IEC 60870-5-104 (IEC104) is a protocol distributed by the International 
Electrotechnical Commission (IEC) that provides a standardized method of
sending telecontrol messages between central stations and outstations, 
typically running on TCP port 2404.

It is used in combination with the companion specifications in the 
IEC 60870-5 family, most notably IEC 60870-5-101, to provide reliable
transport via TCP/IP. 

An IEC104 Application Protocol Data Unit (APDU) consists of one of three 
Application Protocol Control Information (APCI) structures, each beginning
with the start byte 0x68. In the case of an Information Transfer APCI, an
Application Service Data Unit (ASDU) follows the APCI.

The IEC104 inspector decodes the IEC104 protocol and provides rule options
to access certain protocol fields and data content. This allows the user to 
write rules for IEC104 packets without decoding the protocol. 