[Cluster-tech] Add CRC to PC9x

[Dirk Koopman]

Lee Sawkins wrote:
> Hi Dirk
> 
> Is it going to cause you any problems if I add a CRC immediately after
> the last "^" character in the PC9x sentences?  This would be a PKZip
> compatible CRC32.  When I split up the current PC9x sentences I do not
> process data after the last "^" as it is always empty.  Does Spider do
> the same?  This CRC would not include the hops count, since it changes.
> 
> The reason for adding this is to find and discard corrupted packets. 
> This would help stop looping and prevent erroneous data from entering
> the user/node tables.
> 
> It would be great if you could do the same with your PC9x packets.
> 

It will break the software, stop hop count decrementing and be generally 
a nuisance.

In any case a CRC32 is a complete overkill for the job. I notice that 
the problem has gone away (at least for now). I expect someone was doing 
some experimenting. It isn't just you and me that is fiddling, there are 
other (silent) fiddlers out there.

I am inclined to not do it. Sentence corruption is such a rare event 
that I am not, at all, convinced a crc (of any kind) is anything other 
than noise. I am slowly removing as much of that as I can get away with 
as it is.

You have to consider the serious possibility that someone was reading in 
a sentence, parsing it wrongly, putting it back together and sending it 
out. For some reason people seem to think that is a better way of 
dealing with this stuff than keeping the sentence as is, just 
decrementing the hop count and then sending it out. The problem is with 
the splitting up, parsing and regurgitating method is that the crc will 
(almost certainly) be recalculated on the new sentence - which is now 
*different* to the original.

We have had all that in the past with people "adjusting" comment or 
frequency fields. It still goes on, to an extent, somewhere where people 
post in UTF8 and it gets converted somewhere to iso-88xx or vice versa.

I wish people would just leave the bl***y sentences *ALONE*!!!

Dirk