming back. An address object avoids it.
-        Enum_Option.MN_ID_OPTION_TYPE: {'identifier': ipaddress.IPv6Address('::1')},
         # ``(len + 6) % 4`` has to be 0.
         Enum_Option.AUTH_OPTION_TYPE: {'data': b'\xa5\xa5'},
         # Without this ``_make_opt_mesg_id`` reads the clock, and the capture

pcapkit/protocols/internet/mh.py

@@ -7659,7 +7659,16 @@ def _make_opt_mn_id(self, type: 'Enum_Option', option: 'Optional[Data_MNIDOption
             subtype_val = self._make_index(subtype, subtype_default, namespace=subtype_namespace,  # type: ignore[assignment]
                                            reversed=subtype_reversed, pack=False)
 
-        if isinstance(identifier, ipaddress.IPv6Address):
+        # NOTE: The wire format is chosen by ``subtype_val`` (c.f. ``mn_id_selector``),
+        # not by the Python type of ``identifier``, so the width has to be taken from
+        # the former. For the ``IPv6_Address`` subtype the schema always packs a fixed
+        # 16-octet address (:class:`~pcapkit.corekit.fields.ipaddress.IPv6AddressField`
+        # ignores any declared length), so ``identifier`` is normalised to that wire
+        # form here as well, keeping the packed bytes and the declared length derived
+        # from one value instead of two independent computations (c.f. #448).
+        if subtype_val == Enum_MNIDSubtype.IPv6_Address:
+            if not isinstance(identifier, ipaddress.IPv6Address):
+                identifier = ipaddress.IPv6Address(identifier)
             id_len = 16
         elif isinstance(identifier, int):
             id_len = math.ceil(identifier.bit_length() / 8)

tests/protocols/internet/test_mh_unit.py

@@ -498,8 +498,10 @@ def test_mh_option_constructors_cover_known_options_and_dispatch(self) -> None:
         self.assertEqual(proto._make_opt_mn_id(Option.MN_ID_OPTION_TYPE,
                                                subtype=MNIDSubtype.NAI,
                                                identifier='node@example').length, 13)
+        # default subtype is IPv6_Address, which always packs a fixed 16-octet
+        # address regardless of the identifier's Python type -- see #448.
         self.assertEqual(proto._make_opt_mn_id(Option.MN_ID_OPTION_TYPE,
-                                               identifier=0x1234).length, 3)
+                                               identifier=0x1234).length, 17)
         self.assertEqual(proto._make_opt_auth(Option.AUTH_OPTION_TYPE, subtype=AuthSubtype.MN_HA,
                                               spi=7, data=b'ab').spi, 7)
         with self.assertRaises(ProtocolError):
@@ -2289,6 +2291,45 @@ def test_mh_length_derived_addresses_pick_their_family(self) -> None:
                 self.assertEqual(data.ipv4, ipv4)
                 self.assertEqual(data.prefix, ipaddress.ip_address(prefix))
 
+    def test_mh_mn_id_option_length_matches_packed_octets(self) -> None:
+        """The MN-ID option's declared length must count what actually gets packed.
+
+        ``_make_opt_mn_id`` used to size the identifier from the *Python type* of
+        the ``identifier`` argument rather than from ``subtype_val``, which is what
+        actually selects the wire format (see ``mn_id_selector``). For the
+        ``IPv6_Address`` subtype -- the method's own default -- the schema always
+        packs a fixed 16-octet address
+        (:class:`~pcapkit.corekit.fields.ipaddress.IPv6AddressField` ignores any
+        declared length entirely), so a ``str`` or ``int`` identifier, including
+        the no-argument default, declared a ``length`` that disagreed with what was
+        actually packed: ``len(packed) != length + 2``. See #448.
+        """
+        import ipaddress
+
+        from pcapkit.const.mh.option import Option
+        from pcapkit.protocols.internet.mh import MH
+
+        proto = object.__new__(MH)
+
+        # every documented ``identifier`` type, against the subtype that used to
+        # mis-size three of the four -- including the method's own default, which
+        # is itself one of the broken types (``str``).
+        cases = (
+            ('default (no identifier)', {}),
+            ("str '::'", {'identifier': '::'}),
+            ("str '2001:db8::1'", {'identifier': '2001:db8::1'}),
+            ('int 0x1234', {'identifier': 0x1234}),
+            ('bytes (16-octet packed form)',
+             {'identifier': ipaddress.IPv6Address('2001:db8::1').packed}),
+            ('IPv6Address', {'identifier': ipaddress.ip_address('2001:db8::1')}),
+        )
+        for label, kwargs in cases:
+            with self.subTest(label):
+                schema = proto._make_opt_mn_id(  # type: ignore[arg-type]
+                    Option.MN_ID_OPTION_TYPE, **kwargs)
+                self.assertEqual(schema.length, 17)
+                self.assertEqual(len(schema.pack()), schema.length + 2)
+
     def test_mh_redirect_option_rejects_contradictory_flags(self) -> None:
         """:rfc:`6463#section-4.2` allows exactly one of the ``K`` and ``N`` flags.