I'm trying to update/improve someone else's decoder written in Lua.
It's for a simple UDP (and TCP) based protocol.
But I need to be able to get access to the upper layer
to be able to decode it easily.
Can someone provide an example of how to determine if the higher layer was UDP or TCP?
If all you need to know is whether it was called from TCP or UDP, then a typical approach is to create slightly different dissector functions, one for TCP and one for UDP (they can then call a common function, setting a parameter) and register the TCP dissector with TCP and the UDP dissector with UDP. That approach for C dissectors is demonstrated, for example, here:
And it's pretty similar for Lua dissectors using some of the examples linked from here:
John Thacker