They’re a bit of a feature of the connection-oriented nature of TCP as the other reply mentions. If the server process crashes and restarts for example, the client will be told that its previous connection is not valid anymore. Basically TCP lets client and server assume that all bytes put into the socket after connect()/accept() will end up at the other side in that same order. Each time there is an error that violates that assumption, the connection needs to be explicitly “reset”.