RFC793 (original TCP specification) declared that during the
SYN-RCVD state, only pure <RST> segments are to be processed.
However, RFC5961 and the current TCP specification RFC9293
improved this by stating that also <RST,ACK> should first
be checked for acceptability, and if acceptable, may either
trigger a challenge ACK, reset the connection, or get
silently ignored.
This was found with a hyperscaler loadbalancer, which would
validate the reachability of TCP services by frequently performing
a TCP 3WHS, immediately followed by a <RST,ACK>. When the <RST,ACK>
overtakes the final <ACK> of the 3WHS, that RST got ignoried, leading
to exhaustion of TCP tables, as the processing was different between
SYN-RCVD and ESTABLISHED state.