diff options
Diffstat (limited to 'packages/meshbay-node/src/meshbay_node/transport')
4 files changed, 271 insertions, 15 deletions
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/core.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/core.py index d197044..07820b2 100644 --- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/core.py +++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/core.py @@ -15,6 +15,7 @@ from meshbay_common.protocol import MNP from meshbay_node import ops from meshbay_node import transfers as transfers_mod from meshbay_node.transport.webrtc import ( + proactor_udp, # noqa: F401 (patches asyncio's Windows loop on import) sctp_fix, # noqa: F401 (patches aiortc on import) sctp_stats, # noqa: F401 (patches aiortc on import) udp_buffer, # noqa: F401 (patches aioice on import) diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/proactor_udp.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/proactor_udp.py new file mode 100644 index 0000000..38cb97c --- /dev/null +++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/proactor_udp.py @@ -0,0 +1,139 @@ +"""Datagrams moved straight through the socket on Windows' event loop, applied +on import. + +The node runs on the ProactorEventLoop, the one that can start ffmpeg there +(`platform.configure_event_loop`), and so do its connection processes. Its +datagram transport keeps one overlapped operation in each direction and starts +the next only from the completion callback of the one before, which runs on a +later turn of the loop. So a socket sends **one datagram per turn of the loop, +and receives one**, however many are waiting, and a turn lasts as long as +everything else the loop has to do: the other connections, reading the file, +the hub. The busier the node, the slower every connection, with cores to spare. + +Measured on loopback, 1200-byte datagrams, with the loop otherwise busy 0.5 ms +a turn: 1,928 sent a second (2.3 MB/s), and 1,954 received, the kernel dropping +87 % of what arrived; straight through the socket, 88,000 sent, and every +datagram received at the 20,000 a second it was sent at. In the node holding the +connections itself, a browser's download over eight connections ran at 9.1 MB/s +with the node at 45 % of a core and an SCTP round trip of 745 ms on loopback: +that is the queue in front of each socket. In a connection process serving the +command-line client, 42 % of the time was `IocpProactor.sendto`. Linux never +paid for the sending half: the selector loop's transport tries the socket first +and queues only what it refuses. + +Sending: a datagram goes out with a non-blocking `sendto` when nothing is queued +and no overlapped send is pending, so order is kept; anything else, the socket +refusing included, goes through the proactor's own queue as before. + +Receiving: once an overlapped read completes, its datagram is delivered, then +whatever else the socket holds is read with non-blocking `recvfrom`, and only +then is the next overlapped read posted. Never while one is pending, which +could hand a later datagram to the completion port ahead of the ones read here. +The overlapped path turns the ICMP "port unreachable" a send to a silent +address brings back into an empty read; read directly it is a +ConnectionResetError, and it is skipped the same way. + +No effect off Windows, where the class is never used. +""" + +import sys +from asyncio import exceptions, proactor_events + +_Transport = proactor_events._ProactorDatagramTransport +_proactor_sendto = _Transport.sendto + +# Datagrams read straight from the socket after one completion, at most: a +# socket under a flood still lets the loop turn. +DRAIN_MAX = 256 + + +def sendto(self, data, addr=None) -> None: + if (self._write_fut is None and not self._buffer and not self._conn_lost + and isinstance(data, (bytes, bytearray, memoryview)) and data + and (self._address is None or addr in (None, self._address))): + try: + if self._address is not None: + self._sock.send(data) + else: + self._sock.sendto(data, addr) + return + except (BlockingIOError, InterruptedError): + pass + except OSError as exc: + # What the overlapped path does with a failed send. + self._protocol.error_received(exc) + return + _proactor_sendto(self, data, addr) + + +def _deliver(self, data, addr) -> None: + """To the protocol, and whatever it raises reported as a callback's would + be: in CPython's order the next read was posted first, and an exception + here must not end the reading either.""" + try: + self._protocol.datagram_received(data, addr) + except (SystemExit, KeyboardInterrupt): + raise + except BaseException as exc: + self._loop.call_exception_handler({ + "message": "Exception in datagram_received", "exception": exc, + "transport": self, "protocol": self._protocol}) + + +def _drain(self) -> None: + for _ in range(DRAIN_MAX): + if self._conn_lost or self._closing: + return + try: + if self._address is not None: + data, addr = self._sock.recv(self.max_size), self._address + else: + data, addr = self._sock.recvfrom(self.max_size) + except (BlockingIOError, InterruptedError): + return + except ConnectionResetError: + continue + except OSError as exc: + # The overlapped read posted next meets it again, and reports it + # the proactor's way. + self._protocol.error_received(exc) + return + if data: + _deliver(self, data, addr) + + +def _loop_reading(self, fut=None) -> None: + """CPython's own (3.12), but a completed read is delivered and the socket + drained before the next overlapped read is posted, not after.""" + try: + if self._conn_lost: + return + assert self._read_fut is fut or (self._read_fut is None and self._closing) + self._read_fut = None + if fut is not None: + res = fut.result() + if self._closing: + return + data, addr = (res, self._address) if self._address is not None else res + if data: + _deliver(self, data, addr) + _drain(self) + if self._conn_lost: + return + if self._address is not None: + self._read_fut = self._loop._proactor.recv(self._sock, self.max_size) + else: + self._read_fut = self._loop._proactor.recvfrom(self._sock, self.max_size) + except OSError as exc: + self._protocol.error_received(exc) + except exceptions.CancelledError: + if not self._closing: + raise + else: + if self._read_fut is not None: + self._read_fut.add_done_callback(self._loop_reading) + + +if sys.platform == "win32": + _Transport.sendto = sendto + _Transport._loop_reading = _loop_reading diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/sctp_fix.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/sctp_fix.py index 0d45a7d..a7e6f86 100644 --- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/sctp_fix.py +++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/sctp_fix.py @@ -1,12 +1,13 @@ -"""A fix to aiortc's SCTP sender, applied on import (aiortc 1.15). +"""Two fixes to aiortc's SCTP stack, applied on import (aiortc 1.15). -aiortc counts the bytes in flight and sends nothing new while that count is at -or above the congestion window. A chunk the peer acknowledged in a gap block is -marked `_acked` and taken out of the count. If the T3 timer then expires, every -chunk still queued is sent again, that one included, and goes back into the -count with `_acked` still set. When the cumulative ack finally covers it, the -SACK handler skips it because it reads as acknowledged already, and its bytes -stay counted for the life of the association. +**The sender leaked bytes in flight.** aiortc counts the bytes in flight and +sends nothing new while that count is at or above the congestion window. A chunk +the peer acknowledged in a gap block is marked `_acked` and taken out of the +count. If the T3 timer then expires, every chunk still queued is sent again, +that one included, and goes back into the count with `_acked` still set. When +the cumulative ack finally covers it, the SACK handler skips it because it reads +as acknowledged already, and its bytes stay counted for the life of the +association. Each such loss leaks a chunk. On a lossy path (a phone on Wi-Fi) the leak grows past the window, which T3 itself keeps small. From then on the node sends @@ -14,14 +15,49 @@ nothing on that connection: nothing is outstanding, so no timer runs to unstick it, and the association still reads as established. The peer keeps talking and the node keeps answering into a queue that never moves. Live symptom: the photo backup stalled after a few dozen photos, its slot granted and the grant never -delivered (docs/MESHBAY_DESIGN.md §9.12). +delivered (docs/MESHBAY_DESIGN.md §9.12). The fix is the invariant the code +assumed: a chunk being sent again is in flight, so it is not acknowledged. -The fix is the invariant the code assumed: a chunk being sent again is in -flight, so it is not acknowledged. `test_sctp_fix.py` replays the sequence -against aiortc itself. +**The receiver reassembled in quadratic time.** After every DATA chunk, +`InboundStream.pop_messages` walks the reassembly list from its start, through +every fragment of the message still being assembled. A message of n fragments +costs n²/2 steps, and a 1 MiB `file_chunk` is about 870 fragments: measured on a +receiving aiortc, 153 million steps for 400 MB, two thirds of its CPU, and the +whole of why it could not take more than 20 MB/s on loopback. No message can +come out of a reassembly holding no last fragment, so the walk is skipped until +one is there; when one is, the walk is aiortc's own, unchanged. + +The node receives uploads in 48 KB messages, where the walk is short; the +command-line client receives 1 MiB ones and carries the same fix +(`meshbay_cli/client/sctp_fix.py`). + +**The receiver acknowledged every packet.** RFC 4960 §6.2 asks for one SACK +every second packet, and Chromium does that; aiortc sent one at the end of +every packet of data. Each SACK costs its receiver a DTLS decryption and a parse +on the one core that also does everything else — on a node sending a download +to the command-line client, more time than sending the data. In an upload the +node is the receiver and pays the other half: the encryption and the send of a +SACK per packet. A SACK still goes at once when something is out of order or +duplicated (the loss signal the sender is waiting for) or the packet carried +anything other than data; a lone packet is acknowledged within `SACK_DELAY_S`. + +`test_sctp_fix.py` replays all three against aiortc itself. """ -from aiortc.rtcsctptransport import RTCSctpTransport +import asyncio +import contextlib + +from aiortc.rtcsctptransport import ( + SCTP_DATA_LAST_FRAG, + DataChunk, + InboundStream, + InitChunk, + RTCSctpTransport, + parse_packet, +) + +# RFC 4960 §6.2: at most 500 ms; 200 is what implementations use. +SACK_DELAY_S = 0.2 def _flight_size_increase(self, chunk) -> None: @@ -30,3 +66,82 @@ def _flight_size_increase(self, chunk) -> None: RTCSctpTransport._flight_size_increase = _flight_size_increase + + +_add_chunk = InboundStream.add_chunk +_pop_messages = InboundStream.pop_messages +_prune_chunks = InboundStream.prune_chunks + + +def add_chunk(self, chunk) -> None: + _add_chunk(self, chunk) + if chunk.flags & SCTP_DATA_LAST_FRAG: + self._last_frags = getattr(self, "_last_frags", 0) + 1 + + +def pop_messages(self): + # Only ever too high (a pruned last fragment), never too low: a message + # leaves the reassembly with exactly one last fragment, and only here. + if not getattr(self, "_last_frags", 0): + return + for message in _pop_messages(self): + self._last_frags -= 1 + yield message + + +def prune_chunks(self, tsn: int) -> int: + size = _prune_chunks(self, tsn) + self._last_frags = sum(1 for c in self.reassembly if c.flags & SCTP_DATA_LAST_FRAG) + return size + + +InboundStream.add_chunk = add_chunk +InboundStream.pop_messages = pop_messages +InboundStream.prune_chunks = prune_chunks + + +async def _handle_data(self, data: bytes) -> None: + """aiortc's own, down to the SACK at the end: that one may wait for a + second packet.""" + try: + _, _, verification_tag, chunks = parse_packet(data) + except ValueError: + return + if any(isinstance(x, InitChunk) for x in chunks): + assert len(chunks) == 1 + expected_tag = 0 + else: + expected_tag = self._local_verification_tag + if verification_tag != expected_tag: + return + for chunk in chunks: + await self._receive_chunk(chunk) + if not self._sack_needed: + return + self._sack_packets = getattr(self, "_sack_packets", 0) + 1 + if (self._sack_packets >= 2 or self._sack_misordered or self._sack_duplicates + or not all(isinstance(c, DataChunk) for c in chunks)): + await _sack_now(self) + elif getattr(self, "_sack_timer", None) is None: + self._sack_timer = self._loop.call_later(SACK_DELAY_S, _sack_later, self) + + +async def _sack_now(self) -> None: + timer = getattr(self, "_sack_timer", None) + if timer is not None: + timer.cancel() + self._sack_timer = None + self._sack_packets = 0 + if self._sack_needed and self._association_state != self.State.CLOSED: + # The DTLS transport may be gone under a timer that fired late. + with contextlib.suppress(ConnectionError): + await self._send_sack() + + +def _sack_later(self) -> None: + self._sack_timer = None + # Held: a task nobody references can be collected before it runs. + self._sack_task = asyncio.ensure_future(_sack_now(self)) + + +RTCSctpTransport._handle_data = _handle_data diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/worker.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/worker.py index 832428a..c933aba 100644 --- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/worker.py +++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/worker.py @@ -224,9 +224,10 @@ async def _main() -> None: root.handlers[:] = [_Forward(writer.write)] root.setLevel(hello.get("level", logging.INFO)) - # Patches aiortc on import, as the node does for the connections it holds - # itself. + # Patches aiortc, and asyncio's Windows loop, on import, as the node does + # for the connections it holds itself. from meshbay_node.transport.webrtc import ( # noqa: F401 + proactor_udp, sctp_fix, sctp_stats, udp_buffer, |