The reason that the three acks don't cause a fast restransmit is that
  the receive window is updated between the two first acks for 1809. My
  question: Why does a receive window update disqualify the ack as a
  duplicate? The receive window is updated when the receiving process
  gets some data from the socket, i.e., it's a completely local thing.

If the receive window is updated, one can't infer that the ack was due
to another arriving packet - it could simply have been generated by
the TCP receiver reading data.

-- 
        Greg Troxel <gdt@ir.bbn.com>