fix(node): a receive buffer that holds an upload's window

Uploads ran at 6.5 MB/s because the kernel dropped datagrams at the
node's 256 KB socket. Asking for 4 MB gave 19.5 MB/s, no drop.

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

3 files changed, 40 insertions, 1 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 c48e9ca..d197044 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/core.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/core.py
@@ -17,6 +17,7 @@ from meshbay_node import transfers as transfers_mod
 from meshbay_node.transport.webrtc 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)
 )
 from meshbay_node.transport.webrtc.channel import (
     _REPLY_TO,
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/udp_buffer.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/udp_buffer.py
new file mode 100644
index 0000000..8a2db31
--- /dev/null
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/udp_buffer.py
@@ -0,0 +1,34 @@
+"""A larger receive buffer on the node's ICE sockets, applied on import.
+
+aioice asks the kernel for 256 KB per socket. The node reads its sockets from
+Python, more slowly than a line delivers, and a browser sending a file keeps up
+to 1.5 MB in flight (32 chunks of 48 KB, `transport-upload.js`). What does not
+fit in the buffer is dropped by the kernel of the node's own machine
+(`UdpRcvbufErrors`), the browser's SCTP sender takes each drop for congestion,
+and an upload settles around 6.5 MB/s on a line that carries 47
+(docs/MESHBAY_DESIGN.md §5.5).
+
+The buffer has to hold what one upload may have in flight, counted the way the
+kernel counts it, which is about twice the bytes for packets of this size. The
+kernel grants no more than `net.core.rmem_max` allows, and on a machine where
+that is left at its default of 208 KB this changes nothing: `granted()` says
+what a socket really gets, and the node logs it once.
+"""
+
+import socket
+
+import aioice.turn
+
+WANTED = 4 * 1024 * 1024
+
+aioice.turn.UDP_SOCKET_BUFFER_SIZE = WANTED
+
+
+def granted() -> int:
+    """What the kernel gives a socket that asks for `WANTED`, in bytes."""
+    with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as sock:
+        try:
+            sock.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, WANTED)
+        except OSError:
+            pass
+        return sock.getsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF)
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 072c849..832428a 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/worker.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/worker.py
@@ -226,7 +226,11 @@ async def _main() -> None:
 
     # Patches aiortc on import, as the node does for the connections it holds
     # itself.
-    from meshbay_node.transport.webrtc import sctp_fix, sctp_stats  # noqa: F401
+    from meshbay_node.transport.webrtc import (  # noqa: F401
+        sctp_fix,
+        sctp_stats,
+        udp_buffer,
+    )
 
     worker = Worker(reader, writer)
     worker.configure(hello)