Philip Withnall authored Jul 31, 2015 and Olivier Crête committed Jun 03, 2016

The state tracking previously assumed that if a FIN packet was sent, the
other side received it and the preceding packets, and hence it was
correct to sent an RST if an unexpected packet (such as a delayed
SYN-ACK) was received.

In cases where there is high packet loss, this won’t work. For example,
peer A sends a SYN, it is received and peer B replies with a SYN-ACK
which is also received; then peer A sends its data and a FIN, which are
both dropped. Since it hasn’t received anything since the original SYN,
peer B resends its SYN-ACK. If that is received, peer A was incorrectly
treating it as an erroneous packet, and would then send a RST. In actual
fact, it should take this as a signal that the data and FIN packets were
dropped, and should resend them.

TODO: Add unit tests