An application gets a connection reset by peer error when it has an established TCP connection with a peer across the network, and that peer unexpectedly closes the connection on the far end.
That usually happens when the peer crashes, but can also happen with poorly-written applications or frameworks that don't shut their TCP connections down cleanly.
Similar to a connection refused error, a connection reset by peer error is generated when the operating system receives a TCP reset (RST) from the remote system.
Blog: Cloudy with a Chance of TCP Drops
Network Data: Key Concepts
Network Performance Monitoring
Corvil for IT Operations Analytics
Connection Refused
TCP reset (RST)