According to the RLPX handshake spec, one should give clients 2 seconds to disconnect after sending a disconnect packet:

Inform the peer that a disconnection is imminent; if received, a peer should disconnect immediately. When sending, well-behaved hosts give their peers a fighting chance (read: wait 2 seconds) to disconnect to before disconnecting themselves.

However, Aleth closes the socket immediately (via Session::drop). This also means that the disconnect packet may not be sent - this is because Session::disconnect (and all of the functions it calls) is executed on the network thread, whereas the async_write may not execute immediately in which case I suspect it will execute after Session::disconnect / Session::drop:

Here's where the actual socket write takes place (in Session::write):