feat(node): log what a transfer moved when it ends

One INFO line per lease: bytes, duration, rate, and the SCTP
association's window, round trip, losses and T3 expirations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 43 insertions, 0 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/sctp_stats.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/sctp_stats.py
new file mode 100644
index 0000000..338ded9
--- /dev/null
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/sctp_stats.py
@@ -0,0 +1,43 @@
+"""Counts what aiortc's SCTP sender does not, applied on import (aiortc 1.15).
+
+aiortc halves its congestion window on every loss and drops it to one packet
+when the T3 timer expires, and keeps no record of either. Those two counts are
+what say whether a slow transfer was held back by the path: a window that never
+grows past a few hundred KB is losses, and the node could not tell.
+
+A loss is counted when the sender enters fast recovery, so a burst of packets
+dropped together is one loss, which is also one halving of the window.
+"""
+
+from aiortc.rtcsctptransport import RTCSctpTransport
+
+_receive_sack_chunk = RTCSctpTransport._receive_sack_chunk
+_t3_expired = RTCSctpTransport._t3_expired
+
+
+async def _counting_receive_sack_chunk(self, chunk) -> None:
+    recovering = self._fast_recovery_exit is not None
+    await _receive_sack_chunk(self, chunk)
+    if not recovering and self._fast_recovery_exit is not None:
+        self._mb_losses = getattr(self, "_mb_losses", 0) + 1
+
+
+def _counting_t3_expired(self) -> None:
+    self._mb_t3 = getattr(self, "_mb_t3", 0) + 1
+    _t3_expired(self)
+
+
+RTCSctpTransport._receive_sack_chunk = _counting_receive_sack_chunk
+RTCSctpTransport._t3_expired = _counting_t3_expired
+
+
+def describe(pc) -> str:
+    """The association of `pc` as a log suffix, or nothing when it has none."""
+    sctp = getattr(pc, "sctp", None)
+    if sctp is None:
+        return ""
+    srtt = getattr(sctp, "_srtt", None)
+    rtt = f"{srtt * 1000:.0f} ms" if srtt else "?"
+    return (f" sctp cwnd={sctp._cwnd // 1024} KB rtt={rtt}"
+            f" loss={getattr(sctp, '_mb_losses', 0)}"
+            f" t3={getattr(sctp, '_mb_t3', 0)}")