Function send_frame_timed 

async fn send_frame_timed<S, C>(
    socket: &mut ClientSocket<S>,
    message: ServerMessage,
    clock: &C,
) -> Result<FrameWrite>
where
    S: Sink<Message, Error = Error> + Unpin,
    C: WriteClock,

send_frame, timing the socket’s share of the write.

The open’s own line cannot place its remainder without it (#2506): open_ms ends when this returns, so the frame’s transfer is inside it, and a write that yielded is otherwise indistinguishable from the fold, the record’s reads and the serialisation before it.