FIN/ACK closes upstream connection for HTTP, but not for HTTPS

Server NGINX
1

I’m running nginx as a reverse proxy to do SSL and load balancing. The
upstream server behind nginx needs to know when the client closes the
connection.

If I turn ssl off, the connection looks like:

  1. client connects to nginx
  2. nginx connects to upstream
  3. …
  4. client sends FIN/ACK to nginx
  5. nginx sends FIN/ACK to upstream, and to client

With ssl on:

  1. client connects to nginx
  2. nginx connects to upstream
  3. …
  4. client sends FIN/ACK to nginx
  5. nginx waits 60 seconds
  6. nginx sends FIN/ACK to upstream

What’s going on here? Why does nginx wait for a timeout to send the
FIN/ACK
upstream? Why does the behaviour change when I turn ssl on?

Thanks,
Jeremy