Make all peer messages constant message size by rustyrussell · Pull Request #8893 · ElementsProject/lightning

rustyrussell · 2026-02-10T04:17:32Z

Not sufficient, but definitely necessary for avoiding trivial observation.

l3 doesn't just need to know about l2 (which it can get from the channel_announcement), but needs to see the node_announcement. Otherwise: ``` l1, l2 = node_factory.line_graph(2, wait_for_announce=True, # No onion_message support in l1 opts=[{'dev-force-features': -39}, {'dev-allow-localhost': None}]) l3 = node_factory.get_node() l3.rpc.connect(l1.info['id'], 'localhost', l1.port) wait_for(lambda: l3.rpc.listnodes(l2.info['id'])['nodes'] != []) offer = l2.rpc.call('offer', {'amount': '2msat', 'description': 'simple test'}) > l3.rpc.call('fetchinvoice', {'offer': offer['bolt12']}) tests/test_pay.py:4804: ... > raise RpcError(method, payload, resp['error']) E pyln.client.lightning.RpcError: RPC call failed: method: fetchinvoice, payload: {'offer': 'lno1qgsqvgnwgcg35z6ee2h3yczraddm72xrfua9uve2rlrm9deu7xyfzrcgqypq5zmnd9khqmr9yp6x2um5zcssxwz9sqkjtd8qwnx06lxckvu6g8w8t0ue0zsrfqqygj636s4sw7v6'}, error: {'code': 1003, 'message': 'Failed: could not route or connect directly to 033845802d25b4e074ccfd7cd8b339a41dc75bf9978a034800444b51d42b07799a: {"code":400,"message":"Unable to connect, no address known for peer"}'} ``` Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Commit 888745b (dev_disconnect: remove @ marker.) in v0.11 in April 2022) removed the '@' marker from our dev_disconnect code, but one test still uses it. Refactoring this code made it crash on invalid input. The test triggered a db issue which has been long fixed, so I'm simply removing it. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

This requires access to dumpcap. On Ubuntu, at least, this means you need to be in the "wireshark" group. We may also need: sudo ethtool -K lo gro off gso off tso off Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Give us a single "next message" function to call. This will be useful when we want to write more than one at a time. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Do all the special treatment of the message type first. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

This gives us finer control over write sizes: for now we just cap the write size. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

We're doing our own buffering now. We leave the is_urgent() function for two commits in the future though. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

… count. We are about to use them to make our packet size constant, and this will upset the tests. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Messages are now constant. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Changelog-Added: Protocol: we now pad all peer messages to make them the same length.

This replaces our previous nagle-based toggling. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

…ges. Since we delay the others quite a lot (up to 1 second), it's better to consider most messages "urgent" and worth immediately transmitting. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

This is exactly what membuf is for: it handles expansion much more neatly. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Lagrang3 · 2026-02-17T09:11:44Z

The test added on commit "pytest: add fixture for checking packet sizes." won't fail as expected.
I think it is because assert_constant_payload() is only called after the test is completed during cleanup phase.

rustyrussell · 2026-02-18T03:43:02Z

The test added on commit "pytest: add fixture for checking packet sizes." won't fail as expected. I think it is because assert_constant_payload() is only called after the test is completed during cleanup phase.

No, it definitely works (I hit it during debugging!) and I reverted and took off xfail:

tests/test_connection.py::test_constant_packet_size PASSED [100%]
tests/test_connection.py::test_constant_packet_size ERROR [100%]
==================================== ERRORS ====================================
________________ ERROR at teardown of test_constant_packet_size ________________
have_pcap_tools = None
tmp_path = PosixPath('/tmp/pytest-of-rusty/pytest-0/test_constant_packet_size0')
@pytest.fixture
def tcp_capture(have_pcap_tools, tmp_path):
# You will need permissions. Most distributions have a group which has
# permissions to use dumpcap:
# $ ls -l /usr/bin/dumpcap
# -rwxr-xr-- 1 root wireshark 229112 Apr 16 2024 /usr/bin/dumpcap
# $ getcap /usr/bin/dumpcap
# /usr/bin/dumpcap cap_net_admin,cap_net_raw=eip
# So you just need to be in the wireshark group.
cap = TcpCapture(tmp_path)
yield cap
cap.stop()
> cap.assert_constant_payload()
tests/fixtures.py:193: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ self = <fixtures.TcpCapture object at 0x731d573af230>
def assert_constant_payload(self):
tshark_cmd = [
"tshark",
"-r", str(self.pcap),
"-Y", "tcp.len > 0",
"-T", "fields",
"-e", "tcp.len",
]
out = subprocess.check_output(tshark_cmd, text=True)
lengths = [int(x) for x in out.splitlines() if x.strip()]
assert lengths, f"No TCP payload packets captured on port {self.port}"
uniq = set(lengths)
> assert len(uniq) == 1, (
^^^^^^^^^^^^^^

rustyrussell · 2026-02-10T04:17:32Z