[Guy Harris <gharris@xxxxxxxxx>]

Rami AlHasan wrote:

> I would be grateful if anyone can describe to me the meaning of the 
> dissesctor tables, which is used at the parsing of the single-octet IE 
> and Variable-length IE of the Q931 protocol dissector. I did not 
> understans what does this code segment do?

It lets other dissectors request that they be handed the contents of 
particular information elements - in effect, it lets you extend the 
Q.931 dissector without changing the dissector.

> Is it a must to do this?

That depends on whether any dissectors do that.  None of the dissectors 
that come with Ethereal do that, but some people might have private 
add-ons that do.

> what is the effect of ignoring this portion of the code?

If you ignore it, you don't know all the things you can do with the 
Q.931 dissector.

> Please in other words what makes this condition happen and we enter this If statement?

What makes it happen is dissectors that register in one of those 
dissector table for a particular IE number or a particular Q.931 codeset 
and network traces that contain Q.931 packets using that IE number or 
codeset.