Sequence number recovery in stateful devices

A method implemented at least in part by a network security device communicatively coupled to a first device and a second device, the method comprising:

• generating a first packet specifying a first sequence number;
• sending the first packet to the first device;
• determining that a response from the first device has not been received within a threshold amount of time;
• generating, at least partly in response to the determining, a second packet specifying a second sequence number;
• sending the second packet to the first device;
• receiving, from the first device, a third packet specifying a third sequence number;
• and
• storing the third sequence number in association with a communication session between the first device and the second device.

A network security device communicatively coupled to a first device and a second device, the network security comprising:

• one or more processors;
• and
• one or more non-transitory computer-readable media storing computer-executable instructions that, when executed by the one or more processors, cause the one or more processors to perform acts comprising:
• sending, to the first device, a first empty acknowledgment packet specifying a first sequence number;
• receiving, from the first device, a second empty acknowledgment packet specifying a second sequence number;
• and
• storing the second sequence number in association with a communication session between the first device and the second device.