fix(hip): make() without parameters writes Header Length 0, below the RFC 7401 minimum of 4

pcapkit/protocols/internet/hip.py

@@ -570,6 +570,10 @@ def read(self, length: 'Optional[int]' = None, *, extension: bool = False, **kwa
             raise ProtocolError('HIP: invalid format')
         if schema.ver['bit_1'] != 1:
             raise ProtocolError('HIP: invalid format')
+        # Header Length below 4 cannot cover the two HITs, and would leave a
+        # negative parameter length below.
+        if schema.len < 4:
+            raise ProtocolError(f'HIP: invalid header length: {schema.len}')
 
         hip = Data_HIP(
             next=schema.next,
@@ -649,7 +653,9 @@ def make(self,
             parameters_value, total_length = self._make_hip_param(parameters, version=version)
             length = total_length // 8 + 4
         else:
-            parameters_value, length = [], 0
+            # NOTE: Header Length excludes the first 8 octets but always
+            # counts the two 16-octet HITs, hence the minimum of 4.
+            parameters_value, length = [], 4
 
         return Schema_HIP(
             next=next_value,  # type: ignore[arg-type]

tests/protocols/internet/test_hip_parameterless_length_unit.py

@@ -0,0 +1,71 @@
+# -*- coding: utf-8 -*-
+"""HIP writes Header Length 4 when it carries no parameters.
+
+GitHub issue #1189: :meth:`HIP.make <pcapkit.protocols.internet.hip.HIP.make>`
+wrote Header Length ``0`` for ``parameters=None``. RFC 7401 §5.1 counts the
+field in 8-octet units excluding the first 8, so the two 16-octet HITs alone
+make 4 the minimum. The header's own re-parse then computed a parameter length
+of ``-32`` and raised ``ProtocolError``. :meth:`HIP.read
+<pcapkit.protocols.internet.hip.HIP.read>` now rejects a wire Header Length
+below 4 up front.
+
+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`, not at module load, so it
+always belongs to the live :mod:`pcapkit` import.
+
+"""
+
+import unittest
+
+from tests._support import reimport_once_per_class
+
+#: Keyword arguments for a parameterless HIP header with no payload.
+KWARGS = dict(next=59, packet=1, checksum=b'\x00\x00', controls_anonymous=False,
+              shit=0, rhit=0, payload=b'')
+
+
+class TestHIPParameterlessLength(unittest.TestCase):
+    """Pin Header Length for a HIP header without parameters."""
+
+    def setUp(self) -> None:
+        reimport_once_per_class(self)
+
+    def test_make_without_parameters_writes_length_4(self) -> None:
+        from pcapkit.protocols.internet.hip import HIP
+
+        for version in (1, 2):
+            with self.subTest(version=version):
+                proto = HIP(parameters=None, version=version, **KWARGS)
+                self.assertEqual(len(proto.data), 40)
+                self.assertEqual(proto.data[1], 4)
+                self.assertEqual(proto.info.length, 40)
+                self.assertNotIn('parameters', proto.info)
+
+    def test_parameterless_header_parses_back(self) -> None:
+        from pcapkit.protocols.internet.hip import HIP
+
+        for version in (1, 2):
+            with self.subTest(version=version):
+                data = HIP(parameters=None, version=version, **KWARGS).data
+                parsed = HIP(data)
+                self.assertEqual(parsed.info.length, 40)
+                self.assertEqual(parsed.info.version, version)
+                self.assertEqual(parsed.data, data)
+
+    def test_read_rejects_header_length_below_4(self) -> None:
+        from pcapkit.protocols.internet.hip import HIP
+        from pcapkit.utilities.exceptions import ProtocolError
+
+        for value in (0, 3):
+            with self.subTest(len=value):
+                # Next Header, Header Length, Packet Type 1, Version 2, then
+                # checksum, controls and the two HITs, all zero.
+                data = bytes([59, value, 0x01, 0x21]) + bytes(36)
+                with self.assertRaisesRegex(ProtocolError, 'invalid header length'):
+                    HIP(data)
+
+
+if __name__ == '__main__':
+    unittest.main()

tests/protocols/internet/test_hip_unit.py

@@ -226,7 +226,7 @@ def test_hip_read_make_post_init_and_parameter_list_edges(self) -> None:
             parsed_with_param = proto.read(extension=True)
         read_params.assert_called_once_with(8, version=2)
         self.assertEqual(parsed_with_param.parameters[Parameter.ESP_INFO].index, 1)
-        self.assertEqual(proto.make(parameters=None).len, 0)
+        self.assertEqual(proto.make(parameters=None).len, 4)
 
     def test_hip_register_parameter_warns_on_overwrite(self) -> None:
         from pcapkit.const.hip.parameter import Parameter