Wireshark · Wireshark-dev: Re: [Wireshark-dev] Best practice for dissecting modular protocol

[Stephen Fisher <steve@xxxxxxxxxxxxxxxxxx>]

On Dec 18, 2009, at 2:11 AM, Jan Gerbecks wrote:

> -------------------------------------------------------------------------------------------------------------------------------------------------------------
> | FieldID 8bit	| Length 16 bit | Data as specified in FieldId and  
> described by length	| FieldID | Length | Data|---		|
> -------------------------------------------------------------------------------------------------------------------------------------------------------------

This looks just like the common Type-Length-Value (TLV) format (http://en.wikipedia.org/wiki/Type-length-value 
) used in protocols such as Cisco Discovery Protocol (CDP) and many  
others.  Unfortunately, Wireshark does not have built-in routines for  
easily handling TLV data right now.  Try looking at how the CDP  
dissector handles it (epan/dissectors/packet-cdp.c).  Maybe others can  
suggest a better example dissector.

> To dissect this correctly, I could obviously try to define the  
> maximum number of PNRP Ids in the hf_register_info hf[] Array but  
> that doesn't seem like a very elegant solution.

> I had a look at the vnc dissector but it didn't quite solved the  
> same problem.

Yeah.. As the writer of a lot of the VNC dissector, I would say that  
it's probably not a good example as the total length of the nested  
messages in VNC usually isn't known at the beginning of the message,  
so it's kind of a hack to get it to work at all :).


Steve