Connection Refused.
The host refused service. This happens when a server process is not present at the requested address.
Connection timeout. No connection established in specified time. This happens when the destination host is down or when problems in the network result in lost transmissions.
Connection is reset by a partner. The server unexpectedly closed the connection and the request to the server was not fulfilled.
The 500 (Internal Server Error) status code indicates that the server encountered un unexpected condition that prevented it from fulfilling the request.
The 503 (Service Unavailable) status code indicates that the server is currently unable to handle the request due to a temporary overload or scheduled maintenance, which will likely be alleviated after some delay. The server MAY send a Retry-After header field (Section 7.1.3) to suggest an appropriate amount of time for the client to wait before retrying the request.
Note: The existence of the 503 status code does not imply that a server has to use it when becoming overloaded. Some servers might simply refuse the connection.
The 504 (Gateway Timeout) status code indicates that the server, while acting as a gateway or proxy, did not receive a timely response from an upstream server it needed to access in order to complete the request.
https://www.rfc-editor.org/rfc/rfc7231#section-6.3.3
https://docs.oracle.com/cd/E19455-01/806-1017/sockets-9/index.html https://support.postman.com/hc/en-us/articles/6346313956887-Fixing-an-ECONNRESET-error