docs: one datagram per turn of the loop on Windows, and how it hidHEADmain

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

1 files changed, 20 insertions, 0 deletions

diff --git a/docs/MESHBAY_DESIGN.md b/docs/MESHBAY_DESIGN.md
index e10209a..4db06c4 100644
--- a/docs/MESHBAY_DESIGN.md
+++ b/docs/MESHBAY_DESIGN.md
@@ -1531,6 +1531,26 @@ still 20, the client at a full core; 28 with the reassembly fixed; 39 with the
 acknowledgements, 49 by the node's own count of the transfer. An upload from the
 client went from 25 to 29.
 
+**On Windows a datagram goes straight through the socket, in both directions.**
+The node and its connection processes run on the ProactorEventLoop, the one that
+can start ffmpeg there, and its datagram transport keeps one overlapped operation
+per direction, starting the next from the completion callback of the one before,
+on a later turn of the loop. A socket therefore sent one datagram per turn and
+received one, and a turn lasts as long as everything else the loop does: the
+other connections, the file, the hub. That is a ceiling per connection that
+falls as the node gets busier, with cores to spare. With the loop otherwise busy
+0.5 ms a turn: 1,928 datagrams sent a second, and 1,954 received with 87 % of
+what arrived dropped by the kernel. In a node holding its connections itself, a
+browser's download over eight of them ran at 9 MB/s with the node at 45 % of a
+core and an SCTP round trip of 745 ms on loopback, and one of the eight sessions
+was lost on the way. `proactor_udp.py` sends with a non-blocking `sendto` when
+nothing is queued, and after a completed read takes whatever else the socket
+holds before posting the next one; refusals and errors take the proactor's own
+path. Measured on loopback on a two-core machine, 400 MB, with it: the
+command-line client's download 27 to 33 MB/s, its upload 18 to 22 (23 with two
+connection processes), the browser's download over eight connections 9 to 14
+before and 17 to 19 after, where the browser takes the other core.
+
 **A session is open from the end of its handshake, and a reconnect does not
 shorten that.** Until connect() returns, a client sends only what the node takes
 before the proof — `handshake`, `handshake_response`, `keypair_bundle_fetch`,