[OPENSSH] Bad protocol version - network scan or possible attack [0/3]

alert any $EXTERNAL_NET any -> $HOME_NET any (msg: "[OPENSSH] Bad protocol version - network scan or possible attack [0/3]"; content: "Bad protocol version identification"; content:!"UNKNOWN";parse_src_ip: 1; parse_port; normalize; default_proto: tcp; default_dst_port: $SSH_PORT; parse_port; threshold: type suppress, track by_src, count 3, seconds 300; classtype: non-standard-protocol; program: sshd; sid: 5000068; rev:9;)