I came across an old service which listens on a range of TCP ports. Client connections are handled with this scheme:

1. Client connects to the first port
2. Server answers with a port number and closes the connection
3. Client re-connects to the server-supplied port
4. Business happens

Comments indicate that the author was insistent that, once a client connected to a port, that port was "in use". No other clients would be able to connect. Allegedly, the connection attempt would block. That is why the first "welcome" port needed to be "freed" as soon as possible.

Obviously, this does not make much sense today, but it made me wonder: Has there ever been a time or architecture which supported TCP/IP and multithreading, but could not handle more than one connection per TCP port? The application is still running today on a Win2k server.

The source is written in Pascal. No indication of when it was first conceived. My guess is early 90s, targeting DOS.