What exactly does the Nagle algorithm do?

  It groups together as much data as it can between ACK's from the other
  end of the connection.  I found this really confusing until Andrew
  Gierth (andrew@erlenstar.demon.co.uk) drew the following diagram, and
  explained:

  This diagram is not intended to be complete, just to illustrate the
  point better...

  Case 1: client writes 1 byte per write() call. The program on host B
  is tcpserver.c from the FAQ examples.

        CLIENT                                  SERVER
  APP             TCP                     TCP             APP
                  [connection setup omitted]