1.2 Fatal Errors

Certain invalid situations are described in this specification, such as a server sending a Push Notification subscription request to a client, or a client sending a Push Notification response to a server. These should never occur with a correctly implemented client and server, and if they do occur, then they indicate a serious implementation error. In these extreme cases, there is no reasonable expectation of a graceful recovery, and the recipient detecting the error should respond by unilaterally aborting the session without regard for data loss. Such cases are addressed by having an engineer investigate the cause of the failure and fixing the problem in the software.

Where this specification says "forcibly abort", it means sending a TCP RST to terminate the TCP connection and the TLS session running over that TCP connection. In the BSD Sockets API, this is achieved by setting the SO_LINGER option to zero before closing the socket.