Installation guide

6: LAN Message Filtering
DC 900-1325I 83
The ICP header includes a field that specifies the number of bytes in the combined pro-
tocol header and data areas. If a filter function changes the number of bytes in the pro-
tocol data and header area, it must change the byte count field in the ICP header as well.
Failure to do so results in failures at the ICP or the protocol application. Note that the
ICP header is always in network (big endian) byte order. Use the
ntohs library function
to examine the byte count and the
htons library function to modify it.
The ICP header and protocol header structures are defined in the chapter on Client-
Server Protocol in the Freeway Client-Server Interface Control Document.
If the SRA filter function changes the ICP header byte count to zero, the msgmux task
discards the message. Be careful when returning a count of zero. The validity of this
action depends on the protocol and the protocol application. We suggest that you con-
tact Protogate’s Customer Support Department as described on page 14 as to the advis-
ability of eliminating messages in this fashion for the protocol used by your Freeway
configuration.
Figure 6–2: Freeway Server Message Buffer
TSI header
Freeway header
ICP header
Protocol header
Protocol data
.
.
.
3032