Simon's SQL

A couple of cases for this error message.

1. Network device hardware failure.
2. Old Driver version
3. SQL bug. Please check the latest patch.
4. TCP Chimney setting and the SyncAttackProtect setting.
5. AG listener or Endpoint port misconfiguration.
6. Encryption/Decryption issue during data transit.
   1. SSL or TLS cipher mismatch
   2. AG endpoint encryption method discrepancy.
7. During AG or Mirroring failover transit.
8. Network packet loss due to any reason
   1. Firewall blocking for certain case.
   2. Jumbo frame or MTU size misconfiguration.
9. Login Authentication issue.

https://sqlperformance.com/2013/11/system-configuration/ag-connectivity

https://blogs.msdn.microsoft.com/developingfordynamicsgp/2013/12/03/tcp-chimney-setting-and-sql-server-error-tcp-provider-an-existing-connection-was-forcibly-closed-by-the-remote-host/

https://blogs.msdn.microsoft.com/docast/2017/07/27/sql-connectivity-troubleshooting-checklist/

https://technet.microsoft.com/en-us/library/ms187005(v=sql.105).aspx

https://documentation.red-gate.com/clone2/troubleshooting/installation-issues/an-existing-connection-was-forcibly-closed-by-the-remote-host-error

http://sirsql.net/content/2016/11/09/availability-groups-issue-with-2016-cu2/