Microsoft patents network backup system that keeps connections alive during failures

1. A method for managing connection states at an active network appliance and a backup network appliance, the method comprising: receiving, at the active network appliance, a packet associated with a communication session of a VM in a software defined network (SDN), wherein: the active network appliance and backup network appliance are configured to process connections in the SDN, the active network appliance is configured to actively process connections and the backup network appliance maintains connection states such that the backup network appliance can actively process the packet associated with the communication session in response to a failure of the active network appliance, and network interfaces from the VM are provisioned on multiple network interface devices at each of the active and backup network appliances; identifying an active network interface device on the active network appliance associated with the communication session, wherein a backup network interface device on the backup network appliance is also associated with the communication session; forwarding the packet to the active network interface device for processing the packet associated with the communication session; and in response to determining that the active network interface device has failed, causing subsequent packets associated with the communication session to be routed to the backup network interface device associated with the communication session, thereby allowing the backup network interface device to maintain connection state information and avoid performing new connection state processing for the communication session.