diff --git a/pyleco/core/message.py b/pyleco/core/message.py
index affba0fe..42ae377d 100644
--- a/pyleco/core/message.py
+++ b/pyleco/core/message.py
@@ -23,7 +23,7 @@
 #
 
 from json import JSONDecodeError
-from typing import Any, List, Optional, Self
+from typing import Any, List, Optional, Self, Tuple
 
 
 from . import VERSION_B
@@ -53,12 +53,15 @@ class Message:
     def __init__(self, receiver: bytes | str, sender: bytes | str = b"",
                  data: Optional[bytes | str | Any] = None,
                  header: Optional[bytes] = None,
-                 conversation_id: bytes = b"",
+                 conversation_id: Optional[bytes] = None,
+                 message_id: Optional[bytes] = None,
+                 message_type: Optional[bytes] = None,
                  **kwargs) -> None:
         self._setup_caches()
         self.receiver = receiver if isinstance(receiver, bytes) else receiver.encode()
         self.sender = sender if isinstance(sender, bytes) else sender.encode()
-        self.header = (create_header_frame(conversation_id=conversation_id, **kwargs)
+        self.header = (create_header_frame(conversation_id=conversation_id, message_id=message_id,
+                                           message_type=message_type)
                        if header is None else header)
         if isinstance(data, (bytes)):
             self.payload = [data]
@@ -69,9 +72,9 @@ def __init__(self, receiver: bytes | str, sender: bytes | str = b"",
 
     def _setup_caches(self) -> None:
         self._payload: List[bytes] = []
-        self._header_elements = None
-        self._sender_elements = None
-        self._receiver_elements = None
+        self._header_elements: Optional[Tuple[bytes, bytes, bytes]] = None
+        self._sender_elements: Optional[Tuple[bytes, bytes]] = None
+        self._receiver_elements: Optional[Tuple[bytes, bytes]] = None
         self._data = None
 
     @classmethod
@@ -148,6 +151,12 @@ def message_id(self) -> bytes:
             self._header_elements = interpret_header(self.header)
         return self._header_elements[1]
 
+    @property
+    def message_type(self) -> bytes:
+        if self._header_elements is None:
+            self._header_elements = interpret_header(self.header)
+        return self._header_elements[2]
+
     @property
     def receiver_node(self) -> bytes:
         if self._receiver_elements is None:
diff --git a/pyleco/core/protocols.py b/pyleco/core/protocols.py
index c3580f1e..cc6e13c9 100644
--- a/pyleco/core/protocols.py
+++ b/pyleco/core/protocols.py
@@ -76,11 +76,10 @@ def set_properties(self, properties: Dict[str, Any]) -> None: ...
 
     def call_method(self, method: str, _args: Optional[list | tuple] = None, **kwargs) -> Any: ...
 
+
 class PollingActor(Actor, Protocol):
     """An Actor which allows regular polling."""
 
-    polling_interval: float
-
     def start_polling(self, polling_interval: Optional[float]) -> None: ...
 
     def set_polling_interval(self, polling_interval: float) -> None: ...
@@ -121,7 +120,7 @@ def sign_out(self) -> None: ...
 
     def send(self,
              receiver: str | bytes,
-             conversation_id: bytes = b"",
+             conversation_id: Optional[bytes] = None,
              data: Optional[Any] = None,
              **kwargs) -> None:
         """Send a message based on kwargs."""
@@ -131,7 +130,7 @@ def send(self,
 
     def send_message(self, message: Message) -> None: ...
 
-    def ask(self, receiver: bytes | str, conversation_id: bytes = b"",
+    def ask(self, receiver: bytes | str, conversation_id: Optional[bytes] = None,
             data: Optional[Any] = None,
             **kwargs) -> Message:
         """Send a message based on kwargs and retrieve the response."""
diff --git a/pyleco/core/serialization.py b/pyleco/core/serialization.py
index 13a57437..c29cb3df 100644
--- a/pyleco/core/serialization.py
+++ b/pyleco/core/serialization.py
@@ -23,22 +23,34 @@
 #
 
 import json
-import random
-from time import time
-import struct
-from typing import Tuple
+from typing import Optional, Tuple
 
+from uuid_extensions import uuid7  # as long as uuid does not yet support UUIDv7
 from pydantic import BaseModel
 
 
-def create_header_frame(conversation_id: bytes = b"", message_id: bytes = b"") -> bytes:
+def create_header_frame(conversation_id: Optional[bytes] = None,
+                        message_id: Optional[bytes] = None,
+                        message_type: Optional[bytes] = None) -> bytes:
     """Create the header frame.
 
     :param bytes conversation_id: ID of the conversation.
-    :param bytes message_id: Message ID of this message, must not contain ";".
+    :param bytes message_id: Message ID of this message, must not contain.
     :return: header frame.
     """
-    return b";".join((conversation_id, message_id))
+    if conversation_id is None:
+        conversation_id = generate_conversation_id()
+    elif len(conversation_id) != 16:
+        raise ValueError("Length of 'conversation_id' is not 16 bytes.")
+    if message_id is None:
+        message_id = b"\x00" * 3
+    elif len(message_id) != 3:
+        raise ValueError("Length of 'message_id' is not 3 bytes.")
+    if message_type is None:
+        message_type = b"\x00"
+    elif len(message_type) != 1:
+        raise ValueError("Length of 'message_type' is not 1 bytes.")
+    return b"".join((conversation_id, message_id, message_type))
 
 
 def split_name(name: bytes, node: bytes = b"") -> Tuple[bytes, bytes]:
@@ -55,17 +67,15 @@ def split_name_str(name: str, node: str = "") -> Tuple[str, str]:
     return (s.pop() if s else node), n
 
 
-def interpret_header(header: bytes) -> Tuple[bytes, bytes]:
+def interpret_header(header: bytes) -> Tuple[bytes, bytes, bytes]:
     """Interpret the header frame.
 
-    :return: conversation_id, message_id
+    :return: conversation_id, message_id, message_type
     """
-    try:
-        conversation_id, message_id = header.rsplit(b";", maxsplit=1)
-    except (IndexError, ValueError):
-        conversation_id = b""
-        message_id = b""
-    return conversation_id, message_id
+    conversation_id = header[:16]
+    message_id = header[16:19]
+    message_type = header[19:20]
+    return conversation_id, message_id, message_type
 
 
 def serialize_data(data: object) -> bytes:
@@ -86,7 +96,4 @@ def deserialize_data(content: bytes) -> object:
 
 def generate_conversation_id() -> bytes:
     """Generate a conversation_id."""
-    # struct.pack uses 8 bytes and takes 0.1 seconds for 1 Million repetitions.
-    # str().encode() uses 14 bytes and takes 0.5 seconds for 1 Million repetitions.
-    # !d is a double (8 bytes) in network byte order (big-endian)
-    return struct.pack("!d", time()) + random.randbytes(2)
+    return uuid7(as_type="bytes")  # type: ignore
diff --git a/pyproject.toml b/pyproject.toml
index 41dca309..405e9fc1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,6 +19,7 @@ dependencies = [
   "pyzmq",
   "openrpc",
   "jsonrpc2-pyclient",
+  "uuid7",
 ]
 
 [project.optional-dependencies]
diff --git a/tests/core/test_message.py b/tests/core/test_message.py
index 1011a754..6b1d6c30 100644
--- a/tests/core/test_message.py
+++ b/tests/core/test_message.py
@@ -29,10 +29,13 @@
 from pyleco.core.message import Message
 
 
+cid = b"conversation_id;"
+
+
 class Test_Message_from_frames:
     @pytest.fixture
     def message(self) -> Message:
-        return Message.from_frames(b"\xffo", b"rec", b"send", b"x;y",
+        return Message.from_frames(b"\xffo", b"rec", b"send", b"conversation_id;mid0",
                                    b'[["GET", [1, 2]], ["GET", 3]]')
 
     def test_payload(self, message: Message):
@@ -48,7 +51,13 @@ def test_sender(self, message: Message):
         assert message.sender == b"send"
 
     def test_header(self, message: Message):
-        assert message.header == b"x;y"
+        assert message.header == b"conversation_id;mid0"
+
+    def test_conversation_id(self, message: Message):
+        assert message.conversation_id == b"conversation_id;"
+
+    def test_message_id(self, message: Message):
+        assert message.message_id == b"mid"
 
     def test_multiple_payload_frames(self):
         message = Message.from_frames(
@@ -61,8 +70,8 @@ def test_no_payload(self):
         assert message.payload == []
 
     def test_get_frames_list(self, message: Message):
-        assert message.to_frames() == [b"\xffo", b"rec", b"send", b"x;y",
-                                             b'[["GET", [1, 2]], ["GET", 3]]']
+        assert message.to_frames() == [b"\xffo", b"rec", b"send", b"conversation_id;mid0",
+                                       b'[["GET", [1, 2]], ["GET", 3]]']
 
 
 class Test_Message_create_message:
@@ -72,8 +81,8 @@ def message(self) -> Message:
             receiver=b"rec",
             sender=b"send",
             data=[["GET", [1, 2]], ["GET", 3]],
-            conversation_id=b"x",
-            message_id=b"y",
+            conversation_id=b"conversation_id;",
+            message_id=b"mid",
         )
 
     def test_payload(self, message: Message):
@@ -89,25 +98,26 @@ def test_sender(self, message: Message):
         assert message.sender == b"send"
 
     def test_header(self, message: Message):
-        assert message.header == b"x;y"
+        assert message.header == b"conversation_id;mid\x00"
 
-    def test_get_frames_list(self, message: Message):
+    def test_to_frames(self, message: Message):
         assert message.to_frames() == [
             VERSION_B,
             b"rec",
             b"send",
-            b"x;y",
+            b"conversation_id;mid\x00",
             b'[["GET", [1, 2]], ["GET", 3]]',
         ]
 
-    def test_get_frames_list_without_payload(self, message: Message):
+    def test_to_frames_without_payload(self, message: Message):
         message.payload = []
-        assert message.to_frames() == [VERSION_B, b"rec", b"send", b"x;y"]
+        assert message.to_frames() == [VERSION_B, b"rec", b"send", b"conversation_id;mid\x00"]
 
     def test_message_without_data_does_not_have_payload_frame(self):
-        message = Message(b"rec", "send")
+        message = Message(b"rec", "send", conversation_id=b"conversation_id;")
         assert message.payload == []
-        assert message.to_frames() == [VERSION_B, b"rec", b"send", b";"]
+        assert message.to_frames() == [VERSION_B, b"rec", b"send",
+                                       b"conversation_id;\x00\x00\x00\x00"]
 
     def test_message_binary_data(self):
         message = Message(b"rec", data=b"binary data")
@@ -117,6 +127,7 @@ def test_message_data_str_to_binary_data(self):
         message = Message(b"rec", data="some string")
         assert message.payload[0] == b"some string"
 
+
 class Test_Message_with_strings:
     @pytest.fixture
     def str_message(self) -> Message:
@@ -180,13 +191,13 @@ def test_get_frames_list_raises_error_on_empty_sender():
 
 class TestComparison:
     def test_dictionary_order_is_irrelevant(self):
-        assert Message(b"r", data={"a": 1, "b": 2}) == Message(b"r", data={"b": 2, "a": 1})
+        m1 = Message(b"r", conversation_id=cid, data={"a": 1, "b": 2})
+        m2 = Message(b"r", conversation_id=cid, data={"b": 2, "a": 1})
+        assert m1 == m2
 
     def test_distinguish_empty_payload_frame(self):
-        m1 = Message("r")
+        m1 = Message("r", conversation_id=b"conversation_id;")
         m1.payload = [b""]
-        m2 = Message("r")
+        m2 = Message("r", conversation_id=b"conversation_id;")
         assert m2.payload == []  # verify that it does not have a payload
         assert m1 != m2
-
-
diff --git a/tests/core/test_serialization.py b/tests/core/test_serialization.py
index d098aed2..74e8d8fb 100644
--- a/tests/core/test_serialization.py
+++ b/tests/core/test_serialization.py
@@ -28,11 +28,29 @@
 from pyleco.core import serialization
 
 
-@pytest.mark.parametrize("kwargs, header", (
-    ({}, b";"),
+class Test_create_header_frame:
+    @pytest.mark.parametrize("kwargs, header", (
+        ({"conversation_id": b"\x00" * 16, }, bytes([0] * 20)),
+    ))
+    def test_create_header_frame(self, kwargs, header):
+        assert serialization.create_header_frame(**kwargs) == header
+
+    @pytest.mark.parametrize("cid_l", (0, 1, 2, 4, 6, 7, 10, 15, 17, 20))
+    def test_wrong_cid_length_raises_errors(self, cid_l):
+        with pytest.raises(ValueError, match="'conversation_id'"):
+            serialization.create_header_frame(conversation_id=bytes([0] * cid_l))
+
+    @pytest.mark.parametrize("mid_l", (0, 1, 2, 4, 6, 7))
+    def test_wrong_mid_length_raises_errors(self, mid_l):
+        with pytest.raises(ValueError, match="'message_id'"):
+            serialization.create_header_frame(message_id=bytes([0] * mid_l))
+
+
+@pytest.mark.parametrize("header, conversation_id, message_id, message_type", (
+        (bytes(range(20)), bytes(range(16)), b"\x10\x11\x12", b"\x13"),
 ))
-def test_create_header_frame(kwargs, header):
-    assert serialization.create_header_frame(**kwargs) == header
+def test_interpret_header(header, conversation_id, message_id, message_type):
+    assert serialization.interpret_header(header) == (conversation_id, message_id, message_type)
 
 
 @pytest.mark.parametrize("full_name, node, name", (
@@ -53,3 +71,21 @@ def test_dict(self):
         raw = {"some": "item", "key": "value", 5: [7, 3.1]}
         expected = b'{"some": "item", "key": "value", "5": [7, 3.1]}'
         assert serialization.serialize_data(raw) == expected
+
+
+class Test_generate_conversation_id_is_UUIDv7:
+    @pytest.fixture
+    def conversation_id(self):
+        return serialization.generate_conversation_id()
+
+    def test_type_is_bytes(self, conversation_id):
+        assert isinstance(conversation_id, bytes)
+
+    def test_length(self, conversation_id):
+        assert len(conversation_id) == 16
+
+    def test_UUID_version(self, conversation_id):
+        assert conversation_id[6] >> 4 == 7
+
+    def test_variant(self, conversation_id):
+        assert conversation_id[8] >> 6 == 0b10