[Stefano Pettini <spettini@xxxxxxxxxxxxxxxxxxxxx>]

Hello,

I have a question about a relationship between some fields and protocols.

I have protocol A (A over UDP) that can transport protocol B (B over A, 
of course) and protocol C (C over A). Protocol A has extensions and 
variable fields, identified by a number.

There are extensions with a number within a known interval that are 
related to A, but there are also additional extensions with a number 
within another known interval related to the upper layer protocol (B, C 
or other protocols).

For example, in protocol A, the extension numbered 64 contains data 
related to protocol B, if A transports B, or to protocol C, if A 
tranports C.

There's no way to understand the protocol of A's payload. This should be 
done manually by the user, using the "decode as ..." window.

Where should I write the code to dissect those extensions?

If I add their dissector in A, how can I know which is the upper layer 
protocol choosen by the user? Also, I think it's not a good idea to add 
the code to dissect B, C or other protocols in A's dissector.

If I add their dissector in B and C, how can I access bytes of A, a 
lower level protocol? And how can I update A protocol tree?

I think there could be a third way: since B and C are very related to A, 
I could consider them as a single protocol, and write 3 dissector: one 
for A (generic, that doesn't dissect those extensions), one for B/A, one 
for C/A.

Any suggestion?

Thanks.

Stefano