What is the purpose of the keword "established" in the next example:
acces-list 110 permit tcp any 172.30.0.0. 0.0 255.255 established ?
According to documentation it allows to check ACK and RST flags in TCP. (6-bit flag field in TCP-header).
If one of these flags is set, a match occurs. If neither bit is set and the source want to establish a TCP connection, a match will not occur.
So my question is what is the purpose of this and when can you deploy it ?