@@ -225,6 +225,7 @@ Layer-limited extraction: `layer='link'` no longer sets the termination flag on
 - **a breaking change to** `NTPTimestamp`: it and `PMIPv6Timestamp` are `Info` classes in `pcapkit.protocols.data.internet.mh` rather than tuples, and `NTPTimestamp` keeps no alias at its 1.4.x home in `pcapkit.protocols.internet.mh`. As Mappings they no longer index, unpack, compare or hash as a tuple does; `.seconds` and `.fraction` still work, and the dumpers write both as keys ([#1886](https://github.com/JarryShaw/PyPCAPKit/issues/1886), [#1887](https://github.com/JarryShaw/PyPCAPKit/pull/1887)).
 - **a breaking change to** rebuilding from `to_dict()`: the payload is dispatched only from the `__next_name__` and `__next_type__` keys, and the IPv6 extension-header chain only from `__exthdr__`, never from key order, which on PyPy follows an earlier instance's attribute layout and so picked the wrong layer. `to_dict()`, iteration and `len()` carry the dunder keys a new `Info.__included__` names, and `Info.__excluded__` only keeps a key out of `repr`, `str` and the dumps. A hand-written dict needs those keys, `__short_read__` among them, to rebuild, and a custom `Info` lists its own dunder keys in `__included__` ([#2003](https://github.com/JarryShaw/PyPCAPKit/issues/2003), [#2014](https://github.com/JarryShaw/PyPCAPKit/pull/2014)).
 - **a breaking change to** `PCAPNG.context`: the block's section `Context`, which it returned from 1.0.0 through 1.4.x, is `PCAPNG.section_context`, with no alias, and `context` is the caller-supplied `ContextRegistry` every protocol inherits from `ProtocolBase`, on a Section Header Block too. `PCAPNG.section_context` raises `UnsupportedCall` where `context` did: on a block that holds no section, such as a Section Header Block the `PCAPNG` engine has not assigned the section it opens. Code reading `block.context` for the section reads `block.section_context` ([#2172](https://github.com/JarryShaw/PyPCAPKit/issues/2172), [#2205](https://github.com/JarryShaw/PyPCAPKit/pull/2205)).
+- **a breaking change to** HIP's `REG_FAILED` parameter: its first octet is the Failure Type, and it carries no lifetime ([RFC 8003 Section 4.5](https://datatracker.ietf.org/doc/html/rfc8003#section-4.5)), so `pcapkit.protocols.data.internet.hip.RegFailedParameter` replaces `lifetime` with `failure_type`, a `RegistrationFailure`, with no alias, and its `reg_type` holds `Registration` values rather than failure types. `HIP._make_param_reg_failed()` takes `failure_type` in place of `lifetime`, and `reg_failed` takes registration types. The octets are unchanged ([#2309](https://github.com/JarryShaw/PyPCAPKit/issues/2309)).
 
 #### Fixed
 
 
@@ -1489,6 +1489,15 @@ Changed
   :class:`~pcapkit.foundation.engines.pcapng.PCAPNG` engine has not assigned the section
   it opens. Code reading ``block.context`` for the section reads
   ``block.section_context`` (:issue:`2172`, :pr:`2205`).
+* **a breaking change to** HIP's ``REG_FAILED`` parameter: its first octet is the
+  Failure Type, and it carries no lifetime (:rfc:`8003#section-4.5`), so
+  :class:`pcapkit.protocols.data.internet.hip.RegFailedParameter` replaces ``lifetime``
+  with ``failure_type``, a :class:`~pcapkit.const.hip.registration_failure.RegistrationFailure`,
+  with no alias, and its ``reg_type`` holds
+  :class:`~pcapkit.const.hip.registration.Registration` values rather than failure
+  types. :meth:`.HIP._make_param_reg_failed` takes ``failure_type`` in place of
+  ``lifetime``, and ``reg_failed`` takes registration types. The octets are unchanged
+  (:issue:`2309`).
 
 Fixed
 ~~~~~
 
@@ -610,15 +610,25 @@ def __init__(self, type: 'Enum_Parameter', critical: 'bool', length: 'int', life
 
 @info_final
 class RegFailedParameter(Parameter):
-    """Data model for HIP ``REG_FAILED`` parameter."""
+    """Data model for HIP ``REG_FAILED`` parameter.
+
+    .. versionchanged:: 1.5.0
+
+       ``lifetime`` is replaced by :attr:`failure_type`, with no alias, and
+       :attr:`reg_type` holds :class:`~pcapkit.const.hip.registration.Registration`
+       values: ``REG_FAILED`` carries a Failure Type octet ahead of its
+       registration types and no lifetime (:rfc:`8003#section-4.5`,
+       :issue:`2309`).
+
+    """
 
-    #: Registration lifetime.
-    lifetime: 'timedelta'
     #: Registration failure type.
-    reg_type: 'tuple[RegistrationFailure, ...]'
+    failure_type: 'RegistrationFailure'
+    #: Registration types that failed with :attr:`failure_type`.
+    reg_type: 'tuple[Registration, ...]'
 
     if TYPE_CHECKING:
-        def __init__(self, type: 'Enum_Parameter', critical: 'bool', length: 'int', lifetime: 'timedelta', reg_type: 'tuple[RegistrationFailure, ...]') -> 'None': ...  # pylint: disable=unused-argument,multiple-statements,redefined-builtin,super-init-not-called,line-too-long
+        def __init__(self, type: 'Enum_Parameter', critical: 'bool', length: 'int', failure_type: 'RegistrationFailure', reg_type: 'tuple[Registration, ...]') -> 'None': ...  # pylint: disable=unused-argument,multiple-statements,redefined-builtin,super-init-not-called,line-too-long
 
 
 @info_final
 
@@ -49,6 +49,7 @@
 from pcapkit.const.hip.notify_message import NotifyMessage as Enum_NotifyMessage
 from pcapkit.const.hip.packet import Packet as Enum_Packet
 from pcapkit.const.hip.parameter import Parameter as Enum_Parameter
+from pcapkit.const.hip.registration_failure import RegistrationFailure as Enum_RegistrationFailure
 from pcapkit.const.reg.transtype import TransType as Enum_TransType
 from pcapkit.corekit.fields.ipaddress import parse_ip_address
 from pcapkit.corekit.multidict import OrderedMultiDict
@@ -217,8 +218,6 @@
     from pcapkit.const.hip.esp_transform_suite import ESPTransformSuite as Enum_ESPTransformSuite
     from pcapkit.const.hip.nat_traversal import NATTraversal as Enum_NATTraversal
     from pcapkit.const.hip.registration import Registration as Enum_Registration
-    from pcapkit.const.hip.registration_failure import \
-        RegistrationFailure as Enum_RegistrationFailure
     from pcapkit.const.hip.suite import Suite as Enum_Suite
     from pcapkit.const.hip.transport import Transport as Enum_Transport
     from pcapkit.corekit.protochain import ProtoChain
@@ -1998,7 +1997,7 @@ def _read_param_reg_failed(self, schema: 'Schema_RegFailedParameter', *, version
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Type              |             Length            |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-           |   Lifetime    |  Reg Type #1  |  Reg Type #2  |  Reg Type #3  |
+           | Failure Type  |  Reg Type #1  |  Reg Type #2  |  Reg Type #3  |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |      ...      |     ...       |  Reg Type #n  |               |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    Padding    +
@@ -2018,7 +2017,7 @@ def _read_param_reg_failed(self, schema: 'Schema_RegFailedParameter', *, version
             type=schema.type,
             critical=bool(schema.type & 0b1),
             length=parameter_total_len(schema.len),
-            lifetime=datetime.timedelta(seconds=schema.lifetime),
+            failure_type=schema.failure_type,
             reg_type=tuple(schema.reg_failed),
         )
         return reg_failed
@@ -4432,33 +4431,48 @@ def _make_param_reg_response(self, code: 'Enum_Parameter', param: 'Optional[Data
 
     def _make_param_reg_failed(self, code: 'Enum_Parameter', param: 'Optional[Data_RegFailedParameter]' = None, *,  # pylint: disable=unused-argument
                                version: 'int',
-                               lifetime: 'int | timedelta' = 0,
-                               reg_failed: 'Optional[list[Enum_RegistrationFailure | StdlibEnum | AenumEnum | str | int]]' = None,
+                               failure_type: 'Enum_RegistrationFailure | StdlibEnum | AenumEnum | str | int' = Enum_RegistrationFailure.Registration_requires_additional_credentials,  # pylint: disable=line-too-long
+                               failure_type_default: 'Optional[int]' = None,
+                               failure_type_namespace: 'Optional[dict[str, int] | dict[int, str] | Type[StdlibEnum] | Type[AenumEnum]]' = None,  # pylint: disable=line-too-long
+                               failure_type_reversed: 'bool' = False,
+                               reg_failed: 'Optional[list[Enum_Registration | StdlibEnum | AenumEnum | str | int]]' = None,
                                reg_failed_default: 'Optional[int]' = None,
                                reg_failed_namespace: 'Optional[dict[str, int] | dict[int, str] | Type[StdlibEnum] | Type[AenumEnum]]' = None,  # pylint: disable=line-too-long
                                reg_failed_reversed: 'bool' = False,
                                **kwargs: 'Any') -> 'Schema_RegFailedParameter':
         """Make HIP ``REG_FAILED`` parameter.
 
+        .. versionchanged:: 1.5.0
+
+           ``lifetime`` is replaced by ``failure_type``, with no alias, and
+           ``reg_failed`` takes registration types rather than failure types
+           (:issue:`2309`).
+
         Args:
             code: parameter code
             param: parameter data
             version: HIP protocol version
-            lifetime: lifetime
-            reg_failed: registration failure list
-            reg_failed_default: default registration failure
-            reg_failed_namespace: registration failure namespace
-            reg_failed_reversed: reverse registration failure namespace
+            failure_type: registration failure type
+            failure_type_default: default registration failure type
+            failure_type_namespace: registration failure type namespace
+            failure_type_reversed: reverse registration failure type namespace
+            reg_failed: list of registration types that failed
+            reg_failed_default: default registration type
+            reg_failed_namespace: registration type namespace
+            reg_failed_reversed: reverse registration type namespace
             **kwargs: arbitrary keyword arguments
 
         Returns:
             HIP parameter schema.
 
         """
         if param is not None:
-            reg_type = cast('list[Enum_RegistrationFailure]', param.reg_type)
-            lt = math.floor(param.lifetime.total_seconds())
+            failure = param.failure_type
+            reg_type = cast('list[Enum_Registration]', param.reg_type)
         else:
+            failure = self._make_index(failure_type, failure_type_default, namespace=failure_type_namespace,  # type: ignore[assignment]
+                                       reversed=failure_type_reversed, pack=False)
+
             if reg_failed is None:
                 reg_failed = []
 
@@ -4467,12 +4481,10 @@ def _make_param_reg_failed(self, code: 'Enum_Parameter', param: 'Optional[Data_R
                 reg_type.append(self._make_index(reg, reg_failed_default, namespace=reg_failed_namespace,  # type: ignore[arg-type]
                                                  reversed=reg_failed_reversed, pack=False))
 
-            lt = lifetime if isinstance(lifetime, int) else math.floor(lifetime.total_seconds())
-
         return Schema_RegFailedParameter(
             type=code,
             len=1 + len(reg_type),
-            lifetime=lt,
+            failure_type=failure,
             reg_failed=reg_type,
         )
 
 
@@ -186,8 +186,9 @@ def registration_type_list_len(pkt: 'dict[str, Any]') -> 'int':
     Used by the ``reg_request``, ``reg_response`` and ``reg_failed`` fields of
     :class:`RegRequestParameter`, :class:`RegResponseParameter` and
     :class:`RegFailedParameter` respectively, each of which follows a single
-    ``lifetime`` octet with a list of registration type octets sized by the
-    remainder of the parameter.
+    prefix octet (the ``lifetime`` of the first two, the ``failure_type`` of
+    the last) with a list of registration type octets sized by the remainder
+    of the parameter.
 
     Args:
         pkt: Parameter unpacked schema.
@@ -197,7 +198,7 @@ def registration_type_list_len(pkt: 'dict[str, Any]') -> 'int':
 
     Raises:
         FieldValueError: If the parameter's ``Length`` on the wire is too
-            short to hold the ``lifetime`` octet already read, which would
+            short to hold the prefix octet already read, which would
             otherwise underflow the list length below zero.
 
     """
@@ -1171,18 +1172,18 @@ def __init__(self, type: 'Enum_Parameter', len: 'int', lifetime: 'int', reg_resp
 class RegFailedParameter(Parameter, code=Enum_Parameter.REG_FAILED):
     """Header schema for HIP ``REG_FAILED`` parameters."""
 
-    #: Lifetime.
-    lifetime: 'int' = UInt8Field()
+    #: Registration failure type.
+    failure_type: 'Enum_RegistrationFailure' = EnumField(length=1, namespace=Enum_RegistrationFailure)
     #: Registration types.
-    reg_failed: 'list[Enum_RegistrationFailure]' = ListField(
+    reg_failed: 'list[Enum_Registration]' = ListField(
         length=registration_type_list_len,
-        item_type=EnumField(length=1, namespace=Enum_RegistrationFailure),
+        item_type=EnumField(length=1, namespace=Enum_Registration),
     )
     #: Padding.
     padding: 'bytes' = PaddingField(length=parameter_padding_len)
 
     if TYPE_CHECKING:
-        def __init__(self, type: 'Enum_Parameter', len: 'int', lifetime: 'int', reg_failed: 'list[Enum_RegistrationFailure]') -> 'None': ...
+        def __init__(self, type: 'Enum_Parameter', len: 'int', failure_type: 'Enum_RegistrationFailure', reg_failed: 'list[Enum_Registration]') -> 'None': ...
 
 
 @schema_final
 
@@ -13,6 +13,9 @@
   :class:`~datetime.timedelta`, so 197 of the 256 octets could not be rebuilt.
 * #1328 -- the reserved bits of ``Controls`` and of ``ESP_INFO`` were written
   back as zero.
+* #2309 -- ``REG_FAILED`` read its Failure Type octet as a ``lifetime`` and its
+  Reg Type octets as failure types, rather than as :rfc:`8003#section-4.5`
+  lays them out.
 
 Every case builds its own octets in memory and reads no capture. :class:`HIP`
 is imported inside each test, after :func:`~tests._support.reimport_once_per_class`,
@@ -35,6 +38,13 @@ def packet(params: bytes = b'', controls: bytes = b'\x00\x00') -> bytes:
             + controls + bytes(32) + params)
 
 
+def reg_failed(failure_type: int, reg_types: 'list[int]') -> bytes:
+    """Return a ``REG_FAILED`` parameter, zero-padded to 8 octets."""
+    body = (936).to_bytes(2, 'big') + (1 + len(reg_types)).to_bytes(2, 'big') \
+        + bytes([failure_type, *reg_types])
+    return body + bytes(-len(body) % 8)
+
+
 class TestHIPRoundTrip(unittest.TestCase):
     """Pin the rebuilt octets of parsed HIP packets."""
 
@@ -106,6 +116,67 @@ def test_esp_info_reserved_round_trips(self) -> None:
         self.assertRoundTrips(packet(bytes.fromhex('0041000c' 'abcd' '0102')
                                      + bytes.fromhex('0a0b0c0d' '01020304')))
 
+    def test_reg_failed_reads_failure_type_then_registration_types(self) -> None:
+        """#2309: the first octet is a Failure Type, and the rest are Reg Types."""
+        from pcapkit.const.hip.parameter import Parameter
+        from pcapkit.const.hip.registration import Registration
+        from pcapkit.const.hip.registration_failure import RegistrationFailure
+        from pcapkit.protocols.internet.hip import HIP
+
+        wire = packet(reg_failed(1, [1, 2, 3]))
+        param = HIP(io.BytesIO(wire), len(wire), extension=True).info.parameters[Parameter.REG_FAILED]
+
+        # Both registries are integer enums, so ``==`` alone cannot tell a
+        # Failure Type from a Reg Type of the same value: compare the types too.
+        self.assertIs(type(param.failure_type), RegistrationFailure)
+        self.assertEqual(param.failure_type, RegistrationFailure.Registration_type_unavailable)
+        self.assertEqual([type(reg) for reg in param.reg_type], [Registration] * 3)
+        self.assertEqual(param.reg_type, (Registration.RENDEZVOUS,
+                                          Registration.RELAY_UDP_HIP,
+                                          Registration.RELAY_UDP_ESP))
+        # No ``lifetime`` alias is kept: ``REG_FAILED`` has no lifetime.
+        self.assertNotIn('lifetime', param)
+        self.assertFalse(hasattr(param, 'lifetime'))
+
+    def test_reg_failed_parse_then_rebuild_is_byte_exact(self) -> None:
+        """#2309: every Failure Type, with several Reg Types, rebuilds unchanged."""
+        for failure_type in (0, 1, 2, 3, 9, 201, 255):
+            for reg_types in ([], [1], [1, 2, 3], [0, 4, 200, 255, 2, 1, 3]):
+                with self.subTest(failure_type=failure_type, reg_types=reg_types):
+                    self.assertRoundTrips(packet(reg_failed(failure_type, reg_types)))
+
+    def test_reg_failed_make_then_parse_is_byte_exact(self) -> None:
+        """#2309: ``failure_type`` and ``reg_failed`` build the RFC 8003 octets."""
+        from pcapkit.const.hip.parameter import Parameter
+        from pcapkit.const.hip.registration import Registration
+        from pcapkit.const.hip.registration_failure import RegistrationFailure
+        from pcapkit.protocols.internet.hip import HIP
+
+        for failure_type, reg_types, octets in (
+            (RegistrationFailure.Insufficient_resources,
+             [Registration.RELAY_UDP_ESP, Registration.RENDEZVOUS], (2, [3, 1])),
+            (3, [1, 2, 3], (3, [1, 2, 3])),
+            (RegistrationFailure.Registration_requires_additional_credentials, [], (0, [])),
+        ):
+            with self.subTest(failure_type=failure_type, reg_types=reg_types):
+                wire = HIP.__new__(HIP).make(parameters=[(Parameter.REG_FAILED, {
+                    'failure_type': failure_type,
+                    'reg_failed': reg_types,
+                })]).pack()
+                self.assertEqual(wire[40:].hex(), reg_failed(*octets).hex())
+
+                param = HIP(io.BytesIO(wire), len(wire), extension=True).info.parameters[Parameter.REG_FAILED]
+                self.assertIs(type(param.failure_type), RegistrationFailure)
+                self.assertEqual(param.failure_type, octets[0])
+                self.assertEqual([type(reg) for reg in param.reg_type],
+                                 [Registration] * len(octets[1]))
+                self.assertEqual(list(param.reg_type), octets[1])
+                self.assertRoundTrips(wire)
+
+        # The default Failure Type is octet 0, as the removed ``lifetime`` was.
+        wire = HIP.__new__(HIP).make(parameters=[(Parameter.REG_FAILED, {})]).pack()
+        self.assertEqual(wire[40:].hex(), reg_failed(0, []).hex())
+
 
 if __name__ == '__main__':
     unittest.main()